Jinjun Liu

A series active power filter adopting hybrid control approach

The series active power filter (APF) is suitable for compensating a voltage type harmonics-producing load, whereas the control approach it adopts may directly influence its compensation characteristics. This paper first discusses the control approach of detecting source current in terms of the basic operation principle of a series APF, then developing a control approach of detecting load voltage. On the basis of these, a hybrid control approach is proposed. In this approach, the reference signal of the compensation voltage needed by the series APF is obtained by detecting both source current and load voltage. Thus, this approach has the advantages of the first and the second control approaches and, at the same time, it can overcome their respective drawbacks. For practical realization, the control methods of the PWM inverter in the series APF and for regulating its DC side voltage are discussed in detail. A prototype of the series APF is manufactured and corresponding experimental investigation is done. The results show that when the series APF, if adopting the hybrid control approach instead of the other two, compensates for the voltage type harmonics-producing load, its performance can be improved greatly.

Strategies and Operating Point Optimization of STATCOM Control for Voltage Unbalance Mitigation in Three-Phase Three-Wire Systems

Reactive power control through the static compensator (STATCOM) has gained wide attentions due to its outstanding performance. But for a STATCOM with traditional control strategies, unbalanced utility voltages will greatly affect the performance of the STATCOM and, in severe cases, may even cause the shutdown of the STATCOM for overcurrent protection. This paper proposes novel control strategies to ensure normal operation of STATCOM in three-phase three-wire systems when severe system voltage unbalance occurs and, furthermore, to mitigate voltage unbalance at the point of common coupling (PCC) whether it is caused by the utility or load. The control laws, operation principles, compensation characteristics, and operating point optimization of the proposed control strategies are analyzed and compared in detail. The power flow in the STATCOM and corresponding dc-side voltage-control schemes for these control strategies are also introduced. It is shown that the proposed voltage-controlled current source (VCCS) strategy and modified voltage-controlled voltage source strategy are valid for voltage unbalance compensation, especially the VCCS strategy for low rating under the same performance. Finally, the simulation and experimental investigations were carried out with these three parameter-optimized control strategies, respectively, for light and heavy load conditions. The results verified that the current passing through the STATCOM is under control and the voltage at the PCC is more balanced than before compensation. The compensation performance of the STATCOM is thus proven satisfactory

Stability margin monitoring for DC distributed power systems via perturbation approaches

In multi-module power electronics systems, especially DC distributed power systems, the small-signal stability issues are often dealt with by employing Middlebrooks impedance criterion. However, for on-line system stability margin monitoring, directly measuring impedance of the source and load subsystems then making quantitative comparisons is too complicated and difficult. This paper proposes practical and simple methods, which involve applying current or voltage perturbation to the dc side of distributed power systems then only measuring the amplitude of two currents or voltages in order to monitor the stability margin. For both methods (current perturbation and voltage perturbation), an implementation approach that does not employ external voltage or current perturbation source is also presented. All these methods and approaches are equivalent to the impedance measuring and comparing method based on the impedance criterion with different forbidden regions. A comparative evaluation of each method and its corresponding implementation approach is provided.

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.

A Unified Control Strategy for Three-Phase Inverter in Distributed Generation

This paper presents a unified control strategy that enables both islanded and grid-tied operations of three-phase inverter in distributed generation, with no need for switching between two corresponding controllers or critical islanding detection. The proposed control strategy composes of an inner inductor current loop, and a novel voltage loop in the synchronous reference frame. The inverter is regulated as a current source just by the inner inductor current loop in grid-tied operation, and the voltage controller is automatically activated to regulate the load voltage upon the occurrence of islanding. Furthermore, the waveforms of the grid current in the grid-tied mode and the load voltage in the islanding mode are distorted under nonlinear local load with the conventional strategy. And this issue is addressed by proposing a unified load current feedforward in this paper. Additionally, this paper presents the detailed analysis and the parameter design of the control strategy. Finally, the effectiveness of the proposed control strategy is validated by the simulation and experimental results.

Indirect Current Control Based Seamless Transfer of Three-phase Inverter in Distributed Generation

The distributed generation (DG) should supply continuous power to the local critical load, even in the condition of utility outage. And many efforts have been made for the three-phase inverter in the DG to transfer between the islanding mode and the grid-tied mode seamlessly. This paper proposes a transfer strategy based on indirect current control for the three-phase inverter in the DG, which is composed mainly of two cascaded feedback loops, i.e., capacitor voltage loop and external grid current loop, in synchronous reference frame. In the grid-tied mode, the grid current is controlled by regulating the capacitor voltage. When islanding happens, the reference of the voltage loop is fixed by the limiter. As the local load is located in parallel with the filter capacitor in the proposed strategy, the load voltage is always controlled by the capacitor voltage loop directly, and the quality of the load voltage can be guaranteed. Besides, the performance of the capacitor voltage loop is further improved by introducing an inner capacitor current loop. Moreover, the steady state and transient state of the system, including the output of the limiter, are comprehensively analyzed, and then the compensators and limiters are designed. Finally, the proposed control strategy is verified by the simulation and experimental results.

A Study on DC Voltage Control for Chopper-Cell-Based Modular Multilevel Converters in D-STATCOM Application

This paper presents a three-phase transformerless structure of modular multilevel converters based on cascaded pulsewidth-modulated chopper cells in medium-voltage D-STATCOM application. It is suggested for installation on unbalanced medium-voltage power-supply systems to achieve higher performance in improving power quality. This paper proposes a new control strategy based on power-flow analysis with a focus on dc capacitor voltage control that is insulated from the source voltage fault. This control strategy enables the D-STATCOM system to compensate unbalanced nonlinear load even under asymmetrical grid fault conditions and it has no limitations on the cascaded number. Experimental results obtained from a 60-V 3-kVA downscaled prototype verify that this D-STATCOM system performs satisfactory, not only compensating for unbalanced nonlinear load but also keeping the capacitor voltages maintained at the given value even in unbalanced power systems.

Modeling, Analysis, and Mitigation of Load Neutral Point Voltage for Three-Phase Four-Leg Inverter

The three-phase four-leg inverter is very suitable for the high-power uninterruptible power supply application, and the load neutral point voltage (LNPV) should meet the requirement. In this paper, two factors influencing the LNPV were revealed first, which are the switching states and the ratio of the neutral inductance to the phase inductance (k). Then, the 16 switching states can be classified into four groups, and the ones in the first and the second group come to the minimum amplitude of LNPV with a specific ratio k respectively. Third, three methods to mitigate the LNPV were proposed based on the analysis of LNPV. In the first method, switching states in the first group are just utilized, and the LNPV can be eliminated with the ratio k of unity. In the second method, the switching states in the fourth class are avoided, and the LNPV can be reduced. Common mode filter is used in the third method and the LNPV can be mitigated effectively with the ratio k of unity. Furthermore, the power losses in the first and the second method were estimated and were compared with the original situation. Finally, the proposed methods were verified by simulation and experimental results.

A method for inductor core loss estimation in power factor correction applications

Conventional core loss estimation methods exhibit limitations in dealing with important aspects of switching power converter applications such as different duty cycles, discontinuous-conduction-mode, variable switching frequency, or variable duty cycle operation. These limitations are particularly evident when trying to estimate boost inductor core loss in power factor correction circuits. This paper first presents a core loss estimation method that addresses these limitations and then demonstrates an effective technique to estimate core losses in power factor correction circuits. Finally, the authors show examples of how this method can be conveniently incorporated into simulation software to automate the core loss estimation process. The inductor models that are developed to facilitate this automatic core loss estimation and the approaches to implement the calculation in simulation software, especially a program called SIMPLIS, are also provided.

Stability Issues of

Stability issues in hybrid energy storage systems (HESSs) are the major concern, in addition to the control design challenges of individual modules. In this paper, the stability issues of Z + Z type cascade systems in HESS are focused. The stability issues of cascade systems have been studied for many years. Impedance ratio type criterions in the form of ZS/ZL or ZL/ZS have been proposed to solve these stability issues. However, existing ratio type criteria still have problems in some cascade systems. The terminal characteristics of submodule are studied and the different types of cascade system are defined in this paper. The validity of conventional impedance ratio type criterion would be influenced by the right-half-plane zeros (RHZ) in terminal impedance of submodules in the Z + Z type cascade system. Two improved criteria are proposed for the Z + Z type cascade system: one is improved ratio type criterion considering the RHZ in the terminal impedance of the submodule; the other one is sum type criterion which does not need the information about RHZ in terminal impedance of each submodule. Experimental evidences are provided to prove the validity of these two improved criteria.