Preface

Transformer is the important and one of the costliest equipment in the power system. Mainly it is necessary to protect the transformer against 1) External faults 2) Over load (there are other things too; like moisture in paper and oil, core and tank circulating currents, rise in temperature etc. but that is not the part of this topic.). We can achieve both these functions with the help of overcurrent and earth fault relay provided on transformer HV side and LV side. To do it properly we need to coordinate these relays properly for its PSM and TMS.

Higher value of PSM means transformer remains unprotected for overload whereas lower value means transformer can’t be used to its full load capability.

Same thing is with TMS, lower value of TMS may lead to un-necessary tripping of the transformer. Whereas due to selection of higher value for TMS transformer may get exposed to external fault for more time and in some case it may fail too.

When a transformer operating in parallel with other trips; the transformer remained under operation may get overloaded. Also there may be sudden unexpected rise in load on transformer due to tripping of any other elements in the network. Under such situation load is reduced (trimmed) by tripping some of the less important load served by the transformer there by maintaining un-interrupted power supply to important load.

Thus relay coordination and designing proper protection scheme is utmost important and is an art. Proper understanding of IDMT characteristics and behaviour of transformer under fault and load is necessary to adopt these settings properly.

For protection of transformers use of numerical relays is very common. These numerical relays generally having four stages for its operating characteristics. Also all these stages can be assigned to different output contacts. Modern numerical relay may also have facility to set it either directional or non-directional and adopting different settings for forward direction and reverse direction. Possibilities may change manufacturer to manufacturer and depending on these possibility we may adopt different protection schemes.

There should be sufficient margin between HV side as well as LV side Overcurrent pickup settings and load trimming scheme such that overcurrent pickup shall always happens only after operation of load trimming stages.

Setting methodology for transformer HV side relay is same for whether they are operating separately or in parallel. However LV side settings methodology varies depending upon whether transformer operating separately or parallel.

We can have different protection schemes for the transformers depending upon possibilities of customizing relay operational characteristics and configuration of its output contacts.

All these setting schemes methodologies are discussed through next topics.