professor md dutt

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Prof MD Dutt HOD Ex Department SRCT Thuakheda Bhopal MP India

READING MATERIAL FOR B.E. STUDENTS

OF RGPV AFFILIATED ENGINEERING COLLEGES

BRANCH VII SEM ELECTRICAL AND ELECTRONICS

SUBJECT EHV AC AND DC TRANSMISSION

Professor MD Dutt

Addl General Manager (Retd)

BHARAT HEAVY ELECTRICALS LIMITED

Professor(Ex) in EX Department

Bansal Institute of Science and Technology

Kokta Anand Nagar BHOPAL

Presently Head of The Department ( EX)

Shri Ram College Of Technology

Thuakheda BHOPAL

Sub Code EX 7102 Subject EHV AC AND DC TRANSMISSION

UNIT I

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EX 7102

RG PV Syllabus

UNIT I EHV AC AND DC TRANSMISSION

Construction of EHV ac and DC links, Kind of DC links. Limitation and advantages of AC & DC transmission. Principal application of AC & DC transmission . Trends in

EHV AC & DC transmission ,Power handling capacity. Convertor analysis Gartz circuit, firing angle control over lapping.

INDEX

S No Topic UNIT I Page

1 Construction of EHV ac and DC links, Kind of DC links 3- 6

2 Limitation and advantages of AC & DC transmission. 7-8

3 Principal application of AC & DC transmission 9-10

4 Trends in EHV AC & DC transmission ,Power handling capacity 11-13

5 Convertor analysis Graetz circuit, firing angle control over

lapping.

13-16

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1. CONSTRUCTION OF EHV AC & DC LINKS, KIND OF DC LINKS

Introduction

The industrial growth of a nation demands increased consumption of energy. Electrical energy is the most convenient form of energy since it is available to the consumer at the

very instant it is switched on. The other benefits of electrical energy are the ease with which it is generated in bulk and transmitted efficiently and economically over long

distances. This has led to increase in the generation and transmission facilities to meet the increasing demand. The compulsion of supplying energy at reasonable cost has led

to the establishment of remotely located generating stations predominantly the coal fired thermal power stations. Large hydro generating stations are located at several

hundred kilometers from the load centers. Long distance power transfer is possible only with extra high voltage EHV and ultra high voltage UHV transmission system.

Construction of EHV AC LINKS AC system is better from the generation and utilization point of view. The reliability

and flexibility of power transmission by AC is undisputable. That is why AC power transmission at EHV and ultra high voltage UHV are in use in the most of the countries.

The complete AC transmission system has the following parts:-

a) Terminal Sub Stations b) AC transmission lines, with parallel 3ph AC circuits.

c) One or more intermediate Sub stations. d) Static VAR system SVS incorporating thyristor controlled shunt reactance and

shunt capacitance.

Schematic of EHV AC transmission Link

B= Transmission line , 3ph AC double circuit R= Shunt Reactor

D= Shunt compensation or Static VAR source ( SVS) X= Circuit Breakers

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A long transmission link needs at least two parallel 3ph transmission circuits to ensure

reliability and stability during fault condition on any line. It has one or more intermediate sub station for installing series capacitors , shunt reactors, switching and

protection equipments. Generally at interval of 250to 300Kms intermediate sub stations are required. A long Ac transmission line takes lagging KVAr during heavy loads and

leading reactive KVAr during low loads resulting in low voltage at receiving end during heavy loads, high receiving end voltage during light loads.

A long transmission line has series inductive reactance Xl and shunt capacitance

reactance Xc, The series inductive reactance gives a voltage drop IXl, which varies with line/load current I, shunt capacitance supplies reactive power (-Q) which is the

function of voltage V² . Due to distributed series inductance the transmission line absorbs reactive power throughout its length. The reactive power varies with increase in load current. Hence the supply and absorption of reactive power by distributed

inductance and capacitance of line goes varying along the line. Therefore the voltage along the line varies. The voltage variation depends upon load current and its power

factor, Since the substation bus voltage is to be kept within specified limits (±12.5%) hence SVS conventional shunt reactors and shunt capacitors are necessary at terminal

substation for controlling VAr and voltage. The objective of long AC transmission line is to transfer real power ‘P’ with sending

end voltage Ι Vs Ι , receiving end voltage Ι Vr Ι and voltage profile along the line length within specified limits and to maintain synchronous stability and voltage

stability, Conventional AC line has serious limitation in controlling the power flow.

CONSTRUCTION HVDC LINKS Recent developments in conversion equipment have reduced the size and cost and

improved reliability in HVDC links. HVDC systems have the ability to rapidly control the transmitted power. HVDC transmission has the following parts:-

1) Ac network and convertor station at each terminals. 2) Interconnecting HVDC line.

The major component of a HVDC transmission system are convertor stations where conversion from Ac to DC, and from DC to AC are performed.

The station where AC to DC is done is known as rectifier station and DC to AC is known as inverter station. The role of rectifier and inverter station can be reversed by

appropriate convertor control, thus power reversal. A typical HVDC convertor station consists of converter units usually consists of two

three phase converter bridges connected in series to form a 12 pulse converter unit. The total number of valves are 12 in such unit. Each valve is used to switch in a segment of

an AC voltage wave form. The valves can be packaged as single valve , double valve and quadruplet valve arrangements. The largest thyristor device now come with rating of 10KV,5000A. The rating of valve group are limited merely by the permissible short

circuit currents than steady state load requirement.

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The thyristors are triggered by firing pulse to gates, The current through the valve and

DC transmission line is controlled by adjusting the delay angle ( α ) of firing thyrister. Each convertor station has a control system which controls the voltage , current and

power through the DC lines.

TYPES OF HVDC LINKS HVDC links can be broadly classified into following categories.

a) Mono polar Link b) Bi polar Link

c) Homo Polar Link

Mono Polar Link :- A monopolar link has only one conductor and the return path is provided by permanent earth or sea. The line usually operates with –ve polarity w.r.t ground to reduce carona loss and radio interference.

Mono polar links are more economical because of saving in conductor and line loss.

Mono polar is very useful where power is to be transmitted below sea level. In such cases sea is used as second electrode. A metallic return may be considered where earth

resistivity is very high

b) Bi Polar Link:- Each terminal station has two convertors of equal voltage rating which are connected in series on the DC side. The junction between the convertors are

grounded, Normally the currents in the two conductors are balanced. A negligible amount ( about 1% of rated current) of the unbalanced current flows in the ground.

The voltage rating of Bi Polar Link is expressed ± Kv for example HVDC link 810Km long 1500MW, ±500Kv. The neutral points of convertors may be grounded at one or both ends. If grounded at

both ends , each pole can operate independently. If one pole is isolated due to fault on its conductor, the other pole can operate with ground return for transmitting half load.

The direction of power flow can be changed by reversing the polarity of two poles through controls .

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c) Homo Polar Link:- A homo polar link configuration is constituted of two or

more conductor’s. It operates with all the conductor’s having same polarity, usually –ve and return path is provide by ground or metallic return. Usually –ve polarity is preferred

because it causes less carona loss and less radio interference. The advantage of homo polar link is that in the event of fault on one

conductor, the entire convertor can be connected to the remaining healthy conductors. In homo polar configuration the insulation requirement are less stringent than in the bi

polar arrangement . As the ground is continuously used as return path the ground current can cause corrosion of Gas and oil pipe lines that lie with in few kilometers of

system electrodes.

2 LIMITATION AND ADVANTAGES OF AC & DC TRANSMISSION

EHC AC transmission line has the following inherent advantages:-

i) Voltage can be stepped up or stepped down in substation to have economical transmission

ii) Parallel lines can be easily added. iii) AC lines can be easily extended or tapped.

iv) Equipments are simple and reliable. v) Operation of A system is simple and adopts naturally to the synchronously

operating AC system.

EHV AC transmission Limitation i) Reactive losses While transferring power at a lagging power factor there will be

drop in voltage along the line. Where as if the reactive power is leading there is

rise in voltage. The reactive voltage drop or rise and the natural load do not put any restriction on the distance over which the power may be transmitted. But to

fix the voltage which causes limitation in power transmission. ii) Stability Consideration:- The stable condition means the sending end and the

receiving end remains in synchronism with each other. If synchronism is lost the system is called unstable. The stability limit is the maximum power flow without

losing synchronism. P = Vs Vr sinδ

X ( Vs = Sending end voltage, Vr= receiving end voltage, X= series reactance, α =load angle)

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iii) Current carrying capacity of conductors The permissible loading of an EHV AC

line is limited by transient stability limit and line reactance to almost one third of thermal rating of conductors

iv) Ferranti Effect the rise of receiving end voltage for a lightly loaded line is known as “Ferranti Effect”, Shunt reactors in the load end are generally used to

control this voltage rise. v) Number of lines: - A fault on any one phase of a 3phase AC trips all the

3phases. Hence an additional three phase line is always provided to maintain continuity of power flow and transmission stability.

ADVANTAGES DC TRANSMISSION LINE

Various advantages of HVDC transmission are:- i) Cheaper in cost :- Bi polar DC transmission line requires two conductors

while AC system requires 3 wire.

ii) The potential stress is 1/√2 times less in case of DC system compare to AC system of same operating voltage.

iii) The phase to phase and phase to ground clearance and tower size are smaller in case of DC transmission.

iv) No skin effect:- There is no skin effect on DC transmission system. v) Lower transmission losses:- As only two conductors are required in HVDC,

hence I²R losses are low for the same power transfer. vi) Better voltage regulation, As there is no inductance hence voltage drop due to

inductance does not exists. vii) Permissible loading on a EHV AC line is limited by transient stability. No

such limit exists in HVDC lines. viii) Greater reliability A two conductor bi polar HVDC link is more reliable tha

3phase HVAC 3wire line.

ix) It is possible to generate power at one frequency and utilize it at some other frequency.

x) Less dielectric power loss and higher current carrying capacity. Cable have less dielectric loss with HVDC compare to HVAC

xi) Absence of charging current:- Due to absence of charging current in HVDC, power can be transmitted to along distance by cables.

xii) Low short circuit current, In HVAC parallel lines results I larger short circuit currents in the system.

xiii) Lesser carona loss:- the carona losses are proportional to )f+25), f frequency. The carona losses are less in HVDC.

xiv) Lower switching surge level:- The level of switching surges due to DC is lower compare to HVAC.

xv) Reactive power compensation :- HVDC does not require any reactive power compensation, where as HVAC requires shunt or series compensation.

Limitation of HVDC

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1. Costly Terminal equipments : The convertors required at both ends are more

expensive. The convertors have a very little over load capacity and absorb considerable reactive power. The convertors produce a lot of harmonics both on

Dc and AC side and may cause R.I. To remove ripples from the DC output, filtering and smoothening equipments are to be provided. On AC side filters are

to be provided for absorbing the harmonics, and thus further increasing the cost of convertor

2. HVDC circuit breakers comprises of circuit breaking capacitors, reactors etc, which increase cost several times than that of an AC circuit breaker.

3. More maintenance of insulators is required in HVDC system. 4. Circuit breaking in multi terminal DC system is difficult and costlier

5. Voltage transformation is not easier in case of DC system.

3.PRINCIPAL APPLICATION OF AC AND DC TRANSMISSION

Application Of HVDC transmission:- For generation, transmission and utilization of electrical energy, 3 phase AC systems are used universally and have a definite

superiority over DC system. However in following particular applications, HVDC is a strong alternative to EHV AC

transmission. 1. For long distance high power transmission 2. For interconnections (tie lines) between two or more AC systems are having their

own load and frequency control. 3. For back to back asynchronous tie sub station where two AC systems are

interconnected by a convertor station without any AC transmission line in between.

4. For underground or submarine cable transmission over long distance at high voltage .

Application Of HVAC transmission:-When the load on the system is increased ,the voltage at the consumer’s end falls due to increased voltage drop in

1. Alternator’s synchronous impedance 2. Transmission line.

3. Transformer impedance 4. Feeder and distribution.

Modern power utility industry requires a constant voltage to be maintained at the consumer terminal irrespective of the type and magnitude of the load. When power is supplied to a load through a transmission line keeping sending end voltage constant, the

receiving end voltage undergoes variations depending upon the magnitude of load and P.F of the load. Higher the load with smaller P.F the greater is the voltage variation.

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The voltage variation at a node is an indication of the unbalance between the reactive

power generated and consumed by that node .If the reactive power generated is greater than consumed, the voltage goes up.

The voltage control equipment is used at more than one point on the system for two reasons.

1) The power network is very extensive and there is a considerable voltage drop in transmission and distribution system.

2) The various circuits of the power system have dissimilar load.

For very long lines (more than 500Km) Shunt reactors are necessary at intermediate

point to limit the voltage during the light load. Switchable shunt reactors do not give stepless and smooth voltage control. The difficulty is over come by

a) Thyristor controlled shunt reactor TCR

b) Synchronous motor

In loaded power lines, the permanent fixed shunt reactors continuously absorb rated reactive power. These permanent connected shunt reactors also consume active power.

These leads to reduced voltage level and decreased power transmission capacity. With the development Controlled shunt reactors CSR , which is thyristor controlled

high impedance transformer, a stable bus voltage can be maintained by providing variable reactive power based on load variations. Also switching problems related to the

breakers are also avoided. The basic principal of CSR is to control the reactive power by using thyristor valves ,

which can provide necessary speed of switching and control by means of firing angle control. The thyristor valves are connected to the secondary side as it is not economical and practical to use thyristor valves at high transmission voltages such as 400KV.

Basically the CSR consists of high impedance transformer controlled by an anti parallel

pair of thyristor valves as shown in figure. The impedance of the transformer can be varied by varying the firing angle of the Thyristor. The major advantages of CSR are as

follows:- 1) Fully controllable reactive power

2) Reduction in dynamic over voltage limit 3) Increased power carrying capacity of lines

4) Fast response 5) Ability of direct connection to EHV level

6) Minimum Harmonics 7) Compatibility of single phase auto re closure.

STATIC VAr SOURCE SVS The static VAr source SVS, static VAr control SVC utilize shunt reactor and shunt capacitor combination with high voltage high power thyristor for achieving fast,

accurate source of controlled reactive power (Q). during heavy loads , the thyristor for capacitor control are made to conduct for a longer duration in each cycle. During low

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loads , the thyristor in the reactive circuit are made to conduct for longer duration in

each cycle. Thus step less variation of shunt compensation is achived.

APPLICATION OF SVS 1) Better, step less, fast, accurate voltage control of sub station buses over a wide

range of loads by supplying (-Q) capacitive or ( +Q) reactive into the sub station bus.

2) Dynamic compensation of fluctuating reactive loads:- With arc furnaces, rolling mills SVS are used for rapid change in reactive power compensation in

accordance with varying load. This reduces lamp flicker and voltage dips. SVS is commonly used in industrial distribution also.

4 TRENDS IN EHV AC & DC TRANSMISSION

TRENDS IN EHV UHV AC TRANSMISSION

The recent technological developments and application of power semiconductor devices, digital electronics, control equipments and satellite communication have

increased the capabilities of EHV UHV AC transmission lines . The modern trends in AC transmission are

1) Flexible AC transmission systems FACTS employ power electronic based and other static controllers to enhance the controllability of AC transmission lines. They also increase power transfer capability.

2) AC transmission voltages have exceeded the EHV limit. It has got increased to 1150KV and research is going on for 1500KV. 765KV is introduced in INDIA in

the year 2007. 3) Satellite imagery techniques is supposed to enhance the transmission line survey.

Survey techniques are improving through GIS and Airborne Laser Terrain Mapping ALTM

4) Tall and multi circuit towers are proposed to avoid deforestation ,protection of wild life and effective utilization ROW.

5) Recent trend is to use multi conductor bundled conductors having four ,six and eight sub conductors.

6) High temperature endurance conductors will bring in increased loading and higher power transfer capability.

7) Compact and gas insulated sub station of 765KV and 1200Kv will reduce the land requirement.

8) Use of high strength, polymer insulators are also proposed to strengthen the AC

transmission systems.

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TRENDS IN DC TRANSMISSION

The recent technological developments and application of power semiconductor

devices, digital electronics, control equipments and protective equipments have increased the use HVDC transmission lines . These developments have reduced the cost

of convertor stations and improved the reliability and performance of HVDC systems .The modern trends in HVDC transmission are :-

1) With the increase current and voltage rating , the number of semiconductors devices to b connected in series and parallel have come down thereby reducing

the cost of convertors. With the development light triggered thyristors , the reliability of convertor operation got improved.

For better economy , devices such as gate turn off (GTO) thyristors , which can be turned off by the gate signal, are needed. However a GTO requires large amount of current to turn off. MOS (metal oxide semiconductor) controlled

thyristors also known as MCT ,is a device in which a very large line current can be turned off by a small gate current. Also the turn off time for MCT compare to

GTO is one third. MCT is most promising element in HVDC transmission. 2) Convertor Control:- The development of micro computer based control

equipments has made it possible to design convertor controls with automatic transfer between systems during a malfunction . Thus it is possible to carry out

scheduled maintenance of stand by system, when the convertor is in operation and there by reducing the forced outage of control equipments. The micro

computer based control also permits the use of adaptive control algorithms and other specialized approaches for fault diagnoses and protection .

3) DC Breakers:- With the development of HVDC circuit breakers , it is now possible to tap existing DC link or develop new MTDC system. Parallel operation of convertors, which allows some flexibility in future system planning

is also possible decentralized control, of control and protection etc are some coordination of etc are some issues which are under steady

4) Conversion Of Existing AC Lines : The constrains of ROW and to increase the power capacity of the line have forced to seek option of covering existing HVAC

transmission line to DC transmission line. 5) Development of Capacitor commuted convertors:- A series capacitor bank along

with the ZnO varistor in parallel provide compensating commutating reactance and assists the compensation process. The other advantage that commutating

capacitors offer are improved convertor performance, better stability of HVDC system during low AC voltage and AC system disturbance , reduce AC filter

capacitor requirement.

POWER HANDLING CAPACITY As already seen power that can be transmitted P = Vs Vr sinδ ( neglecting the line resistance)

X Or

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P = Vs Vr sinδ Lx

Where as P is the 3Ph power in MW Vs, Vr = voltage at sending end , voltage at receiving end in KV line to line

δ = phase difference between Vs and Vr x positive sequence reactance per phase in ohm/Km

In order to calculate how much power a single circuit transmission line can handle at a given voltage , one has to know the value of positive –sequence line inductance and its

reactance at power frequency. In modern practice , the losses caused by I²R is also gaining importance. Therefore, the use of higher voltages is essential to lower the

required current value. Also the conductor resistance ‘R’ is decreased by using bundled conductors comprising of several sub conductors in parallel. The average values of line parameters for transmission line with horizontal configuration are given below .

1 System Voltage 400KV 765KV 1200KV

2 Average Height in Mtrs 15 18 21

3 Phase Spacing in Mtrs 12 15 21

4 Conductor 2X32mm 4X30mm 8X46mm

5 r , Ohm/Km (20˚) 0.031 0.0136 0.0027

6 x , ohm/Km (50Hz) 0.327 0.272 0.231

7 x/r 10.55 20 85.6

5 CONVERTOR ANALYSIS GRAETZ CIRCUIT, FIRING ANGLE CONTROL OVERLAPPING

The basic HVDC converter is the three phase full wave bridge circuit, This circuit is also known as GRAETZ bridge. This is six pulse convertor and the 12 pulse converter

is composed of two such bridges in series supplied from two different three phase transformers with voltage differing in phase by 30˚. The pulse number of a converters

defined as the number of pulsation cycle of (ripple) of direct voltage per cycle of alternating voltages. The configuration of a given pulse number is selected in such a way that both the valve and converter transformer utilization are maximized. The

Graetz bridge has been universally used for HVDC converters as it provides better utilization of the converter transformer and a lower voltage across the valve when not

conducting.

For the purpose of analysis, the following assumptions are made:-

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a) The AC system including the converter transformer, may be represented by an

ideal source of constant voltage and frequency in series with a lossless inductance representing mainly the transformer leakage inductance.

b) The direct current (id) is constant and ripple free ( because of the large smoothening reactor).

c) The valves have zero resistance when conducting and infinite resistance when not conducting.

The waveform of line voltage and phase voltage of the source is shown in figure.

Let the instantaneous phase voltage be ea = Em sin(ωt+150˚)

eb = Em sin(ωt+30˚) ec = Em sin(ωt -90˚)

Where Em is the peak value of line to neutral voltage. Then the line to line voltage

eac = ea - eb = √3 Em sin(ωt+120˚)

eba = √3 Em sinωt

ecb = √3 Em sin(ωt-120˚)

To ease the understanding of the operation of bridge converter, It is assumed that source

inductance is Lc=0 and no ignition delay.

Three phase, full wave bridge circuit Graetz circuit.

Equivalent Circuit for three phase full wave bridge converter

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WAVE FORMS OF VOLTAGES AND CURRENTS OF GRAETZ BRIDGE

CIRCUIT FIRING ANGLE CONTROL OVERLAPPING

We have earlier assumed Lc=0 and because of this, the current transfer from ad because

of this, the current transfer from one valve to the next valve , when the latter is fired, was instantaneous. Due to leakage inductance of the convertor transformer and the

impedance in the supply network, the current in a valve cannot change suddenly and thus commutation from one valve to the next cannot be instantaneous. Therefore the

transfer of current from one phase to another requires a finite time, called the commutation time or overlap time. The corresponding angle is called overlap angle or

commutation angle is denoted by µ. In normal operation, the overlap angle is less than 60˚, typical full load values are in the

range of 15˚ to 25˚. When the overlap angle is greater than 0˚ but less than 60˚, three valves conduct simultaneously during commutation. However between commutation

only two valves conduct. A new commutation begins every 60˚ and lasts for a period of µ. Therefore the angular period when two valves conduct with no ignition delay (i.e

α=0) is 60˚ - µ, as shown in diagram. During each commutation period, the current in the incoming valves increases from 0 to Id and the current in the outgoing valves

reduces from Id to 0. For simplicity, only the valves conduction periods is identified not the valve currents.

If 60˚ ≤ µ <120˚, an abnormal mode of operation occurs in which alternately three and four valves conduct.

Thus there are three modes of the converter 1. Mode 1 two and three valve conduction (µ<60˚) 2. Mode 2 Three valve conduction (µ=60˚)

3. Mode 3 Three and four valve conduction (µ>60˚)

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Let us analyze the effect of overlap (µ<60˚) by considering the commutation from valve 1 to valve 3 shows the periods of valve conduction. When ignition

delay is included. The commutation begins when ωt=α ( delay angle) and ends when ωt=δ Where δ is extinction angle and is the sum of the delay angle α and

commutation angle µ.

At the beginning of commutation i1 = Id i3 =0 At the end of commutation i1=0 and i3=Id

Effect of ovelap NGLE ON PERIOD OF CONDUCTION OF VALVES

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Voltage wave forms with delay and overlap angle

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