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Instrument Transformers Application Guide

Instrument Transformers Application Guide by ABB
Instrument Transformers Application Guide by ABB

How to specify current transformers?

Important main factors when selecting current transformers are:

Standard (IEC, IEEE international) //

  • Rated insulation level (service voltage)
  • Altitude above sea level (if >1000 m)
  • Ambient temperature (daily temperature or average over 24 hours) − Rated primary current
  • Rating factor (maximum continuous current)
  • Rated secondary current
  • Short-time current
  • Dynamic current
  • Number of cores
  • Burdens (outputs) and accuracies for each core − Pollution level (creepage distance)

Rated insulation level

The current transformer must withstand the operational voltage and overvoltages in the network. Test voltages are specified in the standards in relation to the system voltage. These tests shall show the ability of a current transformer to withstand the overvoltages that can occur in the network.

The lightning impulse test is performed with a wave shape of 1.2/50 μs and simulates a lightning overvoltage.

For current transformers with a system voltage of 300 kV and more the switching impulse test is performed with a wave shape of 250/2500 μs simulating switching overvoltages. It is performed as a wet test. For voltages below 300 kV a wet power frequency test is performed instead.

The dielectric strength of air decreases as altitude increases. Consequently, for installation at an altitude higher than 1000 m above sea level, the external insulation (arcing distance) of the transformer has to be adapted to the actual site altitude.

Note that as far as the internal insulation is concerned, the dielectric strength is not affected by altitude.

Typical designs of CTs
Typical designs of CTs

According to IEC 61869-1 the arcing distance under the standardized atmospheric conditions is determined by multiplying the withstand voltages required at the service location by a factor k.

k = em · ( H – 1000 ) / 8150

where:

  • H = Altitude above sea level in meters
  • m = 1 for power frequency and lightning impulse voltage
  • m = 0.75 for switching impulse voltage

According to IEEE dielectric strength that depends on air should be multiplied by an altitude correction factor to obtain the dielectric strength at the required altitude.

Title:Instrument Transformers Application Guide – ABB
Format:PDF
Size:9.9 MB
Pages:134
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Instrument Transformers Application Guide
Instrument Transformers Application Guide by ABB

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Page edited by E.C. (Google).