Research On The Power Flow Automatic Adjustment Methods In Power System

With the development of the progress of power grid interconnection, the system scale in grid analysis and calculation has increased rapidly in recent years in China. Under such background, more and more difficulties emerge in the practical application of power flow analysis and new contents are endowed to it, which is considered as a comparatively mature technology in power system. At present, the adjustment after the power flow calculation is mainly completed manually according to the operators’experiences. Because of the large scale of the power grid, the adjusting work is time-consuming and of low efficiency. In view of the situation, This study chooses the issues of large grid power flow adjustment and tie-line power flow fluctuation in large weak interconnected system as the main research field, and focuses on the practical methods of unsolvable adjustment of power flow, reasonable adjustment methods of large-scale grid, fluctuation characteristics of tie-line power flow in UHV line-based large weak interconnected system.This study has provided a power flow similar to the system through decreasing the load level based on the principle of the traditional practical unsolvable adjustment of power flow. Then improve the system transmission capacity based on the characteristics of the similar power flow to realize the purpose of power flow unsolvable adjustment. Use the Pδ, PV and QV curves of the system when seeking adjustment methods and find out the weak points of the system through the characteristics of the curve slope. The calculation example of node point 36 and the example of a Provincial Grid reveal that the algorithm is effective and can be applicable to large grid analysis.This study has proposed the adjustment method of the unsolvable power flow based on the normal direction of the solvable limit of the parameter space. The traditional adjustment method of the unsolvable power flow base on the parameter space firstly find the solvable point which owns the shortest distance to the initial state. Then the operators select the adjustment action according to the practical experiences. The method put forward by this paper has considered the constraints of the actual operation in the iterative process of the adjustment, by which a large amount of calculation has been reduced. This method has been proved to be more suitable to the large-scale system. In addition, this paper has introduced the traditional parameter space method and inters point method which are most commonly used by researchers in unsolvable adjustment of power flow and has analyzed the advantages and disadvantages of these two methods in the calculation examples. At last, this paper has also analyzed the application results of this adjustment method in the Polish 2383 bus system.The study transforms the inequality constraint of the system into equality constraint through adding variables during the reasonable adjustment process, thus the reasonable adjustment issue is converted to the solving of system diophantine equations. And the method of optimal multiplier power flow calculation combined with alternating iteration is proposed to solve the system power flow. The calculation amount of this method is really less compared to that of the optimization algorithm. And the calculation scale and convergence of this method is approximate to that of power flow calculation. Therefore, it can be easily applied to actual large system. Furthermore, since the algorithm does not have special requirements for whether initial power flow is solvable or not, system power flow in line with the requirements can be obtained according to the initial power injection situation, thus combining unsolvable adjustment and reasonable adjustment of power flow. The Polish 2383 bus system has been used to prove the efficiency and convergence of the proposed algorithm.The key issue of the algorithm lies in how to seek the static weak points of the system so as to find out the most suitable adjustment method. This paper uses the partial indicator analysis method in static voltage stability analysis as reference and discusses the advantages and disadvantages of partial indicator method. Partial system equivalence method is improved and the different situations of power injection point in partial system are taken into consideration. Through introducing characteristic value of Jacobian matrix, which is an indicator used in grid static voltage analysis, the minimum characteristic value of the system Jacobian matrix after partial equivalence can be used as the indicator of measuring the partial system approximate to extreme. The calculation example proves that the indicator is correct and this method can be applied to practical method of unsolvable adjustment of power flow to improve the efficiency of the algorithm.The paper analyzes the characteristics of tie-line power oscillation in combination with actual oscillation process. Theoretical derivation is conducted to deduce the maximum value of tie-line power fluctuation and related factors influencing the peak value of tie-line power fluctuation are analyzed in the study. It indicates that the peak value of fluctuation is mainly related to the difference between the electromagnetic power mutation values of the two systems at the two sides of the tie-line and the moment of inertia of two interconnected systems. Finally, this paper, in combination with the concept of transfer ratio applied to actual system, discusses relevant laws of the peak value of tie-line power fluctuation after severe disturbance.