Current transformers with capacitive burdens

Capacitors may be included in the secondary circuits of current transformers, to effect filtering or tuning or to compensate for the capacitance of equipment being protected. While, in the past, considerable attention has been given to the behaviour of current transformers feeding resistive and inductive burdens, very little has been given to the performance obtained when there are capacitors in the burden. This paper is devoted to this purpose. Initially, the steady-state behaviour obtained when a sinusoidal current is flowing in the transformer primary winding is examined, with both purely capacitive and series RC burdens connected to the secondary winding. The types of output obtained when core saturation occurs, and the core-flux saturation levels needed to avoid saturation, are determined. The interesting feature revealed is that it is possible, under certain conditions, to have three possible secondary currents for a given primary current, with fixed circuit parameters. This effect is due to ferroresonance, and it is clearly important, as it could seriously affect the performance of associated protective equipment. The later part of the paper deals with the behaviour, during transient conditions on the system, of transformers with capacitive burdens. It is shown that the presence of capacitance in the burden of a transformer causes its core flux to contain a component which changes linearly with time. Such a component causes the inevitable onset of saturation in the event of a system fault which persists for a few cycles, and thus it makes it impossible to design transformers which should be suitable for protective equipment such as that used in association with busbars.