Why a voltage transformer should not be connected in short circuit – explained by equations.

The voltage transformer or potential transformer is called PT or VT, is an instrument transformer that is used for metering and protection purpose. Its function to make a low voltage from the high voltage of a certain ratio, so that the large voltage can be easily measurable or sensed by using this low voltage by low voltage capacity costly equipments like relay, PLC.

The voltage transformer is a transformer that is connected to a very large impedance (use in virtual open circuit mode). A voltage transformer is designed to give the secondary a voltage which is proportional to that applied to its primary.

For a voltage transformer (VT) the ratio of voltage between the primary and the secondary is constant. The secondary voltage is independent of the load. If Z is the load impedance of a VT, then at the secondary, we have:

P = power flow to the secondary, Is = secondary current, Us = secondary voltage (imposed by the primary circuit), if Z increases, then P and I decrease..

Thus, we can put an impedance with a value of between the nominal impedance and infinity across the terminals of a VT without any danger. We can thus operate with a VT in an open circuit without any danger.However, if we decrease Z, the supplied power becomes excessive and the VT is damaged.Never connect a VT in short-circuit.

Think about it- if z=impedance is a large value, power or current will be a certain value, but if z tends to zero, power or current would be an excess value.

One thought on “Why a voltage transformer should not be connected in short circuit – explained by equations.”

1. Pang says:

Very clear and useful. Thanks!