Friday, July 27, 2012

Protection of Voltage Transformers

Voltage transformers rated for system voltage of up to 34.5 kV are protected by a current-limiting fuse. Above this voltage rating, it is difficult to find a fuse of adequate fault-interrupting rating.

The primary fuse current rating is selected to protect the VT against the bolted fault on the secondary terminals. The VT is designed to withstand the mechanical and thermal stresses resulting from a bolted fault at the secondary terminals for a duration of 1 s.

A miniature circuit breaker or a fuse on the secondary is used to protect the VT against overloads. The secondary protective device must carry the rated current at the thermal burden. The secondary protective device is located near the VT. If a number of loads are fed by the VT, each branch is protected by a molded-case circuit
breaker or a fuse not higher than half the rating of the main protective device.

The primary or secondary fuse elements shall not be mechanically weak, as they can open the circuit due to vibration or if dropped accidentally. The primary fuse on the VT connected to an isolated phase bus in a power plant is subject to vibration. 

A current-limiting fuse rated 0.5 A for a 0.3Z VT may fail under such conditions. Discuss with the manufacturer and increase the fuse rating to 0.75 A or 1.0 A. However, ensure that the large fuse protects the VT against the secondary fault current.

 A fused-protected, control voltage transformer.

General Electric Voltage Transformer
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