Haizhen Xu

Synchronization strategy of microgrid from islanded to grid-connected mode seamless transfer

Microgrid should be operated in both grid-connected and islanded mode to ensure high voltage quality and reliability. In the case of continuous uninterrupted power supply, seamless transfer is important between the two modes,and synchronization of the voltage of the point of common coupling (PCC) and utility grid should be finished first to achieve the goal. In this paper varies of synchronization strategies used in different microgrid control structures from islanded mode to grid-connected mode are summarized, and a new method based on droop control is proposed. This method does not affect power sharing during synchronization procedure. Simulation and experiment results are presented to show the feasibility of the method.

Modeling and Resonance Analysis of Multiparallel Inverters System Under Asynchronous Carriers Conditions

With increasing penetration of renewable energy sources into modern power systems, multiparallel inverters are commonly employed in the interface to the utility grid, giving rise to potential resonance problems. In order to better understand the issue, resonance of multiparallel inverters is analyzed under an asynchronous carrier. First, multiparallel inverter equivalent circuits are modeled within a controlled bandwidth. In order to investigate resonance characteristics under an asynchronous carrier, a crossover filter function is used to separate output currents of different frequencies for a two-parallel inverter to analyze the output current frequency response. With this approach, resonance characteristics are focused under asynchronous conditions, and the impact of shifting LCL parameters on resonance is examined. The influence of paralleled inverter number on output currents is examined, and the analysis is expanded to multiparallel inverter systems to determine the resonance characteristics. Experimental results validate the developed models and the proposed resonance characteristics.

Research on the synchronization control strategy for microgrid inverter

The three-loop power virtual synchronous generator (VSG) control methods are commonly used in microgrid inverter (MGI) units to share the loads among the parallel MGI units. In order to achieve stable operation of the MGI units under paralleled or grid-connected mode, a droop synchronization controller is designed in this paper. The difference of phase and amplitude between different MGI units is detected and is used to calculate the output frequency and amplitude of the MGI. The control principle is introduced and the synchronization control block diagram is given. The simulation and experimental results verify that the proposed method is effective in achieving paralleled operation of the MGI units.

A dynamic voltage transient suppression control strategy for microgrid inverter

China Machinery Industry Standard (CMIS) GB/T 12326-2008 prescribed in the islanded operation mode, microgrid inverter (MGI) output voltage dynamic transient range is less than 10% when the resistive load is switched rapidly between 0% and 100%. The three-loop power virtual synchronous generator (VSG) control methods are commonly used in MGI units to share the loads among the parallel MGIs. However, the transient output voltage of the MGI is beyond the CMIS when the resistive load steps sharply. In order to meet the CMIS, an output voltage differential feedback improved algorithm was employed. Comparing to the traditional control method, the improved method could obtain satisfying dynamic transient voltage suppression, resonance inhibition and improve system stability. These theoretical findings were eventually verified experimentally both on simulation and laboratory prototypes.

A reactive power sharing method based on virtual capacitor in islanding microgrid

For the problem that paralleled inverters cannot share reactive power proportional to their power capacity in islanding microgrid. Originally, the effect of inverter output impedance and line impedance to reactive power sharing with conventional droop control is analyzed. Following, a method adjusting the output reactive power by paralleling capacitor at the inverter output is proposed, and the relationship between paralleled capacitors, line impedance and reactive power is analyzed. On the basis, virtual capacitor method is realized to improve reactive power balance. Finally, the proposed method and analysis are verified through simulation.

A hybrid derivative feedback control for VSG-based VSC to improve dynamic response characteristic

With high power Virtual Synchronous Generator (VSG)-based Voltage Source Converter (VSC) and low switching frequency, time delay has become a big issue affecting VSC stability and dynamic response in islanded mode. In this paper, effect of time delay on output impedance is analyzed and a hybrid derivative feedback control is proposed to improve system stability and dynamic response. First step, derivative feedback compensator of control variable is adopted to improve system stability and then, derivative feedback equation of state variable is adopted to improve dynamic response. Time delay is also discussed and compensated to improve dynamic response furthermore. The control method is verified by both analytical model, time simulations and experiment results.

A Reactive Power Sharing Strategy of VSG Based on Virtual Capacitor Algorithm

In the islanded microgrid, distributed generators are controlled with virtual synchronous generator (VSG) strategy to simulate rotor inertia and droop characteristics of synchronous generators, in order to enhance the voltage and frequency support capabilities. Since the capacity and location distribution of each VSG is random, the VSG output impedance, line impedance and its capacity are mismatched, resulting in inaccurate sharing of reactive power. Based on the study of reactive power sharing schemes without communication and system parameters detection, and aiming at the contradiction between reactive power sharing error and voltage control accuracy of existing schemes, a reactive power sharing strategy based on virtual capacitor is proposed. The strategy simulates characteristics of paralleling capacitor at the VSG output terminal, and compensates the output voltage according to adaptive control of VSG output reactive power, thus to reduce reactive power sharing error, and improve the voltage control accuracy meanwhile. The design of virtual capacitor parameters and a two VSG parallel system stability with proposed strategy are analyzed in this paper. The correctness and effectiveness of the proposed strategy is verified by experiments.

Resonance characteristics analysis of paralleled inverters based on Resonance Stability Margin

With increasing penetration of renewable energy sources into modern power systems, multi-parallel inverters are commonly employed in the interface to the utility grid, giving rise to potential resonance problems. Firstly, 2-parallel inverters equivalent circuit model is established based on its control. In order to decouple inverter output current, the components characteristics of inverter output are analyzed in different frequencies, and one of 2-parallel inverter output current under condition of synchronized and asynchronized carriers are worked out. An idea “Resonance Stability Margin” is raised to investigate and evaluate resonance characteristics, variables that represent the stability of the system are found in control system of single inverter, and are used to record when the system achieves the same closed-loop dominant pole position or open-loop phase margin under different conditions. Values of those variables recorded indirectly reflect the degree of resonance in the paralleled inverters system. Experimental results validate the developed resonance characteristics and the proposed resonance stability margin.

Research on power compensation strategy for diesel generator system based on virtual synchronous generator

Due to the large adjust inertia of the diesel generator set (DGS), the quality of power supply would decline when a large load disturbance occurs to the diesel generator system. This paper aims to improve the stability of the diesel generator system voltage amplitude and frequency under impact load conditions using a virtual synchronous generator (VSG) method. Firstly, a mathematical model of DGS is built, the dynamic performances of DGS under a variety of load conditions is analyzed. Secondly, a three-phase energy storage inverter based on VSG control is selected as the parallel power compensator to suppress the system voltage amplitude and frequency fluctuations. Finally, experimental results are given to verify the feasibility and validity of the proposed control strategy.

Virtual impedance design of virtual synchronous generation in microgrid

The adoption of virtual synchronous generation (VSG) control can maintain the stability of voltage and frequency in power electronic interfaced mircogrid. In this paper virtual impedance is added to the VSG control strategy in order to improve the system control performance in resistive-inductive line impedance, the influence of virtual impedance on VSG system stability and dynamic performance is analyzed through the small-signal model. Simulation is presented to verify the conclusion of this paper.