Impedance Measurement of Traction Network and Electric Train for Stability Analysis in High-Speed Railways

Instability and oscillation issues have frequently occurred in high-speed railways due to the impedance mismatch versus frequency between the four-quadrant converter (4QC)-based high-speed train and the traction network (hereinafter train–network). However, solely utilizing the mathematical deduction to quantify the impedances seems to be difficult owing to unknown detailed parameters of both traction network and electric train. This paper proposes a method to measure the equivalent impedances of the traction network and the 4QC of the electric train in the stationary frame for stability analysis. A disturbance circuit consisting of antiparallel insulated gate bipolar transistor (IGBT) modules and an excitation load is adopted by means of the pulsewidth modulation (PWM) signal to drive the IGBTs. Consequently, a desired broad spectral excitation is then generated to measure the output impedance of the traction network. When injecting the harmonics twice, which are linearly independent at the same frequency, the input impedance of the 4QC can be calculated at the corresponding frequency considering the frequency-coupled effect. The proposed method shows a good measurement accuracy. Additionally, the stability and oscillation issues of the train–network system can be then identified using measured impedances. Both simulation and experimental results validate the effectiveness of the proposed measurement method.

I. Introduction

Power electronic converters with a constant power feature may bring a negative incremental impedance to the electric power system [1]– [7]. With increasing adoption of these constant power loads, oscillations and instabilities have been frequently aroused in industrial power systems that may potentially lead to a system collapse in serious operations [4], [5]. As a special power system, traction power system may encounter similar instability issues caused by the interactions between the electric train and traction network [6], [7]. The traction network impedance determines the strength of the network that an enough strong network can endure the negative impedance caused by the constant power loads, and a weak network may easily arise small-signal stability issues [8], [9].