A Proposal to Increase the Operating Capacity of Synchronous Compensators Through Negative Excitation and Effects on Stability and LOE Protection

Synchronous compensators play a fundamental role in the voltage regulation of systems based on the generation or consumption of reactive power. To increase their underexcited operating range, it can operate in the salience region using negative excitation, also known as the reluctance region of the capability curve. According to the study presented in [1], these conditions introduce the machine to an operating point close to the practical stability limit (PSL), where systemic disturbances can affect the compensator angular stability. Firstly in [1], a power system is modeled to evaluate the increase in the operating range of the compensator. In a second moment, a study to define the tap of the step-up transformer is carried out through power flow analyses between the synchronous machine and the system, determining the operational range of the synchronous compensator. In a third moment, a transient angular stability analysis is conducted through the application of short-circuit contingencies and generation withdrawal to verify the angular stability in operation with negative excitation. Finally, the parameterization and testing of the loss of excitation (LOE) protection coordinated with the Underexcitation Limiter (UEL) of the Automatic Voltage Regulator (AVR) is evaluated in this context. The results indicate that it is possible to increase the underexcited operating range of synchronous compensators using negative excitation, at operating points close to the PSL region. Regarding ANSI 40 protection, the proposed adjustments allow a UEL control action coordinated with the protection actuation.