Modeling And Stability Analysis Of VSG Broadband Impedance Considering DC Voltage Loop

The scale of grid-connected generation of renewable energy,mainly based on wind and light,is expanding.The power system is highly electronic.The interaction between power generation equipment and power grid is intensified,which is very easy to cause the problem of wide-band oscillation.As a new interface between renewable energy generation and public power grid,virtual synchronous generator(TGCI)is different from traditional grid-connected inverter(TGCI).It can simulate the external characteristics of synchronous generator,and track the frequency of power grid without phase locked loop.Most of the existing researches regard the dc side of VSG as constant value,while for the grid-connected system of direct drive wind power generator and photovoltaic,the dc-side voltage control is an important part which cannot be ignored.The influence of dc-side dynamic on the stability of VSG grid connection needs further discussion.Therefore,this thesis takes the VSG with dc voltage loop as the research object,and analyzes the grid connection stability from the angle of dq impedance.The main contents and innovation points can be summarized as follows:1)Considering the control loops,sampling and control delay,especially the influence of dc-side voltage,the wide-band dq impedance model of VSG grid connected system in two-phase rotating coordinate system is established.Based on the model,it is concluded that the control loop bandwidth,system power and dc-side capacitance mainly affect the dq impedance of the system below 10 Hz,and the dc voltage loop bandwidth is the main factor;The control effect of dc voltage loop enhances the coupling term in the impedance matrix in the frequency band below 10 Hz,and introduces the negative impedance characteristic,so there is instability risk in the weak current network.2)A 380V / 100 kVA broadband impedance measurement equipment is developed.The measurement frequency range is 3Hz-2kHz.The topology,control method,software and hardware design of the equipment are given in detail.The impedance measurement equipment is used to measure the passive resistive load online,and the experimental results verify the effectiveness of the equipment;By online measuring the dq impedance of the VSG considering the dc voltage loop,the experimental results verify the correctness of the wide-band dq impedance model of the VSG considering the dc voltage loop.3)Considering the influence of grid strength,system power,dc-side capacitance and control loop bandwidth,the grid stability of VSG considering dc voltage loop is studied by using impedance analysis method.It is found that the dc-side capacitance and dc voltage loop bandwidth have important impacts on the stability of VSM grid-connected system under weak grid.Increasing the dc-side capacitance or reducing the dc voltage loop bandwidth can improve the grid-connected stability of the system.Moreover,the influence of dc voltage loop on the grid-connected stability of VSG and TGCI is compared and analyzed.Different from the subsynchronous oscillation caused in VSG,dc voltage loop is easy to induce high frequency oscillation in TGCI.