Dynamic Characteristics Analysis Of UHVDC System With Reactive Power Compensators

The line commutated converter(LCC)based Ultra High Voltage Direct Current(UHVDC)system under hierarchical connection mode(HCM)is a new type of DC transmission scheme that has been developed in 2013.The basic feature of HCM is that on the inverter's end,the UHVDC system is linked to AC networks with different voltage levels(500 kV and 1000 kV).The LCC-UHVDC-HCM system has various advantages such as voltage and power stability,capability of maximum DC power transfer and low investment cost.In addition to many advantages,it has some operational challenges like cascaded commutation failures,longer fault recovery and temporary overvoltage under various disturbances.These can be coped with the installation of various reactive power compensators(RPCs)such as synchronous condenser(SC)and static synchronous compensator(STATCOM)at inverter's end.This thesis aims at investigating the characteristics analysis of LCC-UHVDC-HCM system with various RPCs.Initially,the impact of one AC system's strength on the dynamics of another AC system is analysed.It is examined that the system's strength of one AC source can effect the dynamics of another AC system.Then,the impact of different RPCs on the dynamic characteristics of LCC-UHVDC-HCM system is investigated.The different RPCs considered are SC,STATCOM and combination of both SC and STATCOM(hybrid reactive power compensator,HRPC).Various transient indices like commutation failure immunity index(CFII),commutation failure probability index(CFPI)and fault recovery time(FRT)are undertaken in order to assess the comparative performance of various RPCs.The results indicate that performance of RPCs changes during various disturbances.It is examined that STATCOM is better than other dynamic compensators in improving CFII and CFPI.SC and HRPC show almost same behavior in improving CFII.However,in case of CFPI,HRPC is superior to SC.The performance of SC is better than STATCOM and HRPC in improving FRT under severe fault conditions.However,under low fault conditions,all RPCs have equal performance in reducing the FRT.A novel index termed as "equivalent hierarchical mode short circuit ratio"(EHMSCR)index is proposed in order to quantitatively evaluate impact of RPCs on AC system's strength of LCC-UHVDC-HCM system.The basic theory and analytical framework regarding the proposed index are demonstrated in detail.An equivalent model of LCC-UHVDC-HCM system with AC system's strength modified via EHMSCR is established in order to ensure the application of proposed index.Then,three different case studies are carried out in order to further validate the accuracy and effectiveness of the EHMSCR index.PSCAD/EMTDC is used to confirm the satisfactory performance of the proposed index through comprehensive electromagnetic transient simulations.The results indicate that EHMSCR can accurately evaluate the system's strength of both AC sources(500 kV and 1000 kV)of LCC-UHVDC-HCM system with RPCs.To obtain better results associated with the improvements in dynamics characteristics such as cascaded commutation failures and fault recovery time,a coordinated control between LCC-UHVDC-HCM system and RPCs are presented.It is observed that the presented coordinated control shows greater improvement in case of installing STATCOM at inverter's end,moderate improvement in case of HRPC and no improvement in case of SC.In addition to this,the performance of the coordinated control is investigated under different control modes such as reactive power control(Q-control)and voltage control(V-control)in the outer loop of STATCOM.The analysis shows that coordinated control with V-control mode exhibits better performance in enhancing the dynamic characteristics of LCC-UHVDC-HCM system by improving the CFII,effectively reducing the CFPI and fault recovery time under various AC faults.