Enhanced Stability and Control of Voltage Source Converters With LCL Filter in Weak Grid

Abstract

Voltage source converters show considerable potential in integrating renewable energy sources with the power grid. The stability of LCL -type grid-connected converters is significantly affected by internal resonances within filters. The digital control delay can lead to instability with proportional capacitor current feedback-based active damping (AD). This instability emerges when the resonance frequency nears the critical frequency (fs/6) owing to fluctuations in grid impedance, in weak grid. To address this limitation, a hybrid damping comprising grid current feedback with a damping resistor is proposed to mitigate the effect of digital control delay. A feed-forward point of common coupling (PCC) voltage compensation is proposed without additional sensor requirements to enhance the phase margin (PM). The proposed hybrid damping compensates for the digital control delay, extending the critical frequency region to the Nyquist frequency (fs/2). The proposed design of PCC voltage feed-forward enhances the PM and strengthens the robustness against grid impedance variation. An open loop synchronization is implemented to mitigate the instability raised due to phase locked loop (PLL)-based synchronization (low-frequency instability). The hybrid damping and PCC voltage feedforward controller are designed using a graphical approach. The proposed control achieves a stable operation even when the short-circuit ratio is significantly low ( $\leq 2$ ). An admittance-based stability analysis is performed followed by a closed-loop pole trajectory. To validate the proposed method and controller design, comprehensive simulation studies on a 100-kVA system and laboratory experiments on a 15-kVA prototype are conducted. The proposed methodology is tested for unbalanced voltage conditions and with an outer power loop control.