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This paper explores the issue of the neutral-point oscillation in the four-leg neutral-point clamped inverter. Such an issue is closely associated with the neutral-point current, so this study models the inverter on the dc side to clarify its relationship and presents a three-dimension pulsewidth modulation to eliminate the neutral-point oscillation. In addition, an offset-injection approach to reducing the neutral-point oscillation is also investigated and discussed. The laboratory test results will validate the analysis of the proposed approach, and a discussion will be presented to evaluate the effectiveness of eliminating the neutral-point oscillation.

The neutral-point voltage-balancing control is a critical issue of a neutral-point-clamped converter. The conventional control strategy is to inject the specific zero-sequence voltage to regulate the redundant vector, but it is unacceptable to the three-phase four-wire system. This paper provides a neutral-point voltage-balancing control by the dipolar modulation, and the control freedom of the zero-sequence voltage is flexible. Therefore, the proposed method is suitable for both three-phase three-wire and three-phase four-wire systems. Besides, the proposed method improves the control dynamic in a reactive power system. The proposed neutral-point voltage-balancing control is verified by the mathematical analysis and experimental results. The control dynamic, power loss estimation, and the output current performance are analyzed in this paper.

Common-mode voltage output is closely associated with switching states in three-phase three-wire neutral point clamped converter. Detailed analysis shows avoiding redundant states can effectively improve the common-mode voltage, so this paper proposes a pulse width modulation (PWM) technique with zero redundant state to reduce the common-mode voltage output, and evaluates their performance with the conventional PWM schemes. This paper also takes the compensation of neutral point potential deviation into consideration. Laboratory test results are presented to verify the effectiveness of the proposed scheme.

As more distributed renewable energy sources are installed in the utility grid, the static synchronous compensators are applied to manage the power factor and the grid voltage in the medium-voltage level of the power system. The cascaded multilevel converter with single-star topology is well applied to this application. The dc-capacitor voltage balancing control is a fundamental issue, and it can be accomplished by negative-sequence current and/or zero sequence voltage injection. Unfortunately, the zero sequence voltage injection increases the risk of over-modulation and the negative-sequence current injection results in high peak current. This paper provides the voltage balancing strategy to manage the peak current or the modulation index. Besides, all the asymmetrical grid voltages are considered in the proposed method to satisfy the grid fault operation. Laboratory test results verify that the proposed method limits the peak current or manages the modulation index during the grid fault operations.

This paper presents the flexible dc capacitor voltages balancing control method for star-connected cascaded H-bridge converters. In this paper, the power flows are analyzed by investigating all the converter's control freedoms. Based on the analyses, users can accomplish the dc capacitor voltage balancing with both negative-sequence current and zero-sequence voltage. These two kinds of control options are further integrated together with the defined weighting factor $K$ . The low-voltage ride-through operation of a proposed control method is also discussed in this paper. The control method is verified with a 220-V 1-kVA static synchronous compensators based on the star-connected cascaded H-bridge converters, and the experimental test results show that all the dc voltages are well regulated at the commanded value with different weighting factor $K$ .

The low-voltage ride-through (LVRT) capability is a critical requirement of grid-connected converters of renewable energy resources to uphold the stability of the power system. The step-up transformer that connects the power converter to the grid is exposed to sudden voltage changes throughout the LVRT, and the resulting magnetic flux deviation often leads to severe inrush current when the grid fault clears. The inrush current exerts significant mechanical forces in the transformer and damages its insulation. This paper proposes a control method of the power converter to reduce the risk of inrush current by mitigating the magnetic flux deviation during the LVRT operation. The proposed method also manages the output current of the power converter within the capacity of semiconductor devices to reduce their stress while performing the proposed flux compensation and other LVRT operations. The reliability of the power converter and the transformer can be improved by the proposed control method.

The LCL -based filter has been widely applied to mitigate the size of the inductor in the high-power converter, but it usually leads to resonance in the system. Therefore, an active damping technique based on the virtual resistor is provided in this paper for the LCL -filter system. The literature papers only suppressed the resonance of the LCL -filter and focused on the stability of the internal current control loop with an inductive grid impedance. Therefore, these previous stability analysis methods cannot be suitable for offshore wind farm applications due to the multiple resonance frequency characteristic of the transmission cable and multiparalleled converters. Therefore, this paper analyzes the control stability with both of the internal resonance ( LCL -filter) and external resonance (between the grid impedance and current controller). Besides, the behavior of multiparalleled converter and the multiple resonance frequency of long transmission cable are discussed and analyzed for the offshore wind farm applications. Finally, the laboratory and simulation results are for the proposed method verification.

The LCL-filter is one of the methods to reduce the size of the filter inductor but it leads to resonance in the system. The resonance usually results in unstable system and triggers the over-current protection. An active damping technique based on the virtual resistor is provided in this paper. The concept and the mathematical analysis of the proposed method are explained, and then the stability of the single converter operation and multiple grid-connected converter operation are analysed. Moreover, the simulation and the laboratory experiment results are for the verification.

With more and more distributed energy resources being installed in the utility grid, grid operators start imposing the low-voltage ride-through requirement on such systems to remain grid-connected and inject reactive and/or active current to support grid voltage during fault conditions. This paper proposes a positive and negative sequence current injection method to meet such requirement. In the meantime, the proposed method can reduce the second harmonic ripples in the dc link of the power converter without violating the peak current constraint of the power transistors. The long-term reliability of the converter system can thus be improved.

Common mode voltage output is closely associated with switching states in three-phase three-wire neutral-point clamped inverter. Detailed analysis shows avoiding redundant states can effectively improve the common mode voltage, so this paper proposes a pulse width modulation (PWM) technique with zero redundant states to reduce the common mode voltage output, and evaluate their performance with the conventional PWM scheme. This study also takes the issue of neutral point potential deviation compensation into consideration. Laboratory test results are presented to verify the effectiveness of proposed scheme.

This paper investigates different compensation methods for neutral point potential deviation based on modular multilevel single star neutral point clamped bridge converter. Theoretical analysis indicates injecting an offset for a cell can effectively decrease this deviation, so this research presents different offset-based compensation techniques, and evaluates their output performance. Besides, this research also presents a hierarchical voltage balancing control to efficiently manage the power flow among cells. Laboratory test results are provided to verify their effectiveness.

This paper presents the application of a Delta-Connected Cascaded H-Bridge Converter in unbalanced load compensation. The compensation includes positive sequence reactive power current and negative sequence current compensation. The controller designed based on the detail power flow analysis which can achieve dc capacitor voltage balancing control by zero sequence current injection even when compensating the negative sequence current. But zero sequence current injection may cause high peak phase current which may break converter switches. This paper also proposed Peak Current Limit Control (PCLC) in delta-connected cascaded converter. Laboratory and Simulation test results are provided to validate the proposed PCLC method.