How to Reduce CVA500 CT Saturation Test Duration

Current-transformer (CT) saturation testing verifies whether a CT can reproduce primary current reliably, particularly during fault conditions. For Canadian utility, industrial, and service teams, RCCE can help you select and apply DV Power CVA500 test equipment with practical attention to safe settings, field workflow, and support.

Why the CT knee point matters

The knee point is the secondary-terminal voltage above which a CT begins to saturate. After saturation, secondary current may no longer accurately represent primary current—exactly when protective systems may need dependable measurement most.

The CVA500 supports winding resistance, demagnetization, saturation, turns-ratio and polarity, burden, and insulation testing for CTs, VTs, and CVTs. In a saturation test, it builds an excitation curve to determine knee voltage and knee current.

Use a focused saturation-test setup

For a saturation-only job, primary, burden, and insulation-test cables are not required. In the CVA500 software, select CT, enable multi-tap where applicable, select only Saturation, then use Advanced Test Options to set the start voltage, end voltage, and maximum current.

Observe the voltage and current limits

For a multi-tap CT, configure the maximum test voltage for every selected tap combination. The CVA500 secondary terminals are rated to 2,100 V AC; the induced voltage at every selected combination must remain below that rating. The maximum-current setting is also protective: if it is reached before the selected end voltage, the test stops with an overcurrent condition. The maximum selectable setting is 10 A.

Why the start voltage changes the duration

By default, CVA500 starts at 1 V and increases until it reaches the selected maximum voltage or about 10% above the detected knee point. That conservative default is right when the CT characteristics are unknown. But if a trusted nameplate, earlier result, or asset record identifies the expected knee-point range, starting closer to that range avoids measuring a large low-voltage region that is not needed.

Important: the start voltage must stay below the CT's actual knee point. A start voltage set too high can omit part of the excitation curve and introduce meaningful measurement error.

Published CVA500 results: multi-tap CT

DV Power tested three combinations with rated knee voltages of 100 V, 200 V, and 400 V. The measured knee voltages stayed consistent as the start voltage was raised, while duration fell substantially.

For multi-tap work, select the start voltage with reference to the lowest expected knee voltage among the selected tap combinations.

Published CVA500 results: single-tap CT

For a single-tap CT rated at 490 V, DV Power measured 491.1 V at both a 1 V and a 400 V start. Test duration reduced from 81 seconds to 21 seconds—nearly four times faster.

Practical field checklist

1. Review the CT nameplate and trustworthy previous records.
2. Identify the lowest expected knee voltage for the selected taps.
3. Select a start voltage safely below that value.
4. Confirm every induced tap voltage remains below 2,100 V AC.
5. Run the test and review the complete excitation curve.
6. Return to a lower start voltage if the curve appears incomplete or results differ unexpectedly.

Technical data and case-study results are adapted from DV Power's original CVA500 practical guide.