| Owing to the advantages of rapid and flexible power regulation,large capacity,low loss,and long transmission distance not limited by synchronous operation stability,line-commutated converter based high-voltage direct-current(LCC-HVDC)has been widely used in the fields of large-area power grid interconnection and long-distance large-capacity power transmission.With the increasing scale of large-capacity DC transmission,the coupling between AC and DC,as well as between sending-end and receiving-end system is becoming more and more serious.A single AC grid fault may be transmitted and amplified under dynamic response of the DC system,which further leads to a series of cascading fault reactions.Commutation failure is a common fault during the operation of HVDC transmission system.If the commutation failure is not effectively suppressed,it may cause continuous commutation failures and even DC blocking.Above chain reaction of the DC system will bring huge active and reactive power impact on AC system,which will severely threaten the safe and stable operation of the AC and DC power grid.Thus,making full use of the control ability of the DC system itself and external controllable equipment,and improving the interruption ability of commutation failure caused cascading reaction is of great significance for ensuring the safe operation of the power grid.To this end,this paper has carried out research on the interruption method of commutation failure caused cascading reaction,and the research are carried out from the aspects of prevents the commutation failure,suppression of continuous commutation failure and suppressing the impact of continuous commutation failure on the AC system.The main work of this thesis is as follows:(1)In order to overcome the problem of the lack of quantitative selection of the existing commutation failure prevention control parameters,the existing commutation failure prevention control is optimized and improved from the two aspects of the threshold voltage and trigger angle adjustment.Firstly,by considering the dynamic variation of the DC current in the commutation process and the impact of the fault occurrence time on the commutation process,combined with the DC system control response characteristic equation,the calculation formula of the critical commutation voltage is derived.Secondly,according to the commutation margin control demand under the AC voltage drop of the receiving-end grid,the calculation method of the trigger angle adjustment amount is established.Based on the obtained critical commutation voltage and the trigger angle adjustment amount calculation method,the quantitative basis for the commutation failure prevention control improvement is provided and immunity of the LCC-HVDC system to commutation failure can be improved.(2)In order to avoid the continuous commutation failure of the LCC-HVDC system caused by the voltage drop at reveiving-end gird,the continuous commutation failure suppression problem in the hybrid dual-infeed DC system is studied,and two control methods are proposed.1)In order to quantify the selection of reactive power control variable and the suppression ability of voltage source converter based high voltage direct current(VSC-HVDC)for continuous commutation failure of LCC-HVDC system in hybrid dual-infeed DC system,a continuous commutation failure suppression method based on power control region of VSC-HVDC inverter station is proposed.By establishing the equivalence relationship between the reactive power control variable of VSC-HVDC inverter station and the critical commutation voltage of LCC-HVDC inverter station for continuous commutation failures,taking into account the capacity of VSC-HVDC inverter station and external system constraints,a power controllable region for reactive power control of VSC-HVDC inverter station is proposed,furthermore,a continuous commutation failure suppression method based on the controllable region is proposed to mitigate continuous commutation failures.2)In order to give full play to the suppression ability of the hybrid dual-infeed DC system for continuous commutation failures and take into account the safety requirements of the AC grid,a continuous commutation failures suppression method based on active and reactive power coordination is proposed.A correlation model among extinction angle and DC current of LCC-HVDC inverter and the output reactive power of VSC-HVDC is deduced.Furthermore,by combining the constraints of control variables in VSC-HVDC and LCC-HVDC,a controllable operation region for continuous commutation failure is proposed in hybrid dual-infeed HVDC transmission system,which is based on LCC-HVDC DC current and VSC-HVDC reactive power coordinate plane.Then,an optimization model with the minimum variation of transmitted active power by hybrid dual-infeed HVDC system as the objective function and the controllable operation region for commutation failure as the constraint was established,a continuous commutation failure control method based on the coordination of LCC-HVDC active power and VSC-HVDC reactive power is proposed.(3)In order to reduce the risk of continuous commutation failures of LCC-HVDC system during AC fault recovery stage,a recovery control method considering the dynamic reactive power and commutation margin of the LCC-HVDC system is proposed.The influence of the reactive power external characteristics of DC inverter on the recovery of the AC voltage at the receiving-end grid during AC fault recovery stage is analyzed.The relationship model among the reactive power exchange of the AC/DC system and DC current,trigger advance angle and AC voltage at receiving-end grid is established.Then,the DC current constraint is deduced in order to satisfy the AC voltage recovery target.Furthermore,taking into account the influence of the commutation voltage amplitude and voltage harmonic on the commutation margin during the fault recovery process,the DC current constraint that satisfies the dynamic constraint of the commutation area is established.Based on the obtained DC current constraints and the conventional voltage dependent current order limiter,an AC fault recovery controller is designed to improve the AC fault recovery performance of the LCC-HVDC system.The proposed control method can suppress the influence of the reactive power characteristics of DC system on the commutation voltage recovery and reduce the risk of continuous commutation failures during the recovery process through the online calculation and dynamic adjustment of the DC current reference valure in the fault recovery stage.(4)In order to suppress the influence of continuous commutation failures on the stability of the sending-end AC system,a ride-through control method for continuous commutation failures based on the emergency control of adjacent HVDC is proposed.Firstly,based on the analysis of the commutation failure process of the DC inverter station,an active output model of inverter station under continuous commutation failure is established.Secondly,aming at the constraints of ride through for continuous commutation failures,the impact of a continuous DC power sag on the stability of sending-end system is analyzed.On this basis,a rolling calculation model for power angle and acceleration area variations of the sending-end system during continuous commutation failures is established on the basis of model predictive control theory.Lastly,a ride-through control method for continuous commutation failures is developed utilizing the emergency control of adjacent HVDC.The proposed control method can improve the endurance capability of an AC system against continuous commutation failures and reduce blocking risk of HVDC system during continuous commutation failures.In this paper,benefit exploration is made to improve the suppression ability of chain reaction caused by commutation failure.The research results have good theoretical significance and engineering reference for fully utilizing the controllability of DC system to improve the safety and stability level of AC/DC hybrid power grid. |
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