Stablity Analysis And Control Design Of AC Grid-Connected Voltage Source Converter

With the proposal and promotion of carbon peaking and carbon neutrality goals,it is imperative that traditional power system requires clean energy substitution of power generation and energy consumption.Therefore,the voltage source converters(VSC)based renewable power generation and high voltage directive current(VSC-HVDC)transmission system are widely applied in the generation,transmission,distribution,and consumption of the power system,which transform the traditional power systems into the power electronic devices dominant power systems.Compared with the synchronous and asynchronous generators based on traditional power systems,the grid-tied VSC system exhibits weak damping,low inertia,multi-time scale coupling,and multi-variable coupling characteristics.The grid-tied VSCs are high susceptibility to disturbance,which may lead power systems to run into an unstable region.Wind power generation connected weak AC grid,VSC-HVDC connected weak AC grid,and the VSC-HVDC connected wind power generation are the typical applications of grid-tied VSC,which are highly susceptible to wideband oscillation instability.The control loops of VSC have strong interactions with the weak AC grid,and the interactions would be enhanced when the VSC is applied for the VSC-HVDC connected wind power generation due to the multi-time scale coupling effects between the control loops of VSC-HVDC and wind power generation system.Amounts of field experience show that the interactions lead grid-tied VSC to be unstable with tens of Hz to thousands of Hz oscillation.Since the mechanisms of the wideband oscillation are different from the electromechanical transient time-scale torsional oscillation and electrical oscillation in the traditional power system,it is indispensable to analyze the wideband oscillation with the electromagnetic transient time-scale frequency domain small-signal models.The VSC adopts grid following controls(Constant active/reactive power control)when it is connected to the AC system with synchronous power generation,and the VSC adopts grid forming controls(Constant voltage magnitude and frequency).Based on the applications of VSC,the dissertation focuses on the stability mechanism and enhancement control of grid-tied VSC and renewable generation connected VSC-HVDC systems.Since the control loops have multiple time scales and are coupled,the stability of grid-tied VSC can be analyzed in the electromagnetic transient time scale and electromechanical transient time scale,respectively.In the electromechanical transients time-scale,the dq frame-based short circuit ratio is proposed(DQOSCR),based on which the stability margin of grid-tied can be analyzed by the local measurement instantaneous value.In the electromagnetic transient time-scale,the single-input and single-out(SISO)transfer function is proposed to simplify the modeling and stability analysis of grid-tied VSC,based on which the variation process of grid-tied VSC from stability to instability can be described clearly.Moreover,the output characteristics of grid-tied VSC are reshaped by the enhancement parameter design method,self-stability phase locked-loop design,and feedforward compensation design.The detailed analyses are shown as follows.(1)The electromechanical transient time-scale grid-tied VSC stability analysis.The physical connection between the conventional network topology-based short-circuit ratio based and the power flow-based operating short-circuit ratio is analyzed.The mathematical connection between the critical values of the two types of short-circuit ratio and the static voltage stability critical point is established.Based on the local instantaneous measurements,the extended power flow Jaccobi matrix is modeled,based on which the DQOSCR takes the influence of the control characteristics of the voltage-source converter and the impedance ratio of the AC system into account is proposed.(2)The electromagnetic transient time-scale grid-tied VSC stability analysis.The VSC can be connected to the synchronous AC system and no synchronous AC system for which the SISO analytical open-loop transfer function equations are established for the two cases,respectively.Based on the SISO transfer function,the control loops,main circuit parameters,and the operation state of the system are modeled as the multiplication of typical control loops.The consistency of the SISO transfer function model,impedance model,and state-space model in the stability analysis of the grid-tied VSC is demonstrated.In the weak synchronous AC grid-connected VSC system,the interaction between the grid impedance and PLL is studied with different grid impedance ratios.The permanent magnet directive generator(PMSG)connected VSC-HVDC is taken as an example for the stability analysis of no synchronous AC grid-tied VSC,in which the interaction between PMSG phase-locked-loop and VSC-HVDC outer-loop voltage control is analyzed.(3)The enhancement control for grid-tied VSC stability is proposed.A parameter optimization method considering the interaction between the control loops of the grid-connected converter is proposed.Based on the estimated grid-tied VSC crossing-over frequency,the control parameter can be designed as the requirement of stability.An improved control structure to enhance the self-stability of the phase-locked loop is proposed.By adding the magnitude correction loop to the phase-locked loop,the magnitude provided by the phase-locked loop at the crossing-over frequency of the SISO open-loop transfer function can be decreased without changing the output phase-frequency characteristics of the phase-locked loop,which is useful to improve the stability of grid-tied VSC.A feedforward compensation control without the requirement of parameter design is proposed.The feedforward loop is added in the current reference to compensate phase-locked-loop dynamics under disturbance,which can enhance grid-tied VSC stability without adjusting the control parameters or changing the phase-locked loop structure.