Mission Brief
Which Way Did It Flow?
FortisBC, 25 kV ring bus at the Vernon tie. Two sources back-feed this bus, and last week a fault on the adjacent feeder tripped YOUR healthy tie breaker — a reverse-infeed mis-operation. The plain overcurrent relay couldn't tell the fault was behind it. Your job: commission the directional element (67/67N) so it trips for forward faults and blocks the reverse infeed — and survives a bolted close-in fault that collapses the polarizing voltage.
- Understand directionality — the relay decides from an ANGLE, not a magnitude
- Set the RCA/MTA so forward operates and reverse blocks, with margin
- Choose phase and ground polarizing sources for this grounding/scheme
- Recognise when memory polarization is mandatory (close-in collapse)
- Back-calculate TMS and commission with V+I injection at fault angles
Direction is an angle, not a magnitude
Every element so far tripped on how much current. This one trips on which way it flows. The relay compares the current against a reference phasor (“polarizing”) and asks a single yes/no question: is the current within ±90° of the maximum-torque line? Drag the current below and watch the decision flip as it crosses the zero-torque boundary.
drag the I phasor — operate when it lands within MTA ± 90°
Why you need it — parallel paths
On a radial feeder, current only ever flows one way, so plain 50/51 is enough. The moment you have two sources or parallel feeders, a single fault is fed from both ends. A non-directional relay on the healthy feeder sees that reverse infeed and trips it too — you lose selectivity. The 67 element looks at direction and blocks the reverse-fed relay.
The reference — polarizing
“Within ±90° of what?” — of the polarizing reference. The choice depends on the element and the fault:
- Phase 67 — quadrature (cross-phase) voltage: polarize the faulted phase's current with the other two phases' voltage, which stays healthy for a single fault.
- Ground 67N — zero-sequence voltage (3V0), zero-sequence current (neutral CT), or negative-sequence (I2/V2). Which one is available depends on the grounding and the VT/CT scheme.
The trap — close-in faults collapse Vpol
A bolted three-phase fault right at the relay drives the local voltage toward zero — including the polarizing voltage. With no reference, a plain voltage-polarized element goes blind exactly when it's needed most. The fix is memory polarization: the relay remembers the pre-fault positive-sequence voltage for a few cycles and polarizes against that. If a fault retains very little voltage, memory polarization is mandatory.