Research Of Low Frequency Oscillation In AC/DC Hybrid System Containing VSC-MTDC

With the exhaustion of traditional energy and people’s awareness of environmental issues,the incorporatation of large-scale renewable energy resources has become a research hotspot at present.Voltage source converter based high voltage direct current(VSC-HVDC)is one of key techniques of renewable energy integration because of its advantages,such as independent adjustment of active and reactive powers,two-direction controlled,power supply ability to passive systems as well as no communication failure,etc.Voltage source converter based multi-terminal direct current system(VSC-MTDC)is a system developed on the basis of the traditional two-terminal DC system,which consists of three and more VSC stations.Compared with the traditional two-terminal system,VSC-MTDC has a better flexibility,economy and reliability,which is more suitable for application scenes such as the interconnection of offshore wind turbines,island power transmission and city power distribution network,etc.With the development together with incorporatation of multi-terminal,high-capacity and high voltage VSC system,the small-signal stability problem of AC/DC hybrid system containing VSC-MTDC becomes increasingly outstanding.Several investigations have shown that the dynamics of phase locked loop(PLL)may cause the instability of the whole system if adopting vector current control(VCC)and connected to weak AC systems.Therefore,it is of great significance to investigate the influence and suppression of VSC-MTDC on low frequency oscillations to ensure the secure and stable opration of AC/DC system.This paper firstly analyzes the difference of VSC-MTDC with the traditional two-terminal system,i.e.,the difference of DC topologies and connection types and thus investigates the power flow calculation algorithm,electromechanical transient modeling and linearized model of AC/DC system containing VSC-MTDC.And then,the classical damping torque analysis(DTA)theory was entended to derive the close-loop damping torque index(DTI)model and applied to the analysis of VSC-HVDC supplementary damping controller(VSDC).Considering the characteristics of wide-area measurement system(WAMS),DTA was applied to the design of VSDC and suppression of low frequency oscillation and some conclusions are derived.The main innovation work of this paper can be summarized as follows:(1)This paper studied the steady-state model of VSC station and MTDC and pointed out that the biggest difference is there may exist direct current point of common coupling,DC-PCC(DC-PCC)which is considered into the power flow calculation model of this paper.Then different control strageties and mechansims of VSC-MTDC are investigated and the respective power flow calculation equations are deduced.On this basis,a novel power flow calculation algorithm of AC/DC system containing VSC-MTDC is proposed,which can be applied to different DC topologies and connection types.The algorithm can combine the astringency of unified iterative method and the efficiency of alternating iteration method,whose mechanism is similar to PQ decoupled load flow method of AC system.It neglects the complex coupling relationship of systems and inaccurate boundary conditions which will not enlarge the error,prevents the algorithm trapped into some iteration and gurantee its flexibility and astringency.For some scenes such as the interconnection of area grids through VSC-MTDC and integration of offshore wind turbines,the algorithm can keep its astringency and efficiency no matter whether the AC system is weak or not.(2)This paper presents the traditional elecrtomechanical transient simulation model of two-terminal system and analyzes the reason why the model is no longer feasibile for VSC-MTDC.Similar to the mechanism of admittance matrix,the quasi-admittance matrixes of VSC-MTDC are derived and the unified model to update DC-PCC voltages and DC line currents is proposed.The model can conduct the unified calcultaion of DC-PCC voltages and DC line currents which is clear and easy to realize.The quasi-admittance matrixes keep constant in every interation period and thus will not cause extra calculations.The linearized state-space model of VSC-MTDC is illustrated including MTDC,VSC station and the interaction with AC system.The respective equations are deduced to conduct the linearization of AC/DC system containing VSC-MTDC.Based on the unified model,the simulation of DC system and the whole AC/DC system can be conducted.Different from the traditional imporved Euler method,the simulation of DC system must iterate to the given accuracy in this paper in order to reduce the truncation error and keep the the simulation accuracy.(3)At present,the model of DTI is calculated when the amplication parameter is zero and the modes are open-loop eigenvalues.Thus,the DTI obtained are open-values without damping controllers in fact.The variation of system modes is a nonlinear process and obviously the open-DTI is not accurate.This paper derives the closed-loop DTI model which can make the analysis more precise.On this basis,a new sensitivity is proposed and a novel supplementary damping controller design method is derived based on this sensitivity.The new index is more visualized and has a clearer physical meaning which provides a new method for controller parameter online adjusting when the operation condition is varying.Then the DTI theory of VSDC is derived and applied to the design of VSDC including the selection of installation location,tunnel and input signal and parameter tuning.(4)The development of WAMS is greatly helpful for oscillation suppression of large-scale interconnected power systems.However,the time-delay and data-loss characteristics of WAMS have threatened the stable operation of power grids.In this paper,data-loss is added into the sampling model considering time-delay to derive the unified model of sampling data according to its mathematical expectation.Based on second-order Pade approximation,the eigenvalue calculation model of power system considering time-delay and data-loss characteristics is deduced which can be applied to analyze the detailed influence of time-delay and data-loss on low frequency oscillation and conduct the design of supplementary damping controller combined with phase compensation method.Simulation results show the proposed method makes a good performance to damp oscillations and improve the small-signal stability of both pure AC system and AC/DC hybrid system.