The Continuous Commutation Failure Suppression Method And AC Protection Optimization Strategies In AC/DC Hybrid System

With the construction of HVDC transmission project,a large scale HVAC/DC hybrid grid has formed in China.Compared with the traditional AC grid,there are new challenges in AC/DC hybrid system.The AC-side fault at the receiving end can cause the commutation failure or even continuous commutation failure of the HVDC system,which affects the correct action of AC protection and threatens the safe and stable operation of the AC and DC system.Therefore,the AC/DC hybrid system was taken as the research object in this paper,through theoretical derivation and simulation verification,the continuous commutation failure suppression method and AC line protection optimization strategies were studied.Firstly,the continuous commutation failure mechanism of HVDC caused by ACside fault was analyzed,and a continuous commutation failure suppression method based on dynamic current deviation control was proposed.Starting from the mechanism of commutation failure,the method made use of the relation between the extinction angle increment and the slope of current deviation ramp function,and adopted the second order generalized integrator to detect the dynamic change of threephase voltage and zero-sequence voltage at the receiving converter bus in real time.Based on this,the upper limit of the extinction angle increment in current deviation control ramp function was regulated to change its slope under different ac faults,thus implementing the adaptive adjustment of the extinction angle setting.Simulations in PSCAD/EMTDC prove that the proposed method can inhibit the continuous commutation failure and improve the performance of HVDC systems.Secondly,the adaptability of the traditional fault-component current based differential protection in AC/DC hybrid system was analyzed,and it may reject to act due to DC commutation failure and fault resistance under the unsymmetrical earth fault,so an AC superimposed-current differential protection immune to DC commutation failures was proposed.The cosine value of zero-sequence-current phase difference at both ends of the transmission line was used to compensate the differential current amplitude in traditional differential protection criterion based on fault-component currents,such that the internal and external faults can be identified accurately.The phase selection function considering the differential current amplitude difference between fault and healthy phases was constructed to discriminate the fault phase.Therein,the fault type identification criterion using the total sum of half-cycle sampling values of zero sequence current at both ends of the line was constructed to ensure that the proposed protection scheme only works during unsymmetrical earth fault.Simulation results verify the feasibility of the proposed protection scheme,and show that the proposed protection scheme can protect the whole length of the line reliably without the influence of DC commutation failure and fault resistance.Thirdly,the adaptability of the distance protection in AC/DC hybrid system was analyzed: when the DC commutation failure caused by the single-phase grounded fault via fault resistance on the AC side,the traditional distance protection may reject to act due to the factors such as the fault resistance and the equivalent current injected into the AC side by the DC system.Based on the isolation effect of convertor transformer on zero sequence current,a distance protection optimization strategy based on the ratio of the amplitude of the zero-sequence-current vector sum and difference at both ends of the line was proposed: the optimization criterion was constructed by combining the relation of amplitude ratio and 1 under internal and external faults,and then the logic operation was carried out with the traditional distance protection criterion to decide whether the protection act or not.Simulations in PSCAD/EMTDC show that this optimization strategy is not affected by fault resistance,fault location and DC commutation failure,and can reliably identify internal and external faults.Moreover,it does not require strict synchronization of data transmitted at both ends,and can assist to solve the problem of distance protection rejection in AC/DC hybrid system.Finally,the main research contents and results were summarized.