Zone 2 Trip on a Healthy Parallel 220 kV Line During a Single Line-to-Ground Fault
A 220 kV double-circuit line is 92 km long, with both circuits on the same towers, solidly grounded at both ends. Each circuit has a numerical distance relay with quadrilateral characteristics and a CT ratio of 1200/1 A. Settings are: Zone 1 = 0.8 x ZL at 0 s, Zone 2 = 1.2 x ZL at 0.4 s, Zone 3 = 1.8 x ZL at 1.2 s; power-swing blocking and load encroachment are enabled. There is no teleprotection channel; the scheme is stepped distance only. During a dry, clear night, a single line-to-ground fault occurred on Circuit 1 at 62% of the line from Bus A. Circuit 1 tripped on Zone 2 from both ends at 400 ms. Circuit 2 also tripped at 380 ms on its Zone 2 earth-fault element, although it had no fault. Line load was 240 MW and the voltage at Bus A dipped to 0.62 pu during the fault. Post-fault insulation resistance, ratio, breaker timing and relay self-tests on Circuit 2 were all normal. The operator wants both circuits charged immediately; the SDO wants the trip explained first.
1. Explain the most probable mechanism of the Circuit 2 trip and identify the evidence in the fault records that would confirm it.
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The fault location is the key. At 62% from Bus A, the fault was beyond Zone 1, so Circuit 1 was correctly cleared by Zone 2 at 0.4 s from both ends. Circuit 2 was healthy, so its Zone 2 earth element must have seen an apparent impedance inside reach that was not a real fault.
Mechanism: zero-sequence mutual coupling. Both circuits share towers. During the earth fault, the zero-sequence current in Circuit 1 induces a longitudinal voltage in the zero-sequence circuit of Circuit 2, through the mutual impedance Z0M between the parallel circuits. Circuit 2 carries no fault current, but its relay measures a modified zero-sequence voltage and current, and the computed apparent impedance drops below the Zone 2 earth reach. Because the fault persisted for 0.4 s, the element timed out and tripped a healthy circuit. The high load and the 0.62 pu dip are incidental and do not explain an earth-fault element pick-up; load encroachment would affect phase elements, not the earth element.
Confirming evidence. In the Circuit 2 event record: earth-element pick-up coincident with the Circuit 1 fault inception, a small zero-sequence current in Circuit 2 (tens to a few hundred amps), and an apparent impedance just inside 1.2 x ZL that collapses when Circuit 1 clears. Replay the COMTRADE files and compare measured apparent impedance with a short-circuit study run both with and without Z0M included.
2. What protection changes do you make before recharging, and how do you judge between a temporary blocking measure and a permanent fix?
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Permanent fixes, in order of preference.
- Enable zero-sequence mutual compensation with the correct Z0M magnitude and sign, and verify it by replaying the recorded event; this is the direct correction for the observed mechanism.
- Because there is no teleprotection, Zone 2 is doing the primary clearing duty and must remain fast. The robust fix is a permissive over-reaching transfer trip or directional comparison channel, so an internal fault is cleared in a few tens of milliseconds and the Zone 2 earth element can be given a longer time.
- If no channel is available soon, increase the earth-fault Zone 2 time to at least the parallel-circuit clearing time plus a margin, and re-check coverage at the remote bus and coordination with downstream relays.
Temporary measure. Disabling the Zone 2 earth element (retaining the phase elements and Zone 3 as backup) removes the false trip but leaves reduced earth-fault sensitivity and slower backup, so it needs written risk acceptance and a short duration.
Judgement. Do not simply reduce Zone 2 reach: that creates a protection gap at the remote line end. Select the measure based on the confirmed mechanism, not on the trip being inconvenient.
3. Do you restore Circuit 2 immediately? State the conditions you would attach and the residual risk if the line is charged before the settings are corrected.
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Restoration. Circuit 2 may be charged once its own insulation, CTs, breakers and relay are proven healthy, which the post-fault tests already indicate. But the priority order is: confirm the cause from the fault records, apply the Z0M compensation or the temporary blocking measure, and only then charge.
If charged before correction, the residual risk is a repeat: any future earth fault on Circuit 1 will again push Circuit 2's earth element into Zone 2, tripping both circuits and losing a double-circuit corridor, possibly with generation rejection or load shedding at 240 MW transfer. That is a credible system risk and must be recorded.
Conditions to attach. Written approval for any protection downgrade; a shorter review window (for example 72 hours) with a named owner; both circuits on the same protection configuration verified by test injection; operating instructions to the control centre noting that an earth fault on either circuit will trip both until the fix is in; and a check that the busbar and breaker-failure schemes still coordinate with the revised Zone 2 timing. If the fix cannot be implemented within the window, keep one circuit on a deliberately conservative earth-fault setting and accept reduced backup protection with formal sign-off.