A converter based on virtual synchronous generator technology in microgrid

As an important part of electric multiple units (EMU), the auxiliary power supply system plays an important role in the normal operation of the vehicle. In order to improve the reliability of the auxiliary power supply system, the virtual synchronous generator (VSG) technology is applied to the parallel control system of auxiliary inverter. The essence of VSG technology is to control the inverter to simulate the working principle of synchronous generator, so as to obtain similar operation characteristics of synchronous generator. The auxiliary inverter controlled by VSG technology can avoid the disadvantage of hard external characteristics of power electronic devices. Its rotating inertia can obviously prevent frequency fluctuation, and the system has high stability and reliability. In this paper, the frequency and voltage of the auxiliary inverters are controlled independently with VSG technology, so that the output voltage can be controlled stably and the power quality can be improved. Finally, the MATLAB simulation results show that the parallel control of auxiliary inverter based on VSG achieves the expected goals.

In recent years, virtual synchronous generator (VSG) technology has been more and more used in grid-connected inverters of PV power generation systems. Photovoltaic inverter based on VSG can improve the voltage support capacity of the power grid, but it increases the difficulty of the power grid elaborate modeling and simulation. This paper researched the modeling and parameter identification of the PV inverter based on VSG. In this paper, the PV inverter simulation model based on VSG technology was set up in PSCAD and the effective voltage support capability of VSG technology for the power grid was verified. The mathematical model of the inverter based on VSG was established in MATLAB and important parameters were identified based on the improved differential evolution algorithm. Finally, the performance of the modeling method and parameter identification method used in this paper were tested by considering various perturbations.

This study constructs a novel virtual synchronous generator system based on a transfer function, and optimizes the parameters of the model by using the improved whale algorithm to improve the frequency control ability of virtual synchronous generator. Virtual synchronous generator technology helps to solve the problem that the integration of large-scale renewable energy generation into the power system leads to the deterioration of system frequency stability. It can maintain the symmetry of grid-connected scale and system stability. The virtual synchronous generator technology makes the inverter to have the inertia and damping characteristics of a synchronous generator. The inverter has the inertia characteristics and damps to reduce the frequency instability of high penetration renewable energy power system. The improved whale algorithm is efficient to find the best combination of control parameters and the effectiveness of the algorithm is verified by microgrid and power system. The results show that the proposed frequency coordination control scheme suppresses the frequency deviation of power system and keep the system frequency in a reasonable range.

A large number of distributed generations (DGs) connected to the distribution network have a significant impact on the stability of the power system due to their low inertia. Meanwhile the DGs are generally linked to the end of the distribution network, where power quality problems are very common. This paper applies virtual synchronous generator (VSG) technology in three-phase four-leg inverter. A strategy of harmonics compensation is utilized to reduce the harmonic pollution caused by unbalanced and nonlinear load. A parameter selection method based on the dynamic response characteristics of VSG control is proposed. The inner current control loop is used so that the inverter output current could track the reference value accurately. The validity and correctness of the control strategy are proved by the simulation results.

Artificial neural network-based virtual synchronous generator for frequency stability improving of grid integrating distributed generators

Low frequency oscillation (LFO) is dangerous for power systems secure operation. Over the past decades, installations of power system stabilizer (PSS) on synchronous generator (SG) exciter or flexible AC transmission systems (FACTS) are widely-used methods in electrical power industry to suppress power system oscillation and improve the system oscillation stability. But with the rapid increase of renewable energy sources, more and more power electronic converters with seriously different dynamic characteristics from that of SGs’ are connected and operated in the power system. Thus, the future power system is very likely to generate more complex oscillation phenomenon and system stability problems are maybe more serious than ever before. Under these circumstances, adding supplementary control on these existing power electronic converters is becoming an attractive choice. In this paper, inspired by working principle of the PSS, an improved virtual synchronous generator (VSG) control strategy is presented to damp LFO actively via adaptively adjusting its parameter values of virtual inertia and damping coefficient, according to deviation value and changing rate of its output active power. This idea takes full advantage of flexible adjustability of VSG’s parameters. The effectiveness of the proposed VSG strategy is proved by simulation results, which indicates remarkable performance in fast damping of LFO.

In recent years, the increasing penetration of distributed generation in microgrids challenges the control and coordination of energy resources. Especially in microgrids with virtual synchronous generator (VSG)-controlled converters and conventional synchronous generators (SG), the inherent inertia difference (i.e., the VSG and SG) results in a poor transient performance when the VSG and/or loads are cut in/out. Thus, this paper explores the transient performance of microgrids with parallel VSG and SG systems. More importantly, a novel pre-synchronization control method is proposed to eliminate the phase jump while meeting the requirements in case of closures or re-closures of generation units. A small-signal dynamic model is presented, and accordingly, the VSG inertia and its damping can be designed considering the capacity ratio of VSG and SG units. In addition, with the power angle stability analysis, an active power provision strategy is introduced to suppress the transient power oscillation due to the inertia difference. Finally, the feasibility of the proposed methods is verified by simulations on a microgrid consisting of parallel VSG and SG units.

Photovoltaic power generation is mainly connected to the grid by grid-connected inverter. Compared with the traditional synchronous generator, it has the advantages of flexible control and quick response, but there is also lack of inertia and damping. With the increasing of the permeability of the photovoltaic power supply, the rotating spare capacity and the moment of inertia in the power system are relatively reduced, which poses a severe challenge to the safe and stable operation of the power grid. The introduction of photovoltaic virtual synchronous generator makes the grid connected inverter have similar operation characteristics of synchronous generator and improve the stability of power system. In this paper, a hardware in loop simulation test scheme based on RT-LAB is proposed for PV virtual synchronous generator, and a simulation test platform is set up to study the testing method of the performance of the PV virtual synchronous generator. Based on this platform, the characteristics of the PV virtual synchronous generator, such as the inertia and the primary frequency modulation, are carried out. The test results show that the simulation platform is correct.

The technology of Virtual Synchronous Generator (VSG), which can make the inverter connected to distributed generation work like traditional synchronous generator, plays an important role in improving the stability of the power system because of its good voltage and frequency supporting capability. In this paper, an improved VSG control method is proposed, which can ensure seamless transfer between grid-connected and stand-alone modes, and achieve accurate tracking control of active and reactive power. The control topology and mathematical model of VSG are constructed firstly based on the rotation equation and electrical equation of synchronous generator; then, the corresponding VSG active-frequency droop control, reactive-voltage droop control and inner loop voltage-current control are designed, and the influences of key parameters are analyzed through frequency domain method. Finally, two simulations with different operation modes and loads are conducted to verify the effectiveness in Matlab/Simulink environment, and the results show that the performance is well.

Due to various implementations of the voltage control, a virtual synchronous generator (VSG) may have different behaviors. This paper compares several voltage control strategies used in the VSG as well as the technology applied in a traditional synchronous generator. The performances are evaluated by the small-signal model and frequency-domain analysis, where the relationship and differences among various control strategies are revealed. The experimental results verify the theoretical analysis. The outcome of this paper is a reference to properly design the voltage control of a VSG and to understand the differences between a VSG and an actual synchronous generator.

Virtual synchronous generator (VSG) technology for integration of distributed energy resources attracts increasing attentions for its enabling inverters to simulate the inertia and damping characteristics of synchronous generators and improve the stability of the system. In this paper, a decentralized VSG-based adaptive coordinated control strategy is proposed for islanding microgrids consisting of photovoltaic generators combined with battery energy storages in DC side (PV/BES-VSG). By the proposed method, the droop characteristics of VSGs can be adaptively adjusted according to the DC bus voltage. In this way, the local controllers of PV/BES-VSG units can switch operating modes automatically without the need of a central controller, so that the power sharing among PV/BES-VSG units is allocated according to the maximum output power of PVs and the limit of charging/discharging power of BES instead of merely rated capacity of the inverters. To test the proposed method, an islanding model of microgrid with two PV/BES-VSG units in parallel is built in Matlab/Simulink. The simulation results show that by the proposed control strategy, the coordination control between PV and BES, and between PV/BESVSG units can be effectively realized with maximum use of PV power under the premise of the rational distribution of power.

With people’s attention to environmental protection, clean energy has become an important research and development direction. Among them, photovoltaic power generation has many advantages, such as simple process, no fuel consumption, no noise, no pollution and so on. The power grid capacity is becoming larger and larger, and has a great impact on the environment. Therefore, the grid connection of photovoltaic power generation will cause major problems for the planning, operation and dispatching of power grid. Virtual synchronous generator (hereinafter referred to as VSG) technology can simulate the inertia, primary frequency regulation and voltage regulation characteristics of synchronous generator, which has become an important way to improve the dynamic frequency response ability of the system. Therefore, VSG technology has become an important research technology of photovoltaic grid connected system, among which FM method will also become an important research direction. Firstly, this paper analyzes the VSG algorithm and its basic characteristics. Finally, this paper analyzes the control scheme of overall primary frequency regulation of photovoltaic power station (hereinafter referred to as PPS).

The lack of inertia and damping mechanism of photovoltaic (PV) grid-connected systems controlled by maximum power point tracking (MPPT) poses a challenge for the safety and stability of the grid. Virtual synchronous generator (VSG) technology has attracted wide attention, since it can make PV grid-connected inverter present the external characteristics of a synchronous generator (SG). Nevertheless, traditional PV-VSG is generally equipped with an energy storage device, which leads to many problems, such as increased costs, space occupation, and post-maintenance. Thus, this paper proposes a two-stage improved PV-VSG control method based on an adaptive-MPPT algorithm. When PV power is adequate, the adaptive-MPPT allows the PV to change the operating point within a stable operation area to actualize system supply-demand, matching in accordance to the load or dispatching power demand; when PV power is insufficient, PV achieves traditional MPPT control to reduce power shortage; simultaneously, improved VSG control prevents the DC bus voltage from falling continuously to ensure its stability. The proposed control approach enables the two-stage PV-VSG to supply power to loads or connect to the grid without adding additional energy storage devices, the effectiveness of which in off-grid and grid-connected modes is demonstrated by typical simulation conditions.

Because of the increasing use of renewable energy technology, distributed power sources and greatly varying loads can be critical factors in power system destabilization. This problem has hampered the adoption of renewable energy; thus, researchers have studied various control method types that enable an inverter to be operated as a synchronous generator (SG). Some of these methods, referred to as Virtual Synchronous Generators (VSGs), are designed to provide virtual inertia. Using VSG technology, we can decrease the introduction of fossil fuel-based power, which allows a variety of energy sources to be combined. In this study, we derived and analyzed the frequency system dynamics of microgrids, and show that a VSG expressed as a first-order equation can stabilize the frequency of a grid without causing resonance among the generators and loads. Furthermore, we derived excitation system dynamics, and observed that a VSG applying constant impedance can reduce the voltage fluctuations in the system. The results were verified in laboratory experiments and through a simulation using EMTP-RV. The simulation test results, which were generated using data acquired from an actual photovoltaic facility, indicated that the VSG could effectively suppress system deviations caused by sudden weather change.

The conventional synchronous generator (SG) can regulate the active and reactive power for frequency and voltage stability, when the system fluctuates. In renewable energy generation system (RES), the stability problem of RES can be solved by adding virtual inertia and damping in the control strategy of the grid-connected inverter. The grid-connected inverter and renewable energy system with this kind of control strategy, called as virtual synchronous generator (VSG), acts as a synchronous generator. In this paper, the topology of grid-connected PV array and energy storage system is proposed. The VSG used in grid-connected inverter, is modelled as stator and rotor model by stator voltage equation and rotor motion equation, respectively. Meanwhile, the VSG control mechanism, based on droop control and model of VSG, can be divided as active power-frequency and reactive power-voltage control. Besides, the system stability is analyzed based on small signal model, for the design of inertia and damping. The simulation model is designed by MATLAB/Simulink for the implementation of VSG. In simulation, it is compared that the dynamic regulation performance of VSG with different control coefficient, such as virtual inertia, virtual damping, droop coefficient and integral coefficient. The optimization for regulation performance in simulation provides an appropriate selection method of the control coefficient for VSG.