Study On The Stable Operation Of AC/DC Power System Based On Voltage Source Converter

With the exploitation and utilization of large-scale renewable energy resource and the increase in the share of renewable generation in the grid,the significant changes have happened in the operational characteristics of the power network.The features of modern power system including following sections,fossil energy as the main body and renewable energy as a supplement,Bulk Power Generation,Stand-off delivery,Centralized scheduling control and management.Especially in long distance power transmission and interconnected power systems,the HVDC Transmission has played a very important role.Compared to LCC-HVDC,line commutate converter based high voltage direct current,VSC-HVDC,voltage sourced converter based high voltage direct current,not only has good controllability and fast response speed,can be available to supply for passive receive terminal network,but also can rapidly independent regulate the active power and reactive power,improve the transmission capacity of existing communication network and keep the system from voltage,power angle instability,so it can be used to improve the controllability and stability of AC system,and has the ability of fast failure recovery.At the same time,VSC-HVDC technology works without reactive power compensation problems,or commutation failure,having the characteristics of low harmonic level,easy power flow reversal and small area covered.VSC-HVDC technology is especially suitable for the renewable energy power supply,the large-scale electric field grid connection,non synchronous AC power grid interconnection,passive islanding power supply,city power grid capacity increasement etc..For the moment,VSC-HVDC has been vigorously developed though it in the transmission capacity share is less.This is because of its own unique characteristics and thanks to rapid advances in power electronics technology.Twenty years have passed since the first VSC-HVDC system has been put into operation,the transmission power of the VSC-HVDC system from an initial increase 3 MW to 3000 MW today.Meanwhile from the initial development of ±10kV to ±350kV in DC rated voltage,even ±500kV in certain project under construction.With the voltage grade of VSC-HVDC system gets higher and the transmission power is bigger and bigger,the stability problem of AC/DC power system is threatened more frequently.Beginning with the interface of AC/DC power system,this paper is mainly concerned about the mathematical model of AC/DC power system and the steady state performance of AC/DC power system is studied emphatically.The main achievements of this thesis are listed as following:(1)The quasi-steady-state model of VSC-HVDC systemThe general VSC-HVDC system modeling method is to make appropriate simplification at physical level before mathematical modeling.In this thesis,detailed mathematical models of the main electrical components of VSC-HVDC system(including AC transformer,AC filter,commutation reactor,DC circuit and capacitor)are established based on dq0 coordinate system.The transfer function of the converter is proposed,and then transfer function of each element is built when needed.Finally,the model simplification technique is used to simplify VSC-HVDC system model and its control system according to requirement of detail level.(2)Direct current control parameter with double loop structureThe design of direct current control parameters with double loop structure is divided into two parts: inner loop control parameter design and outer loop control parameter design.Firstly,in original quasi-steady-state model of VSC-HVDC system using the simplified technique PAMRA,the inner loop current controller is reduced to one order system,which is series connecting with inner PI part to forme first order closed-loop transfer function.As a result,the PI controller parameters of the inner loop current control system can be determined by the response speed requirements of the inner loop current control system.Then according to different control method,the inner loop current controller methods are established,through active & reactive power PI controller and AC&DC voltage PI controller to input of input(inner loop current controller output)-output(inner loop current controller input)model about inner loop current controller.Among them,the last two are established by small signal model.Finally,the PAMRA technology is used to simplify the model once more.If the model is reduced to two order system,parameters of PI controller in outer loop control system can be calculated by explicit expressions of two order system in frequency domain and time domain index.Compare with the ABB HVDC Light model,it shows that the designed method is effective.(3)An alternate iteration power flow solutionThe control equation LCC-HVDC system is mainly decide by the LCC internal variable and DC network internal variables,so it is easy to solve the AC and DC equations alternately;and in the four kinds of control modes of VSC-HVDC,constane active power control equation,constant reactive power control equation and constant AC voltage control equation are ralted to the active power,reactive power,AC voltage amplitude,while constant DC voltage control equation is associated with DC voltage.Two control equations must come from active power control equations and the reactive power control equation respectively.Therefore,compared with LCC-HVDC system,VSC-HVDC system is bound up the AC system much more.Based on the steady-state model of VSC-HVDC system,an alternate iteration power flow solution is put forward by full consideration of DC system control mode and AC/DC coupling characteristics.The basic idea of this method are:first of all,according to the control mode and the initial value of DC system,directly solve for transfer to communication network as the given value(power and voltage),and then given the AC network flow calculation.After the DC iteration convergence in AC network,check whether the last the given value has changed,if changed,according to the new value to recalculate,if not,then iterative convergence results for the results of flow calculation.The application effect and efficiency of this solution is verified by simulation.(4)The supporting strength quantitative index of AC system for VSC-HVDC system unitThe VSC-HVDC system can provide controllable active power and reactive power for the AC network,and the traditional power decoupling control is able to power decoupling.Therefore,the reactive power is the part of “all contribution” for the VSC-HVDC system unit.So in this,this paper presents apparent power equivalent effective short circuit ratio index,it can be used quantify the supporting strength index of AC system for VSC-HVDC system unit.Apparent power equivalent effective short circuit ratio works through self-impedance,mutual-impedance of network nodes,the votage vectors of nodes,and the active power and reactive power of all VSC system units.The results of the simulation andexperiments as given prove the workability of apparent power equivalent effective short circuit ratio being used to characterize the supporting strength quantitative index of AC system for VSC-HVDC system unit.(5)Analysis of stable operation of VSC-HVDC system unitBased on the equivalent steady-state power flow equation of the system network at the common node of AC and DC netwoek,the steady state safety constraints of VSC operation are formed from four aspects: the power flow equation solvable,the system controllable,the static voltage stability and the power angle stability.And then the criterion of stable operation is established.The regularity of the change of the stable operation criterion with apparent power effective short circuit ratio is also studied;the main factors that can not be stable when VSC-HVDC is connected to weak AC network are analyzed.Finally,a numerical method to calculate the effective apparent short circuit ratio under various control modes is presented and the accuracy of proposed method is verified by PSCAD simulation.(6)On line voltage stability evaluation of AC/DC hybrid systemAn online evaluation method for static voltage stability AC system is improved,which is promoted to AC/DC hybrid power system.This method is based on the solution of power flow equation,considering the relative influence of branch line to be measured,using the synchronous measurement of branch power flow,voltage and some other electrical quantities,calculated by simple on-line voltage stability index measured node.At the same time,the active power margin and reactive power margin of the active power and the on-line voltage stability reactive power index are also presented,the voltage stability index of active power and reactive power is considered as the on-line voltage stability margin index.The method is simple and rapid,and has certain practical value in engineering.