| China has built the world’s largest AC-DC hybrid power grid with the highest voltage level.At the same time,the coupling between the AC network and the DC system is becoming closer,fault evolution mechanism is more complicated,and the risk of systematic fault is gradually increasing.It is a major and urgent national demand to effectively ensure the safe operation of large-scale AC-DC hybrid power grid.This paper fully explores the coupling mechanism of AC-DC hybrid power grids in multiple dimensions,explores the interaction effects of different control variables,and conducts a series of studies on the transient characteristics and control strategies of the converter.The main content and results of the thesis research are as follows:(1)The rapid prediction method and mitigation measure of single feed-in LCC-HVDC commutation failure are proposed.Starting from the mechanism of commutation failure,the commutation voltage-time area is used as the detection criterion,and two-parameter sinusoidal curve fitting based on the least squares method is used to realize the real-time calculation of the commutation area.Therefore,a fast prediction method of commutation failure based on voltage waveform fitting is proposed.On this basis,the formula for calculating the critical commutation area is further deduced and used to quantify the physical relationship between the severity of the fault and the quantity of advanced firing.An advanced firing control strategy based on critical commutation area is proposed.In order to further improve the ability of the DC system to withstand commutation failure and clarify the cooperation between advanced firing control and advanced current control,an advance current control based on commutation area theory is introduced for reducing the required commutation area.(2)The mechanism analysis and mitigation measure of single feed-in LCC-HVDC fault-recovery continuous commutation are proposed.Firstly,the causes of continuous commutation failure are sorted out and classified.Secondly,the control strategy switching sequence of inverter-side converters during the fault recovery process of commutation failures is analyzed,and the whole recovery process is clearly clarified by using the converter’s steady-state operating curve.The coupling mechanism and variation trend of relevant electrical quantities are deduced,and the mechanism of fault-recovery continuous commutation failure is revealed.In order to further improve the coordination of VDCOL and current deviation control,a limited VDCOL strategy is proposed.By real-time sensing of the inverter-side converter bus voltage,current deviation control of the inverter is fired in advance,and thus,effective mitigation of continuous commutation failure is achieved.(3)A boundary condition analysis method for simultaneous commutation failure of multi-infeed DC transmission system is proposed.First,the active power and reactive power characteristic equations of the inverter-side converter under three different control modes are derived.At the same time,in order to achieve the decoupling of the AC and DC power characteristic equations,this paper simplified the inverter side power characteristic equations to concern only one variable,namely the effective value of the inverter-side converter bus voltage.Secondly,by analyzing the steady-state power flow of the receiving AC grid,an analytical calculation method for multi-infeed interaction factor is proposed.Finally,taking the minimum turn-off angle as the criterion of commutation failure judgment and considering the effect of DC current change on commutation failure,the simultaneous commutation failure interaction factor index is defined,and a simultaneous commutation failure boundary condition identification method which considers the inverter voltage interaction is proposed.(4)The fault evolution mechanism of multi-infeed HVDC transmission system and the mitigation measures of continuous commutation failure are proposed.On the basis that the inverter power supply is equivalent to a voltage-controlled current source,a fault analysis model of the multi-infeed DC transmission system receiving grid is established,and the power-frequency expression of the converter bus fault voltage is derived.For minor and severe fault conditions,the relationship between certain electrical quantities in the AC and DC systems is established.Phasor diagram was used to qualitatively analyze the influence of the variation of equivalent current source of the inverter station on the voltage of the converter bus,and the fault evolution mechanism of the multi-infeed DC transmission system was revealed.Based on above analysis,the mechanism of continuous commutation failure in the multi-infeed HVDC transmission system is deeply analyzed,and the mitigation measures of continuous commutation failure based on inverter-side delayed firing angle control are proposed. |
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