Analysis and Suppression of High-Frequency Resonance and Propagation Mechanism in Multiterminal Flexible Direct Converter Station

doi:10.12204/j.issn.1000-7229.2024.02.003

Abstract

Abstract:

To address the issue of high-frequency resonance in multiterminal flexible direct current (DC) transmission systems caused by unloaded line input, a simplified impedance model is established for MMC in the high-frequency range. Based on the impedance analysis, the electrical mechanism of high-frequency resonance in a single MMC caused by an unloaded line input was analyzed. The analysis shows that owing to the unloaded line input, the MMC AC-side equivalent impedance exhibits negative damping in the high-frequency range, leading to high-frequency resonance. Joint modeling was then conducted for the receiving converter stations under different control methods. By analyzing the propagation path of high-frequency resonance energy between stations and its impact on the operation of receiving MMCs, research has shown that high-frequency resonance on the AC side of MMC can affect other MMCs through DC lines. The degree of the impact is related to the external control method. To solve the problem of high-frequency resonance, a suppression method involving the addition of a bandstop filter to the voltage feedforward was adopted to eliminate the negative damping of the MMC at the high-frequency resonance point, complete the impedance reshaping of the oscillating MMC, effectively suppress the high-frequency resonance of the MMC, and reduce the negative impact of high-frequency resonance energy propagation on other MMCs. Finally, the accuracy of the MMC high-frequency resonance analysis and the effectiveness of the corresponding suppression method were verified using the RT-LAB5600 real-time online simulation platform.

Key words: modular multilevel converter multiterminal direct current (MMC-MTDC), high-frequency resonance, impedance analysis, energy propagation path

CLC Number:

TM71

CHEN Jikai, SUN Chongbo, LI Yang, ZHANG Jiayang. Analysis and Suppression of High-Frequency Resonance and Propagation Mechanism in Multiterminal Flexible Direct Converter Station[J]. ELECTRIC POWER CONSTRUCTION, 2024, 45(2): 26-36.

Figures/Tables 17

Fig.1 Typical high-frequency resonance phenomenon of MMC

Fig.2 Four terminal MMC-MTDC system simulation model

Fig.3 Structure block diagram of MMC station controller

Fig.4 MMC high-frequency simplified model

Fig.5 Equivalent circuit point of unloaded line input

Table 1 Main parameters of simulation platform

Fig.6 Equivalent impedance Bode diagram of MMC AC side for unloaded line input

Fig.7 Block diagram of high-frequency resonance effect of sag station on DC voltage station

Fig.8 Bode diagram of open-loop transfer function

Fig.9 MMC simplified control model with additional bandstop filter

Fig.10 Influence of bandstop filter on impedance characteristics of flexible DC system

Fig.11 Equivalent impedance Bode diagram of MMC AC side after impedance improvement

Fig.12 Voltage and current waveform of PCC before and after unloaded line input

Fig.13 Simulation waveform and FFT before and after the unloaded line input

Fig.14 Simulation waveform and FFT before and after enabling high-frequency resonance strategy

Fig.15 Simulation waveform of voltage and current on the DC side of MMC during high-frequency resonance

Table 2 Voltage and current fluctuations on the DC side of each MMC

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