final report -group-41
TRANSCRIPT
Calculation and sizing of equipment installed at HIS based PAKISTAN
REFINERY LIMITED grid station with short circuit calculation and over
current earth fault protection.
Prepared by:
PROJECT ADVISORS
Internal Advisor
(Lecturer NEDUET)
External Advisor
(Assistant Manager KESC)
Department of Electrical Engineering
N.E.D.University of Engineering & Technology
Karachi-75270
BATCH 2008-2009
MUHAMMAD MUBEEN MAHMOOD (EE-143)
MUHAMMAD HASSAN UL HAQ
TALHA ALI QASMI
B.E (EE) PROJECT REPORT
2
Calculation and sizing of equipment installed at HIS based PAKISTAN
REFINERY LIMITED grid station with short circuit calculation and over
current earth fault protection.
B.E (EE) PROJECT REPORT
Prepared by:
MUHAMMAD MUBEEN MAHMOOD (G.L)………………….EE-143
ALI AHMED …................................................………………….EE-167
HABIB ALI KHAN…...............………………………………….EE-169
MUHAMMAD UMER FAROOQUE KHAN………………..…EE-188
Project Advisors
Internal Advisor
MUHAMMAD HASSAN-UL-HAQ
(Lecturer NEDUET)
External Advisor
TALHA ALI QASMI
(Assistant Manager KESC)
Department of Electrical Engineering
NED University of Engineering & Technology
Karachi-75270
BATCH 2008-2009
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ACKNOWLEDGMENT
First and foremost we would like to thank Allah almighty for his infinite blessings which
helped us and give us strength to complete this project.
Secondly we would like to recognize the support of our internal advisor Mr. Hassan-ul-
Haq who guided us through all the levels of our project and directed us to the path of the
project completion.
Thirdly we would like to thank our external advisor Mr.Talha Ali Qasmi who helped us
on every step, takeout time from his extremely busy schedule and arranged numerous
meetings, and provided us component specifications on our requests, checked the report
completely and properly and furthermore he answered our questions where our
intelligence failed. Altogether, Mr. Talha’s efforts, patience and knowledge made this
project a worthwhile for us!
We would like to express my appreciation towards my parents & members of KESC for
their kind co-operation and inspiration which help us in completion of this project
would not have come into existence without their efforts, patience and their believe in
our abilities.
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ABSTRACT
The above mention project is basically the study and simulation project which is performed by us under the supervision of KESC
Our project is at the HIS based PRL Grid station. The scope of this project is to size the equipments along with earthing and protection practices of the concerned grid station.
Our extent of work is to calculate and size various equipments such as CTs, VTs, Power and auxiliary transformers, Capacitor and D.C battery banks etc. Along with the simulation Short circuit fault analysis and earthing design on Electrical Transients Analysis Program (ETAP).
Our main objective of doing this project is to lucid our view and get a practical exposure of how equipment sizing and design is done industrially. Also, the calculations of the project will help KESC officials to verify the work done by the electrical contractors of the project i.e. ABB.
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CONTENTS
TOPIC PAGE N.O
CHAPTER 1. INTRODUCTION
CHAPTER 2. BACKGROUND
CHAPTER 3. SECURITY
1. EARTHING 18
1.1 SOIL CHARACTERISTICS 18
1.1.1 SURFACE MATERIAL 18
1.1.2 SOIL RESISTIVITY AND PARAMETERS 19
1.1.3 RELATION BETWEEN SURFACE MATERIAL AND SOIL LAYER 20
1.1.4 SELECTION OF WEIGHT_____________________________________________20
1.2 DEFINITIONS OF PARAMETERS 20
1.3 DESIGN PROCEDURE 24
1.4 CALCULATIONS 26
1.4.1 Data Given 26 1.4.2 Conductor’s Size ____27 1.4.3 Tolerable Step And Touch Potentials 28 1.4.4 Ground Resistance/ Number Of Conductors 29 1.4.5 Grid Current 30 1.4.6 Decrement factor 31
1.5 METHODS TO LOWER GROUND CALCULATIONS 31
1.6 ETAP SIMULATION RESULTS 32
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CHAPTER 4. OPERATION
2. CAPACITOR BANK
2.1 NEED OF CAPACITORS 34
2.2 CALCULATIONS 35
3. POWER TRANSFORMER 38
3.1 TYPE OF CORE 38
3.2 SERVICE ALTITUDE______________________________________________________________40
3.3 HOTSPOT FACTOR 40
3.4 PARAMETERS 41
3.5 SIZING OF POWER TRANSFORMER 45
4. AUXILIARY TRANSFORMER
4.1 LOAD CALCULATIONS 46
5. INSTRUMENT TRANSFORMER
CURRENT TRANSFORMER 48
5.1 DESIGN PARAMETERS 49
5.2 ERRORS ___________________________________________________________51
5.3 CT SIZING_________________________________________________________________53
CAPACITIVE VOLTAGE TRANSFORMER __________________________________ 63
5.4 DEFINITIONS 63
5.5 SPECIFICATIONS 64
5.6 SIZING 65
CHAPTER 5. PROTECTION 6. CABLES
6.1 TYPES OF CABLES 68
6.2 SPECIFICATIONS 70
6.3 CALCULATIONS 7. CIRCUIT BREAKERS
7.1 INTRODUCTION 75 7.2 SPECIFICATIONS 75 7.3 SELECTION 79
8. SHORT CIRCUIT ANALYSIS
8.1 DIFFERENCE BETWEEN OVERLOAD AND SHORT CIRCUIT 80 8.2 CAUSES OF SHORT CIRCUIT 80 8.3 SHORT CIRCUIT CURRENT IN OTHER ELEMENTS 81
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8.4 SHORT CIRCUIT CALCULATION 82
8.5 SOURCES OF SHORT CIRCUIT CURRENT 85
8.6 REACTANCES OF ROTATING MACHINES 86
8.7 SHORT CIRCUIT ANALSIS BY ETAP 89
9. OVERCURRENT AND EARTH FAULT PROTECTION
9.1 INTRODUCTION 90
9.2 SIMULATION RESULTS 92
9.3 PROTECTION CURVE 96 9.4 SIMULATION REPORT 97
10. DC BATTERY BANK
10.1 DEFINITIONS 98
10.2 DESIGN CONSIDERATIONS 99
10.3 DESIGN PROCEDURE 100
10.4 CELL SIZING CALCULATIONS 102
CONCLUSION
REFERENCE 108
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CHAPTER 1
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INTRODUCTION
There are many considerations in carrying out Electrical Projects. The most important
are reliability and economic considerations. While working on this project, our basic
objective was to design earthing, protection systems and size the grid equipments to
ensure smooth and economic operation. These efforts not only reduce the cost of the
project but also lessen future maintenance costs.
This project was under the supervision of KESC Officials. We started off with earthing
system design in which, earthing mesh was designed according to site specifications and
grid requirements. Next, sizing of equipments like as CTs, VTs, Power and auxillary
transformers, Capacitor and D.C battery banks etc. was done. This was critical because,
inaccurate calculations would have led to disastrous costs and mal-operations.
Furthermore, Capacitor banks were sized to improve power factor upto 0.95. Then
came the short circuit fault analysis which is a major protection concern for electrical
engineers.The reliability and safety of electric power distribution systems depend on
accurate and thoroughh knowledge of short-circuit fault currents that can be present,
and on the ability of protective devices to satisfactorily interrupt these currents.This
computational Knowledge helps us for planning, design, operation, and troubleshooting
of electrical power systems.
Cables and Circuit Breakers were then sized according to the results of the shorts circuit
simulations of ETAP.
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CHAPTER 2
11
BACKGROUND
This project basically belongs to KESC. It is being done for Pakistan refinery limited grid station which is in stages of installation. The project’s results will be ratified by the engineers working for KESC. The plant has: •Two 132 kV line bays •Two 132 kV power transformer bays •One 132 kV coupler bay •Two 132/11 kV power transformers (40MVA each) •Twenty Six 11 kV outgoing feeders •Two 11 kV incoming feeders The control building is with 11kV metal enclosed switchgear
PROBLEMS AND THEIR SOLUTIONS:
The problems are solved keeping in mind the economical and reliable aspects of the
project.
Earthing Design:
Earthing mesh’s design and calculations entailed an exhaustive study from IEEE Std 80-2000 (Revision of IEEE Std 80-1986). Then simulation was performed on E-tap of earthing. The basic concept of earthing is that we lay a mesh of conductors beneath the earth surface and a few ground rods to eject the current into the earth. During this designing, a number of things like groundresistance, step and touch potentials are needed to kept within permissible and safe bounds. However we have designed by keeping the ground resistance 0.283 Ω fixed, and was successful in obtaining the same desired value of ground resistance. We did encounter some problems in meeting the desired values of ground resistance, step and touch potentials, but the ground resistance was lowered by adding additional grounding rods. This in turn also brought the step and touch potentials to the desired value.
Sizing the Power Transformer and its solution:
We had to design the power transformer which best suited for the site because the load
was varying. The transformer was sized such that KESC specifications were strictly
followed. The type of stacking we used is step lap stacking. There are three principle
benefits of step-lap, firstly reduced material costs, as less core steel is required for the
same losses, secondly faster assembly of the core and thirdly lower noise.
In our t/f at normal conditions ONAN (oil natural air natural) type of cooling will be
applicable but after ambient temperature which is 50°C according to KESC specification
ONAF (oil natural air force) type of cooling is used. The maximum temperature that can
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be sustained by transformer and its equipment at normal condition. Up till this
temperature our transformer is working satisfactory. According to KESC specs this
temperature is 50°C.
The maximum temperature or the highest temperature that can be sustained by
transformer winding is the hotspot temperature. The local hot spot temperature is 20°C
greater than the normal temperature of winding. According to KESC specs this
temperature is 68°C.
Capacitor Bank Size For Correcting the Power factor :
Since we were dealing with highly inductive load, therefore it was necessary to use pfi
plants as it is not recommended due to economic reasons because cost of generation
increases in order to minimize the reactive part we place the capacitor because
capacitance of capacitor cancel the effect of inductive part
By improving power factor we get many advantages some of this mentioned below:
Lower utility fees by:
(a). Reducing peak KW billing demand:
(b). Eliminating the power factor penalty:
Increased system capacity and reduced system losses in your electrical
system
Increased voltage level in your electrical system and cooler, more efficient
motors
We connect our capacitor near to the load in delta connection because cost in delta
decreases because its size decreases Cy =3C∆
Rating of Circuit Breaker :
A circuit breaker is equipment that breaks a circuit either manually or automatically
under all conditions at no load, full load or short circuit
The rating of CIRCUIT BREAKER depends on two currents i.e. short circuit and normal
current we find out these through the calculation and from simulation through ETAP.
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Short Circuit Calculation :
We have also done the short circuit analysis and calculation in our project so to avoid
the short circuit faults occurrence in our project. Since we know that whenever a fault
occurs on a network such that a large current flow in one or more phases and a load is
bypassed, a short circuit occurrs.
A fault may occur on a power system due to number of reasons .Some of the common
causes are,
Earthquakes
Snow frost
Falling of a tree along a line
Vehicles colliding with supporting structures
Birds shortening the line
Small animals like rats, lizards etc. enter switchgears to create fault.
Insulation breakdown
Lightning
High speed winds
The types of Short circuit faults are classified as
Symmetrical Faults
Unsymmetrical Faults
The unsymmetrical faults can further be classified as;
Single line-to-ground (L-G) fault
Line-to-line(L-L) fault
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Double lint-to-ground(L-L-G) fault
Three phase short circuit (L-L-L)fault
Three phase-to-ground(L-L-L-G) fault
IMPACT OF SHORT CIRCUIT:
The consequences are variable depending on the type and the duration of the fault, the
point in the installation where the fault occurs and the short-circuit power.
As it is quite obvious from the chart that short circuit current is so severe for the
power system
Short circuit current is about 10 times to that of full load current for each of the
equipment used in the power system.
If that much current pass through the system when it results in following
consequences.
At the fault location, the presence of electrical arcs, resulting in damage to
insulation, welding of conductors, fire and danger to life.
On the faulty circuit Electrodynamics forces acts, resulting in deformation of the
bus bars and Disconnection of cables.
Excessive temperature rise due to an increase in I2R losses can damage or melt
the insulation of wire.
All equipment and connections (cables, lines) subjected to a short circuit undergo
strong mechanical stress (electrodynamics forces) which can cause breaks, and
thermal stress which can melt conductors and destroy insulation. Etc.
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Current Transformer
CURRENT TRANSFORMER which is also referred to as 'CT' is ” is primarily used for
measuring line current since it is not possible to use line current directly for
measurement and relaying purpose as there may occur insulation problem due to its
high value. 'CT' is a piece of electrical equipment which converts line current (primary
current) in to small standard current values which are suitable for the connected
devices. These connected devices may be measuring instruments, relays for protection
purpose or other devices etc.
In the secondary circuit, instrument transformers reproduce the current or voltage
owing to its primary circuit within the prescribed limits along with the phase relations.
On the next stage, current transformers then transform their current or voltage levels
into a level than can be safely utilized. In this two windings primary and secondary
winding .primary has single turn while secondary has many turns Cross section of
primary winding is greater as compare to Secondary winding Because primary winding
is consist of single turn and secondary winding is consist of many turns, CT primary
current is greater than secondary, so primary energized also secondary energized so
we have to use greater cross section in primary. If we use lesser cross-section wire at
the primary so because of high current at primary it will damage due to excessive
heating.
BATTERY BANK DESIGNING:
Battery banks are used to supply backup DC Supply.
A Battery is a device that can change chemical energy into electrical energy by reaction
of certain chemicals. Electrons from one kind of chemical travel to another under as a
consequence of the chemical reaction, this causes an electric current that can power a
load.
Batteries have three basic parts:
1. Anode: It is the positively charged electrode that attracts the negative ions.
2. Cathode: It is the negatively charged electrode that attracts the positive ions.
3. Electrolyte: It is a liquid medium which acts as medium to conduct electricity.
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The maximum and minimum permissible system voltages decide the number of cells in
the battery. It has been normal practice to use 9–10, 18–20, 36–40, 92–100, or 184–200
cells for system voltages of 12, 24, 48, 125, or 250 V, correspondingly.
OVERCURRENT AND EARTH FAULT PROTECTION:
When relatively high current, above the normal operating current, flows in the system
for certain period of time it is called overcurrent. The power system is capable to bear
the overcurrent for certain time.
A fault may occur between the phases and phases and ground. The faults which cause
the short circuit currents to flow through the earth are called earth faults or ground
faults.
Over current and earth fault protection simulation is done on ETAP. The simulation
includes protection of
3-phase symmetrical fault
Line to ground fault
Line to line fault
Line to line to ground fault
All the faults have extensively been discussed in the short circuit report.
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CHAPTER 3
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EARTHING 1.1SOIL CHARACTERISTICS Soil and surface material selection is an important consideration for the earthing design of a substation. The geometry of the grid, likewise other parameters, also depends upon the surface material and soil type as it directly affects the mesh voltage. Moist soil is a good electrical conductor with some resistance ‘r’ and also acts as a dielectric between the two buried conductors but since the charging current is insignificant as compared to the leakage current i.e. the current that flows through the protective ground conductor to ground, so the earth can be modelled as the pure resistance. The grid resistance and the voltages such as step touch and mesh depends directly on the soil resistivity. The variation in the resistivity may cause the variation in the voltage gradients within the grid. We have considered the uniform soil assumption for the calculations of required voltages so that to employ single value of resistivity.
1.1.1 Surface Material: A surface material is used in order to increase the contact resistance between human feet and the ground. The surface material should be considered of high resistivity material in order to increase the contact resistance between the human feet and earth. This lowers the danger of potential gradients which affect the human in the vicinity of the substation. The data for surface material is
• Surface Material type. Gravel • Resistivity. 8534.4 ohm.m • Depth. 0.1m
Gravel is often used for the surface material as it has high resistivity. It is composed of
unconsolidated rock fragments and due to which it actually distorts the potential gradients caused by the ground conductor which may add up with the potential gradient of the adjacent conductor and be dangerous. Gravel is very useful in retarding the evaporation of moisture and limits the drying of topsoil layers during long dry weather periods. Covering the surface with a material of high resistivity is very valuable in reducing shock currents. A layer 0.1–0.15 m thick decreases the danger factor (ratio of body to short-circuit current) by a ratio of 10:1, as compared to the natural moist ground. The shallower depth of just 0.1 reflects the uniform soil assumption vertically. Increasing the depth of surface material to 1m will further decrease the shock possibility and further increases the contact resistance but then it would account in the increased ground resistance. The range of resistivity values of the surface material depends upon certain factors which include kinds of stone, size, condition of stone i.e clean or with fines, moisture contents etc. moisture contents lowers the resistivity and then the account is kept of
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using the rock samples of typical type for certain areas. Certain factors such as porosity, resistivity of pore fluid and the percentage of conducting minerals i.e. clays, graphite, sulphides, contained within the sediment determine the resistivity of the rock.
1.1.2 SOIL RESISTIVITY AND PARAMETERS: Ground resistance is an important factor for the grid design which depends upon the layers that have been laid beneath the surface material. The most commonly used soil resistivity models are the uniform soil model and the non-uniform model i.e two-layer soil model and multi-layer for complex soil conditions. Two-layer soil models are often a good approximation of many soil structures. A uniform soil model should be used only when there is a moderate variation in apparent resistivity. In homogeneous soil conditions, the uniform soil model may be reasonably accurate. If there is a large variation in measured apparent resistivity, the uniform soil model is unlikely to yield accurate results. A more accurate representation of the actual soil conditions can be obtained by use of a two-layer model. The two-layer model consists of an upper layer of finite depth and with different resistivity than a lower layer of infinite thickness. A two-layer soil model can be represented by an upper layer soil of a finite depth above a lower layer of infinite depth. The representation of a ground electrode based on an equivalent two-layer earth model is sufficient for designing a safe grounding system. Sand/dry soil is selected as the top layer material with the resistivity of 30 ohm.m and the depth of the upper layer is of 5m. Chosen the depth of 5m and having the grounding electrode length of 3m, the electrode now behaves same when the electrode is in the upper layer of uniform soil of resistivity ρ1. This actually matters when the two layers are of different resistivities. The bottom layer material chosen is also sand/dry soil with the resistivity of 30 ohm.m, ideally which is of 1000 ohm.m. The depth of the lower layer is infinite. A comparison below shows the difference when the two layers of different resistivity are considered. The variation of soil characteristics greatly influence the performance of grounding grid as it effects the ground resistance, GPR, step and touch voltages. When the upper layer is more resistive than the lower layer then the ground resistance will be less than that of the upper layer. In case of touch and step voltages, the voltages will be less as compared when the lower layer is more resistive. If the underlying soil is of low resistivity than that of surface material, as in our case,
than only some grid current will flow in the thin layer of surface material and thus the
current through the body can be lowered considerably.
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1.1.3 Relation between surface material and soil layer: If the underlying soil has a lower resistivity than the surface material, only some grid current will go upward into the thin layer of the surface material, and the surface voltage will be very nearly the same as that without the surface material. The current through the body will be lowered considerably with the addition of the surface material because of the greater contact resistance between the earth and the feet. However, this resistance may be considerably less than that of a surface layer thick enough to assume uniform resistivity in all directions. The reduction depends on the relative values of the soil and the surface material resistivities, and the thickness of the surface material. If the underlying soil has a higher resistivity than the surface material, a substantial
portion of the grid current will go upward into the thin layer of surface material. The
surface potentials will be altered substantially due to the concentration of current near
the surface.
1.1.4 50kg or 70kg Weight selection:
The weight is used to calculate the Tolerable Step and Touch Potentials.50 kg weight
give more strict calculations of step and touch voltages (At 50 kg, resistance will be less
of a person, so more current can flow through it, so to minimize the current we should
have lower value of tolerable touch potential, so calculation becomes strict). Whereas
using 70kg for calculations gives optimized but harmless results. We are using 70 kg
body weight for the calculations.
1.2 DEFINITIONS OF PARAMETERS Ambient temperature of Soil: Ambient temperature refers to the temperature in a room, or the temperature which surrounds an object under discussion. Our concern is with soil, so the ambient temperature of soil calculated and found at site is about 30ºC.
Df:
It is the factor by which a transient during a fault dies out. It determines the rms
equivalent of asymmetrical current in a fault time Tf. It accounts for the effect of initial dc
offset (during system transient condition), and its attenuation during the fault.
Tf: It is the time (in seconds) for the duration of fault. It helps in determining the decrement factor.
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Tc: This time (in seconds) is the clearing time. It is the duration of fault current for sizing ground conductors.
Ts: This is the duration (in seconds) of shock current to determine permissible levels for the human body. The fault duration (tf) (tc) and shock duration (ts) are normally assumed to be equal.
Range: Typical values for tf and ts range from 0.25 s to 1.0 s.
Ifg: It is the rms value of the fault current to ground in kA. In our grid the value of Ifg is 40 kA.
X/R: It is the ratio between Reactance and resistance. It is used in determining the Decrement Factor Df.
Sf: It is the Current Division Factor. It is a factor in percent, relating the magnitude of fault current to that of its portion flowing between the grounding grid and the surrounding earth.
Cp: Cp stands for Corrective Projection Factor. It is a factor in percent, accounting for the relative increase of fault currents during the station’s lifespan.
If there exists no future growth in the system then, Cp = 100%. Cp= 1.25 as per recommendations of IEEE 665-1995, page number 12
Touch Potential (Ep): It is the potential difference between the ground and the hand of a person in contact with a grounded structur
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Step Potential (Es): It is the potential difference between the feet of a person standing near a grounded structure without being in contact with it. The value of the maximum safe step potential and touch potential depends on the
resistivity of the top layer or surface material, and on the duration of the current flow.
For example, for a substation with a 0.1 m layer of crushed rock and current flowing for
0.5 s, the maximum value of the safe step potential is approximately 3100 V and the
maximum safe touch potential is approximately 880 V. In our case with gravel as surface
material having resistivity of 8534.4 ohm-m and depth of 0.1 m, and with duration of
current flow 0.5s, tolerable touch potential is 2185.362 V, whereas the tolerable step
potential is 8075.42. This shows that on changing the surface material while keeping
depth of surface material and current flow duration constant, step and touch potentials
vary
The value of step and touch potential must be in ranges to avoid the critical amount of shock energy from being absorbed before the fault is cleared and the system de-energized
Ground Potential Rise GPR: This voltage is equal to the product of maximum grid current IG and grid resistance Rg.
For normal operating conditions the value of GPR is near zero (0). But when fault occur the value of GPR rises due to the difference in potential between fault site and another remote ground.
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Rg: It is the substation ground resistance in Ω. Range:
Ground resistance is usually about 1 Ω or less. In smaller distribution substations, the usually acceptable range is from 1 Ω to 5
Ω.
Reflection factor:
It is basically the part of radiant energy that is reflected from a surface. It relates a
reflected wave to an incident wave. In grounding we consider it due to abrupt changes in
soil resistivities.
Reflection factor (k) = -0.993
This value ranges from -1 to +1.
Surface layer derating factor:
It is used to calculate the ground resistance of the foot in the presence of a finite
thickness of surface.
Surface layer derating factor (Cs) = 0.691
Derating Factor:
It is used to calculate the effect of dc offset during a fault.
Decrement factor (Df) = 1.414
Kh: It is the adjustment weighting factor that determines the effects of grid depth.
Ki: It is the adjustment factor for grid geometry,
Kii It is the adjustment weighting factor for the effects of inner conductors on the corner mesh. Kii = 1
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Km: It is the Spacing factor for mesh voltage. Km = 0.639
Ks: Spacing factor for step voltage Ks = 0.260
Parameters of rods and conductors: Lx: 60m It is the length of conductors in X-direction Ly: 40m It is the length of conductors in Y-direction Number of Conductors in X Direction: 10 Number of Conductors in Y Direction: 10 Depth of Conductors: 1 m Conductor Size: 150 mm2
Conductor Material Used: Copper commercial hard drawn Number of Rods: 8 Rod Diameter: 17 mm Rod Length: 3m Arrangement Of Rods: Rods throughout grid area Rod Material: Copper clad steel rod
1.3 DESIGN PROCEDURE:
These steps are taken from IEEE Std 80-2000-Section 16.4.
Step 1: The property map and general location plan of the substation should provide good estimates of the area to be grounded. A soil resistivity test, will determine the soil resistivity profile and the soil model needed (that is, uniform or two-layer model). Step 2: The conductor size is determined. The fault current 3I0 should bethe maximum expected future fault current that will be conducted by any conductor in the grounding system, and the time, tc, should reflect the maximum possible clearing time (including backup). Step 3: The tolerable touch and step voltages are determined. The choice of time, ts, is based on the judgment of the design engineer.
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Step 4: The preliminary design should include a conductor loop surrounding the entire groundedarea, plus adequate cross conductors to provide convenient access for equipment grounds, etc.The initial estimates of conductor spacing and ground rod locations should be based on the current Ig and the area being grounded. Step 5: Estimates of the preliminary resistance of the grounding system in uniform soil can be determined. For the final design, more accurate estimates of the resistance may be desired. Computer analysis based on modelling the components of the grounding system in detail can compute the resistance with a high degree of accuracy, assuming the soil model is chosen correctly. Step 6: The current IG is determined. To prevent overdesign of the grounding system, only that portion of the total fault current, 3I0, that flows through the grid to remote earth should be used in designing the grid. The current IG should, however, reflect the worst fault type and location, the decrement factor, and any future system expansion. Step 7: If the GPR of the preliminary design is below the tolerable touch voltage, no further analysis is necessary. Only additional conductor required to provide access to equipment grounds is necessary. Step 8: The calculation of the mesh and step voltages for the grid as designed can be done by the approximate analysis technique or by the more accurateIEEEIN AC SUBSTATION GROUNDING STD 80-2000 Step 9: If the computed mesh voltage is below the tolerable touch voltage, the design may be complete (see Step 10). If the computed mesh voltage is greater than the tolerable touch voltage, the preliminary design should be revised (see Step 11). Step 10: If both the computed touch and step voltages are below the tolerable voltages, the design needs only the refinements required to provide access to equipment grounds. If not, the preliminary design must be revised (see Step 11). Step 11: If either the step or touch tolerable limits are exceeded, revision of the grid design is required. These revisions may include smaller conductor spacing, additional ground rods, etc. Step 12: After satisfying the step and touch voltage requirements, additional grid and ground rods may be required. The additional grid conductors may be required if the grid design does not include conductors near equipment to be grounded. Additional ground rods may be required at the base of surge arresters, transformer neutrals, etc. The final design should also be reviewed to eliminate hazards due to transferred potential etc.
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1.4 CALCULATIONS
1.4.1 Data Given
Ground Resistance (Rg) = 0.283 (design for it)
No. of Ground rods= 8 Rod diameter= 17 mm Rod length= 3m Total length of Ground rods= 24 m Arrangement Of Rods: Rods throughout grid area Rod Material: Copper clad steel rod
Length of conductors in X-direction (Lx)= 45m length of conductors in Y-direction (Ly)= 60m
Depth Of Conductors= 1 m Conductor Material: Copper commercial hard drawn
Total Fault current (lfg)= 40kA Max. Grid current= 20.627 kA X/R= 10
Surface Material: Gravel Surface Material Resistivity= 8534.4 ohm.m Surface Material Depth= 0.1m
Upper layer Material: sand Upper layer Resistivity=30 ohm.m Upper layer Depth= 5 m
Lower layer material: sand Lower layer Resistivity= 30 m
Soil Ambient Temperature=30 degree C
Corrective Projection factor (Cp) = 100%
Current division factor (Sf)= 50 %
Reflection factor (k) = -0.993
Decrement factor (Df) = 1.031
Fault time (Tf) =Clearing Time (Tc) =Shock time (Ts) = 0.5 s
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1.4.2 Conductor’s Size
√
) ... IEEE Std 80-2000 (Revision of IEEE Std 80-
1986)-Section 11.3.1-Eq 37 =Rms Current in KA =40
= ?
= Maximum Allowable temperature in Celcius = 500
= Ambient temperature in Celcius = 30
= Reference temperature for material constants in Celsius = 20
=
0.00381
= 1.78
= 0.00413
= 1/ in Celcius = 242
= Tclearing time in s = 0.5
= Thermal capacity per unit volume in J/ .Celcius) =
√
√
So this area of conductor makes it implicit that our conductors have the ability to
conduct 40KA fault current in worst scenario.
However, we are using the conductor size 150 owing to the following reasons:
To give mechanical strength to the conductor.
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Soil promotes corrosion; this causes a gradual reduction in the conductor’s cross
section. In order to compensate for this reduction during the design life of the
conductor, it is prudent to choose a larger conductor size.
To make the conductor capable of carrying short time surges caused by lightning.
Relay malfunctions can cause fault current to flow for time greater than clearance
time. The conductor size must be adequate for the backup time because longer
the fault current flows, more is the heating and more should be the area for heat
dissipation for safe operation.
It is more economical to include an adequate margin in the conductor size when
thinking of future growth, rather than installing additional conductors at later
stage.
1.4.3 Tolerable Step and Touch Potentials:
For 70 Kg Body weight, we have,
√ ----(i) …IEEE Std 80-2000 (Revision of IEEE Std 80-
1986)-Section 8.3- Eq 30.
… IEEE STD 80-2000 (Revision of IEEE Std 80-1986)-Section 7.4-Eq
27
= Surface layer derating factor =?
= Resistivity of earth beneath the surface material in Ω-m = 30
Ω-m = 8534.4
0.1
= Shock time in s = 0.5
= Tolerable Potential difference between the feet at grounding site =?
From (i)…
29
√
Also,
=
√ … IEEE Std 80-2000 (Revision of IEEE Std 80-1986)-
Section 11.3-Eq 33
=
√
1.4.4 Ground Resistance/ Number of Conductors:
√
… IEEE STD 80-2000 (Revision of IEEE Std 80-1986)-Section 14.2-Eq 51
= 0.283
= 30
= are occupied by ground grid in = 45*60
= ?
We want to keep the ground resistance of 0.28 Ω, for that,
√
The area available for grounding and its apt mesh design is shown below:
30
Mesh: 10X10
So for this we have placed 10 conductors in X direction and 10 in Y direction.
X Direction:
1 conductor=45 m
10 conductors=450 m
Y Direction:
1 conductor = 60 m
10 conductors = 600 m
Total length of conductors placed in grid = 450 + 600
Total length of conductors placed in grid = 1050 m
Total Number of conductors = 10 + 10
Total Number of conductors = 20
1.4.5 Grid Current
… IEEE STD 80-2000 (Revision of IEEE Std 80-1986)-Section 15.1.6-Eq 65
31
1.4.6 Decrement factor
√
= ?
= Ambient temperature in Celcius = 30
√
√
1.5 METHODS TO LOWER GROUND RESISTANCE
Some of the methods that lower ground resistance include:
Adding additional ground rod/rods, so that the parallel combination of added
rods decreases the effective ground resistance.
Adding more grid conductors.
Using longer rods, in cases where low resistance soil lies at considerable depth,
and the soil at hand just a few feet below the earth has high resistivity. This is
because moisture contents are more in earth at great depths from the surface.
Increasing the grid depth.
Increasing grid area
At places where deep digging is impractical for rods (if rock strata obstruct
digging), it is feasible to treat the soil around the rods with some chemicals that
would reduce the ground resistance. Examples of suitable noncorrosive
materials are magnesium sulphate, copper sulphate, calcium sulphate and
ordinary rock salt. The least corrosive is magnesium sulphate, but rock salt is
cheaper and does the job.
Reducing the spacing between the grid conductors.
32
1.6 ETAP SIMULATION RESULTS:
33
CHAPTER 4
34
CAPACITOR BANK
2.1 NEED OF CAPACITOR BANK
Capacitor bank is used to improve the power factor, because low power factor has
following drawbacks,
Power factor is inversely related to the current, so at low power factor, we
require larger cables, switch gears, transformers, alternators etc, which makes
capital cost very high.
Higher currents cause more copper losses which decreases the efficiency of the
system
More voltage drop occurs at higher currents.
More voltage drop occurs at low power factor, and thus the voltage regulation
becomes poor.
Power factor can also be improved by synchronous condenser. When we have induction
motors as load, it is more economical to use them in overexcited mode to improve the
power factor.
Reactive Power Needed By the Capacitor:
Phasor diagram as seen by the load
P = Real Power
S´ = Apparent Power at new
(improved) power factor.
S = Apparent power at old (low)
power factor
=Old Power factor angle
= New Power factor angle
35
Q´ = Reactive power supplied by the source to the load
Qc = Reactive power supplied by the capacitor bank to the load
Q = Reactive power needed by the load
Qc = Q- Q´
2.2 CALCULATIONS:
Designing a capacitor bank for:
a) 75% inductive load
b) 15% inductive load
c) 50% inductive load.
a)
We know that the grid station maximum capacity is 40MVA. We employ 2 transformers
of 20MVA each.
When the load 75% inductive it means that the power factor is 0.25 because 0 power
factor isfor complete inductive and 1 power factor for complete resistive loading.
P=Scosθ
P= (20MVA) (0.25)
P=5MW
36
We know
Qc= P (tanθ1 – tanθ2)
Our initial p.f is 0.25,
;
We want to increase our p.f to 0.95 through a capacitor bank,
Qc = 17MW (tan75.522- tan18.194º)=17.720 MVar/Phase
For V=132V
=
µF/phase
For V=11kv
=
C∆=466.152 µF/phase
b) P=Scosθ
When 15% load then it means that our p.f is 0.85
P= (20 MVA)(0.85)
P =P=17 MW
Qc= P (tanθ1 – tanθ2)
Qc = 17MW (tan31.788º - tan18.194º)
= 17MW (0.256)
𝛩 = 31.788
𝛩 = 18.194
𝛩 ⁰
𝛩 ⁰
37
C∆=
C∆ =
C∆=
We connect the capacitor bank in delta (∆) because we know that for same Qc if we
connect our bank in Y (star) its rating is 3 times of delta (∆).
CY= 3C∆
c) When load is inductive that means that the power factor is 0.5
Thus,
Since,
Therefore,
Now,
Qc = 4.36 MVAr/phase
C∆= 796.50 nF/phase C∆ = 114.696 µF/phase
38
POWER TRANSFORMER
3.1 TYPE OF CORE: (Core Type - according to KESC specs) There are two basic types of transformers categorized by their winding/core configuration: Shell Type : A shell type of transformer is one in which the primary winding is wound on first and
then over it secondary winding is wound (per phase). In it more winding material is required a compared to core material.
Core Type : In core type transformer, on one limb primary winding is wound and on the other limb
secondary is wound. In it more core material is required as compared to the winding material.
39
THREE OR FIVE LIMB CORE: (Three - according to KESC specs) We are using a three phase- three limb transformer. The limbs are the structure on which the
primary and secondary windings are wounded. The height of the transformer is not an issue so
go for three limbs transformer. For certain applications, such as mines, tunnelling machines, and
problems associated with overhead clearances for rail-transport in which height of the
transformer is a limitation we go for reduced yoke depth and greater limbs. The KVA capacity is
the function of both, volume of core and coil and the volume of coil depends upon the height of
winding so by reducing the height and increasing the volume of the core we can achieve the
equivalent KVA. The middle three limbs are used for winding while the extremes are used just to
increase the volume of the transformer and provide a return flux path external to the winding.
The figure on the left shows a 3-phase three limb transformer and on the right a 3-phase five
limbs transformer:
STEPPED LAPPED CORE: This is one of the arrangements of the limb to yoke joint. A simple overlap is one in which same lengths of sheets are used for overlapping in both yoke and limb construction. In a step-lapped core as many as five different plate lengths can be used so that the core can have a five-step overlap. Advantage: This arrangement allows the flux transfer to be gradual through the joint and it allows a proper flux transfer, therefore providing a lower corner loss. Disadvantage: The disadvantage of this arrangement is that, since we are using different lengths to make steps, the sheets of larger lengths cut from the roll of sheet imply that this arrangement requires greater cost.
40
3.2 SERVICE ALTITUDE :(1000 meters Max - according to KESC specs) The mounting height of transformer is chosen with respect to the sea level. The maximum height for normal operation is 1000m above sea level. Above 1000m the normal operating temperature range of the transformer varies. If the height of the transformer is more than 1000 m above sea level then the ambient temperature will change accordingly eg: -5 ⁰C- 28 ⁰C temperature ranges may shift to -2 ⁰C to 31⁰C Karachi is about 5 to 120 feet (1.5 to 37 meters) above sea level. Impact of Altitude: According to Boyle’s law, pressure is inversely proportional to volume.so when altitude of the transformer increases, the oil pressure decreases and consequently its volume increases. Hence the boiling point of oil decreases. This adversely affects the overall transformer efficiency.
TEMPERATURE RISE LIMITS AT ALL TAP CHANGER SETTINGS:
Specification Value Description
Oil / top 45K
The oil when heats up, its
density decreases and so it
comes up.
Windings / average 50K This is the average
temperature of windings
Windings / hot spot 63K
This is the maximum
temperature of winding at the
top section which the
windings can withstand.
3.3 HOT SPOT FACTOR (H): (1.3 for Core type- according to KESC specs) The hot spot temperature is the maximum temperature at any point of the winding which exists at the top section of the winding. The loading of a transformer greatly depends upon this winding withstand temperature. With temperature and time the insulation level disintegrates and the tensile strength of the winding degrades. Thus the insulation becomes brittle and remains no longer capable of handling high short circuit current. It can be referred as the aging of transformer. The curve below shows the relation of the aging of transformer with the hot spot temperature. The IEC and IEEE standard hotspot winding temperature is 95 ⁰C and 97 ⁰C respectively. By implying certain method, (the curve shows the relation of a Kraft paper) ,and by chemical treatment of the Kraft paper we can increase the hotspot temperature which will enhance the life a transformer.
41
RATED VOLTAGES FOR INSULATION CLASS: A standard set of voltages which the insulating material can withstand. These standard
voltages are different for different classes for eg:
For insulation class E124 the rated insulation levels are as follows:
Primary
kV
145
Secondary kV
24
Tertiary
kV
24
(IEC 60076-3 Power transformers - Insulation levels Section 3.2)
3.4 PARAMETERS:
PRINCIPLE TAPPING: (132KV- according to KESC specs) Tapping at which rated voltage (132KV in our case) is obtained is called principle tapping. A typical Power Transformer consists of 25 taps, where tap-13- is the principle tapping. The taps below the principle tap 13cause the primary voltage to decrease progressively
as the taps approach the lower part of the windings and vice versa.
42
RATED CURRENT: The maximum current the windings can withstand without getting thermally damaged.
The rated current of the winding changes with respect to the taps due to the fact that the
power remains same. The rated current for the tapped winding of greater voltage is less
than the rated current of the tapped winding where the voltage is reduced. By this we
can observe that the I2R losses are increased in the tapping of less voltage and more
current.
VOLTAGE VARIATION RANGE: (±20 KV- according to KESC specs) The maximum and minimum voltages that can be obtained from the tapping. In this case the voltage variation range is ±20 KV that means the we can obtain the voltage from 112KV to 152 KV. TAPPING RANGE: (±15%- according to KESC specs) The percentage range of increase or decrease in the rated voltage (132KV) as we move from tap 1 to tap 25. In our case, tapping range is ±15% (of 132KV) i.e 112 KV and 152 KV.
NUMBER OF STEPS: (±12 x 1.25 Steps - according to KESC specs) The percentage increase or decrease in rated voltage (132KV) between successive taps. In our case there, are total 25 taps and each tap accounts for a ±1.25 %( of the rated voltage) change in the voltage.
TAPPED WINDING: (HV) We normally draw taps from the higher voltage side because current is lesser in that winding and therefore it makes the tap changing convenient, because lower currents are easier to handle.
IMPEDANCE VOLTAGE-PRIMARY/SECONDARY (31.5/40MVA basis): Impedance voltage of a Power Transformer is the amount of voltageVpr applied to the primary side while secondary is short-circuited, to produce full load current in the secondary side. The ratio of the "Vpr" and nominal voltage of the transformer is called impedance voltage. The impedance voltage depends upon the position of taps, and for accurate calculations of the forces it is essential to use the impedance corresponding to the tapping position being considered. Tapping above the principle tap increases the impedance of the winding being used and consequently increases the impedance voltage. While taping below the principal tap causes the impedance of the winding to decrease and thus the impedance voltage decreases too. For normal tapping configuration the percentage change in impedance due to tapping is of the order of 10%, and if this condition not fulfilled the force will be in error by an amount up to ±20%. Note: Impedance voltage of a Power Transformer is usually listed on the transformer nameplate, expressed as a percentage. It is calculated by conducting a short-circuit test of the transformer.
43
According to KESC specs following are the Impedance voltage values at different Tap positions:
tap changer position -13- % 16
tap changer position -25- % 18
VECTOR GROUP SYMBOL :( YNyn0 (d11)-according to KESC specs) It defines the connection type of primary and secondary side of a transformer. D stands for delta connection while Y stands for star connection. The YNd11 shows the primary is star connected and the secondary is delta connected and the secondary leads primary by 30⁰. N represents that the primary connected star has a neutral. It describes the phase angle displacement between the primary and secondary so for parallel operation of transformers the vector group of two transformers must be same.
44
MAGNETIC FLUX DENSITY AT RATED VOLTAGE & FREQUENCY: Machines normally operate near knee points. Magnetic flux at rated voltage and
frequency in our transformer core is 1.6 Weber/m2. If this flux is increased further in
our case, it means that the core will saturate and that is going to lower the efficiency.
Conversely, the value of flux if decreased in our case, it will cause low coupling between
the primary and secondary, ultimately lowering the efficiency.
LOAD LOSSES (40MVA basis): Series resistance loss: Transformer upon being loaded gets lower voltage at terminals due to the drop in the series resistor in the transformer model. Copper losses: Due to the flow of current I2R losses occur in both secondary and primary windings. Note: Load Losses are determined by the short-circuit test of the transformer.
POWER FREQUENCY WITHSTAND VOLTAGE: The r. m. s. value of sinusoidal power frequency voltage that the equipment can withstand during tests at rated frequency that is 50 Hz for a specified time that is for 60seconds. -According to KESC specs following are the Power Frequency withstand voltages: Primary winding (line/neutral) 275/275 KV
Secondary winding (line/neutral) 50/50 KV
Tertiary winding 50 KV
LIGHTNING IMPULSE WITHSTAND VOLTAGE: Impulse withstand voltage is the voltage which is produced due to certain switching operations and its value is much greater than power frequency withstand voltage. It is to be kept in mind that the lightning withstands voltage is the voltage level that insulation can withstand during surge or lightning strikes. It is normally used in testing. -According to KESC specs following are the Lightning Impulse Withstand Voltages: Primary winding (line/neutral) 650/650 KV
Secondary winding (line/neutral) 125/125 KV
Tertiary winding 125 KV
45
COOLING METHOD:
The cooling of transformer is of two types; 1. Oil natural air natural (ONAN)
Oil is circulated within the transformer to avoid the rise of temperature beyond a certain limit. The natural air outside the pipes also helps to maintain the temperature of the transformer not to exceed a certain limit. If the temperature within the transformer exceeds 50 ⁰C then we go for ONAF cooling.
2. Oil natural air forced (ONAF) Beyond 50⁰C we go for forced air cooling plus the oil is still being circulated in the pipes. There are cooling fans below or on the side of the transformer which sense the temperature rise and starts working. This decreases the temperature rise and the keeps the temperature within the limits. By ONAF technique we can increase the efficiency of transformer.
3.5 SIZING OF POWER TRANSFORMER:
To determine the MVA rating of the transformer that should be used we require the following
data.
Load=33 KW
Power factor= cos𝜙=0.95
Voltage= 132/11 KV
Now for future expansion of the grid, we are employing a factor of 1.15 i.e 15 % expansion,
So we employ a transformer rated at 40 MVA.
46
AUXILIARY TRANSFORMERS
These are the power transformers that provide power to the station’s auxiliaries during
normal operation. It provides the most economical power to the station as it is directly
connected to the main generation unit hence reducing the effect of line losses due to less
distance. These are basically step down transformers. The HV side transformer voltage
corresponds to the voltage of the generating unit and the LV side voltage is stepped
down to 6.6KV.
4.1 LOAD CALCULATIONS LIGHTING SYSTEM:
No of lights = 450
Power factor =0.93
Wattage = 60W
VA =
Total kVA = 450 x 64 =29 KVA
CEILING FAN:
Number of Fans = 100
Power factor =0.9
Wattage = 80W
VA of ceiling fan=
= 89 VA
Total KVA =100X89 = 9 KVA
AIR CONDITIONERS:
No of Air Conditioners = 20
Power Factor = 0.8
Wattage = 4800 W
VA of Air conditioner =
6 KVA
Total KVA=20x6000= 120 KVA
INDUCTION MOTOR:
No of Induction Motors=15
Power Factor =0.88
Wattage of induction motor: 746w
VA of induction motor=
=848 VA
Total KVA=15x848= 13KVA
47
REFRIGERATOR:
No of refrigerators= 10
Power Factor = 0.7
Wattage of Refrigerator = 600w
VA of refrigerator =
=857 VA
Total KVA of Refrigerator = 86 KVA
MISCELLANEOUS LOAD:
Other load = 20KVA
Total KVA of all Loads= 29+9+120+13+86+20
Total KVA of all Loads= 277 KVA
Since our total load is 277 KVA, Keeping in mind the design margin we will use auxiliary
transformer of 500 KVA for smooth operation and future expansion prospects.
48
INSTRUMENT TRANSFORMER:
An instrument transformer is a device used to transform high voltages or high currents
to low voltages or currents which can be utilized by the metering devices. The basic
applications of an instrument transformer are for metering and protection purposes.
CURRENT TRANSFORMER: An instrument transformer used to provide low secondary currents proportional to the
current flowing in the primary winding. Current transformers are commonly used in
protective relays and metering in the electrical power industry where they allow safe
measurement of large currents in the presence of high voltages. Current transformer is
designed in a way that the circuitry is protected from the high voltages.
Principle of operation: The current transformer principle
works on variable flux. The
principle of current transformer
follows the B-H curve due to the
non-linearity of the iron core. When
the saturation region is attained the
increase in current and voltage
does not remain linear thus,
producing an error. The secondary
current of an ideal transformer is
equal to the product of the primary
current and the turn ration of the
primary and secondary. But practically, there is a slight difference as some of the
primary current or the primary ampere-turn is utilized for magnetizing the core, thus
leaving less than the actual primary ampere turns to be "transformed" into the
secondary ampere-turns. This unintentionally introduces an error in the transformation.
Types of current transformer based on the application:
The primary applications of a current transformer are
described below:
1. Metering current transformer:
The value of the line current i.e. at the primary is reduced to
very low amperes for metering purposes. A metering CT has
a very sharp I-V curve. This is because instruments and
meters work accurately at low currents. And in metering CT
at primary, rated current flows, it can go up to 120% of the
49
rated current. So by having a sharp curve, current will be limited that will enter the
meters because transformers should not be saturated for the purpose of avoiding loss of
efficiency, therefore metering CT is designed to saturate at low. Special nickel-alloy
metal having a very low magnetizing current is used in order to achieve the accuracy.
2. Protection Current transformer:
The protection of the electrical equipment is required at the fault conditions by reducing
the line current to very low amperes. Large fault currents at
the primary may induce greater errors and is reduced by
avoiding the saturation level to ensure the proper operation
of the relay when the current is the multiple of the normal
operating current. Relays should not be operated under
normal operating conditions; therefore the curve of
Protection type CTs is not sharp. This allows the fault
current (Very high current) that appears at the primary of
CT to be reflected at it secondary without saturation so that
relay is operated only at fault conditions. Had the curve
been sharp, the relays would have been tripping under normal currents because
transformer would have been having saturation point at very low current(not a fault
current), and transformers are operated below saturation point for avoiding the loss of
efficiency. In the protection type CTs, the current at primary can reach up to 2000% of
the rated current.
5.1 DESIGN PARAMETERS:
1. CT KNEE POINT VOLTAGE
The point on the magnetization curve after which a further 10% increase in the voltage
of secondary side causes 50% increase in the excitation current. The transformer is said
to be in the saturation region and the accuracy of the transformer is reduced.
2. CT SECONDARY CIRCUIT VOLTAGE
The voltage appearing at the terminals of the secondary is called CT secondary circuit
voltage.
3. CT BURDEN
The burden is usually expressed as the apparent power in volt-amperes VA absorbed at a specifiedpower factor and at the rated secondary current.
4. CT PRIMARY CURRENT
The maximum line current that appears at the primary of the current transformer is
called the CT primary current. The average and the peak value must be known.
50
5. CT SECONDARY CURRENT
The current that appears at the secondary of the current transformer after undergoing
the transformation process depending upon the turn ratio of the secondary and primary.
6. ACCURACY CLASS
If a Transformer is 5P20, then its accuracy class is 5P. This means that composite error
will be ± 5% and within this range the efficiency will not suffer significantly, if the
primary current increases 20 times
7. ACCURACY LIMIT FACTOR
The accuracy limit factor is the multiple of primary rated current for which the error is
guaranteed less than 5 or 10% depending on whether the accuracy class is 5P or 10P
respectively, and within this error range, the efficiency of the CT doesn’t suffer This is
the value of primary current up to which the CT complies with composite error
requirements. This is typically define as 5P5, 5P10, 5P15 and 5P20 etc., so these right
side numbers 5, 10 or 15 and 20 means that the composite error of the CT has to be
within specified limits at 5, 10, 15 or 20 times the rated primary current.
8. RESISTANCE OF CT
It is the secondary winding dc resistance corrected to 75 C.
Why it is called DC: The performance of a current transformer used in protective
relaying is largely dependent on the total burden or impedance in the secondary circuit
of the current transformer. The current transformer core flux density (and thus the
amount of saturation) is directly proportional to the voltage that the current
transformer or secondary must produce. So for a given amount of secondary current, the
larger the burden impedance becomes, the greater is the tendency of the current
transformer to saturate.
The current transformer burden impedance of most electromechanical relays decrease
as the secondary current increases because of saturation in the magnetic circuits of the
devices. At high saturation, the burden impedance approaches its DC resistance.
9. MAXIMUM FAULT CURRENT
The maximum line current flowing at the fault condition i.e. 40 KA.
10. OVER DIMENSIONING FACTOR
A factor assigned by the purchaser to indicate the multiple of rated secondary current
occurring under power system fault conditions, inclusive of safety factors, up to which
the transformer is required to meet performance requirements.
51
11. Dynamic current
The peak value of the primary current which a transformer will withstand, without being damaged electrically or mechanically by the resulting electromagnetic forces, the secondary winding being short-circuited
12. SHORT TIME THERMAL CURRENT
The R.M.S. value of the primary current which a transformer will withstand for one second without suffering harmful effects, the secondary winding being short-circuited.
13. Rated burden:
The apparent power of the secondary circuit in Volt-amperes expressed atthe rated
secondary current and at a specific power factor (0.8 for almost all standards). Example
30VA, 20VA etc.
14. Over current factor:
The rated over current factor is a figure by which the rated primary current must be
multiplied in order to obtain the rated accuracy limit of primary current.
5.2 ERRORS IN CT:
1. Ratio error
The secondary current is less than the expected current according to the turn ratio. The
decrease in magnitude arises due to the fact that the actual transformation ratio is not
equal to the rated transformation ratio. It is also called current error and mathematically
can be expressed as
2. Composite error:
For relaying, the difference between the instantaneous primary current and instantaneous secondary current multiplied by the turn ratio is the composite error. In case of metering, it is the square root of the sum of the squares of the ratio error and phase error.
Where, Kn is the rated transformation ratio; Ip is the r.m.s. value of the primary current; ip is the instantaneous value or the primary current; isis the instantaneous value of the secondary current;
52
T is the duration of one cycle.
Following is a graph showing the relation between Composite error ε [%] and Primary
current I1 [A]:
From graph we can see that 160 A is the limit primary current, after which composite
error starts significantly increasing such that even a small increase in current results in
large amount of increase in composite error. This causes the transformer efficiency to
decrease.
3. Accuracy error
The difference between the actual accuracy limit factor and the rated accuracy limit
factor
What is actual and rated accuracy limit factor: Rated accuracy limit current is the
value of primary current up to which the CT will comply with the requirements for
composite error while the actual accuracy limit factor differs from the rated accuracy
limit factor and is proportional to the ratio of the rated CT burden and the actual CT
burden
4. Phase error
The phase difference between the primary voltage and the reversed secondary voltage
vectors is known as phase error.
53
5.3 CT SIZING: The key to CT dimensioning is symmetrical short circuit current and transient dimensioning factors:
Kscc = ALF - rated symmetrical short-circuit current factor
K'ssc = Effective ALF - effective symmetrical short-circuit current factor
Ktd - transient dimensioning factor
Kscc and K’scc
The relationship between voltage and current of the current transformer is only linear
before the knee point. If we have a CT “5P20”, this signifies that the relationship will be
linear till the current is 20 times the rated current. This linear limit is given by Kssc,
i.eKssc=20. Kssc’ accounts for the burden (resistance of the relay), resistance of the CT
windings and resistance of the leads.
Ktd
Ktd is the transient performance of the relay factor. This is given by the manufacturer.
Rct - secondary winding dc resistance at specified temperature
Rb - rated resistive burden of the relay
R'b - Rleads + Rrelay; this is the connected burden The CTs are required to supply appropriate currents to the relay to make them operate. The coherent working of CT and relay is dependent upon the factors Kssc’ and Ktd. Correct functioning is achieved by ensuring the following is valid:
Issc max - maximum symmetrical short-circuit current
Ipn - CT rated primary current
The right hand side of the equation shows the multiplying factor that when multiplied by the primary current gives the current that will operate our relay. And the actual ALF on the left should be greater than the factor on the right hand side because we are incorporating the worst case in our consideration, because if current more than the current (that operates the relay) comes, the CT doesn’t become inefficient because of getting saturated.
54
SPECIFICATIONS OF CTS:
55
FOR 132KV COUPLER CURRENT TRANSFORMER –B1:
For Core 2 to 5:
Data:
Ω
Calculation:
NOW,
Now to check the condition,
; THE CT SIZE IS SUITABLE
56
FOR 132KV TRANSFORMER CURRENT TRANSFORMER –B1: For Core 2 to 4:
Data:
Ω
Calculation:
NOW,
Now to check the condition,
(X)
The Condition is not satisfied therefore choosing a suitable value for ‘n’ (Over Current
Factor of CT):
i.e n= 3.5;
Now to check the condition,
; THE CT SIZE IS NOW SUITABLE
57
For Core 5:
Data:
Ω
Calculation:
NOW,
Now to check the condition,
; THE CT SIZE IS NOW SUITABLE
58
FOR 132KV OHL/UGC CURRENT TRANSFORMER –B1:
For Core 2 to 4:
Data:
Ω
Calculation:
NOW,
Now to check the condition,
(X)
The Condition is not satisfied therefore choosing a suitable value for ‘n’ (Over Current
Factor Of CT)
i.e n= 7;
Now to check the condition,
; THE CT SIZE IS NOW SUITABLE
59
For Core 5:
Data:
Ω
Calculation:
NOW,
Now to check the condition,
; THE CT SIZE IS SUITABLE
60
FOR 11KV INCOMMER CURRENT TRANSFORMER –B1: For Core 2 and 3:
Data:
Ω
Calculation:
NOW,
Now to check the condition,
; THE CT SIZE IS SUITABLE
61
FOR 11KV FEEDER CURRENT TRANSFORMER –B1: For Core 2:
Data:
Ω
Calculation:
NOW,
Now to check the condition,
(X)
The Condition is not satisfied therefore choosing a suitable value for ‘n’ (Over Current
Factor Of CT):
i.e n= 3.5;
Now to check the condition,
; THE CT SIZE IS NOW SUITABLE
62
FOR 11KV COUPLER CURRENT TRANSFORMER –B1: For Core 1:
Data:
Ω
Calculation:
NOW,
Now to check the condition,
; THE CT SIZE IS SUITABLE
63
CAPACITIVE VOLTAGE TRANSFORMER:
5.4 DEFINITIONS: Rated voltage factor: The multiple factor of the rated Primary voltage to determine the maximum voltage at which the transformer’s thermal conditions are within the requirements for a specified time and also within the accuracy requirements Rated Primary Voltage: The value of primary voltage at which the transformer operates at optimum efficiency with minimum losses. Rated Secondary Voltage: The value of secondary voltage at which the transformer operates at optimum efficiency with minimum losses Rated Output: The value of the apparent power in VA at a specified power factor that the transformer should supply to the secondary circuit at the rated secondary voltage and with the rated burden when connected to the transformer. The standard values of rated output (in volt-ampere) at a power factor of 0.8 lagging are: 10, 15, 25, 30, 50, 75, 100, 150, 200, 300, 400, 500 VA.
Burden: The Apparent power absorbed by the secondary circuit in VA at a specific Power factor and at rated secondary voltage is called the burden. It can also be described as the admittance of secondary circuit (Siemens) with the power factor (leading or lagging) Accuracy Class: It is the allowable range of percentage composite error when the primary current reaches a certain multiple of its rated value (as specified by the Rated Accuracy limit Factor) Um: The highest equipment voltage
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Impulse withstand current (peak): The value of the peak current that can be withstood by a device when it isin operating condition. Power frequency withstand voltage The windings that are designed to withstand the highest equipment voltage shall also be able to withstand a voltage that will appear on the windings because of lightning impulse. This voltage that has to be tolerated is called the Power Frequency Withstand Voltage Limit of Temperature Rise: Transformers having a specified voltage factor (say 1,5 for 30 s or 1,9 for 30 s) shall be tested such that after the application of 1,5 times rated voltage for 30s, the temperature rise shall not exceed by more than 10 K. The respective voltage factor will be applied after the application of 1,2 times rated voltage fora time sufficient to reach stable thermal conditions.
5.5 SPECIFICATIONS:
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5.6 SIZING:
The basic idea for CVT sizing is that we have to keep the connected burden of CVT less
than its rated burden. In our case, the above mentioned CVT’s have rated burden of
100VA. Normally in Pakistan CVT’s are OVERSIZED due to standards set by
KESC/WAPDA which are quite old and not revised.
DATA:
Relay Burden=0.25 VA
Length= 140m lead length
Area= 6 mm2 of lead
Rct=3.1 ohms
Vs=57.73 V
CALCULATION:
I= (
) =
=18.6 A
Lead Wire Burden in VA=
Where,
I= Secondary current in Amps
L= Lead wire distance in meter
A= Cross sectional area of wire
Lead Wire Burden in VA=
=
=15.2 VA
Total connected burden= Relay burden+ Lead wire burden
Total connected burden= 0.25+ 15.2
Total connected burden=15.4 VA< 100 VA
Hence both CVT’s are suitable.
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CHAPTER 5
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CABLES
6.1 TYPES OF CABLES:
Low Tension Cables:
These cables are used for voltage levels up to 1000V. Impregnated paper is the most
important insulation material used in them.
High Voltage Cables:
These cables are used for transmission up to 11 KV. These cables are belted, contain
multi-cores, inter-core insulation, screens,inner sheath, bedding, armour, Outer
sheath. PVC and XLPE are used as insulations in them.
Extra High Voltage Cables:
The cables are for voltage above 66 KV. Above their operational voltages, there is a
danger of breakdown of dielectric due to the existence of void spaces. Void spaces must
therefore be filled with fillers to prevent their ionization and use the cable for higher
voltages.
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6.2 SPECIFICATIONS:
ALUMINUM CONDUCTOR:
1. Al conductor has lesser weight as compared to copper.
2. Al is less costly as compared to Copper.
SEMI-CONDUCTING CONDUCTOR AND INSULATION SCREEEN:
There are actually two semi-conductive layers on a HV cable. One is between the actual
conductors and the XLPE. The other semi-conductive layer is outside the XLPE
insulation.These screens are earthed along with the earthed metal sheath.
The semi-conductor is used to proportionally distribute the electrical stresses over a
large area. Most conductors consist of multiple strands of copper/aluminum. The outer
edge of the conductor bundle is not even. It has several crumples on the outer edge
where the individual strands meet each other. This increases the electricalstress (up to
20%) on the insulation causing untimely failure. The internal semi-conductor makes a
uniform voltage level for the XLPE where it meets the conductor strands.
XLPE INSULATION (Cross-Linked Polyethylene):
It is a basically a thermo-plastic material. It has a tendency to become brittle at
temperature below 0ᵒC.
Why have we employed XLPE insulation?
It has high operating temperature.
They are lesser complex to joints.
Their installation is easier relatively.
This insulation possesses high electrical strength and low losses per cross
section.This implies greater mechanical strength.
XLPE softens high temperature, thus restricting the short circuit and high
overload currents.
METALLIC SCREEN/SHEATH:
The metallic screen shall consist of one or more tapes. We have used copper tape as a
metallic screen.
Such Metallic sheaths are provided due to the following reasons:
1. To prevent ingress of moisture.
2. To protect against mechanical vibration and shocks.
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If don’t need to provide protection against mechanical shocks then we
can employ PVC based compounds as sheathing materials.
3. To serve as an earth path.
For cases in which PVC is employed as a sheathing material, then such
sheath will not be able to serve as an earth path. Since fault currents
are very large, therefore we have used copper(having high electrical
conductivity) as sheathing material to provide an eath path.
BEDDING:
After the sheathing material, bedding compounds of fibrous material are used to
provide a circular shape to the cable. We have employed PVC bedding.It facilitates heat
dissipation.
ARMOURING:
It is applied over bedding material to provide mechanical strength to the cables. Cables
which are subjected to heavy mechanical stresses should be armoured with galvanized
steel. It alse provides protection of sheath from mechanical damage. We have employed
Aluminum wires as armour.
The magnetic material in the alternating magnetic field of a single core cable exhibits
excessive losses. Due to this Single core cables are kept unarmoured or even if they are
armoured, non-magnetic materials are used. In multicore cables, the net alternating
magnetic field is zero which reduces the heating losses in the armour to zero. Aluminum
is used as an armoured material due to its nonmagnetic properties, high conductivity
and mechanical stress.
OUTERMOST SHEATH:
The outer PVC sheath is used to protect the internal sheaths from corrosion. To protect
the corrosion of the outermost sheath, we employ a sheath of plastic material (Poly Vinyl
Chloride) for the outermost sheath.
CONDUCTOR:
Conductors are stranded. That is each core is divided into a small number of conductors
and are grouped together and spiraled in opposite directions to provide greater
strength.
Why stranding is done?
To minimize skin effect
To provide flexibility to the conductor.
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ELECTRICAL RESISTANCE OF CONDUCTOR: Resistance measurements shall be made on all conductors. The dc resistance is also called the geometric resistance of the conductor that is just due to the material properties, whereas AC resistance is the sum of Dc, Skin effect and Proximity effect resistances. The D.C resistance of each conductor at 20 °C shall not exceed the appropriate maximum value specified in IEC 60228. And for concentric conductors, the resistance shall in accordance with the national standards. VOLTAGE WITHSTAND: It is the voltage for a specified time and at a particular frequency that when applied on cable, the insulation doesn’t breakdown. PARTIAL DISCHARGE: Partial discharge (PD) is a dielectric breakdown of a small portion of an electrical
insulation system under high voltage stress. When partial discharge is initiated, current
pulses will appear and persist for Nano-seconds to a micro-second, then disappear and
reappear repeatedly. The usual way of quantifying partial discharge magnitude is in
Pico-coulombs.
Partial discharge is caused by non-uniformity ,voids, cracks, or inclusions within an
electrical insulation and, since the dielectric constant of the void is considerably less
than the surrounding dielectric partial discharge appears. If such void areleft
undetected, they can eventually lead to the full breakdown of the insulation system.
6.3 CALCULATIONS
1)CONTINUOUS CURRENT RATING CALCULATION:
√
Where,
∆θ= It is the difference between maximum operating temperature and ambient
temperature
= 90 C- 35 C = 55 C
=Dielectric Loss =0.474
R= ⁄
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= Loss factor of sheath or screen= 0.076
Thermal resistance between sheath and conductor= 0.435 ⁄
= Thermal resistance of outer covering (Serving) = 0.095 ⁄
= External Thermal resistance =1.473 ⁄
I= Continuous current rating
By solving we get,
I= 907.5 A
2)CALCULATIONSOF EMERGENCY CURRENT RATINGS:
√
⁄ [
⁄ ]
⁄
⁄
⁄
⁄
⁄
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2.1) CALCULATIONS FOR Positive and Negative Sequence Impedance:
3.1.1) Positive and Negative Sequence Resistance= R ⁄
R = +
Where,
= A.C Resistance of conductor = 0.03099 ⁄
= Resistance of Sheath = 0.05400 ⁄
= Reactance of Sheath
For :
= 4πf ×
×
Where,
= Distance between conductors = 300 mm
Internal Diameter of Sheath = 74.4 mm
External Diameter of Sheath =79.6 mm
By Solving,
= 0.12900 ⁄
FOR R:
By solving,
R= 0.07694 ⁄
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2.1.2) Positive and Negative Sequence Reactance= X ⁄
X = 4 f ×
× -
Where,
= Geometric mean distance between three conductors √ =
377.98 mm
= distance between conductor A & B =300mm
= distance between conductor B & C =300mm
= distance between conductor C & A =300mm
= Geometric mean radius of one conductor = 0.772
d= Diameter of conductor = 34 mm
13.12 mm
By Solving,
X= 0.1014 ⁄
Positive and Negative Sequence Impedance =
= R+ j X = 0.0769 + j 0.1014 ⁄
2.2) CALCULATIONS FOR ZERO SEQUENCE IMPEDANCE:[Z0(Ω/Km)]
Zero Sequence Impedance = Ω/km
=
2.2.1) Impedance of conductors = Ω/km
(
)
Where,
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= AC Resistance of Conductor =0.03099 Ω/km
= Equivalent earth resistance = 0.17771 Ω/km
= Distance to equivalent earth return path = 853440 mm
√
2.2.2) Impedance of sheath = Ω/km
Where,
√
2.2.3) Mutual impedance between conductors and sheath = Ω/km
where,
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CIRCUIT BREAKERS
7.1 INTRODUCTION Circuit breaker is a device which is used to open the circuits when abnormal conditions appear in them. It is a necessary part of the power system protection. When a fault comes the relay senses it and signals the breaker to trip. The design specifications are selected such that the breaker may not get damaged. FUNCTION: The functions of circuit breaker are:
The circuit breakers must be able to open the contacts automatically as soon as the relay signals its trip coil.
It must be able to withstand the normal operating full load current. It must be able to withstand heavy fault current for few seconds such that it may
not get damaged the second the fault occurs. The arcing medium must be able to withstand the high voltages between the
contacts when they are opened. It must be able to close the contacts automatically when the fault is removed.
MECHANISM: When the fault occurs and high magnitude short circuit flows, the relay signals the circuit breaker tripping coils, the contacts get separated slowly. The gap formed by the separation of pair conducting contacts in the circuit breaker becomes conductive due to ionizing of electrically neutral surrounding gas.The high voltage that exists between the contacts while getting separated initiates an arc which may be of such intensity that it may melt away the contacts. Efficient arc quenching medium must be used so to de-ionize the electrons near the contacts to avoid the arc. CLASSIFICATION BASED ON THE MEDIUM OF ARC EXTINCTION:
Air circuit breaker Oil circuit breaker Vacuum circuit breaker SF6 circuit breaker
7.2 SPECIFICATIONS RATED VOLTAGE (145 KV) The maximum voltage for the proper operation of the circuit breaker such that the designing and insulation may not get damaged of the breaker. It is the highest rms value of the voltage for which the breaker is designed. RATED NOMINAL CURRENT (3150 A) The maximum current the breaker could withstand for the continuous supply.
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TYPE OF THE OPERATING MECHANISM(SPRING) The mechanism consists of two springs commonly known as the opening and the closing springs. The mechanical energy required for the opening of breaker is always stored in the opening spring such that a closed circuit breaker is always ready to get opened. A motor drive is used to drive the spring charging gear which stores the energy in the opening coil immediately after the closing of the breaker. The motor drives DC universal drives. The spring mechanism is not at all manual operation and due to its fast breaking time, it is widely used.
NUMBER OF BREAKS PER PHASE (1) Multiple breaks are used when the time to separate the contacts is not fast, the voltage becomes too high across the contact. The multiple breaks together act as voltage divider so that the contactsdon’t get damage due to high voltage that exists between a single contacts. In our case the breaking is not an issue plus we have employed an efficient quenching medium so we have used a single break per phase.
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TYPE OF INSULATION MEDIUM (SF6) The insulation medium chosen for the extinction of arc when the contacts are opened is sulphur hexafluoride (SF6). It is a thermally stable gas and is non-flammable, non-poisonous and odorless.It serves a better quenching medium than air and oil. RATED INSULATION WITHSTAND VOLTAGE
Atone minute power frequency (275kV)
The maximum insulation level the circuit breaker tolerate at the rated frequency such that it may not get damage.
At impulse (50ms/650kV)
The tolerable insulation level of the circuit breaker when a lightning impulse occurs.
Rated duration of short-circuit (3 sec) The time taken by the circuit breaker to withstand the heavy short circuit current before it gets tripped. RATED INSULATION MEDIUM PRESSURE (6.4 BAR) The rated pressure of the insulation material within the enclosure of the circuit breaker. It depends upon the installation height of the breaker and above 1000m,variation in the pressure must be controlled. RATED TIME QUANTITIES
Opening Time 28 ms
The time taken by the contacts to open completely from the initiation of the opening of the contacts.
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Arcing Time 12 - 22 ms
The time interval between the instant of the first initiation of an arc and the instant of final arc extinction.
Total Breaking Time
The total time required to isolate the fault current from the system. It is the sum of the opening and arcing time.
Closing Time.... 70ms
The time taken by the contacts to join in order to close the circuit for so that the operation of the system starts.
ELECTRICAL LIFE OF THE CIRCUIT BREAKER AT THE RATED BREAKING CAPACITY BEFORE AN OVERHAUL: An overhaul is time of maintenance. The time frame the circuit breaker requires in order to operate efficiently without the need of maintenance. NO OF TRIP COILS (2) The electromagnetic coil which on being energized by the protective relays opens the breaker contacts when fault occurs. RATED SHORT CIRCUIT BREAKING CURRENT (40 KA) The short circuit current the breaker should be able to withstand for a very short time interval such that it may not get damage the instant the fault appears. CREEPAGE DISTANCE (4950 MM) The shortest distance along the surface of a solid insulating material between two conductive contacts when they are opened. YEARLY LEAKAGE RATE OF INSULATION MEDIUM: It is the leakage of insulation gas(SF6) during one year. It is below 0.1% per year (Mentioned by Siemens) hence minimizing the environmental impact of the gas and no need for replenishment during the life of the switch gear INSTALLATION LOCATIONS (INDOOR / OUTDOOR)
The indoor and outdoor installation of the circuit breaker depends upon the arc quenching medium for the circuit breaker.In indoor the quenching medium used is SF6 oil or vacuum, while for outdoor installation of the breaker air, is our default quenching medium. In outdoor the spacing between the contacts is not a constraint due to which air can be used as the quenching medium to avoid arc. On the other hand, in indoor installation the contact spacing is a constraint, so in order to avoid the arcing, we prefer a more efficient quenching medium.
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7.3 CIRCUIT BREAKER SELECTION CRITERIA
Breaker selection is done on the basis of short circuit current that flows through a point
on the breaker installation, the rated capacity of the short circuit and the rated voltage
the circuit breaker can withstand. A value above the rated current and rated voltage is
chosen for the safety purpose. The manufacturing companies have a chart which shows
all the above discussed specifications and we can choose our desired circuit breaker
accordingly.
A Sample circuit Breaker selection table is shown in the following figure:
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SHORT CIRCUIT ANALYSIS
The short circuit current is the high magnitude current that flows through a power
system due to abnormal conditions occurring in the system which causes a severe
damage to the system equipment if necessary steps are not employed. The selection and
determination of power system protection equipment signifies the short circuit current
analysis.
8.1 DIFFERENCE BETWEEN OVERLOAD AND SHORT CIRCUIT:
Overload
An overload occurs when too many devices are being operated and the source is being
burdened. For example if a motor is rated at 5A but at the overload condition, it will be
burdened and will draw more current to feed the load. As the motor draws above it
rated current it will get heat up and the winding may get damage. The system may able
to run on overload condition for some time without getting damage.
Short circuit:
Short circuit condition occurs when two bear conductors comes in to contact with each
other or with the ground. High magnitude current flows, the resistance drops to almost
zero and due to which the voltage also becomes zero. The short circuit current is
thousand times the normal operating current and must be isolated from the system as it
occurs to avoid the damage to the equipment.
8.2 CAUSES OF SHORT CIRCUIT:
Short circuit can be caused by:
Over voltages due to switching or lightning surges
Contamination of insulation-by salt sprays, pollution ,moisture content etc
Mechanical causes – Overheating, abrasion, corrosion etc
Faults on Transmission line:
Mostly, faults occur on overhead lines because they are exposed to the elements of
nature.
60-70% faults occur on them. Wind topples the transmission line, also ice puts lot of
stress on the transmission lines. These factors normally cause fault. Also trees fall on
transmission line, causing the occurrence of failt.
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8.3 SHORT CIRCUIT CURRENT IN OTHER ELEMENTS:
Short circuit probability in other elements is:
Cables: 10-15%
Circuit Breakers: 10-12 %
Generators: 10-15%
Motors: 10-15%
Transformers: 10-15%
Consequences of Short Circuit:
The consequences of short circuit are:
High current flows, many times high as the rated current, this high current
can cause exorbitant heating and fire.
Thermal/Mechanical damage occurs in the transmission lines, transformers,
motors, generators, cables, busbaretc depending upon where the fault takes
place
Discontinuity in operation
Types of Short Circuit:
Line to ground Line to Line Line to line to ground 3
phase Ground
75-80% 5-7% 10-12%
8-10%
a-symmetrical fault a-symmetrical fault a-symmetrical fault a-symmetrical
fault
In 3phase ground fault, highest current flows among all the other faults.
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8.4 SHORT CIRCUIT CALCULATION: Power system has variousequipmentthat contribute as per their own behaviourin the
scenario of fault
E: System voltage
Z:Component’s impedance
Rt= Wire’s Resistance.
Rt F
Rf
%R = (Rf/Rt)*100
I = Full load current
V = System Voltage
for reactance;
Short Circuit KVA
Although the potential at the fault is zero but we usually express the short circuit
current in terms ofshort circuit KVA. The product of normal system voltage and the
short circuit current at the point of fault occurred is called the short circuit KVA. In other
words, the product of pre fault voltage and post fault current.
Let,
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As we know that,
Therefore,
Characteristics of Short Circuit Current:
Consider an RL circuit-shorted line.
)
After switch gets closed, we observe the following equation
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i(t) =Instantaneous current that we get by natural and forced response.
The above equation has DC component which causes a-symmetry, it dies out with time.
It also contains time varying sinusoidal component
Z= impedance
√
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8.5 SOURCES OF SHORT CIRCUIT CURRENT: There source for short circuit current.
1- Generators
2- Synchronous Motors
3- Induction Motors
1. GENERATOR:
Suppose a short circuit takes place on a circuitry that is powered by an alternator, the
alternatorstillgenerates voltage as the excitation is still powering the generator, also, the
rotor is rotated at constant speed by the prime mover.This voltage feeds the fault point
with an abnormally high current. The impedance upto the fault point from the alternator
and the internal impedance of the generator restrict the fault current
2. SYNCHRONOUS MOTOR:
Voltage falls when short circuit current takes place. This retards the synchronous. The
motor now no longer gives energy to the load. The load is now driven by inertia.The
synchronous motor now starts acting as an alternator as the terminal voltage is
significantly reduced. . The short circuit current is restricted by the impedance between
the motor and the fault point.
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3. INDUCTION MOTOR:
The effect of induction motor at the time of short circuit is a little different in
comparison of the synchronous motor due to the fact that the induction motor doesn’t
have the field circuit and it works on the normal operating principle of synchronous
starting. As the short circuit occurs, the terminal voltages are removed so as the 3-phase
stator current. The motor will keep rotating due to inertia due to which voltages will be
induced in the stator winding and it will start feeding the fault. The current goes until
the flux decays to zero. Thus we can conclude that the induction motor feeds the fault for
just few cycles.
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8.6 Reactances of rotating machines The short circuit current has two components dc and ac. The dc component decreases
exponentially. The decaying of the current is due to the fact that the reactance of the
machine keep decreasing exponentially and thus attaining the steady state reactance
also called the synchronous reactance. The reason behind this is the fact that the
inductor can’t change its flux instantaneously therefore the inductance will decrease due
but gradually. A time will come that it will gain inductance that will limit the current to a
steady state. The three reactances of the rotating machines are discussed below.
a) Sub transient Reactance (X’’d): The current flows during the first 3 cycles of
fault is determined by the sub-transient reactances. Since, the current is high
initially therefore it is the minimum of the 3 reactances due to which the current
is almost ten times that of the normal operating conditions. Though the time
period is very less but it is the most important factor for the protection system
design.
b) Transient Reactance (X’d): The machine starts to gain flux for the next 4-5
cycles due to which the reactance increases. This reactance is called the transient
reactance. The current falls but at a rate slower than the time period of the sub-
transient reactance. The value of current in this period is about 5 times of the
steady state short circuit current.
c) Synchronous Reactance (Xd): This is the reactance when the steady state short
circuit current starts flowing in the system. It is the greatest of the 3 discussed
reactances. This value is obtained when the reactance equals the synchronous
reactance of the machine. It is the most significant value and is used for the short
circuit calculations.
This implies that,
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8.7 SHORT CIRCUIT FOR TRANSFORMERS: The impedance of the transformer helps to determine the maximum fault current passes
through a transformer under fault conditions. Transformer doesn’t alter the system
voltage and neither has it produced it. The short circuit current flowing through a
transformer is determined by by the help of its secondary reactance, the reactance of the
generator and the system to the transformer terminals plus the reactance of the
transformer circuit till the fault point. The short circuit current is limited by the
transformer’s impedance. By multiplying the reciprocal of the impedance timed the full
load current. Thus, if a transformer has 10% impedance, the reciprocal of 0.1 is 10.Thus,
the maximum short circuit current is 10 times the full load current.
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8.8 SHORT CIRCUIT ANALSIS BY ETAP
The simulation report for only 3-phase symmetrical fault at bus-1 report has been
attached. Similar tasks can be carried out for different faults at different buses.
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OVERCURRENT AND EARTH FAULT PROTECTION
9.1 Introduction
When relatively high current, above the normal operating current, flows in the system
for certain period of time it is called overcurrent. The power system is capable to bear
the overcurrent for certain time.
A fault may occur between the phases and phases and ground. The faults which cause
the short circuit currents to flow through the earth are called earth faults or ground
faults.
Over current and earth fault protection simulation is done on ETAP. The simulation
includes protection of
3-phase symmetrical fault
Line to ground fault
Line to line fault
Line to line to ground fault
All the faults have extensively discussed in the short circuit report.
The overcurrent protection simulation is a complex task done under the title of
STAR DEVICE COORDINATION ANALYSIS
Star is a complete module for selectivity and is a protection coordinator for systems. It represents a new concept for the performance of both dynamic and steady coordination of device, their protection, and their testing.
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STAR MODE TOOLBAR
FAULT INSERTION (PD SEQUENCE-OF-OPERATION)
Protective Device (PD) Sequence-of-Operation executes by introducing a fault on certain SLD using the Fault selection button in the Star mode.
In our simulation we have used the static load rather the lumped load. The static load doesn’t contribute to the short circuit current. So protection and relay coordination of the loads is insignificant. We have performed the protection coordination of the transformer and the generators. The important terms that are used for the simulation are
FULL LOAD CURRENT:
The full load current or FLA is the maximum current a generator can draw at normal condition. For protection input data, we have used 125% of 175 FLA so that our relay start sensing from 220 A.
PICKUP TIME: The time after which our relay start sensing when the sensing current flows for certain time.
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MAX/MIN CURRENT: The buses are faulted individually and the currents flowing from the buses are noted. Minimum and maximum currents are noted by which we set the curve and achieve the desired protection setting.
9.2 SIMULATION RESULTS
1. 3-PHASE SYMMETRICAL FAULT
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2. LINE TO GROUND FAULT
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3. LINE TO LINE FAULT
95
4. LINE TO LINE TO GROUND FAULT
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9.3 PROTECTION CURVE:
The curve below shows the settings of the relay. The setting is basically the coordination
of the four relays that needs to be operated at the given fault and at a particular bus. The
settings for all the faults are done in accordance with the same curve.
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9.4 SIMULATION REPORT
The simulation report for only 3-phase symmetrical fault at bus 38 report has been
attached. Similar tasks can be carried out for different faults at different buses.
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DC BATTERY BANK
A Battery is a device that can change chemical energy into electrical energy by reaction
of certain chemicals. Electrons from one kind of chemical travel to another under as a
consequence of the chemical reaction, this causes an electric current that can power a
load.
Batteries have three basic parts:
4. Anode: It is the positively charged electrode that attracts the negative ions.
5. Cathode: It is the negatively charged electrode that attracts the positive ions.
6. Electrolyte: It is a liquid medium which acts as medium to conduct electricity.
10.1 DEFINITIONS:
Ampere Hour
One ampere-hour is equal to a current of one ampere flowing for one hour. It is a unit-
quantity of electricity used as a measure of the amount of electrical charge that may be
obtained from a storage battery before it requires recharging.
Available Capacity: (IEEE Std 1115™-2000 (R2011) – 3.1)
The capacity for a given discharge time and ending-of-discharge voltage which can be
withdrawn from a cell within the specific conditions of operation.
Battery Duty Cycle: (IEEE Std 1115™-2000 (R2011) – 3.2)
The load which is supposed to be supplied for specific time periods.
Full Float (Constant Potential) Operation: (IEEE Std 1115™-2000 (R2011) – 3.3)
Operation of a dc system with the parallely connected battery, load, and with the
battery charger supplying the normal dc load plus any self-discharge or charging current
required by the battery. (The battery provides current only when the load surpasses the
output of charger.)
Period: (IEEE Std 1115™-2000 (R2011) – 3.4)
An interval of time in the battery duty cycle during which the load is assumed to be
constant for purposes of cell sizing calculations.
Rated Capacity : (IEEE Std 1115™-2000 (R2011) – 3.5)
The capacity assigned to a cell by its manufacturer for a specific constant current
discharge, with a given discharge time, at a specified electrolyte temperature, to a given
end-of-discharge voltage. The conditions used to establish rated capacity are based on a
constant current charge, in accordance with IEC 60623 (1990-03) [B2]
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Ampere-Hour Capacity
It is basically the Ampere hours that can be supplied by the battery on a single
discharge.It depends on the following factors:
Quantity of electrolyte
Discharge rate
Density of electrolyte
Temperature
Age
No, design and dimensions of electrodes
Life history of the battery
12.1 DESIGN CONSIDERATIONS:
Minimum cell voltage: (IEEE Std 1115™-2000 (R2011) – 6.2)
The minimum battery voltage is equal to the least system voltage plus any voltage drop between the battery terminals and the load. The minimum battery voltage is then used to calculate the permissible minimum cell voltage as follows:
Minimum cell voltage =
Charging time as limiting factor: (IEEE Std 1115™-2000 (R2011) – 6.1.2) The time available to charge the battery can affect both the number of cells and the cell size. The time required for a charge decreases as the charging voltage per cell increases, assuming that the charging equipment can supply the high current necessary early in the recharge cycle. If the maximum charging voltage is limited, it is essential to select the number of cells that can be charged in the time available. This, in turn, may require using a larger cell than would otherwise have been necessary. Limits are supplied by the battery manufacturer for charging current and voltage. Rounding off: (IEEE Std 1115™-2000 (R2011) – 6.1.3) If the results of calculations through formula given in shown in 6.2 indicate a need for a cell of fractional value, we can round that result off to the nearest whole number of cells. Temperature derating factor ( Tt): (IEEE Std 1115™-2000 (R2011) – 6.2.1) The available capacity of a cell is affected by its operating temperature. The standard temperature for stating cell capacity is 25 °C. If the least expected electrolyte temperature is below standard, choose a cell large enough to have the desired capacity available at the lowest expected temperature. The battery manufacturer should be consulted for capacity derating factors for various discharge times and temperatures. If the lowest expected electrolyte temperature is above 25 °C, generally there is no noticeable increase in the available capacity.
100
Design margin:
It is prudent design practice to provide a capacity margin to allow for unforeseen
additions to the dc system, and less-than-optimum operating conditions of the battery
due to improper maintenance, recent discharge, ambient temperatures lower than
anticipated, or a combination of these factors.
Capacity Rating Factor (Kt) : The capacity rating factor, Kt, is the ratio of rated ampere-hour capacity (at a standard time rate, at 25 °C, and to a standard end-of-discharge voltage) of a cell, to the amperes that can be delivered by that cell for t minutes at 25 °C and to a given end-of-discharge voltage. Kt factors are available from the battery manufacturer, or may be calculated from the following formula:
10.2 DESIGN PROCEDURE: Number of cells: (IEEE Std 1115™-2000 (R2011) – 6.1)
The maximum and minimum permissible system voltages decide the number of cells in
the battery. It has been normal practice to use 9–10, 18–20, 36–40, 92–100, or 184–200
cells for system voltages of 12, 24, 48, 125, or 250 V, correspondingly.
We can calculate the no. of cells by the following formula:
Number of cells =
According to the data of the battery bank,
Number of cell =
= 183.33 = 184 cells
SIZING METHODOLGY: (IEEE Std 1115™-2000 (R2011) – 6.1)
The initial calculations are based on a trial selection of cell range. On the basis of this initial assumption, we further size the battery bank to achieve optimum cell type and size appropriate enough for the application. The capacity obtained from the first calculation acts as a guide for optimum sizing. The cell designated for a specific duty cycle must have enough capacity to carry the combined loads during the bduty cycle. To determine the required cell size, it is necessary to calculate, from an analysis of each section of the duty cycle , the maximum
101
capacity required by the combined load demands (current versus time) of the various sections. The first section analysed is the first period of the duty cycle.
A worksheet has been designed for sizing battery bank according to the following
procedures:
a) Fill in necessary information in the heading of the worksheet. The temperature and
voltage recorded
are those used in the calculations. The voltage used is the minimum battery voltage
divided by the
number of cells in the battery.
b) Fill in the amperes and the minutes in columns (2) and (4) as indicated by the section
heading notations.
c) Calculate and record the changes in amperes as indicated in column (3). Record
whether the changes
are positive or negative.
d) Calculate and record the amount of time in minutes from the start of each period to
the end of the
section as indicated in column (5).
e) Record in column (6) the capacity rating factors Kt, and in column (7) the
temperature derating
factors Tt, for each discharge time calculated in column (5).
f) Calculate and record the cell size for each period as indicated in column (8). Note the
separate subcolumns
for positive and negative values.
g) Calculate and record in column (8) the subtotals and totals for each section as
indicated.
h) Record the maximum section size [the largest total from column (8)] in item (9), the
random section
size in item (10), and the uncorrected size (US) in items (11) and (12).
102
i) Enter the design margin (≥1.0) in item (13) and the aging factor (≥1.0) in item (14).
Combine items
(12), (13), and (14) as indicated and record the result in item (15).
j) When item (15) does not match the capacity of a commercially available cell, the next
larger cell is
required. Show the result in item (16).
k) From the value in item (16) and the manufacturer’s literature, determine the
commercial designation
of the required cell and record it in item (17).
10.3 CELL SIZING CALCULATIONS:
Battery duty cycle diagram
103
CELL SIZING DATA
DISCHARGE CURRENTS FOR EX CELL RANGE MANUFACTURED BY EXIDE PAKISTAN LTD
CELL TYPE
RATE Ah 1s 60s 15min 30min 60min 90min 120min 180min 300min 480min
EX396P 369 878 627 334 266 207 166 138 107 72 45.4
EX392P 392 927 666 355 282 220 177 147 113 76.4 48.2
EX415P 415 984 705 375 299 233 187 155 120 80.9 51
EX438P 438 1041 743 396 315 246 198 164 127 85.4 53.9
EX461P 461 1090 786 417 332 258 208 173 133 89.9 56.7
EX505P 505 1197 857 457 364 283 228 189 146 98.5 62.1
EX555P 555 1317 942 502 400 311 250 208 161 108 68.3
EX625P 625 1480 1062 565 450 350 282 234 181 122 76.9
EX690P 690 1635 1175 624 497 387 311 258 200 135 84.9
EX740P 740 1756 1260 669 533 415 334 277 214 144 91
EX830P 830 1968 1409 754 598 465 374 311 240 162 102
EX920P 920 2181 1565 833 663 516 415 345 266 179 113
EX965P 965 2287 1643 876 695 541 435 361 279 188 119
EX1040P 1040 2464 1770 941 750 583 469 390 301 203 128
EX1150P 1150 2726 1954 1041 831 645 519 431 333 225 141
EX1220P 1220 2896 2074 1106 882 684 550 457 353 238 150
EX1390P 1390 3299 2365 1257 1000 776 627 521 402 271 171
Period Load Total Amperes Duration (min) Capacity Removed
1 L1+L2 320 0.08 (5sec) 0.43
2 L1+L3 100 29.92 49.87
3 L1+L2+L4+L5 280 30 140
4 L1+L3+L4 200 60 200
5 L1 40 59.42 39.61
6 L1+L6 120 0.58 (35) 1.16
7 L7 100 1 1.67
TOTAL 432.74
104
Since the total capacity removed accrding to our duty cycle, matches with the rated
ampere hours of EX438P cell type, therefore, we will now do the calculations of capacity
rating factors (kt) for the selected cell type as shown in the table below.
CALCULATION OF CAPACITY RATING FACTORS (KT) FOR EX438P CELL TYPE
Discharge time (t) (min)
time t1
(min)
Time t2
(min)
Amperes
for t1 (1)
Amperes for
t2 (2)
Factor Kt1
for t1
[438/(1)]
Factor Kt2
for t2
[438/(2)]
Factor Kt
for time t
0.083 0.017 1 1041 743 0.421 0.59 0.432
0.583 0.017 1 1041 743 0.421 0.59 0.518
30 30 - 315 - 1.39 - 1.39
59.92 30 60 315 246 1.39 1.78 1.779
60 60 - 246 - 1.78 - 1.78
90 90 - 198 - 2.212 - 2.212
119.92 90 120 198 164 2.212 2.671 2.67
120 120 - 164 - 2.671 - 2.671
150 120 180 164 127 2.671 3.449 3.06
179.92 120 180 164 127 2.671 3.449 3.448
180 180 - 127 - 3.449 - 3.449
105
CELL SIZING WORKSHEET:
Minimum Cell Voltage: 1.10 V
(1) Period
(2) Load
(amps)
(3) Change In Load
(amperes)
(4) Duration or
Period ( Mins)
(5) Time to end of section (minutes)
(6) Capacity
rating factor at t Min rate
(Kt)
(7)
Required Section Size (3) x (6) = Rated Amp hours
Pos. Values
Neg. Values
Section 1 - First period Only - if A2 is greater than a1, go to Section 2
1 A1= 320 A1-0 = 320 M1= 0.08 t=M1=0.08 0.432 138.24 ***
Sec1
TOTAL 138.24 ***
Section 2 - First two periods Only - if A3 is greater than A2, go to Section 3
1 A1= A1-0 M1= t=M1+M2= 2 A2= A2-A1 M2= t=M2= Sec2
***
TOTAL
Section 3 - First three periods Only - if A4 is greater than A3, go to Section 4
1 A1=320 A1-0=320 M1= 0.08 t=M1+M2+M3=60 1.78 569.6 2 A2=100 A2-A1=-220 M2= 29.92 t=M2+M3=59.92 1.779
391.38
3 A3=280 A3-A2=180 M3= 30 t=M3=30 1.39 250.2 Sec3 Subtotal 819.8 391.38
TOTAL 428.42 ***
Section 4 - First four periods Only - if A5 is greater than A4, go to Section 5 1 A1=320 A1-0=320 M1= 0.08 t=M1+M2+M3+M4=120 2.671 854.72
2 A2=100 A2-A1=-220 M2= 29.92 t=M2+M3+M4=119.92 2.67
587.4
3 A3=280 A3-A2=180 M3= 30 t=M3+M4=90 2.212 398.16 4 A4= 200 A4-A3= -80 M4= 60 t=M4 = 60 1.78
142.4
Sec4 Subtotal 1252.88 445
TOTAL 807.88 ***
Section 5 - First five periods Only - if A6 is greater than A5, go to Section 6 1 A1= A1-0= M1= t=M1+M2+M3+M4=M5= 2 A2= A2-A1= M2= t=M2+M3+M4+M5= 3 A3= A3-A2= M3= t=M3+M4+M5= 4 A4= A4-A3= M4= t=M4+M5 = 5 A5= A5-A4= M5= t=M5= Sec5 Subtotal
TOTAL
Section 6 - First six periods Only - if A7 is greater than A6, go to Section 7 1 A1=320 A1-0=320 M1= 0.08 t=M1+M2+M3+M4+M5+M6=180 3.449 1103.68
2 A2=100 A2-A1=-220 M2= 29.92 t=M2+M3+M4+M5+M6=179.92 3.448
758.56
3 A3=280 A3-A2=180 M3= 30 t=M3+M4+M5+M6=150 3.06 550.8 4 A4= 200 A4-A3= -80 M4= 60 t=M4+M5 +M6=120 2.671
213.68
5 A5=40 A5-A4= -160 M5=59.42 t=M5+M6=60 1.78
284.8
6 A6=120 A6-A5=80 M6=0.58 t=M6=0.58 0.518 41.44 Sec6 Subtotal 1695.92 1257.04
TOTAL 438.88 ***
Section 7 - First seven periods Only - if A8 is greater than A7, go to Section 8 1 A1= A1-0= M1= t=M1+M2+M3+M4+M5+M6+M7= 2 A2= A2-A1= M2= t=M2+M3+M4+M5+M6+M7= 3 A3= A3-A2= M3= t=M3+M4+M5+M6+M7= 4 A4= A4-A3= M4= t=M4+M5 +M6+M7= 5 A5= A5-A4= M5= t=M5+M6+M7= 6 A6= A6-A5= M6= t=M6+M7= 7 A7= A7-A6= M7= t=M7= Sec7 Subtotal
TOTAL
***
- Random Equipment Load
R AR= 100 AR-0= 100 MR= 1 t=MR= 1 0.59 59 ***
106
CALCULATIONS BY THE RESULTS OF CELL - SIZING - WORKSHEET: - Maximum Section Size (7) 807.88+ Random Section Size (8) 59.0 = Uncorrected
Size (US) (9) 866.88.
- US (9) 866.88 x Design Margin (10) 1.10 x Aging Factor (11) 1.25 = (12) 1191.96.
- When the cell size (12) is greater than a standard cell size, the next larger cell is
required.
THEREFORE,
Required cell size is approximately (12) 1195 Ampere Hours. Therefore cell (13) X1220
is required.
107
CONCLUSION:
The study and sizing of the equipment installed at the HIS based grid station have been
successfully carried out. The calculations and the simulation results have been
authenticated by the KESC engineers. Initiating from the soil constraints that occurred
along with the problems of selection of number of rods and conductors, power
transformer MVA sizing, capacitor bank size for powerfactor improvement, current
transformer and capacitive voltage transformer sizing and calculation for protection
purpose have been approved by the KESC engineers. The load to be driven by the
auxiliary transformer in the grid station, the cable sizing of the power cables in
accordance with the temperature rise and fault current have been calculated for the best
operation. Also, the short circuit analysis and overcurrent earth fault protection
simulations have been carried out on ETAP for the protection purpose and the circuit
breaker selection.
108
REFERENCES:
IEEE Std 80-2000 (Revision of IEEE Std 80-1986)IEEE research
papers on earthing of a grid station.
http://www.esgroundingsolutions.com/about-electrical-
grounding/what-is-step-and-touch-potential-and-resistance-to-
ground.php
http://constructionmanuals.tpub.com/14026/css/14026_73.htm
Power Systems by AshfaqHussain
Short circuit analysis, protection design and CT sizing of Orangi Thermal Power Station using Etap-B.E. (e) project report by TahaImtiaz Ali
IEC-60044-1(Current transformer)
Alstom’s Calculation Report Of - CT Sufficiency For Micom P139
http://www.electrical-design-
tutor.com/transformercalculations.html
http://www.electrical4u.com/electrical-transformer/instrument-
transformer.php
https://sites.google.com/site/gauravgaurali/experiments
RET 670 ABB Relay manual
Alternate method to size a CT(The IEC 60044 Method)
Instrument Transformers Application Guide by ABB.
J And P –The Transformer Book.
IEC 60076-10 Power transformers - Determination of sound levels
IEC 60076-8 Power transformers - Application guide
109
IEC 60076-5 Power transformers - Ability to withstand short circuit
IEC 60076-4 Power transformers - Guide to the lightning impulse and switching impulse testing
IEC 60076-3 Power transformers - Insulation levels, dielectric tests
and external clearances.
IEC 60076-2 Power transformers - Temperature rise
IEC 60076-1 Power transformers – General
http://www.electropedia.org/substation/cables.php
IEEE 485-1997 Battery Bank Sizing
Engineering Encyclopedia-Saudi Aramco Desktop Standards
IEC 62271- Cable Sizing
LS cables- reference LSGS-06-PC0241
IEC 60502-1 High Voltage Cables
IEC 60502-2 Selection Of Cables