Echo logic returns the received permission to the other terminal when the local relay cannot see the fault, because its breaker is open or its fault current contribution is weak. The strong terminal can then trip without delay in a POTT scheme. Weak-infeed logic adds, under voltage supervision, the trip of the weak terminal itself.
What echo and weak infeed solve
A POTT scheme, or its DCUB unblocking variant, only accelerates the trip if both terminals see the fault forward and send each other permission. There are two situations in which a terminal cannot do so:
- Open breaker: the line is energised from one end only. The open terminal has no current, its overreach does not pick up and it sends no permission.
- Weak infeed: the terminal has little source behind it (a small transformer, a solar or wind plant, a weak network). For an internal fault it contributes so little current that its overreaching elements do not detect it reliably.
In both cases the strong terminal sees the fault and sends permission, but receives nothing and clears in zone 2. Echo logic solves the problem by returning the received permission. Weak-infeed logic adds the trip of the weak terminal itself. Both are part of the permissive overreaching teleprotection schemes.
How echo works
The weak or open terminal receives the permission from the strong terminal and returns it as a short pulse if certain conditions are met:
- It has received the permission for a minimum time (to rule out noise).
- Its overreach has not picked up.
- Its reverse elements see no fault: if the fault is behind it, it must return nothing.
- In the open-breaker variant, the breaker is open.
The strong terminal receives the echo, has its overreach picked up and trips. The total time includes two channel transits plus the validation delay.
An application guide by the WECC Relay Work Group (2021) points out that echo widens the exposure of the scheme: if the reverse element fails to detect an external fault, a power swing or a high-current emergency transfer, the echo returns an undue permission. It therefore recommends checking the reverse elements with particular care wherever echo is enabled.
Echo lockup. If two terminals have echo, they could return the permission to each other indefinitely. Implementations limit the pulse duration and block it for a time after sending it; on three-terminal lines, an SEL paper describes cutting the echo once the relay already receives permission from all the other terminals.
How weak infeed works
Echo lets the strong terminal trip, but the weak terminal remains connected and feeding the fault, even if with little current. Weak-infeed logic turns the received permission into a local trip. The WECC guide lists these supervision conditions:
- No reverse element picked up.
- Phase-to-phase undervoltage or residual (zero-sequence) overvoltage present.
- Local breaker closed.
- Remote permission received for a specified time.
Voltage supervision is the key: an internal fault near the weak terminal depresses its voltage even if the current is small. An SEL technical paper notes that this supervision adds security without a speed penalty, and warns that if weak infeed is enabled at every terminal by default, the scheme is no longer purely permissive and resembles a blocking scheme, with the same dependence on the reverse elements.
| Condition | Echo | Weak infeed |
|---|---|---|
| Remote permission | Received for a minimum time | Received for a specified time |
| Reverse elements | Seeing no fault | None picked up |
| Local overreach | Not picked up | Not a listed condition |
| Local breaker | Open, in the open-breaker variant | Closed |
| Voltage | Not a condition | Phase-to-phase undervoltage or residual overvoltage |
| Result | Returns the permission as a short pulse | Local trip |
Channel and timing
Echo doubles the channel time for the strong terminal’s trip. With analogue power line carrier, short echo pulses can be a challenge; digital channels handle them without difficulty. If the channel is lost, there is no echo and the scheme is left with zone 1 and time-delayed zone 2.
Advantages and limitations
Advantages: clearing without delay over 100% of the line with an open or weak terminal, without changing the reaches.
Limitations: echo and weak infeed depend on the reverse elements seeing every external fault and abnormal condition; voltage supervision can get complicated with the loss-of-potential logic at very weak terminals.
Where it is used
It is usually needed on high- and extra-high-voltage lines connecting inverter-based generation plants, whose fault current contribution is limited by the inverter control, and on radial lines or lines with small transformers at one end. Whether it is needed, and with which settings, is decided by the protection study with the actual contribution of the plant. The connection of solar and wind plants to the transmission system is a typical case.
How each manufacturer implements it
SEL (AcSELerator QuickSet) includes in its POTT logic echo, also for an open breaker, and voltage-supervised weak-infeed tripping. Siemens SIPROTEC 4 7SA (DIGSI) has a weak-infeed function that is set to off, echo only, or echo and trip; the trip requires a received permission (POTT or unblocking) and a drop in phase-to-ground voltage. Hitachi Energy Relion 670 (PCM600) groups them with current reversal guard in a weak-end infeed block (ZCRWPSCH). GE Vernova (Multilin, EnerVista) offers permissive echo and weak-infeed provisions in its pilot schemes. The MiCOM P44x (GE Vernova MiCOM Agile and Schneider Electric Easergy MiCOM) include open-breaker echo and weak-infeed tripping in their POR scheme. Timers, voltage supervision and the way the echo is blocked differ.
Why it fails or misoperates
- False trip by echo: a reverse element that did not see an external fault or a swing.
- Weak terminal does not trip: voltage supervision mis-set or blocked by the loss-of-potential logic.
- Echo not sent: breaker reported closed because of a miswired auxiliary contact.
How it is tested
Secondary injection simulates the open terminal and the weak terminal, permission receipt, echo send and its blocking, and weak-infeed tripping with voltage supervision. The complete scheme is validated with an end-to-end test with one terminal simulating a low contribution.
Does your line to a solar plant clear in zone 2 faults that should be instantaneous? At Tiza Group we review the echo logic and the settings remotely. See our remote relay configuration service or contact us.
References
- WECC Relay Work Group, Application Guide for Echo Keying Logic on Permissive Overreaching Transfer Trip Schemes, 2021.
- Kasztenny, Mynam, Fischer and Guzmán, Permissive or Blocking Pilot Protection Schemes? How to Have It Both Ways, SEL, Western Protective Relay Conference, 2020.
- IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines.
- IEC 60834-1, Teleprotection equipment of power systems – Performance and testing – Part 1: Command systems.
Frequently asked questions
When is echo logic needed?
In POTT or DCUB schemes, when a terminal may have its breaker open or not enough fault current for its overreaching element to see internal faults. Without echo, that terminal sends no permission and the strong terminal clears in zone 2.
What is echo lockup?
It is the risk of two relays echoing the signal back and forth indefinitely. Echo logic limits the pulse duration and blocks it for a time after it is sent, or cuts it once permissions from all terminals have been received.
Why is weak-infeed tripping supervised by voltage?
Because an internal fault near the weak terminal depresses its voltage even if the current is small. Requiring undervoltage or zero-sequence overvoltage confirms that there is a fault and avoids tripping on a spurious permission.