Assessment Of Safety And Economy Of Three-phase Unbalance For Low Voltage Distribution Network

With the rapid development of the world economy,traditional energy sources such as fossils are increasingly depleted.Facing the problem of shortage of traditional energy supply and environmental pollution,energy saving and emission reduction is listed as the basic national policy in China.The power industry is the focus of energy saving and emission reduction in China,and the losses of low-voltage distribution network accounts for up to 60% of losses of the total grid,so its energy saving and losses reduction are widely concerned.In addition to the factors of equipment itself,power quality also has a greater impact on its losses.Among them,three-phase unbalance is one of the important factors of high losses in low-voltage distribution network,which is widespread and serious in low-voltage distribution network,and has a significant impact on the economy and security of low-voltage distribution network.At present,the three-phase unbalanced treatment methods are more diversified,but the evaluation index of the treatment effect is single,which can't fully reflect the advantages and disadvantages of the treatment schemes,and the selection of the treatment schemes is blind.In view of above problems,the following works are accomplished in this paper:(1)The different influences of three-phase unbalance on low-voltage distribution network are expounded.From the perspective of the decision-maker(power companies),the influences of three-phase unbalance on line losses,transformer losses and its insulation life loss are studied.The influence of unequal amplitude and phase asymmetry of three-phase current on line losses is analyzed in detail,and the relationship between unbalance degree of three-phase current,average load rate with transformer losses is calculated quantitatively.(2)On the basis of detailed analysis of heat generation and heat dissipation process inside transformer,according to the original calculation model of transformer hot-spot temperature based on thermal circuit model and the quantitative relationship between three-phase unbalance degree,average load rate with losses,the original calculation model of hot-spot temperature is improved.Meanwhile,a general model for calculating hot-spot temperature under unbalanced three-phase current is proposed.This model is applied to the calculation of transformer life loss,which can improve the calculation accuracy of hot-spot temperature and life loss,and it is verified by simulation.(3)Based on the different influences of three-phase unbalance on low-voltage distribution network and power quality standards and relevant operating regulations,a comprehensive evaluation index system with hierarchical structure is built,which selects relevant index from the perspective of safety and economy to reflect the influences of three-phase unbalance on the low-voltage distribution network.(4)In order to realize the comprehensive evaluation of the effects of different treatment schemes and considering the preference information of decision-maker for different indicators,a new combination weighting method is formed by combining the subjective weighting method based on decision-makers' preference information with the objective weighting method in this paper.The Grey Relational Evaluation model is applied to evaluate the safety and economy of the schemes comprehensively,so as to guide the optimization of the schemes.(5)The evaluation method proposed in this paper is used to evaluate the treatment schemes of modified IEEE33 buses distribution network and actual low-voltage distribution network,and its feasibility and validity are verified.