Research On Correlation Characteristics Of DC System Fault Evolution And DC Control Protection Under AC Fault

As more and more DCs are connected to the AC grid,my country has formed the world's largest AC and DC hybrid grid with the highest voltage level.In high-voltage DC transmission based on grid voltage commutation,the converter components are easily affected by AC voltage fluctuations.A single AC system failure may induce cascading failures such as DC commutation failure,subsequent commutation failure,and DC blocking.This fully exposes the complexity of the evolution of DC faults in current AC-DC hybrid power grids,and poses new challenges to the analysis and control of the safety and stability of AC-DC hybrid power grids.Therefore,there is an urgent need to clarify the correlation characteristics of DC system fault evolution and DC control protection,so as to improve the resistance of DC commutation failure and DC blocking to AC faults.After the AC fault,the evolution process of the DC system fault can be summarized as: the first DC commutation failure after the AC fault is unavoidable;if the DC control participates in the adjustment,if the control is improper,it may cause subsequent commutation failure;after the failure of multiple commutation failure evolution It may trigger the DC protection to malfunction and cause the DC to be blocked.Therefore,to study the correlation characteristics of DC system fault evolution and DC control protection under AC faults,it should be specifically developed to study the correlation between DC commutation failure and AC faults,the correlation between subsequent commutation failure and DC control,DC blocking and DC control protection Of relevance.To study the correlation between commutation failure and AC failure.The mechanism is used to analyze the impact of AC faults on the state change of AC and DC systems and the commutation process,and the relationship between the commutation failure and AC faults is excavated by combining the fault recording of the inverter station after an AC fault in a typical AC/DC hybrid power grid.The analysis shows that AC fault is the root cause of DC commutation failure,which affects the commutation process by affecting the commutation voltage,DC current,and control system behavior.The discrete nature of commutation failure comes from the randomness of the AC fault moment and the game relationship between the control effect of the control system and the rise of the DC current.This leads to a large rangeof fuzzy areas in commutation.It is suggested that the input variable of the control system should be added with DC current index and the AC fault detection criterion should be optimized to accelerate the response speed.Finally,based on the AC-DC correlation characteristics,an improved strategy for commutation failure assessment is given.To study the correlation between subsequent commutation failure and DC control.Combined with the basic control structure of DC,electrical and timing analysis of the subsequent commutation failure of DC under AC faults is conducted to reveal the correlation between subsequent commutation failure and DC control.The analysis shows that the severe electrical fluctuation after the first DC commutation failure determines that the subsequent commutation failure is closely related to the DC control that detects the electrical fluctuation.The risk of subsequent commutation failure during DC faults comes from two aspects: One is that the rectifier side VDCOL ignores the AC side of the inverter side and raises the DC current reference;The second is that the introduction of the DC current deviation in the comparison of the fixed-off angle controller may mask the true trend of the change in the off angle.Accordingly,it is recommended to establish an interaction mechanism between the rectifier side and the VDCOL on the inverter side to limit the rectifier side VDCOL to increase the DC current reference value when the inverter side AC fault is not removed and the inverter side DC voltage is not restored.To study the correlation between DC blocking and DC control protection.Combining the failure of commutation and the action of DC control protection,an electrical timing analysis of the process of DC blocking caused by improper coordination of control and protection is performed to reveal the correlation between DC blocking and DC control protection.The analysis shows that the failure of the single-bridge commutation will produce the differential current used for the bridge differential protection,and the failure of the double-bridge commutation will produce the differential current used for the differential protection of the valve group.In the subsequent commutation,the Y bridge is more prone to commutation failure than the D bridge.Clarified the cause of DC blocking caused by improper control and protection coordination: Section I of bridge differential protection will return after preventing subsequent commutation failure,weakening its limiting effect on the DC current reference value,resulting in the DC current reference value being converted to constant power controlThe large value of the output will then cause the commutation failure of the double bridge at the same time and trigger the misoperation of the second stage of the differential protection of the valve group.From the perspective of avoiding DC blocking caused by AC faults,it is recommended to limit the reference value of the DC current of the constant power control output after the bridge differential protection section I returns.This work was supported by the National Grid Smart Grid Joint Fund Project(U1766213)and the National Natural Science Foundation of China(51677073).The correctness and effectiveness of some of the research results have been obtained in actual DC engineering PSCAD / EMTDC with detailed control and protection modules.The simulation model was verified.