Information for the Safe Operation of Voltage Transformers

Operation conditions for potential transformers

1. When the secondary terminals are connected to the measuring or protection devices, one of the terminals should be earthed for safety as seen in Figure VT-1.
2. The base plate must be earthed.
3. The secondary circuits must not be short-circuited during operation.
   Otherwise the voltage transformers will be destroyed.
4. If any of the secondary windings of a voltage transformer will not be used, then it must be left open with one of the terminals connected to the earth as seen in Figure VT-2.
5. For single phase transformers, the neutral terminal of the primary “N” must be earthed in the earthed (neutral) systems as seen in Figure VT-3.

Other important points and notes
When using single pole insulated inductive voltage transformers it is very important to to be aware that, if a circuit is being closed or during the decaying period of an earth fault ferroresonance may occur.

Ferroresonance can lead to the overheating and thermal destruction of the voltage transformer or high levels of voltages may be induced. In general, ferroresonance can be eliminated by the use of an appropriate resistor. The resistor is placed as a burden in opendelta circuit formed by three voltage transformers‘ delta windings. The opendelta circuit must always be earthed only at one point as seen in Figure VT-4.
The open-delta connection can also be used for earth-fault monitoring with appropriate devices.

As the number of cable systems is increasing in the energy distribution systems, the protection of voltage transformers have become very important for the uninterrupted operation of the system without any failure and/or down time. For that reason it is always recommended to use open-delta windings in single phase inductive voltage transformers.
The use of open-delta windings may not be sufficient for the protection of voltage transformers by itself in some cases. An energy systems design engineer shall always use proper surge arresters, avalanche diodes, limiters and/or their combinations for the survivability of the distribution system after a fault or disturbance.