Zone acceleration in distance protection: how it works

What zone acceleration is

Zone acceleration is an underreaching teleprotection scheme originally designed for switched distance relays, where a single measuring unit changed its reach in steps. The idea:

• Zone 1 at one end (underreaching, around 80 % of the line) trips its breaker and sends a signal to the remote end.
• The remote end, on receiving it, accelerates its protection: it extends the reach of its measuring zone to an overreach, or bypasses its zone 2 delay, and trips without waiting if the fault is inside.

It is one of the device 85 logics of ANSI/IEEE C37.2 that complement distance protection 21. The comparison with the other schemes is in the guide on teleprotection schemes.

How it works: logic and signals

Sequence for a fault at 85 % of the line from A:

1. Zone 1 at B picks up, trips its breaker and sends the signal.
2. A receives the signal and extends its measuring zone (or enables its zone 2 without delay).
3. The extended measurement sees the fault and A trips.

The difference from the PUTT scheme is subtle but practical. In PUTT, the receiving end already has an overreaching element picked up and trips as soon as the permissive signal arrives. In classic acceleration, the signal changes the reach of the measurement, which must evaluate the fault again: that time adds to the channel time. In modern numerical relays, with all zones measuring in parallel, the practical difference shrinks, and acceleration is often implemented as “zone 2 without delay on received signal”, which in practice looks very much like PUTT.

As in every underreaching scheme, 100 % coverage requires the zone 1 reaches of both ends to overlap, and the send signal must be extended for a short time so it is not cut off if the local breaker opens first.

The variant with no channel: zone 1 extension

One variant uses no communication and is usually called zone 1 extension. It works in coordination with automatic reclosing 79:

1. With the recloser ready, zone 1 runs with an extended reach, beyond the remote end.
2. A fault anywhere on the line (and somewhat beyond) trips without delay.
3. The recloser closes the breaker and zone 1 returns to its normal reach.
4. If the fault persists, the protection, now at normal reach, clears it selectively.

It gives up selectivity on the first trip: a fault on the next line can open this line for a moment, but the reclose restores it. It makes sense in networks where most faults are transient and no channel is available. Some manufacturers also offer other local acceleration logics that use no channel.

Channel and timing

With a channel, acceleration asks the same of it as a permissive scheme: short transmission time and good dependability. A false signal alone does not trip, because the extended measurement must also see the fault. If the channel is lost, the scheme stops accelerating and zone 1 plus time-delayed zone 2 remain.

Advantages and limitations

Advantages:

• Simple logic, with no echo and no current reversal guard.
• The zone 1 extension variant gives speed without a channel.

Limitations:

• Slower than PUTT or POTT in its classic form, because of the new measurement after the signal.
• The zone 1 reaches must overlap: it does not suit short lines or weak-infeed ends.
• Zone 1 extension without a channel loses selectivity on the first trip and depends on the recloser.

Where it is used

Classic acceleration shows up mainly in older high-voltage installations with electromechanical or static switched distance relays, and in some utility practices kept by tradition. Zone 1 extension is found on subtransmission and high-voltage lines without a channel, where the utility accepts a non-selective first trip in exchange for speed.

How each manufacturer implements it

In modern numerical relays, acceleration rarely appears as a scheme with that name. Siemens SIPROTEC 4 7SA (DIGSI) has a controlled overreach zone, Z1B, which can be enabled by teleprotection (PUTT mode with Z1B) or by the recloser before the first cycle. Hitachi Energy Relion 670 (PCM600) includes a local acceleration logic (ZCLCPLAL) separate from the scheme communication logic. SEL, GE Vernova (Multilin) and MiCOM P44x relays (GE Vernova MiCOM Agile and Schneider Electric Easergy MiCOM) do it with their scheme logic or with programmable logic that bypasses the zone 2 delay on the received signal. That is why, when an old scheme is migrated to a new relay, the original intent has to be rebuilt, not just the reaches copied.

Why it fails or misoperates

• Non-selective trip: zone 1 extension active while the recloser was blocked or out of service.
• End-zone fault cleared in zone 2: zone 1 reaches that do not overlap, signal received late or send pulse too short.
• Accelerated false trip: zone 1 at the sending end overreaching because of capacitive voltage transformer transients or a wrong k0.

How it is tested

Secondary injection verifies the zone 1 send, the extended reach or the accelerated zone 2 on received signal, and the interaction with the recloser in the variant without a channel. The complete scheme is validated with an end-to-end test synchronized by GPS at both ends.

Inherited an acceleration scheme and not sure how to carry it over to a new relay? Our team reviews the logic and settings remotely. See remote relay configuration or contact us.

References

• IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines.
• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
• CIGRE, Technical Brochure 192, Protection using telecommunications (2001).
• Blackburn and Domin, Protective Relaying: Principles and Applications.

Frequently asked questions