Studies On Fault Characteristic And Novel Differential Protection For AC System Connected To Inverter Station

High-Voltage Direct-Current(HVDC)transmission technology is an effective measure to achieve cross-regional and large-capacity power transmission,which is helpful to alleviate the problem of uneven energy distribution and coordinate economic development.Therefore,there are many HVDC transmission projects built in China,and the hybrid operation of AC and DC systems is becoming more common.For the AC system connected to inverter station,the AC system and the DC system influence each other.On the one hand,a fault in AC system can change the operating status of inverter station or even cause commutation failure,which is not conducive to the stable operation of hybrid system On the other hand,affected by the control system and the phenomenon of commutation failure,the output characteristics of inverter station are obviously non-linear,then the fault characteristics of the AC power grid at receiving end and the fault detection of protection device are influenced.In response to this problem,the paper explores the variation mechanism of fault characteristics,analyzes the fault characteristics,and proposes adaptive protection method for the AC system connected to inverter station.The main research contents and contributions of this thesis are as follows:(1)Variation mechanism analysis for the fault characteristics of the AC system connected to inverter station.According to the power characteristics of the inverter under different working conditions,the characteristics of reactive/active power distribution,inverter output current phasors and corresponding fault components,and the sequence networks' structure of the AC system with inverter station are analyzed.Then,the voltage and current phasors' relationship at both ends of faulty line are discussed for different AC grid structures.Studies have shown that,the change of the reactive power distribution between inverter station and AC system caused by the change of inverter's working status is the direct cause of the variation of fault characteristics;the existence of non-faulty lines can reduce the variation caused by inverter station and improve protection's working environment.(2)Action behavior and adaptability analysis for traditional protection principles.Combining the theoretical research and simulation analysis,the trajectories of current phasor,measured impedance,fault direction on the AC line connected to inverter station are described detailly under different fault conditions and different line structures.Then the action behaviors of traditional protection principles are analyzed,such as distance protection,current differential protection,and directional pilot protection.Analysis shows that the response characteristics of inverter station to AC fault will affect the fault detection ability of traditional protection principle,resulting in the refusal or mis-operation of distance protection and directional pilot protection,and the ability against to transition resistance of current differential protection is reduced;the protection principles based on zero sequence electrical quantity have good adaptability.(3)The virtual active power differential principle based on fault components and the integral instantaneous active power differential principle are proposed.The reactive power distribution in the AC system with inverter station changes with the inverter's working status,leading to the variation in the characteristics of voltage and current phasor,while the active power characteristics of faulty line are relatively stable,and the inverter station has little influence on active power differential principle.Based on this,the principle of active power differential protection is studied,and the virtual active power differential principle based on fault components is proposed to deal with the dead zone problem,and the principle of integral instantaneous active power differential protection is further proposed to improve response speed.Simulation results can verify the validity of proposed principles,which can be used to detect the faults in the AC transmission lines connected to inverter station.(4)A novel differential protection scheme based on active power is constructed,which can be applied to the AC grid connected to inverter station.The performance requirement for the protection used for AC power grid connected to inverter station is analyzed;by introducing a voltage threshold into distinguishing different fault scenarios,a comprehensive application scheme of active power differential criteria,integral instantaneous active power differential criteria,and integral instantaneous active power differential criteria based on fault components is constructed.Based on the AC/DC hybrid power grid simulation platform in RTDS,simulations can verify the proposed protection scheme's adaptability to the AC system contain inverter station,under various fault conditions and different AC grid structures.Theoretical analysis and simulation results show that the proposed protection scheme is not affected by the working status of inverter station,can accurately detect faulty lines,and ensure that the non-fault lines do not mal-operate;is with good ability against transition resistance and without dead zone problem.