Understanding Line Distance protection (21)
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Understanding Line Distance protection (21)

February 28, 2020


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 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 of 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. 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 1does 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 bug 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 cause it helps us
get noticed, which also helps us keep putting out free contents like this video.
Have a great day.

Only registered users can comment.

  1. Hi I was wondering, what criteria would determine a communication based schedule like pott, or Ducb etc.
    I know line differential is used when line SIR is very high and distance based protection cannot be applied.

    You also mentioned that grid can't handle large duration of fault contribution.
    Can you please give a real world situation,

  2. Excellent presentation. I have a question. Line distance protection advantage is its independence on change of the system beyond of the zone of protection (because is based on impedance) but is always described in scenarios where power flows in both directions, there are various zones of protection and a pair of relay are used, one at each extreme of the line. But in cases where there are just one zone of protection and unidirectional energy flow (for example in my case, a 40 KM 115kV single line feeding a radial/terminal substation), does it make sense to use line distance protection just at power source side?.

  3. I have found a good post on how distance relay and distance protection works at-
    http://newtechworld.net/distance-protection-relays/

  4. Thank you for this presentation!
    I'd like to know whether there are softwares that allow to test and simulate distance peotection.
    Thanks again

  5. Thank you very much for the presentation! I would like to know how to calculate the magnitude of the worse case when the fault falls on zone 3 of relay.
    thank you!

  6. hi your videos are very helpful..am working in electrical protection designing..plz give me a video link that you mentioned as1st video of distance protection

  7. Nowadays distance relays modeld like D60 P742, REL670 are coming with 5 zones. what's the advantage of increasing this zones. Line differential are used only if the lines are less than 10km . How to communicate with remote end relay

  8. So far so good. Vids 1 & 2 seem very good. I've picked up a few things. Thanks for creating these vids and sharing them!

  9. Fantastic. Thank you so much for this video.
    it’s very useful to understand how distance relay work.
    More videos please.

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