Coordination Prevention Of Commutation Failure And Emergency Power Control For Ultra High Voltage Direct Current Transmission System

With the rapid development of ultra high voltage direct current(UHVDC)transmission project,the problem of weak HVDC-carrying capacity of AC system is prominent,UHV AC/DC hybrid system presents ‘Strong HVDC and Weak AC System' characteristics.The AC and DC systems are interdependent in ‘Strong HVDC and Weak AC System' grid,and the coupling relationship between AC and DC systems is close.The coupling relationship between AC and DC systems in hybrid power grid is mainly manifested as follows: AC system provides reactive power and voltage support for DC system to ensure it commutation successfully;conversely,the change of power transmitted by DC system will affect the stability of AC system.In order to ensure the safe and stable operation of the ‘Strong HVDC and Weak AC System' hybrid power grid,it is necessary to study the mechanism of interaction between AC and DC systems,and put forward control measures to improve the stability of the hybrid power grid.Firstly,this paper studies the impact of AC system on DC system.Based on UHVDC hierarchical access mode,it studies the leading factors of commutation failures of high voltage and low voltage valves.The coordination control of commutation failure preventions(CFPREVs)under UHVDC hierarchical access is proposed.The coordination control advances the start time of CFPREV of valve groups accessed into non-fault layer AC grid and improves the ability of the valve groups to resist commutation failure.Based on the UHVDC multi-feed system,the influence of CFPREV on commutation process in multi-feed system is studies.A coordination control of CFPREVs in UHVDC multi-feed system is proposed.The coordination control can adjust the output of the CFPREV adaptively,avoids commutation failure of other DC system.Simulation models are built based on the system in planning and construction,and the validity of the above conclusions is verified by simulation studies.Then,this paper studies the impact of DC system on AC system.An improved DC emergency power control is designed.In order to improve the accuracy of simulation research,a hybrid AC-DC electromechanical-electromagnetic transient hybrid simulation is built based on ADPSS.In the electromagnetic part of the hybrid simulation model,an improved DC emergency power control is designed.The control can restrain the oscillation in the forward and backward swing process of power angle and improve the stability of AC grid.