In the DUTT (direct underreaching transfer trip) scheme, zone 1 at each end trips its breaker and, at the same time, sends a trip command to the remote end, which opens without checking anything on its own. It is the simplest scheme, but a false command on the channel opens the line, so it needs a channel with the highest security.
What the DUTT scheme is
The DUTT (direct underreaching transfer trip) scheme is the simplest teleprotection scheme for lines protected by distance protection 21. It uses zone 1 at each end, which is set short of the remote bus (around 80 % of the line), and adds a communication channel:
- When zone 1 at end A sees a fault, it trips its breaker and sends a command to end B.
- End B trips its breaker just by receiving the command, without checking with its own measurements.
In ANSI/IEEE C37.2 terms, the pilot communication logic is device 85; in IEC 61850 the scheme is modeled with logical node PSCH. It belongs to the family of teleprotection schemes.
How it works: logic and signals
For the scheme to cover 100 % of the line, the zone 1 reaches of both ends must overlap. If a fault is close to B, outside zone 1 of A, zone 1 of B sees it, B trips and sends the command to A, which trips at once.
Sequence for an internal fault at 85 % of the line from A:
- Zone 1 at B picks up (the fault is 15 % from B).
- B trips its breaker and keys the send.
- The command travels over the channel.
- A receives the command and trips its breaker without waiting for zone 2.
Zone 1 at A does not see the fault. Without the scheme, A would trip in zone 2, with its delay. For an external fault, beyond bus B, no zone 1 picks up and nothing is sent: selectivity depends entirely on zone 1 underreaching.
An important detail: if the local breaker opens very fast and zone 1 resets, the command is cut short. That is why the send signal is usually held for a short time after zone 1 resets, to make sure it arrives complete.
Channel and timing
Since the received command trips without supervision, DUTT treats every reception as a direct trip. IEC 60834-1 classifies direct trip commands as the most demanding in security: a false command caused by noise, interference or human error opens the remote breaker with no fault present. The channel must also have good dependability, although transmission time is less critical than in a blocking scheme.
In practice, dedicated digital channels are used (direct optical fiber, a multiplexer with an IEEE C37.94 interface) or teleprotection equipment with high-security command encoding.
Advantages and limitations
Advantages:
- Simple logic, easy to understand and to test.
- Works with a weak-infeed end: the end that does not see the fault trips anyway on the command.
- Not affected by current reversal on parallel lines, because zone 1 underreaches.
Limitations:
- Security depends entirely on the channel: any false command trips.
- The zone 1 reaches must overlap: on short lines, where zone 1 is small or unreliable, the scheme leaves gaps.
- With high fault resistance, zone 1 can underreach even more and leave faults without acceleration.
For these reasons many utilities replaced DUTT with the PUTT scheme, which uses the same signal but requires the receiving end to confirm the fault.
Where it is used
DUTT appears on long high-voltage lines, where the zone 1 reaches overlap with ample margin, with very secure digital channels. It is also found in older installations or as part of a combined scheme, for example when the direct command is also used to transfer trips from other protections, as in direct transfer trip (DTT). At extra-high voltage it is less common as the main scheme: there POTT, DCB and line differential dominate.
How each manufacturer implements it
GE Vernova (Multilin, EnerVista) offers DUTT as a pilot scheme of its own in relays such as the D60. In SEL relays (AcSELerator QuickSet) the selectable schemes are POTT, DCUB and DCB, so DUTT is built with programmable logic: zone 1 to the channel and the received input assigned to the trip equation. Siemens SIPROTEC (DIGSI) and Hitachi Energy Relion 670 (PCM600) handle it within their teleprotection logic or as remote direct tripping, depending on the version. MiCOM P44x relays (GE Vernova MiCOM Agile and Schneider Electric Easergy MiCOM) include direct transfer trip among their aided schemes. In every case the scheme name, the send extension time and the way the channel is supervised differ: there is no universal setting.
Why it fails or misoperates
- Remote trip with no fault: spurious command from channel noise, a wiring error on the receive input, or a test done without isolating the send.
- Trip for an external fault: zone 1 overreaching (capacitive voltage transformer transients, a wrong residual compensation k0, mutual coupling on parallel lines). Since the command trips without supervision, an error at one end opens both.
- End-zone fault not accelerated: zone 1 reaches that do not overlap, or high fault resistance.
- Command that does not arrive: send signal too short compared with the opening time of the local breaker.
The first thing to check is the event log from both ends, with synchronized time stamps: who sent, who received and in what order.
How it is tested
At each end, secondary injection verifies the zone 1 reach, the send keying and its extension, and the trip on receipt. The complete scheme is validated with an end-to-end test: two GPS-synchronized test sets simulate the same fault and the time is measured from zone 1 pickup at one end to the trip at the other, channel included.
Did your line open on a transfer trip command and it is not clear where it came from? Our team reviews the events and settings at both ends remotely. See remote relay configuration 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 C37.236-2013, Guide for Power System Protective Relay Applications over Digital Communication Channels (listed by IEEE as inactive-reserved since 2024; its revision is in progress).
- 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
What is the difference between DUTT and PUTT?
In both, zone 1 sends the signal. In DUTT the remote end trips just by receiving it; in PUTT it also needs its own overreaching element to see the fault. PUTT is more secure against false commands on the channel.
Does DUTT work with a weak-infeed end?
Yes, for faults within reach of zone 1 at the strong end: the weak end trips on the received command even if its relay does not detect the fault. That is one of the few advantages of DUTT over PUTT.
What happens in a DUTT scheme if the channel is lost?
The scheme stops accelerating trips and the line is left with instantaneous zone 1 and time-delayed zone 2 at each end. The larger risk is not losing the channel but a false command caused by noise, which opens the remote breaker.