Transformer Protection

Transformer Protection : Part 1

1. Introduction 

       The transformers are static devices without having any rotating part and are totally enclosed. Hence the chances of fault occurring on transformers are much rare as compared to the fault occurring on generators. Similarly possibilities of running on abnormal conditions are also less in transformers compared to generators.

       But though the fault possibility is rare, if fault occurs, the transformer must be quickly disconnected from the system. The rare faults if not cleared quickly can get developed into the major faults which may be very serious for the transformer. Hence the protection must be provided to the transformers against possible faults.

       The use of series fuses is very common in case of small distribution transformers instead of circuit breaker. Hence it is not necessary to install any automatic protective relaying equipments with the distribution transformers. But the power transformers having large ratings always need some type of automatic protective relaying equipments, to give protection against the possible faults.

2. Possible Transformer Faults

       The generators are subjected to the number of faults and abnormal conditions but the transformers are not. The various possible transformer faults are,

1. Overheating             2. Winding faults

3. Open circuit             4. Through faults

5. Over fluxing 

Let us discuss these faults

2.1 Overheating

       The overheating of the transformer is basically of sustained overloads and short circuits. The permissible overload and the corresponding duration is dependent on the type of transformer and class of insulation used for the transformer. Higher loads are permissible for very short duration of time. The overloading which continues for longer time is dangerous as it causes overheating of the transformer. Similarly the failure of the cooling system, through rare, is another possible cause of overheating.

       Generally the thermal overload relays and temperature relays, sounding the alarm are used to provide protection against overheating. Similarly temperature indicators are also provided. On the transformers, when temperature exceeds the permissible limits, the alarm sounds and the fans are started. The thermocouples or resistance temperature indicators are also provided near the winding. These are connected in a bridge circuit. When temperature exceeds the limiting safe value, the bridge balance gets distributed and alarm is sounded. If the corrective action is not taken within certain period of time then the circuit breaker trips.

2.2 Winding Faults

       The winding faults are called internal faults. These faults are,

i) Phase to phase faults

ii) Earth faults

iii) Internal faults

       The overheating or mechanical shocks cause to deteriorate the winding insulation. If the winding insulation is weak, there is a possibility of short circuit between the phases or between the phase and ground. Also the possibility of short circuit between the adjacent turns of the same phase winding is also possible.

       When such an internal fault occurs, the transformer must be quickly disconnected from the system. If such a fault persists for longer time, there is possibility of oil fire. The differential protection is very commonly used to provide protection against such faults. But this protection is not economical for the transformer below 5 MVA for which an overcurrent protection is used. For the high capacity transformers in addition to main differential protection, the overcurrent protection is also provided as a backup protection. For earth fault protection, the restricted earth fault protection system, neutral current relays or leakage to frame protection system is used.

2.3 Open Circuits

       The open circuit in one of the three phases is dangerous as it cause the undesirable heating of the transformer. A separate relay protection is not provided for the open circuits as open circuits are much harmless compared to other faults. In case of such faults, the transformer can be manually disconnected from the system.

2.4 Through Faults

       Through faults are the external faults which occur outside the protected zone. Through faults are not detected by the differential protection. If the through faults persists for long period of time, the transformer may get subjected to the thermal and mechanical stresses which can damage the transformer. The overcurrent relays with undervoltage blocking, zero sequence protection and negative sequence protection are used to give protection against through faults. The setting of the overcurrent protection not only protects the transformer but also covers the station bus bar and portion of a transmission line. Such a protection acts a backup protection for the differential protection.

2.5 Overfluxing

       The flux density in the transformer core is proportional to the ratio of the voltage to frequency i.e. V/f. The power transformers are designed to work with certain value of flux density in the core. In the generator transformer unit, if fault excitation is applied before generator reaches its synchronous speed then due to high V/f the overfluxing of core many result. Higher core flux means more core loss and overheating of the core. The saturation of magnetic circuit is also the probable cause for the oiverfluxing operation. The V/f  relay called volts/hertz relay is provided to give the protection against overfluxing operation. This relay does not allow exciting current to flow till generator reaches to a synchronous speed and runs to produce voltage of proper frequency. The overfluxing relays with enough time lag also can be provided.

       Apart from these fault, some other faults like tap-changer faults, high voltage surges due to lightning and switching, incipient faults i.e. slow developing faults may also occur in the transformers. The Buchhloz realy is used for oil immersed transformers to give the protection against incipient faults.

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