Overcurrent protection using signals derived from saturated measurement CTs

Abstract

A conventional inverse time overcurrent relay measures the current flowing into a fault using protection current transformers (CTs). If the current is greater than the operating threshold the relay trips its circuit breaker after an appropriate time. The protection CTs are designed to provide a proportional replica of the primary fault current. This can be achieved because protection CTs are rated to cope with the maximum possible fault current and worst case DC offset. Hence the protection CT has to be physically large and supply a low burden secondary load. In the case of using an inadequate size of CT or high burden secondary load, the CT may saturate. The saturation of the CT may cause distortion of the secondary current. As a result the relay may measure the distorted secondary current which does not represent the actual fault current. This may affect the operating time and result in maloperation of a time graded protection scheme. The objective of this paper is to design and evaluate an overcurrent relay suitable for use with iron-core measurement CTs. This type of CT is not designed to cope with the fault current and DC offset. This implies that during a fault the CT causes the distortion in the secondary current due to magnetic saturation. The relay analyses the distorted current and predicts the magnitude of the primary current using an algorithm based on cross-correlation. A prototype relay was tested on. an analogue test bench configured so that deep saturation of the CTs occurred during a fault.