Digital Low Pass Filter Optimization for STATCOM Control in Variable Frequency Weak Grid

Weak grid condition with a relatively large internal impedance is a common problem in standalone grids, e.g., a microgrid. Compared to strong grids, special considerations on system stability and dynamic performance are demanded in weak grids, especially the ones with variable frequency alternating current (VFAC). For example, the grid is supplied by a variable frequency permanent magnet generator. The traditional dq-frame control does not work well when the control bandwidth is limited, especially when the fundamental frequency is relatively high, e.g., >50Hz. Introducing a digital low pass filter (LPF) as an improvement in the closed-loop control has been widely used, but there is a lack of LPF design optimization guidance under the VFAC case, since the existing methods are mostly empirical. In the case of a weak VFAC grid with a static reactive power compensator (STATCOM), this paper uses a small-signal analysis and root locus technique to mathematically model the stability issue to optimize the LPF. The proposed optimization approach can obtain the system stability and improve the current quality, which is verified through both simulation and experimental tests.