This simulator models a 10 MVA, 69/13.8 kV, Dyn11, 60 Hz power transformer fitted with a tap-changing mechanism. Switch between an on-load tap changer (OLTC) and a de-energized tap changer (DETC), run the regulator in manual or fully automatic mode, and watch how tap position, winding impedance and load power factor together determine the secondary voltage.
• Operate tab: a real-time 3D transformer and tap-mechanism model with a casing toggle (cutaway view), home view reset, auto-rotate camera and an expand control, plus a mechanical tap-position strip and raise/lower LV-voltage buttons. • Control desk: run-time playback, advance 1 s / advance 60 s time-step buttons, isolate-primary and energize-primary controls, a tap-changer type selector (OLTC vs. DETC), a regulator-mode selector (manual vs. automatic voltage regulation), and a leading-power-factor load toggle. • Regulation & ratios tab: a secondary-voltage history chart (actual, measured, target and deadband-limit traces over the last 300 simulated seconds), a tap-position history chart across all 17 positions (−8 to +8), automatic-voltage-regulator controls with adjustable band and delay, a formula panel following the calculation step by step, and a tap-to-voltage characteristic sweep across all 17 settled positions at the current load and power factor. • Experiments & diagnostics tab: a diagnostic condition selector (jammed drive, reversed drive direction, lost VT measurement, VT calibration bias) with an adjustable measurement-bias slider, a drive-alarm reset, guided experiments, a 20-check verification bench, and an exportable event log. • Learn & assess tab: lessons on tap-changer operation, model-scope notes, external references, and a knowledge-check quiz.
A transformer's turns ratio sets its voltage ratio between primary and secondary windings. A tap changer lets that ratio be adjusted in small discrete steps — in this model, 17 positions each changing HV turns by 1.25% of nominal — to compensate for supply voltage drift or load-dependent voltage drop. An on-load tap changer (OLTC) can switch taps while the transformer stays energized and carrying current, using a transition mechanism (diverter switch and transition impedances) so current is never fully interrupted during the changeover. A de-energized tap changer (DETC) has no such mechanism and can only be moved when the transformer has been fully isolated from all sources — it is used for coarser, infrequent adjustments rather than continuous regulation.
In automatic voltage regulation mode, the regulator continuously compares measured secondary voltage to a target and only issues a tap command once the error has stayed outside a deadband for longer than a configured time delay, which prevents the mechanism from hunting on every small fluctuation. Because the model includes winding impedance rather than just the ideal turns ratio, actual secondary voltage under load also depends on load current magnitude and power factor, not tap position alone.
The secondary voltage history chart overlays four traces — actual voltage, the (possibly biased or lost) measured voltage the regulator sees, the target setpoint, and the deadband limits — so you can see directly whether the regulator is chasing a distorted reading or holding tightly to a healthy one. The tap position history chart shows the discrete, stepped nature of tap changes: each move is a full step to an adjacent one of the 17 positions, with mechanical travel time modeled explicitly, and the old ratio holds until travel completes.
The tap-to-voltage sweep chart shows what secondary voltage would result at every one of the 17 tap positions under your current supply, load and power factor — useful for understanding why the regulator picks the tap it does, and how much headroom remains before hitting the end of the tap range. The formula panel below it walks through the same calculation the sweep uses, including winding impedance, so the relationship between tap position and delivered voltage is never just the bare turns ratio.
An on-load tap changer (OLTC) can change taps while the transformer remains energized and carrying load, using a make-before-break or resistor/reactor transition mechanism to avoid interrupting current. A de-energized tap changer (DETC) can only be operated after the transformer is fully isolated from all sources, and is typically used for infrequent, planned ratio adjustments.
The regulator compares measured secondary voltage against a target value using a deadband, expressed as a percentage around the target. A tap change command is only issued once the measured voltage has stayed continuously outside that deadband for longer than a set time delay; returning inside the band, or the error reversing direction, resets that delay timer.
Tap changers are most commonly placed on the high-voltage winding because the current there is lower, which allows smaller, less expensive tap-changer contacts and mechanisms. In this model, the convention is that a positive tap position adds primary (HV) turns, which lowers the secondary voltage.
A jammed drive fails to move the tap position and times out after 10 seconds of no movement, latching a drive alarm; a reversed drive direction also latches an alarm on the first mismatch it detects. A lost voltage transformer (VT) measurement or a VT calibration bias affects only the automatic regulator's view of secondary voltage — it does not change the actual physical turns ratio, which is a purely mechanical function of tap position.