GeneralPAC- Making power system intuitive

This video was brought to you by GeneralPAC.com, making power systems Intuitive, Open and Free for Everyone, Everywhere. Consider subscribing and supporting through patreon.com/GeneralPAC. This is a mechanism for you to support us financially so we can continue making high quality power system video tutorials. Our corporate sponsor for this topic is AllumiaX.com from Seattle, Washington. Contact them for industrial and commercial power system studies.

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

In this video, we will demonstrate the advantage of using Per-units in Power systems and how it makes it easy to calculate and compare different equipment.

By expressing the impedances of different electrical components in per units, it is easier to compare their characteristics. It’s worth mentioning that we’re specifically talking about comparing impedances in per-units because the size of an electrical machine varies, then its internal impedance, voltage drop and losses also vary. For example, a primary circuit reactance of 0.1 ohms would be much higher for one transformer and on the other hand much lower for the other transformer; and the driver for this is the Voltage, Power, and various other ratings of the two transformers. However, if that equipment (whether it be a machine, transformer, generator, capacitor bank, etc) would have its electrical quantities converted into per unit values, the different voltage drops and levels disappear and the impedance can be compared over the same scale. Now this is a huge advantage of using per-units in power systems.

Let’s consider an example from the book of Electric Power System by B.M. Weedy (5th Edition).

Example 2.3, pg. 62, Electric Power System by B.M. Weedy (5th Edition)

Let’s ignore the friction and windage losses and efficiency and focus on calculating the % Losses instead. We will employ two methods to do that, one is by normal calculations and the other is through per-units

1: Power method

Total resistance = 0.1 + 0.15 = 0.25 ohms

Pout = V × I = 200V ×100A = 20,000W

Plosses = I2 × R= (100V)2 × 0.25Ω = 2,500W

%Losses =
Plosses / Pout
× 100 =
2,500 / 20,000
× 100 = 0.125 × 100 = 12.5%

Let’s solve the same example by way of per-units.

Method 2: Per-Unit method

We know that to covert a power system into per units, we require base values and actual values. In the case of these machines we will take the ratings as the base values and we will get the base voltage and the base current. We will now find the per unit impedance for the machine:

Total resistance = 0.1 + 0.15 = 0.25 ohms

Per unit impedance =
Actual impedance / Base impedance
=
Actual impedance / Base voltage ⁄ Base current
=
0.25Ω / 200V ⁄ 100A
=0.125 p.u

Let’s draw a table and compare the results using the two calculation methods:

We can see that the results are exactly the same! This means that under the given ratings, the per unit impedance is equal to % Losses. Thus, if we employ either of the methods in comparing more than one machines, we can determine which machine is efficient. The difference is that by using per unit calculations, we don’t have to perform extra steps and calculations as we did in the first method.

This concludes that if we want to compare different electrical components, we could simply calculate the per unit impedances and find out the desirable component for our power systems! Manufacturers usually specify the impedance of a component in in percents or per units on the base of the nameplate rating.

In this video we saw how per unit systems can ease the comparison of different electrical equipment in a power system. In the upcoming videos we will be dealing with an example for per unit systems in transformer calculations.

We hope, you have a continued interest in this topic and series as a student or professional. We also hope you find this content useful and enlightening. Please consider subscribing to GeneralPac.com or becoming a patron on patreon.com/generalpac.

Thankyou.

Greetings from the GeneralPAC Team!

We make high-quality Power Systems Video Tutorials on complex topics that are free and open to everyone!  Thank you so much for supporting us through Patreon so can continue doing good and valuable work.

What is Patreon and why do we use it?

Patreon is a fantastic portal that allows our fans and community to make monthly contribution (like Netflix subscription) so we can continue creating high-quality power systems video tutorials. In return, you get access to incredible perks like voting on future topics, getting your questions answered, access to VIP Q/A webinars with the creators of GeneralPAC, and much more! We THANK YOU for supporting us

Why do we need your support?

An incredible amount of time and effort is needed to develop high-quality video tutorials. Each video (Part 1 for example) takes approximately 10 hours to complete which includes learning the concept ourselves, brainstorming creative ways to teach and explain the concepts, writing the script, audio recording, video recording, and editing. It's no wonder why Hundreds-of-Thousands of people have watched, liked, subscribed, and left positive comments on Youtube channel. Your support truly makes all the difference.

Become a patron today!