Instrument Transformer Testing

Instrument transformers (ITs) for metering purposes must have a high accuracy up to class 0.1 for accurate billing. Hence, it is essential to inspect and calibrate them at regular intervals.

ITs for protection purposes supply protection relays and must also function accurately at a multiple of the nominal current. A regular inspection should ensure that ITs, together with the connected relays, react quickly and correctly in the event of a fault and guarantee the highest possible security of supply.

The importance of IT tests is often underestimated. Risks such as confusing ITs for metering and protection or mixing up connections can be reduced significantly by testing before initial use. At the same time, damages to the 

interior of an instrument transformer, caused for example during shipping, can be recognized easily. Also changes in an IT, caused for example by aging insulation, can be identified at an early stage.

Our test systems provide high-precision, automatic testing and assessment of CTs to IEC 60044-1, IEC 60044-6, IEC 61869-2, IEEE C57.13 and IEEE C57.13.6 standards; VTs to IEC 60044-2, IEC 60044-5, IEC 61869-3, IEC 61869-5, IEEE C57.13 and IEEE C57.13.6 standards; combined transformers to IEC 60044-3 and of IEC 61850 standards.

Solutions

Partial Discharge Measurement and Analysis on Instrument Transformers

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Partial Discharge Measurement and Analysis on Instrument Transformers

Partial discharges (PD), as defined by IEC60270, are localized dielectric discharges in a partial area of a solid or liquid electrical dielectric insulation system under high voltage field stress (high voltage). Partial discharges in an instrument transformer (IT) deteriorate its insulation and can lead to failure of the IT.

You can detect partial discharges already in their early stages with our testing systems. This allows you to draw conclusions regarding the insulation condition. At the same time, further measures such as maintenance work or additional diagnostic measurements can be initiated.

Moisture Analysis on Instrument Transformers

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Moisture Analysis on Instrument Transformers

Moisture in the solid part of the paper insulation is one of the most important factors that impact the condition of instrument transformers (ITs). Moisture from the atmosphere during installation or repairs can easily ingress. Over time, increasing moisture content causes the aging of the oil-paper insulation.

Moisture in oil-paper insulation can reduce the dielectric strength or accelerate the aging of the cellulose (so-called depolymerization) and lead to eventual failure.

Therefore, determining the moisture content in the insulation is an important measure to ensure the reliability and lifetime of current and voltage transformers. Since it can be problematic to take oil samples from ITs, which can lead to rather inexact measurement results, our testing systems use dielectric measurements for moisture determination.

Dissipation/Power Factor Measurement (Tan Delta) on Instrument Transformers

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Dissipation/Power Factor Measurement (Tan Delta) on Instrument Transformers

An insulation failure is often the reason for an instrument transformer (IT) malfunction.

Factors which possibly accelerate the insulation's aging process:

  • Moisture ingression due to broken seals
  • Heat caused by the ambient temperature or sunlight
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Older insulation can lead to high losses in the IT, which generates further heat. Since ITs are not actively cooled, old or degraded insulation can result in a failure or even an explosion. Fragments from the explosion represent a serious hazard for persons in the substation as well as for surrounding equipment.

Dissipation/power factor (tan(δ) delta) measurements help you determine the insulation condition. When using our testing systems, you will also be able to determine the dissipation factor at various frequencies and detect any signs of aging at an early stage. Thus, an IT can be replaced before any further damage occurs.

Common Electrical Tests on Instrument Transformers

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Common Electrical Tests on Instrument Transformers

Our test systems let you carry out several standard tests quickly:

  • magnetization characteristic
  • saturation behavior (ALF/FS)
  • transformation ratio and its accuracy
  • polarity
  • load
  • winding resistance
  • withstand voltage or remanence 
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The results give you a good overview of your instrument transformers’ (ITs’) condition to determine further necessary measures.

Former accuracy class 0.1 tests of ITs required a complex on-site test, or sometimes a complete disassembly and transport to a test laboratory. However, you can perfectly use OMICRON’s powerful and compact test solutions for field tests. With their high accuracy, they can also be used partially for the calibration of ITs during metering.

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