PUTT Scheme: Permissive Underreaching Transfer Trip

What a PUTT scheme is

The PUTT scheme (Permissive Underreaching Transfer Trip) improves on DUTT with a local condition: the terminal that receives the signal trips only if it also sees the fault.

• Zone 1 at each terminal (underreaching, about 80% of the line) trips its breaker and keys the permissive signal.
• The remote terminal trips without delay if it receives the permission and its overreaching element (usually zone 2) has picked up.

It is a scheme of device function 85 in ANSI/IEEE C37.2, modelled in IEC 61850 with the PSCH logical node, and it complements distance protection 21. Its place among the other schemes is summarised in the guide on teleprotection schemes.

How it works: logic and signals

The trip logic at the receiving end is an AND gate: permission received AND zone 2 picked up → trip. Zone 2 keeps its timer as backup; the scheme only bypasses the delay when the permission arrives.

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

1. Zone 1 at B and zone 2 at A pick up.
2. B trips its breaker and sends permission to A.
3. A receives the permission; since its zone 2 already sees the fault, it trips without waiting for the zone 2 delay.

For an external fault beyond bus B, zone 2 at A picks up, but no zone 1 sees the fault and no one sends permission: A waits for its zone 2 delay and the protection of the next line clears first.

As with DUTT, 100% coverage requires the zone 1 reaches of both terminals to overlap. The send signal must also be held for a short time after zone 1 resets, so that it is not cut off if the local breaker opens before the remote end confirms.

Channel and timing

A false command from the channel cannot trip on its own, because the receiver requires seeing the fault. For that reason a permissive channel calls for good dependability and short transmission time rather than the extreme security of a direct trip: a permission that arrives late turns an accelerated trip into a zone 2 trip. IEC 60834-1 treats permissive commands as intermediate in security between blocking and direct tripping.

If the channel is lost, PUTT does not overtrip: it simply stops accelerating and the line is left with instantaneous zone 1 and time-delayed zone 2.

Advantages and limitations

Advantages:

• More secure than DUTT against spurious commands.
• Simple logic, with no need for echo or current reversal guard.
• Good performance on long lines, where the zone 1 reaches overlap with margin.

Limitations:

• On short lines zone 1 becomes very small and may not overlap: gaps appear with no acceleration.
• It does not suit weak-infeed terminals: the weak end picks up neither its zone 1 (it does not send) nor its zone 2 (it does not accept the permission). Without additional logic, faults are cleared in zone 2.
• With high fault resistance zone 1 underreaches further and may not send.

Where it is used

PUTT is common on HV and EHV lines of medium to long length, with channels independent of the line (optical fibre, multiplexer, radio). On short lines, or with weak terminals such as solar plants connected to the transmission or subtransmission system, the POTT scheme with echo, directional comparison or line differential are preferred. A historical variant is zone acceleration, where the permission extends the zone 1 reach instead of being supervised by an element that has already picked up.

How each manufacturer implements it

GE Vernova (Multilin, EnerVista) offers PUTT as a dedicated pilot scheme in relays such as the D60. In the SEL-421 (AcSELerator QuickSet) the selectable schemes are POTT and its variants, DCUB and DCB, so a PUTT is built with programmable logic equations. Siemens SIPROTEC 4 7SA (DIGSI) includes it among the modes of its distance teleprotection in two variants: permission supervised by protection pickup or by the overreaching zone Z1B. Hitachi Energy Relion 670 (PCM600) selects it as the permissive underreach mode of its scheme communication logic. The MiCOM P44x (GE Vernova MiCOM Agile and Schneider Electric Easergy MiCOM) call it PUR (permissive underreach) within their aided schemes. What changes is the element that supervises the permission, the send extension and the channel supervision.

Why it fails or misoperates

• End-zone fault cleared in zone 2: zone 1 reaches that do not overlap, a weak terminal, high fault resistance or a permission that arrived after the delay.
• Accelerated false trip: zone 1 overreaching at the sending end (capacitive voltage transformers, wrong k0, mutual coupling) combined with zone 2 at the receiving end, which does see the external fault.
• Send not issued or not received: send and receive contacts crossed between relay and teleprotection equipment, input polarity, or a channel in alarm.

Check the event records of both terminals first, with synchronised time stamps: zone 1 at the sender, send time, receive time and zone 2 status at the receiver.

How it is tested

Secondary injection verifies the zone 1 and zone 2 reaches, send keying by zone 1 and the accelerated trip on receive plus zone 2. The complete scheme is validated with an end-to-end test: the same fault, GPS-synchronised at both terminals, at several points along the line, including the end zone of each terminal.

Did your line clear in zone 2 a fault the PUTT scheme should have accelerated? At Tiza Group we review the events and settings of both terminals remotely. See our remote relay configuration service or contact us.

References

• 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.
• IEEE C37.2, Standard for Electrical Power System Device Function Numbers.
• IEC 61850-7-4, logical node models (PSCH).
• CIGRE, Technical Brochure 192, Protection using telecommunications (2001).
• Blackburn and Domin, Protective Relaying: Principles and Applications.

Frequently asked questions