VA Technology 
Protection schemes are commissioned on the assumption that the instrument transformers feeding them behave. A current transformer that saturates below the fault current, or a voltage transformer wired with reversed polarity, defeats a correctly set relay every time. CT/PT testing proves that interface before relay injection begins.
The work splits into two stages. The transformers are measured first; the relay is injected afterwards with the quantities they will actually deliver. Skipping the first stage leaves a failure mode that no amount of relay testing reveals.
What the excitation curve shows
An excitation test applies a variable voltage to the secondary winding with the primary open. Secondary current rises slowly while the core is linear, then climbs sharply as the core approaches saturation. Each point on the curve pairs a secondary voltage with an exciting current, and the shape belongs to the core, not to the turns ratio.
A ratio check proves the turns relationship at low flux only. The excitation curve shows that the core has the material and cross-section the design promised, and that the winding has no shorted turns. A transformer with shorted turns shows a low knee point and a high exciting current long before a ratio measurement drifts off tolerance.
Reading the knee point
The knee is the point where a small increase in voltage drives a large increase in exciting current, where the curve leaves its linear region. Protection settings are written against the knee-point voltage of the core, so this measurement decides whether the transformer stays linear through an external fault.
Reaching that voltage with a conventional source needs a bulky variable supply and a safe working distance. A CT/PT testing instrument that derives high equivalent excitation voltages from a low-power output makes the same measurement practical at the panel, which is the difference between a test that gets done and one that gets deferred.
Ratio, polarity and burden checks
Ratio measurement compares the injected primary current with the secondary current at a defined tap. Polarity matters because every differential, directional and distance element assumes a direction of current flow; a transformer installed backwards inverts the measured angle and the element misoperates. Check polarity at the transformer terminals rather than at the relay, so wiring errors stay separate from transformer errors.
Secondary winding resistance and connected burden decide whether a protection core can drive the relay through a fault. A burden above the rated value pulls the operating point up the excitation curve toward saturation. Measure both with the transformer isolated from the relay and its load, as part of the same CT/PT testing sequence.
| Check | Method | What a failure looks like |
|---|---|---|
| Excitation and knee point | Variable voltage across the secondary | Knee below design value; early saturation |
| Ratio | Injected primary current against secondary current | Ratio error outside rated accuracy |
| Polarity | Current direction compared at the terminals | Reversed angle; differential and directional elements misoperate |
| Winding resistance | DC measurement across the secondary | High resistance from loose terminals; a low value suggests shorted turns |
| Burden | Connected load measured with the transformer isolated | Burden above rating; operating point moves toward saturation |
From CT/PT testing to relay injection
Once the transformers pass, the relay is injected with secondary quantities that represent the worst case they can deliver. The VACTP200P analyzer runs the checks above as one automatic sequence, with ratio, polarity, winding resistance and burden completed in seconds and printed on site. Its derived knee-point voltage reaches 40 kV.
Injection follows with a set matched to the scheme. The VAE-430 covers three-phase overcurrent, directional, frequency and distance work, with 3 × 0–40 A AC current and 8 binary inputs for trip and logic checks. The VAE-660 adds the six current channels that transformer and generator differential schemes need, plus 1 × 0–180 A at 1000 VA for high-burden relays. Both sit in the secondary test instruments range, and process-bus schemes are covered in the IEC 61850 testing guide.
Frequently asked questions
What is the knee point of a CT?
It is the point on the excitation curve where the core stops behaving linearly. Beyond it, a small rise in secondary voltage drives a large rise in exciting current, and the secondary current no longer reproduces the primary current.
Why is polarity testing necessary?
A relay measures the direction of current, and a reversed transformer reverses that measurement. Through-fault current then looks like an internal fault to a differential scheme, and a directional element points the wrong way. The check takes seconds and prevents a misoperation that is hard to diagnose later.
Does a ratio test prove the CT will not saturate?
No. Ratio is measured at low flux and says nothing about the core above the knee point. Only the excitation curve and the knee-point voltage show how the core behaves under fault current, and whether the connected burden keeps the operating point below that knee.
CT/PT testing fixes the inputs to everything downstream, so the commissioning checklist places it ahead of the trip tests. Send the nameplate data and knee-point values to our engineering team to match an analyzer configuration to a scheme, or request a quote for a complete secondary testing package.
