| The traditional HVDC transmission technology based on thyristor plays a very important role in the strategy of West-East electricity transmission project in China.However,with the continuous construction of HVDC transmission projects,the DC drop points will be more and more in Eastern load center and the problem of commutation failure on the inverter will become increasingly prominent,which seriously threatens the safe and stable operation of the power system.The hybrid HVDC transmission system that the rectifier adopts line commuted converter(LCC)and the inverter adopts modular multilevel converter(MMC)not only meet the needs of the strategy of West-East electricity transmission project,but also completely eliminates the problem of commutation failure on the inverter.Therefore,it has been gradually applied in China’s actual projects.LCC-MMC hybrid HVDC as a novel HVDC transmission technology,its operation and control technology is not mature enough,which needs continuous research and improvement.The hybrid HVDC transmission system needs to work at the condition of low DC voltage in the harsh environment,the LCC partial valve group withdrawing or the failure of the AC system in the rectifier side.Therefore,this paper revolves around the pseudo bipolar and true bipolar Hybrid HVDC transmission system that the inverter adopts hybrid MMC(HMMC)composed of fifty percent half bridge sub-module(HBSM)and fifty percent full bridge sub-module(FBSM),and the control strategy of the fault in the rectifier side is studied through theoretical analysis and simulation verification.When the system operates at low DC voltage,the valve segment voltage mutation will cause the valve segment to bear the high voltage change rate.In view of this problem,the alternate arrangement strategy of HBSM and FBSM in each arm is put forward.To facilitate the flexible adjustment of the number of each sub module and meet the change of control strategy in the simulation system,a modulation and voltage sharing module of HMMC is customized.Meanwhile,the low DC voltage operation capability of the hybrid HVDC transmission system under traditional control strategy is analyzed,and the result is verified by simulation.Under the traditional control strategy,the two seed module’s capacitor voltages in each arm may not be balanced because of the unidirectional arm current when the system operates at low DC voltage condition caused by the fault in the rectifier side,etc.In view of this problem,taking the pseudo bipolar hybrid HVDC transmission system as an example,an improved circulation control strategy is proposed,and the selection of the amplitude and phase instruction value of the circulation component is derived in detail.Then an improved circulation controller based on vector control is designed.At the same time,a fault transient current suppression strategy based on virtual resistance and the limitation of current instruction is proposed in order to solve the problem that the DC current and HMMC AC current oscillation in the early stage of failure.And the effectiveness of the proposed control method is verified by simulation analysis.Applied to large capacity HVDC transmission system,HMMC usually adopts true bipolar structure.According to the characteristics of the true bipolar HMMC topology,the control strategy under the fault in the rectifier side is studied from the point of the DC component instruction value of the arm voltage.First,An improved selection method of the arm voltage DC component instruction value for the traditional true bipolar symmetric HMMC is put out,which expands the DC voltage range that the traditional true bipolar symmetric HMMC can operate.Secondly,the topology of the true bipolar asymmetric HMMC is proposed to avoid the unbalance of the two seed modules’ capacitor voltages in each arm.And the selection method of the arm voltage DC component instruction value is given.Meanwhile,the effect of the proposed control strategy and topology on the equipment of HMMC valve side is analyzed,and a ture bipolar hybrid HVDC simulation model is established in PSCAD/EMTDC to validate the proposed control strategy.At last,the research work is summarized and the next step of research needed is pointed out. |
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