三相不平衡与线损的量化分析

Quantitative Analysis Between Three-phase Unbalance and Line Losses

三相不平衡影响系统运行效能,以负序、零序和正序分量有效值比值度量负序、零序三相不平衡率,使得不平衡率不具有唯一性。线损增加是三相不平衡带来的主要影响,亟待进行量化评估。在分析序分量和相量大小关系的基础上,提出采用三相正序分量有效值的平方和与总量平方和之比度量三相平衡率,统一了含零序和不含零序系统的三相平衡率和不平衡率度量形式。论证了在电源电压对称的情况下,传递相同功率,不平衡系统正序电流分量大小等于平衡系统的电流。对于三相三线制系统,不平衡系统与平衡系统的线损比值为平衡率的倒数,并给出了根据量测值计算线损增量的方法;对于三相四线制系统,不平衡系统线损由端线线损和中线线损两部分构成,不平衡系统端线线损与平衡系统的比值为平衡率的倒数,中线线损根据零序电流单独计算,并给出了根据量测值计算线损增量的方法。算例显示,提出的度量方法能够更好地评价三相不平衡,并反映与线损的量化关系。

Three-phase unbalance affects operation efficiency of power system. Three-phase unbalance ratio is not unique as it can be defined as RMS(root mean square) of negative sequence or zero sequence divided by positive sequence component, namely, negative sequence unbalance ratio and zero sequence unbalance ratio. Line loss increasing is main influence of three-phase unbalance. It needs quantitative analysis. Based on analyzing value relationship of sequence component and phasor, this paper proposes to measure the three-phase balance ratio using squares of the three-phase positive sequence component RMS divided by squares of the phasor RMS. So, the three-phase balance and the unbalance ratio are unified no matter with or without zero sequence component. It is demonstrated that unbalance system current positive sequence component is equal to balance system, if two systems have symmetrical voltage source and transmit same power. For three-phase three-line system, line losses ratio between unbalance system and balance system is equal to reciprocal of balance ratio. It also gives method that can calculate line losses ratio based on measurement values. For three-phase four-line system, unbalance system line losses compose of two parts. One is phase-line losses, anther is neutral line losses. Phase line losses ratio between unbalance and balance system is equal to reciprocal of balance ratio. Neutral line losses need to separate calculation based on zero sequence current. Line losses ratio also can calculate based on measurement values. Examples show that the proposed balance ratio can evaluate three phase unbalance well. It also can evaluate quantitative relationship between three-phase unbalance and line losses.