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In recent years, virtual inertia algorithms are mostly applied to inverters of high-penetration distributed generation systems to improve system equivalent inertia. When distributed generators (DG) based on traditional first-order virtual inertia (TFVI) algorithm operates in weak grid connected mode, there is contradiction between its output active power steady state and dynamic characteristics adjustment. Although the virtual inertia with improved structures such as first-order virtual inertia based on differential compensation (DCFVI) and second-order virtual inertia (SVI) have better active power control accuracy and power oscillation damping ability simultaneously, its frequency support ability in high permeability distributed generation system is weaker due to its fast frequency response and small inertia in the initial stage of response, which is easier to cause under-frequency load shedding. In this paper, firstly, the frequency characteristics of the DG based on the above virtual inertia algorithms operating in islanded mode are analyzed, and the active power steady-state and dynamic characteristics of the DG operating in grid connected mode are studied as well. Afterwards, based on the active power steady-state and dynamic characteristics comparison and summary of DG grid-connected system with each improved virtual inertia algorithm, through optimization of the second-order virtual inertia structure, an optimized second-order virtual inertia (OSVI) algorithm is proposed, which can not only maintain the active power control accuracy and suppress power oscillation, but also can provide larger frequency support capability. Finally, the effectiveness of the proposed control strategy is verified by simulation.

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.

In view of the contradiction between active power steady-state and dynamic characteristics regulation of VSG based on the existing first-order virtual inertia algorithm operating in grid- connected mode and multi-paralleled mode, a virtual inertia control strategy based on multi-stage lead-lag compensation is proposed in this paper. By cascading a multi-stage lead-lag link in the first -order virtual inertia forward channel, the control freedom of VSG grid-connected and paralleled system is increased, so that the zero and poles can be freely configured. Furthermore, a first-order lead-lag link is taken as an example to analyze its effect on the output active power steady-state and dynamic characteristics of VSG grid-connected system and paralleled systems. Through reasonable configuration of the second-order virtual inertia parameters, the system damping ratio can be increased and the power oscillation can be eliminated while its steady-state characteristics are not changed under the circumstance of a large inertia. Experimental results verify the effectiveness of the proposed strategy.

Photovoltaic(PV) inverters have been widely used in large-scale PV power generation systems to reduce the system mismatch and increase the output power. With the increasing installed capacity of PV power stations, the number of string PV inverters increases, which brings about risk of inverter resonance and influences the quality and stability of multi-parallel inverters grid-connected system. Thus, the problems of multi-parallel inverters resonance have become a hot research topic. This paper is focused on the case of grid-connected string PV inverter systems, and the grid-connected PV inverter resonance where resonance suppression strategy will be analyzed and discussed. Firstly, the structure of string PV inverter and multi-parallel inverters system are presented. Secondly, discussions are made about the model of multi-parallel inverters system and its resonance characteristics based on the multi-input-multi-output(MIMO) system, Norton equivalent circuit and carrier synchronization. Thirdly, two strategies of “inverter level” and “system level” are provided to summarize the multi-parallel inverters resonance suppression strategy. Finally, the development tendency of resonance suppression strategy and conclusions are prospected.

The resonant stability problems of LCL filter have become big challenges for grid-connected inverter in weak grid. This paper takes the effect of non-minimum phase time delay on the ratio between resonance frequency and sampling frequency into account and use it to design the LCL filter. Both LCL resonance frequency and inverter-side inductor and filter capacitor related L 1 C resonance frequency are limited to a specific stability region considering the following factors: different computation delay, inductance variation and grid impedance variation in a large range. Associated with the maximum current ripple expression for inverter-side inductance calculation, three expressions can establish a simple and robust method to calculate LCL filter parameters without active damping. Simulation and experimental results are provided to validate the correctness of the developed model and control scheme.

This paper explores the impact of the output impedance on the power-sharing stability between parallelized microgrid inverters (MGIs) operating with virtual synchronous generator (VSG) method. In these systems, a virtual output impedance is usually added to the control loop of each MGI to improve the power sharing, regardless of line impedance unbalances. The time derivative of the MGI output current is usually used to implement the virtual impedance, which makes the system highly sensitive to the output current with high slew rate. To solve this, the virtual impedance is designed based on a cascaded second order general integrator (CSOGI) scheme, which is less sensitive to the output current noise, avoids to perform the time derivative function, achieves better output voltage total harmonic distortion. The proposed strategy is demonstrated in detail and validated with a two 100kVA-MGIs parallel system under linear and nonlinear loads.

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.

This paper presents a state of charge (SOC) based control strategy for virtual synchronous generator (VSG) when connecting to the grid. The VSG is operated in voltage source mode, the output frequency of VSG is calculated by considering the virtual inertial and the power reference together. The power reference is acquired by adding the SOC control module and the frequency control module, the proportion of each module is determined by applying a weight coefficient, if the SOC control module is in the majority, the VSG is more inclined to maintain the SOC, otherwise the VSG is more inclined to stable the frequency. Considering the dynamic response and stability of control system the select range for the weight coefficient is designed. Simulation results are presented in order to validate the proposed approach.

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.

This paper concentrates on the load disturbance in three phase microgrid inverter (MGI) with an isolated transformer based on virtual synchronous generator (VSG) control. The transformer can provide the VSG with inductive output impedance when placed behind the LC filter. But the load disturbance may lead to steady-state error and poor dynamic performance with only capacitor voltage feedback control. The capacitor voltage differential feedback control is proposed to damp the output LC filter resonance and thus increases the system stability margin. The output current differential feedforward control is proposed to eliminate the influence of load disturbance then obtains error-free steady-state output and excellent dynamic responses. These theoretical findings have been eventually verified experimentally both on simulation and laboratory prototypes.

This paper proposes a novel seamless transfer method for microgrid connected fuel cell inverters (FCIs) between voltage source and current source modes. In order to protect the fuel cell, the FCI output power should be restricted when the microgrid frequency is greatly disturbed. In this sense, the FCI is transfered from voltage source mode to constant current source mode. Meanwhile, the FCI is transfered from constant current source mode to voltage source mode until service can be restored. The proposed method can cooperatively work with well developed virtual synchronous generator (VSG) control algorithm so that these FCIs are able to share load among them as well as subsist under transient events of the system. Theoretical analysis and experimental results validate effectiveness of the proposed method.

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.

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.

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.

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.

In this paper, a general active damping method based on capacitor voltage detection for stable control of LCL-filter grid-connect inverter is proposed. Three active damping methods are derived from the general form. Root locus and bode diagrams are used to analysis these three active damping methods. And in the case of different control delay, robustness of three active damping methods are compared. Integrated the advantages and disadvantages of each method, and eventually come to a relatively better active damping method. Finally, each active damping method is verified by simulation and experiment.