| Flexible HVDC transmission based on voltage source converter(VSC)and modular multilevel converter(MMC)is able to independently control active power and reactive power and to supply a passive system without commutation failures.Therefore,flexible HVDC technology is especially suitable for fields like large-scale distributed energies integration,grid asynchronous interconnection,offshore wind power generation and passive power generation integration,however,with few studies on flexible HVDC line protection schemes.Furthermore,problems and shortcomings reveal when the HVDC transmission protection principles are applied to flexible HVDC transmission.This dissertation investigates these key issues,with the results and conclusions as follows:(1)The topology feature and hierarchical control strategy of converter are studied.The simulation model of flexible DC system is established in the electromagnetic transient simulation software PSCAD/EMTDC.The key component model and parameter configuration of flexible DC system are designed in detail,and the reliability of the simulation model is verified.(2)The characteristics of transient current after line faults in flexible DC system are analyzed.The changing features of DC line current are extracted by mathematic tools of waveform signal distance and moving average filter.The criterion of DC fault identification is constructed by using waveform difference of boundary currents,and a longitude protection scheme for DC line is proposed based on boundary transient current signal distance.The protection scheme is validated by simulation in MMC-HVDC.The proposed scheme is clear in principle with small cost of computation,and able to protect the whole line.(3)Types,advantages and disadvantages of fault current limiters are reviewed.The working principle of resistance fault current limiters is analyzed and the model is built in PSCAD/EMTDC.The effectiveness of resistance fault current limiters in flexible DC system is thus validated.(4)The DC fault transient features of VSC in flexible HVDC is analyzed,together with the DC fault voltage and current derived.A single-terminal protection scheme based on reactor voltage is proposed and verified in a multi-terminal VSC-HVDC system.The scheme has fast action,strong anti-interference ability,low sampling rate and can be used as the main protection of DC line.The proposed protection scheme is verified by simulation in multi-terminal VSC-HVDC.It is fast in action and stable against interference,has a low requirement on sampling frequency,and can be used as the main protection of DC line.Finally,the full text is summarized and further research trend is pointed out. |
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