| In order to take full use of the superiorities of both line commutated converter(LCC)and modular multilevel converter(MMC),hybrid utral high voltage direct current system(Hybrid-UHVDC)is formed by flexible combination of two kinds of converter.The hybrid-UHVDC technology has broad application prospect and development potential in China’s cross-regional large-scale absorption clean energy,boosting the “dual carbon” goal,and the new power system construction with clean energy as the main body.Among those hybrid-UHVDC topologies,LCC and MMC series hybrid-UHVDC topology provides a more economical,flexible and convenient pathway for long-distance and bulk-capacity overhead line transmission.This hybrid-UHVDC topology has been applied by the ±800 k V/8000 MW Baihetan-Jiangsu hybrid cascade UHVDC transmission(BJHC-UHVDC)project,which is being planned and constructed by State Grid Corporation of China(SGCC).BJHC-UHVDC project,as a typical topology engineering application of LCC and MMC series hybrid-UHVDC topology,will play an important engineering demonstration role on leading technological innovation in the future long-distance and bulk-capacity overhead line UHVDC project construction,the flexible transformation of conventional LCC-HVDC projects and other fields.However,BJHC-UHVDC system still has a series of key technical issues that have not been well solved,and there is an urgent requirement for in-depth and systematic research on BJHC-UHVDC system.Combined with the practical project,this thesis attempts to carry out systematic research on BJHC-UHVDC system from the following aspects:(1)Efficient MMC electromagnetic transient simulation modelAs MMC contains a large number of power electronic switching devices,the detail degree and computational efficiency of MMC electromagnetic transient model are the most important factors to be considered in the electromagnetic transient simulation modeling of hybrid-UHVDC system.However,the MMC electromagnetic transient model cannot combine the computational accuracy and simulation efficiency.Thereby,an MMC based half-bridge submodule(HBSM)electromagnetic transient model with simple modeling method,high simulation accuracy and obvious acceleration effect is developed.Firstly,two equivalent circuits of the MMC arm are derived based on arm equivalent: one is based on the arm current,which reflects the relationship between the arm equivalent output voltage;the other is based on the average capacitor current,which models the charging and discharging of the arm equivalent capacitor.Then,based on the trapezoidal integration rule and the nested fast and simultaneous solution,the MMC electromagnetic transient model is developed,which can accurately represent the dynamic responses in both de-blocked and blocked modes.Finally,the accuracy and computational efficiency of the proposed model are verified against the MMC Thevenin equivalent model and average value model based on the electromagnetic transient simulation(2)Overall control and operation characteristics of the BJHC-UHVDC systemThe overall control and steady-state operation characteristics of BJHC-UHVDC system are the basis and premise for the follow-up research.Firstly,the basic control strategy of BJHC-UHVDC system is designed,and the U-I operation characteristics are analyzed based on the steady-state mathematical model.Then,the DC fault characteristics and fault ride-through strategy of BJHC-UHVDC system are analyzed.Then,on the basis of analyzing the reason of power imbalance and unplanned power return among MMC units,two improved coordination control strategies among MMC units are proposed.Finally,the electromagnetic transient simulation results verify the effectiveness of the proposed basic control strategy and the two coordinated control strategies.(3)Fault ride-through strategy under sending-end AC fault for BJHC-UHVDC systemWith the limitation of MMC voltage modulation ratio,when the sending-end AC fault is extremely acute,due to the unidirectional conductivity thyristor,the current cut-off will unavoidably exist,resulting in power transmission interruption and enormous impacts on both AC and DC systems.Hence,a sending-end AC fault ridethrough control strategy based on adaptive MMC DC voltage reduction is designed.Firstly,under the varying severity of sending-end AC faults,the U-I operation characteristics are meticulously analyzed.Then,based on the enlarged MMC operation range with the third harmonic voltage injection and reactive power dynamic adjustment,a sending-end AC fault ride-through strategy is proposed to adaptively regulate the MMC DC voltage relying on fault severity.Finally,the effectiveness and feasibility of the proposed coordinated control strategy is verified through several simulation scenarios of varied fault severity.(4)Fault ride-through strategy under receiving-end AC fault for BJHC-UHVDC systemUnder receiving-end AC fault,the inverter LCC may suffer the commutation failure,and three MMCs power delivery will also be impeded,resulting in vast surplus power at the DC-side inverter.Consequently,the sub-modules are compelled to overcharge,and eventually,the sub-module capacitor voltages may exceed these safety thresholds.The valve-side single-phase-to-ground fault in internal MMCs also force severe upper-arm sub-module overvoltage.In order to suppress the sub-module capacitor overvoltage,by employing the MMC-based embedded battery energy storage system(BESS),a fault ride-through strategy under receiving-end AC fault for BJHC-UHVDC system is proposed.Firstly,the inducements of sub-module overvoltage are analyzed.Then,on the basis of introducing the basic structure and operation principle of MMC-BESS,a fault ride-through strategy is proposed.Finally,compared with the controllable metal oxide arrester(MOA)dopted in BJHC-UHVDC project,the effectiveness of MMC-BESS in suppressing the sub-module capacitor overvoltage and improving the system fault ride-through is verified.(5)Research on the harmonic transfer and resonance characteristics for BJHC-UHVDC systemDue to the LCC nonlinear switching characteristics,a large number of harmonics are generated in the DC side.When the harmonic current flows into MMC,the normal operation of MMC is affected through the sub-module switching.Besides,MMC has low DC-side impedance characteristics in the low frequency range,which may aggravate the DC loop low frequency resonance of the BJHC-UHVDC system.Firstly,considering the circulation suppression control,based on average switching function,the harmonic transfer characteristics for DC side harmonic current in MMC is deduced.In addition,with the electromagnetic transient simulation,the influence of DC side harmonic current on the harmonic components of MMC electrical parameters is studied and verified,such as sub-module capacitance voltage,arm voltage,AC output voltage,and so on.Then,based on the time domain simulation,using the test signal method,the DC-loop low frequency resonance characteristics of BJHC-UHVDC system are studied.Finally,the DC side impedance equivalent model of BJHC-UHVDC system is established.Based on the steady-state frequency domain analysis method,the DC-side impedance corresponding to the DC circuit impedance equivalent model is calculated,and the correctness of the DC-loop impedance equivalent model is verified by comparing with the time domain simulation results. |
