Admittance Decomposition for Assessment of APF and STATCOM Impact on the Low-Frequency Stability of Railway Vehicle-Grid Systems

Dynamic compensators, such as active power filter (APF) and static synchronous compensator (STATCOM), can be used to mitigate the low-frequency oscillations (LFO) in high-speed railway vehicle-grid systems. However, the small-signal impedances of APF and STATCOM are affected by that of vehicles, which makes it challenging to characterize the impedance of these devices directly and ensure stable operation. The traditional stability analysis is based on terminal impedances of source and load subsystems, which does not always provide sufficient insight into LFO suppression. In this article, a novel idea based on $dq$-frame admittance decomposition is proposed to investigate the impact of APF and STATCOM on the system stability. The proposed approach is demonstrated in simulation and experimental results. It is shown that the control paths of the APF and STATCOM for measuring the vehicle current introduce their coupling with the vehicle. The resulting coupling admittance of these devices plays a significant role in the load admittance reshaping and determining their ability to enhance the system stability and suppress the LFO.

I. Introduction

With the increasing penetration of multiple electric units and locomotives (herein referred to as vehicles) in high-speed railway systems, the power quality and stability of the overall vehicle-grid system may become compromised [1]. The so-called low-frequency oscillation (LFO) events, ranging from a fraction to several Hertz, have been known to appear in such railway vehicle-grid systems, and received great attention in recent publications. As explained in [2] and [3], the incidents of LFO tend to stem from the interaction between the grid and vehicles, and usually appear when multiple vehicles happen to be at the railway station with only auxiliary loads running. In such scenario, the high fluctuating grid voltage tends to trigger the system protection and increase the vehicles’ unscheduled downtime.