Analysis and compensation of circulating currents errors in modular multilevel converters in low‐frequency operation

Abstract

Errors in the injected high-frequency circulating currents are originally investigated for modular multilevel converters (MMCs) in low-speed drive operation. The mathematical analysis reveals that the magnitude and phase errors in the injected circulating currents cause serious energy imbalance in the MMC arms, resulting in large voltage fluctuations in the submodules. Furthermore, these fluctuations get larger when the circulating currents are injected at higher frequencies, which reduces the drive system stability and damages the submodule capacitors. A remedy approach is proposed to minimise the error effects by adjusting the circulating current command. A full-scale system comprising a 1 MVA MMC and a 1 MW induction machine is simulated to verify the theoretical analysis. The experimental investigation of a down-scale MMC feeding an inductor is also conducted to validate the theoretical results.

Suppressing the Capacitor Voltage Fluctuations in Low Frequency Operation of Modular Multilevel Converters

“…Therefore, it is prudent to use high common-mode frequencies to reduce the size of submodule capacitors. However, an increase in frequency may cause errors in the circulating currents, and these errors result in undesired voltage fluctuations in the submodules at the output frequency [14]. In our earlier work, a technique to mitigate these voltage fluctuations due to the error has been proposed [14].…”

“…However, an increase in frequency may cause errors in the circulating currents, and these errors result in undesired voltage fluctuations in the submodules at the output frequency [14]. In our earlier work, a technique to mitigate these voltage fluctuations due to the error has been proposed [14]. The proposed technique improves the suppression of capacitor voltages at a low output frequency by compensating the circulating current error using two parameters α * (amplitude compensation) and θ * (phase compensation).…”

“…To simplify the technique proposed in [14], we proved in [15] that the same objective can be achieved by only one additional parameter in the circulating current reference instead of two. As a result, the proposed technique becomes simpler, and is able to enhance the suppression of voltage fluctuations in the MMC capacitors.…”

“…This paper is based on our earlier work presented in [14] and [15] with the following additional contributions: (i) The reasons for circulating current errors are explained when the common-mode frequency increases. (ii) A method for computing the circulating current error is introduced and a clear relationship between the error and the compensation parameter is established.…”

“…(iii) Improvements in suppressing the voltage fluctuations are analyzed and validated in the time and frequency domains. (iv) Computer simulations for a medium-voltage MMC drive are performed instead of a lowvoltage drive [14], [15]. (v) Experimental results showing a comparison of the old and proposed techniques are added in this paper.…”