Zha Xiaoming

Development of iterative learning control strategy for active power filter

The paper introduces iterative learning control strategy to active power filter (APF). The control strategy can incorporate feedback and feedforward into the controller of APF and it can enhance the tracking speed, simplify the configuration of the controller and obtain a fast and simple algorithm. The strategy with good robustness can realize self-optimizing and self-stabilizing of the system. It is suitable for the hybrid passive filter and APF The simulation and experiment results are given in the paper.

A novel control strategy for Dc voltage stability and balancing in cascade STATCOM based on droop characteristic

Cascade STATCOM is constructed by cascading several identical H-bridge (single-phase full bridge) inverters, each with their own isolated dc bus. Since DC capacitors of cascade STATCOM are separate, their voltage may not be exactly balanced during operation periods. This paper presents a droop characteristic of dc voltage of H-Bridge inverters, and a novel control strategy for dc voltage stability and balancing in cascade STATCOM based on this characteristic has realized. The validity and feasibility of this control strategy is verified by simulation results, and has been used on a practical engineering of ±4.33Mvar STATCOM. Based on this, a basic insurance and good platform is provided for the study and development of cascade STATCOM, and has very important reference value for other cascade power conversion devices.

Research on parallel operation of distributed inverter based on variable droop coefficient

In the numerous strategy of distributed inverters parallel operation, PQ droop control is an effective method. But traditional droop control method can result in considerable output voltage frequency and amplitude deviation under the condition of high-power output. Therefore this paper proposed an improved droop control method, which using large droop coefficient when the output is small in order to ensure output power balance. When the output is large, droop coefficient becomes smaller in order to reduce output voltage frequency and amplitude deviation. In the meantime, the droop coefficients change has followed the principle which the inverter output power balance need to follow. This new method can improve stability and reliability of the micro-grid. Simulation results show the effectiveness of this control method.

The small signal model for wireless parallel operation of inverters

In this paper, a modelling method for the parallel operation of inverters in a micro-grid system is proposed. Conventionally, the droop method has been applied to achieve a simple structure and no-interconnections among power sources. The small signal model of the wireless parallel inverter allows obtaining the behaviour of the system in function of frequency and voltage droop. The model is simple and eases to unstainable, and can give reference to the next step for the dynamic stability analysis and dynamic equivalency in power system. Simulation has been carried out on the system with two inverters in parallel. The results have validated the model developed.

Resonance analysis and suppression of system with multiple grid-connected inverters

The resonances of grid-connected system with multiple inverters would enlarge harmonics which bring challenges to the performance of inverters and power quality of the grid. The impedance model of the system is established and the resonant mechanism and characteristics is analyzed. From the analysis result, a global resonance suppression strategy based on virtual admittance is proposed which only reshapes the grid impedance around the resonance point, and it has little influence on fundamental wave control. The capacity of the harmonic suppression unit is decreased while the suppression effect is assured. The simulation results verified the correctness of theoretical analysis and the effectiveness of the proposed method.

The topology and control of a new hybrid power filter

A new topology of hybrid power filter is proposed in this paper, which consists of a C-type filter and an active filter. The application of C-type filter not only reduces the rating of active filter but also prevents the resonance occurring significantly when proper parameters are used. The Iterative Learning Control (ILC) method is applied to the realization of harmonic compensation. In this paper, both the topology and the control method are discussed. Experimental results based on the proposed topology and control method are presented.

Modeling and resonant characteristics analysis of multiple paralleled grid-connected inverters with LCL filter

This paper establishes the model of multiple paralleled grid-connected inverters with LCL filter and analysis its resonant characteristics and influencing factors. The analysis results show that multiple paralleled grid-connected inverters have more than one resonant frequency, and the resonant frequency are closely related to the number, type and composition ratio of parallel inverters. Meanwhile the lower resonance frequency decreases with the increasing number of parallel inverters, and mainly affected by the control parameters and the grid strength when the amount of inverters is fixed. Simulation waveform and experiment results verify the availability of the model and the accuracy of analytical results.

A fundamental wave recursive DFT algorithm applied in the inverter's pre-synchronization

This paper presents a voltage fundamental wave recursive DFT algorithm applied in grid-connected inverter pre-synchronization. The algorithm not only is able to reliably track the voltage fundamental phase, frequency and amplitude, but also reduces the calculating consumption to guarantee real-time algorithm. Based on the equal-angle interval sampling technique (EAISP), the algorithm can adjust the sampling interval according to the fundamental phase angle feedback. So it can achieve adaptive voltage-frequency tracking to reduce the spectral leakage and enhance the fundamental wave synchronization parameter accuracy. With respect to the traditional phase-locked loop synchronization method, the algorithm can precisely track the fundamental wave while harmonics and grid voltage zero-offset are serious to reduce the impact on the micro-grid and the inverter itself when grid-connected operation. The simulation results show its validity and feasibility.

Lagrange Modeling and a Novel Passivity-Based Control of a Single-phase inverter Applied in Neutral-Line Active Power Filter

In this paper, we apply passivity-based control theory in a single-phase pulse-width modulation (PWM) voltage-source inverter using Euler-Lagrange(EL) dynamics equation. The EL model derived from the Lagrange function and EL equation indicates the accordance with the time-domain model of state equation. If proper damping was injected into the system through the controller designed, Energy-dissipative properties of the model could be fully retained, i.e., the energy storage function H(x) and its differential H dot , which denotes the change rate of H(x), meet the request of the Passivity theory. Based on the EL model, passivity-based controllers are then synthesized using the techniques of energy shaping and damping injection, which have the features of enhanced robustness, guaranteed zero steady-state error, faster response and good stability at moment of load disturbance. Furthermore, we introduce a method of a single-phase inverter applied in the neutral-line active power filter system through a novel topology. Finally, simulation results on a 10 kVA neutral-line active power filter provide verification of the salient features above.

An improved observer for the estimation of synchronous generator damper current

An improved observer is designed for the estimation of synchronous generator damper current. Compared with existing damper current observer, the improved observer has higher accuracy. Furthermore, initial value selection and absolute stability domain of the observer are discussed in this paper. The improved observer has larger absolute stability domain which gives less pressure to simulation step selection. The condition of well-performed observer even when initial value and startup time of observer are changed is also given. By estimating damper current of synchronous generator in IEEE first bench case for SSR, simulation results testify the validity of improved damper current observer.