继电保护原理与应用(第三版)3.1-3.5(15)

The open-type phasor diagram permits easy documentation of all possible currents and voltages, some of which are not convenient in the closed-type phasor diagram. The delta voltage Vab, representing the voltage (drop) from phase a to phase b, is the same as Van - Vbn. Similarly, Vbc = Vbn - Vcn and Vca = Vcn - Van.
开放式相量图允许轻松记录所有可能的电流和电压，其中一些在封闭式相量图中不方便。表示从a相到b相的电压（压降）的电压增量Vab与Van-Vbn相同。类似地，Vbc=Vbn-Vcn和Vca=Vcn-Van。
As indicated, the closed-type phasor diagram can lead to difficulties. As seen in Figure 3.3b, its shape lends itself mentally to an assumption that the three vertices of the triangle represent a, b, and c phases of the power system, and that the origin 0 represents n = g. Questions arise with this closed-type phasor diagram about why Van = Vag has its phasor arrow as shown, because the voltage drop is from phase a to neutral; similarly for the other two phases. Also why Vab, Vbc, and Vca are pointing as shown, for they are drops from phase a to phase b, phase b to phase c, and phase c to phase a, respectively. It would appear that they should be pointing in the opposite direction.