Zeng Liu

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.

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.

Infinity-Norm of Impedance-Based Stability Criterion for Three-Phase AC Distributed Power Systems With Constant Power Loads

This paper presents a stability criterion for three-phase AC distributed power system (DPS). While the source output impedance and the load input admittance under synchronous reference frame are generally investigated to predict the stability of the three-phase AC DPS, the infinity-norms of the impedance and admittance are innovatively adopted in the proposed criterion to improve the computational complexity and the conservatism. Meanwhile, the computational complexity and the conservatism of the proposed criterion are analyzed and compared with existing ones. Furthermore, the terminal characteristics of the studied three-phase AC DPS composed of an LC filter and a three-phase boost rectifier, which cover the source output impedance and the load input admittance, are comprehensively modeled. Finally, the effectiveness of the proposed criterion is validated by experimental results.

A Unified Virtual Power Decoupling Method for Droop-Controlled Parallel Inverters in Microgrids

The coupling between the active and the reactive power of the droop-controlled inverter is critical in the low-voltage microgrid. The conventional virtual power-based control strategy can decouple the active and the reactive power of the inverter, whereas the active power of the load is probably not equally shared among the parallel inverters in the microgrid. To tackle this issue, this paper proposes an improved virtual power-based control method for the droop-controlled parallel inverters with a unified rotation angle in the power transformation. Meanwhile, the optimal value of the unified rotation angle is rigorously derived through investigating the relative stability of the system by innovatively adopting Routh Stability Criterion. Moreover, the proposed method is enhanced by introducing low-pass filters in the coupling path for further mitigating the coupling between the active and the reactive power of the inverter. Additionally, the stability analysis and parameter design for the proposed method are comprehensively presented. Finally, the effectiveness of the proposed method is validated by the simulation and experimental results.

Comparison of Z-source inverter and traditional two-stage boost-buck inverter in grid-tied renewable energy generation

Z-source inverter and traditional two-stage buck-boost inverter are compared with respect to the voltage stress and maximum current stress of semiconductor devices and the inverter efficiency in grid-tied renewable energy generation application. The operation principles of the two inverters are introduced firstly, considering the two different PWM strategies used in Z-source inverter. Based on the principles, voltage stress and maximum current stress of semiconductor devices in both topologies are derived and compared. Then the efficiency of two inverters are evaluated and compared. The analysis and comparison results show that the traditional two-stage buck-boost inverter bares smaller voltage and current stress of semiconductor device and has higher efficiency than Z-source inverter under the same input and output conditions.

Output Impedance Modeling and Stability Prediction of Three-Phase Paralleled Inverters With Master–Slave Sharing Scheme Based on Terminal Characteristics of Individual Inverters

Paralleled inverters are widely employed as the power source in the AC distributed power system, whose output impedance and stability status are required in the impedance-based stability analysis of the whole system. This paper presents an output impedance model as well as a stability prediction method for three-phase paralleled inverters with master-slave sharing scheme. While the output impedance of three-phase paralleled inverters is generally modeled under synchronous reference frame (SRF), the terminal characteristics of individual inverters operating standalone are innovatively adopted in the proposed output impedance model, with no need for knowledge about inner parameters of the inverters. Furthermore, the stability criterion is derived with this model according to the generalized Nyquist criterion, where the stability of paralleled inverters can be predicted by investigating characteristic loci of two return ratios. Meanwhile, the terminal characteristics of individual inverters controlled under SRF is comprehensively modeled. Finally, the proposed output impedance model and stability criterion were experimentally verified.

Output impedance modeling and stability criterion for parallel inverters with average load sharing scheme in AC distributed power system

The stability criterion of cascaded AC system is usually used to check the stability of AC distributed power system (DPS) by dividing it into a source and a load subsystem. However, the output impedance (admittance) of these subsystems should be obtained, and the stability of subsystems should be checked before applying stability criterion of cascaded AC system. Parallel operation of inverters in source subsystem complicates the output impedance of source subsystem, and can cause source subsystem operating alone instable. In this paper, the output impedance of the inverters operating in parallel with average load sharing scheme is modeled by the I/O character of each inverter operating stand-alone, and a stability criterion was proposed to predict the stability of parallel inverters based on Generalized Nyquist Criterion (GNC). The output impedance model and stability criterion proposed for parallel inverters were verified by simulation results.

Modeling, analysis and mitigation of load neutral point voltage for Three-phase four-leg inverter

The Three-phase four-leg inverter is very suitable to be used in the high power UPS, and the load neutral point voltage should be low to meet the requirement of the UPS. In this paper, the load neutral point voltage is modeled. Two factors influencing the load neutral point voltage are revealed, which are the switching states and the ratio of neutral inductance to phase inductance. Then, the load neutral voltage is analyzed. Sixteen switching states can be grouped into four classes, and each class is with the same load neutral point voltage. Switching states of the first and the second class come to the minimum valve of zero with the special ratio of k. Thirdly, three methods to mitigate the load neutral point voltage are proposed, and the benefit and the limitation of each method are analyzed. Finally, the proposed methods are verified by simulation and experiment.

Loss Oriented Evaluation and Comparison of Z-Source Inverters Using Different Pulse Width Modulation Strategies

The pulse width modulation strategies of Z-Source inverter can be classified into two categories according to the two different shoot through state insertion ways. This paper focuses on a loss analysis of Z-source inverter using the both PWM strategies. Based on the semiconductor device loss model, the losses evaluation formula are derived firstly for both cases, then a loss-oriented comparison of the two PWM strategies is conducted for a typical Z-Source inverter considering the different operation situations such as different input voltages, modulation depth, switching frequency, output power ratio.