AC circuit breaker design is reliant upon current zero crossing within rated time for successful interruption of current. It has however been shown on recent transmission designs that a circuit breaker on a circuit with a shunt reactor may have to interrupt fault currents with high and slowly decaying DC components that result in the zero-missing phenomenon (ZMP), i.e. the current zero crossing not being realised within the rated time for the circuit breaker to interrupt the fault current.  This situation is pronounced where there is low fault current infeed through the circuit breaker attempting to clear the fault. In this case, the lower peaks of the symmetrical current component oscillating about the DC offset remain above zero. This issue raises questions which require further investigation, such as:

• What are the transmission design characteristics that make circuit breakers susceptible to the ZMP?

• What is the likelihood that the circuit breaker will be faced with this situation?

• What are the likely consequences of the ZMP?

• If the circuit breaker successfully interrupts the current, what happens next?

• Can a switching strategy or some cost-effective mitigation be devised?

This call aims to address the questions above through detailed investigation of the phenomenon and its impacts followed by investigation and development of mitigation options.