Abstract
Low frequency oscillation (LFO) in the electric multiple units (EMUs)-traction network cascade system (ETNCS) can lead to traction blockade and abnormal operation. Using the Chinese CRH3 EMUs as an example, a new LFO suppressing method by applying the energy storage (ES) device on EMUs is proposed in this paper and the ES is connected to the DC-link on board. The equivalent model of the ETNCS is established at first. The critical condition of LFO in the ETNCS is analyzed via impedance ratio criterion and Bode diagram. Then, the proposed on-board bidirectional DC-DC converter integrated with the ES system is controlled via a virtual DC machine (VDCM) strategy, and the VDCM small signal model derivation and the parameter design are carried out. Using the Bode diagram of the impedance ratio, the system stability with the ES device controlled by the proposed VDCM scheme is analyzed again; simulation results verify the effectiveness of the control algorithm to suppress the LFO adaptively. Compared with the traditional PI controller, the system steady-state and dynamic performance can be improved with the proposed method, including decreased overshoot, fluctuation, response time, and increased gain margin of the DC-link voltage.
Highlights
Due to the electrical parameter mismatch of the electric multiple units (EMUs) and the traction network, network voltage and current fluctuations, and corresponding overvoltage or over-current traction blockade caused by Low frequency oscillation (LFO) occurred frequently in recent years [1]–[6]
Network-side parameters relate with the capacity of traction substation and the length of the traction network section, which are the representation of the source internal impedance; while the number of running trains, control strategy, topology and control parameters of the Four-Quadrant Converters (4QCs) or the Line-Side Converter (LSC) of certain train are the contribution terms on the train-side impedance
In order to suppress the LFO, the DC side power compensation strategy of LSC based on on-board energy storage is proposed
Summary
Due to the electrical parameter mismatch of the electric multiple units (EMUs) and the traction network, network voltage and current fluctuations, and corresponding overvoltage or over-current traction blockade caused by Low frequency oscillation (LFO) occurred frequently in recent years [1]–[6]. T. Li et al.: New Low-Frequency Oscillation Suppression Method Based on EMU On-Board Energy Storage Device because of the EMU parallel connection. Li et al.: New Low-Frequency Oscillation Suppression Method Based on EMU On-Board Energy Storage Device because of the EMU parallel connection In another word, the source is relatively ‘weak’ compared to the ‘strong’ load when the LFO appears. System stability estimation based on the coupling impedance of the cascade system was proposed Both the analysis and the estimation only considered the internal current control loop. It is difficult to perform the parameter setting and system stability analysis for this nonlinear controller Another adaptive control strategy of the LSC has been studied to suppress LFO, i.e., a direct power control based on Model Prediction Control (MPC) [22]. Substituting (3) into (5), the impedance ratio of the ETNCS can be written as, TM
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