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A framework for power system restoration Lindenmeyer, Daniel

Abstract

The problem of restoring power systems after a complete or partial blackout is as old as the power industry itself. In recent years, due to economic competition and deregulation, power systems are operated closer and closer to their limits. At the same time, power systems have increased in size and complexity. Both factors increase the risk of major power outages. After a blackout, power needs to be restored as quickly and reliably as possible, and consequently, detailed restoration plans are necessary. In recent years, there has also been an increasing demand in the power industry for the automation and integration of tools for power system planning and operation. This is particularly true for studies in power system restoration where a great number of simulations, taking into account different system configurations, have to be carried out. In the past, these simulations were mostly performed using power-flow analysis, in order to find a suitable restoration sequence. However, several problems encountered during practical restoration procedures were found to be related to dynamic effects. On the one hand, simple rules that help to quickly assess these problems are needed. On the other hand, accurate modeling techniques are necessary in order to carry out time-domain simulations of restoration studies. In this work, the concept of a new framework for black start and power system restoration is presented. Its purpose is to quickly evaluate the feasibility of restoration steps, and if necessary, to suggest remedial actions. This limits the number of time-consuming time-domain simulations, based on the trial-and-error principle, and helps to efficiently find feasible restoration paths. The framework's principle is to subdivide the problem of assessing the multitude of different phenomena encountered during a restoration procedure into subproblems. These can be assessed by simple rules formulated in the frequency and Laplace domain. This thesis concentrates on the initial stages of restoration where three major problem areas are identified. The system frequency behavior after the energization of loads is assessed using analysis in the Laplace domain and simplified generator control system models. The occurrence of overvoltages is assessed in the frequency domain. Sensitivity analysis is used in order to find the most efficient network change that can be applied to limit overvoltages. In case time-domain simulations need to be carried out, a method based on Prony analysis and fuzzy logic helps to limit the overall calculation time. Problems related to motor starts are evaluated by rules formulated in the frequency and Laplace domain. For these studies, a new induction motor parameter estimation method is developed that helps to build more accurate motor models. All the proposed rules are validated using time-domain simulations based on actual system data. Of crucial importance in the restoration process is the black and emergency start of large thermal power plants with small hydro or gas turbines. These cases represent islanded system conditions with large frequency and voltage excursions that need careful investigation. It is shown how they can be simulated using the Electromagnetic Transients Program (EMTP) by means of an emergency motor start case whose simulation results are compared to measurements.

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