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Line protection

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Relay A · line 1
Plant and grid

Sequence of events cycles

    Relays
    Schemes

    Inside the two relays on line 1

    Each end of the line has its own relay, and each one draws its own R–X plane with itself at the origin, looking down the line. The same fault lands at a different place on each: a fault near the plant is close to Relay A (the plant end of line 1) but near the far end for Relay B (its town end). The red ⚡ tick marks where the fault really is; the big dot is where the relay measures it.

    Relay A

    Relay B

    How to read the R–X plane

    The plane shows impedance, in primary ohms, as the relay works it out from its own voltages and currents. The thick line from the origin is the protected line, out to the far bus (100%); where another line carries on beyond that bus, the dashed line goes on to its far end. For a solid fault, the faulted loop lands right on the line at the fault's distance, which is how the relay knows how far away the fault is. Fault resistance pushes it to the right.

    The circles are the zones. A point inside zone 1 (green, 80% of the line) trips at once. Zone 2 (amber, 125%) reaches past the far bus into the next line, so it waits 18 cycles unless the pilot channel says the other end sees the fault too. Zone 3 (red) waits 60 cycles. It looks backwards by default, as many pilot schemes set it: it reaches 100% of the line behind the relay, so a fault on the bus behind it lands inside it, and in DCB it is the element that keys the block. Set it to look forward instead and it reaches 240% ahead, as backup for the next line. Drag the white handles on the line to change a zone's reach (arrow keys work too). Zoom with the + and − buttons in the corner, a pinch or Ctrl+scroll, or a double-click; drag the background to pan, and ⤢ fits the zones again. The reaches are the same fraction of each relay's own line at every relay.

    The relay forms six loops: three ground loops (A-G, B-G and C-G, with zero-sequence compensation) and three phase loops (A-B, B-C and C-A). The big dot is the loop the relay picked as the faulted one; the small grey dots are the others, and the dotted trail is where the big dot has been. Under normal load every loop is far out to the right, off the plot. A fault behind the relay lands below and to the left of the origin, where the dashed reverse circle of the blocking scheme sits.

    Ground overcurrent: time–current curves

    The inverse-time element (51G) trips after a time that depends on how far the ground current 3I0 is above its pickup: the further above, the faster. The instantaneous element (50G) trips at once above its own level. Each relay has its own settings; these are the selected relay’s. Drag the diamonds: the pickup along the top, the time dial on the curve, the 50G level along the bottom. The dashed curve is the relay it is graded against, the next one out along the chain: for the same fault the selected relay must wait longer, by at least 18 cycles, so the nearer relay clears it first. The coloured dots are the two relays of the selected line, each on its own curve, with the ring filling as its 51G timer runs. Both elements are directional (67G): ground current coming from behind the relay does not count.

    · To watch 51G clear a fault by itself, use Try a fault ▸ Ground overcurrent alone.

    What the selected relay measures voltages solid, currents dashed: Relay A, line 1

    A healthy system is three equal voltages 120° apart with small currents. In a ground fault the faulted phase's voltage collapses and its current grows and lags it by nearly the line angle; the other two barely change. That difference is what the loops on the R–X plane are made from.

    Display options
    About the relays

    Each relay measures the three phase voltages at its bus and the three phase currents into its line. From them it forms six loop impedances: three ground loops (Va ÷ (Ia + k0·3I0), with k0 the zero-sequence compensation factor of the line) and three phase loops ((Va − Vb) ÷ (Ia − Ib)). For a bolted fault, the faulted loop reads exactly the impedance of the line between the relay and the fault, so the relay knows how far away the fault is.

    Zones are self-polarised mho circles, each a fraction of the relay's own line, supervised by a direction element polarised by memory (the positive-sequence voltage from before the fault). Zone 1 reaches 80% and trips a cycle after it picks up. Zone 2 reaches 125%, into the next line, and waits 18 cycles. Zone 3 waits 60 cycles: by default it looks backwards, 100% of the line behind the relay (local backup for its bus, and the element a DCB scheme keys its block from); turned forwards it reaches 240%, as remote backup for the next line, and DCB then uses a separate reverse circle (150% behind). Ground overcurrent is directional (67G), so current from behind a relay does not count. 50G is set per relay at 125% of the ground current it sees for a fault on the far bus (at least 1,000 A), so it never reaches past it. 51G follows an IEC very-inverse curve from a 300 A pickup, with each relay's time dial set so that it waits at least 18 cycles longer than the next relay out along the chain. Breakers open 3 cycles after a trip.

    The substations have their own protection. Bus differential (87B), one relay under each yard, adds up the current coming into each bus through its breakers: what passes through cancels out, while a fault on the bus leaves its current over. When that is over 800 A and over half the biggest current through any one of its breakers, it trips every breaker on that bus, 1.5 cycles after it picks up (click its badge for its chart); transformer differential (87T) trips a faulted transformer's 138 kV breakers and its 13.8 kV breaker. Breaker failure (50BF) watches every breaker that has been told to trip: one still closed 9 cycles later is declared failed, and every breaker around it is tripped, with a transfer trip to the far end of a line straight on its other side. Breakers opened by these three are not reclosed. Reclosing follows a fast trip (zone 1, pilot or 50G, or a zone 2 timer trip when the other end tripped fast): 36 cycles after the line goes dead, the end towards the grid closes onto it from its live bus; 15 cycles later the other end closes once the synchronism check passes (within 20°, 0.1 Hz and 10%), waiting up to 5 s; in a two-breaker terminal the second breaker follows 30 cycles later, synchronism-checked too. For 10 cycles after any close, switch-onto-fault trips at once on anything zone 2 sees, either way, so a permanent fault locks the line out.

    Three pilot schemes share a channel that takes about one cycle. POTT (permissive overreaching transfer trip): each end keys permission when its zone 2 sees the fault, and a relay trips at once on zone 2 plus received permission; if the channel is lost the relay simply waits for its zone 2 timer. DCB (directional comparison blocking): each end keys a block when its reverse element sees a fault behind it, and a relay in zone 2 trips after a 1.8-cycle coordination wait unless a block has arrived; this still trips fast when the fault has killed the carrier, but with a dead channel it will trip for faults on the next line. DCUB (directional comparison unblocking): a guard tone is sent all the time and shifts to trip for a forward fault; losing the guard without a trip (the fault has knocked out the carrier) opens a 9-cycle window in which zone 2 may trip, after which the pilot is blocked. Try each with the channel working, lost on the faulted line, and dead.

    The model samples every third of a cycle, so that is the smallest step you can take: hold Shift and press → to move on by one sample. Simplifications: the relays are ideal (no CT or VT errors, no filtering delay), the zones are plain mho circles without load encroachment or power-swing logic, a breaker either opens on all three poles or sticks on all three, there is no dead zone between a breaker and its current transformer, and the reverse element is a plain mho rather than a directional element with its own reach.

    Further reading at SEL (Schweitzer Engineering Laboratories), whose published papers these schemes follow:

    Changes apply at once.

    ← Generator lab: run the plant that feeds these lines · Next, what the rotors do while a fault waits to be cleared: the stability lab → · The maths behind faults, step by step: the faults course →