Multi-Objective Optimal Reactive Power Flow Considering Static Voltage Stability

In the electricity market, for the environment and economy limit, the system current transfer capability is used furthest and possible close to its limit in the load peak time, the system voltage stability margin is very lower than ever and easy to collapse when the system meets the serious contingencies. Optimal Reactive Power Flow (ORPF) is an effective measure to hold the security and economy balance of the power system operating. To reduce the active power loss and improve the voltage quality are the general ORPF main objectives, and the system voltage stability is considered rarely. But the system voltage stability margin may be changed in the ORPF process. If the system voltage stability is considered in ORPF, the voltage margin can be improved and the collapse probability will be small. So the multi-objective ORPF incorporating static voltage stability is studied in this dissertation, the main research work is summarized as follows:(1) The ORPF security and economy hybrid model is put out based on considering the active power loss, voltage quality and voltage stability margin, the validity of minimal eigenvalue of the voltage margin index used in ORPF is analysed. For ORPF is a typical non-linear programming problem with the characteristics of multi-objective, uncertainty, multi-restriction and discreteness, a multi-objective self-adaptive immune algorithm (MOAIA) is proposed to resolve the model in this dissertation, the main idea of the proposed algorithm includes two parts, firstly, the partial affinity and global affinity are defined to evaluate the antibodies affinity to the multi-objective functions, this part can avoid the weight factor selection; secondly, self-adaptive crossover, mutation and clone rates of the antibodies are used to keep the antibodies diversity, hence the proposed algorithm can achieve the dynamic balance between individual diversity and population convergence. The test systems results show the model is right and the algorithm is feasible.(2) The system reactive power reserve capacity is an important index for the voltage stability margin, so a new ORPF model is put out, whose objectives are to improve the system reactive reserve capacity and voltage level and decrease the active power losse. The system reactive power reserve capacity is not the sum of the generators reactive power reserve capacity. In order to calculate the system reactive power reserve capacity, the generators locations in power system and their value to support voltage stability must be considered. The system reactive power reserve distributor factor is defined and can be calculated based on two steps, firstly, the spatial electrical distance is defined and the immune-medoid clustering algorithm is proposed for reactive power/voltage control network partitioning, the total distance of the generator to the other buses is looked as the index to support the region voltage stability; secondly, the region reactive power reserve demand index is calculated based on the region stability margin. IA is used to resolve the model optimal solution and tested in the 84-bus system, the results indicate the system reactive power reserve capacity model is right, and the network partitioning algorithm is feasible.(3) Available Transfer Capability (ATC) is not only an important factor to evaluate the system stability, but also a key signal to distribute the power system resource, so ORPF with improving the system ATC is studied. A novel calculation method of ATC considering voltage stability is proposed, in the line P-Q voltage stability plane, the limit point of line stable operating can be gotten according to the line power flow distributor factor, while the position of the line thermal limit and the voltage stability of P-Q curve is considered. The system ATC can be calculated by one time power flow, so this method calculating burden is very small. At the time, the ORPF affection to the system ATC is analysed. The test systems results show the ATC calculating method is right and improving the system ATC is feasible in ORPF.(4) In the electricity market, the generator companies and the grid companies have the different economic benefit, in order to encourage the generator companies to support the reactive power auxiliary service, the grid companies will pay for the service, the grid companies should decrease the payment by the technology way, so the daily ORPF is studied. Relative to the static ORPF, the daily ORPF should consider the reactive power equipments action times and the load change. The system load integrative change is defined to reveal the buses load change and its curve is divided to satisfy the equipments action times constrain, the clustering method is used to select the system section for ORPF, in this way, the daily ORPF is looked as several system section static ORPF. A new simply way to calculate the generator opportunity cost is proposed and can avoid the general method shortcoming. The mathematic model of daily ORPF is put out whose objective function is to decrease the system active power loss and reactive power ancillary service, while the voltage stability is looked as constrain conditions when the system meets the line contingences. In two test systems, the load curve dividing method and the model of daily ORPF are tested, the results show the load curve dividing method is feasible and the model is right.