Research On Safety Correction Control Of Large Power Grid Based On Transmission Line Switching

Static problems such as line power flow overload,node voltage violation,and high network loss are already important factors that threatening the safe operation of the power grid,affecting power quality,and reducing economic benefits.Compared with traditional control methods such as cutting machine,load shedding and reactive power regulation,the transmission line switching measures have the advantages of high economic benefits,easy operation and small system impact.Considering the actual needs of power grid companies and the relatively few domestic research results in the field of transmission line switching in China,this dissertation focuses on the transmission line switching measures for safety correction control of large power grids.The main research contents include:(1)Aiming at the problem of line overload,a stage solution strategy of transmission line switching scheme considering look-ahead evaluation is proposed.This novel method is based on a three-stage strategy,including screening,ranking,and detailed analysis and assessment stages to balance the relationship between computational efficiency and solution accuracy by combines the linear with the nonlinear methods.The simulation results based on 39-bus system and 2746-bus large-scale system show that the proposed strategy is effective and fast,and can meet the needs of online applications.(2)Aiming at the problem of node voltage violation,a transmission line switching control strategy considering N–1 security constraints is proposed.Stage 1adopts the sensitivity method to quickly identify the effective candidate transmission lines.Stage 2 deduces and estimates node voltage variation ?V_i-km based on PQ power flow decomposition model to give the priority of pre-sequence line schemes,and evaluates a set of pre-switching line schemes.Stage 3 uses the Newton-Raphson method and the Look-ahead algorithm for detailed analysis and evaluation,respectively.And a set of high-quality transmission line switching schemes are evaluated by comprehensive N-1 verification results.The simulation results show the advantages of the proposed strategy in solving accuracy and computing efficiency.(3)Aiming at the system active network loss has been neglect while using transmission line switching control measures to alleviate line overload and eliminate node voltage violations,a staged solution strategy including screening,detailed analysis and quality evaluation of switching schemes is proposed to reduce of active power loss.The test results based on the IEEE 300-bus system show that the proposed solution strategy has advantages in computational efficiency and accuracy.(4)Aiming at the problem of the static voltage stability of the system after transmission line switching,a fast method for obtaining saddle node bifurcation points with variable step size is proposed.The sensitivity method is adopted to ensure that the step size can be self-adjusted with the change of slope at the operating point;the dichotomy method is incorporated into the step size adjustment algorithm to deal with the situation that the predicted operating point i+1 is separated from the upper half of the P-V curve;and the local curve fitting technology is used to improve the accuracy of the solution.The simulation results show that the proposed method has faster calculation speed and higher accuracy,and is suitable for large power grid on-line application.(5)Aiming at the problem of the transient stability of the system after transmission line switching,a transient stability margin assessment method based on transient energy is proposed.The sinusoidal prediction method is introduced to fit the residual power angle characteristic curve,and the transient stability margin of the system after the application of the high-quality scheme is evaluated.The results show that the assessed high-quality scheme did not cause transient instability.