MRQ1 – Field failure relay

The field failure relay MRQ1 protects synchronous generators against operation outside the stable operation area due to loss of excitation. When partial or complete loss of excitation occurs on a synchronous machine, reactive power flows from the system into the machine and the apparent impedance as viewed from the machine terminals goes into the negative X region in the R-X diagram. The MRQ1 detects the low or under impedance condition and trips the generator circuit breaker, thus preventing damage due to out of step operation and system instability. The under impedance measurement provides two elements with separate impedance and time settings. Therefore setting according to the dynamic and steady state stability curve is possible. MRQ1 calculates the momentary impedance value from the generator current and voltage and compares this value with the two settings of the under impedance elements. Under impedance circle no. 1 reproduces the steady state stability area of the generator. Element no. 1 may be used for alarm purposes and corrective measures like boost excitation. Element no. 2 reproduces the dynamic stability area of the generator. The time delay is set to a lower value. It provides fast clearing on complete loss of field and backs up element no. 1. Element no. 2 should trip the generator circuit breaker quickly. The following factors determine the setting of the two elements: Stability diagram of the generator, excitation system of the generator and the system configuration.

FEATURES & FUNCTIONALITY

The field failure relay MRQ1 protects synchronous generators against operation outside the stable operation area due to loss of excitation. When partial or complete loss of excitation occurs on a synchronous machine, reactive power flows from the system into the machine and the apparent impedance as viewed from the machine terminals goes into the negative X region in the R-X diagram. The MRQ1 detects the low or under impedance condition and trips the generator circuit breaker, thus preventing damage due to out of step operation and system instability. The under impedance measurement provides two elements with separate impedance and time settings. Therefore setting according to the dynamic and steady state stability curve is possible. MRQ1 calculates the momentary impedance value from the generator current and voltage and compares this value with the two settings of the under impedance elements. Under impedance circle no. 1 reproduces the steady state stability area of the generator. Element no. 1 may be used for alarm purposes and corrective measures like boost excitation. Element no. 2 reproduces the dynamic stability area of the generator. The time delay is set to a lower value. It provides fast clearing on complete loss of field and backs up element no. 1. Element no. 2 should trip the generator circuit breaker quickly. The following factors determine the setting of the two elements: Stability diagram of the generator, excitation system of the generator and the system configuration.