This simulator models a four-terminal protected bus zone — two infeed circuits and two outgoing feeders, each with its own CT, feeding a bus differential relay that compares the signed sum of terminal currents against a percentage-restraint boundary. Adjust feeder loading, source sharing, internal and external fault fixtures, CT attenuation and polarity, then watch the relay decide whether to trip every breaker in the zone.
• A real-time 3D model of the four-terminal bus zone — two infeed CTs, two feeder CTs, the protected busbar, the bus differential relay and all zone breakers — with home view, focus-selected-part, toggleable enclosure, auto-rotate and expand controls, tappable components with callouts, and numbered labels matching a companion diagram. • Ten live controls: feeder 1 current, feeder 2 current, source 2 share percentage, internal bus-fault current, external feeder-1 fault increment, feeder-1 CT attenuation, a reverse feeder-2 CT polarity toggle, restraint slope, minimum spill pickup, and a feeder-2-connected toggle. • Play/pause, single-step and larger-step time controls, plus a playback-speed selector from 0.1x to 60x laboratory speed. • Apply fault fixture, remove fault fixture and reset zone breakers actions, with a live sequence narrative and per-component status. • Six live metrics: measured incoming current, measured outgoing current, absolute spill, restraint, operate boundary, and signed terminal-current sum. • A Current balance tab with two charts, the full model equations, and snapshot measurements. • An Experiments tab with four guided scenarios (external fault with correct CTs, internal bus fault, external fault with CT attenuation, polarity error), a model-verification bench of independent automated checks, and a timestamped event log with a copyable trial report. • A Learn & assess tab with guided lessons, a knowledge-check quiz and a written scope/reference statement.
Bus differential protection relies on Kirchhoff's current law: for a healthy bus, the signed sum of every current crossing its protection boundary should be close to zero, because whatever current enters through the infeed CTs also leaves through the feeder CTs. An external fault, even a very large one, still respects this balance — the same fault current that flows in through one boundary point flows back out through another, so the relay should restrain.
An internal bus fault breaks that balance: fault current is drawn from the sources but never leaves through a measured feeder, so it shows up as unmatched "spill" current. The relay compares this spill against a percentage-restraint boundary — a bias term proportional to the total current flowing through the zone — rather than a fixed threshold, so that heavier normal or through-fault loading does not falsely trip the relay. When spill exceeds that boundary for long enough, every breaker feeding the zone must open, not just one feeder, because any energized source could still be feeding an internal fault.
The equations panel shows Ispill = the signed sum of terminal currents, Idiff = |Ispill|, Ibias = half the sum of the absolute terminal currents, and the trip boundary = max(pickup, slope × Ibias), with a trip declared 25 ms after spill exceeds that boundary and zone breakers opening 50 ms later. The CT-attenuation and reversed-polarity controls let you see how a measurement error, not a real internal fault, can push spill across the operate boundary — restraint reduces this risk but is a security measure, not a way to eliminate measurement error entirely.
This model uses four scalar, co-phasal terminal-current fixtures for one representative phase, with source sharing imposed directly by the share-percentage control. It does not implement check zones, dynamic zone selection or CT saturation-detection algorithms, so it should be read as a teaching model of the balance principle rather than a certified relay's full logic.
It compares the signed sum of currents at every terminal crossing the protected bus boundary. For a healthy bus or an external through fault, the same current that enters the zone through one terminal leaves through another, so the signed sum stays near zero. An internal bus fault draws current that never leaves through a measured feeder, producing unmatched "spill" current that the relay detects.
The trip boundary is calculated as max(pickup, slope × Ibias), where Ibias is proportional to the total current flowing through the zone. This restraint scales the operate threshold up as loading or through-fault current increases, which prevents heavier normal current flow or measurement tolerances at high current from falsely tripping the relay.
Yes. The simulator includes a feeder CT attenuation control and a reverse-polarity toggle for exactly this reason — an attenuated or wrongly-polarized CT measurement can create apparent spill current that crosses the operate boundary even though the physical fault is outside the protected zone, or there is no fault at all.
An internal bus fault can be fed by every energized source connected to that bus. Opening only one feeder would leave the fault still energized by the remaining sources, so the relay must command every modeled breaker in the zone to open to fully isolate the fault.