training report at pgcil hvdc ss

8/12/2019 Training Report at PGCIL HVDC Ss

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SUMMER TRAINING

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ACKNOWLEDGEMENT

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EXECUTIVE SUMMARY

Electric power transmission was originally developed with direct current. The availability

of transformers and the development and improvement of induction motors at the

beginning of the 20thCentury, led to greater appeal and use of a.c. transmission. But, later

realized .c. transmission is more practical when long distances were to be covered or where

cables were re!uired.

The purpose of this study is to e"amine the #$C systems % understand the transmission

system for transmitting huge chun& of power to various remote locations where generation is

not feasible. This study also provides various theories regarding #$C systems and it also

covers the pivotal role played by PGCIL in handling #$C systems in 'ndia.

(ther )spects such as Transmission *lanning Criteria is also included which is basic

re!uirement of any #$C *ro+ect carried out by *C'- in 'ndia. 't covers various factors such

as *ower e!uirement, Type of Conductor, -ine -imit, Tariff, % etc. which are necessary to

understand beforehand installing any #$C system. #$C system/s protection e.g. lightning

stri&e1 % human safety measure are briefly discussed .

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Table of ContentsABOUT PGCIL.........................................................................................................5

'ntroduction.................................................................................................................(b+ectives....................................................................................................................

Establishment of Transmission 3ystem.......................................................................41 ABOUT HVDC..............................................................................9

'ntroduction: ..95.5 )$)6T)E3 ($E )C ................................................................................50

5.2. -'7'T)T'(63888........................................................................................55

2A HVDC BIPOLE CONVERTER............................................................................122.5. C(7*(6E6T3.........................................................................................................52

9THE THEORY OF CONVERTERS HVDC............................................... 88..........5

9.5 C(6$ET( C(6T(-..................................................................................................5

9.2 C(77:T)T'(6 *(CE33.............................................................................................549.9 #$C C(6;':)T'(63..............................................................................................549.< C(6$ET(3 )**-'C)T'(63......................................................................................20

! TRANSMISSION PLANNING CRITERIA..............................................................................25


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POWER GRID CORPORATION OF INDIA LIMITED "HISTORY#:$

I%&'()*+&,(%:

P(-' G',) C('/('0&,(% ( I%),0 L,,&)POWERGRID3Founded 23 October 19921 is a

6avratna state?owned electric utility company head!uartered in urgaon, 'ndia. *owerrid was

incorporated on (ctober 29, 5A@A under the companies )ct, 5A> as the 6ational *ower

Transmission Corporation -imited, with the responsibility of planning, e"ecuting, owning

operating and maintaining the high voltage transmission systems in the country. The total

revenue of company is 4,09.@ crore 200A1 with net income 2,0


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ischarge all functions of planning and coordination relating to 'nter?3tate Transmission

3ystem with?

o 3tate Transmission :tilities

o Central overnment

o 3tate overnment

o enerating Companies

o egional Electricity Boards

o )uthority

o -icensees

o Transmission -icensees

o )ny other person notified by the Central overnment on this behalf.

E"ercise supervision and control over the 'nter?3tate Transmission 3ystem.

Efficient (peration and 7aintenance of Transmission 3ystems.

EstablishFaugment and operate all egional -oad ispatch Centers and Communication

facilities.

To facilitate private sector participation on Transmission system through 'ndependent

*rivate Transmission Company, Goint $entures.

To assist various 3EBs and other utilities in up gradation of s&ills % sharing of e"pertise

by organizing regular conferences, tailor?made training wor&shops directed towards

specific technological and (%7 areas and e"tending laboratory facilities for testing

purposes etc.

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estoring power in !uic&est possible time in the event of any natural disasters li&e super?

cyclone, flood etc. through deployment of Emergency estoration 3ystems.

D4(/%&04 S&0;:

The phased development of *(HE' at the time of its formation was foreseen as followsI

*hase?'I Transfer of Transmission facilities along with related manpower from Central F

CentreJ3tate Goint $enture (rganizations.

*hase?''I Transfer of e"isting egional Electricity Boards and egional -oad ispatch

Centers together with associated communication facilities.

*hase?'''I Establish *ower *ool to facilitate e"change of power between 3tatesFegionsleading to formation of 6ational *ower rid.

E&074,,000 &m now.

There are regional gridsI

6orthern egionI

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elhi, #aryana, #imachal *radesh, Gammu )nd Kashmir, *un+ab, a+asthan

:ttaranchal and :ttar *radesh

Eastern egionI

Bihar, Ghar&hand, (rissa 3i&&im and Hest Bengal.

Hestern egionI

adra and 6agar #aveli, aman and iu, Chhattisgarh, oa u+arat, 7adhya *radesh

and 7aharashtra.

3outhern egionI

)ndhra *radesh, Karnata&a, Kerala, *ondicherry and Tamil 6adu.

6orth?Eastern egionI

)runachal *radesh, )ssam, 7anipur, 7eghalaya, 7izoram, 6agaland and Tripura

*(HE'/s networ&, as at 3eptember, 200@, comprises of over >A,000 circuit &m of high

voltage transmission lines and 55> sub?stations spread across the country. The inter?regional

power transfer capacity of 6ational rid has been enhanced to about 54,000 7H from 5


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H,;< V(4&0; D,'+& C*''%& "HVDC#

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) 2==? 2==@

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S*7&0&,(% "%*7'# @ A9 50< 552

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proportional to the s!uare of the current. #owever, power is also proportional to voltage, so for

a given power level, higher voltage can be traded off for lower current. Thus, the higher the

voltage, the lower the power loss. *ower loss can also be reduced by reducing resistance,

commonly achieved by increasing the diameter of the conductor but larger conductors are

heavier and more e"pensive.

#igh voltages cannot be easily used in lighting and motors, and so transmission?level voltage

must be reduced to values compatible with end?use e!uipment. The transformer, which only

wor&s with alternating current, is an efficient way to change voltages. *ractical manipulation of

C voltages only became possible with the development of high power electronic devices such

as mercury arc valves and later semiconductor devices, such as thyristors, insulated?gate bipolar

transistors 'BTs1, high power capable 7(3;ETs power metalJo"ideJsemiconductor field?

effect transistors1 and gate turn?off thyristors T(s1

A)0%&0; 0%) L,,&0&,(% ( 04&'%0&,%; +*''%& &'0%,,(%

The advantage of #$C is the ability to transmit large amounts of power over long distances

with lower capital costs and with lower losses than )C. epending on voltage level and

construction details, losses are !uoted as about 9 per 5,000 &m. #igh?voltage direct currenttransmission allows efficient use of energy sources remote from load centers.

'n a number of applications #$C is more effective than )C transmission. E"amples includeI

:ndersea cables, where high capacitance causes additional )C losses.

Endpoint?to?endpoint long?haul bul& power transmission without intermediate LtapsL, for

e"ample, in remote areas

'ncreasing the capacity of an e"isting power grid in situations where additional wires are

difficult or e"pensive to install *ower transmission and stabilization between unsynchronised )C distribution systems

Connecting a remote generating plant to the distribution grid

3tabilizing a predominantly )C power?grid, without increasing prospective short circuit

current

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educing line cost. #$C needs fewer conductors as there is no need to support multiple

phases. )lso, thinner conductors can be used since #$C does not suffer from the s&in

effect

;acilitate power transmission between different countries that use )C at differing

voltages andFor fre!uencies 3ynchronize )C produced by renewable energy sources

-ong undersea cables have a high capacitance. Hhile this has minimal effect for C

transmission, the current re!uired to charge and discharge the capacitance of the cable causes

additional '2 power losses when the cable is carrying )C. 'n addition, )C power is lost

to dielectric losses.

#$C can carry more power per conductor, because for a given power rating the constant

voltage in a C line is lower than the pea& voltage in an )C line. 'n )C power, the root means!uare 731 voltage measurement is considered the standard, but 73 is only about 45 of

the pea& voltage. The pea& voltage of )C determines the actual insulation thic&ness and

conductor spacing. Because C operates at a constant ma"imum voltage without 73, this

allows e"isting transmission line corridors with e!ually sized conductors and insulation to carry

2A more power into an area of high power consumption than )C, which can lower costs.

Because #$C allows power transmission between unsynchronised )C distribution systems, it

can help increase system stability, by preventing cascading failure from propagating from one

part of a wider power transmission grid to another. Changes in load that would cause portions of

an )C networ& to become unsynchronized and separate would not similarly affect a C lin&,

and the power flow through the C lin& would tend to stabilize the )C networ&. The magnitude

and direction of power flow through a C lin& can be directly commanded, and changed as

needed to support the )C networ&s at either end of the C lin&. This has caused many power

system operators to contemplate wider use of #$C technology for its stability benefits alone

D,0)0%&0;

The disadvantages of #$C are in conversion, switching and control. ;urther operating an#$C scheme re!uires &eeping many spare parts, which may be used e"clusively in one

system as #$C systems are less standardized than )C systems and the used technology

changes fast.

The re!uired static inverter are e"pensive and have limited overload capacity. )t smaller

transmission distances the losses in the static inverters may be bigger than in an )C

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transmission line. The cost of the inverters may not be offset by reductions in line construction

cost and lower line loss.

'n contrast to )C systems, realizing multiterminal systems is comple", as is e"panding e"isting

schemes to multiterminal systems. Controlling power flow in a multiterminal C system

re!uires good communication between all the terminals power flow must be actively regulated

by the control system instead of by the inherent properties of the transmission line. #igh

voltage C circuit brea&ers are difficult to build because some mechanism must be included in

the circuit brea&er to force current to zero, otherwise arcing and contact wear would be too great

to allow reliable switching. 7ulti?terminal lines are rare

C(& (


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in order to provide the necessary bloc&ing voltage capability.Thyristors used for #$C valves

are amongst the largest semiconductors of any type produced for

any industry. ;igure shows an @. &$ thyristor with an active silicon diameter of 55 mm

whichstarts life as a silicon ingot of 52 mm diameter, hence such thyristors are often referred

to as52 mmD thyristors1.

3uch components are e"pensive and there may be many thousand such components in a #$Cstation. 7oreover, they are !uite delicate and re!uire a great many additional components to

control.

2. Convertor Transforer: The converter transformer is the interface betweenthe )C system and the thyristor valves. Typically the #$C converter transformer issub+ected to a C voltage insulation stress as well as the )C voltage stress normally

e"periencedby a power transformer. it is important that the converter transformer be

thermally designed to ta&e into consideration both the fundamental fre!uency load and

the )C harmonic currents that will flow from the converter through the converter

transformer to the )C harmonic filters.

!.AC ,4&': The )C harmonic filters are typically composed of a high voltageconnected capacitor ban& in series with a medium voltage circuit comprizing air?cored

air?insulated reactors, resistors and capacitor ban&s. These filters are used to limit theimpact of )C harmonics and reactive power generated by Convertors .

".S((&


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a. Po%er Contro&: If the power demand is chaned then the power order wi!! ramp to the new

power transfer !e"e! at a rate of chane #$nown as the %ramp rate&' pre(se!ected b) the operator* +)pica!!)

the ma,imum power !imit is defined b) an o"er!oad contro!!er which is continuous!) ca!cu!atin the

therma! capabi!it) of the con"erter station e-uipment*

b. 're()en*y Contro&: . / scheme can contro! the . fre-uenc) of an . s)stem b)

automatica!!) adustin the power bein de!i"ered into that . s)stem in order to ba!ance the !oad with the

supp!)* +he fast power contro! b) the / reduces the under(fre-uenc) or o"er(fre-uenc) which can

resu!t from a chanin !oad in a sma!! power s)stem such as an is!and !oad*

c. Prote*tion: a #$C converter station the types of protection utilized fall into

two categories

M Conventional )C1 substation protection

M C protection

)C connected e!uipment such as converter transformers and )C harmonic filter

components, along with feeders and busbars, are protected using conventional )C protection

relays. The converter, along with the C circuit, is protected using hardware and software.

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+.DC fi&ters: Converter operation results in voltage harmonics being generated at theC

terminals of the converter, that is, there are sinusoidal )C harmonic components superimposed

on the C terminal voltage. This )C harmonic component of voltage will result in )C

harmonic current flow in the C circuit and the field generated by this )C harmonic current

flow can lin& with ad+acent conductors,such as open?wire telecommunication systems, and

induce harmonic current flow in these other circuits. 'n a bac&?to?bac& scheme, these harmonics

are contained within the valve hall with ade!uate shielding and, with a cable scheme,

the cable screen typically provides ade!uate shielding. #owever, with open?wire C

transmission it may be necessary to provide C filters to limit the amount of harmonic current

flowing in the C line. The C filter is physically similar to an )C filter in that it is connected

to the high voltage potential via a capacitor ban& other capacitors along with reactors and

resistors are then connected to the high voltage capacitor ban& in order to provide the desired

tuning and

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T


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as the driving voltage. The lea&age inductances of phases 5 and 9 will be the reactances which

determine the current. This is simply a phase to phase short circuit of the transformer. The short

circuit current flows through valve 9 in the forward direction and through valve 5 counter to the

forward direct current. )s soon as the short circuit current has achieved the amplitude of the

direct current the composite current is zero1, valve 5 e"tinguishes. )t this point, thecommutation process has ended and valve 9 is carrying the entire direct current. Curve of the

direct voltage during commutation is along the average value of the voltages of valves 5 and 9.

C*''%& +(%&'(4

Current control mainly determines

3teady state transmission power

Changes in transmission power according to size and rate of change

The dynamic behavior of the system including temporary overload

-imitation of transient overcurrents determined by amplitude and duration

The loading of all essential components of an #$C system, with the e"ception of filter

circuits, is determined by the direct current or an alternating current proportional to the direct

current. Therefore current control is also a very essential protective function.

'n #$C two point systems, the rectifier generally assumes the tas& of the current control. 't

is occasionally advantageous to assign the current control function to the inverter. #owever,

since the current control of the rectifier is needed as a proactive function, it is advantageous

to also use it for this purpose during normal operation. Then it is always active and monitors

itself.

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HVDC C(%,;*'0&,(%

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'n a common configuration, called monopole, one of the terminals of the rectifier is connected

to earth ground. The other terminal, at a potential high above, or below, ground, is connected to

a transmission line. The earthed terminal may or may not be connected to the corresponding

connection at the inverting station by means of a second conductor.

'f no metallic conductor is installed, current flows in the earth between the earth electrodes at

the two stations. Therefore it is a type of single wire earth return. The issues surrounding earth?

return current include

Electrochemical corrosionof long buried metal ob+ects such as pipelines :nderwater earth?return electrodes in seawater may produce chlorine or otherwise affect

water chemistry.

)n unbalanced current path may result in a net magnetic field, which can affect magnetic

navigational compasses for ships passing over an underwater cable.

These effects can be eliminated with installation of a metallic return conductor between the two

ends of the monopolar transmission line. 3ince one terminal of the converters is connected to

earth, the return conductor need not be insulated for the full transmission voltage which ma&es

it less costly than the high?voltage conductor.

BIPOLAR

'n bipolar transmission a pair of conductors is used, each at a high potential with respect to

ground, in opposite polarity. 3ince these conductors must be insulated for the full voltage,

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transmission line cost is higher than a monopole with a return conductor. #owever, there are a

number of advantages to bipolar transmission which can ma&e it the attractive option.

:nder normal load, negligible earth?current flows, as in the case of monopolar

transmission with a metallic earth?return. This reduces earth return loss and environmentaleffects.

Hhen a fault develops in a line, with earth return electrodes installed at each end of the

line, appro"imately half the rated power can continue to flow using the earth as a return path

operating in monopolar mode.

3ince for a given total power rating each conductor of a bipolar line carries only half the

current of monopolar lines, the cost of the second conductor is reduced compared to a

monopolar line of the same rating.

'n very adverse terrain, the second conductor may be carried on an independent set of

transmission towers, so that some power may continue to be transmitted even if one line is

damaged.

) bipolar system may also be installed with a metallic earth return conductor.

Bipolar systems may carry as much as 9,200 7H at voltages of NF?>00 &$. 3ubmarine cable

installations initially commissioned as a monopole may be upgraded with additional cables and

operated as a Bipole.

C('(%0 D,+


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considerable power loss, create audible and radio?fre!uency interference, generate to"ic

compounds such as o"ides of nitrogen and ozone, and bring forth arcing.

Both )C and C transmission lines can generate coronas, in the former case in the form of

oscillating particles, in the latter a constant wind. ue to the space charge formed around the

conductors, an #$C system may have about half the loss per unit length of a high voltage )C

system carrying the same amount of power. Hith monopolar transmission the choice of polarity

of the energised conductor leads to a degree of control over the corona discharge. 'n particular,

the polarity of the ions emitted can be controlled, which may have an environmental impact

on particulate condensation. *articles of different polarities have a different mean?free

path.1 6egative coronas generate considerably more ozone than positive coronas, and generate it

further downwind of the power line, creating the potential for health effects. The use of

a positive voltage will reduce the ozone impacts of monopole #$C power lines.

A//4,+0&,(%

O',-

The controllability of current?flow through #$C rectifiers and inverters, their application in

connecting unsynchronized networ&s, and their applications in efficient submarine cables mean

that #$C cables are often used at national boundaries for the e"change of power. (ffshore

windfarms also re!uire undersea cables, and their turbines are unsynchronized.

AC %&-(' ,%&'+(%%+&,(%

)C transmission lines can only interconnect synchronized )C networ&s that oscillate at the

same fre!uency and in phase. 7any areas that wish to share power have unsynchronized

networ&s. #owever, #$C systems ma&e it possible to interconnect unsynchronized )C

networ&s, and also add the possibility of controlling )C voltage and reactive power flow.

) generator connected to a long )C transmission line may become unstable and fall out of

synchronization with a distant )C power system. )n #$C transmission lin& may ma&e it

economically feasible to use remote generation sites. Hind farms located off?shore may use

#$C systems to collect power from multiple unsynchronized generators for transmission to

the shore by an underwater cable.

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'n general, however, an #$C power line will interconnect two )C regions of the power?

distribution grid. 7achinery to convert between )C and C power adds a considerable cost in

power transmission. The conversion from )C to C is &nown as rectification and from C to

)C as inversion. )bove a certain brea&?even distance about 0 &m for submarine cables, and

perhaps >00J@00 &m for overhead cables1, the lower cost of the #$C electrical conductorsoutweighs the cost of the electronics.

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The ob+ective of system planning is to evolve a power system with a level of performance

characterised by an acceptable degree of ade!uacy and security based on a trade?off between

costs and ris&s involved. 'nsofar as power transmission systems are concerned , there are nowidely adopted uniform guidelines which determine the criteria for transmission planning vis?s?

vis acceptable degree of ade!uacy and security. The criteria generally depends on the factors

such as availability of generation vis?O?vis demand, voltage levels, and configuration of the

system, control and communication facilities and resource constraints. *ractices in this regard

vary from country to country. The common theme in the various approaches is the acceptable

system performanceD. Even though the factors affecting system performance are probabilistic in

nature, deterministic approach has been used most commonly, being rather easy to apply. ;or

adopting probabilistic approach, long operating e"perience and availability of reliable statistical

data regarding performance of system components, namely e!uipment failure rate, outage

duration, etc, are essential. 3uch data are presently being compiled by a few utilities but these

are still inade!uate to go in for a totally probabilistic approach. #ence it is considered prudent

to adopt a deterministic approach for the present with a committed thrust towards progressive

adoption of probabilistic approach.

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P40%%,%; P


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The choice shall be based on cost, reliability, right?of?way re!uirements, energy losses,

down time in case of upgradation and reconductoring options1

'n case of generating station close to a ma+or load centre, sensitivity of its complete

closure with loads to be metto the e"tent possible1from other generating stations is also

studied.

'n case of transmission system associated with 6uclear *ower 3tations there shall be two

independent sources of power supply for the purpose of providing start?up power

facilities. ;urther the angle between start?up power source and the 6** switchyard

should be, as far as possible, maintained within 50 degrees.

The evacuation system for sensitive power stations viz., 6uclear power stations shall

generally be planned so as to terminate it at large load centres to facilitate islanding of the

power station in case of contingency.

Contingency is the temporary removal of one or more system elements from service. The

cause or reason for such removal may be a fault , planned maintenanceFrepair etc.

5. 3ingle Contingency J The contingency arising out of removal of one system element

from service.

2. ouble Contingency J The contingency arising out of removal of two system elements

from service. 't includes a FC line, two 3FC lines in same corridor or different corridors,

a 3FC line and a transformer etc.

9. are Contingency J Temporary removal of complete generating station or complete sub?

station including all the incoming % outgoing feeders and transformers 1 from service,

#$C bipole and stuc& brea&er condition.

Hhere only two circuits are planned for evacuation of power from a generating station,

these should be two single lines instead of a double circuit line.

eactive power flow through 'CTs shall be minimal. 6ormally it shall not e"ceed 50 of

the rating of the 'CTs. Hhenever voltage on #$ side of 'CT is less than 0.A4 pu, no

reactive power shall flow through 'CT.

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ThermalF6uclear enerating units shall normally not run at leading power factor.

#owever, for the purpose of charging, generating unit may be allowed to operate at

leading power factor as per the respective capability curve.

'nter?regional lin&s shall, in the present conte"t, be planned as asynchronous ties unless

otherwise permitted from operational consideration.

L(0) G%'0&,(% S+%0',(

The load?generation scenarios shall be wor&ed out so as to reflect in a pragmatic manner the

daily and seasonal variations in the load demand and generation availability.

L(0) )0%)

The profile of annual and daily demands will be determined from past data. These data willusually give the demand at grid supply points and for the whole system identifying the

annual and daily pea& demand.

A+&, /(-'

The system pea& demands shall be based on the latest reports of Electric *ower 3urvey

E*31 Committee. 'n case these pea& load figures are more than the pea&ing availability, theloads will be suitably ad+usted substation wise to match with the availability.

The load demands at other periods seasonal variations and minimum loads1shall be derived

based on the annual pea& demand and past pattern of load variations.

;rom practical considerations the load variations over the year shall be considered as underI?

5. )nnual *ea& -oad J 't is the simultaneous ma"imum demand of the system being

studied. 't is based on latest Electric *ower 3urvey E*31 or total pea&ing power

availability, whichever is less.

2. 3easonal variation in *ea& loadscorresponding to high thermal and high hydro

generation1

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9. 7inimal load J 't is the e"pected minimum system demand and is determined from

average ration of annual pea& load and minimum load observed in the system for the last

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units. 'n case of nuclear units the minimum level of output shall be ta&en as not less than 40

of the rated capacity.

eneration dispatches corresponding to the following operating conditions shall be considered

depending on the nature and characteristics of the system.

)nnual *ea& -oad

7a"imum Thermal generation J 't is the condition when hydro generation is lownot

necessarily minimum1and thermal generation is &ept ma"imum to meet seasonal pea&

loadsnot necessarily annual pea& load1.'n other words it is the condition when the gap

between monthly pea& demand and hydro power availability is ma"imum.

7a"imum #ydro generation J 't is the condition when hydro power availability is

ma"imum during the year. 't is also &nown as #igh #ydro condition.

)nnual 7inimum -oad

3pecial area dispatches J 't is the condition when power output from all the generating

stations located in an area in close pro"imity1 is &ept at the ma"imum feasible level.

7a"imum ;easible level of a generating station is the ma"imum power output when all

the units in a power station are in service, assuming no planned or forced outages.

#owever, in case of power stationFcomple" where si" or more units e"ist, for every si"

units one unit Jsecond largest?is assumed to be under annual planned maintenance.

3pecial dispatches corresponding to high agricultural load with low power factor,

wherever applicable.

(ff pea& conditions with ma"imum pumping load where *umped 3torage stations e"ist

and also with the inter?regional e"changes, if envisaged.

Complete closure of a generating station close to a ma+or load centre.

The generation dispatch for purpose of carrying out sensitivity studies corresponding to

complete closure of generating station close to a ma+or load centre shall be wor&ed out by

increasing generation at other stations to the e"tent possible &eeping in view the

ma"imum li&ely availability at these stations, ownership pattern, shares etc.

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P',,74 L,% L(0),%; L,,&

*ermissible line loading limit depend on many factors such as voltage regulation, stabilityand thermal capacity etc. Thermal capacity of a line refers to the amount of current that

can be carried by a line conductor without e"ceeding its design operating temperature.

3urge 'mpedance -oading 3'-1 means a unit power factor load over a resistance line

such that series reactive loss 'P2Q1 along the line is e!ual to shunt capacitive gain

$P2Q=1. :nder these conditions the sending end and receiving end voltages and current

are e!ual in magnitude but different in phase position. Hhile 3'- gives a general idea of

the loading capability of the line , it is usual to load the short lines above 3'- and long

lines lower than 3'- because of the stability limitations1.line loading can also be shown

in terms of surge impedance loading of uncompensated line1as a function of line length

assuming a voltage regulation of and phase angular difference of 90 degrees between

the two ends of the line. 'n case of shunt compensated lines, the 3'- will get reduced by a

factor &, where

& R s!rt 5?degree of compensation1

;or lines whose permissible line loading as determined from the curve is higher than the

thermal loading limit, permissible loading limit shall be restricted to thermal loading

limit.

Thermal loading limits are generally decided by design practice on the basis of ambient

temperature, ma"imum permissible conductor temperature, wind velocity, etc. 'n 'ndia,

the ambient temperatures obtaining in the various parts of the country are different and

vary considerably during the various seasons of the year. esigns of transmission line

with )3C conductors in E#$ systems will normally be based on a conductortemperature limit of 4 deg Celsius. #owever, for some of the e"isting lines which have

been designed for a conductor temperature of > deg Celsius the loading shall be

correspondingly reduced. 'n the case of )))C conductors, ma"imum conductor

temperature limit will be ta&en as @ deg Celsius.

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T/('0'6 O'(4&0;

These are power fre!uency overvoltages produced in a power system due to sudden load

re+ection, single?phase?to?ground faults etc.


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be such that under steady state condition, switching on and off of the reactors shall not

cause a voltage change e"ceeding . The standard sizes7$)1 of reactors areI?

9 % @0 at &v 5?ph units1 0, >9 % 550 at @00 &$

;i"ed line reactors may be provided to control Temporary *ower ;re!uency overvoltage

Safter all voltage regulation has ta&en place within the limits defined, under all probable

operating conditions.

-ine reactors switchableFcontrolledFfi"ed1 may be provided if it is not possible to chargeE#$ line without e"ceeding the voltage limits defined. The possibility of reducing pre?

charging voltage of the charging end shall also be considered in the conte"t of

establishing the need for reactors.

S&0&,+ VAR C(/%0&,(% "SVC#

3tatic $) compensation shall be provided where found necessary to damp the powerswings and provide the system stability under conditions defined. The dynamic range of

static compensators shall not be utilised under steady state operating conditions as far as

possible.

S*7$S&0&,(% P40%%,%; C',&',0 The re!uirements in respect of E#$ sub?stations in a system such as the total load to be

catered by the sub?station of a particular voltage level, its 7$) capacity, number of

feeders permissible etc. are important to the planners so as to provide an idea to them

about the time for going in for the adoption of ne"t higher voltage level sub?station and

also the number of substations re!uired for meeting a particular !uantum of load.

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Keeping these in view the following criteria have been laid down for planning an E#$

substationI

The ma"imum fault level on any new substation bus should not e"ceed @0 of the rated

rupturing capacity of the circuit brea&er. The 20 margin is intended to ta&e care of the

increase in short?circuit levels as the system grows. The rated brea&ing current capability

of switchgear at different voltage levels may be ta&en asI?

592 &$ ?? 2F95 &)

220 &$ ?? 95.F


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) stuc& brea&er condition shall not cause disruption of more than four feeders for 220

&$ system and two feeders for &$ systems.

T'0%,,(% E%;,%',%;

M08(' C(/(%%& ( T'0%,,(% L,%

Conductor

Tower esign and foundation

Earth wire

'nsulators

31


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#ardware ;ittings

)ccessories

CONDUCTORS

BUNDLE CONDUCTOR SELECTION AND OPTIMIATION

3ize, Type and Configuration of conductor influencesI?

Tower and its geometry

;oundations

(ptimum spans

ating and configuration of 'nsulator string

'nsulator 3wings

round clearance

-ine interferences li&e electric field at ground, corona, radio % T$ interference, audible

noise etc.

CONDUCTOR SELECTION SCENARIO

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SCENARIO A

3election of character for a transmission line of identified voltage level and specified

minimum power flow but power flow capacity becomes ruling factor in selection ofconductor size low voltage lines1.

SCENARIO B

3election of conductor for a transmission line with identified voltage level and a specified

minimum power flow but voltage level become ruling factor in selection of

conductorFconductor bundle size E#$F:#$ lines1.

SCENARIO C

3election of conductor for high power capacity long distances transmission lines where

selection of voltage level and conductorFconductor bundle size are to be done together to

obtain most optimum solution #$C Bipole1

CONDUCTOR BUNDLE SELECTION METHODOLOGY

*rimary set of conductor bundleFsizes identified to start optimization

*arameters li&e insulation re!uirement, limits for corona, '$, )6, thermal ratings, line

losses and statutory clearances identified

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etailed analysis of various alternatives in respect of following to be carried out to select

the configuration

( Basic insulation design and insulator selection

( Tower configuration analysis

( Tower weight and foundation analysis

( Capital line cost analysis and span optimization

( -ine loss calculation

( Economic evaluations*H1 of alternatives

( Comparison of interference performance( Cost sensitivity analysis

CONDUCTOR OPTMIATION PROCEDURE

*'7)= 3E-ECT'(6

Thermal rating of the conductorFconductors

7anufacturing facilities

E"pense of other utilities

3ystem voltage alternatives

Construction convenience

-ine loss considerations

Terrain conditions and ground profile

3pan length re!uirements

34


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ight (f Hay limitations

CONDUCTOR SELECTION DESIGN CONSIDERATIONS

B)3'C C(63'E)T'(63 6(6 $)')B-E1

51 -oading condition and reliability level for the transmission line.

21 'nsulator co?ordination

91 -imit load condition for structure, conductor, insulator, and hardware as well as limit

conditions for swing of conductor and insulator strings.

1 *arameters for economic evaluation

CONDUCTOR SELECTION FOR SPECIAL TRANSMISSION SYSTEM

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:*)T'6 (; -'6E3

( 3ag of the selected conductor at ma"imum operating temperature should not e"ceed

the sag of the original conductor

( 6o e"tra loadings on the structure at various design considerations

:*)'6 (; -'6E3

( -ine interference in respect of '$, T$', )6, E;, 7; etc. 3hould be within

acceptable limits

( Conductor surface gradient within acceptable limits

( )symmetric bundle

C(7*)CT -'6E3

( -owest possible sag and swing for re!uired !uantum of power

( Considerations involved in upgradingFup rating

CONDUCTOR BUNDLE SELECTION: ESTIMATION OF TOWER

WEIGHTS AND FOUNDATION VOLUMES

;or each alternative of conductor and insulator configuration

T(HE HE'#T E3T'7)T'(6

*reliminary tower design studies conducted

36


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Estimation based on regression analysis and empirical formulae

;(:6)T'(6 $(-:7E E3T'7)T'(6

*reliminary foundation design studies conducted

C(6:CT( B:6-E (T'7'U)T'(6I TEC#6(?EC(6(7'C )6)-=3'3

51 Capital cost of line

Cost of each item, construction cost

21 Cost of line losses

)nnual loss cost R )nnual demand cost N )nnual energy -oss Cost

91 esults of economic evaluation*H or )nnual Cost basis1


38/49

)-- )-:7'6':7 )--(= C(6:CT( )))C

ood Conductivity

#igh Tensile 3trength

3uperior Corrosion resistance compared to )C3

'mproved strength to weight ratio resulting in lower sag

-ower electrical losses

)luminium Conductor 3teel 3upported )C33 Conductor1

3imilar to )C3 e"cept )luminium

3teel Core #igh 3trength1 carries most of the load and hence less sag compared to

conventional )C3 conductor under emergency loadings.

Can be operated at 200 degree C without loss of strength

'mproved Conductivity

Better self damping characteristics

Compact Conductors

)luminium wiresFstrands shaped trapezoidal

'ncreased )luminium area and hence higher current carrying capacity

38


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'6$) Conductors

Core made of alloy of 'ron. 6ic&el having low thermal coefficient of e"pansion5F9rdthat

of steel1

)fter certain transition temperature all load transferred to the core and hence lower sagcompared to )C3 after transition temperature

Can be operated up to 2000C

DESIGN OF TOWERS

SALIENT DESIGN CONDITIONS

The reliability of transmission line towers depends on the appropriate selection of design

criteriaFparameters.

Climatic conditions play an important role in determining the reliability of transmission

line tower.

) significant number of transmission line failures can be the result of wind speed

e"ceeding design limits due to deficiencies in selection of design parametersFcriteria.

EARTHWIRE

;unctions of Earth wire

To protect conductor against lighting flashovers

To provide a path for fault current

39


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-ighting ;lashovers

irect ;lashover

(ccurs due to shielding failure with lighting on the conductor, flashover ta&ing place

across the insulator string from conductor to ground.

Bac& ;lashover

(ccurs due to high towering resistance with a high voltage at the grounded tower cross

arm compared to conductor, resulting in a flashover across the insulator string from

ground to conductor.

7a"imum allowable fault current '1 through earthwire mainly depends upon

)rea of Earth wire)1

7a"imum permissible temperature

Time of short circuitt1

' varies proportional to ) and inversely proportional to s!rt t1

HARDWARE FITTINGS AND ACCESSORIES FOR CONDUCTOR

EARTHWIRE

#)H)E ;'TT'63

;or attachment of insulator string to tower

?shac&les, Ball clevis, =o&e *late, Chain lin&

;or attachment of insulator string to the conductor

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3uspension % tension assembly

;ittings li&e ?3hac&les, soc&et clevis, chain lin&

;or protection of insulator string from power follow current

)rcing horn

;or ma&ing electric field uniform and to limit the electric field at live end

Corona control ringFrading ring

;or fine ad+ustment of conductor sag ?3ag )d+ustment plate, Turn Buc&le

#)H)E ;'TT'63? esign

)rcing #orn

The air gap is maintained for satisfactory performance under actual field conditions.

;or power follow current

=o&e *late

To withstand mechanical loads?Thic&ness % shear edge maintained

To maintain sub conductor spacing

Corona Control ingFrading #orn

To cover at least one live and insulator disc

To cover hardware fittings susceptible for CoronaF'$

3uspension )ssembly

? 3haped to prevent hammering between clamp % conductor

? To minimize static % dynamic stress in conductor under various

loading conditions

? 7inimum level of coronaF'$ performance

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? ;or slipping of conductor under prescribed unbalanced conditions

between ad+acent conductor spans

Tension )ssembly

( To withstand loads of atleast A of conductor :T3

( To have conductivity more than that of conductor

3ag )d+ustment *lateFTurn Buc&le

( To ad+ust sag upto 50mm in steps of >mm.

C(& E%;,%',%;

W


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:ndergrounding is more e"pensive, since the cost of burying cables at transmission voltages is

several times greater than overhead power lines, and the life?cycle cost of an underground

power cable is two to four times the cost of an overhead power line.

A7&'0+& +(& &,0&

51 *reliminary 3urvey % 3oil 'nvestigation

21 -and )c!uisition for 3ubstation and % Compensation

91 Cost of compensation for transmission lines

Compensation towards trees, crops % *TCC

Compensation towards forest


44/49

551 Custom uty

521 'nterest uring Construction 'C1

S*7&0&,(% E*,/%&

51 '3 3ubstation E!uipment.

21 Circuit Brea&ers

91 'solators

1 3urge )rrestersI They should be provided near line entrances, transformers so as to achieve

proper insulation coordination. These shall be fitted with pressure relief devices and

diverting ports suitable for shattering of porcelain housing providing path for the flow of

rated currents in the event of arresters failure.

S*7&0&,(% S*//('& 0+,4,&,

51 )C % C *ower supplies

21 ;ire ;ighting 3ystemI ;ire fighting system in general conforms to fire insurance regulations

of 'ndia. The fire fighting system is proposed with both )C motor % diesel engine drivenpumps. )utomatic heat actuated emulsifying system is proposed for transformers and

reactors.

91 (il evacuating, filtering, testing, % filling apparatusI To monitor the !uality of oil for the

satisfactory performance of transformers, shunt reactors and for periodical maintenance

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necessary oil evacuating, filtering, testing and filling apparatus would be provided at new

substations.


46/49

wider geographic area. emote and low?cost sources of energy, such as hydroelectric power or

mine?mouth coal, could be e"ploited to lower energy production cost.

The capital cost of electric power stations is so high, and electric demand is so variable, that it is

often cheaper to import some portion of the needed power than to generate it locally. Because

nearby loads are often correlated, electricity must often come from distant sources. Because of

the economics of load balancing, wide area transmission grids now span across countries and

even large portions of continents. The web of interconnections between power producers and

consumers ensures that power can flow, even if a few lin&s are inoperative.

The unvarying or slowly varying over many hours1 portion of the electric demand is &nown as

the Vbase loadV, and is generally served best by large facilities and therefore efficient due to

economies of scale1 with low variable costs for fuel and operations, i.e. nuclear, coal, hydro.

enewable sources such as solar, wind, oceanFtidal, etc. are not considered Vbase loadV but canstill add power to the grid. 3maller and higher cost sources, such as combined cycle or

combustion turbine plants fueled by natural gas are then added as needed.

-ong distance transmission allows remote renewable energy resources to be used to displace

fossil fuel consumption. #ydro and wind sources canLt be moved closer to populous cities, and

solar costs are lowest in remote areas where local power needs are minimal. Connection costs

alone can determine whether any particular renewable alternative is economically sensible.

Costs can be prohibitive for transmission lines, but various proposals for massive infrastructure

investment in high capacity, very long distance super grid transmission networ&s could berecovered with modest usage fees.

#igh voltage direct current #$C1 is used to transmit large amounts of power over long

distances or for interconnections between asynchronous grids. Hhen electrical energy is

re!uired to be transmitted over very long distances, it is more economical to transmit using

direct current instead of alternating current. ;or a long transmission line, the lower losses and

reduced construction cost of a C line can offset the additional cost of converter stations at each

end. )lso, at high )C voltages, significant although economically acceptable1 amounts of

energy are lost due to corona discharge, the capacitance between phases or, in the case of buried

cables, between phases and the soil or water in which the cable is buried.

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E%,'(%%&04 0%) S(+,04 A/+&

F('& I%(4%& C40'0%+

)s per the practice, preliminary route selection is done by *(HE' based on such

documents as ;orest )tlas and the survey of 'ndia maps using beeD line method, followed

by field verification through wal& over survey. )ll possible steps are ta&en to avoid the route

alignment through the forests. 'n case where it becomes unavoidable due the geography of

the terrain, the alignment is made in such a way that the route through forests is barest

minimum.

;or the selection of optimum route, following points are to be ta&en into considerationI

The route of the proposed transmission line does not involve any human rehabilitation

)ny monument of cultural or historical importance is not generally affected.

The route does not create any threat to the survival of the community.

't does not affect any public utility services li&e playground , school, other

establishments, etc

't does not pass through any sanctuaries, national par&, etc.

't does not infringe with the areas of natural resources.

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S(+,04 I* RR 0*'

)s per the prevailing law, land below transmission line is not re!uired to be ac!uired and

only land for substation is ac!uired. *(HE' is following the practice of land

management to minimize the land re!uirement to the barest minimum. enerally 20 to 90

hectares of land is re!uired for constructing a substation depending upon the type of the

voltage level. Even for this 20 to 90 hectare land, *(HE' try to locate the substation

on government land as far as possible and in the absence of govt. land private land is

ac!uired. 'n order to insure the indigenous Tribal1 people do not suffer adverse affects,

utmost care is ta&en to avoid ac!uisition of land belonging to tribal community. 'n spite of

that, *(HE' has developed an indigenous people Tribal1 evelopment *lan '**1

which ensure that they receive culturally compatible social and economic benefits for any

adverse affects.

R, A%046,

R%* R,

The capital cost of the transmission system comprises of

i1 )n e!uity component

ii1 ) loan component

This is recovered through the annual transmission charges consisting of return re!uired for

the e!uity, an interest for the loan component together with the depreciation charges, the (

% 7 charges and interest on wor&ing capital from the beneficiaries as per 6otification in

proportion to the benefits derived by them. These are recovered in monthly fi"ed charges

from the beneficiaries. 'n addition to annual charges 'ncome Ta", ;E$ and incentives, etc.

as per notification would also be payable.

The Bul& *ower Transmission )greement B*T)1 which cover the payments for

transmission charges for all the e"isting pro+ects as well as those that may be included in

future after approval by CE) already e"ists.

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R;*40&('6 R,

B*T)s have the provision that the transmission tariff for new F e"isting transmission assets

commissioned as well as the additional tariff payable due to additional capitalization fromyear to year, etc. shall be computed by *(HE' based on norms F methodology

followed in the (' notification dated 5>.52.A4 in accordance with norms to be specified by

the Central Electricity egulatory Commission CEC1 as amended from time to time.

T0',

The cost of electric power is normally given by the e"pression a N bQ&H N cQ&Hh1 per

annum, where a is a fi"ed charge for the utility, independent of the power output b depends on

the ma"imum demand of the system and hence on the interest and depreciation on the installed

power station and c depends on the units produced and therefore on the fuel charges and the

wages of the station staff.

Tariff structure may be such as to influence the load curve and to improve the load factor.

Tariff should consider the pf power factor1 of the load of the consumer. if it is low, it ta&es

more current for the same &Hs and hence T and Transmission and istribution1 losses are

correspondingly increased. The power has to install either pf correcting improvement1 devices

such as synchronous condensors, 3$C 3tatic $ar Compensator1 or voltage regulating

e!uipment to maintain the voltages within allowed limits and thus the total cost increases. (ne

of the following alternatives may be used to avoid the low pf I

5. To charge the consumer based on K$) rather than KH.

2. ) pf penalty clause may be imposed on the consumer.

training report at pgcil hvdc ss

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