What is the function of negative sequence impedance in power system protection?

Negative sequence impedance is the impedance offered to negative sequence currents in an unbalanced three-phase system. Negative sequence currents arise during unbalanced faults, such as single-line-to-ground faults, line-to-line faults, and double-line-to-ground faults. Symmetrical components are used to analyze unbalanced faults, breaking down the unbalanced currents and voltages into three sets of balanced components: positive sequence, negative sequence, and zero sequence. Negative sequence impedance plays a crucial role in power system protection by providing information about the type and location of unbalanced faults. Negative sequence relays are specifically designed to detect negative sequence currents. These relays can be used to trip circuit breakers and isolate the faulted section of the system, preventing further damage and maintaining system stability. The magnitude and angle of the negative sequence impedance can be used to distinguish between different types of unbalanced faults. For example, a single-line-to-ground fault typically produces a larger negative sequence current than a line-to-line fault. The negative sequence impedance can also be used to locate the fault. By comparing the negative sequence impedance measured at different points in the system, the location of the fault can be estimated. Negative sequence impedance is also used in generator protection to detect unbalanced loading, which can cause overheating of the generator windings. By monitoring the negative sequence current, the generator can be tripped if the unbalanced loading exceeds a certain threshold.