Voltage stability analysis of a power system network comprising a nuclear power plant

Master Thesis

2018

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As recently as 2016, the performance of South Africa’s power utility has shown that it is not resilient enough to withstand the consequences of a power system blackout. Blackouts are defined as being a form of power system instability that can be brought about by a variety of abnormal network scenarios. The most common modes of failure are grouped under the term power system stability. In this dissertation, the different modes of power stability that can affect a nuclear power station will be investigated and discussed. The particular phenomenon that will be focused on, however, is the effect that voltage instability has on the ability of generators and loads to perform their standard functions, thus ensuring a secure power system. To investigate the effect that voltage instability has on a nuclear power station, this dissertation will look at relevant literature on the topic. In addition, by extracting from common examples of national and international occurrences of voltage stability, this dissertation will record the effects that this phenomenon has on the security of a power system, in particular on nuclear power plants. To model the network containing a nuclear power plant for the evaluation of voltage stability, the different mathematical models of the generation plant are presented, which include: the automatic voltage regulator, power system stabilizer, governor, nuclear reactor, and excitation system. Also presented are mathematical models of network equipment such as under voltage tap changers and the dynamic loads that are of interest when evaluating voltage stability. The models used for evaluation of the voltage stability phenomenon affecting a nuclear power plant and the surrounding integrated power system are built in the Digsilent PowerFactory® software. The scenario for evaluation is based on a voltage stability event that occurred around at the Koeberg nuclear power system situated in the Western Cape province on South Africa on 15 October 2003. It is commonly accepted that voltage stability can be evaluated at a steady state level by performing power versus voltage (PV) analysis to determine the voltage buses vulnerable to voltage collapse, and reactive power versus voltage (QV) analysis to determine the critical reactive devices required to avert a voltage instability event. The scenarios that are evaluated for voltage stability are divided into two sections: i) a PV and QV analysis as per the event that occurred on 15 October 2003 and ii) present-day voltage stability indices for PV and QV if mixed with a generation such as renewable energy sources that include wind, solar, biomass and concentrated solar power (CSPs). The result reveals the vulnerabilities of the nuclear power plant and the surrounding integrated power system due to a voltage instability event. Some of the solutions proposed include a review of the typical power system protection schemes — such as under and overvoltage detection scheme — that are used. In the study, PV and QV curves provide v good indications of the state of critical busbars and the reactive power reserve margins available before instability can potentially settle in. Simulations confirmed the effectiveness of critical equipment installed in the Western Grid and the effect on their electrical parameters such as torque and the slip on motors.
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