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UBC Theses and Dissertations

Aggregation of voltage and frequency dependent electrical loads Louie, Kwok Wai

Abstract

Electrical loads play a very important role in the behaviour of an electric power system. Since there is a tremendous number of different loads in the system, representing each load with its own model becomes impractical for system level studies. This thesis deals with the issue of aggregating loads to simplify system level studies. Six new and accurate aggregate static load models, a novel EMTP based load model, and four very accurate aggregate induction machine models have been developed. The proposed aggregate load models are voltage and frequency dependent and accommodate the different data formats of individual loads. By including the information of voltage and frequency dependence, the models can be used in larger ranges of studies than the conventional aggregate static load models, thus resulting in more accurate representations. The valid voltage range of the models is about 75% to 125% of rated voltage and, the valid range of frequency of the models is about 85% to 115% of rated frequency. The proposed EMTP load model represents a load with basic circuit elements. The model consists of two varied turns ratio transformers and two varied admittance RLC circuits. It represents the voltage dependence and the frequency dependence of a load separately, resulting in a much simpler load representation than a conventional load model. The model not only improves the accuracy of load representations, bus also broadens the EMTP application in studies other than the transient analyses, such as power flow studies. The proposed aggregate induction machine models have been developed based on the specifications and circuit parameters of individual machines. Since the specification of the machines are the most basic information of the devices, they provide a natural and accurate representation of the machines. The circuit parameters of the machines reflect the behaviour of the devices, they can be used to compose the machines under high and low frequencies, resulting in a simple and accurate machine representation. These two different aggregate models accommodate the data availability of individual machines. To verify the validity of the proposed load models, field tests from published literature are compared with computer simulations. The simulation results of some test systems obtained i) by the proposed load models, ii) by the conventional load models, and iii) by solving the original systems without aggregating their circuit components are also compared. The results of these comparisons prove that the proposed load models can represent loads in power systems more accurately than the existing load models. The proposed load models have been developed into computer software packages with the high level computer programming language Ada 95.

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