Yingjie He

A Novel DC Voltage Control Method for STATCOM Based on Hybrid Multilevel H-Bridge Converter

This paper presents a transformerless static synchronous compensator (STATCOM) system based on hybrid multilevel H-bridge converter with delta configuration. This hybrid multilevel STATCOM is characterized by per-phase series connection of a high-voltage H-bridge converter operating at fundamental frequency and a low-voltage H-bridge converter operating at 5 kHz without any other circuit for dc voltage control. A new control strategy is proposed in this paper with focus on dc voltage regulation. Clustered balancing control is realized by injecting a zero-sequence current to the delta-loop, while individual voltage control is achieved by adjusting the fundamental content of ac quasi-square-waveform voltage of high-voltage converter. A downscaled experimental prototype rated at 100 V and 3 kVA is constructed in authors laboratory. Experimental results show that the hybrid multilevel STATCOM performs satisfactory not only improving efficiency and waveform quality, but also compensating reactive power and negative-sequence current while maintaining dc voltage at the given value.

An Improved Repetitive Control for Active Power Filters with Three-Level NPC Inverter

Due to the difficulty of handling nonlinear loads for traditional active power filter(APF) with two-level inverter in high voltage high power grid, this paper researches three-level active power filters. This paper presents an improved repetitive scheme to control the shape of ouput current waveform. The design process of each part in the repetitive controller is presented. Q(z) is designed as a notch zero phase shift filter which degrade the high order harmonic severely with close-to-unit gain at low and medium frequency band. The revised filter, the phase compensator and the repetitive gain is also given. The control method has fast dynamic convergence rate and little static misadjustment. The experiment results illustrate that the performance of the proposed approach is satisfactory.

Deadbeat Control with a Repetitive Predictor for Three-Level Active Power Filters

Three-level NPC inverters have been put into practical use for years especially in high voltage high power grids. This paper researches three-level active power filters (APFs). In this paper a mathematical model in the d-q coordinates is presented for 3-phase 3-wire NPC APFs. The deadbeat control scheme is obtained by using state equations. Canceling the delay of one sampling period and providing the predictive value of the harmonic current is a key problem of the deadbeat control. Based on this deadbeat control, the predictive output current value is obtained by the state observer. The delay of one sampling period is remedied in this digital control system by the state observer. The predictive harmonic command current value is obtained by the repetitive predictor synchronously. The repetitive predictor can achieve a better prediction of the harmonic current with the same sampling frequency, thus improving the overall performance of the system. The experiment results indicate that the steady-state accuracy and the dynamic response are both satisfying when the proposed control scheme is implemented.

An novel deadbeat control method for cctive power filters with three-level NPC inverter

Three-level NPC inverter has been put into practical use for years especially in high voltage high power grid. This paper researches three-level active power filters(APF). In this paper the mathematical model in d-q coordinates is presented for 3-phase 3-wire APFs. The dead-beat control scheme is obtained by using the state equations. Canceling the delay of one sampling period and providing the predictive value of the harmonic current is key problem of the deadbeat control. Based on this deadbeat control, the predictive output current value is obtained by the state predictor. The delay of one sampling period is remedied in this digital control system by the state predictor. The predictive harmonic command current value is obtained by the repetitive predictor synchronously. The repetitive predictor can achieve better prediction of the harmonic current with same sampling frequency, thus improves the overall performance of the system. The experiment results indicate that the steady-state accuracy and dynamic response are both satisfying when the proposed control scheme is implemented.

Research on control system of DC voltage for active power filters with three-level NPC inverter

Due to the limitation of voltage capability and current capability of power devices, it is very difficult to handle nonlinear loads for traditional active power filters(APF) with two-level inverter in high voltage high power grid. Three-level NPC inverter has been put into practical use for years especially in high voltage high power grid. The application of such converter to APFs is hindered by the problem of voltage unbalance of DC capacitors that leads to system instability. This paper presents a close-loop digital control algorithm of DC voltage for this APF. The PI controller regulates DC voltage in the outer-loop controller. In the current-loop controller, this paper researches the issues of neutral-point voltage control from the space vector PWM (SVPWM) point of view in detail. Then this paper proposes a simple neutral-point voltage control method. The experiment results illustrate that the performance of the proposed approach is satisfactory.

A Comprehensive Study on Equivalent Modulation Waveforms of the SVM Sequence for Three-Level Inverters

According to the number of segments, the space vector modulation (SVM) can divide into conventional SVM sequence (CSVMS) and special SVM sequence (SSVMS). It is well known that SVM can be equivalently realized by carrier-based pulse width modulation (CBPWM) with a zero component injection in both two-level and three-level inverters. For eight segment CSVMS in three-level inverters, many papers have ever got the injected zero component equation. But similar approach does not work for SSVMS, which has more than eight segments, e.g., 10, 12, or 14. What more, the accurate injected zero component has never been achieved so far. To solve above problems, therefore, a generated decomposition method of modulation wave has been proposed in this paper. By strict theoretical derivation, the same correlation can be extended to SSVMS and the corresponding zero component expression can be acquired. Based on this method, this paper puts a lot attention on the essential feature explanation, mathematical proof, and gives a detailed process to get the zero component equation, using it can realize SSVMS by CBPWM. The simulation and experimental results illustrate that the generated decomposition method of modulation wave is correct, and the unified theory of CBPWM and SVM is verified.

Control strategy study of STATCOM based on cascaded PWM H-bridge converter with delta configuration

This paper describes a transformerless static synchronous compensator (STATCOM) system based on cascaded pulse-width-modulation (PWM) H-Bridge converter with delta configuration. It is intend for dynamically compensating reactive power and negative-sequence load current, as well as for improving power quality and reliability of power distribution system. In this paper, attention is paid to reactive-power control, negative-sequence current compensation, along with voltage-balancing control consisting of clustered balancing control and individual balancing control. Clustered balancing control that keeps the dc voltage of each cluster equal to the dc mean value of all the three clusters is achieved by introducing a zero-sequence current flowing through the loop of the three delta-connected clusters in three phases. The aim of this control scheme is to fully extend power-quality- improving capability of STATCOM system. A 100-V 3-kVA downscaled laboratory system is designed, constructed and tested, using large-capacity electrolytic capacitors as dc-voltage sources. Experimental results obtained from the laboratory system verify the viability of this cascaded multilevel PWM STATCOM system.

A PI Control Algorithm with Zero Static Misadjustment for Tracking the Harmonic Current of Three-Level APFs

Tracking harmonic current quickly and precisely is one of the keys to designing active power filters (APF). In the past, the current state feedback decoupling PI control was an effective means for three-phase systems in the current control of constant voltage constant frequency inverters and high frequency PWM reversible rectifiers. This paper analyzes in detail the limitation of the conventional PI conditioner in the APF application field and presents a novel PI control method. Canceling the delay of one sampling period and the misadjustment for tracking the harmonic current is the key problem of this PI control. In this PI control, the predictive output current value is obtained by a state observer. The delay of one sampling period is remedied in this digital control system by the state observer. The predictive harmonic command current value is obtained by a repetitive predictor synchronously. The repetitive predictor can achieve better predictions of the harmonic current. By this means, the misadjustment of the conventional PI control for tracking the harmonic current is cancelled. The experiment results with a three-level NPC APF indicate that the steady-state accuracy and dynamic response of this method are satisfying when the proposed control scheme is implemented.

Theoretical Analysis and Control of DC Neutral-point Voltage Balance of Three-level Inverters in Active Power Filters

In recent years, multilevel technology has become an effective and practical solution in the field of moderate and high voltage applications. This paper discusses an APF with a three-level NPC inverter. Obviously, the application of such converter to APFs is hindered by the problem of the voltage unbalance of DC capacitors, which leads to system instability. This paper comprehensively analyzes the theoretical limitations of the neutral-point voltage balancing problem for tracking different harmonic currents utilizing current switching functions from the space vector PWM（SVPWM）point of view. The fluctuation of the neutral point caused by the load currents of certain order harmonic frequency is reported and quantified. Furthermore, this paper presents a close-loop digital control algorithm of the DC voltage for this APF. A PI controller regulates the DC voltage in the outer-loop controller. In the current-loop controller, this paper proposes a simple neutral-point voltage control method. The neutral-point voltage imbalance is restrained by selecting small vectors that will move the neutral-point voltage in the direction opposite the direction of the unbalance. The experiment results illustrate that the performance of the proposed approach is satisfactory.

A design method of hybrid cascade multilevel structure for active power filter application in moderate-voltage grid

Nowadays, hybrid cascade multilevel technology has attracted a increasing concern, especially in moderate-voltage and high-power applications. In this pa-per, a design method about how to design main circuits of active power filters(APF) with hybrid cascade multilevel structure is proposed. Through careful research, the combination of these cascade multilevel H-bridge modules is properly chosen and the number of high-frequency modules is reasonably selected. Simulation results validate the rightness of the proposed method.