Distance zone 1 is set not to reach the opposite end, so a fault near one end is cleared instantly by that end and by the other only after the zone 2 time, unless a teleprotection scheme speeds up the trip. When the remote end clears slowly on a line that has teleprotection, the typical causes are a failed channel or a signal not received, a weak source without echo logic, an active current reversal guard, or a resistive fault the overreaching zone did not see.
Symptom
During a fault on a high or extra-high voltage line, the end closest to the fault opened in zone 1 time and the remote end did so well afterward, in zone 2 time or even through a backup protection. The fault stayed fed from one side during that interval, with an effect on stability, on equipment and on the users of the remote substation.
Distance protection 21 sets its zone 1 below the line length. Errors in the CTs (current transformers), the VTs (voltage transformers) and the line impedance do not allow a fault at the end of the line to be told apart with confidence from one just past the remote bus. That decision leaves at each end a strip in which the fault is seen only in zone 2, time-delayed. Teleprotection exists to close that gap: each end tells the other what it sees and both trip fast for internal faults. If the remote end cleared slowly, either the line has no teleprotection or it did not work for that fault.
Possible causes, from most to least likely
| Cause | Why it happens | What gives it away |
|---|---|---|
| Line without teleprotection | The protection has only stepped zones | The remote end’s time matches its zone 2 time |
| Failed channel or signal not received | Failure of the teleprotection equipment, multiplexer or fiber; blocked signal | Channel alarm; send recorded at one end with no receive at the other |
| Weak source at one end without echo logic | The weak end does not detect the fault and does not return permission | The strong end sent permission and waited; the weak one tripped when the strong one opened |
| Current reversal guard | After a fault on a parallel line, the guard blocks tripping for a time | Fault on the parallel line just before, or current reversal recorded |
| Resistive fault outside the overreaching zone | Fault resistance pushes the impedance outside the remote end’s characteristic | The remote end’s overreaching zone did not pick up |
| Scheme disabled or in test | Test blocks open, function switched off after maintenance | No send or receive in the events |
| CVT transient | The capacitor voltage transformer distorts the voltage in the first cycles | Delay of the distance element on close-in faults with low voltage |
| Slow remote breaker | The relay tripped on time, but the breaker was late | Fast trip command and delayed opening; possible 50BF operation |
What to check first
- Events from both ends aligned in time: pickup of each zone, teleprotection send and receive, trip and breaker opening. See event report and sequence of events.
- Teleprotection scheme of the line: POTT, PUTT, DCB, DCUB or other, and its echo and guard logic. See POTT scheme.
- Alarms from the channel and the teleprotection equipment before and during the fault.
- Current infeed from each end and the impedance seen by each relay, with the estimated fault resistance.
- Events on parallel lines immediately before the fault.
- Does the line have teleprotection enabled?
- No: Expected behavior: the remote end clears in zone 2
- Yes: Did the remote end receive the signal from the other end?
- No: Failed channel, teleprotection equipment or scheme in test
- Yes: Did the remote end's overreaching zone pick up?
- No: Weak source without echo or resistive fault outside its characteristic
- Yes: Current reversal guard active or slow remote breaker
How to confirm
- Reproducing the fault in both relays with the records from both ends, by synchronized injection. It shows whether the scheme should have tripped fast with those settings. See end-to-end tests.
- Short-circuit calculation of the fault with its estimated location and resistance, to see what impedance each end should have measured and with what infeed.
- Teleprotection channel test: transmission time, integrity and alarms.
- Remote breaker timing if the relay issued the command on time. See circuit breaker operating time test.
How to keep it from coming back
- Teleprotection on the lines where the required clearing times or stability demand it, with a scheme chosen for the line and the sources.
- Echo and weak-infeed logic where one end has little source. See echo and weak-infeed logic.
- Current reversal guard set with the real times of the channels and breakers. See current reversal guard.
- Channel supervision with an alarm and automatic fallback to a backup scheme when it drops.
- End-to-end testing after every change to the relays, the channel or the communications equipment.
How each scheme behaves when the channel fails
The choice of scheme determines what happens if the signal does not arrive. The difference explains why the same failed channel produces slow clearing on one line and an unwanted trip on another:
| Scheme | What each end sends | Fast trip condition | With the channel down |
|---|---|---|---|
| Direct underreaching transfer trip (DUTT) | Trip command when its zone 1 picks up | Receiving the command | No acceleration; demands a very secure channel because a spurious signal trips |
| Permissive underreaching transfer trip (PUTT) | Permission when its zone 1 picks up | Receiving permission and seeing the fault in zone 2 | No acceleration; remote clearing stays in zone 2 |
| Permissive overreaching transfer trip (POTT) | Permission when its overreaching zone picks up | Receiving permission and seeing the fault in that zone | No acceleration; remote clearing stays in zone 2 |
| Directional comparison blocking (DCB) | Block when it sees the fault in the reverse direction | Not receiving a block after a coordination time | May overtrip on external faults, because the block is missing |
In permissive schemes, loss of the channel degrades speed, and the backup is the stepped behavior of the zones; in the blocking scheme the degradation is in security. That is why blocking schemes are complemented with channel supervision and with logic that decides what to do if there is no guard signal. See POTT scheme.
Why zone 1 reach does not cover the whole line
Zone 1 reach is set to a fraction of the line impedance, usually around 80 to 90 %, to leave margin for CT and VT errors, zero-sequence impedance and calculation uncertainty. Up to 10 to 20 % of the line at each end falls outside zone 1, and in that strip each relay sees the fault only in zone 2. Faults in that strip are the ones that one end clears fast and the other slowly when there is no teleprotection or it is not available. It is a design limitation, not a relay defect, and it is solved with teleprotection or with line differential protection.
Capacitor voltage transformers and operating speed
CVTs include a ferroresonance suppression circuit that, on a sudden voltage collapse, produces a transient during the first cycles. When the ratio of source impedance to line impedance is high, the voltage during the fault is low and the transient can delay the impedance measurement. Relay manufacturers compensate for that condition with specific logic, whose behavior is described in their manual; if a distance relay trips slower on faults very close to the bus with a CVT, compare that delay with what the manufacturer declares before changing zones.
What not to do
- Do not extend zone 1 beyond the line to “speed up” the remote end: selectivity with neighboring lines is lost.
- Do not shorten the zone 2 time without reviewing coordination with adjacent lines.
- Do not test the channel with teleprotection in service without coordinating the blocking of the outputs.
- Do not analyze just one end: slow clearing is understood only with the records from both.
When to call us
If a line cleared slowly at one end, send us the records from both ends: we quote the trip analysis, a remote service available worldwide, and tell you why it was not accelerated. Our team reviews and adjusts the zones, the teleprotection, the echo and the guard through remote relay configuration; if the scheme and channel must be tested with both ends, we do it with end-to-end tests, in Colombia or elsewhere on request. Write to us.
Related guides
- Teleprotection schemes
- POTT scheme
- Echo and weak-infeed logic
- Current reversal guard
- Distance protection 21
- Zone acceleration
References
- IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines.
- Blackburn and Domin, Protective Relaying: Principles and Applications.
Frequently asked questions
Why did one end of the line trip fast and the other take longer?
Because the fault was inside zone 1 of one end and outside that of the other. Without a working teleprotection scheme, the far end trips only when its zone 2 time expires. That is the expected behavior of distance protection without a channel, and it is exactly what teleprotection is meant to prevent.
What is sequential tripping on a line?
It is when one end does not detect the fault until the other one opens. It happens with weak sources or with faults that, seen from one end, fall outside its zones because of the infeed from the other end. When the strong end opens, the current distribution changes and the weak end sees the fault and trips, with a delay.
Is slow clearing at the remote end a compliance problem?
It can be. Grid codes and utility requirements set maximum fault clearing times, and the system operator reviews events. Clearing in zone 2 time on a line that was supposed to have accelerated tripping usually calls for an explanation.