Current transformer (CT) can cause serious damage

Current transformer (CT) can cause serious damage

Open circuit condition in a current transformer (CT) can result in dangerous over voltage condition at the secondary terminals of the CT. An open circuit CT especially of high ratio and carrying high currents can produce secondary open circuit voltage in the range of few kilo volts.

Current transformer (CT) can cause serious damage to equipment in close proximity, but the potential risk to human life during an explosion and the aftermath is an even greater concern. In an explosion caused by overstressed dielectric or hydrogen generated by partial discharge activity, for example, the ceramic parts of the external insulator are blown to bits, scattering debris across an area of up to 200 m (650 ft). Although the number of CT explosions is small, the effects of explosions can be immense, especially when human lives are at stake. The Israel Electric Corp. (IEC) launched a research project to find a simple and effective means to prevent this phenomenon.

In normal operation, the secondary winding of the CT must remain closed through a burden circuit, or short circuit. The leads of the secondary winding may only be opened during an inter-turn over voltage test. The test current at system frequency in the primary winding must be no more than the rated current, and the peak induced voltage between the leads of the secondary winding must be no more than 4.5 kV for 60 sec. The voltage may be measured and recorded by a high impedance measurement device connected to the leads of the secondary winding.

In the event of the CT’s secondary winding circuit being opened, due to a protection relay failure or any other reason, a no-load condition exists on the secondary winding circuit. As a result, the CT behaves like a current generator connected to a high impedance load, or open circuit. Consequently, the CT behaves like an ignition coil, producing extremely high voltages. This is a hazard that, in rare circumstances, has resulted in fatalities.

Partial discharge is most likely to occur in the oil insulation between the secondary winding leads, between these leads and the tank, and in one of the bushings between the turns on the winding. The level of partial discharge is dependent on the amplitude of the induced voltage.