Vector control strategies to enable equal frequency operation of the modular multilevel matrix converter

Abstract

The modular multilevel matrix converter (M 3 C) is a power converter topology for ac to ac conversion that is suitable for high-power applications. The control of this converter is complex, particularly if ac system frequencies are similar. In these cases, the floating capacitors can present large voltage oscillations. Therefore, this study presents a new vector control system to enable the operation of the M 3 C when the frequencies at the input and output are virtually the same. The effectiveness of the proposed method has been validated using simulations and experimental results from a prototype M 3 C power converter with 27 cells. 11Introduction The modular multilevel matrix converter (M 3 C) is a power converter topology for direct ac-ac conversion first proposed for wind energy conversion systems [1]. This power converter has advantages over traditional topologies including modularity, a simple extension to enable operation at high voltage levels with options for redundancy, control flexibility and enhanced waveform quality [2]. Lately, the M 3 C has been also proposed for drives [3, 4], large power wind turbines [5, 6] and grid-connected applications [7]. The M 3 C is characterised by a cascade connection of full-bridge power cells forming a cluster. The direct ac-to-ac connection of two ac ports is achieved using the nine clusters of the converter, as shown in Fig. 1. The capacitor voltage of each power cell is floating and can charge-discharge during the operation of the converter. Therefore, one of the most important control tasks is to maintain the voltage of each capacitor within an acceptable range [2]. This converter is suitable for low-speed high-power applications because lower circulating currents and common-mode voltage are required to mitigate the oscillations in the capacitors, in comparison to other topologies such as the modular multilevel converter [2]. However, the M 3 C has an inherent problem when the frequencies at the input and the output of the converter are very similar or equal. This situation can result in oscillations in the floating capacitor voltages. Cascade control systems based on decoupled modelling of the M 3 C have been proposed [3, 4]. These previously proposed approaches use circulating currents in the converter along with common-mode voltage control to regulate the voltages across the floating capacitors. When the input-port frequency is low in comparison to the output-port frequency, referred to as low-frequency mode (LFM), the average components of the capacitor voltages are controlled using either the circulating currents or the common-mode voltages [3-6, 8]. When the input-port frequency is close or similar to the output-port frequency, referred to as equal frequencies mode (EFM), mitigation signals are included to compensate the oscillations in the floating capacitor voltages [7, 9]. These mitigation signals are pre-defined offline and can increase the peak current flowing through the converter. In this context, this paper presents vector control strategies to enable EFM operation of the M 3 C. This proposal considers nested control systems to regulate the floating capacitor voltages and the input-output ports. In EFM, circulating currents and common-mode voltage are used to form the mitigation signals that can reduce the oscillations in the floating capacitor voltages to zero. The effectiveness the proposed control strategies is validated through experiment and simulation results from a prototype converter rated at ∼5 kVA with 27 operational cells. 22Mathematical representation of The M 3 C The dynamics of the M 3 C can be represented by a decoupled model obtained using the double αβ0 transformation [3, 4]. This procedure enables a decoupled representation of the voltages-currents and power-capacitor voltages of the M 3 C as follows: 2.1 Voltage-current model of the M 3 C In (1), the representation of the M 3 C converter after applying the αβ0 transformation to both ports of the converter is presented. The model is represented by nine independent equations. Variables v xy , and its associated i xy x ∈ {α, β}, y ∈ {α, β}, represent internal cluster voltages of the converter and the so-called circulating Fig. 11 Modular multi-level matrix converter topology