Necessity of Differential Protection

A transformer is a static device that transfers electrical energy from one circuit to another by electromagnetic induction without the change in frequency. The transformer, which can link circuits with different voltages, has been instrumental in enabling universal use of the alternating current system for transmission and distribution of electrical energy. The transformer is an honorary electrical "machine" in which the flux changes occur by variation in currents with time, instead of by motion. Various components of power system, viz. generators, transmission lines, distribution networks and finally the loads, can be operated at their most suited voltage levels. As the transmission voltages are increased to higher levels in some part of the power system, transformers again play a key role in interconnection of systems at different voltage levels. Transformers occupy prominent positions in the power system, being the vital links between generating stations and points of utilization. As such protection of the transformer from internal / external faults becomes essential. Instantaneous isolation of the faulty transformer prevents major transmission disturbance and will help to minimise the damagesinside the transformer itself.

Various protections are provided for the transformer according to its voltage level and capacity. All transformers having capacity more than 5 MVA and voltage ratings above 33kV are provided with Differential Protection. Differential protection is main protection along with restricted earth fault protection, Over voltage, Over / Under frequency, Over current & Earth fault protection along with various auxiliary protections are provided for the transformers in EHV network. To reduce the effects of thermal stress and electro dynamic forces it is advisable for the overall protection package to minimise the time that a fault is present within a transformer. Identification and isolation of transformer from network in case of internal fault will avert major damages in the transformer thereby reducing the cost of repairs & restoration.

Transformer faults are generally classified into four categories:

− Winding and Terminal faults − Core faults

- On-load tap changer faults

- Tank and transformeraccessory faults

− Abnormal operating conditions such asover voltage,over fluxing and overload − Sustained or un-cleared external faults

The Figure – 1.0 provides the statistics of transformer faults. While designing the transformer protection package, all these conditions needs to be considered individually.

To provide effective protection for faults within a transformer and security for normal operation and external faults, the design and application of transformer protection must consider factors such as:

− Magnetising Inrush current

− Winding arrangements

− Winding connections

− Connection ofprotection secondary circuits