Understanding Base Value Calculations: Line vs Phase Voltage Explained"

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Discussion Overview

The discussion revolves around the calculation of base values in polyphase AC circuits, specifically focusing on the choice between using line-to-line voltage and phase voltage for determining base values such as current, impedance, and power in a per-unit system.

Discussion Character

  • Technical explanation, Conceptual clarification

Main Points Raised

  • One participant questions whether to use line-to-line voltage or phase voltage for base value calculations, seeking clarification on the reasoning behind the choice.
  • Another participant expresses confusion about the terms "line-to-line voltage" and "phase voltage," indicating a lack of understanding of the initial question.
  • A third participant suggests that the question pertains to polyphase AC circuits, specifically three-phase systems, although they admit to limited knowledge on the topic.
  • A fourth participant explains the typical practice in the per-unit system, stating that total three-phase power is taken as base power, line-to-line voltage as base voltage, and line current as base current. They also note the relationship between these quantities and how to derive base impedance from them.

Areas of Agreement / Disagreement

The discussion includes varying levels of understanding and knowledge about the topic, with no consensus reached on the initial question regarding the use of line-to-line versus phase voltage.

Contextual Notes

Participants have not fully defined the terms used, and there may be assumptions about prior knowledge of polyphase systems that are not explicitly stated.

anon6912
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When calculating base values for current, impedance etc
do we use the line to line voltage or the phase voltage?

What's the reason for using one over the other?
 
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What is a "line to the line voltage"? What is "phase voltage"? I don't understand your question.
 
My best guess is that he's asking about polyphase AC circuits, likely 3-phase.

I don't know much about 3-phase though.
 
If you are talking about per-unit system, this is the usual practicce.
Take Total 3-phase power S, as base power,
Take Line-to-Line voltage V, as base Voltage V.
Take Line-Current I, as base Current
Take an Impedance Z, as base impedance

But since you can only select two Base quantities independently, If you say take S and V as the initial base quantities (which is the usual practice) then
I = S / sqrt(3)*V

Now, the Impedance Z is assumed to be that impedance Z, which when connected in Star fashion in a 3 phase line with line-to-line voltage *V* will result in line Current *I*.

So, I hope that now you can find the base impedance Z, from the base voltage V and base Power S.
 

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