The transformer is one of the major equipment in power system. It is a static device, totally enclosed and usually oil immersed, and therefore the fault occurs on them are usually rare. But the effect of even a rare fault may be very serious for a power transformer. Hence the protection of power transformer against possible fault is very important.
The fault occurs on the transformer is mainly divided into two type external faults and internal fault. External fault is cleared by the relay system outside the transformer within the shortest possible time in order to avoid any danger to the transformer due to these faults. The protection for internal fault in such type of transformer is to be provided by using differential protection system.
Differential protection schemes are mainly used for protection against phase-to-phase fault and phase to earth faults.The differential protection used for power transformers is based on Merz-Prize circulating current principle. Such types of protection are generally used for transformers of rating exceeding 2 MVA.
Connection for Differential Protection for Transformer
The power transformer is star connected on one side and delta connected on the other side. The CTs on the star connected side are delta-connected and those on delta-connected side are star-connected. The neutral of the current transformer star connection and power transformer star connections are grounded.
The restraining coil is connected between the secondary winding of the current transformers. Restraining coils controls the sensitive activity occurs on the system. The operating coil is placed between the tapping point of the restraining coil and the star point of the current transformer secondary windings.
Normally, the operating coil carries no current as the current are balanced on both the side of the power transformers. When the internal fault occurs in the power transformer windings the balanced is disturbed and the operating coils of the differential relay carry current corresponding to the difference of the current among the two sides of the transformers.Thus, the relay trip the main circuit breakers on both sides of the power transformers.
Problem Associated with Differential Protection System
When the transformer is energizing the transient inrush of magnetizing current is flows in the transformer. This current is as large as 10 times full load current and its decay respectively.This magnetizing current is flows in the primary winding of the power transformers due to which it causes a difference in current transformer output and it makes the differential protection of the transformer to operate falsely.
To overcome this problem the kick fuse is placed across the relay coil. These fuses are of the time-limit type with an inverse characteristic and do not operate in short duration of the switch in the surge. When the fault occurs the fuses blow out and the fault current flows through the relay coils and operate the protection system. This problem can also be overcome by using a relay with an inverse and definite minimum type characteristic instead of an instantaneous type.