Research On Mechanism Analysis And Mitigation Strategies For The Evolution Process Of HVDC Commutation Failure In Complicated Power Systems

Commutation failure(CF)is a common fault of line-commutated-converter-based high voltage direct current(LCC-HVDC),which threatens the safe and stable operation of power systems by influencing the active power transmission,voltage stability and other aspects.Since multiple HVDC systems are connected into the ac power system,the scale of power system as well as influence degree of CF is gradually increasing,and the stable operation of complicated ac/dc hybrid power system is facing greater risks and challenges.However,current researches on the mechanism analysis,evaluation criteria and real-time control of CF are still insufficient,and the whole cascading reaction process and mitigation strategies of CF in complicated power systems still need to be further studied.Therefore,complicated CF issues are firstly classified and defined in this dissertation.And the cascading reaction and corresponding physical mechanism of the whole process from the ac fault in the receiving-end system to the power angle instability in the sending-end system are deeply analyzed,based on which the blocking methods and mitigation strategies are proposed:(1)Considering the definitions and concepts of CF issues in the current researches are confused and ambiguous,the scientific classification and concept system of CF are proposed.On this basis,the influence of CF on system stability and the evolution process are analyzed and elaborated,then the physical mechanism as well as influencing factors of the whole CF cascading reaction process are deeply researched;(2)Aiming at the problem of accuracy and applicability of CF critical value under different fault conditions and operating conditions,a novel control method based on the boundary voltage interval is proposed for the first CF prevention.The boundary voltage interval is derived and defined,and the dc current order is adjusted in real-time according to the CF risk and operation state reflected by the boundary voltage,so as to prevent the first CF following the ac fault and reduce the risk of subsequent CF under serious ac faults;(3)Since the impact of commutation voltage harmonics on subsequent CF is inadequately considered in current researches,a control method considering voltage harmonics is proposed for subsequent CF mitigation.By predicting the commutation voltage as well as the extinction angle,and introducing the predicted value of extinction angle in the subsequent commutation process into the original control strategy of the inverter,the control accuracy of extinction angle control and effect on subsequent CF mitigation are improved;(4)Considering that very few studies have focused on the cascading CF in multiple HVDC systems caused by the reactive power interaction,a real-time multiple HVDC control method is developed to mitigate the cascading CF.Based on the characterization of real-time voltage states at the ac busbars of inverters,the commutation failure prevention(CFPREV)and voltage dependent current order limiter(VDCOL)are controlled with coordination by considering the reactive power interaction,and the cascading CF is effectively mitigated owing to the proposed coordinated control of valve firing angle and dc current;(5)In view of the problem that few studies have focused on the real-time control methods of transient stability under CF power impact,a real-time control method is proposed to improve the transient stability of the send-end ac system after suffering the CF power impact.Based on the mechanism analysis of transient instability in the send-end ac system caused by CF,the required tripped generation capacity for maintain the power angle stability is then quantified according to the relative kinetic energy and residual acceleration area of the generator rotor movement,to improve the transient stability of sending-end system under the CF power impact.In this dissertation,some beneficial explorations are made focusing on the mechanism and coping strategies of the whole process of CF in complicated power systems.The research results provide well theoretical significance and engineering reference for dealing with the CF issues and ensuring the stable operation of ac/dc hybrid power systems.