Research On Optimal Strategies Of Short Circuit Current Limiter Deployment For Large-scale Power Systems

The continuous increasing demand for electric power leads to rapid development of power grids. As a result, the fault-current levels increase year by year, and at some points it may even exceed the maximum short-circuit ratings of circuit breakers. Therefore, how to reduce the fault-current levels and how to select current-limiting measures is drawing great attentions to both academia and industry. The commonly used solutions such as area separations, bus splitting, using series reactors or fault current limiters (FCL), using high-impedance transformers, transformers with neutral grounding via a small reactor and so on, are chosen by engineering experience in traditional methods. Obviously, these methods are cumbersome and can take a lot time to find a feasible solution, and can hardly obtain an optimal solution. In this paper, various types of current-limiting measures were analyzed and researched, and a new methodology using computer technology to obtain optimal current-limiting deployments was proposed. This thesis mainly obtains the following results:1. The changes of the system admittance matrix associated with each type of the measures were analyzed in detail and a unified model to evaluate the measures was established. Thus the mathematical model to optimize current-limiting measures for power grid was easily formulated. Preferences for current-limiting measures were introduced and by adjusting the preference factors it can improve the feasibility of the optimal current-limiting deployment.2. A discrete particle swarm optimization algorithm with immune (IA-DPSO) was designed to solve the problem. The algorithm introduced the technology of clone and mutation in immune algorithm. These two operators enable the diversity of the swarm to be preserved to discourage premature convergence.3. A technique to reduce search space of the problem was given. The potential location of current-limiting measures was selected by sensitivity analysis, and the original problem was decomposed into two sub-problems according to the characteristics of current-limiting measures. This method reduces the number of variables. It can accelerate the calculation speed and improve the computational efficiency, and avoid the algorithm from dimension disaster problem as well.4. An optimization strategy for neutral grounding reactor (NGR) of transformer was proposed. The mathematic model considered the economic factors, the effects of NGR to zero-sequence protections, power systems insulation levels and so on. Thus it was able to meet the needs of the actual network in selection for NGR. An implicit enumeration method was used to solve the problem. The method was simple, and it could find the optimal solution and multiple sets of feasible solutions as well.