Direct transfer trip (DTT, or intertrip) is a command that a protection sends over a channel to open a breaker in another substation, without the receiving relay having to see the fault. It is used when the remote protection cannot detect the condition: breaker failure, a fault in a reactor or transformer with no breaker of its own, or tripping of generation to prevent an island.
What direct transfer trip is
Direct transfer trip (DTT, also called intertrip) is the simplest teleprotection command: the received signal trips. It compares neither directions nor zones. A local protection detects a condition that the remote end cannot see and orders it to open its breaker.
It differs from the DUTT scheme in where the command comes from. In DUTT the command comes from zone 1 of the line protection itself; in DTT it comes from another protection: breaker failure, reactor or transformer protection, anti-islanding or overvoltage protection. And it differs from permissive schemes, such as the PUTT scheme, in that the receiving end checks nothing on its own. Its place among the other schemes is covered in the guide on teleprotection schemes.
When it is used
Breaker failure (50BF). If the line breaker at end A does not open, breaker failure protection 50BF trips the adjacent breakers in substation A. If the fault is on bus A, end B keeps feeding it through the line and its distance protection only sees it in zone 2, with a delay. If the fault is on the line, B has already tripped on its own protection, but its recloser would re-energize the fault through the stuck breaker. In both cases DTT opens B at once and blocks its reclosing.
Reactors and transformers with no breaker of their own. A shunt power reactor connected directly to the line, or a transformer connected to the line with no high-voltage breaker, is isolated by opening the line breakers. Its differential or mechanical protection trips locally and sends DTT to the remote end, because an internal fault in that equipment may be invisible to the remote line protection.
Distributed generation and anti-islanding. When the utility opens the feeder or line that connects a plant, the plant may be left feeding an island. DTT from the utility substation orders the plant breaker to open. It is common in generation interconnections to distribution and transmission systems.
Overvoltage and equipment protection. For example, an overvoltage at the open end of a long line may require opening the end that energizes it.
How it works: logic and signals
- The local protection (50BF, 87R, 87T, 63, anti-islanding) detects the condition.
- It activates the command input of the teleprotection equipment, usually a dedicated command separate from the permissive commands.
- The remote equipment delivers the command at its output.
- The received command goes straight to the trip circuit, often through a lockout relay 86 that also prevents reclosing.
- 01Local protection50BF, 87R, 87T, 63 or anti-islanding detects the condition
- 02SendDedicated command of the teleprotection equipment
- 03Remote receiveThe remote equipment delivers the command at its output
- 04TripStraight to the trip circuit, often through an 86 that prevents reclosing
Some utilities add a minimal local supervision (for example, a current or voltage detector) for extra security when the channel is not fully reliable. That changes the nature of the scheme and must be justified in the study.
Channel and timing
IEC 60834-1 places direct tripping at the most demanding end of security: a false command opens a breaker without any check. It also requires high dependability, because a lost command leaves a fault uncleared from that end. Transmission time, on the other hand, is less critical than in blocking or permissive schemes, because DTT usually follows a decision that already has its own delay, such as the 50BF timer.
That is why teleprotection equipment encodes direct trip commands with more redundancy, validates them for longer and logs them with a time stamp. Digital channels use dedicated commands over IEEE C37.94 or equivalent interfaces; critical applications use two channels over separate routes.
Advantages and limitations
Advantages:
- Clears conditions that no remote protection can see.
- Simple logic that is easy to audit.
- Independent of reaches and directions.
Limitations:
- Security rests entirely on the channel and on test discipline.
- A poorly isolated test sends real trips to another substation.
- If the command does not arrive, the remote backup clears with a delay.
How each manufacturer implements it
Line relays from SEL, Siemens SIPROTEC, Hitachi Energy Relion 670, GE Vernova (Multilin) and MiCOM P44x (GE Vernova and Schneider Electric) have direct trip receive inputs, with their own validation timer and reclose-blocking logic; the names and the way they are assigned differ. In many substations DTT does not go through the line relay at all: the output of the teleprotection equipment acts directly on the lockout relay. With IEC 61850 between substations, the command can travel as a GOOSE message if the channel and the architecture allow it.
Why it fails or misoperates
- Remote opening with no fault: 50BF or transformer test without isolating the DTT send, noise on an analog channel, inputs wired to the wrong command.
- Command that does not arrive: wrong command assignment in the multiplexer, channel in alarm with no warning, validation time longer than the command duration.
- Reclosing onto the fault: DTT that does not block reclosing at the remote end.
Check the event log of the teleprotection equipment at both ends: it is the record that shows whether the command was sent and whether it arrived.
How it is tested
Each source of the command is verified (50BF, reactor protection, transformer, anti-islanding), along with the command assignment on the channel, the transmission time, and the trip and reclose blocking at the remote end. The coordinated test between substations is done as an end-to-end test, with both ends isolated from the real breakers until the logic is confirmed.
Did a remote substation open on a transfer trip that nobody can explain? Our team reviews the events and the logic remotely. See remote relay configuration or contact us.
References
- IEC 60834-1, Teleprotection equipment of power systems – Performance and testing – Part 1: Command systems.
- IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines.
- IEEE C37.119-2025, Guide for Power System Circuit Breaker Failure Protection (supersedes the 2016 edition).
- 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).
- CIGRE, Technical Brochure 192, Protection using telecommunications (2001).
- Blackburn and Domin, Protective Relaying: Principles and Applications.
Frequently asked questions
Why does DTT need more security than a permissive scheme?
Because the received command opens the breaker with no local check at all. A false command caused by noise or a test error opens the line, usually with a three-pole trip and reclosing blocked. In a permissive scheme, by contrast, the receiving end must also see the fault.
Does DTT block reclosing at the remote end?
Usually, yes. If the command comes from a breaker failure or from a fault in a transformer or reactor, reclosing would re-energize the fault. How it is blocked depends on the relay logic and on the utility's practice.
What happens if the DTT command does not arrive?
The fault is not cleared from the remote end by that path and the backup has to act, for example remote distance zone 2 or 3, with its delay. That is why DTT also needs high dependability and, in critical applications, redundant channels.