继电保护原理与应用(第三版)3.1-3.5(11)

电压可以是下降或上升。如果不能清楚地指出这两种做法的意图，或者在电路图中混用这两种做法，可能会造成许多混乱。这可以通过将标准化为一种做法来避免。由于电压降在整个电力系统中更为常见，因此所有电压都会显示出来，并且总是被认为是在正半周期内从较高电压降到较低电压。在许多国家，这个惯例与电压是V、E还是U无关。在这本书中，V是用来表示，它总是一个电压降。
The consistent adoption of only drops throughout does not need to cause difficulties. A generator or source voltage becomes a minus drop because current flows from a lower voltage to a higher voltage. This practice does not conflict with the polarity of equipment, such as transformers, and it is consistent with fault calculations using symmetrical components.
始终只采用drops并不需要造成困难。由于电流从较低的电压流向较高的电压，发电机或电源电压变成负电压降。这种做法与设备（如变压器）的极性不冲突，并且与使用对称组件的故障计算一致。
Voltages (always drops) are indicated by either (1) a letter designation with double subscripts; or (2) a small plus () indicator shown at the point assumed to be at a relatively high potential. Thus, during the positive halfcycle of the sine wave, the voltage drop is indicated by the order of the two subscripts used, or from the ‘‘’’ indicator to the opposite end of the potential difference. This is illustrated in Figure 3.2a, where both methods are shown. It is preferable to show arrows at both ends of the voltage-drop designations, to avoid possible confusion. Again, it is most important to recognize that both these designations in the circuit diagrams, especially if arrows are used, are only location and direction indicators, not phasors.