An Improved Q-Axis Current Control to Mitigate the Low-Frequency Oscillation in a Single-Phase Grid-Connected Converter System

Abstract

An electric railway system is a typical single-phase grid-connected converter system, and the low-frequency oscillation (LFO) phenomenon in electric railway systems has been widely reported around the world. Previous research has indicated that the LFO is a small-signal instability issue caused by impedance mismatching between the traction network system and electric trains. Therefore, this paper proposes an improved q-axis current control method to reshape the train’s impedance. The proposed method can be implemented easily by relating a reverse q-axis reactive current directly to the reference of the q-axis current under the dq current decoupled control. Moreover, considering the additional q-axis reactive current control, a small-signal impedance model of a train–network system is built. The impedance-based analysis results indicate that the proposed q-axis reactive current feedback control can increase the magnitude of the train’s impedance, which is beneficial to enhancing the system’s stability. Finally, this paper employs experimental results to verify the effectiveness of the proposed method.

An Anti-Islanding Protection Method Based on Voltage-Synchronous Impedance Angle Measurements

“…The proposed method starts islanding detection when the system frequency change rate exceeds ±0.02 Hz/s, and it sets the detection signal strength at 5% of the DG output frequency's current amplitude. According to the technical requirements of the distributed power supply grid connection [3,31], the voltage protection action criterion is configured as U ≥ 1.1 or U ≤ 0.85, and the frequency protection action criterion is f ≥ 50.5 Hz or f ≤ 49.5 Hz. To simplify the data analysis, the To verify the effectiveness of the proposed method, the following is a comparison and analysis of the proposed method with OFP/UFP and OVP/UVP in terms of grid disturbance identification and islanding identification.…”