Electromagnetic Transient Characteristics And Control Technology Of UHV Half-wave Length AC Transmission

Worldwide energy transmission planning urgently needs to develop ultra-long distance,large capacity transmission mode.UHV half-wavelength AC transmission(HWACT)with its remarkable technical and economic advantages,provides a special technical mode for it.However,the advantages of HWACT are based on steady operation.When switching operations or system faults occur,the HWACT may have serious electromagnetic transient problems,including system overvoltage,secondary arc current(SAC)and recovery voltage(RV),transient recovery voltage(TRV)of circuit breakers,their control technologies are the key problems that restrict the engineering application of HWACT.Therefore,this dissertation deeply studies the generation mechanism,regular characteristics and solutions of the core electromagnetic transient problems faced by UHV HWACT.The main research contents and innovative results achieved are as follows:The generation mechanisms of traditional overvoltages in HWACT system are studied,including power frequency overvoltage,switching overvoltage and grounding short-circuit overvoltage.By constructing a source-to-grid UHV HWACT system,the characteristics of the traditional overvoltages are analysed,including the amplitude of the overvoltage and its distribution along the line,the waveform characteristics and the duration characteristics,etc.According to grounding short-circuit overvoltage on series resonance condition,the control scheme of metal oxide arresters(MOAs)installed along the line and active grounding in the middle of the line is proposed.There is a new type of overvoltage phenomenon in half-wave length transmission line(HWLL)caused by power fluctuation(PF),so this paper defines it as power fluctuation overvoltage(PFO),which belongs to temporary overvoltage category.Based on uniform transmission line,electromechanical transient and electromagnetic transient simulation,the generation mechanism of PFO is expounded.The time scale of PFO extends from microsecond or millisecond of traditional overvoltages to second level in the case of single-phase instantaneous grounding.The amplitude of PFO is always high,the duration is long,and the coverage on HWLL is wide.The power angle characteristic of the generator is the key factor to show the characteristics of the above PFO.The convention which is generally not using arresters limit temporary overvoltage should be broken for HWLL.Thus,considered from overvoltage and equipment tolerance,the mitigations of PFO are proposed for UHV HWLL,which are configuring MOAs differentially along HWLL and with using of inputting high speed grounding switch(HSGS)along the line during fault period,or generator tripping after fault.Thus,the theory and control technology of HWACT overvoltages are further completed.With the analyses of frequency and time domain,the steady state,transient and distribution characteristics of high amplitude SAC and RV for HWLL are analysed due to the mechanism of power frequency resonance.Considering the influence factors such as arc resistance,transmission power,system network structure and so on,the unique SAC and RV characteristics of UHV HWLL are explained,which are different from conventional transmission lines.The configuration scheme of HSGSes along the HWLL is put forward for undermining the resonance condition,and the working conditions of induction voltage and current of HSGS during breaking are determined.Thus,the secondary arc theory of HWLL may be further completed,and the scheme of HSGS can be optimized.The characteristics of breaking short-circuit current and TRV for HWLL circuit breaker during breaking the different fault locations are studied.Then the electromagnetic transient analysis method for actual breaking short-circuit current of circuit breakers is provided.It indicates that the TRV peak and TRV rising rate under the remote fault locations are both high because of the serious resonance overvoltage of its line side superimposed with the high-frequency oscillation voltage of its source side,which far exceeds the breaking capacity of the UHV circuit breaker.The installation of MOAs along the HWLL and opening resistor in circuit breakers are proposed for inhibiting TRV as well as the inhibition levels.The method of MOAs and HSGSes configuration along the HWLL are studied as well as the determination method of the rated induction voltage and current of HSGS,including MOA rated voltage,MOA installation position and configuration quantity,HSGS installation position and configuration quantity,and the coordinating action strategy between HSGSes.The limitation requirements of UHV HWACT system including grounding short-circuit overvoltage and PFO are proposed.Thus,the overall control scheme and suppression effect of electromagnetic transient problems for UHV HWAVT system are obtained.