High Voltage Power Transformer Protection

Substation | Source: Asian Business Daily

Most of the Power transformers of 132/11KV and above are of Star-Star vector grouping with the neutral solidly earthed. There are a few transformers with delta-star (delta on HV side). The 33/11KV and 11KV/415V Transformers are of delta-star (delta on HV side).

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The types of faults that the transformers are subjected to are classified as:

Through Faults:

These are due to overload conditions and external short circuits. 

• Time graded O/C & E/F relays are employed for external short circuit conditions. Fuses are provided for Distribution transformers.

Internal Faults:

• Electrical Faults: - Faults that cause immediate serious damage such as phase to earth or phase to phase faults, short circuits between turns of HV&LV windings, etc. 
• Incipient Faults: - These are initially minor faults, causing slowly developing damage. Such as a poor electrical connection of conductors of the breakdown of insulation, etc.

The following relays are employed to protect the transformer against internal faults. 

• Buchholz relays 
• Differential relays 
• REF relays. 
• Over fluxing relays

Buchholz Relays:

Whenever a fault in the transformer develops slowly, heat is produced locally, which begins to decompose solid of liquid insulating materials and thus to produce inflammable gas and oil flow. This phenomenon has been used in the gas protection relay or popularly known as Bucholz relay. 

This relay is applicable only to the so-called conservator type transformer in which the transformer tank is completely filled with oil, and a pipe connects the transformer tank to an auxiliary tank or " Conservator" which acts as an expansion chamber.

Buchholz Relay Trip Control Circuit

Differential Relays

A Differential relay compares the currents on both sides of the transformer. As long as there is no fault within the protected equipment (Transformer), the current circulates between the two CTs and no current flows through the differential element. But for internal faults the sum of the CTs secondary currents will flow through the differential relay making it to operate.

Percentage Differential Relay in a Two Terminal Circuit

Two basic requirements that the differential relay connections are to be satisfied are: 

1. It must not operate for load or external faults. 
2. It must operate for internal faults. 

As on-load tap change facilities are invariably provided in the grid transformers, any departure from the nominal tap position will result in spill currents in the relay circuits. 

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Further, the CTs are often of different types and have dissimilar magnetization characteristics, again resulting in spill current during heavy through fault conditions. To avoid unwanted relays operation under the above two conditions a "Percentage Bias" differential relays are used.

Restricted Earth Fault

This relay is operative only for the internal faults of the transformer and thus fast operating timer can be achieved. 

Restricted Earth Fault Protection Circuit

• An external fault on the star side will result in current flowing in the line CT of the affected phase and a balancing current in the neutral CT and current in the relay is zero and hence relay is stable. During an internal fault, the line current on the line CT gets reversed and hence relay operates. 

The arrangement of residually connected CTs on the delta side of a transformer is only sensitive to earth faults on the delta side because zero sequence currents are blocked by the delta winding.

Over fluxing Protection 

Over fluxing conditions in a transformer can occur during system over-voltage and/or under frequency conditions (V/F). 2. The Overfluxing condition does not call for high-speed tripping. The tripping can be delayed depending on the overflux withstand capability of the transformer. Relays with definite time delay (nearly 30Sec.) and inverse characteristics are being employed.

Reference: 

Title: Handbook for Protection Engineers

Author: Mohammed Fasil

Document: PDF | pages 35-45 | Download