Zixin Li

An Improved Pulse Width Modulation Method for Chopper-Cell-Based Modular Multilevel Converters

In this paper, an improved pulse width modulation (PWM) method for chopper-cell (or half-bridge)based modular multilevel converters (MMCs) is proposed. This method can generate an output voltage with maximally 2N+1 (where N is the number of submodules in the upper or lower arm of MMC) levels, which is as great as that of the carrier-phase-shifted PWM (CPSPWM) method. However, no phase-shifted carrier is needed. Compared with the existing submodule unified pulse width modulated (SUPWM) method, the level number of the output voltage is almost doubled and the height of the step in the staircase voltage is reduced by 50%. Meanwhile, the equivalent switching frequency in the output voltage is twice that of the conventional SUPWM method. All these features lead to much reduced harmonic content in the output voltage. What is more, the voltages of the submodule capacitors can be well balanced without any close-loop voltage balancing controllers which are mandatory in the CPSPWM schemes. Simulation and experimental results on a MMC-based inverter show validity of the proposed method.

A novel inner current suppressing method for modular multilevel converters

Ideally, the inner (the upper or lower arm) current of a modular multilevel converter (MMC) is ideally assumed to be the sum of a dc component and an ac component of the fundamental frequency. However, as ac current flows through the submodule (SM) capacitors, the capacitor voltages fluctuate with time. Consequently, the inner current is usually distorted and the peak/RMS value of it is increased compared with the theoretical value. The increased currents will increase power losses and may threaten the safe operation of the power devices and capacitors. This paper proposes a closed-loop method for suppression of the inner current in an MMC. This method is very simple and is implemented in a stationary frame, and no harmonic extraction algorithm is needed. Hence, it can be applied to single-phase or three-phase MMCs. Besides, this method does not influence the balancing of the SM capacitor voltages. Simulation and experimental results show that the proposed method can suppress the peak and RMS values of the inner currents dramatically.Ideally, the inner (the upper or lower arm) current of a modular multilevel converter (MMC) is assumed to be the sum of a dc component and an ac component of the fundamental frequency. However, this current is usually distorted and the peak/RMS value of it is increased compared with the theoretical result. This is because ac current flows through the submodule (SM) capacitors and the capacitor voltages fluctuate with time. The increased currents will increase power losses and may threaten the safe operation of the power devices and capacitors. This paper proposes a novel close-loop method for suppression of the inner current in MMC. This method is very simple and is implemented in stationary frame, and no harmonic extraction algorithm is needed. Hence, it can be applied to single-phase or three-phase MMC. What is more important, this method does not influence the balancing of the SM capacitor voltages. Simulation and experimental results show that the proposed method can suppress the peak and RMS values of the inner currents dramatically. Meanwhile, the harmonic contents in the output current can also be suppressed satisfactorily even when the SM capacitor voltage ripple factor is as large as about ±19%. Therefore, the proposed method can also be adopted to reduce the SM capacitance requirement.

Single-Loop Digital Control of High-Power 400-Hz Ground Power Unit for Airplanes

In this paper, the influence of one-sample delay for sampling and computation in digital control on the bandwidth of the inner current loop of a 400-Hz ground power unit (GPU) is analyzed first. The results show that it is difficult and even impossible for high-power 400-Hz GPUs to maintain low total harmonic distortion content in the output voltage with the conventional proportional-integral-based double-loop control. To improve the performance, resonant controllers with parallel structure which are widely used in active power filters are applied to the single-loop control of the 400-Hz GPU. The parameter design criterion for the parallel resonant controllers is discussed in the discrete time domain. Meanwhile, adoption of proportional gain in the single-loop control is investigated. The results show that it can improve the performance little and may cause instability problems. Comparisons between different control methods for the 400-Hz GPU are also made, and the single-loop control method in this paper seems to be the most suitable one in terms of simplicity and performance. Experiments on a 16-b fixed-point DSP-controlled 90-kVA 400-Hz GPU prototype show satisfactory results of the single-loop method feeding linear/nonlinear and balanced/unbalanced loads.

Control of Three-Phase Boost-Type PWM Rectifier in Stationary Frame Under Unbalanced Input Voltage

In the last decade, many methods for control of three-phase boost-type pulsewidth modulation (PWM) rectifier under unbalanced input voltage conditions have been studied and presented. These methods use the sequential components of input voltages, pole voltages, and input currents of the rectifier to analyze and control the instantaneous powers, of which dual (positive-sequence and negative-sequence rotating) frame control is the most common structure. Anyway, this paper analyzes the instantaneous powers of the PWM rectifier in two-phase stationary frame. Based on this analysis, the input-power-control, input-output-power-control, and output-power-control methods for PWM rectifier under unbalanced voltage conditions are proposed in single stationary frame. Compared with the existing methods, simplicity may be the major advantage of the method in this paper. Rotating transformation and phase detection of the input voltages are both eliminated. Moreover, sequential component extraction of the control variables is also not necessary, leading to much reduced time delay to the control system. Experimental results on a 9-kVA PWM rectifier show validity and effectiveness of the proposed methods.

A Novel Single-Phase Five-Level Inverter With Coupled Inductors

In this paper, a novel single-phase five-level inverter is proposed using coupled inductors. This inverter can output five-level voltage with only one dc source. No split of the dc voltage capacitor is needed, totally avoiding the voltage balancing problem in conventional multilevel inverters. The level of the output voltage is only half of the dc-link voltage in all conditions, leading to much reduced dv/dt . This inverter is based on the widely used three-arm power module and the voltage stresses on all the power switches are the same, making it very easy to construct. Operation mechanism of this inverter is analyzed and the possible switching patterns are investigated. Based on these analyses, a novel optimized modulation scheme is presented. With this modulation method, no dc components exist in the inductor currents, which is very helpful for minimization of the inductors. Simulation and experimental results show the validity of the proposed inverter together with the optimized modulation scheme.

Power Module Capacitor Voltage Balancing Method for a ±350-kV/1000-MW Modular Multilevel Converter

This letter focuses on the power module (PM) capacitor voltage balancing of a

Design of Power Electronic Transformer Based on Modular Multilevel Converter

\pm 350

A three-phase 10 kVAC-750 VDC power electronic transformer for smart distribution grid

-kV/1000-MW modular multilevel converter (MMC). For this MMC, the current flowing through the IGBTs will exceed the nominal current of the adopted commercial high-voltage (3.3 kV or higher) IGBT products, i.e., 1500 A. As for the PM voltage balancing of this MMC, the switching frequency must be kept low to reduce the losses and the accurate calculation of the PM switching frequency is of great importance to the design of the cooling system. This letter presented a low-switching frequency PM balancing method by introducing the balancing adjusting number (BAN). The PM switching frequency calculation method is also presented with different BANs. Computer simulation and the real-time control hardware in the loop test results based on RT-LAB show the effectiveness of the proposed PM balancing method and the accuracy of the switching frequency calculation method under the rated power.

A new digital control method for high performance 400 Hz ground power unit

Power electronic transformer acts as a key component of the future automation in smart grid, which is the replacement of conventional distribution transformers by a power transformer. In this paper , the optimum design of a power electronic transformer(PET) based on topology of Modular multilevel converter (M2C). M2C is an emerging and highly attractive topology intended for high- or medium-voltage power electronic devices. In the design process, the modulation scheme of M2C is analyzed. A control method for balancing the M2C DC buses has been introduced. The PET includes a dual-active-bridge DC/DC converter. The DC-DC converter realizes magnetic isolation of PET and reduces the transformer size. Meanwhile, both primary-side and secondary-side switches can achieve the zero-voltage-switching(ZVS) .The PET performs power transformers functions, at the same time, which has advantages such as power factor correction, elimination of voltage sag and swell, and reduction of voltage flicker in load side.

A new single-phase five-level inverter with no problem of voltage balancing

In this paper, a power electronic transformer (PET) topology for distribution grid is presented based on modular multilevel converter (MMC). The presented topology is of three-phase type and have two dc voltage terminals, i.e. the high-voltage dc terminal and the low-voltage dc terminal. So, it can be connected directly to high-voltage direct current (HVDC) systems and renewable energy systems simultaneously. This feature makes it possible to perform as an “energy router” in smart grids. What is more important, the proposed PET can substantially reduce (by 44.4% for the studied case) the number of medium frequency transformers (MFTs), compared with the existing solutions. Computer simulation and experimental results on a 10 kV prototype show validity of this PET.