Teleprotection is the use of a communication channel between relays at the ends of a transmission line so that each end can decide, within a few cycles, whether a fault is inside the protected line and trip without waiting for time-delayed backup zones. Teleprotection schemes (also called pilot schemes or communication-aided schemes) such as POTT, PUTT, DCB and direct transfer trip combine local distance or directional elements with signals received from the remote end, sent over fiber optics, power line carrier or microwave. The result is high-speed clearing for 100 percent of the line, which improves stability margins, reduces equipment damage and enables high-speed reclosing.
Why teleprotection is needed
A distance relay at one end of a line cannot tell precisely whether a fault near the far end is just inside or just outside the line, because of measurement errors, line impedance uncertainty and infeed effects. That is why Zone 1 is set to underreach (typically a portion of the line) and trips instantly, while Zone 2 overreaches the remote bus and trips with a time delay to coordinate with adjacent protection. Without communication, a fault near one end is cleared instantly there but with Zone 2 delay at the other end. On transmission systems that delay can be unacceptable for stability, for limiting damage, and for single-pole tripping and reclosing schemes.
Teleprotection solves this by exchanging simple information (typically a permissive, block or trip command) so that both ends trip at high speed for any internal fault. It is also used to trip remote breakers directly, for example for breaker failure or for transformer faults where there is no local high-side breaker.
Main teleprotection schemes
POTT: permissive overreaching transfer trip
Each end uses an overreaching element (Zone 2 distance or a forward directional ground element) to send a permissive signal to the remote end. An end trips when its own overreaching element sees a forward fault and it receives the permissive signal from the other end. For an internal fault both ends see forward and both send, so both trip. For an external fault, the end behind the fault sees reverse and does not send, so the other end does not trip.
Practical POTT schemes need additional logic:
- Current reversal guard: on parallel lines, the fault current direction on the healthy line can reverse when one end of the faulted line opens. A transient block prevents a false trip during that reversal.
- Echo and weak infeed logic: if one end has little or no fault current (weak source or open breaker), it cannot detect the fault. Echo logic returns the received permissive so the strong end can trip, and weak infeed logic can also trip the weak end on undervoltage.
POTT is secure because a missing channel prevents tripping by the scheme (backup zones still operate), and it is the most common choice on fiber and digital channels.
PUTT: permissive underreaching transfer trip
The permissive signal is sent by the underreaching Zone 1, which trips locally anyway. The remote end trips if its overreaching element picks up and it receives the permissive. PUTT is simple and secure, but it relies on Zone 1 detecting the fault from at least one end, so it may not cover high-resistance faults or weak-infeed situations as well as POTT, and on short lines Zone 1 may not reach far enough to be useful.
DCB: directional comparison blocking
In a blocking scheme, each end sends a block signal when it sees a reverse fault. An end trips if its overreaching element sees a forward fault and no block signal arrives within a short coordination time. For internal faults no block is sent, so the channel is not needed to trip, which makes DCB dependable when the channel itself is affected by the fault. This is why DCB has traditionally been paired with on-off power line carrier, where the signal must travel over the faulted line. The trade-off is security: if the block signal is lost during an external fault, the scheme can overtrip. The coordination timer also adds a small delay compared with permissive schemes.
DCUB: directional comparison unblocking
Unblocking schemes, typically used with frequency-shift power line carrier, behave like permissive schemes but allow tripping for a short window when the guard signal is lost without a trip signal, on the assumption that the loss was caused by the internal fault attenuating the carrier. They balance the security of permissive logic with the dependability needed on carrier channels.
DTT: direct transfer trip
Direct transfer trip sends a trip command that causes the remote breaker to open without local supervision by a fault detector. Typical applications include breaker failure protection, transformer or shunt reactor faults on lines without a local high-side breaker, and anti-islanding of generation. Because a spurious DTT command trips a line unconditionally, it needs the highest channel security, often with dual channels, coded commands or supervision where the application allows.
Line current differential (87L)
Strictly speaking, current differential is a different protection principle, but it depends on the communication channel just as much. Each end sends current samples or phasors to the other, and the relays compare them directly. It is simple to set and handles complex lines well, but it needs a digital channel with low and predictable delay, and channel asymmetry (different delay in each direction) must be handled, typically with time synchronization or channel-based compensation.
| Scheme | Signal sent | Trips when | Channel loss effect | Typical channel |
|---|---|---|---|---|
| POTT | Permissive from overreaching element | Local forward overreach plus permissive received | Scheme does not trip (secure) | Fiber, digital multiplexed, microwave |
| PUTT | Permissive from Zone 1 | Local overreach plus permissive received | Scheme does not trip (secure) | Fiber, microwave, carrier |
| DCB | Block from reverse element | Local forward overreach and no block after coordination time | May overtrip on external faults (dependable) | On-off power line carrier |
| DCUB | Trip or guard, with loss-of-guard logic | Permissive received, or short window after guard loss | Brief trip window, then blocked | Frequency-shift power line carrier |
| DTT | Direct trip command | Command received | No trip; spurious command would trip | Fiber, microwave, dual channels |
| 87L | Current data | Differential criteria met | Usually falls back to backup elements | Fiber, direct or multiplexed |
Communication channels
Fiber optics
Fiber is now the preferred medium. Relays can use dedicated fibers connected directly between relays, or a multiplexed connection through the utility’s SDH, SONET or packet network. IEEE C37.94 defines a standard optical interface between relays and multiplexers. Fiber is immune to electromagnetic interference and to the fault itself, but the path matters: OPGW on the protected line shares the right-of-way with the line, and redundant routes avoid losing both protection channels in one event. Packet-switched networks need careful engineering of latency, jitter and path symmetry before they carry differential protection.
Power line carrier
Power line carrier (PLC) couples high-frequency signals onto the line conductors through coupling capacitors and line tuners, with line traps blocking the signal from entering the substation bus. It uses the protected line itself, which makes it independent of third-party infrastructure but means an internal fault can attenuate the signal. That is why carrier is associated with blocking and unblocking logic. PLC remains common on existing transmission systems and as a second, diverse channel.
Microwave and radio
Licensed microwave links carry teleprotection for many utilities, especially where fiber is not available. Path availability depends on fading, rain and line of sight, so system design must address dependability, and the radio equipment must be powered from substation DC with appropriate redundancy.
Other channels
Copper pilot wires and leased telephone circuits were common in the past and still exist in places, but copper is exposed to ground potential rise and induced voltages, and leased circuits are being withdrawn by carriers. Migration of these schemes to fiber or radio is a frequent project. These choices are part of substation telecommunications engineering.
Performance requirements
IEC 60834-1 describes teleprotection equipment performance in three related terms:
- Security: the ability to avoid an unwanted command in the presence of noise or interference.
- Dependability: the ability to deliver a command when one is sent.
- Transmission time: the delay from command input at one end to output at the other.
These parameters trade off against each other. The application determines the priority: DTT emphasizes security, blocking schemes emphasize speed and dependability, and permissive schemes sit in between. Utilities and transmission owners usually set required channel times and redundancy in their protection standards, and NERC reliability standards may apply to protection system maintenance for bulk electric system elements.
Designing a teleprotection scheme
- Define the line characteristics: length, source strength at each end, parallel lines, series compensation, tapped loads or generation.
- Select the protection principle and pilot scheme (POTT, DCB, 87L, or a combination with redundant systems).
- Select channels and redundancy, ideally with diverse paths for primary and secondary systems.
- Specify the interface: hard-wired contacts, relay-to-relay digital communication, IEC 61850 GOOSE over a WAN, or IEEE C37.94 to multiplexers.
- Define logic details: echo, weak infeed, current reversal guard, channel failure alarms and fallback behavior.
- Calculate relay settings for the elements that drive the scheme, coordinating timers with channel delays.
- Document the scheme with logic diagrams and test procedures for both ends.
Testing teleprotection
- Channel tests: verify end-to-end propagation delay, bit error rate or signal levels, alarms and loop-back behavior, and path asymmetry for differential schemes.
- Command tests: send each command (permissive, block, DTT) and verify reception, timing and alarm handling.
- End-to-end scheme tests: use time-synchronized test sets at both ends to inject simultaneous fault quantities for internal and external faults, weak-infeed and current reversal cases. This is the only way to prove the complete scheme, including logic in both relays and the channel.
- Commissioning and maintenance: repeat key tests after firmware changes, channel rerouting or setting changes, and review event records after real faults to confirm scheme timing.
Common mistakes
- Choosing a blocking scheme on a channel that can lose the block signal during external faults without considering the overtrip risk.
- Omitting current reversal guard on parallel lines or echo logic on weak-source terminals.
- Routing primary and backup channels over the same fiber cable or multiplexer.
- Ignoring channel delay asymmetry on rerouted networks for differential protection.
- Testing each end separately and skipping a true end-to-end test.
- Leaving channel failure alarms unmonitored by SCADA, so a failed channel goes unnoticed until a fault.
How PWM helps with teleprotection
PWM designs and commissions teleprotection and line protection schemes for transmission and generation interconnection projects, including scheme selection, logic, relay settings, channel interfaces and end-to-end testing, within our protection, control and supervision services. If you are migrating legacy carrier or pilot-wire schemes to fiber or need end-to-end testing, contact our protection team.
Frequently asked questions
What is teleprotection?
Teleprotection is the use of a communication channel between protective relays at the ends of a line so they can exchange permissive, blocking or trip signals. It allows high-speed tripping for faults anywhere on the line instead of waiting for time-delayed backup zones.
What is the difference between POTT and PUTT?
In POTT, the permissive signal is sent by an overreaching element, so both ends key the channel for internal faults. In PUTT, the signal is sent by the underreaching Zone 1. POTT generally handles high-resistance faults and weak-infeed terminals better, with added echo and current reversal logic.
Why is DCB used with power line carrier?
In directional comparison blocking, the channel only needs to work for external faults, when a block signal is sent. For internal faults no signal is needed, which suits power line carrier because the fault on the protected line may attenuate the carrier signal.
What is direct transfer trip used for?
Direct transfer trip opens a remote breaker without local fault supervision. It is used for breaker failure, transformer or reactor faults without a local high-side breaker, and anti-islanding of generation. It requires very high channel security.
How is a teleprotection scheme tested?
Channels are tested for delay, errors and alarms, and each command is verified. The complete scheme is proven with time-synchronized test sets at both line ends injecting simultaneous fault quantities for internal, external, weak-infeed and current reversal cases.
