Impacts of Voltage Feedforward Control on Impedance Characteristics and Stability of MMC-based STATCOM

Recently, the voltage feedforward control is widely used in the MMC-based static synchronous compensator (STATCOM) to improve its performance of starting, disturbance rejection, suppression of background harmonics and unbalance in the grid voltage. However, this control may lead to a challenge to the system stability, due to its large impact on the frequency characteristics of STATCOM. Hence, sequence impedance modeling and analysis method are used in this paper to understand the effect of the voltage feedforward control on the system stability for providing a basis of impedance shaping design. The frequency-domain small-signal impedance models of STATCOM with a typical voltage feedforward control in the dq-frame and αβ-frame are developed by double-harmonic linearization, respectively. The effects of the voltage feedforward control in the two frames on the impedance characteristics and system stability are analyzed and compared. A cased study based on a wind farm in Western China is presented to confirm the analysis by simulations.