Study On Post-Fault Electromagnetic Transient Issues For UHV High Series Compensation Lines

In order to solve the transmission capacity limitation issue for power delivery channel of the large scale energy base in China,it is necessary to adopt more than 40% high series compensation(SC)technology in the long distance UHV AC transmission line.Under the influence of series compensation capacitance and rated voltage rise,when UHV high series compensation line occurs inner fault,the short-circuit current through the series compensated line is greater and the residual voltage across the series capacitor is higher,which cause the electromagnetic transient process excited by high amplitude current and voltage to be more serious in the event of a fault occurrence and removal.It stresses more significant adverse effects on electrical shock of the circuit breaker(CB)and neutral point reactor for shunt reactor,and also to electromagnetic problems such as system overvoltage,secondary arc extinguishing.The seurity of main equipment and power grid is threatened,which make it need further research on the influence mechanism and characteristics,and put forward some effective solutions.In this disertation,taking the occurrence time of the four types of post-fault electromagnetic transient problems of high series compensation lines,the main research work and innovative achievements are obtained as follows:(1)Based on the overvoltage protection action characteristics including the saturation of metal oxide varistor(MOV)and Gap trigger,the equivalent mathematical models for series compensator of UHV series compensation line are established.The physical process of the fault current conversion before and after MOV and Gap action is proved,and also the variation characteristics and influence factors of the AC and DC component for series compensation line CB short-circuit current.The occurrence mechanism and conditions of zero crossing delay for series compensation circuit breaker short-circuit current are put forward.In order to solve the short circuit current zero crossing delay problem for the opposite side of the circuit breaker for SC under unilateral arrangement caused by the MOV action,several methods are put forward,such as optimizing the series compensation layout pattern,shortening the SC bypass time,adjusting the both sides line breakers tripping sequence and so on.The methods can effectively suppress the zero crossing delay phenomenon on series compensation CBs short-circuit current,and avoid the failure risk of UHV SC circuit breaker clearing short-circuit current.(2)The characteristics of macroscopic statistical distribution of peak value via short-circuit current of transient recovery voltage(TRV)across CB fracture during switching after short circuit current zero crossing for UHV high series compensation line are putforward,and also of the microscopic characterization parameter of waveform rise rate and peak value time.The conclusion is cleared that the TRV switching capability of existing UHV CBs can not cover all the fault scenarios of high series compensation line.The peak time delay phenomenon of the CB TRV waveform caused by the SC bypass is found and an improved TRV evaluation method is proposed,which solves the problem that the existing TRV evaluation standard is not suitable for the lines installed with SC.In view of the fact that TRV exceeds the standard when the CB clears multiphase fault,solution is put forward in two aspects including comprehensive comparison of TRV optimal suppression measures and increase of the UHV CB swithing expected TRV test requirements.It provides a technical scheme for the reliable switching of the CB under the condition of high series compensation line.(3)The generation mechanism of overvoltage occurred in the sound phase and adjacent lines when UHV high series compensation lines happens to be fault and disconnection is noted.The fault clearing switching overvoltage simulation method covering multiple influence factors for UHV high series compensation lines is proposed.Statistical research on fault clearing switching overvoltage values and distribution characteristics along line is carried out,and conclusion is drawn that the overvoltage amplitude of the fault clearing switching overvoltage of the long distance UHV series compensation line may exceed the standard control level of UHV AC system.The inhibition effects of various measures on the amplitude of fault clearing overvoltage and insulation flashover rate are analyzed.A comprehensive evaluation method for the fault clearing overvoltage is established,which is based on the integration of overvoltage limit and line insulation flashover control.The best measures to restrain the fault clearing overvoltage of the high series compensation lines are put forward.(4)Considering the difference of SC working states,the thevenin equivalent circuit after UHV SC line CB breaking single phase fault is established.The impact machanism of seacondary arc current and neutral point reactor electrical stress occurred after the UHV SC line single phase fault disconnection by series compensation degree increase,high saturation and SC bypass is revealed.The characteristics of secondary arc current and recovery voltage,neutral point reactor voltage and current for UHV high series compensation line are defined,and the influence of high series compensation on CB single reclosing time setting,neutral point reactor insulation level and current tolerance are evaluated.It is proposed that shunt reactor saturation characteristic must be considered when researching UHV series compensation lines secondary arc current and neutral point reactor electrical stress issue,especially for high series compensation degree.The influence of the movement speed on the secondary arc current and neutral point reactor electrical stress by linkage bypass is analyzed,and the target control time is specified.On the basis of active suppression method,the withstand current parameter optimization design requests for UHV neutral point reactor is proposed form the passively lifting equipment parameters aspect,which can guarantee the equipment safety of neutral point reactor in UHV high series compensation application scenarios.