Transmission Line Protection (21)
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Transmission Line Protection (21)

December 26, 2019


Thanks for taking the time to watch the 1st
video in our End-to-End Testing series. End-to-end testing builds on traditional transmission
line protection, so we are going to start with some basic principles from the Relay
Testing Handbook series to make sure everyone is on the same page before we delve into the
end-to-end testing details in future videos. Power lines transmit energy from one location
to another and are the lifeblood of the electrical system. There are many problems that can cause
a power line to fail such as: * lightning strikes,
* animals spanning two lines, * equipment failure, or
* just plain human error. Your traditional overcurrent protection worked
well when the power flowed in only one direction because the fault can be located by current
magnitude. The relay closest to the fault will trip first in a properly coordinated
system, because of the large current flowing into it and its smaller pickup setting.
Current can flow in any direction on an electrical grid, so it is almost impossible to set all
of the relays to operate correctly for every possible scenario. Even if you were to achieve
the impossible and set every relay perfectly, everything would get thrown out of whack the
next time the grid is changed to add a new generator or customer.
Imagine that you are a utility design engineer in the dirty 30�s and you keep losing the
entire electrical grid for every fault, no matter where it is on the grid. What you really
need is a relay that will operate for any fault on the transmission line it is protecting,
and ignore faults anywhere else on the grid. You can measure the voltage at any point in
the system using PTs, and you can also measure the current entering and leaving the power
line with CTs mounted at each end. You can use the current and voltage with your standard
electrical formulas to calculate: * Phase Current (51P)
* Residual Current (51G or 51N) * Phase Voltage (27P)
* Residual Voltage (59G or 59N) * Directional Overcurrent (67)
* Power (32) * Resistance/Impedance (21)
* Any symmetrical component (Q) or, * Differential current (87)
Will any of these values help you determine whether a fault is on the transmission line
or not? [Pause]
We already know that overcurrent and voltage elements are not selective enough for this
application. Symmetrical Components are mostly extensions of current and voltage that are
good at identifying what kind of fault occurred, but not where the fault is. Power and directional
overcurrent will tell us what direction the fault is, but they are not selective enough
to determine if the fault is on the power line or somewhere further down in the system.
The perfect solution is differential protection. Differential protection monitors the current
entering and leaving the transmission line and sums them together. If the currents cancel
each other out, the differential relay will not trip because the system is either operating
normally, or there is a fault that is not on the transmission line. If the current and
voltage leaving the transmission line do not cancel each other out, the fault must be on
the transmission line and the relay will open both circuit breakers. This looks like a great
solution Too bad this is the 1930s and the technology
necessary to transmit three-phase analog signals across large distances in real time doesn�t
exist. That leaves you with impedance, which is a
pretty good second best solution. In order for this solution to work, you need to know
the impedance of the transmission line. You could measure it with fancy equipment, but
it is usually easier to calculate it using: * Size of the wire
* Length of the transmission line * And Spacing between wires
Let�s imagine that we�ve run through the calculations and miraculously, the spacing
and type of wire works out to one Ohm per mile. The transmission line you want to protect
is 10 miles long, so the entire impedance of this line is 10 Ohms. If we applied a dead
short at the end of this line, the relay would measure the voltage and current flowing through
the relay, apply Ohms Law, and calculate 10 Ohms. If there was a fault 50% down the line,
the relay would measure 5 Ohms. You have come up with a great solution to the problem that
can be applied to any transmission line as long as you can calculate the line impedance.
You�ve gone into your workshop with this information and created an impedance relay
that will only look for faults in the forward direction. Your creation has an adjustable
pickup system so that you can apply this relay to any transmission line, and now it needs
a setting for the 10 Ohm power line we started with. What pickup setting in Ohms will provide
the best protection for this transmission line? [Pause]
Did you choose 10 Ohms? That is a great setting for a perfect transmission line in a textbook,
unfortunately we live in the real world and there are a couple of problems with a setting
set to 100% of the transmission line: * The first problem is that we calculated
the transmission line impedance, and there will always be errors and assumptions that
will affect the calculations . * Even if we could account for the inherent
inaccuracy with the calculations, the transmission line impedance will change with temperature
and spacing between wires. Seasons and wind can change the transmission line impedance.
* What signals are you using to measure the real-time impedance? Those CTs and PTs aren�t
perfect, especially the CTs. What is standard percent error allowed for a protection class
CT? [Pause] * If you guessed 10%, you are correct up to
a point. Protection class CTs can have errors up to 20% when the current exceeds 20x its
rating. All of these problems add up, and if we set
the impedance protection to be 100% of the transmission line, the real-world impedance
may be larger or smaller than our calculations. If the real impedance is smaller than our
setting, our relay may trip for a fault on an adjacent power line somewhere else in the
system�the very thing we were trying to prevent. Most impedance relays are set to
operate instantaneously with a setting somewhere between 70-90% of the line impedance to make
sure that the relay only trips when a fault occurs on the protected line. This protection
is usually called the Zone 1 protection. What happens if the fault occurs on the last
30% of the transmission line? You could add another relay to trip at 100% of the line
with a time delay so that if the relay incorrectly detects a fault on an adjacent line, the relay
allows the adjacent line relay time to trip first. If the fault is actually on the line,
the relay will trip after a short time delay. In fact, you could purposely set this Zone
2 relay to over-reach into the other transmission lines in order to provide 100% protection
for this transmission line and back-up protection for adjacent lines if those relays fail. Most
Zone 2 impedance elements are set at 120% of the transmission line they are protecting
with a 15-25 cycle delay to allow time for the adjacent relays to trip.
Zone 1 and Zone 2 protection overlap and provide 100% protection of the transmission line,
and backup protection for adjacent lines. Remember that there is another relay on the
other side of the transmission line looking in the other direction, and all of these zones
overlap to provide a pretty good protection scheme. Any fault in the region where the
two Zone Ones overlap will trip both breakers almost instantaneously� and a fault outside
that region will be cleared instantaneously on one side with a maximum time delay of 25
cycles on the other side. You can now apply this protection to any transmission
line to provide reliable and selective protection. Click the subscribe button to get automatic
updates whenever we post a new video. The next video in this series will demonstrate
this protection scheme in action. If you want more information, check out the
description below for links to other videos, The Relay Testing handbooks that this video
was based from, and online training courses. Please like this video if you found it useful.
It helps us get noticed, which means that we can afford to keep posting free videos
for you.

Only registered users can comment.

  1. thank you, great work it helps me so much, would you please make another video about the transformer and generator protection

  2. please add more videos very informative and easy to follow, bought the books but this video tutorial is easy to understand

  3. Great Video. The animations and graphics get someof the more complex principles across far better than any powerpoint….including ours.

  4. Could you do a video on how the logics are done in Distance protection numerical relays.. Say Autoreclosure logic especially for a one and half breaker scheme and all the other logics

  5. Its quite helpful. I have been reading about distance protection for last 3 days but was confused, this video really helped me.

  6. how do you know that the relay will detect the short circuit current as 8 A and voltage is 40 V at the 50% of the transmission line?

  7. This is the best video I have seen on distance protection relays. With a little prior reading and this video, I'm sure anyone will be able to get a holistic understanding. Thanks, look forward to more!

  8. i dont understand at time 7.08 min they said that at last 30 % of line1 if fault occurs adjacent relay operate first.why this ? adjacent relay is not issue na main issue is to protect genrator side (substation side). first relay 1 has to operatr?

  9. This is the very good and clear video for explanation of the distance protection relays. I will recommend it for a young people starting to study this theme.

  10. Great video. It cleared my doubt about the zones.
    Can you please explain which relay will cover a fault in the part between the bus and the CT first?

  11. Anyone meets some testing and commissioning Protection relays WhatsApp group.? To get me on them
    Best Regards

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