electrical transmission line

---By Dhananjay Jha, Engineer (E), SJVN

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ELECTRICAL TRANSMISSIO

N Line


DEFINITION

Electrical transmission system is the means of transmitting power from generating station to different load centres.


INDIAN TRANSMISSION NETWORK gridmap\powergrid_map.pdf


TO LOOK INTO Transmission Network

Conductor

Earthwire

Inuslator

Tower


DISCUSSION ON Network

Transmission line model

Characteristics

Conductors

Insulators

Earthwire


TRANSMISSION NETWORK

Generating Station

Transmission Line

Sub-Stations


LOAD FLOW Associated system

Voltage magnitudes & angle

Active and reactive power


ABCD MODEL OF TRANSMISSION LINE


2-PORT NETWORKA transmission line can be represented by a 2-port network – a network that can be isolated from the outside world by two connections (ports) as shown:

2-PORT NETWORKSIf the network is linear, an elementary circuits theorem (analogous to Thevenin’s theorem) establishes the relationship between the sending and receiving end voltages and currents as

S R R

S R R

V AV BII CV DI

Here constants A and D are dimensionless, a constant B has units of , and a constant C is measured in siemens. These constants are sometimes referred to as generalized circuit constants, or ABCD constants.

2-PORT NETWORKS –SHORT LINES

•Upto 80 Km

•Shunt capacitance is neglected and resistance & inductance are lumped together.

•Therefore, IS = IR = I

Hence the ABCD constants for the short transmission line model, are 1

01

AB ZCD

MEDIUM-LENGTH TRANSMISSION LINE

Considering medium-length lines (80 to 250 Km-long).

•The shunt admittance is also included for calculations. However, the total admittance is usually modeled ( model) as two capacitors of equal values (each corresponding to a half of total admittance) placed at the sending and receiving ends.



The current through the receiving end capacitor can be found as :

2 2C RYI V

And the current through the series impedance elements is :

2ser R RYI V I


From the Kirchhoff’s voltage law, the sending end voltage is

2 12ser RS RC R RRZI V Z I I V YZV V ZI

The source current will be1 1 2 1 1

22 2 4S R RC ser C C R S R RY YI I I I I V ZYV ZYI Y II V

Therefore, the ABCD constants of a medium-length transmission line are

12

14

12

ZYA

B ZZYC Y

ZYD

If the shunt capacitance of the line is ignored, the ABCD constants are the constants for a short transmission line.

LONG TRANSMISSION LINE• For long lines, it is not accurate enough to approximate the

shunt admittance by two constant capacitors at either end of the line. Instead, both the shunt capacitance and the series impedance must be treated as distributed quantities.

• The voltages and currents on the line is found by solving differential equations of the line.

LONG TRANSMISSION LINE

sinh'

tanh 2'

2

dZ Zd

dY Y

d

Here, Z is the series impedance of the line, Y is the shunt admittance of the line, d is the length of the line, is the propagation constant of the line.

yz where y is the shunt admittance per kilometer and z is the series impedance per km.

LONG TRANSMISSION LINE

The ABCD constants for a long transmission line are

' ' 12'

' '' 14

' ' 12

Z YA

B ZZ YC Y

Z YD

TRANSMISSION LINE CHARACTERISTICSAC voltages are usually expressed as phasors.Load with lagging power factor.

Load with unity power factor.

Load with leading power factor.

For a given source voltage VS and magnitude of the line current, the received voltage is lower for lagging loads and higher for leading loads.

TRANSMISSION LINE CHARACTERISTICS

In overhead transmission lines, the line reactance XL is normally much larger than the line resistance R; therefore, the line resistance is often neglected. We consider next some important transmission line characteristics…

1. The effect of load changesAssuming that a single generator supplies a single load through a transmission line.

Assuming that the generator is ideal, an increase of load will increase a real and (or) reactive power drawn from the generator and, therefore, the line current.

1) If more load is added with the same lagging power factor, the magnitude of the line current increases but the current remains at the same angle with respect to VR as before.

TRANSMISSION LINE CHARACTERISTICSThe voltage drop across the reactance increases but stays at the same angle.

Assuming zero line resistance and source voltage to be of constant magnitude:

voltage drop across reactance jXLI will stretch between VR and VS.

Therefore, when a lagging load increases, the received voltage decreases.

2) An increase in a unity PF load, on the other hand, will slightly decrease the received voltage at the end of the transmission line.

Vs = VR + jXLI


3) Finally, an increase in a load with leading PF increases the received (terminal) voltage of the transmission line.


TRANSMISSION LINE CHARACTERISTICS In a summary:

1. If lagging (inductive) loads are added at the end of a line, the voltage at the end of the transmission line decreases .

2. If leading (capacitive) loads are added at the end of a line, the voltage at the end of the transmission line increases.



The voltage regulation of a transmission line is

VReg = (Vnl - Vfl )/ Vfl

where Vnl and Vfl are the no-load and full load voltages at the line output.


2. Power flow in a transmission lineThe real power input to a 3-phase transmission line can be computed as

,3 cos 3 cosin S S S LL S S SP V I V I

where VS is the magnitude of the source (input) line-to-neutral voltage and VLL,S is the magnitude of the source (input) line-to-line voltage.

Similarly, the real output power from the transmission line is

,3 cos 3 cosout R R R LL R R RP V I V I The reactive power input to a 3-phase transmission line can be computed as

,3 sin 3 sinin S S S LL S S SQ V I V I


The apparent power input to a 3-phase transmission line can be computed as

,3 sin 3 sinout R R R LL R R RQ V I V I

And the reactive output power is

,3 3in S S LL S SS V I V I

And the apparent output power is

,3 3out R R LL R RS V I V I


A simplified phasor diagram of a transmission line indicating that IS = IR = I.

Further it can be observed that the vertical segment bc can be expressed as either VS sin or XLIcos. Therefore:

sincos S

L

VI

X


Therefore, the power supplied by a transmission line depends on the angle between the phasors representing the input and output voltages.

The maximum power supplied by the transmission line occurs when = 900:

max3 S R

L

V VPX

This maximum power is called the steady-state stability limit of the transmission line. The real transmission lines have non-zero resistance and, therefore, overheat long before this point. Full-load angles of 250 are more typical for real transmission lines.

Then the output power of the transmission line equals to its input power: 3 sinS R

L

V VPX



Few interesting observations can be made from the power expressions:

The maximum power handling capability of a transmission line is a function of the square of its voltage. For instance, if all other parameters are equal, a 220 kV line will have 4 times the power handling capability of a 110 kV transmission line.

Therefore, it is beneficial to increase the voltage. However, very high voltages is limit by other factors.


The maximum power handling capability of a transmission line is inversely proportional to its series reactance, which may be a serious problem for long transmission lines. Some very long lines include series capacitors to reduce the total series reactance and thus increase the total power handling capability of the line.

In a normal operation of a power system, the magnitudes of voltages VS and VR do not change much, therefore, the angle basically controls the power flowing through the line. It is possible to control power flow by placing a phase-shifting transformer at one end of the line and varying voltage phase.


3. Transmission line efficiency

The efficiency of the transmission line is

100%out

in

PP


4. Transmission line ratingsOne of the main limiting factors in transmission line operation is its resistive heating. Since this heating is a function of the square of the current flowing through the line and does not depend on its phase angle, transmission lines are typically rated at a nominal voltage and apparent power.

5. Transmission line limitsSeveral practical constrains limit the maximum real and reactive power that a transmission line can supply. The most important constrains are:

1. The maximum steady-state current must be limited to prevent the overheating in the transmission line. The power lost in a line is approximated as

23loss LP I R

The greater the current flow, the greater the resistive heating losses.


2. The voltage drop in a practical line should be limited to approximately 5%. In other words, the ratio of the magnitude of the receiving end voltage to the magnitude of the sending end voltage should be greater than 95%.

This limit prevents excessive voltage variations in a power system.

3. The angle in a transmission line should typically be 300 ensuring that the power flow in the transmission line is well below the static stability limit and, therefore, the power system can handle transients.



Any of these limits can be more or less important in different circumstances.

In short lines, where series reactance X is relatively small, the resistive heating usually limits the power that the line can supply.

In longer lines operating at lagging power factors, the voltage drop across the line is usually the limiting factor.

In longer lines operating at leading power factors, the maximum angle can be the limiting f actor.


SELECTION OF TRANSMISSION VOLTAGE

Standard Voltage - 66,110,132, 220, 400 KV or above

Selection Criterion of Economic Voltage – Quantum of power to be evacuated Length of line Voltage regulation Power loss in Transmission Surge impedance level


Economic Voltage of Transmission of Power

NcKVALV

*1506.15.5

V = Transmission voltage (KV) (L-L).L = Distance of transmission line in KM

KVA=Power to be transferred Nc= Number of circuits

Empirical Formula


conductors


ALUMINIUM CONDUCTOR STEEL REINFORCED Aluminium Conductor Steel Reinforced consists of a

solid or stranded steel core surrounded by one or more layers of strands of 1350 aluminium. The high-strength ACSR 8/1, 12/7 and 16/19 standings', are used mostly for overhead ground wires, extra long spans, river crossings, etc. The inner core wires of ACSR is of zinc coated (galvanized) steel.


ACSR CONDUCTORS

Nomenclature :- Al/Steel/dia.

Panther :- 30/7/3.0 mm

Zebra :- 54/7/3.18mm

Snowbird :- 42/3.98 + 7/2.21mm

Moose :- 54/7/3.53mm


OTHER CONDUCTORS

AAC (All Aluminium Conductor)

AAAC (All Aluminium Alloy Conductor)

AL-59

ACCC (Aluminium conductor composite core)


CONDUCTORS- COMPARISON

ACSR Moose

AAAC Moose Al-59

Dia. (mm) 31.77 31.05 31.50Cross sectional area (sq-mm)

597 570 586.59

Ambient Temperature (deg.C)

40 40 40

Current carrying capacity(A) at 75 C

728 699 759

At 95 C NA 952 976SAG (m) at 85 C 13.26 14.15 14.52


SELECTION OF CONDUCTOR SIZE

Mechanical Requirement

Tensile Strength(For Tension)

Strain Strength(For Vibration)

Mechanical Requirement

Electrical Requirement


ELECTRICAL REQUIREMENT

o Continuous current rating.o Short time current carrying rating.o Voltage dropo Power losso Minimum dia to avoid coronao Length of line


SHORT TIME RATING

According to short time rating conductor size is given by-

Where A=area of conductor(mm2) IF= fault current(KA) t= fault duration(1 sec.)

tIA F **58.7


CORONA Visual corona voltage in fair weather

condition is given by-

V0= corona starting voltage, KV(rms) r= radius of conductor in cm D= GMD equivalent spacing between conductors

in cm m= roughness factor

= 1.0 for clean smooth conductor=0.85 for stranded conductor

rDn

rrmV log)3.01(1.210


Voltage gradient at the surface of conductor at operating voltage-

rDLog

V

n

g 30

Corona discharge form at the surface of conductor if g0≥ corona starting gradient i.e.

rrmg )3.01(1.21

0

(rms kv/cm)


SOME OTHERS CONSIDERATION IN CONDUCTOR SELECTION

River crossing

Weight/ Dia. - Less Weight/Dia ratio conductor swing more.

INSULATORIn overhead transmission lines, the conductors are suspended from a pole or a tower via insulators.


INSULATORInsulator are required to support the line conductor and

provide clearance from ground and structure.

Insulator material- High grade Electrical Porcelain

Toughened Glass

Fiber Glass


DESIGN OF INSULATION System over voltage factors shall be

evaluated.

Due to switching Power frequency Lightning


Switching over-voltage :

Switching off of long lines on no load Energizing lines of no load

DESIGN OF INSULATION


DESIGN OF INSULATION Power frequency over voltage

Loaded line interrupted at one end Occurrence of fault Open line suddenly connected to load


DESIGN OF INSULATION


INSULATORType of Insulator-

Pin type insulator

Suspension insulator

Strain insulator


INSULATOR Pin Type Insulator : Used for

transmission and distribution of electric power at voltages up to 33 kV


INSULATOR

Suspension : For voltages greater than 33 kV, it is a usual practice to use suspension type insulators


INSULATOR

Strain : A dead end or anchor pole or tower is used where a straight section of line ends, or angles off in another direction.


INSULATOR STRING Disc insulator are joint by their ball pins and

socket in their caps to form string.

No of insulator disc is decided by system

voltage, switching and lighting over voltage

amplitude and pollution level.

Insulator string can be used either suspension

or tension.

Swing of suspension string due to wind has to

be taken into consider.

Fig. single string

Fig. Double string


EARTH WIREEarth wire provided above the phase conductor across the line

and grounded at every tower. It shield the line conductor from direct strokes

Reduces voltage stress across the insulating strings during lightning

strokes

Design criterion: Shield angle

25°-30° up to 220 KV

20° for 400 KV and above

Earth wire should be adequate to carry very short duration lightning

surge current of 100 KA without excessive over heating


tIA 5A= Area(in mm2) of conductorI =current in KAt = Time in second

Area of Steel Wire = 3*A(mm2)

For EHV line it is suggested as 70 mm2 (7/3.66 mm).

ACSR is used as earth wire (12/3.0 mm AL+7/3.0 mm steel)

OPGW

EARTH WIRE


EARTH WIRE (OPGW) Optical Ground Wire

Advantages :

Serves the dual purpose of ground wire and communication.

High speed data transmission.


EARTH WIREOPGW


SMART GRID


SMART GRID A Smart Grid is an electricity network that can

intelligently integrate the actions of all users connected to it – generators, consumers and those that do both – in order to efficiently deliver sustainable, economic and secure electricity supplies. European Smart Grid Technology Platform

SG3 defines Smart Grids as the concept of modernizing the electric grid. The Smart Grid is integrating the electrical and information technologies in between any point of generation and any point of consumption. Smart Grid Working Group 3


SMART GRID The aim of smart grid is to provide real-time

monitoring and control, and thus improve the overall efficiency of the entire system apart from inclusion of renewable energy resources into the system.


SMART GRID Presently, the Indian electricity system

faces a number of challenges:

Shortage of power Power Theft Poor access to electricity in rural areas Huge losses in the grid Inefficient power consumption Poor reliability


SMART GRID The Smart Grid is a transition of the present

energy system into a new era of reliability, availability and efficiency.

The smart grid vision involves a uniformly integrated communication system with the present power system.


SMART GRID- INDIA The India Smart Grid Task Force (ISGTF) is an

inter-ministerial group set up under the chairmanship of Shri Sam Pitroda in September 2010 to serve as Government's focal point for activities related to Smart Grid and to evolve a road map for Smart Grids in India.


SMART GRID-INDIA Indian Smart Grid Forum (ISGF) was set up in

2010 to provide a mechanism through which academia, industry, utilities and other stakeholders could participate in the development of Indian smart grid systems and provide relevant inputs to the government’s decision-making.


SMART GRID-PILOT PROJECTS UHBVN,Haryana CESC, Mysore TSECL, Tripura KSEB, Kerala Electricity department-Govt. of Puducherry UGVCL, Gujrat AP CPDCL, Andhra Pradesh APDCL, Assam MSEDCL, Maharashtra CSPDCL, Chattisgarh HPSEB, Himachal Pradesh PSPCL, Punjab WBSEDCL, West Bengal JVVNL, Rajasthan


SMART GRID-PILOT PROJECTS HPSEB, Himachal Pradesh

Project Area- Kala Amb No. of Consumers-650 Total Cost- 17.85 Cr. MoP Share-Rs. 8.92 Cr.


SMART GRID-PILOT PROJECT Benefits Envisaged

Shifting peak loadReduction in penalties Reduction in outages

Status- RFP (Request for Proposal) issued on 25.08.2014.

electrical transmission line

Engineering