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Why we use Per-Units in Power Systems, Part 2a.

This is the first part of the series on answering the question, Why we use Per-Units in Power Systems. We have introduced per unit systems in the previous series and discussed how we derive them. In this series, we will demonstrate the significance of using per-unit systems in calculations.

Per-Unit systems are employed in power systems analysis due to a number of reasons. We will list them here and describe them briefly.

  1. Easy calculations/comparisons
  2. When we are given two equipment to compare, say two generators, we have to choose one for our power system. We have to compare qualitatively and quantitatively and here is when per unit systems come handy for the quantitative part. We use per unit values of the system to determine the generator requirements and then decide which will be the best fit for our power system. This is described in more detail in Part 2b.

  3. Transformer (referring to the sides)
  4. For transformers, we usually make its equivalent by referring to low voltage side or the high voltage side. After that, we calculate the different impedances. However, when we are using per unit values for impedances in a transformer, they remain the same for the low voltage side and the high voltage side. Therefore, calculating one value of per unit impedance is enough and the lengthy process can be omitted. A complete derivation of this has been done to prove this aspect in Part 2c.

  5. Multiple voltage levels
  6. When we are dealing with a power system that has different voltage levels in different areas of the network, we prefer to employ per unit calculations. Similar to what we did in Part 1c of this series where we solved an example of a power system having two transformers to vary voltage levels in the three different regions. Converting such system into its per-unit equivalent simplifies the calculations. We will be investigating this in detail in Part 2d.

  7. Elimination of √3
  8. So, it’s obvious that by mentioning √3, we are talking about calculations involving three phase systems and employing common Wye connected systems vs Delta connected systems. We saw some formulas and derivations in Part 1d of this series that showed us how √3 got canceled out of three phase calculations. By doing so, those calculations became more simple. We’ll look deeper into this in the upcoming video Part 2e.

In this video we discussed the advantages, per-unit systems bring in, to analyze a power system.

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