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Please Explain the Principle of Weak InFeed Echo Permissive Over Reach Transfer Trip Schemes?

Rahim submitted this question to the Ask Chris section of the RelayTraining.com website.

Most line distance protection schemes assume that the system will always be able to supply enough amps to make the relays trip. However, there are many scenarios on transmission systems across the world where you could have a strong source on one end of a transmission line with a weak source on the other, and this configuration could prevent the protective relay(s) from operating the way we would expect them to.

The scenario that is easiest to understand, and explain, occurs when we connect a generating plant, or generator, to the transmission system. The drawing below shows this scenario with the generator breaker open and a fault on the transmission line close to BKR1.

Permissive Overreach Transfer Trip Scheme operating with a weak infeed

If this was a normal system, RLY-2 would:

  • see a Zone 2 (Z2) fault,
  • send a Z2 Permissive Trip,
  • receive the Permissive Trip from the other side, and
  • open BKR2 after a short time delay.

RLY-1 would:

  • see a Zone 1 (Z1) and Zone 2 fault,
  • send a Z2 Permissive Trip, and
  • open BKR1 instantaneously.

We are simulating a weak infeed from the generator by opening BKR3 which means that RLY-1 does not see the fault and does nothing while RLY-2 opens BKR2 after the normal time delay for Zone 2. (around 20 cycles)

This is not an ideal situation.  There is a fault on the line and BKR1 is still closed.  If we bring the generator up to speed and close the generator breaker, we will close the generator onto a fault.  The fault was also connected to the system for 20 cycles, which was plenty of time to cause an unnecessary disruption to the system. We have the technology to do better, and a Weak Infeed protection scheme can open BKR1 when a fault occurs on the line, and speed up the trip time of BKR2.

We need RLY-1 to recognize that there is a fault on the line, even if the generator breaker is open.  Fortunately, there are numerous ways for relays to communicate information about a fault. The first method uses the generator breaker’s (BKR3) 52a contact.  We know that the left side of the circuit in our example is guaranteed to be weak if BKR3 is open, so we can create an “Echo” permissive signal that sends a permissive trip to RLY-2 when RLY-1 receives a permissive AND the generator breaker is open as shown in the drawing below.

Permissive Over Reach Transfer Trip Scheme with Weak Infeed Protection using 52a contacts

The fault hasn’t changed since the first example, but both breakers opened after a very short time delay with the new Weak Infeed logic. In this scenario, RLY-2 would:

  • see a Zone 2 fault, and
  • send a Z2 Permissive Trip.

RLY-1 would:

  • receive the Permissive Trip from the other side,
  • see that BRK3 is open,
  • send a permissive Echo to RLY-2, and
  • open BKR1 based on the Weak Infeed logic.

RLY-2 would:

  • receive a permissive echo from RLY-1,
  • still see Zone-2, and
  • then open BRK2 based on a permissive trip instead of waiting for the Zone-2 time delay.

This system works great when BKR-3 is open, but generators typically do not have a lot of fault capacity compared to the system. The generator side could still be considered weak when the breaker is closed because of its relatively weak ability to contribute to a fault. We would be right back at square one if this was the only aspect of Weak Infeed protection schemes, and the generator breaker was closed.

Generator protection schemes can’t use traditional inverse time overcurrent (51) protection because generators can’t produce enough fault current to make them effective. Some generator relays do use modified 51-elements for system backup protection by applying what we’ve learned about faults over the last 100 years or so. Everyone knows that a fault will cause the measured current to increase in proportion to the severity of the fault, but that doesn’t happen in isolation. The faulted voltage is also affected.

What happens to the faulted voltage during a fault?

The faulted voltage will decrease.  The actual fault voltage will be proportional to the distance from the potential transformers (PTs) the relay uses to measure the system voltage. The fault voltage will be zero if the fault is on top of the PTs, and will grow as the fault gets farther away. Therefore, we can set an undervoltage element to operate if the voltage drops enough to signal a fault that is closer to the weak side.  That undervoltage can be added to the Weak Infeed logic as shown below.

Permissive Over Reach Transfer Trip Scheme with Weak Infeed Protection using 52a contacts and 27 undervoltage

The fault hasn’t changed since the first example, but both breakers opened after a very short time delay with the new Weak Infeed logic even though the generator breaker is now closed. In this scenario, RLY-2 would:

  • see a Zone 2 fault, and
  • send a Z2 Permissive Trip,

RLY-1 would:

  • receive the Permissive Trip from the other side,
  • recognize that a fault is on the line because the Undervoltage (27) element operated,
  • send a permissive Echo to RLY-2, and
  • open BKR1 based on the Weak Infeed logic.

RLY-2 would:

  • receive a permissive echo from RLY-1,
  • still see Zone-2, and
  • then open BRK2 based on a Permissive trip.

We now have a flexible solution, but have introduced a new problem.  What will happen if the fault is on the generator side?

Permissive Over Reach Transfer Trip Scheme with Weak Infeed Protection using 52a contacts and 27 undervoltage misoperation

BKR1 and BKR2 would both operate because none of the conditions in our Weak Infeed logic have changed. We need a way to block the infeed logic when the fault is not on the protected transmission line. Can RLY-1 tell whether the fault is on the line, or behind RLY-1 near the generator?

The Zone-3 protection element is set to detect faults behind the relay, and we need to add it to the Weak Infeed protection scheme to prevent the Echo signal from being sent if the fault is behind the relay as shown in the following diagram.

Permissive Over Reach Transfer Trip Scheme with Weak Infeed Protection using 52a contacts and 27 undervoltage with Z3 impedance blocking

In this scenario, all of the previously described components are in place, which means the Weak Infeed logic would normally send an Echo signal, but the relay’s Zone-3 element has picked up and prevented the Echo permissive from being sent. This new piece to our logic scheme now prevents the Weak Infeed scheme from operating for faults that are not on the transmission line. Some engineers try to make it even more secure by adding a Zone-2 block for Weak Infeed logic as well.

If we move the fault back onto the line, Zone-3 does not pickup and the Weak Infeed Echo protection scheme works normally.

Permissive Over Reach Transfer Trip Scheme with Weak Infeed Protection using 52a contacts and 27 undervoltage with Z3 impedance blocking

These are the basic essentials of Weak Infeed Echo Permissive Protection schemes.  I hope I was able to answer Rahim’s question adequately. Please feel free to comment below if you have more questions about this scheme, or have something new to add.

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About the Author Chris Werstiuk

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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