Comprehensive Power Quality Assessment Approach And Its Application To Offshore Wind Farms

As one of the renewable power generation resources and low-carbon power supply, wind power has been developed widely across the world, especially offshore wind power. As the offshore wind can provide more consistent and strong wind, the offshore wind turbines operate at higher capacity factors. Although it has these benefits, wind speed characterizes the variation, intermittence, and randomness. And confused flowing and tower-shadow effect are also the influencing factors of wind power unit. All of these can bring fluctuation of wind turbine output which will lead to power fluctuations, which affect the power quality. In addition, offshore wind power incorporates into the weak distribution network, which will influence and interfere with the operation of power network. As the wind power penetration into the grid is increasing quickly, the influence of wind turbines on the power quality is becoming an important issue. Under the environment of electric power market, the market mechanism is pricing by quality and high quality and high price. And because of the evaluation for rewards and punishment, people are paying attention to the problems of power quality increasingly. It is necessary to researching on the power quality assessment of offshore wind farm.Firstly, the thesis analyzes the problems of power quality and the index system of power quality. We introduce the simulation model of offshore wind farm and the acquisition of the data for power quality indexes. We also analyze and assess the power quality from two aspects, which are the wind conditions and the installed capacity of offshore wind farm. Secondly, we analyze research status of power quality assessment all over the world and summarize the advantages and disadvantages. Then we use three methods to evaluate the power quality. We acquire the judgmental matrix by using analytic hierarchy process (AHP) and obtain the index weights for power quality assessment by combining the compatibility matrix methodology and entropy weight method with connection degree modeling in Set Pair Analysis (SPA) theory. The compatibility matrix method can effectively address the challenge that it is difficult to inspect the consistency by judgmental matrix. And the results will be more objective by using entropy weight method. The power quality index weights determination can vary for different experts. We adopt SPA theory to cope with and minimize the certainty and uncertainty which are introduced from the differences of subjective knowledge of experts. Two comprehensive evaluation methods are adopted to rank the power quality indexes based on the calculated index weight. One is the approach which is based on statistical and fuzzy judgmental method, and the other is the approach that is based on Technique for Order Preference by Similarity to Ideal Solution (TOPSIS). From application examples, it is showed that the proposed approaches can assess the power quality of offshore wind farm objectively, comprehensively, and reasonably. It has fine application value.At last, we propose a new ordering method to assess power quality, which is a method combining TOPSIS and Vague theory. Vague theory considers evidence in both membership and non-membership aspects. The degree of membership is described by triples which contains quality aspect, poor quality aspect and uncertainty aspect of power quality indexes. Thus, vague theory has stronger capacity to express uncertain information than the traditional fuzzy sets. The best sorting result can be gained by applying this method. Examples prove that the model is reasonable and effective.