继电保护原理与应用(第三版)3.1-3.5(16)

如图所示，封闭式相量图可能会导致困难。如图3.3b所示，它的形状在心理上有助于一个假设，即三角形的三个顶点代表电力系统的a、b和c相，原点0代表n=g。这个封闭型相量图产生了问题，即为什么Van=Vag有如图所示的相量箭头，因为电压降是从a相到中性点的；其他两相也是如此。还有为什么Vab、Vbc和Vca如图所示指向，因为它们分别是从a相到b相、从b相到c相、从c相到a相的滴。看起来他们应该指向相反的方向。
The phasors shown on this closed phasor diagram (see Figure 3.3b) are absolutely correct and must not be changed. The difficulty is in combining the circuit diagram with the phasor diagram by the mental association of a, b, and c with the closed triangle. The open type avoids this difficulty. This also emphasizes the desirability of having two separate diagrams: a circuit diagram and a phasor diagram. Each serves particular, but quite different, functions.
此闭合相量图（见图3.3b）上显示的相量绝对正确，不得更改。难点在于通过a、b、c与闭合三角形的心理联系，将电路图与相量图结合起来。开放式避免了这个困难。这也强调了有两个单独的图的可取性：一个电路图和一个相量图。每一种服务都有特定的功能，但有很大的不同。

FIGURE 3.3 Phasor diagram for a typical three-phase circuit operating with balanced or symmetrical quantities. (a) Circuit diagram showing location and assumed directions of current and voltage drops. I and V are locational and directional indicators, not phasors. (b) Phasor diagrams showing current and voltage magnitudes and phase relations.