Your image

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.

Power Systems Protective Device Coordination Part 5.

In part 4, we talked about the “inverse time characteristics” strategy and how we can use it to achieve coordination. In part 5, we will be discussing the time characteristic curves in more detail.

If this video was helpful for you, please consider subscribing to GeneralPAC.com. Our goal is to make Power Systems intuitive.

Time-current characteristic curves are used to show the time taken by the breaker to trip for a range of overcurrent conditions. They define the operating characteristics of a protective device. The x-axis shows the current level, and the y axis is used to show the time taken by the protective device before the tripping action is initiated.

To explain this concept, let us again consider a facility with a 50 KVA, 60Amps static load.

As discussed in previous parts, all breakers should be rated at some percentage above the full load amps of the downstream load. Breaker “C” is rated at 80 Amps at 480V, Breaker “B” is 120 Amps at 480 volts. And Breaker “A” is rated 100 Amps at 4.16 kV. The breaker amp rating is much greater when we compare all the breakers at the same voltage level.

To demonstrate how time-current characteristic curves can be helpful in achieving coordination, let’s consider a fault between Breaker “B” and Breaker “C”. For this case, let’s assume both Breakers to be “Thermal Magnetic” Breakers.

We will now plot the curves of Breaker “B” and Breaker “C” on the same axis. We can see that Breaker “B” picks up at a higher amp value, while Breaker “C” picks up at a lower amp value. This results in a shift between the curves and allows coordination.

Now that we understand the basics, let’s go into more detail and talk about the different regions of a time-current characteristic curve. For a thermal magnetic breaker. There is a long-time region which starts from the pickup amp value. This region includes the protection against overloads from 110 to 120 percent of the amp rating. There is a fixed, non-adjustable time delay for each Breaker, after which, it enters the instantaneous region.

This instantaneous region could begin from about 200 to 300 percent of the amp rating, and is used for protection against high abnormal currents. The instantaneous region has a very small delay of a few cycles, after which, the tripping action of the Breaker is initiated.

This is one way of achieving protection coordination using inverse time characteristics and the breaker amp ratings. However, there is a flaw with using thermal magnetic breakers for coordination purposes. If we look at the curves again, we can see that the instantaneous regions of Breaker “B” and Breaker “C” are overlapping after a certain current level.

Let’s say this level is at 10000 Amps. What this means, is that for any fault current above 10000 Amps, there is a high risk of both breakers tripping simultaneously. This would result in a power blackout on the 480V bus. This type of overlap is particularly true for Thermal Magnetic Circuit Breakers which have a fixed amp rating, some instantaneous rating adjustment, and fixed curve. This will be discussed in more detail later.

The solution to eliminating this overlapping region lies with the use of breakers with electronic trip units. Their operating characteristics are highly adjustable for ensuring better protection coordination.

In Part 6, we will be discussing the operating characteristics of electronic circuit breakers in more detail. If you found this video helpful, please consider liking and subscribing to GeneralPAC.com. Thank you.

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!