This simulator models a window-type current transformer (CT) — a single primary conductor passing through a magnetic core wound with many secondary turns, feeding a meter or relay burden. Adjust the primary current, winding ratio and secondary circuit, then watch how burden, polarity and core saturation affect the measured secondary signal.
• A real-time 3D model of the window-type CT with a toggleable enclosure, home-view reset, auto-rotate orbit, expandable 3D view and pause/play animation control. • Adjustable primary current, winding turns ratio and nameplate settings via the source & nameplate control panel. • Secondary circuit mode selector: meter connected (burden), shorting link closed, or open secondary — a virtual energized-fault fixture. • A reversible S1/S2 secondary polarity checkbox. • Four live scope charts: current fidelity (ideal vs. measured secondary current), core flux density with saturation-onset markers, secondary terminal voltage, and the excitation (B-H) characteristic curve. • A calculated-quantities panel with ratio error, phase displacement and THD, plus the governing equations. • A tabbed workflow across four pages — CT workbench, Signals & equations, Experiments & tests, and Learn & assess. • Guided experiments, a 'Model verification bench' that runs built-in model checks, a diagnostic challenge exercise with a diagnosis selector, and a knowledge-check quiz with feedback and reset. • A session log recording actions, and an export-results-to-JSON button.
A CT works on ampere-turn balance: the primary current times its (usually single) turn must be matched by the secondary current times its many turns, so Np·ip − Ns·is corresponds to the magnetizing effort driving the core's flux. Because the secondary has far more turns than the primary, a large primary current is stepped down to a small, standardized secondary current that a meter or relay can safely measure.
The secondary voltage that develops depends on the burden — the total impedance of the meter, leads and winding resistance. Under normal 'meter connected' operation, the CT behaves close to an ideal current source. If the secondary is shorted, current simply flows through the short instead of the meter. If the secondary is opened while the primary is still energized, all of the primary ampere-turns go into magnetizing the core, driving it into saturation and producing a large, illustrative voltage spike — which is why this condition is treated as a fault fixture rather than a normal operating mode.
The current fidelity chart compares the primary current referred to the secondary (amber) against the actual measured secondary current (mint) over one settled cycle — the closer these tracks match, the lower the ratio error. The flux density chart (violet) shows how close the core is running to the modeled saturation onset (±Bk, dashed lines); as burden or primary current rises, flux swings closer to that knee and the fidelity chart starts to distort.
The governing relationships are Np·ip − Ns·is = ℓ·H(B), e = Ns·A·dB/dt, and e = is·(Rw + Rlead + Rburden), with rated burden resistance equal to rated VA divided by rated Is². Ratio error is reported as (measured RMS / ideal RMS − 1) × 100%, phase displacement is the meter fundamental's angle relative to the primary reference, and THD is computed from the sampled cycle excluding the fundamental. This is a generic educational CT model — not a manufacturer device, accuracy-class certification, or a validated safe-voltage prediction for the open-secondary condition.
A current transformer relies on ampere-turn balance: the primary current times its turns must equal the secondary current times its turns. With a single-turn primary passing through a window-type core and many secondary turns, a large primary current is transformed into a small, standardized secondary current (typically 1 A or 5 A) that meters and relays can safely measure.
A CT is a current source, not a voltage source. If the secondary circuit is opened while primary current still flows, all of the primary ampere-turns go into magnetizing the core, driving it into deep saturation and producing very high, potentially dangerous, secondary voltage spikes. This simulator includes a virtual open-secondary fault fixture to illustrate the effect without real risk.
Burden is the total impedance connected to the CT secondary, including the meter or relay, lead resistance and winding resistance. Rated burden impedance equals the rated VA divided by the rated secondary current squared. Excessive burden increases the voltage the CT must develop, which can push the core toward saturation and increase ratio and phase error.
Ratio error is the percentage difference between the measured secondary RMS current and the ideal value predicted by the turns ratio. It grows as the core approaches saturation, which happens when the magnetizing flux density crosses the modeled saturation onset. Higher burden, higher primary current or wrong CT sizing all push the operating point closer to that saturation knee.