Research On Virtual Synchronous Generator (VSG) In The Multi-Energy Complementary Microgrid

The capacity of distributed energy resources (DERs) in power system is increasing rapidly for the past few years, and a high penetration level will bring new influences and challenges to the stable operation of power system. As a special kind of distributed generation system, the multi-energy complementary microgrid (MECM) is a power system with a variety of complementary DER units and has been widely favored because of its high efficiency, reliability and economy advantages. However, compared to the conventional power plants, in which the synchronous generators (SGs) dominates,the converter based DER units have either very small or no rotating mass which is beneficial to the grid frequency stability. Virtual synchronous generator (VSG) is a control strategy applied to the converter of distributed generator (DG) to support power system stability by imitating the behavior of the SG.Due to the control and application of VSG in the MECM, meanwhile, under the support of the National Science and Technology Support Program of China (863 Program) "Research on the Key Technology and Core Equipment Development of Photovoltaic Microgrid" (2015AA050607), and the industry research cooperation project "Research on the Key Technology and Demonstration Application of Virtual Synchronous Generator" sponsored by Sungrow Power Supply Co. Ltd., this dissertation highlights some detailed and deep studies on the load adaptability, grid adaptability, frequency stability and operational stability of VSG in the MECM by using theoretical research, experimental verification and demonstration application. Some novel and important conclusions are drawn as follows:1) A survey of the generational background, operational characteristic and control structure of MECM are introduced. On the other hand, the technical classifications,application fields and key problems of VSG are summarized. In the presented literature review, the VSG control strategy applied in this paper is given. In addition, the small signal models of VSG operating in both gird-connected and islanded modes are established to analysis the effects of main parameters of the virtual inertia and virtual damping on the stability of VSG operating in both grid-connected and islanded modes.2) In order to solve the problem that the power quality of MECM is susceptible to the impulse loads, unbalanced and nonlinear mixed loads, a multiloop control strategy for VSG with only the output voltage feedback is proposed. This control strategy achieves very fast dynamic response as well as possess good steady state performance at load step change. On the other hand, the virtual impedance control strategy based on a cascaded general-integrator (CGI) scheme is proposed to suppress the DC- and harmonic- components of the output currents effectively. As a result, the problem of harmonic amplification caused by the virtual impedance is solved. Meanwhile, a virtual harmonic impedance control strategy is proposed and added to the basic VSG controller to compensate for the system distorted voltage flexibly. The proposed scheme can operate at compensation, rejection, and uncontrolled modes by selecting different virtual harmonic impedance. In addition, the PI plus multi-resonant (PIR) voltage controller is used to compensate for the unbalanced and distorted output voltages of MECM.3) In order to solve the problem that the frequency stability of MECM is deteriorated under fluctuations by loads, a novel frequency hierarchical control scheme suitable for MECM is presented with the frequency hierarchical control structure and the coordinated control strategy, which are suitable for different characteristics of VSG and diesel generator set (DGS) for improving the frequency stability and flexibility of the islanded MECM. Meanwhile, based on the analysis of the dynamic power angle response of the constant parameters (CP)-VSG, self-tuning (ST)-VSG is proposed to continuously search for optimal parameters during the operation of the ST-VSG in order to minimize the maximum frequency deviation (MFD) and rate of change of frequency(RoCoF). In addition, the ST-VSG with negative virtual inertia concept is applied to improve the frequency stability. Besides, in order to suppress the dynamic frequency fluctuation of DGS, the VSG with the differential feedforward of output current (DFOC)is proposed to improve the frequency stability. As a result, the VSG-based DFOC can further increase the damping of MECM, reduce the overshoot, MFD and RoCoF of the system frequency.4) In order to solve the problem that the grid adaptability of MECM is significantly affected by the grid voltage and frequency, an improved VSG control strategy based on the adaptive mode switching between PQ control current-source mode and VSG control voltage-source mode is proposed. Furthermore, the grid adaptability of two existing solutions is analyzed. One is the improved droop control method by dynamically adjusting the droop coefficients, which has limited regulating range. The other is the translational droop curve control method, which may cause the output power of VSG to fluctuate and further lead the system to be unstable. The analyzed results show that the VSG-based adaptive mode switching can be much more adaptive to interface MECM into the utility. In addition, to solve the problems existing in the practical application of the MECM, the black start control strategy as well as the seamless switching control strategy based on VSG are studied and verified by means of experimental results.5) Two experimental platforms of MECM based on VSG are introduced. One is a Hundred-Kilowatt-Level Photovoltaic-Battery-Diesel MECM. The other is a Megawatt-Level Photovoltaic-Battery-Diesel MECM. And the experimental results from the two experimental platforms are displayed to confirm the feasibility and validation of the above proposed control strategies. Furthermore, the related research results have been applied in the Tibet Couqin Multi-Energy Complementary Isolated Microgrid Demonstration Power Plant Project. Again, the field tested results from the demonstration project are utilized to confirm the feasibility and effectiveness of the proposed control strategies.