Rongxiang Zhao

Objective-Oriented Power Quality Compensation of Multifunctional Grid-Tied Inverters and Its Application in Microgrids

Multifunctional grid-tied inverters (MFGTIs) have been paid much attention to handle the commonly concerned power quality issues of the microgrids. An MFGTI can not only interface the renewable energy resource into the utility grid, but also can compensate the harmonic and reactive current in the microgrid as an auxiliary service. However, the apparent capacity of an MFGTI for power quality compensation is limited. Therefore, how to enhance the power quality of the microgrid by optimal utilization of the limited and valuable capacity becomes a technical challenge. In this paper, two optimal control objectives of MFGTIs are presented based on a comprehensive power quality evaluation algorithm by means of analytic hierarchy process theory. One objective aims to obtain the expected power quality using minimal apparent capacity of the MFGTI. Another objective focuses on enhancing the power quality as well as possible in the given available apparent capacity condition. The two proposed strategies are compared in performance, and the paper also discusses how to use them in practice for the best performance. Experimental results performed on a microgrid in the laboratory confirm the validation and feasibility of the proposed optimal control strategies.

Simple passive lossless snubber for high-power multilevel inverters

A passive lossless snubber circuit for multilevel inverters is proposed in this paper. The topology is simple and requires no extra control circuit. In order to reduce the high-voltage stress on power switches with this snubber circuit, an improved snubber circuit is presented by adding separate low-power direct current voltage sources into the original one. The operating principles and design considerations are described in detail in this paper. A prototype of a three-phase three-level diode-clamped inverter with the improved passive lossless snubber is built and tested. The simulation and experimental results indicate that not only can it realize the soft switching operation of the three-level inverter with low-voltage stress but also the topology and the control are simple.

Modeling, Modulation, and Control of the Three-Phase Four-Switch PWM Rectifier Under Balanced Voltage

The modeling, modulation, and control of the three-phase four-switch (TPFS) PWM rectifier are investigated in this paper. Three space vector pulse width modulation methods using different equivalent zero vectors are developed, where sector identification and the trigonometric function are not required. Then, the high-frequency model for the current ripple analysis is proposed, and the effects of three SVM approaches on the ac current ripple are investigated. According to the analytical results, the method introducing the smallest current ripple is selected. With the optimized SVM approach, a control-oriented model, considering the capacitor voltage oscillation and deviation, is built in the dq synchronous frame to facilitate the controller design. Furthermore, a control strategy implementing the proportional controller is developed to eliminate the capacitor voltage deviation. Meanwhile, the dual-loop control of the TPFS is not affected by the proposed strategy as the capacitor voltage deviation is eliminated. Finally, a novel linear modulation index function is defined to reject the low-frequency harmonic current introduced by the overmodulation. Experimental results demonstrate that excellent current performance is achieved with comprehensive considerations of the modeling, modulation, and control strategy.

The Comprehensive Design and Optimization of the Post-Fault Grid-Connected Three-Phase PWM Rectifier

The comprehensive design and optimization, including the optimized modulation approach, detailed modeling, and reliable control algorithm, is presented in this paper to guarantee the stable operation of the postfault three-phase pulsewidth modulation (PWM) rectifier. The effects of the space vector modulation approaches on the capacitor current are investigated. Based on the analytical results, the method introducing the smaller capacitor current is selected as the optimized modulation approach, which is of paramount importance to avoid the permanent failure of the dc-link capacitors. Then, with the optimized modulation approach, a control-oriented model for the post-fault PWM rectifiers is derived based on the α - β stationary frame, where the capacitor voltage oscillation and deviation are taken into account. The proposed model-based controllers do not require the phase-locked loop (PLL), and the elimination of the PLL means that the post-fault rectifiers can never lose synchronization with the grid. Furthermore, an equation of the required dc voltage for linear modulation is proposed, and using the proposed equation, a correct dc voltage is selected to reject the overmodulation. The experimental results validate the effectiveness of the proposed comprehensive design and optimization. Finally, a conclusion can be drawn from the experimental results-that the dc-link capacitor current and the linear modulation should be taken into account when selecting the dc voltage in the post-fault operation.

Hybrid Space Vector Modulation Strategy for Torque Ripple Minimization in Three-Phase Four-Switch Inverter-Fed PMSM Drives

Three-phase four-switch (TPFS) inverters are generally applied as cost-reduction topologies for permanent-magnet synchronous motor (PMSM) drives because of their reduced number of switching devices. However, undesirable torque ripples are produced by the inverter-fed PMSMs due to the application of nonsinusoidal voltages. Because the torque ripples are strongly influenced by the employed pulse width modulation (PWM) strategy, two commonly used switching sequences in TPFS inverter-fed PMSM drives are fully investigated based on the root mean square value of the torque ripples, in which the effects of the different equivalent zero vectors on the torque ripples are presented. Then, a hybrid space vector modulation (SVM) strategy is proposed to minimize the torque ripples by alternatively using the two equivalent zero vector synthesis approaches during a fundamental period. The sector division of the proposed hybrid SVM strategy is determined by the location of the stator current vector, which is quite different from the methods used in other SVM methods. Then, a simplified sector identification method is proposed to reduce the computational burden. The experimental results demonstrate that the proposed hybrid PWM strategy can effectively reduce torque ripples in TPFS inverter-fed PMSM drives.

A novel control strategy for input-parallel-output-series inverter system

This paper presents the topology structure and control method for input-parallel-output-series (IPOS) inverter system which is suitable for high input current, high output voltage and high power applications. In order to ensure the normal operation of IPOS inverter system, the control method should achieve input current sharing (ICS) and output voltage sharing (OVS) among constituent modules. Through the analysis in this paper, input current sharing is automatically achieved as long as output voltage sharing is controlled. IPOS inverter system is controlled by three-loop control which is composed of outer common output voltage loop, inner current loops and voltage sharing loops. Simulation results show that this control strategy can achieve low total harmonic distortion (THD) in the system output voltage, fast dynamic response and good output voltage sharing performance.

Performance Analysis of the Zero-Voltage Vector Distribution in Three-Phase Four-Switch Converter Using a Space Vector Approach

The ac current ripple, the common voltage (CMV), and the current stress on the dc-link capacitor are defined as the critical performance indicators of the three-phase four-switch (TPFS) converter, and the effect of the zero-voltage vector distribution on these crucial indicators is investigated comprehensively in the proposed paper. A unified space vector modulation (SVM) approach, including the special pulsewidth-modulated (PWM) signal logic, is developed to facilitate the performance analysis of the zero-voltage vector distribution. Then, a novel approach based on the nonorthogonal stationary frame is proposed to facilitate the sector identification. Furthermore, because of the asymmetry of the harmonics of the three-phase output voltages, an overall current performance indicator, the total three-phase ac current ripple, is defined to assess the overall performance of the three-phase ac current. In addition, the dc-link capacitor current and CMV per sampling period are investigated; different ineffective duty ratio allocations result in the varied CMVs and capacitor currents. Moreover, due to the complexity of the Fourier analysis method, the root-mean-square (rms) values of the crucial indicators are adopted to evaluate the relative merits of various zero-voltage vector distributions. The proposed rms value equations are straightforward for the evaluation of the effect of the zero-voltage vector distribution on these crucial indicators. Experimental results confirm the accuracy and usefulness of the proposed performance analysis.

Single-phase virtual synchronous generator for distributed energy sources

Virtual synchronous generator (VSG) in single-phase to interface distributed renewable energy is investigated in this paper. Mathematical models and numerical analysis are utilized to illustrate the features of the VSG. Enhanced control strategy is presented to ensure the performance of the VSG. Besides, a second order generalized integer (SOGI) is employed to calculate the instantaneous output power of the VSG in virtual αβ frame. By the means of a phase-locked loop based scheme, the VSG can seamlessly transform between islanded and grid-tied modes, which can meet the requirements of micro-grid. At last, the validity and effectiveness of the proposed approach are verified by the simulated results using PSCAD/EMTDC.

A Capacitor Voltage Balancing Method With Fundamental Sorting Frequency for Modular Multilevel Converters Under Staircase Modulation

A fundamental frequency-sorting algorithm with staircase modulation is proposed to balance the floating capacitors for modular multilevel converters. The driving pulses are assigned to the submodules at every fundamental period according to their charging capabilities for the capacitors. The charging capabilities of the driving pulses can be evaluated by sorting the voltage increments of the capacitors or derived from the symmetrical characteristic of the curve between the voltage increments and the pulse numbers. With this method, all the power devices switch only once per fundamental period, which is suitable for high-power applications. Meanwhile, the sorting frequency decreases to the fundamental frequency. Hence, a large number of calculation resources can be saved. Moreover, it does not need to measure the arm currents so that several current sensors can be saved and the communication protocol between the central and local controllers can be simplified. At last, a three-phase simulation platform with 20 submodules per arm and a down-scaled experimental prototype with eight submodules in each arm are built to validate the proposed voltage-balancing method.

Wireless parallel control of three-phase inverters based on virtual synchronous generator theory

In this paper, virtual synchronous generator (VSG) theory is investigated and utilized to control parallel inverters in a micro-grid, which is capable of sharing the local loads without communication lines. The mathematical model of VSG is built to describe the electromagnetic and mechanical characteristics of a synchronous generator. To make the model suitable for parallel inverters application, droop control is embedded to ensure the active and reactive power regulations. A small signal model is built to analyze dynamic and steady property of inverters. As a result, different VSGs with different parameters can share the power of local loads according to their capacities, and can meet the performance of different response time. At last, the validity and effectiveness of the proposed approach are verified by the simulated results using PSCAD/EMTDC.