Modeling Of Voltage Source Converter And Stability Analysis Of Grid-connected System In Current Control Timescale

With the rapid and sustained development of the power electronics technologies,the penetrations of the voltage source converter(VSC)keep increasing,which has been widely applied to the renewable energy generations,the high voltage DC transmissions,the flexible AC transmissions systems and the motor speed regulation.Due to the different response times of the inner controllers in VSC,the dynamics of the power systems induced by VSC show the features of multiple timescales,which are completely different from the conventional power systems.In AC current control(ACC)timescale,the voltage amplitude/frequency dynamic issues caused by VSC have brought great challenges to the security and stable operation of the power systems.However,relevant studies on VSC modeling and small-signall stability analysis still lack enough universalities and mechansims.In terms of VSC modeling,the existing state-space models and the impedance models are intractuable to intuitively reflect how VSC exchanges the active/reactive power with the grid and its effects on the voltage amplitude/frequency dynamic of the power systems.In terms of the small-signal stability analysis,relevant studies focus on the single VSC scenarios and cannot fully explain these voltage amplitude/frequency dynamic issues induced by multi-VSC.Though,several studies have devoted to analyzing the stability of the multi-VSC scenarios,they can only tell the stable results of the multi-VSC system and still lack the profound cognitions of the interacting mechanisms among multi-VSC.Furthermore,most studies seldom consider the effects of VSC’s DC-link voltage control(DVC)timescale dynamics on the dynamic interactions and small-signal stability of the VSC-dominated power systems.The basis of simplifying the dynamic analysis of the VSC-dominated power systems is still lacking.Therefore,this paper has proposed the VSC internal voltage motion equations in ACC timescale based on the internal voltage motion equation modeling method in ACC timescale.Furthermore,based on the self-stability/en-stability interacting analysis method,the interacting mechanisms and the small-signal stability of the VSC-dominated power systems are analyzed.The details of this paper are as follows:(1)The basic control structure of VSC is introduced in detail.According to the different sizes of the energy storage elements and the different response times of the corresponding controllers,the dynamics induced by VSC can be classified into the DVC timescale dynamics and the ACC timescale dynamics.Then,the basic connotations and challenges of the dynamic issues of the VSC-dominated power systems in ACC timescale are emphaticly described.(2)The basic concept of the internal voltage motion equation is elaborated.Based on the unbalanced active/reactive current excations and the internal voltage amplitude/frequency responses,the internal voltage motion equation of VSC without taking the dynamics of the DVC timescale into consideration is proposed in ACC timescale.This model can intuitively reflect how VSC exchanges the active/reactive power with the grid and its effects on the voltage amplitude/frequency dynamic of the power systems.This model can also be used to analyze the voltage amplitude/frequency dynamic of the multi-VSC system.Furthermore,the effects of the current controller parameters and the PLL parameters on the time-varying characteristics of the actual active/reactive reference current and the characteristics of unbalanced active/reactive current excitation-internal voltage amplitude/frequency response VSC are analyzed.(3)The interaction analysis method based on the self-stability/en-stability is detailedly depicted.Then,the single-input single-output(SISO)equivalent method of the multi-input multi-output(MIMO)linear time-invarient system is proposed.This SISO equivalent method can help the engineers to reuse the simple and mature methods of the SISO system to analyze the dynamic issues of the complex MIMO system.Moreover,the equivalent SISO system reserves the interacting paths among the key variables of the original MIMO system,which can be applied to analyze the interactions among the multi-VSC.Eventually,based on the proposed SISO equivalent method,the interacting mechanism and the small-signal stability analysis of the VSC-dominated power systems without taking the dynamics of the DVC timescale into consideration are illustrated with typical scenerios.(4)Based on the internal voltage motion equation modeling method in ACC timescale,the internal voltage amplitude/frequency motion equation of VSC with taking the dynamics of the DVC timescale into consideration is proposed.The proposed VSC model can be used to study the effects of the DVC timescale dynamics on VSC’s characteristics.Based on the proposed VSC model,the effects of the DC-link voltage controller parameters on the time-varying characteristics of the actual active/reactive reference current and the unbalanced active/reactive current excitation-internal voltae amplitude/frequency response characteristics of VSC are analyzed.The results indicate that the effects of the DVC timecale dynamics on the time-varying characteristics of the active/reactive reference current mainly focus on the low and medium frequency bands.Moverover,the effects of the DVC timecale dynamics on the unbalanced active/reactive current excitation-internal voltae amplitude/frequency response characteristics of VSC can be neglected.(5)Based on the SISO equivalent method of the MIMO system,the equivalent SISO system of the VSC-dominated power systems with taking the dynamics of the DVC timescale into consideration is derived.The equivalent SISO system reserves the interacting paths of the VSC-dominated power systems The interacting paths not only contains the interacting paths through the DVC timescale links,but also contains the interacting paths through the ACC timescale links.Therefore,the equivalent SISO system can be used to analyze the effects of VSC’s DVC timescale and ACC timescale dynamics on the interacting mechanisms and the small-signal stability of the VSC-dominated power systems.Eventually,the interacting mechanisms and the small-signal stability analysis of the VSC-dominated power systems with taking the dynamics of the DVC timescale into consideration are illustrated with different scenerios.The results indicate that the DVC timescale dynamics of VSC should be considered if the active/reactive reference current induced by the DVC timescale dynamics cannot be neglected relative to the active/reactive feedback current,which can be serverd as the basis of simplifying the dynamic analysis of the VSC-dominated power systems.