Study On Protection And Reclosing Technology Applicable For Transmission Lines Connected With Centralized Inverter-interfaced Generators

In recent years,renewable energy power sources represented by photovoltaic power and wind power have become a main development direction for global energy transformation with rapidly increased installed capacity.According to the different modes of interconnection between renewable energy power sources and external AC power grids,they can be divided into two categories which are the inverter-interfaced generator(IIG)and doubly-fed induction generator.IIG,which are gradually occupying a dominant position in renewable energy power sources,mainly include photovoltaic power sources,direct-drive wind turbine generators and voltage-source converter high-voltage direct current(VSC-HVDC)connected offshore wind power plants.As the power generation mode,circuit topology,and control strategy of IIG are quite unique,its fault characteristics are very different from traditional synchronous motors,which have many adverse effects on the relay protection and automatic reclosing of power grid,thus threatening the safe and stable operation of the power system.Funded by the national key research and development program,a series of research,which focus on the fault current characteristics of IIG,protection of AC tie line connected with IIG,DC line protection for VSC-HVDC connected offshore wind farms and reclosing technology applicable to transmission lines connected with IIG,are carried out in this dissertation.The study on IIG's fault current characteristics is an important basis for the fault analysis and protection research of power grid including IIG.However,in the existing research,the influence of the DC voltage fluctuation of IIG during fault period is generally ignored,and the study on the fault characteristics of VSC-HVDC connected offshore wind farms is relatively lagging.Aiming to solve these problems,a unified analysis method for solving the fault current of different types of IIG is proposed by taking account of the DC bus voltage fluctuation,and a theoretical calculation model of its transient and steady state is established,according to the characteristics of the topology and low voltage ride-through(LVRT)control strategy of IIG.The correctness of the theoretical method is verified by the simulation results and LVRT experiment data.Furthermore,the effects of several factors including proportional integral controller parameters,depth of fault voltage sag,and load level on IIG's fault characteristics are analyzed.The obtained results can provide an important reference for the control parameter design and protection research of power grids including IIG.The performance of protections on IIG farm's tine lie,which serves as an important channel for power transmission to the grid,is directly affected by the complex short-circuit current characteristics of IIG.Especially for the distance protection,its performance is seriously degraded,and potential safety hazards are prominent.In this dissertation,the existing issues in the application of traditional phase selection components to the tie line of IIG farm are analysed according to the fault characteristics of IIG.Furthermore,the sequence voltage expressions in the case of different types of faults,which take account of the influence of the transition resistance,are theoretically derived.Hereby,a novel phase selection method by using sequence-voltage phase comparison and phase-voltage amplitude comparison is proposed.The simulation results show that the proposed method is accurate and reliable,and has a strong tolerance towards large fault transition resistance.In terms of impedance measurement components,the applicability of traditional impedance components to the tie line of IIG farm is systematically analyzed.Aiming to solve the existing problems such as weak tolerance towards transition resistance,the possibility of steady-state overstepping misoperation in the case of forward external faults and the possibility of misoperation in the case of anti faults,etc,the improvement measures and engineering application suggestions are proposed,including adding novel directional components,adopting negative sequence voltage polarization and blocking the zone I distance protection,etc.The effectiveness of the proposed scheme is verified by digital simulation results.As a type of IIG,VSC-HVDC connected offshore wind farms have long-distance DC transmission lines.When a pole-to-ground fault occurs,it will be difficult to identify the faulty line due to the small fault current.To solve this issue,starting from the two-terminal VSC-HVDC connected offshore wind farm,a theoretical method for solving the pole-to-ground fault current is proposed,and the variation characteristics of transient fault current are summarized and analyzed,which are further deduced to the multi-terminal VSC-HVDC connected offshore wind farm.On this basis,a novel protection scheme for pole-to-ground faults by using single terminal quantities is proposed.To identify internal and external faults,the proposed scheme comprehensively employs the amplitude and direction characteristics of the transient current of the positive and negative lines on the same terminal.The digital simulation results show that the proposed scheme is fast and reliable,and has a strong tolerance towards large fault transition resistance.Since IIG does not have strong voltage regulation capability and physical inertia which is conducive to frequency maintenance,the solutions of blocking three-phase reclosing and tripping IIG before reclosing are generally used for its tie line protection,resulting that plenty of IIG exit operation or the power supply of local load is interrupted in the case of transient faults.To solve this issue,according to the islanding control strategy of IIG and the principle of phase-locked loop(PLL),a vector relationship model of IIG port actual voltage,PLL calculation voltage,grid-connected point voltage and so on before and after reclosing is established.Based on this,the influences of reclosing phase difference,amplitude difference and frequency difference on reclosing impulse voltage and current are analyzed,and a synchronous check reclosing strategy applicable to the tie line of IIG is proposed,which can effectively improve the success rate of three-phase reclosing without tripping IIG.For single-phase reclosing,according to non-full phase operation control strategies of IIG,the sequence network analysis models are established,and a theoretical calculation method of the gird voltage is proposed,which takes account of different neutral point grounding modes of step-up transformer.It is revealed that when the neutral point of step-up transformer is grounded through a gap,it may cause overvoltage during the non-full-phase operation,leading to the safety hazard of quitting operation of IIG farms.On this basis,the applicable scope and application suggestions of single phase reclosing are given by analyzing the main factors of the overvoltage level,in order to play its role better and improve the power supply reliability of power grid.At the end of this dissertation,systematical summary about the research work is given and the further research directions are pointed out.