A practical method for the calculation of unbalance factor caused by asymmetric loads in a three phase three-wire distribution system was presented. By introducing the term of sequence capacity of an asymmetric load, the current unbalance factor is the ratio of negative-sequence capacity to the positive-sequence capacity of the load, and the voltage unbalance factor at the point of common coupling is the ratio of negative-sequence capacity to the short-circuit capacity of the distribution system. The functional relation between the sequence capacity and the phase-to-phase power of the load was established to simplify the calculating process of unbalance factor. Examples show that the propose method is simple and convenient, and especially suitable for the unbalance factor assessment at the plan stage of industrial distribution systems.%三相不平衡度的估算是贯彻国家标准、提高电能质量的基础,故提出一种三相三线配电系统中由不对称负荷引起的三相不平衡度的实用计算方法.在引入了三相不对称负荷的序容量的概念之后,负荷电流不平衡度等于负荷负序容量与正序容量之比,由该不对称负荷在某公共连接点引起的三相电压不平衡度等于负荷负序容量与系统短路容量之比;建立了三相不对称负荷的序容量与负荷相间功率之间的函数关系,简化了三相不平衡度的计算过程.算例表明该方法简便易用.该方法适合于工业配电网在设计规划阶段对三相不平衡度的估算.
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