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 Over-Reaching Transfer Trip (POTT) 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 fifth video in our end-to-end testing series. We’ll be looking at a Permissive Over-reaching Transfer Trip, or POTT, 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 have not watched the previous videos, stop now and click this link to watch them first, so you can follow along.
Now that the introductions are out of the way, we can start by decoding the term POTT:
- The “P” stands for “Permissive.” A Permissive scheme tells the other relays protecting a line that they can trip faster if they ALSO detect a fault in the correct direction. All relays must agree that there is a fault on the line before a Permissive trip is allowed, 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 “O” stands for “Over-reaching”. Zone-2’s pickup impedance is typically set larger than the protected line, which means at least one relay must measure a Zone-2 fault for this scheme to work.
- “TT” means that at least one relay is sending a Transfer Trip signal to the other relays in the scheme.
You’re looking at an animation of a traditional POTT scheme 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.
Relay-1 will trip instantaneously because of the Zone-1 pickup, but it will also send a POTT signal to the other relay because it has detected a Zone-2, or potential over-reaching, fault on the line. We call Zone-2 an Over-reaching condition because it is purposely set somewhere around 120% of the line as we described in the previous videos. Zone-2 is purposely set to detect faults in the section between the Zone-1 limit and the rest of the line, AND it provides backup protection for faults on other lines in the forward direction. This means that Zone-2 will detect faults that may not be on the transmission line this relay is installed to protect.
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 normally trip after a 20-40 cycle Zone-2 time delay in a standard impedance protection scheme, but Relay-2 received a Permissive Over-Reaching Transfer Trip from Relay-1. This POTT signal from the other relay gives Relay-2 PERMISSION to trip faster IF it also detects a Zone-2 pickup. The permissive signal AND Zone-2 pickup means that Relay-2 will trip after a short communication time delay, which is usually less than 3 cycles.
Let’s look at a fault that is closer to Relay-2, but this time we’ll compare it to a standard distance protection scheme.
This fault is a mirror image of the previous one with the standard distance scheme on the top of the screen, and the POTT scheme shown on the bottom of the screen. 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.
Relay-2 will trip instantaneously in both protection schemes because Zone-1 will always trip first because it has no intentional time delay.
Relay-1 should trip in 20-40 cycles in the normal protection scheme at the top of the screen because it has detected a Zone-2 fault, which means that the fault could be on the line the relay is supposed to protect, OR it could be on the line Relay-4 is installed to protect. Therefore, it is going to wait for Relay-4 to trip in case the fault is on Relay-4’s line. If Relay-4 doesn’t operate, Relay-2 will trip after a time delay.
The POTT scheme on the bottom of the screen will NOT wait for Relay-4 to operate because it is getting feedback from Relay-2 that indicates that the fault is on the line. If Relay-1 and Relay-2 both detect a Zone-2 fault, that means the fault should be in the overlapping region between the two relays, which is 100% of the line. Relay-2 will trip after a short communication time delay and remove the fault from the system.
A POTT scheme has pretty simple operating characteristics. If all the relays protecting a line detect a Zone-2 pickup, then the fault must be on the line; therefore, there is no reason to wait for the normal 20-40 cycles.
However, I bet the POTT schemes installed at your sites are more complicated than what I’ve described here because POTT schemes have a glaring weakness that we can demonstrate in this animation:
This animation can also be found on our website and depicts POTT schemes installed on parallel lines. Current flows from right to left under normal conditions. Then a fault occurs on one line.
Relay-1 detects a Zone-1 and Zone-2 fault, while Relay-2 detects a Zone-2 fault. The standard POTT scheme logic applies. Relay-1 will trip with no intentional time delay, and Relay-2 should trip after a short communication delay. It looks like a standard fault for Relay-1 and Relay-2, but let’s look closer at Relays 5 and 6.
Relay-5 detects a Zone-3 fault because there is a parallel path for current to flow into the fault and the fault appears to be behind Relay-5. Reverse zones are typically used to detect reverse faults in communication-assisted trip schemes and don’t trip anything, or they have long time delays of 60-120 cycles. Relay’s 1 and 2 are going to isolate the fault long before Relay-5’s Zone-3 gets a chance to trip anything. So far so good.
Relay-6 could detect a Zone-2 fault, which has a 20-40 cycle time delay, so it probably won’t have a chance to trip either. BUT… it will send a Permissive trip signal to Relay-5.
Now let’s see how the relays respond to this fault.
Relay-1 trips instantaneously as we predicted, but there was a source connected to Relay-3, which is now the primary source for fault current.
Relay-2 still detects a Zone-2 fault and had received permission to trip for the POTT scheme, so it’s primed to trip after a short communication delay.
The current suddenly changed direction in Relay-5, so it no longer detects a Zone-3 fault and it could now detect a Zone-2 fault. Zone-2 has a long time delay, so that shouldn’t be a problem because Relay-2 should be tripping momentarily. BUT… Relay-6 was sending a POTT permissive before Relay-1 tripped. Relay-5 detects a Zone-2… AND it could still be receiving a POTT signal from Relay-6 because communication signals will always be slower than locally processed information, such as the Zone-2 pickup. This means Relay-5 could be primed for a POTT operation.
We now have a race between Relay-2 and Relay-5. If Relay-5 wins that race, we could lose both lines for a fault on the one line, which could be a major problem.
This weakness is inherent in any over-reaching communication-assisted trip scheme, and most schemes add additional logic to minimize this problem as shown in this revised drawing.
Zone-3 is looking in the reverse direction and is connected to a new drop-out timer, which is connected to a NOT symbol on the POTT Scheme. All of the relays detect the same zones when all breakers are closed in this new hybrid scheme, and now:
- Relay-1 detects a Zone-1 Pickup and will trip instantaneously. Relay-1 also sends a POTT permissive trip signal to Relay-2 because it detects a Zone-2 pickup, and it did NOT detect a Zone-3 pickup in the last 5 cycles.
- Relay-2 should trip on POTT after a short communication delay because Relay-2 detects a Zone-2 pickup, AND it’s receiving a POTT permissive signal from Relay-1, AND it has NOT detected a Zone-3 pickup in the last 5 cycles.
- Relay-5 detects a Zone-3 reverse fault and sends a block signal to its POTT scheme.
- Relay-6 detects a Zone-2 pickup and sends a POTT permissive signal to Relay-5.
All the relays operate normally so far.
Breaker-1 operates with no intentional time delay, which will cause a sudden current reversal through relays 5 and 6. Relay-5 now detects a Zone-2 fault, and it could detect a POTT permissive because Relay-6 may not have had time to release its previous permissive signal yet. This was a problem in the old scheme, but Relay-5 detected a Zone-3 reverse fault a moment before, so the Drop Out timer will hold the Zone-3 input to the POTT scheme on. The NOT logic gate reverses the input and the POTT scheme cannot operate for 5 cycles, which should give Relay-6 plenty of time to release its POTT permissive signal.
We no longer have a race between Relays 2 and 5. Relay-2 will operate after a short time delay and the fault will be isolated from the system without affecting the non-faulted line. The scheme’s weakness has been beaten into submission with logic!
The dirty little secret of communication-assisted trip schemes is they all have the same operating characteristics under normal operating conditions. All end-to-end tests should be performed on either side of each protective zone to make sure the protected line is isolated faster when a communication-assisted trip scheme is applied.
However, POTT schemes require additional tests that prove that the Zone-3 timer is appropriate for the application by simulating a phase reversal from both directions.
Thanks for watching this video. I hope you have some new insights into how Permissive Over-reaching Transfer Trip communication-assisted trip schemes work.
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 in The Relay Testing Handbook: End-To-End Testing using the other link in this video, or reading the description below.
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