Research On Design Technology For Capacitor Commutated Converter Based HVDC Transmission System

The line commutated converter(LCC)uses the semi-controlled power electronic devices,which may cause commutation failure(CF)risks when the AC voltage at the inverter side drops to a certain degree.The CF will lead to interruption of transmission power and threaten the stable operation of the power system in severe AC fault cases.The LCC is not able to operate effectively when connected to extremely weak AC systems.To address this issue,the voltage source converter(VSC)technology and the capacitor commutated converter(CCC)technology were developed almost simultaneously.In the early stages,the VSC technology faced challenges in voltage boosting and had limited power transmission capabilities,while the CCC technology presented greater potential at that time.The CCC is constructed as a conventional LCC modified with a series capacitor between the commutated transformer and the thyristor valve in each phase.The CCC topology has been applied in the Garabi Project in Brazil,the Rapid City Project in the U.S.and the Rio Madeira Project in Brazil.It possesses the high value of academic research and engineering application.Compared to the LCC,the CCC technology can enhance the immunity to the first CF,reduce the reactive power compensation capacity,improve the power factor and lower the load rejection overvoltage.In the system design scheme of the CCC based high voltage direct current(CCC-HVDC)transmission system,the main circuit parameter calculation and the AC filter design are two critical modules.To meet the system design requirements of the CCC-HVDC,this paper conducts a systematic study on the design technology for the CCC,including the main circuit parameter design,the AC harmonic current calculation,the AC filter design and the continuous CF mitigation method.(1)This paper proposes a main circuit parameter design method suitable for the CCC,which provides the steady-state operating parameters for the system design of the CCC-HVDC systems.Firstly,a two-terminal LCC-CCC HVDC transmission system is introduced.Then,the steady-state mathematical model of the CCC is derived and the principle for selecting commutation capacitor values is proposed.The calculation method and detailed procedure for determining main circuit parameters are provided,followed by the presentation of the calculation result for a case study.The result shows that the CCC has the characteristic of reducing the reactive power compensation capacity.Finally,at the rated DC power level,the simulation results from the PSCAD/EMTDC and the calculation results from the proposed method are compared to verify the accuracy of the proposed calculation method for main circuit parameters.(2)This paper proposes a time-domain piecewise analytical AC harmonic current calculation method suitable for the CCC,which can fastly and accurately calculate both characteristic and non-characteristic harmonic currents considering various asymmetric factors.It meets the fast computation requirements for the system design of the CCC-HVDC systems,which needs to consider the operating conditions in the order of hundreds of thousands.Firstly,detailed mathematical models are presented for both the commutation process and the non-commutation process of the CCC.The calculation formula for the overlap angle is derived.Then,the analysis of how to calculate AC harmonic currents considering various asymmetric factors is given.Based on a CCC simulation model built in the PSCAD/EMTDC,the accuracy of the proposed time-domain piecewise method is validated by being compared to the characteristic harmonic currents of the existing analytical calculation method under ideal conditions.The accuracy of the proposed method is also verified by being compared to the AC harmonic currents obtained from the time-domain simulation under nonideal conditions.Finally,the influence of the asymmetric factors on the overlap angle calculation and the influence of the commutation capacitor variation on the commutation parameters are studied.(3)This paper proposes an AC filter design method for the CCC at the inverter side.Firstly,the fundamental principle of the AC filters is explained.Then the impedance-frequency characteristics of the tuned filters and the damping filters are presented.Subsequently,the detailed steps for calculating the performance and the steady-state rated values of the AC filters are proposed.Finally,a case study of the AC filter design at the inverter side of the CCC is presented.By comparing with the AC filter configuration capacity of the LCC under the same conditions,it is showed that the CCC can reduce the number of the parallel capacitors in the AC filter,which further reveals the characteristic of the CCC in reducing the reactive power compensation capacity.(4)This paper proposes a novel converter topology called static synchronous compensator based CCC(SCCC),which is designed to reduce the risk of continuous CFs.Firstly,the topology and the mathematical model of the submodule cascaded STATCOM are introduced,which can achieve the functionality of the reactive power compensator and the active power filter through the designed controller.Then,the mechanism of the continuous CF in the CCC is studied.Next,a comparative analysis of the power transmission curves under the steady-state operation is conducted among the LCC,the CCC and the SCCC.Additionally,the maximum power curves for the SCCC under different short-circuit ratios and capacities are provided and analyzed.Finally,simulation models are built in the PSCAD/EMTDC for the LCC,the CCC and the SCCC.For these three types of converters,a comparative analysis of the transient characteristics under the severe single-phase-to-ground fault and the mild three-phase-to-ground fault is performed.The commutation failure immunity index(CFII)and the commutation failure probability index(CFPI)are given to compare the mitigation degree of the first CF among the three converters.The results validate that the SCCC not only reduces the probability of the first CF but also mitigates the risk of the continuous CF.