Abstract:Aiming at the regulation contradictions between output active power steady-state and dynamic characteristics when the conventional virtual synchronous generator(VSG)based on first-order link virtual inertia is connected to the utility grid, a VSG control strategy based on differential compensation virtual inertia is proposed. Through the incorporation of a differential link in the forward channel of conventional VSG control structure, the control strategy accelerates the power response speed, increases the damping of the system, and reduces the power impact on the energy storage unit of VSG during the dynamic process, while ensuring the maintenance of the output active power steady-state control precision. Simulation and experiments have validated the effectiveness of the proposed strategy.

Abstract:The virtual synchronous control strategy for the doubly-fed induction generator(DFIG)is a promising control technique to provide inertial frequency and voltage support when DFIG is integrated to the power grid. However, the existing virtual synchronous control strategy for DFIG mainly focuses on mimicking the electromechanical dynamic behavior of the synchronous generator, while neglecting the electromagnetic transient responses of DFIG. Firstly, the electromagnetic transient characteristics of virtual synchronous control-based DFIG during symmetrical grid faults are analyzed. Two severe flaws of existing virtual synchronous control strategy are revealed, that is, the incapability of fully imitating the transient characteristics of synchronous generators, or limiting the overcurrent in DFIG rotor. Then, a transient voltage compensation virtual synchronous control strategy for DFIG is proposed. The main principle is compensating for the transient component of the control voltage of rotor side converter to weaken the influence of transient electromotive force in rotor windings to suppress the overcurrent in DFIG rotor. Finally, a simulation analysis is made to compare the performances of existing and proposed virtual synchronous control strategies to show that the proposed virtual synchronous control strategy can not only enhance the advantage of providing inertial support, but also effectively limit the overcurrent in DFIG rotor and the oscillations of electromagnetic torque, with the capability to offer reactive power support to power grid. All of these improve the possibility for DFIG to implement fault ride through and the ability of power system to recover from grid faults.

Abstract:For the two levels of two-way charging machine, a kind of charge/discharge control strategy is proposed based on the virtual synchronous machine(VSM)in microgrid. First of all, the VSM is applied to control the first-level AC/DC converter, while introducing the virtual inertia, damping and excitation to make the charging machine possess the same operation characteristics as the synchronous machine. Secondly, an auxiliary frequency modulation algorithm is designed by using the active power-frequency operation curve to calculate the “correction of the active power reference” and change the power reference, which makes electrical vehicles participate in the primary and secondary frequency regulation of microgrid. Based on the MATLAB/Simulink, the microgrid model with electrical vehicles is developed. The results show that electric vehicles controlled by the proposed strategy can transmit electricity to the power system, which supports the inertia, damping, voltage and frequency, realizes the non-differential regulation of frequency in microgrid at independent mode, and improves the dynamic stability of microgrid.

Abstract:As the penetration of renewable energy increases continuously, the inertia of synchronous power grid and the ability of primary frequency regulation are declining. If renewable energy power generators adopt the virtual synchronous generator(VSG)technology, they can make due contributions to slow down the rate of system frequency variation and reduce the system frequency deviation under the impact of large power shortage. Aiming at the current situation of unclear functional orientation of inertia support and primary frequency regulation in the research on VSG, it is urgent to define the functional orientation demand of both inertia support and primary frequency regulation of VSG in its application in the large power grid. Firstly, this paper analyzes the inertia support function of VSG and its physical meaning. Then the expression formula of inertia support power of VSG is deduced. Secondly, the distinction between the inertial support function and the primary frequency regulation function of VSG is analyzed in detail. Thirdly, this paper simulates and analyzes the roles of the VSG using different control functions in the dynamic characteristic of system frequency changing caused by power imbalance in the large-scale synchronous grid. Finally, it is pointed out that, in a large synchronous power grid, the impact brought by reduction of primary frequency regulation ability is more serious than that brought by the decrease of system inertia. The power system needs the continuous power support of VSG for the primary frequency regulation more than short-term inertia support power.

Abstract:For the control of virtual synchronous generator(VSG)of grid-connected inverter, an approach is proposed to estimate the produced inertia and damping of VSG. Thus, the capability of VSG to participate in power grid adjustment can be quantified. Based on the second-order nonlinear model of VSG and its small signal perturbation processing ability near the working point, a linearized model for VSG is proposed. The discrete model of the linearized model is derived and its relation with inertia and damping parameters of VSG is given. The error magnitude and effective range of the linearized model are proposed. Using the output power step disturbance of VSG, the modal information on power oscillation of VSG is energized. Then, the least-square method is utilized to estimate actual inertia and damping parameters of VSG. Finally, by taking the normal control and VSG control into account, simulated results and experimental measurements have confirmed the possibility and effectiveness of the proposed model and approach. This work is supported by Chongqing Research Program of Basic Research and Frontier Technology(No. cstc2016jcyjA0108), Fundamental Research Funds for the Central Universities(No. 106112015CDJXY150005)and State Grid Corporation of China(No. SGHADK00PJJS1500060).

Abstract:With the continuous increasing penetration of distributed energy in isolated microgrid, the inertia of system is getting lower gradually. Virtual synchronous generator(VSG), which can simulate the inertia of synchronous generator and plays a vital role in damping dynamic frequency variations, has been gradually applied in microgrid. Frequency of island microgrid will oscillate severely and even may exceed its safety constraints when the power imbalance caused by fluctuations of renewable energy output or loads switching happens. To deal with these problems, a virtual synchronous generator control method based on model predictive control is proposed. Firstly, the predictive model, which aims at optimizing the weighted combination of frequency deviation and VSG output with frequency change rate as the constraint is established. Then, the systematic control strategy is designed according to physical quantities, such as frequency, VSG output voltage and current, the increment of active power needed is calculated for changing the power reference of VSG. Meanwhile, the solution of the predictive model is given, and the convergence of the algorithm is analyzed, which provides the basis for selecting key parameters. The correctness and effectiveness of the proposed control strategy are verified by the simulation results of two typical cases.

Abstract:With the increasing penetration of wind energy and due to the large-scale utilization of wind turbines, the present power grid is developing into a system with the loss of rotational inertia, thus it brings about various stability problems. However, as the virtual synchronous generator(VSG)technology can mimic the characteristics of the conventional synchronous generator, the equivalent inertia and damping coefficient of wind turbines and the penetration rate of wind energy in the grid are improved. Firstly, the research prospect of VSG technologies which applied in the field of wind turbine control are introduced. Then, the coordinated control of maximum power point tracking(MPPT)and frequency support is discussed, as well as the energy balance between the actual turbine shaft and virtual shaft is revealed. Also, the designs and stability analysis of key parameters of VSG controlled wind turbines are elaborated. The specific applications in wind turbines are illustrated via the simulations on a permanent magnetic synchronous generator(PMSG)-based wind turbine and a doubly-fed induction generator(DFIG)-based wind turbine. Moreover, the feasibility and prospect of the VSG controlled wind turbines in the area of large wind farm aggregation and specific control methods are addressed. Finally, some remaining difficulties and perspective solutions are also demonstrated.

Abstract:Virtual synchronous generator(VSG)technology is widely concerned because it can help renewable power generation equipment equipped with frequency and voltage support capabilities similar to synchronous machines. The models and control strategies of VSG are now focused on and VSG is mainly applied in the microgrid. Started with the difficulties encountered in the demonstration project of the large-capacity grid-connected renewable VSG, combined with the relevant technical problems of renewable energy development, the safe and efficient operation problems of the grid-connected VSG in the complex conditions are summarized as the grid-connected adaptability of VSG, including grid-connected stability, fault ride-through, and frequency support based on rotor inertia of wind power VSG. For various aspects of grid-connected adaptability problems, the existing research results and limitations are summarized, relevant research progress is specially introduced, and the future research directions are prospected.

Abstract:The virtual synchronous generator(VSG)realizes the virtual inertia control of the converter by simulating the swing equation of traditional synchronous generator, which also suffers the transient instability under large disturbance. This paper analyzes the transient stability of the VSG connected to the infinite bus. Firstly, the large signal model of the inverter controlled by the VSG is established and the system order is reduced based on the singular perturbation theory. The correctness of the model is verified by comparison with the electromagnetic transient simulation model. On this basis, the cell mapping method is introduced to analyze the global nonlinear characteristics of the reduced order model. The region of attraction in the phase plane, which is represented by the virtual angular velocity and the virtual phase angle is found and the key factors that affect the stability of the VSG under large disturbance are analyzed. The results of the analysis are compared with the results of the small signal analysis, which shows that system parameters, such as the internal impedance of the power grid, the feeding power, the virtual inertia, the damping coefficient and the filter parameters, have significant influences on the transient stability of the VSG under large disturbance. Finally, based on MATLAB/Simulink simulation, the rationality of the proposed method is verified.