Transient Performance Evaluation of Grid-forming Control for Railway Traction Converters Considering Inter-Phase Operation

To address the small-signal oscillation issue, the grid-forming (GFM) control scheme can be an alternative to the more conventional grid-following (GFL) control to regulate the single-phase four-quadrant converter (4QC) in railway traction applications. However, unlike GFL control, the GFM control scheme cannot guarantee start-up without inrush current. The situation become even more critical when take the unique interphase operation scenario of railway vehicle. This paper evaluates the transient performance of GFM-controlled 4QC in the inter-phase operation scenario. A phase-lock-loop (PLL) based solution is developed for rapid pre-synchronization to eliminate the inrush current. The results indicate that with the proposal together with proper design of the frequency-regulator, the GFM-controlled 4QC can start up in a rapid and smooth manner.

I. Introduction

In modern electrified railway traction system, the single-phase four-quadrant converters (4QCs) are usually adopted as the front-end converters of the propulsion system, which regulate the DC link voltage with bidirectional power flow capability. The state-of-the-art control scheme of 4QC is the vector current control, which includes DC voltage controller (DVC) cascaded with the current controller and adopts phase-locked loop (PLL) for synchronization [1]–[4]. This control scheme can be identified as grid-following (GFL) control. However, due to the long distance and large line impedance of traction power supply network, the voltage at the point of common coupling (PCC) is affected by the active and reactive power. The PLL and DVC will induce negative impact on the small-signal stability of the vehicle-catenary system [5] [6]. Hence, the railway traction system suffers from small-signal instability problems, such as low-frequency oscillation, harmonic instability and high frequency resonance [1]–[4].