| With the expansion of the installed scale of distribute generator(DG)such as photovoltaic and wind power,the penetration rate of voltage source converter(VSC)with good controllability and flexible operation in power grid is gradually increasing.However,the capacity space of synchronous generator with inertia is occupied by the large-scale new energy generation,which leads to the power system facing the problems of inertia reduction and insufficient frequency modulation capability.Virtual synchronous generator(VSG)technology has become the current research hotspot,which can provide the necessary inertia,damping,frequency and voltage regulation support for new energy grid.In this paper,several critical technologies of VSG are deeply investigated,including power-frequency loop modeling of multi-VSG parallel/grid-connected system,quantitative analysis of interaction between VSG control loops,low-frequency oscillation modal analysis and suppression of multi-machine grid-connected system,etc.The specific contents are as follows:1)The existing P/ω "admittance" model can be conveniently applied to analyze the stability of VSG parallel system,but its application in multi-machine system is restricted because it does not consider the power-frequency response under input disturbance.Therefore,in this paper,the modeling method is proposed that the active power-frequency control loop of VSG is equivalent to a two-terminal network with the first and second P/ω "admittance".Next,considering the influence of grid impedance,an extended P/ω "admittance" model of multi-VSG grid-connected system is established,which can fully describe the active power and angular frequency response of each VSG.Furthermore,based on the node P/ω "admittance"matrix,it is deduced that the output power and angular frequency of VSG are affected by three kinds of factors:the input disturbance of its own active power command,the interaction with other VSG and the disturbance of grid frequency.Subsequently,through numerical analysis and frequency domain method,the oscillation characteristics and variation rules of VSG output power and angular frequency are comparatively studied when different system parameters change.Finally,the simulation study verifies the correctness and effectiveness of the above model and theoretical analysis.2)To further expand the applicability of P/ω "admittance" model,a generalized admittance modeling method of VSG power-frequency loop based on circuit theory is proposed.The rotor dynamics model of VSG is analogized to a circuit network by using the principle of electromechanical analogy,so as to realize the one-to-one correspondence between the synchronous torque link,damping torque link and inertia link of VSG and the resistance,inductance and capacitance in RLC network,and then the power-frequency oscillation of VSG can be analyzed by using circuit theory.Based on element duality,the active-frequency control loop of VSG is simplified and equivalently to a two-terminal network,and then the generalized admittance model of multi-machine parallel system is obtained.Besides,the equivalence between the generalized admittance model and the state-space model is proved by theoretical derivation and simulation verification.Wtat’s more,the applicability of this modeling method to VSG with different damping terms and different inverter control strategies are investigated,and four cases are designed to study the dynamic response characteristics of the system under different control strategy combinations.Finally,the effectiveness of the generalized admittance model is verified by simulation and experiment.3)Based on the multi-VSG grid-connected system model,it can be deduced that the response of VSG output active power and angular frequency under disturbance consists of three parts,one of which is the interaction between VSG control loops.In addition,under the disturbance of active instruction,there are cases where the interaction between VSG control loops is greater than its own disturbance response.Therefore,a method based on the principle of relative gain array(RGA)is proposed to analyze the interaction between different control loops of VSG.Based on the generalized admittance model of multi-VSG grid-connected system,the transfer function matrix of VSG output active power and angular frequency response is derived,and the interaction between VSG control loops is analyzed by RGA principle.Then,by comparing the numerical analysis method with the frequency-domain method,the variation law of the interaction between VSG control loops is studied when different system parameters changing,and determines the frequency range where the interaction is easy to occur,which can provide reference for the design of low-frequency oscillation suppression strategies.On this basis,Prony algorithm is applied to identify low-frequency oscillation parameters,and the correctness of RGA principle analysis is verified.4)VSG technology can provide virtual inertia,damping,frequency and voltage support for the system,but it also introduces the problem of low-frequency oscillation in traditional power systems.To solve this problem,a modal evaluation method is proposed to obtain the oscillation frequency and oscillation influence range of the system by establishing the loop impedance matrix of multi-VSG grid-connected system.Firstly,the generalized admittance model of multi-machine system is built.Secondly,drawing lessons from the difference of series-parallel resonance characteristics in power system,the node admittance matrix and loop impedance matrix of the system are listed separately for modal analysis,which verifies the applicability of modal analysis method,and the participation degree of each VSG during oscillation is determined by sensitivity analysis method.Then,by comparing with the frequency-domain method,the change rule of critical oscillation modes of the system is investigated when different system parameters changing.Furthermore,the characteristics of low-frequency oscillation of voltage-source controlled VSC(VC-VSC)and current-source controlled VSC(CC-VSC)are compared and studied.Simultaneously,the suppression methods of low-frequency oscillation of VC-VSC and CC-VSC are presented respectively,which provide technical support for solving the low-frequency oscillation problem in the application of high-permeabion new energy grid-connected projects. |
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