Circulating Current Suppression in Modular Multilevel Converters With Even-Harmonic Repetitive Control

Abstract

Due to the voltage mismatch between the phase legs and the DC bus in Modular Multilevel Converters (MMCs), the differential current in MMCs is inherently subjected to circulating even order harmonics. Repetitive control based active harmonic suppression methods can be adopted to eliminate such harmonics. Nevertheless, conventional repetitive controllers have a relatively slow dynamic response because all the sampled errors in the past one cycle have to be stored, which causes a response delay for one fundamental period. This paper proposes an improved repetitive control scheme that exclusively copes with even order harmonics based on the circulating current characteristics of MMC systems. The design details of the even harmonic repetitive control scheme according to the harmonics characteristics are provided. The proposed even-harmonic repetitive control scheme requires halved data memory to store error samplings and the delay introduced by the repetitive controller is also reduced. According to the frequency domain analysis, the even-harmonic repetitive control features faster convergence rate, greater low-frequency gains, higher crossover frequency, and higher tolerance against system frequency deviation, while possessing the same even-order harmonics suppression capability and stability as conventional ones. Simulation and experimental results are presented to show the steady-state harmonics suppression capability, dynamic response, and disturbance tolerance of the proposed even-harmonic repetitive control scheme.

Control of MMC-Based STATCOM as an Effective Interface between Energy Sources and the Power Grid

“…where the circulating current (which is flowing between the DC link and each phase leg) is supposed to have DC and second harmonic components [30,31], while i x is considered as the sinusoidal current of phase x at the fundamental frequency. On the other hand, the instant number of inserted SMs in each arm determines the arm voltage and thus directly affects the output voltage as well.…”

A new control strategy forMMCunder grid voltage unbalance condition

“…According to (8), we can get Figure 5 are waveforms under target 1. According to (8), we can get Figure 5 are waveforms under target 1.…”

“…Columns 1, 4, 5, and 6 of formula (8) can be obtained, the expression following is obtained: Eliminate the reactive power double base frequency ripples.…”

“…To eliminate negative sequence current components and keep AC currents balanced. According to (8), we can get Figure 5 are waveforms under target 1. From Figure 5 that the Lyapunov-function-based control strategy and the conventional PI controller designed for the target 1, we can know that both methods can achieve better control effects.…”

“…1 A mathematical model is established based on the topology of MMC in Dekka et al 2 and Debnath and Chinthaval. 3 Cui et al, 4 Bergna-Diaz et al, 5 and Li et al 6 are control strategies for the external MMC, and Ji et al, 7 Yang et al, 8 Geddada et al 9 are the control methods of the internal circulating current under grid voltage balance. These methods achieve good results when the grid voltage is balance, but when the grid voltage is unbalanced, both the external control method and the internal circulating current control methods will lose their effect and are no longer suitable.…”