Reactive Power Optimization Calculation In Multi-area Power Systems Based On External Network Equivalent

With increasing enlargement of power systems, the conventional centralized optimization algorithm is difficult to meet requirements of online analysis. If the large-scale optimization problem is decomposed into multiple sub-problems by adopting decomposition and coordination algorithm, the problem scale can be reduced and a new way to solve the difficult problem of online reactive power optimization computation is provided. According to hierarchical and partitioning structure of power systems and dispatch characteristics of power companies in our country,this paper presents a new decomposition and coordination method based on nonlinear primal-dual interior point method and external network equivalent technique to solve reactive-power optimization problem in multi-area power systems and computational speed is enhanced.The large-scale interconnected power grid is partitioned into several areas by using Ward and REI equivalent technique. The equivalent admittance matrices of external networks need to be recalculated repeatedly and hence computational efficiency is reduced when transformer radios or branch admittances are changed in external system because the Ward and REI equivalents belong to static equivalent. The external equivalent network is corrected quickly by using inverse matrix modification lemma and branch equivalent injection power model, this kind of method correcting equivalent network is suitable for Ward and REI equivalents, and thus the computational efficiency of correcting external equivalent network is highly enhanced. Two kinds of equivalent algorithm are adaptive to the large systems with strong or weak coupling among different areas.In application of Ward equivalent method to solve multi-area reactive-power optimization problem, the multi-area power system is partitioned into master and slaver areas firstly, and boundary buses are defined as different bus types respectively corresponding to different areas, and hence the transfer method of phase angles from master area to slaver areas is determined. Secondly the initial value of equivalent injection power is obtained according to power-flow calculation at basic operating state. Then computational efficiency of the equivalent admittance matrix of external network due to change of transformer ratios is highly enhanced during optimization process by means of fast correction technique of the external equivalent network. Moreover a kind of external coordination strategy which is suitable for reactive-power optimization calculation is introduced and better coordination effects are obtained with less communications.In application of REI equivalent method to solve multi-area reactive-power optimization problem, REI equivalent network is corrected to reflect the change effects of external areas based on X-REI idea, whole power system coordination optimization is implemented only by exchanging voltages at the boundary buses. The proposed algorithm obtains the partitions on the basis of REI equivalent and satisfies reactive-power response in the external area due to the disturbance occurred in the inner area, and also the computational accuracy of decomposition and coordination based on external equivalent technique is improved greatly. The coordinated communication is low and good coordinative results are gained by introducing X-REI method into outer coordinated calculation. The simulation results show that optimization iteration effect and computation accuracy will be better if the conflation of REI buses becomes more delicate, especially the different kinds of buses with reactive-power sources are integrated respectively, otherwise the optimization iteration effect and computation accuracy will become poor, even iteration is not convergent.On the basis of above research, the parallel reactive-power optimization model is established based on external network equivalent, and the parallel calculation model and procedure are presented. The parallel reactive-power optimization calculation is implemented by using Matlab parallel computing platform. According to parallel simulation results, a detailed comparison and analysis between two parallel algorithms based on Ward and REI equivalent is conducted. The results also show that the multi-area decomposition and coordination algorithms are suitable to implement parallelization by means of external network equivalent and the computational speed is improved obviously in large-scale power systems.