EASUN REYROLLE LIMITED
POWER SYSTEM PROTECTION COURSE
REACTOR PROTECTION
EASUN REYROLLE LIMITED The Copyright and other intellectual property rights in this document, and in any model or article produced from it (and including any Registered design rights) are the property of Easun Reyrolle Ltd. Neither this document nor any part of it shall be reproduced or modified or stored in another form, in any data retrieval system without the permission of Easun Reyrolle Limited, nor shall any model or article be reproduced from this document unless Easun Reyrolle Limited consent. Easun Reyrolle Limited Plot No.98, SIPCOT Industrial Complex HOSUR - 635 126 TAMILNADU Telephone : + 91 - 4344 - 76962, 76966, 76995, 77901, 77902 Fax : + 91 - 4344 - 76397 Email :
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INTRODUCTION This paper briefs about the applications and protection requirements of shunt reactors. Shunt reactors are required in both EHV transmission line and long HV transmission lines and cables. Shunt reactor applications Shunt reactance is required to compensate the large capacitive reactance in transmission lines. The capacitor produces VAR generation, which the system generally cannot absorb. The VAR generation increases as the square of the voltage and a function of line length and the conductor configuration. It is necessary 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 well. System switching and operation may require different amounts of VAR absorption and even, at times, some VAR generation. Shunt reactance for VAR control is obtained by: Fixed shunt reactors Switched shunt reactors and capacitors Synchronous condensers Fixed shunt reactors are generally used for EHV and long HV lines and for HV cables. Switched shunt reactors and capacitors and Synchronous condensers are applied in the underlying system and near load centers. Shunt reactors are vary greatly in size, type, construction and application. There capabilities ranges from 3 to 125MVA, at voltage levels from 4.6kv to 765kv. They can be single phase or three phase, oil or dry type, with air or gapped iron cores. The connections may be 1. 2. 3.
Directly to the transmission circuit To the tertiary winding of a transformer bank that is part of the line. To the low voltage bus associated with the line transformer bank.
Line reactors which are connected directly or through a disconnect switch, are a part of the transmission Circuit breakers are seldom used. The neutrals of the reactors are3 solidly grounded or grounded through a neutral reactor. Reactor faults require that all line terminals be open. When connected to the tertiary of a transformer bank, circuit breakers are generally used, either in the supply or in the neutral. Opening the neutral breaker does not isolate the reactor fault. Tertiary applications are operated either ungrounded or grounded through impedance. Line operation without a reactor can result in a very high over voltage when load is lost, such as when one end is opened. This factor encourages the use of direct connected reactors to avoid accidental loss of service should load be lost. Line connected reactors are generally included within the line protection zone and are often well protected by the line relays adjacent to the units. Separate reactor relays are recommended since the remote terminal may not detect the reactor fault. These relays can be applied with CTs sized to the reactor MVA and should include some way of transfer tripping the remote line terminals- especially on long lines or where the remote terminal is a relatively weak source.With separate reactor relays , the line relays provide additional backup. Tertiary connected reactors can be included in the transformer bank differential zone. Separate reactor protection relays are recommended. Where practical, the transformer protection zone overlap should be used as back up. Line side reactor breakers allow the protection to be separated, so that the bank need not be tripped for reactor faults. In such cases the possibility of high voltage during operation without the reactors should be examined. The protective techniques commonly used for reactor primary and back up protection are: a. Rate of rise pressure (applicable to oil units with sealed gas chamber above the oil level. b. Over current (three phase and / or ground) c. Differential (three phase or ground only) Other protections such as distance, negative sequence and current balance have been used to a limited extend. Rate of rise pressure protection Rate of rise pressure protection provides the most sensitive protection available for light internal faults. Tripping is recommended, although the protection is sometimes used for alarm only. An alarm operation should be monitored carefully since there are cases where a fault left no tangible evidence after the first pressure relay -2-
operation but later developed into a severe fault even on the severe fault. The pressure relay was distrusted because of the initial assumed false operation. Rate of rise pressure protection can be used as separate primary protection only if the line protection or transformer differential protections available for faults outside the reactor tank and for backup protection. Rate of rise pressure protection is of course, not applicable to dry type units. Overcurrent Protection Overcurrent phase and ground protection for reactors is shown in Figure 1. To avoid operation on transients, the phase-type time-Overcurrent units (51) are set at 1.5 times rated shunt reactor current; the instantaneous units (50) are set at five times rated current. The ground relay unit (51N) can be set at 0.5 to 1.0 A and relay (50N) at five times more than the 50N setting. Both ground units should be set above the zero sequence current (3I 0) contribution of the reactor for faults outside the reactor protection zone. This setting will avoid operation on line deenergizing oscillations. If the rector is connected to an ungrounded system, 50N and 51N should be omitted. This scheme requires only one set of current transformers. Differential Protection Separate phase differential relays (87), as shown in Figure 2, are applicable for either three-phase or singlephase reactor units. With single-phase units, the separate differential relays aid in identifying the fault. The relays detect both winding and bushing faults. Since the relays will see magnetizing inrush as a 'through' condition, generator differential relays can be used. The scheme shown in Figure 3 provides an excellent combination of phase instantaneous and time-Overcurrent with ground differential. For single-phase reactors, phase faults, which do not involve ground, cannot occur at least within the tank. Therefore, the three 50/51 relays are back protection, which could be omitted. The generator differential relays can also be used for the ground differential (87N) where the shunt reactor is grounded and connected to an ungrounded system . Reactors on Delta System On delta systems, shunt reactors are usually connected to the tertiary of a power transformer associated with the line. Since most faults will involve ground, the units or the associated system are grounded through high resistance for detection purposes. Neutral resistance grounding is shown in Figure 4, and voltage transformer grounding in Figure 5. To limit both transient overvoltage and ground fault current, the resistor is sized so that IOR equals IOC. Since the system capacitance to ground is very large, the impedance of the associated system is essentially negligible and is not shown in the zero sequence diagrams. While the primary current for a ground fault is quite small, the secondary current will be large. If 59N is used for alarm instead of tripping, the secondary current may exceed the continuous thermal rating of the voltage transformers. The 59N relay provide sensitive protection. For tripping, therefore, the 59N relay operate for 3E 0 voltages of 5.5 V or more. For alarm, the relay operates at 15.9V or more. For alarm applications, the continuous 3E 0 voltage should not exceed the rating unless a series resistor is used to limit the voltage across the relay to its rating. Phase protection for Overcurrent or differential relay schemes can obtain three-phase reactors. Overcurrent protection is the same as for Figure 1, without 50N/51N; differential protection is as shown in Figure 2, without 50N/51N. The arrangements offer little protection for single-phase reactors unless a second ground fault should develop in another unit. Although including the reactor within the transformers differential circuit provides some phase-fault protection, it offers no ground-fault protection with high-impedance grounding. Event the phase-fault protection is limited, since the current transformers of the transformer differential are sized for transformer capacity and not for the smaller reactor MVA. Low impedance or solid grounding of the reactors may be used. In this case, either the 50N/51N neutral Overcurrent relay (Figures 4 and 5), or the 87N ground differential should be applied.
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Turn-to-Turn Faults Light turn-to-turn faults are extremely difficult to detect. While the rate-of-rise of pressure relay offers the greatest sensitivity, its application is limited. The reactors must be oil-type, and the fault must cause sufficient pressure change to operate the unit. While transformer action in a turn-to-turn, there is very little current change at the terminals of the unit. The effect is equivalent to an autotransformer with a shorted secondary. The impedance change that will occur in one phase can be represented by Symmetrical Components as a shunt unbalance. As shown in Figure 6, impedance ZA of phase a is not equal to the other two phases, shown with a total reactor impedance of ZB. For this condition, the sequence net works are connected as shown in Figure 6 Because of the transformer action, the change in impedance of the total phase circuit for a shorted turn is difficult to calculate. As a rough estimate, assume a change of 3 percent in the phase with the shorted turn. Also assume that the fault has not yet involved ground or other phases. Given these assumptions, and neglecting phase angles, shunt capacitance, and transformer action, negative and zero sequence currents will be less than 1 per cent. Removing the ground from the units does not change the positive and negative sequence currents significantly, although it does eliminate the zero sequence. The magnitudes of the currents are largely a function of the total reactor impedance; the source impedance is relatively low compared to the reactor impedance. The small unbalances and sequence currents associated with turn-to-turn faults generally are no larger than the normal or tolerable unbalances. Consequently, there seems to be no reliable 'handle' to distinguish between the intolerable and tolerable conditions. While special schemes or relays have been reported, they will probably require very careful "custom" applications and could well be subject to false operation. As the turn-to-turn fault spreads to more turns, the current will increase. Negative sequence relay can be used for this purpose. The relay should be applied with a timer to avoid operation on system transients and external faults, and should be disabled when the breaker is opened. This latter safeguard avoids possible operation on low frequency line oscillations after the line is deenergized. With very little resistance in the line, such oscillations can last an appreciable time. Duobias M relays are used for differential protection, Argus relays for overcurrent and earth fault protection, and THR 1PE is used for impedance protection.
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