Dewei David Xu

A Capacitor Voltage-Balancing Method for Nested Neutral Point Clamped (NNPC) Inverter

A capacitor voltage-balancing method for a nested neutral point clamped (NNPC) inverter is proposed in this paper. The NNPC inverter is a newly developed four-level voltage-source inverter for medium-voltage applications with properties such as operating over a wide range of voltages (2.4-7.2 kV) without the need for connecting power semiconductor in series and high-quality output voltage. The NNPC topology has two flying capacitors in each leg. In order to ensure that the inverter can operate normally and all switching devices share identical voltage stress, the voltage across each capacitor should be controlled and maintained at one-third of dc bus voltage. The proposed capacitor voltage-balancing method takes advantage of redundancy in phase switching states to control and balance flying capacitor voltages. Simple and effective logic tables are developed for the balancing control. The proposed method is easy to implement and needs very few computations. Moreover, the method is suitable for and easy to integrate with different pulse width modulation schemes. The effectiveness and feasibility of the proposed method is verified by simulation and experiment.

Control Strategy With a Generalized DC Current Balancing Method for Multimodule Current-Source Converter

Higher power applications require a multilevel converter to meet high power ratings. A multimodule current-source converter (CSC) can provide a multilevel current waveforms and at the same time eliminate the bulky transformer at the ac side. However, it is required for the multimodule CSC to use multiple dc-link inductors for the purpose of power balance among different modules. Thus, one key issue in the design of the multimodule CSC is to balance the current through different dc-link inductors. This paper focuses on studying the control strategy of the multimodule CSC and a generalized current balancing method is presented. As presented, zero vectors are distributed based on the deviation of the dc-link inductor currents and the comparison of the capacitor voltages. Therefore, the current imbalance problems, not only those among the upper inductors but also those among the lower inductors are solved. Moreover, since the current balancing algorithm is not dependent on module numbers, the presented control strategy is very suitable for modularization. Especially, with the feedback of the capacitor voltages, the resonance arising from the inductor-capacitor (LC) filter is damped. In addition, a design method of the dc-link inductors is also derived. Finally, simulation and experimental results show the validity of the propositions.

Reference-Trajectory-Optimized SVM for High-Power Current-Source Converters to Improve Harmonic Performance and Reduce Common-Mode Voltage

For medium-voltage high-power drives fed by current-source converters (CSCs), a transformerless configuration has benefits in both cost and volume. Removing the isolation transformer necessitates a common-mode choke to undertake the major portion of the drives common-mode voltage (CMV) stress, which would otherwise cause premature failure of the motor insulation system. On the other hand, as device switching frequency in high-power CSCs is normally limited to a few hundred hertz, improving harmonic performance via modulation has always been a challenge. Selective harmonic elimination (SHE) is a preferred modulation scheme in such a system owing to its ability to eliminate several unwanted low-order harmonic currents. Alternatively, conventional space vector modulation (SVM) provides continuous modulation index adjustment capability, but its output current contains low-order harmonics with high magnitudes that may arouse harmful resonances in the system. Aiming at reducing CMV and improving harmonic performance at the same time, this paper proposes a new SVM-based modulation strategy for high-power CSCs using synthesized reference trajectory on the hexagon in the αβ plane. Along with the proposed strategy, two methods of reference trajectory optimization (RTO) are investigated. By introducing a coefficient to the duty-cycle function for RTO, the first method can remove an extra low-order harmonic component, or minimize the weighted total harmonic distortion. The second method, with a different approach of RTO, eliminates the unwanted low-order harmonics by combining the proposed SVM with the SHE. The proposed concepts are verified by both simulation and experimental results.

Optimal Space Vector Sequence Investigation Based on Natural Sampling SVM for Medium-Voltage Current-Source Converter

The device switching frequency of current-source converters (CSCs) in high-power medium-voltage (MV) drive applications is normally around 500 Hz. Conventional space vector modulation (SVM) features fast dynamic response but its output contains low-order harmonics with high magnitudes. Recently, a natural-sampling-based SVM (NS-SVM) with superior low-order harmonics performance has been proposed for MV CSCs. This study further investigates the low-order harmonics performance of different space vector sequences for MV CSCs based on NS-SVM. Dwell times equations based on NS-SVM for each space vector sequence with minimized switching frequency are analyzed and derived; comparisons of different vector sequences in terms of low-order harmonics and load/grid-side total demand distortion performance are thoroughly conducted. Among all the investigated sequences, sequence 2 (SQ2) is the best one; its linear two-step dwell times equations are provided for online implementation. Simulations and experiments are provided to verify its performance.

Common-mode voltage reduction for medium-voltage current source converters by optimizing switching sequences

Common-mode voltages (CMVs) can lead to premature failure of the motor insulation system in medium-voltage (MV) current source drives. This paper proposes a novel space-vector modulation (SVM) based gating strategy that can significantly reduce the peak CMV by optimizing the switching sequences for MV current source converters (CSCs). In the proposed method, the switching sequence of one side of the CSC is fixed and the sequence of the other side is selected to produce the minimum peak CMV. The optimization process is based on the CMV calculation for all the possible switching sequence combinations. To validate the proposed concept, different SVM based control schemes for CSCs are investigated. The results show the effectiveness of the proposed concept. In addition, the proposed strategy can be easily implemented in software and requires no extra hardware. By using this method, the CMV stress of the motor is reduced significantly and the size/weight for the common-mode choke can also be reduced.

A Simple and Cost-effective Precharge Method for Modular Multilevel Converters by Using a Low-Voltage DC Source

A simple and cost-effective precharge method for modular multilevel converter (MMC) by using a low-voltage direct current (dc) source is proposed in this paper. The submodule (SM) capacitors in MMC are required to be precharged to their nominal voltage values at start-up to ensure MMC work normally. Conventional methods perform the precharging either through grid side or dc side by using costly high-voltage bypass breakers, charging resistors or a high-voltage dc source. Voltage of the high-voltage dc source must be equal to SM capacitor voltage and could be as high as thousands of volts. By contrast, the proposed method employs a very low voltage dc source on dc bus through a series-connected blocking diode, and requires no charging resistor and bypass breakers. It takes advantages of the existing power devices, arm inductors, and SM capacitors in MMC, and configures them into boost circuit for the precharging. The proposed method is simple and cost-effective. Moreover, the characteristics of the boost circuit render the proposed method flexible. For instance, the voltage selection of the dc source is flexible and not restricted to a fixed value. The SM capacitors can be charged in different groups. In addition, the method is applicable to different MMC SMs, including SM with different capacitor voltages. The effectiveness of the proposed method is verified through simulation and experiment.

The Optimal PWM Modulation and Commutation Scheme for a Three-Phase Isolated Buck Matrix-Type Rectifier

This paper first starts with reviewing several commonly practiced PWM schemes for the three-phase isolated buck matrix-type rectifier. Then, an optimal six-segment PWM scheme (“Type A”) is proposed. The analysis shows “Type A” PWM scheme has lower duty-cycle loss, maximum output inductor current ripple, and minimum switching loss comparing with other PWM schemes when the mosfet devices are employed. In addition, a low input current total harmonic distortion (THD) can be achieved with duty-cycle compensation. Finally, the steady-state analysis of duty-cycle loss, inductor current ripple, and THD are all compared and verified by the experimental results for “Type A” PWM and eight-segment PWM (“Type E”).

Minimization of Filter Capacitor for Medium-Voltage Current-Source Converters Based on Natural Sampling SVM

Current-source converters (CSCs) used in high-power (megawatt level) medium-voltage (MV) (2.3–6.6 kV) drives require three-phase capacitors to assist the commutation of switching devices and to filter out harmonics of the converter output PWM current. Different types of modulation schemes with different harmonic performance lead to different capacitor selections. Conventional space vector modulation (SVM) featuring high dynamic performance cannot be used in practice even with the help of active damping, which can effectively damp the possible LC resonance introduced by conventional SVM. Recently, a natural-sampling SVM (NS-SVM) is proposed for MV CSC drives. It features superior low-order harmonics and has been highly considered for MV CSCs in practice. This study, therefore, focuses on two issues. One is why conventional SVM cannot be used in practice even with the help of active damping. On this basis, the second is capacitor minimization based on NS-SVM. Finally, experimental verification is provided.

A New Configuration Using PWM Current Source Converters in Low-Voltage Turbine-Based Wind Energy Conversion Systems

Current source converter (CSC)-based wind energy conversion systems (WECSs) are recently gaining increasingly attention due to its reliable short-circuit protection. However, all existing CSC-based configurations are proposed for medium-voltage (MV) (3–4 kV) wind turbine systems, but not for low-voltage (LV) (690 V) systems which are dominating the market of WECS. The biggest challenge for using CSCs in LV systems is no proper switching devices in the market. This paper proposes a new configuration, where a proven modular LV voltage source converter is used as the generator-side converter to be compatible with an MV CSC. By such a design, MV CSCs are successfully used in LV systems with all advantages being inherited. In addition, this configuration features high reliability and reduced size and weight, thanks to the adoption of the modular converter. The performance of the configuration under normal and fault conditions are investigated. Apart from fulfilling traditional control objectives in WECS, a current balancing control is added to ensure safe operation. Additionally, limits and constraints of the configuration using in practice are discussed. Finally, simulation and down-scaled experimental results are presented.