Design Improvement and Tuning of Excitation System ST5B With PSS3B and Coordinated OEL in Hydropower Plant

The article analyzes and suggests the tuning and design improvement in the excitation system for the 1400 MW Tehri–Koteshwar hydropower plant (HPP) to mitigate the severe issues of active power oscillations leading to tripping of generating units. The static excitation system model ST5B, power system stabilizer model PSS3B, and over excitation limiter (OEL) are designed for each synchronous machine in the power plant. The coordination of ST5B, PSS3B, and OEL during critical transient operating conditions is analyzed. The proper coordination of these components of excitation system provides better transient and voltage stability. A new thermal-based OEL and modified power system stabilizer (PSS) designs are implemented to enhance the system's voltage and transient stability. The advantage of improved transient response by using different transient gain reduction (TGR) blocks formed by the lead-lag filter in ST5B during normal and limiting excitation operating conditions is presented. The ST5B and PSS3B dynamic analysis and tuning are performed using bode frequency response in offline mode. The robustness of the modified excitation system and limiters together with the added advantage of different TGR blocks in ST5B are verified using real-time digital simulator. A PSS-improved design and thermal-based OEL implemented for this HPP offer better transient and voltage stability as compared to the existing excitation system.