EFFECTS OF CURRENT TRANSFORMER SATURATION ON OVERCURRENT PROTECTION COORDINATION

The function of a CT is to produce a secondary current that is proportional in magnitude and in phase with the primary current. This secondary current is applied to protective relays of compatible range and load (or burden) characteristics.

When CTs are operated at or near the knee of their excitation curve, small increases in current magnitude can cause the flux density to increase substantially and cause saturation. When saturation occurs, the secondary current wave shape becomes distorted, and the signal to the protective relays is no longer proportional to the current input.

In some cases of severe saturation, the output current of the secondary could be near zero on one or more phases. Depending on the level of distortion of the secondary waveform and the design of the relay, the operation can be affected.

For electromechanical induction disk relays, the effect of CT saturation is to slow the rotational speed of the disk. When the CT becomes saturated, the actual secondary relay current is less than it should be, its wave shape is distorted, and the relay operates more slowly. This condition leads to longer trip times and possible miscoordination.

Saturation can occur in CTs used to measure low-voltage ground-fault current, especially in underdesigned core-balance CTs in backup ground-fault relay applications. Saturation has also occurred in the solid-state low-voltage trip devices that use current sensors (which should not be confused with CTs, except for the fact that they reduce phase currents to a value compatible with their devices electronic circuitry).

These current sensors form a residual circuit for the measurement of ground-fault current. Normal equipment-starting current or downstream phase faults may produce an unbalanced current that can cause a false groundfault current trip.

In most industrial systems, CT saturation is significant only in circuits with relatively low ratio CTs and high magnitude fault currents. In most cases, these circuits feed utilization equipment; therefore, relays with instantaneous settings below the CT saturation point can be applied.

As one progresses back toward the source, the CT ratios get larger at the same voltage level. Also, the CTs have more turns; develop higher voltages; and, therefore, are less likely to saturate when standard burdens are applied. Saturation of CTs due to the dc component of an asymmetrical fault current can cause a delay in the operation of some instantaneous relays. It can also cause false tripping of residually connected instantaneous ground-fault relays.