Which parameter must be monitored by a synchronous check relay before closing an open circuit breaker after it has tripped?

The synchronous check relay is a crucial component in ensuring the safe operation of circuit breakers in power systems. When a circuit breaker has tripped and is being prepared for reclosure, it is essential to verify that the system conditions are appropriate to avoid re-energizing a faulted circuit.

Monitoring frequency is critical because it indicates the operational state of the electrical grid. If the frequency is unstable or significantly different from the system's nominal value, it can suggest that there are ongoing issues within the grid that must be addressed before re-closing the breaker. A failure to assess frequency could lead to severe disturbances, such as oscillations or overloads, potentially causing further equipment damage or system failure.

While voltage magnitudes, phase angles, and current levels are also important parameters in assessing the condition of the electrical network, frequency is the primary indicator used by synchronous check relays to ensure safe reclosure after a trip. The relay must confirm not only that the frequency is within acceptable limits but also that the voltages are synchronized and stable, and the other parameters are aligned before the circuit breaker is closed. Thus, monitoring frequency is a fundamental step in maintaining system reliability and preventing accidents during the power restoration process.