What it is and when you need it
Current and voltage transformer tests verify that these devices step the system current and voltage down to safe values with the ratio, polarity and accuracy that relays, meters and control equipment expect. All protection depends on them: a well-set relay fed by a miswired or saturated CT makes the wrong decisions.
They are done in these cases:
- Commissioning, to confirm the selected tap, the installed polarity and the wiring up to the panel.
- Relay or settings changes, when the ratio in use or the connected burden changes.
- Diagnosis, after a misoperation of a differential or directional element, or a nearby fault.
We test bushing CTs, free-standing CTs, inductive VTs and capacitor voltage transformers (CVTs), in substations and in industry. It is part of our primary equipment testing service.
What is included
- Ratio of each core and each tap in use, with ratio and phase error.
- Polarity of each core, verified against the drawing and against the direction the relay expects.
- Excitation curve and knee point of the protection cores.
- Secondary winding resistance, which enters the saturation calculation.
- Real burden of the secondary circuit, measured from the terminal box to the relay or meter.
- Insulation resistance between windings and to ground.
- For VTs: ratio and polarity of each secondary winding and verification of the circuit up to the panel.
How we work
- Planning. From the nameplate data we build a table per core: ratio, class, rated burden, specified knee point and the destination of each secondary. That way we test what the project needs and not only what the nameplate says.
- Safety. The primary equipment is locked out and grounded. CT secondaries are never left open with current in the primary; we short-circuit them at the terminal block whenever they are not connected to the test set.
- Execution. We use calibrated multifunction test sets (for example the Omicron CPC 100), which measure ratio, polarity, excitation, winding resistance and burden from a single unit. The excitation curve is taken by raising the voltage on the secondary past the knee and bringing it down in a controlled way.
- Demagnetization. After measuring resistance with direct current, the core keeps residual magnetism. We demagnetize it before energizing or repeating the excitation test.
- Analysis and report. We compare each result with the nameplate and with what the project specifies for that core.
Criteria and standards
Instrument transformers are governed by the IEC 61869 series (part 1 general requirements, part 2 inductive CTs, part 3 inductive VTs) and by IEEE C57.13. For protection CTs, IEEE C37.110 guides the verification of performance under saturation.
- Ratio and phase: the measured error is compared with the nameplate, between cores and with the factory. The accuracy class (for example 0.2 or 0.5 for metering and 5P or 10P for protection) is only certified by the accuracy test of IEC 61869-2 or IEEE C57.13, at the currents and burdens the standard sets.
- Knee point: it is determined with the definition of the standard that governs the CT, because IEC and IEEE define it differently, and it is compared with the specified value.
- Burden: on protection cores, the real burden must not exceed the rated burden; if it does, the CT saturates sooner. On metering cores it must stay within the burden range of its class (in IEC 61869-2, 25 % to 100 % of rated): a very low burden can also take the core out of its class.
- Polarity: it must match the drawing and the convention used by the protection. There is no tolerance for an error here.
- Insulation: it is compared between cores and against earlier measurements.
When a criterion depends on the manufacturer’s datasheet, the report states the source. If the real burden or the knee point compromises the protection, we flag it so the setting or the wiring can be reviewed.
What you receive
- Report per CT and VT, with a section per core: ratio, error, polarity, excitation, resistance and burden.
- Excitation curves with the knee point marked.
- Compliance table against the nameplate and against what the project specifies.
- Observations on taps, wiring or burdens that affect the protection.
- Test files for future comparisons.
Why Tiza Group
- One team accountable from start to finish that knows both the CT and the relay it feeds.
- Experience in substations and industry, in medium, high and extra-high voltage.
- Field judgment from design to commissioning: we verify that each core serves the function it was assigned.
- Calibrated test equipment for all CT and VT tests.
Field work is done in Colombia, or elsewhere on request. Contact us to plan your CT and VT tests.