继电保护原理与应用(第三版)3.1-3.5(10)

在电路图中，电流或磁通量的表示用（1）字母，如I或u，用箭头表示假定的流动方向；或（2）用双下标表示，用下标的顺序表示假定的流动方向。因此，假设该方向为正弦波正半周期内的流动方向。此惯例如图3.2a所示。因此，在正半周期内，电路中的电流假设为从左向右流动，如is所用箭头方向所示，或如Iab、Ibc和Icd所用下标所示。单个下标（如Is）便于指定电路各部分中的电流，并且没有方向指示，因此方向箭头必须与这些相关联。Iab、Ibc或Icd不需要箭头，但通常用于增加清晰度和方便性。重要的是要认识到，在这些电路名称中，箭头并不表示相量。它们仅被假定为方向和位置指示器。

FIGURE 3.2 Phasor diagram for the basic circuit elements: (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.
图3.2基本电路元件的相量图：（a）显示电流和电压降位置和假设方向的电路图。I和V是位置和方向指示器，而不是相量。（b） 显示电流和电压大小以及相位关系的相量图。
3.2.5.2 Voltage电压
Voltages can be either drops or rises. Much confusion can result by not clearly indicating which is intended or by mixing the two practices in circuit diagrams. This can be avoided by standardizing to one practice. As voltage drops are far more common throughout the power system, all voltages are shown and are always considered to be drops from a higher voltage to a lower voltage during the positive half-cycle. This convention is independent of whether V, E, or U is used for voltage in many countries. In this book, V is used and as indicated, it is always a voltage drop.