Ferroresonance of Grounded Potential Transformers on Ungrounded Power Systems

Potential transformers that have their primary windings connected in Y and grounded, on an otherwise ungrounded circuit, may oscillate with the distributed capacitance to ground of the circuit following a voltage disturbance. This oscillation, termed here as ferroresonance, may be of several frequencies and will persist if energy losses are not sufficiently high. The amount of losses required to damp out these oscillations is influenced to some extent by the ratio of the applied voltage to that at which the transformer saturates. One method to increase the energy losses and thus protect the transformer and circuit against the results of sustained oscillations is to load resistively the secondaries of the potential transformers either individually or by connecting the secondaries in delta with the loading resistor in one corner of the delta. The required broken-corner delta resistance for the more critical (one-half fundamental) frequency may be estimated from the empirical equation RB = 100La/N2 where RB = broken-corner delta resistance, ohms La = potential transformer primary inductance during saturation, millihenrys (inductance of primary viewed as separate coil in air) Another method using Fig. 14 has been given. Both methodsare predicated on the transformer being fluxed for line-to-line voltage. Maximum transformer primary currents during ferroresonance in the one-half fundamental frequency mode have been estimated with the use of Figs. 9 and 14 Comparison of required loading and maximum primary currents with the results of laboratory tests on two types of potential transformers has shown good agreement.