Reactor Protn

8/10/2019 Reactor Protn

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EASUN REYROLLE LIMITED

POWER SYSTEM PROTECTION COURSE

REACTOR PROTECTION

EASUN REYROLLE LIMITED

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INTRODUCTION

This paper briefs about the applications and protection requirements of shunt reactors. Shunt reactors arerequired in both EHV transmission line and long HV transmission lines and cables.

Shnt reactor application!

Shunt reactance is required to compensate the large capacitive reactance in transmission lines.The capacitor produces VAR generation !hich the s"stem generall" cannot absorb. The VAR generationincreases as the square of the voltage and a function of line length and the conductor configuration. #t isnecessar" to absorb this VAR and provide voltage control at both terminals during normal operation. High over

voltage on sudden loss of load must be limited as !ell. S"stem s!itching and operation ma" require differentamounts of VAR absorption and even at times some VAR generation.

Shunt reactance for VAR control is obtained b"$

%i&ed shunt reactors

S!itched shunt reactors and capacitors

S"nchronous condensers%i&ed shunt reactors are generall" used for EHV and long HV lines and for HV cables. S!itched shunt reactorsand capacitors and S"nchronous condensers are applied in the underl"ing s"stem and near load centers.Shunt reactors are var" greatl" in si'e t"pe construction and application. There capabilities ranges from ( to1)*+VA at voltage levels from ,.v to /*v. The" can be single phase or three phase oil or dr" t"pe !ith

air or gapped iron cores. The connections ma" be

1. 0irectl" to the transmission circuit). To the tertiar" !inding of a transformer ban that is part of the line.(. To the lo! voltage bus associated !ith the line transformer ban.

ine reactors !hich are connected directl" or through a disconnect s!itch are a part of the transmission 2ircuitbreaers are seldom used. The neutrals of the reactors are( solidl" grounded or grounded through a neutral

reactor. Reactor faults require that all line terminals be open.

3hen connected to the tertiar" of a transformer ban circuit breaers are generall" used either in the suppl" orin the neutral. 4pening the neutral breaer does not isolate the reactor fault. Tertiar" applications are operated

either ungrounded or grounded through impedance.ine operation !ithout a reactor can result in a ver" high over voltage !hen load is lost such as !hen one endis opened. This factor encourages the use of direct connected reactors to avoid accidental loss of service shouldload be lost.ine connected reactors are generall" included !ithin the line protection 'one and are often !ell protected b"the line rela"s ad5acent to the units. Separate reactor rela"s are recommended since the remote terminal ma" not

detect the reactor fault. These rela"s can be applied !ith 2Ts si'ed to the reactor +VA and should include some!a" of transfer tripping the remote line terminals- especiall" on long lines or !here the remote terminal is arelativel" !ea source.3ith separate reactor rela"s the line rela"s provide additional bacup.

Tertiar" connected reactors can be included in the transformer ban differential 'one. Separate reactor protectionrela"s are recommended. 3here practical the transformer protection 'one overlap should be used as bac up.

ine side reactor breaers allo! the protection to be separated so that the ban need not be tripped for reactorfaults. #n such cases the possibilit" of high voltage during operation !ithout the reactors should be e&amined.The protective techniques commonl" used for reactor primar" and bac up protection are$

a. Rate of rise pressure 6applicable to oil units !ith sealed gas chamber above the oil level.b. 4ver current 6three phase and 7 or ground8c. 0ifferential 6three phase or ground onl"8

4ther protections such as distance negative sequence and current balance have been used to a limited e&tend.

Rate o$ ri!e pre!!re protection

Rate of rise pressure protection provides the most sensitive protection available for light internal faults. Trippingis recommended although the protection is sometimes used for alarm onl". An alarm operation should be

monitored carefull" since there are cases !here a fault left no tangible evidence after the first pressure rela"

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operation but later developed into a severe fault even on the severe fault. The pressure rela" !as distrustedbecause of the initial assumed false operation.Rate of rise pressure protection can be used as separate primar" protection onl" if the line protection or

transformer differential protections available for faults outside the reactor tan and for bacup protection. Rateof rise pressure protection is of course not applicable to dr" t"pe units.

O)ercrrent Protection

4vercurrent phase and ground protection for reactors is sho!n in %igure 1. To avoid operation on transients the

phase-t"pe time-4vercurrent units 6*18 are set at 1.* times rated shunt reactor current9 the instantaneous units6*:8 are set at five times rated current. The ground rela" unit 6*1;8 can be set at :.* to 1.: A and rela" 6*:;8 atfive times more than the *:; setting. through>

condition generator differential rela"s can be used.

The scheme sho!n in %igure ( provides an e&cellent combination of phase instantaneous and time-4vercurrent!ith ground differential. %or single-phase reactors phase faults !hich do not involve ground cannot occur atleast !ithin the tan. Therefore the three *:7*1 rela"s are bac protection !hich could be omitted.

The generator differential rela"s can also be used for the ground differential 6=/;8 !here the shunt reactor isgrounded and connected to an ungrounded s"stem

. Reactor! on Delta Sy!te"

4n delta s"stems shunt reactors are usuall" connected to the tertiar" of a po!er transformer associated !ith theline. Since most faults !ill involve ground the units or the associated s"stem are grounded through highresistance for detection purposes. ;eutral resistance grounding is sho!n in %igure , and voltage transformergrounding in %igure *. To limit both transient overvoltage and ground fault current the resistor is si'ed so that#4Requals #42. Since the s"stem capacitance to ground is ver" large the impedance of the associated s"stem isessentiall" negligible and is not sho!n in the 'ero sequence diagrams. 3hile the primar" current for a groundfault is quite small the secondar" current !ill be large. #f *?; is used for alarm instead of tripping the

secondar" current ma" e&ceed the continuous thermal rating of the voltage transformers.

The *?; rela" provide sensitive protection. %or tripping therefore the *?; rela" operate for (E:voltages of*.* V or more. %or alarm the rela" operates at 1*.?V or more. %or alarm applications the continuous (E:voltage should not e&ceed the rating unless a series resistor is used to limit the voltage across the rela" to itsrating.

@hase protection for 4vercurrent or differential rela" schemes can obtain three-phase reactors. 4vercurrentprotection is the same as for %igure 1 !ithout *:;7*1;9 differential protection is as sho!n in %igure ) !ithout*:;7*1;. The arrangements offer little protection for single-phase reactors unless a second ground fault shoulddevelop in another unit.

Although including the reactor !ithin the transformers differential circuit provides some phase-fault protectionit offers no ground-fault protection !ith high-impedance grounding. Event the phase-fault protection is limitedsince the current transformers of the transformer differential are si'ed for transformer capacit" and not for thesmaller reactor +VA.

o! impedance or solid grounding of the reactors ma" be used. #n this case either the *:;7*1; neutral

4vercurrent rela" 6%igures , and *8 or the =/; ground differential should be applied.

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Trn/to/Trn :alt!

ight turn-to-turn faults are e&tremel" difficult to detect. 3hile the rate-of-rise of pressure rela" offers the

greatest sensitivit" its application is limited. The reactors must be oil-t"pe and the fault must cause sufficientpressure change to operate the unit. 3hile transformer action in a turn-to-turn there is ver" little current changeat the terminals of the unit. The effect is equivalent to an autotransformer !ith a shorted secondar". The

impedance change that !ill occur in one phase can be represented b" S"mmetrical 2omponents as a shuntunbalance. As sho!n in %igure impedance A of phase a is not equal to the other t!o phases sho!n !ith atotal reactor impedance of

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Reactor Protn