中点形成变压器的分析

本文分析了一种用于三相三线制形成三相四线制的变压器——中点形成变压器(NFT),详细阐述了NFT的原理、结构特点以及NFT在电路中的数学模型。NFT从原理上讲是一个变比为1的自耦变压器,其原副边使用一套绕组。原边接成Y接法,副边接成Y_0接法,从而形成了一个物理中点,完成三相三线制到三相四线制的转换。在三相负载不平衡时,使用NFT会使三相输出电压不平衡。本文一方面从理论上分析了这种不平衡程度同NFT的性能参数(零序阻抗)之间量的关系,从而在理论上提出了NFT的设计依据;另一方面又提出了旨在减少NFT零序阻抗的铁芯结构、铁芯材料、绕组联接及排列等特点。根据分析结果,具体设计与制做了一个15kVA的NFT;并结合实验进行了验证。最后又把NFT模型化,提出了NFT在电路中的数学模型,为使用NFT的电力装置的系统仿真创造了条件。

The Analysis of Neutral Point Forming Transformer

This paper makes an analysis of a transformer referred to as a neutral forming transformer (NFT) used to convert a three-phase three-line system to a three-phase four-line one,and makes a formulation of its principle,features and mathematical model. NFT is a self-coupling transformer with converting ratio 1 : 1 ,its primary windings and the second windings are the same, and are connected in Y/Y0 , so that one physical neurtal point is formed. Under unbalanced three-phase-load conditions,NFT 11 make the three-phase-output voltages unbalanced. On one hand ,the paper makes a study of the quantitative relation- ship between the degree of unbalance and the parameters of NFT (zero-sequence impedance),and on the other hand,it suggests the characteristics of core geometry,core material and winding arrangement which aim at reducing the zero-sequence impedance. According to the study,one prototype of 15kVA is designed,made and tested. Finally,a mathematical model of NFT is proposed,which creates condition for the simulation of the whole system including-NFT.