The OMICRON CPC 100 injects up to 800 A AC to measure the ratio of current transformers and up to 2 kV to measure their excitation and knee point and the ratio of voltage transformers. Its low-level inputs make it possible to measure bushing CTs and low-power transformers. It is a field test with primary injection, which verifies the transformer as equipment and not just its connection to the relay.
What is tested and why it matters
A current transformer or voltage transformer with a defect gives no warning: the relay simply receives a wrong signal. A CT that saturates earlier than expected delays or prevents a trip; a wrong ratio makes an overcurrent or differential relay see a current that does not exist. That is why, at commissioning, instrument transformers are tested as equipment, before closing the secondary circuit to the relay.
The OMICRON CPC 100 is a portable primary injection system that the manufacturer offers for this purpose, among others. This guide describes what each of its outputs contributes to CT and VT tests; for dedicated low-voltage tests there are the CT Analyzer for current transformers and the VOTANO 100 for voltage transformers.
What each output provides
| Output or input | Use the manufacturer gives it on instrument transformers |
|---|---|
| 800 A AC | Prolonged primary injection and current transformer ratio |
| 2 kV AC | Excitation current and CT knee point analysis; primary injection on voltage transformers |
| 300 V AC input | Measuring the CT and VT ratio on the low-voltage side |
| 3 V AC input, 10 MΩ | Ratio of bushing CTs and tests of low-power transformers |
| 10 V DC input | Resistance measurements |
| Frequency from 15 to 400 Hz | Measuring under conditions other than the mains frequency |
Current transformer tests
Ratio. A known primary current is injected and compared with the one that appears in the secondary. It detects shorted turns, wrongly connected taps and ratios that do not match the nameplate. What is verified and why is explained in ratio, polarity and vector group test.
Excitation and knee point. An increasing voltage is applied to the secondary and the current drawn by the core is recorded. The curve shows where saturation begins and whether the core has any damage; it is the basis for knowing whether the CT sustains the fault current its relay expects. The guide on CT excitation and knee point explains the curve and how to read it, and the CT saturation tool lets you estimate the margin for a specific case.
Winding resistance and burden. The winding resistance enters into the calculation of the voltage the CT can deliver, and the connected burden (cables and relays) determines whether it works within its class. The CT burden tool helps estimate that burden.
Polarity. It is the first thing checked by whoever connects a differential or directional protection. A polarity error is found at commissioning with a simple check, or later with a false trip. For this the manufacturer offers a manual accessory for checking the polarity and wiring of instrument transformers and trip circuits.
Voltage transformer tests
The 2 kV AC output is used to inject into the primary of inductive voltage transformers and measure their ratio with the 300 V input on the secondary side. This verifies the actual ratio, not just the nameplate one. The winding resistance of the voltage transformer is another check of the same set.
For the class accuracy of an inductive or capacitor voltage transformer there are systems of another type, which measure ratio error and phase displacement; the manufacturer has a specific one, with its own guide.
Which standard governs these tests
The IEC 61869 series defines the requirements for instrument transformers: part 2 for current transformers and part 3 for inductive voltage transformers. In the IEEE sphere, C57.13 sets the requirements and C57.13.1 is the field testing guide for protection CTs. The numeric acceptance criteria depend on the class of the transformer and the standard that governs it, and are set in the test plan.
What can distort the result
- Remanence. A CT magnetized by a previous measurement with direct current or by a previous fault gives a different curve. Demagnetize before measuring excitation.
- Poorly isolated secondaries. A secondary connected to the relay or to another CT in parallel changes the measurement; that is why testing is done with the circuit isolated and the test blocks open.
- Noise in the substation. In energized yards the interference is real; it is a reason to measure with equipment with good noise suppression and at frequencies away from the mains frequency.
How a CT and VT visit is planned
Before going to the field, have at hand the nameplate of each transformer (ratios, classes, accuracy burden, limit factor), the diagram of the secondaries and the connected burden. With that, what is measured on each core is defined: a protection core of a differential relay calls for excitation and saturation margin; a metering core of a commercial boundary calls for ratio and accuracy. A visit without that plan ends up measuring everything on every core, or worse, forgetting the core that mattered.
The order is also decided: first the measurements that do not magnetize the core, then the excitation ones, and demagnetization at the end. The temperature and the condition of the connections are also noted, because a result without conditions cannot be compared with one five years from now.
Related IEC 61869 tests
The CT and VT tests covered by a set like this are explained in the guides on interturn overvoltage, Rct and Ts, and applied and induced voltage.
CT and VT tests on request
Do you need this test? We do it for you: depending on your case we recommend the right test and test set and include it in the quote. That way you run the test your case really needs, without paying for tests you do not need. This field work is done in Colombia, or elsewhere on request.
See the scope in current and voltage transformer testing and request your quote through the form.
Related guides
- Tests on primary substation equipment
- Terminals and secondaries of the current transformer
- Test blocks and bay isolation
- Primary and secondary injection
- Medium-voltage current transformer
- OMICRON CT Analyzer: current transformer testing
- OMICRON VOTANO 100: voltage transformer testing
- OMICRON CPC 100: power transformer testing
- Interturn overvoltage in a CT: test A and B
- Rct and Ts of a CT: resistance and time constant
- Applied and induced voltage in a voltage transformer
References
- OMICRON electronics, CPC 100 product datasheet (omicronenergy.com/en/products/cpc-100).
- IEC 61869-2, Instrument transformers – Part 2: Additional requirements for current transformers.
- IEC 61869-3, Instrument transformers – Part 3: Additional requirements for inductive voltage transformers.
- IEEE Std C57.13, Standard Requirements for Instrument Transformers, and IEEE Std C57.13.1, Guide for Field Testing of Relaying Current Transformers.
Frequently asked questions
What is measured on a current transformer with the OMICRON CPC 100?
According to the manufacturer's datasheet, the ratio with primary injection of up to 800 A AC and the excitation current with the 2 kV AC output, from which the excitation curve and the knee point are obtained. Which tests are done on a given visit depends on the class of the CT and its function, protection or metering.
What is the CPC 100 3 V low-level input for?
The manufacturer describes it for sensitive ratio measurement of bushing CTs and for tests of low-power instrument transformers. These are cases where the secondary signal is very small and an ordinary input does not measure it with the required accuracy.
Does testing the CT with the CPC 100 replace testing the connection to the relay?
No. Testing the transformer and verifying the wiring up to the relay answer different questions: the first confirms that the CT is compliant, the second that the signal reaches the relay with the right polarity and ratio. A complete commissioning does both.