Technical Case Study 1

MV Circuit Breaker selection- Delayed zero axis crossing for high DC constant

As per IEC 909-0 Page 79, Delayed zero axis phenomenon is not studied. Extract is shown below.

In a near-to-generator short circuit, the short-circuit current behaves generally as shown in figure 2. In some special cases it could happen that decaying short-circuit current reaches zero for the first time, some cycles after the short circuit took place. This is possible if the d.c. time constant of a synchronous machine is larger than the subtransient time constant. This phenomenon is not dealt with in this standard.

In one of the IEEE paper, the medium voltage circuit breaker selection for electrical system (when used for generator applications) with high-magnitude X/R ratiosand zero-axis crossings of short circuit current beyond the interrupting time of the standard circuit-breaker is analyzed.

generator short circuit current percent dc component

Attached calculation (with IEC explanation) is a good reference for the MV circuit breaker selection for the power plant where we observe high X/R ratio and high %DC component near to generator. Sometimes we may end up with nearly or more than 100% DC component and breaker selection becomes critical due to high DC time constant.