Research On Generalized Inertia And Reactive Power Sharing Strategy Of VSG

With the deterioration of environment,energy problems and large-scale application of renewable energy,especially the penetration rate of renewable energy power generation is getting higher and higher,making the inertia of the grid decreases.As a result,it is difficult to maintain short-term power balance to suppress frequency fluctuations.For this reason,the virtual synchronous generator(VSG)control strategy is applied in power system with a power electronics interface.By simulating the inertia and frequency/voltage regulation characteristics of a synchronous generator,the system voltage and frequency support capabilities are increased.However,in terms of frequency regulation,the existing VSG control is based on first order virtual inertia VSG strategy.Although first-order virtual inertia VSG increases system inertia and restrains frequency fluctuations,while the problem of contradiction between its active power steady-state and dynamic characteristic adjustment exists;in terms of voltage regulation,the existing VSG control is based on Q-U droop control strategy.The mismatch of VSG line impedance and their rated capacity leads to steadystate reactive power sharing error when they are operated in islanded mode,and the contradiction between reactive power sharing error and voltage control accuracy exists in the power sharing schemes of increasing virtual impedance and Q-U droop coefficient.With the support of national 863 program "photovoltaic micro-grid key technology research and core equipment development"(2015AA050607)and other national projects,and relying on virtual synchronous generator production and research projects launched by Sungrow Power Supply Co.,Ltd.In this paper,the main contents and innovations of this paper are as follows:(1)The background,research status,difference with synchronous generator of VSG are studied,the application field and the key problems of VSG are summarized.(2)The existing VSG models are compared and analyzed,and based on the VSG algorithm adopted in this paper,the small signal models of VSG connected to gird and paralleled in islanded mode are established.In addition,the basic algorithms such as voltage closed loop control,virtual impedance algorithm and pre-synchronization algorithm are introduced.Aiming at the phase difference transition of original algorithm,a pre-synchronization strategy based on multiplicative phase detector is proposed to shorten the program running time and pre-synchronization process.(3)Based on the summary of some typical virtual inertia and damping coefficient optimization strategies,the contradiction between active power steady-state and dynamic characteristic adjustmentof VSG with first order virtual inertia under islanded mode,gridconnected mode and paralleled mode is analyzed.The contradiction between reactive power sharing error and voltage control accuracy of open-loop reactive power sharing schemes such as increasing droop coefficient and increasing virtual impedance algorithm is analyzed,and the influence of droop coefficient and virtual impedance on system stability is studied.(4)Aiming at the contradiction between steady-state and dynamic characteristic adjustment,the first-order virtual inertia based on differential compensation is proposed firstly.By means of adding a leading compensation link on the basis of the original firstorder inertia,the system damping ratio is increased,power overshoot is reduced,and power response speed is improved.(5)Aiming at the problem that the inertia of first order virtual inertia based on differential compensation in the initial stage of response is small,according to the idea of increasing adjustment freedom to improve system performance,the concept of generalized virtual inertia is proposed,and the expression of generalized virtual inertia is given.Then taking second-order virtual inertia as an example,the characteristics of VSG with second order generalized virtual inertia under islanded mode,grid-connected mode and paralleled mode are analyzed.Second order generalized virtual inertia reduces power overshoot and enlarges initial response stage inertia while maintaining steady-state characteristics.(6)Aiming at the contradiction between reactive power sharing error and voltage control accuracy of the existing open-loop reactive power sharing scheme involving increasing Q-U droop coefficient and virtual impedance algorithm,a reactive power sharing strategy based on virtual capacitor is proposed.The strategy simulates characteristics of paralleling capacitor at VSG output terminal,thus to reduce reactive power sharing error and improve voltage control accuracy meanwhile.Based on the stability analysis of a two VSG paralleled system with the proposed algorithm,an improved virtual capacitor algorithm integrated with virtual impedance is proposed to enhance system stability.(7)Experimental platform of microgrid is introduced and the basic characteristics of VSG operated in various modes,the improved pre-synchronization algorithm,first-order virtual inertia based on differential compensation and reactive power sharing algorithm based on virtual capacitor are verified,and the relevant research results have been applied in the Tibet Couqin smart microgrid.