Converter-Driven Low-Frequency Stability Analysis and Compensation in Weak-Grid-Tied VSCs

This paper analyzes converter-driven low- frequency oscillations of current controlled weak-grid-tied voltage-source converters (VSCs) and proposes an effective method to improve the dynamic behavior. First, the specific single-input single-output (SISO) open-loop transfer function from the active power to the active power reference is obtained. From the SISO transfer function, it is revealed that the low- frequency oscillation frequency mainly depends on the active power control (APC)’s bandwidth in the outer loop. Moreover, it is revealed that the PLL brings phase lag near the frequency of the PLL bandwidth in weak-grid conditions, which results in the smaller phase margin and the weaker damping. By fully taking advantage of transfer functions, a novel reset control based compensator is presented in the outer loop without changing the control structure. The PI+CI control increases the phase margin/oscillation damping by resetting the integrator once the power error signal crosses zero. Bode plots and time-domain simulation results validate the efficacy of the proposed method in mitigating low-frequency oscillations.

I. Introduction

Voltage source converters (VSCs) are the preferred technology to interface renewable power generations such as wind power (WP) and solar photovoltaic (PV) and enable the transmission of bulk amount of power over long distances using high voltage direct current technology (VSC-HVDC) [1]. With the increasing penetration of the power electronics based renewable power generations, the weak grids will become a common scenario in the renewable power generation dominant new power system, as the penetration of the synchronous generators decreases. Though with the advantage of high flexibility, stability issues arise when VSC converters are connected to a weak grid [2], [3].