Research On Emergency Control Strategy Of Sending-end System After Large Capacity DC Blocking

The high-voltage direct current(HVDC)transmission project connects the energy center and the load center,which has realized the effective allocation of resources,and will play an important role in achieving "carbon peak" and"carbon neutrality".However,as the scale effect of the power electronic equipment in the AC/DC hybrid system,the dynamic characteristics of the power grid are more complex and changeable,the transient energy impact caused by the fault is greater,the converter equipment fault tolerance is insufficient,the DC line fault,the converter station fault and AC system fault,may all cause DC blocking.After the DC transmission system is locked,bulk of power surplus will be accumulated in the sending-end AC system.The resulting power flow transfer may cause the AC channel to reach its power transmission limit.At this time,the power angle of the sending-end and receiving-end generators will continue to swing,which will eventually lead to the system power angle instability problem.Therefore,the research on emergency control strategy of the sending-end AC system after the DC blocking is significant.This article mainly studies from the following aspects:(1)In order to give full play to the transient stability control of non-fault DC transmission after DC blocking in the multi-feed DC sending-end system,a coordinated emergency control of multi-HVDC for enhancing transient stability is proposed.Firstly,based on the transient energy function,the influencing factors of the unbalanced energy distribution are analyzed;Then,in order to accelerate the absorption of the unbalanced energy accumulated in the dominant generator of sending-end and receiving-end system,the multi-DC power support effect evaluation index is established;thus,the DC support sequence table is dynamically updated when the machine changes.The simulation results verify the effectiveness of the strategy.(2)To solve the difficulties of quantifying the control amount of the generator-tripping and the optimization of generator-tripping location after DC blocking,a method for forming the generator-tripping control strategy based on the graph approximation method is proposed.First,the different effects of DC blocking faults and short-term power shocks on the transient stability of the sending-end system is analyzed;and then a rapid quantification method for the total amount of generator-tripping control based on the graphical approximation method is proposed.Finally,the allocation strategy of the generator-tripping amount under the upper limit of the tripping ratio based on the proposed evaluation index of generator-tripping effect.The simulation results show that the method can effectively improve the transient stability of the system.(3)Aiming at the difficulty to estimate the sensitivity of the discrete action behavior control of the generator-tripping,the generator-tripping control strategy based on the likelihood ratio gradient estimation is proposed.Firstly,considering the migration of the dominant unstable equilibrium point caused by the generator-tripping,a modified AC/DC hybrid system transient stability margin model is established.Then,the control sensitivity of the discrete action of the generator-tripping based on the likelihood ratio gradient estimation method is obtained.Finally,the generator-tripping control strategy is established,and according to the solved results,the corresponding generator-tripping location and tripping amount is obtained.The simulation results show that this strategy achieves a better transient stability control effect with a lower control cost.(4)Aiming at the complex stability problem of the real system after DC blocking,a coordinated control strategy considering multiple stability constraints is established.Firstly,the influence process of DC modulation and emergency generator-tripping control measures on the frequency change,voltage deviation of the sending end system and the safety of the AC system is analyzed,so as to form the corresponding constraint conditions.Then,in order to minimize the emergency control cost,and take the transient power angle stability as the primary constraint conditional consideration of the coordinated control optimization model from multiple stability constraints was established.Finally,the optimal emergency control strategy is solved.The effectiveness of the coordinated control strategy is verified by simulation.