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.
