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Understanding Distance Protection (21) Video

The last video in our end-to-end testing series gave you a general overview of distance protection, and why we use it in modern electrical systems.  It is important to know how a regular distance protection (21) scheme will react to different faults around the relays, and how those reactions are different from communication-assisted protection schemes.  This video focuses on one line to demonstrate the differences between standard distance protection and communication-assisted trip schemes.

We use the free end-to-end animations located on our blog to apply a series of faults on and around a transmission line to see how a traditional distance protection scheme will react to the different fault locations. We then perform the same tests on a Permissive Overreach Transfer Trip Scheme (POTT impedance protection) to see how the standard distance protection scheme differs from the communication-assisted impedance protection.

After watching this video, you should be able to predict what will happen before you apply any fault scenario during your end-to-end test procedures. (http://youtu.be/xyPN10Ge9YE).

You can get more information about these topics in The Relay Testing Handbook #9: Testing Line Distance Protection (21) (which is a part of The Relay Testing Handbook: Principles and Practice) and The Relay Testing Handbook: End-to-End Testing.  You can save some money by getting the complete Relay Testing Handbook series.

Understanding Line Distance protection (21) Transcript

Here is the transcript of the video for those who would prefer to read the content

Thank you for watching the second video in our End-to-End Testing series. The previous video provided a broad overview of distance protection across an electrical system and why distance protection is used.

Today we’re going to zoom in on one transmission line and see what happens when a fault occurs at different locations on the line. End-to-end relay testing plans typically apply a fault on either side of a protective zone to make sure that the relay operates correctly in each zone. We’re going to apply the same series of tests that would normally be applied to a communication-assisted protection scheme to see what the two standard line distance relays will do in each scenario.

We will be performing our tests on Relay 1 and Relay 2, which have identical protection settings used to detect faults on the transmission line between them. Both relays have Zone 1 protection that is set to operate up to 80% of the line as shown by this black line for Relay 1 and this blue line for Relay 2. Zone 1 will pick up in each relay if the fault occurs above the lines indicating Zone 1.

Both relays also have Zone 2 protection that reaches 120% down the line as shown by the Zone 2 black line for Relay 1 and this blue line for Zone 2. As we discussed in the previous video, Zone 1 is set at 80% of the line to compensate for any CT accuracy and calculation errors, and Zone 2 will protect the other 20% of the line with a 20-cycle time delay.  Zone 2 also gives us the added benefit of back-up protection for Relay 4 and Relay 3. For the purposes of this video, we will be pretending that someone forgot to turn the battery charger back on after the station batteries in Relays 3 and 4 were tested, so relays 3 and 4 will never operate.

All of the relays also have Zone 3 protection looking in the reverse direction. Any fault above this Zone 3 line for Relay 1 will cause Zone 3 to pick up in Relay 1, and any fault above the Zone 3 blue line for Relay 2 will cause Zone 3 to pick up in Relay 2.

If we simulate a fault very close to Relay 1, Zone 1 and Zone 2 will both pick up. This is one thing that you really have to think about when you’re doing end-to-end testing.  We often think in terms of what trips a relay, but communication-assisted tripping schemes share information when elements pick up, not when they trip. Both zones are going to pick up if the fault is closer to one side. If we look at Relay 2, it’s not going to see Zone 1 because the fault is beyond the Zone 1 settings.  Relay 2 will see a Zone 2 pickup as soon as the fault starts.

Relay 1 is going to operate first because there’s no intentional time delay set for Zone 1 and Relay 1 detects a Zone 1 pickup. The description above the breaker indicates zero cycles here, but it’s really somewhere between zero and three cycles. Zone 2 still has a source on the other side feeding the fault, and so Relay 2 Zone 2 will trip about 20-cycles later.

When you’re doing an end-to-end test, you simulate a fault on one side of the protection zone to make sure it operates correctly, and then you move to the other side of the protection zone to see what happens when the fault location changes. When we move the fault to the other side of Relay 2 Zone 1, Relay 1 is still going to detect Zone 1 and Zone 2 pick-ups because the fault is inside both of those zones.  Relay 2 is going to detect the same thing because we’re in the overlapping region between the two relays’ Zone 1 protection. When we click the “What happens next” button, we can see that Zone 1 and Zone 2 protection are both picked up in both relays. That means that both relays are going to operate instantaneously, because we’re in that overlapping region.

If we move the fault closer to Relay 2, you can see that, again, we’re in the overlapping region between Zone 1 and Zone 2. This means that both relays will detect a Zone 1 and Zone 2 pickup. Zone 1 has no intentional time delay, so both relays operate instantaneously.

If we move the fault a little bit closer to Relay 2, we can see that Relay 2 is going to detect a Zone 1 and Zone 2 pickup, but Relay 1 is only going to detect a Zone 2 fault. So, Relay 2 is going to operate almost instantaneously on Zone 2, and Relay 1 is going to operate about 20-cycles later.

Based on these tests, whenever there is a fault on the protection line, the worst case scenario is that one relay is going to operate instantaneously, and the other relay is going to operate about 20-cycles later.

So let’s see what happens when we move the fault outside of the line. This fault occurs behind Relay 1, so Relay 1 will detect a Zone 3 fault because the fault is behind it.  Relay 2 is going to detect a Zone 2 fault because we’re still within Zone 2 for Relay 2. Relay 3 is disabled so it’s not going to operate at all. Relay 1 Zone 3 has a 60-cycle time delay, and Relay 2 Zone 2 has a 20-cycle time delay, so Relay 2 will operate first and isolate the fault.  Relay 1 does not get a chance to operate at all.

If we move the fault outside of Relay 2’s Zone 2 protection, the only element that’s going to pick up is Relay 1 Zone 3. It’s operating as the worst case back-up scenario, and it will trip after 60-cycles to clear from the rest of the system. This is, of course, a worst case scenario. Relay 3 normally would have operated.

If we move to the other side of the line, you can see that the opposite happens. Zone 2 on Relay 1 is going to pick up, and Zone 3 on Relay 2 will also pick up. Zone 2 has a smaller time delay, so the breaker connected to Relay 1 is going to operate first.

If we move the fault outside of Relay 1 Zone 2, Relay 1 is going to ignore that fault completely because it’s outside of its zone of protection. But Relay 2 is going to detect a Zone 3 fault, and it’s going to trip in about 60-cycles.

If we move the fault completely outside of both zones, nothing will happen because it’s outside of the zones of protection for both relays.

Now that we know what will happen with normal line distance protection with no communication, let’s see what happens when we apply a communication scheme. This drawing depicts a POTT, or permissive over-reaching transfer trip, scheme which is the most common kind of communication-assisted protection scheme that there is.

If the fault occurs next to Relay 1. The relays will detect the fault as if there was no communication enabled because those settings haven’t changed. Relay 1 detects a Zone 1 and Zone 2 fault, and Relay 2 detects a Zone 2.

A POTT scheme communicates Zone 2 information between relays. If either relay detects a Zone 2 fault, it will send a signal to the other relay.  If the other relay also detects a Zone 2 fault, the fault MUST be between the two relays, and there is no reason to delay in this case.  Therefore, if a POTT scheme detects Zone 2 in both relays, both relays will operate as quickly as they can.  In this scenario, Relay 1 will still trip instantaneously because of the Zone 1 pickup, but Relay 2 should operate in less than 6 cycles because both relays detected a Zone 2 fault.  The 6 cycles could be any small time delay to allow for communication delays.

When we compare that to what happened with the regular distance protection, the Zone 1 or Relay 1 operated instantaneously just like on the communication-assisted  protection scheme but the other relay operated in 20-cycles. All of the communication equipment, extra settings, and extra time necessary to install, set, and test a communication-assisted trip scheme is used to reduce the amount of time a fault stays on the electrical grid by about 17 cycles.  This may not seem like a big deal, but 17 cycles seems like forever in electrical terms. All communication-assisted trip schemes, such as DUTT,  DCB, DCUBS, etc., have the same end result. So it really doesn’t matter what scheme you test, they all trip faster if a fault is between the two PTs or on the line compared to a regular distance protection scheme.

Thanks for watching this video all the way to the end. We’ll be reviewing the most commonly applied communication-assisted protection schemes in future videos, so I hope you’ll subscribe to this channel to get automatic updates.

In the meantime, you can visit us at relaytraining.com where we have online training classes and The Relay Testing Handbook series these videos were based on.

Please like and subscribe because it helps us get noticed, which also helps us keep putting out free content like this video.

Have a great day.

About the Author Chris Werstiuk

Chris is an Electrical Engineering Technologist, a Journeyman Power System Electrician, and a Professional Engineer. He is also the Author of The Relay Testing Handbook series and founder of Valence Electrical Training Services. You can find out more about Chris here.

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