A teleprotection scheme (ANSI 85) combines the protection at both terminals of a line with a communication channel to clear faults on 100% of the line without delay. In permissive schemes the remote terminal trips only if it receives the signal and also sees the fault; in blocking schemes it trips if it sees the fault and receives no block; in direct transfer trip it trips on the signal alone.
What a teleprotection scheme is
A distance protection 21 without communication can only trip without delay for faults inside its zone 1, which is set short of the line length (around 80%) so as not to trip for faults on the remote bus. Faults in the end section are cleared in zone 2, with a deliberate delay. At high and extra-high voltage that delay can compromise system stability and single-pole reclosing.
A teleprotection scheme solves the problem with a channel between the two terminals. Each relay tells the other what it sees, and with that information both trip without delay for any internal fault. In the ANSI/IEEE C37.2 table the pilot communication function is 85; in IEC 61850 the scheme logic is modelled with the PSCH logical node. The equipment that carries the commands is explained in the guide on teleprotection equipment.
The schemes, one by one
| Scheme | What keys the send | What the receiving end needs to trip | If the channel is lost |
|---|---|---|---|
| DUTT | Zone 1 (underreach) | The received signal alone | Zone 1 + time-delayed zone 2 remain |
| PUTT | Zone 1 (underreach) | Received signal and its overreaching zone picked up | Zone 1 + time-delayed zone 2 remain |
| POTT | Forward overreaching zone | Received signal and its overreach picked up | Zone 1 + time-delayed zone 2 remain |
| DCB | Reverse element (sends block) | Overreach picked up, no block received, after the coordination timer | Keeps tripping for internal faults; may overtrip on external ones |
| DCUB | Overreach (as in POTT) | Overreach picked up and signal received, or loss of guard within a short window | Short trip window, then the scheme is blocked |
| Zone acceleration | Zone 1 | The signal extends the reach or bypasses the zone 2 delay | Zone 1 + time-delayed zone 2 remain |
| Phase comparison | Polarity of the local current | Currents in phase at both terminals | Depends on whether it works as blocking or permissive |
| DTT | Another protection (50BF, reactor, transformer) | The received signal alone | The command does not arrive: backup acts |
Two logics almost always complement the overreaching schemes: echo and weak-infeed logic, for terminals with an open breaker or little fault current, and current reversal guard, for parallel lines.
Permissive versus blocking
In a permissive scheme the signal means “I also see the fault in front of me”. The receiver needs the signal to accelerate the trip. If the channel fails, the scheme stops accelerating but does not overtrip: it loses dependability, not security.
In a blocking scheme the signal means “the fault is behind me, do not trip”. The receiver trips when it sees the fault in the forward direction and no block arrives. If the channel fails, the scheme keeps tripping for internal faults, but an external fault within the overreach can cause a false trip: it loses security, not dependability.
This difference explains the historical choice. With power line carrier (see line trap and power line carrier) the channel travels over the same line that faults, and an internal fault can attenuate the signal just when it is needed. That is why many utilities used DCB over power line carrier and permissive schemes over channels independent of the line. Today, continuously supervised digital channels and numerical relays with self-diagnostics have reduced that dependence, and the choice also weighs each utility’s experience and practice.
Security, dependability and transmission time
IEC 60834-1 defines the performance of teleprotection command equipment with three competing criteria:
- Security: the channel must not deliver false commands due to noise or interference.
- Dependability: a real command must arrive.
- Transmission time: how long the command takes to arrive.
Each scheme calls for a different balance. Blocking needs the shortest transmission time and high dependability, because a late or lost block causes a false trip; channel security matters less, since a false command only blocks. Permissive calls for short time and good dependability, and a false command cannot trip on its own because the receiver must also see the fault. Direct transfer trip demands maximum security, because a false command opens a breaker with no local supervision; time is less critical. Reference values depend on the channel type and the edition of the standard.
Where it is used and how to choose
Teleprotection is common on high- and extra-high-voltage lines, where fast clearing over the whole line is a requirement of the system operator. Selection criteria:
- Short lines: zone 1 becomes very short or unreliable; POTT, DCB or line differential are preferred.
- Long lines: PUTT is a natural option because the zone 1 reaches of both terminals overlap with margin.
- Weak-infeed terminals or terminals with an open breaker: POTT with echo and weak infeed.
- Parallel lines: POTT or DCB with current reversal guard.
- Channel over the line itself (power line carrier): DCB or DCUB.
- Dedicated digital channel (fibre, IEEE C37.94, multiplexer): POTT, PUTT or line differential.
How each manufacturer implements it
All the distance relays of the main manufacturers include the classic schemes, but they group and name them differently. GE Vernova (Multilin, EnerVista) offers them as separate pilot schemes in relays such as the D60 (DUTT, PUTT, POTT, hybrid POTT, blocking and unblocking). SEL (AcSELerator QuickSet) has settings for POTT, DCUB and DCB, and in the SEL-421 also two-channel and per-phase POTT variants; other schemes are built with programmable logic. Siemens SIPROTEC 4 7SA (DIGSI) selects a teleprotection mode (PUTT, POTT, unblocking or blocking, among others) and handles echo, weak-infeed tripping and transient blocking against current reversal separately. Hitachi Energy Relion 670 (PCM600) separates the scheme communication logic (ZCPSCH) from the current reversal and weak-end infeed logic (ZCRWPSCH). The MiCOM P44x, sold today by GE Vernova (MiCOM Agile) and Schneider Electric (Easergy MiCOM), group them as aided schemes: PUR, POR and blocking, with echo and weak-infeed tripping. Each one uses different timers and inputs, so two relays of different brands at the ends of the same line need a careful review of the logic.
Why a teleprotection scheme fails
The most frequent causes in the field are: reach settings that do not meet the scheme rules (insufficient overreach, or a reverse element shorter than the remote overreach), inputs and outputs crossed between relay and teleprotection equipment, timers poorly coordinated with the actual channel time, echo or current reversal logic disabled where it was needed, and channels with noise or variable delay.
How it is tested
Testing each relay on its own does not prove that the scheme works. The definitive test is the end-to-end test: two GPS-synchronised injection sets simulate the same fault at both terminals, and the send, receive, trips and timing of the complete scheme are verified, including the channel.
Did your teleprotection scheme overtrip or fail to accelerate, and the reason is not clear? 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 C37.236-2013, Guide for Power System Protective Relay Applications over Digital Communication Channels (listed by IEEE as inactive-reserved since 2024; a revision is under way).
- 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
Which teleprotection scheme is better, POTT or DCB?
It depends on the channel. POTT is more secure against external faults and is preferred with dedicated digital channels. DCB keeps tripping for internal faults even if the channel is lost, which is why it was historically chosen with power line carrier, whose channel travels over the same line that faults.
What happens to line protection if the teleprotection channel is lost?
In permissive schemes the line is left with instantaneous zone 1 and time-delayed zone 2: faults near the remote terminal are cleared with delay. In a blocking scheme the protection keeps tripping for internal faults, but it can overtrip for external faults.
Is teleprotection the same as line differential 87L?
No. Teleprotection exchanges simple commands (permission, block or trip) between relays that decide with their own measurements. 87L exchanges the measured currents and compares magnitude and phase at both terminals; it needs a digital channel of higher capacity.