Splitting Control Of Sending End Power Grid With E/UHVDC Connection

With the increasing proportion of renewable energy and DC transmission system accessing the sending grid,the operational characteristics of the grid become more and more complex,which brings a series of problems such as poor grid security and stability,low reliability,limited DC operation,and weak grid connection capabilities for new energy sources.The "strong DC and weak AC" characteristics of the sending-end power grid are prominent.In severe cases,the power grid may not be able to withstand the electrical quantity fluctuations and large-scale power transfer problems caused by disturbances.However,the existing research mainly focuses on the first and second line of defense control,and there is less research on the grid destabilization problem.On the other hand,for the sending grid with multiple DC accesses,the conventional splitting schemes and control methods may not be able to meet the system splitting requirements.In order to ensure the stable operation of the power system and reduce the load and DC power loss,this paper proposes a control model for the splitting of the sending grid with DC access,and the restoration control measures for the operation of the sending grid from interconnecting to isolated grid based on graph theory method and convex optimization theory.The main work of the paper are as follows.Firstly,with the objective of minimizing the unbalanced power,a splitting model of the sending end power grid based on a mixed-integer second-order cone programming is proposed.The model has the following characteristics:1)mathematically,the proposed model is a convex optimization problem,which can guarantee a global optimal solution theoretically and has high computational efficiency;(2)physically,the proposed model considers both active and reactive power balance problems in the splitting process,and can accurately and quickly obtain the optimal splitting section.Therefore,it is suitable for the splitting control of large-scale DC transmission-end power grids and is beneficial to the system One-step recovery control.Secondly,for a transmission network with multiple loops,the linearized power flow model in the MI-SOCP splitting model lacks the strong constraint that the sum of loop phase angle differences is zero during the relaxation process.In order to make up for the lack of accuracy,this paper proposes a power flow calculation model that takes this constraint into account based on the application background of the subject.Then the model is incorporated into MISOCP in the splitting model.The optimal splitting section that guarantees the stable operation of the DC system is selected based on the DC multi-output short-circuit ratio index using the multi-slotting scheme solution model.Lastly,for the transient frequency stability problem caused by active power imbalance in the process of unlisting to solitary network operation of the power grid at the sending end,the corresponding safety and stability coordination control measures are proposed respectively based on the sub-network grid structure and unbalanced power size after unlisting,combined with the control measures of interlocking cutter and high circumference cutter on the network side,primary frequency regulation on the unit side and DC power modulation.Simulation verifies the feasibility of the strategy proposed in this paper.