Detailed Wideband Impedance Modeling and Resonance Analysis of Grid-Connected Modular Multilevel Converter

Abstract

Modular multilevel converter (MMC) tends to cause resonance instability in interconnected systems. Current studies on the stability of MMC mainly focus on high-voltage and large-capacity power systems, while the impedance modeling process of MMC ignores the harmonics of the submodule (SM) capacitor voltages and the influence of voltage feedforward control. The applicability of the MMC impedance model with fewer submodules is doubtful. According to the circuit structure and control mode, the AC-side sequence impedance model of the MMC is established, which takes into account the capacitor voltage balance control of submodules and voltage feedforward control. Based on the RT-Lab control hardware-in-loop (CHIL) test platform, the MMC impedance frequency scanning was carried out to verify the accuracy of the impedance modeling method. The influence of control parameters in different frequency bands on the impedance characteristics of MMC was studied. The AC terminal voltage feedforward causes phase lag in the mid-/high-frequency range and increases the risk of resonance. In the low-frequency range, the dynamics and control of capacitor voltages reduce the impedance magnitude of MMC. Using the resonance phenomenon of an MMC connected to a weak grid as an example, the high-frequency resonance mechanism caused by control parameters is analyzed from the perspective of the negative damping effect. The simulation results show that the detailed wideband impedance model can improve the accuracy of the stability analysis results.

Stability Enhancement Method of Standalone Modular Multilevel Converters Based on Impedance Reshaping

“…However, when interfacing with the impedance of the power line, MMCs tend to exhibit oscillatory behavior [10][11][12]. Recent commissions of MMC-HVDC projects have highlighted relevant stability issues.…”

Variable Reactance Criteria to Mitigate Voltage Deviations in Power Transformers in Light- and Over-Load Conditions

“…Apart from this, significant load shedding can provoke over-voltages and may even imply voltage instability [3]. Furthermore, harmonics, resonances, switching operations and faults may lead to the amplification of voltage levels due to non-linear loads, power electronic devices, insulation failures or ground faults, among others [4][5][6].…”

“…If a transformer which works in parallel with another transformer is subjected to over-load conditions, as its X sc is increased, the overload can be transferred to the second transformer to achieve a better share of the overload or even to cancel the overload for the complete set of transformers, A and B, working in parallel. This comes from the condition of equal voltage drops at both transformers, as indicated in (5).…”