A wind turbine generator based long transmission network with series capacitance may suffer sub-synchronous oscillations at low wind speed and high compensation. This paper presents an enhancement of sub-synchronous resonance (SSR) stability caused due to extensive series compensation of transmission line using linear quadratic regulation (LQR) based approach for a static synchronous series capacitor (SSSC) in Doubly Fed Induction Generator (DFIG). An extensive use of series compensation even at light wind can cause the prone conditions for invoking SSR in windfarms. Among all the signals of the network, the electromagnetic torque produced by DFIG reflects SSR instability predominantly. Hence the torque signal of DFIG is modulated and voltage injected by SSSC is regulated to dampen the SSR oscillations in the torque signal using the LQR approach. For modulating the torque signal, the actuating efforts are varied by SSSC. The impact of LQR based damping approach for improvement in SSR mode at high compensation levels and low wind speed is examined using eigen value analysis and time domain simulations. The proposed method can effectively improve SSR stability even at low wind speeds and high compensation levels.

Damping Controller, Linear quadratic regulation, sub-synchronous resonance, wind farms

This paper presents the design procedure and ability of LQR with SSSC for the enhancement of SSR alleviation. The LQR-based damping control scheme for the DFIG-based series compensated network minimizes the error of electromagnetic torque of DFIG as it reflects SSR instability superiorly. To alleviate SSR oscillations, the LQR controls the voltage injected by SSSC in order to modulate the torque signal. Since the response of DFIG torque depends on stator and rotor currents, the effectiveness of the proposed approach is shown by choosing the weight matrices to damp SSR oscillation quickly. The eigenvalue results show that LQR shifts and detunes SSR mode and damping ratio has increased significantly. Time domain results also verify the proposed approach's effectiveness by exhibiting the damped and stable response of the study system after incorporating LQR control.

Chirag Rohit, Pranav B. Darji and Hitesh R. Jariwala (2022). Linear-Quadratic Regulator-based SSR Mitigation Scheme for SSSC Controller in DFIG-based SERIES Compensated Network. International Journal on Emerging Technologies, 13(1): 56–64.