Energy balancing of the Modular Multilevel Matrix Converter based on a new transformed arm power analysis

Abstract

This paper presents a transformed arm power analysis of the Modular Multilevel Matrix Converter (M3C). It enables the energy balancing in the whole frequency range for high power variable-speed drive applications. Four balancing directions are identified for the active power exchange between the converter arms with minimal internal currents. At critical operating points a zero sequence voltage is used. Additionally, the reactive power components can be used to perform a real time calculation of the energy pulsation in all four balancing directions to improve the control performance. A low voltage prototype with 5 cells in each of the nine arms has been realized to verify the theoretical analysis.

A Branch Current Reallocation Based Energy Balancing Strategy for the Modular Multilevel Matrix Converter Operating Around Equal Frequency

“…In the M3C there are four independent inner circulating currents [1]- [4]. These circulating currents are independent of input side currents (i u , i v , i w ) and output side currents (i r , i s , i t ).…”

“…Each branch is formed of a cascaded connection of full-bridge (H-bridge) converter cells. The topology enables direct AC-AC bidirectional power conversion and ensures that the average three-phase input and output waveforms are sinusoidal with controllable displacement factor on the input side [1]- [4]. In common with other members of the modular multilevel cascade converter (MMCC) family, the voltage ratings of the M3C can be increased by adding additional cascaded cells.…”

“…However, the reactive power at the input side is not allowed in some applications as the input side is normally connected to the electrical grid. The work described in [4] introduces some common voltage to avoid the need for reactive power at the input side, a similar technique to the mitigation control of the MMC at low-speed range [7]. However, the reference for the common voltage is difficult to design and may cause over-modulation.…”

An Optimal Full Frequency Control Strategy for the Modular Multilevel Matrix Converter Based on Predictive Control

“…However, it is difficult to get the global optimum solution due to the complexity of the problem, so a sub-optimized solution is designed and used in this paper. Firstly, neglecting all the equality and inequality constraints in (22), then the optimized additional circulating current references satisfy (25), where combined with the injected CMV of ii  in (25) can reduce the value of function J to zero (i.e. no capacitor-voltage fluctuation).…”

“…However, when the input and output voltage magnitudes get closer, part of the load-side reactive power needs to be gradually applied at the supply-side. The work in [25] and [26] introduced both common-mode voltage (CMV) and circulating currents for the balancing control at equal-frequency point, a similar technique to the mitigation control of the MMC at low-speed range [11]- [13]. However, the references for the CMV and circulating currents are difficult to design and it requires an extra margin on branch voltage stresses for the CMV injection.…”