Virtual Synchronous Generator(VSG)technology has broad application prospects in various distributed power generation units and integrated wind-solar-storage AC/DC micro-grids.In order to satisfy the system frequency stability,sufficient virtual inertia must be added to comply with the maximum rate of change of frequency(ROCOF)constraint.However,the increase of virtual inertia will deteriorate the transient characteristics of active power loop when VSG is connected to the grid.Current solutions include the introduction of damping correction links,the use of alternating virtual inertia,the increase of virtual impedance,and general droop control,etc.But these methods have disadvantages such as sacrificing frequency stability,complex nonlinear control,cumbersome parameter design process,and increasing output voltage harmonics,and the research process is only carried out for a single VSG.Therefore,it is necessary to study new strategies for improving VSG transient characteristics that the frequency stability is unchanged,the tolerance of nonlinear load is strong,the parameter design process is simple,and also apply it to the suppression of multi-VSG transient circulation current.Aiming at the shortcomings of existing methods,this paper proposes two new strategies for a single VSG based on improving virtual inertia and extended virtual impedance to improve the transient characteristics of active power loop.Subsequently,the complete mathematical model of multi-VSG parallel system and the equivalent inertia expression of PCC bus are deduced for the first time to verify the improvement effect of new strategies on transient circulation current of multi-VSG system and the frequency stability of PCC bus.Except for the simulation verification of multi-VSG parallel system in Chapter 5,the theoretical derivation in other chapters all have corresponding experimental verification.The main research contents of the thesis include:(1)The research is aiming at the active power loop's transient characteristics of a single VSG and solve the difficulties of parameter design.Firstly,combine with the power flow equation,an accurate mathematical model of VSG that takes into account the virtual inertia and excitation regulator is established.Secondly,based on the constraints of frequency,voltage stability and synchronous frequency resonance,a parameters' design method of VSG power loop is proposed.Finally,a theoretical analysis of transient characteristics on the three occasions of stand-alone mode,grid-connected mode and micro-grid parallel mode is carried out respectively,and the validity of the theoretical analysis is verified by experiments.(2)Aiming at the existing problem of cumbersome parameters' design process in the improvement of VSG transient characteristics,a new strategy for improving virtual inertia of VSG with forward parameters' design process is proposed.Although this method is inherently equivalent to the generalized droop control,it greatly simplifies the parameter design process.Firstly,based on the Bode diagram characteristics of traditional VSG system,basis and constraint conditions for the improvement of active power loop's phase margin are proposed.Secondly,two improvement strategies based on virtual inertia are given in detail,which can add a pair of pole-zero to the loop gain to increase the degree of design freedom,and a forward parameters' design process based on the mathematical model of improved system is given.Finally,the effectiveness of proposed method in improving the transient characteristics of VSG is verified by comparative experiments.(3)In order to improve the VSG's ability to resist frequency disturbances of the micro-grid,a new strategy for improving the active power loop's transient characteristics of a single VSG based on extended virtual impedance is proposed.Different from the traditional virtual impedance strategy to improve the transient characteristics of VSG,the extended virtual impedance with added frequency characteristics has much higher adaptability to harmonic current than the traditional virtual impedance.When the steady-state value of virtual impedance maintains the same,the extended virtual impedance has a better suppression effect on the overshoot of active power.Firstly,the limitations of traditional virtual impedance are analyzed.The realization principle and parameter design method of extended virtual impedance are proposed.Secondly,in order to avoid the influence of increasing power frequency output impedance on the droop characteristics of reactive power loop and the PCC bus voltage error caused by line voltage drop,a reactive power steady-state error compensation strategy and bus voltage estimator based on the VSG system are proposed.Finally,the comparative experiments verify the advantages of extended virtual impedance compared with traditional virtual impedance in improving the transient characteristics of VSG system,as well as the effect of improving the transient characteristics of frequency disturbance in the micro-grid.(4)Aiming at the existing problems of multi-VSG parallel state space mathematical model,which cannot analyze the time-domain transient circulation current of the system effectively,insufficient consideration of disturbance variables,lack of quantitative analysis of bus frequency stability,and have difficulties in truly expanding to three-VSG and above systems,the PCC bus equivalent virtual inertia expression of multi-VSG parallel system and the complete mathematical model of multi-VSG parallel system which considers all the disturbance variables and can analyze the time-domain transient characteristics are presented for the first time,also verifies the effect of two VSG improvement strategies on the suppression of transient circulation current in multi-VSG system when impedance is mismatch.Firstly,the expression of PCC equivalent virtual inertia when multi-VSG connected in parallel is deduced,and the constraint condition of parameters matching on multi-VSG parallel system is proposed.Secondly,based on the mathematical model of multi-VSG parallel system,the effect of two VSG improvement strategies on the transient characteristics of parallel system is compared and analyzed when line impedance is mismatch.In addition,in order to simulate the synchronization principle of synchronous generator,a phase regulator and an amplitude regulator are added to the VSG system,so that the hot swap is realized by accurately and quickly tracking the grid voltage.Finally,through the MATLAB/Simulink simulation platform,the mathematical model of multi-VSG parallel system and the improvement effect of transient characteristics are analyzed and verified.The results of the paper can provide theoretical basis and technical support for the parameter design of VSG in micro-grid,the improvement and optimization of transient characteristics,the modeling of multi-VSG parallel system,the virtual inertia equivalent method and the suppression of transient circulation current in multi-VSG parallel system. |