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Understanding Permissive Under-Reaching Transfer Trip (PUTT) Communication Assisted Trip Schemes Video

Here is the latest video describing the Permissive Under-Reaching Transfer Trip Communication-Assisted Trip Schemes used in modern distance protection.

You can follow along with this animation via the Can You Predict What Happens in a Permissive Under-Reaching Transfer Trip (PUTT) Scheme? post found under the Testing handbooks / Book Extras menu.

You can also get more information about End-to-End Testing and all of the communication-assisted trip schemes via The Relay Testing Handbook: End-to-End Testing.

Here’s the video:

Here’s a transcript:

Welcome to the fourth video in our end-to-end testing series.  We’ll be looking at a Permissive Under-reaching Transfer Trip, or PUTT, communication-assisted trip scheme in this video.

I’m going to assume that you’ve watched the previous videos in this series; so I won’t rehash what to look for in this animation. If you haven’t watched the previous videos, stop now and click this link to watch them first, so you can follow along.

We talked about DTT and DUTT teleprotection schemes in the previous video, and we’re talking about a PUTT scheme today.  These acronyms can be confusing, so let’s break down the code to help you figure out what they  mean.

If the scheme starts with the letter D, the D can stand for “Direct” or “Directional.”

Relays do not share information back and forth in a “Direct” scheme.  One relay simply tells the other relay to trip, and the other relay follows the command.

The TT at the end of an acronym stands for “Transfer Trip.”  This means that a remote relay is sending a trip signal to the other side.

If I told you we were going to test a DTT scheme, you could translate the acronym to mean “Direct Transfer Trip.”

  • “Direct” tells you that you don’t need fancy GPS or IRIG connected equipment to test this scheme.
  • “Transfer Trip” tells you that one relay will send a trip signal to the other.
  • There could be another letter telling you which Zone to apply, but it is missing from this acronym, so that means that there will be an external signal initiating the Transfer Trip; like a breaker fail relay, or 52b signal that will isolate both sides of a line if one breaker operates.

If I told you we were testing a DUTT scheme, you could infer that:

  • “Direct” means no fancy time-synched test equipment will be required on both ends.
  • “U” stands for “Under-reaching”. Therefore one relay will need to measure a Zone-1 signal to send a trip to the other side. A relay test-set will be required for this test.
  • “Transfer Trip” tells you that one relay will send a trip signal to the other.

We’re looking at a PUTT scheme today, and:

  • The “P” stands for “Permissive.” A Permissive scheme tells the other relay that it is allowed to trip faster if it ALSO detects a fault in the correct direction. Both relays must agree that there is a fault before a Permissive trip, unlike the direct scheme that would send a trip signal if only one relay detected a fault. Permissive schemes share information back and forth, so you will need your fancy GPS and/or IRIG connected equipment on ALL sides of the line.
  • The “U” stands for “Under-reaching”. Therefore, at least one relay must measure a Zone-1 fault for this scheme to work.
  • “TT” means that one relay is sending a Transfer Trip signal to the other.

We’re looking at a PUTT animation that you can find on our website, relaytraining.com.  There should be a link on the screen right now that you can open in a new window if you want to follow along. The link can also be found in the description below.

Which elements will pick up in Relay-1 if a fault occurs close to Relay-1 as we show here?

Which elements will pick up in Relay-2?

Zone-1 AND Zone-2 will pick up in Relay-1 because the fault is closest to Relay-1, while only Zone-2 will pick up in Relay-2.

Which relay will trip first?

Relay-1 will trip instantaneously because of the Zone-1 pickup, but it will also send a PUTT signal to the other relay because it has detected an Under-reaching fault on the line.  We call Zone-1 an Under-reaching condition because it is purposely set somewhere between 75 to 90% of the line; so that it will never operate for a fault that is not on the line.

The fault is still on the line even though Relay-1 tripped and current is flowing through Relay-2. How long will it take before Relay-2 trips?

Relay-2 would trip after a 20-40 cycle Zone-2 time delay in a normal impedance protection scheme, but we’re using a PUTT scheme here. Relay-2 receives a Permissive Under-reaching Transfer Trip signal from Relay-1. If Relay-2 receives permission to trip from Relay-1, AND it detects a fault on the line via the Zone-2 pickup, it is allowed to trip faster after a small communication time delay.

Relay-2 trips in a significantly shorter amount of time using a PUTT scheme.

Let’s look at another fault on the line.

This fault is a mirror image of the previous one. Which elements will pick up in Relay-1 and 2?

This time Relay-2 sees a Zone-1 and Zone-2 pickup, while Relay-1 sees a Zone-2 pickup.

Why do I keep harping on these easy questions about pickup in these examples?

Communication-assisted protection schemes like PUTT and POTT use pickup detection to decide what to do, and most relay testers are laser-focused on what trips inside the relay; so I want to make sure that you think in pickup terms, and not trip terms, when evaluating end-to-end test results.

Which relay will trip first, and how long will it take to trip the other relay?

This time Relay-2 will trip instantaneously because it detected a Zone-1 fault.

and Relay-1 should trip in 20-40 cycles because it detects a Zone-2 fault, which means that the relay does not know whether the fault is on the line, or not. However, Relay-1 confirmed that the fault was on the line with its PUTT signal, therefore Relay-2 has permission to trip after a short time delay, and then indicate a communication trip on its front panel.

Now let’s look at how the Permissive Under-reaching Transfer Trip scheme compares to a regular distance protection scheme.

Our standard protection scheme is at the top of the animation, and the PUTT scheme is on the bottom. Watch what happens in each scheme as I cycle through the original fault.

We have a fault that is closer to Relay-1.  Which elements will pick up in each relay?

Did you choose correctly?

Notice that both schemes have identical operating characteristics so far. Relay-1 sees a Zone-1 and Zone-2 fault, while Relay-2 sees a Zone-2 fault.

Which relay should trip first?

Relay-1 trips first in both schemes because they both detect a Zone-1 fault.

When would Relay-2 trip in the standard and PUTT schemes?

Relay-2 trips after 20 cycles in the standard scheme and almost instantaneously using the PUTT scheme because there were two pairs of eyes that verified that the fault was on the line. The Zone-2 delay only exists because one Zone-2 detection by itself can never be sure whether the fault is on the line, or outside the zone of protection.

Will the two schemes perform differently if the fault happens 50% down the line?

Any fault in the overlapping region of Zone-1 will trip instantaneously because both relays see Zone-1.  There is no need for fancy teleprotection schemes for those faults.

What happens when the fault is not on the line and relays 3 and 4 are disabled?

Which elements will pick up in each relay?

Relay-1 will detect a Zone-3 fault, and Relay-2 will detect a Zone-2 fault.

What will happen next?  Will Relay-1 operate? Will Relay-2 operate as a normal protection scheme, or will it trip faster using a PUTT scheme?

Relay-2 will operate after its usual Zone-2 time delay because it will not get permission from Relay-1 to trip faster.

That’s the dirty little secret of communication-assisted trip schemes. They are only installed to trip faster if the fault is beyond Zone-1 to the end of the line.  All this time, money, effort, and extra testing technology saves us 20-40 cycles inside a very narrow window. Why do we do it?

The primary reason is system stability.  There have been studies of the power system that show a relationship between fault duration and system stability. That 20 cycles of fault current could cause a much larger outage for every extra cycle the fault stays on the line, so we want it off as soon as possible.

Thanks for watching this video. I hope you have some new insights into how a Permissive Under-reaching Transfer Trip communication-assisted trip scheme works.

You can play with this animation, and more, by following the link on the screen.  You can also get more information about testing these schemes via The Relay Testing Handbook: End-To-End Testing book via the other link in this video, or in the description below.

As always, please like this video and subscribe to our channel to let Google know we have good stuff.  It helps us get noticed and allows us to keep putting out free content like this with no ads.

Don’t forget to have fun out there.


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About the Author

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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