AC-AC Modular Multilevel Converter Research Articles

Control method of an AC-AC MMC for open-end stator winding wind turbine generation system

With the increasing of power rating and voltage level of wind turbine, the traditaional low voltage converter is being replaced by multilevel converter. This paper proposes a new ac-ac modular multi-level converter (MMC) connecting the open-end stator winding permanent magnet synchronous generator (OW-PMSG) to AC power grid. The working principle of the proposed topology is analyzed. Under different grid conditions, a unified capacitor voltage balance control method and a current control method are proposed. Aiming at the low grid voltage ride through problem, a distributed braking chopper circuit and the corresponding control strategy is given. Simulation is conducted for verification. Compared with the back to back (BTB) MMC for OW-PMSG, the number of capacitors and arm inductors of the proposed topology are reduced by 43% and 33%. Moreover, the number of the state variables of the average capacitor voltages and currents of the proposed topology are reduced by 33% and 47%, reducing the control complexity. The proposed topology also has higher efficiency than the BTB MMC for OW-PMSG. Compared with the other ac-ac MMC topologies for OW-PMSG, the combined power rating of the power devices of the proposed topology is much lower and thus the proposed topology has higher power density.

Operation and control schemes of a novel direct AC-AC modular multilevel converter

AC-DC-AC modular multilevel converter (MMC) and direct AC-AC MMC are two kinds of interfaces to integrating AC motors into power grid, and they are easily damaged from large capacitor voltage fluctuation with varying motor speed. Up to now, none of the methods could balance the reliability and power density of MMC-based motor drives system. To find a better solution to this problem, a novel direct AC-AC MMC topology is proposed in this paper. Operating principle of this topology is introduced and relative multi-loop feedback control method is designed for it. Then, simulation and experiments are conducted for verification. Compared with the traditional MMC with high-frequency injection method, the proposed topology has no common-mode voltage problem and the current stress is reduced by 38.5%. Compared with the half-bridge-based AC-DC-AC MMC, the number of capacitors and inductors of the proposed topology is reduced by 20.8% and 25%. In comparison with the direct AC-AC MMC (modular multilevel matrix converter), the number of power devices of the proposed topology is reduced by 28%. The proposed topology has higher reliability and power density.

A Single-Phase Five-Branch Direct AC–AC Modular Multilevel Converter for Railway Power Conditioning

This paper introduces a single-phase five-branch direct ac-ac modular multilevel converter (MMC) with the capability of equal frequency conversion for railway power conditioning applications. By analyzing the branch energy balance mechanism, the basic operating principle of the proposed MMC is clarified and the control strategy is also proposed. Comparisons between the proposed MMC and the traditional modular multilevel matrix converter (MMMC) on branch voltage and current ratings as well as capacitor dimension requirements are also presented. The effectiveness of the proposed topology and the control strategy is validated on a down-scaled prototype. Both the theoretical analyses and the experimental results reveal that for compensating low power factor traction loads in Scott transformer-based railway power systems, the proposed MMC has decreased branch current stresses and capacitor voltage fluctuations compared to the traditional MMMC, and the required total submodule numbers of these two MMCs are comparable.

Full Degree of Freedom Based Control Scheme of the Single-Phase Direct AC–AC Modular Multilevel Converter for Railway Power Conditioning Under Asymmetric Branch Conditions

This paper conducts a thorough analysis on the circuit dynamics of the single-phase direct ac-ac modular multilevel converter (MMC) to address the branch energy divergence problem in the railway power conditioning application scenario. Based on the analysis, a full degree of freedom (DOF) based control scheme is proposed. By controlling all the DOFs in a coordinated manner, extended branch power controllability can be achieved, which enables each branch's energy to be regulated individually without deteriorating the terminal currents. As a result, the control flexibility and reliability of the MMC can be effectively enhanced in the presence of asymmetric branch conditions. The proposed control scheme is implemented in a cascaded control structure with well-established algorithms. The frequency-domain stability analysis is also presented to guide controller parameter design. The effectiveness of the proposed control strategy is validated through the experiments under various conditions.

Predictive Control of AC–AC Modular Multilevel Converters

Multilevel converters can reach medium-voltage operation increasing the efficiency of high-power applications. Among the existing multilevel converter topologies, the modular multilevel converter (MMC) provides the advantages of high modularity, availability, and high power quality. Moreover, the main advantage compared to cascaded multilevel converters is the lack of an input transformer which results in a reduction of cooling requirements, size, and cost. One of the drawbacks of this topology when used as an ac-ac converter is the input and output frequency components in the control loop, resulting in a more complex controller design. In this paper, a single-phase ac-ac MMC predictive control approach is proposed. The controller minimizes the input, output, and circulating current errors and balances the dc voltages. Experimental results show the performance of the proposed predictive control scheme.