Instrument Transformers: CT and VT Selection for Metering and Protection

A CT/VT specification guide covering ratio, accuracy class, burden, knee-point voltage, transient performance, insulation level and IEC 61869 digital secondary

1. Measurement and Protection Are Different CT Jobs

IEC 61869-1:2023 applies to newly manufactured instrument transformers for measurement, protection and control, including analogue and digital secondary signals. A metering core is selected for accuracy around normal load; a protection core is selected to remain useful during high fault current and transient conditions. Combining both duties in one unspecified core creates avoidable saturation and billing errors.

2. Core Selection Matrix

RequirementMetering coreProtection core
Primary objectiveLow ratio and phase error near declared loadStable reproduction during faults
Typical dataRatio, class, rated burden, extended currentRatio, class, ALF or transient class, burden and Vk
Main failure modeAccuracy drift or excessive burdenSaturation, delayed relay operation or distorted waveform
Commissioning proofInjection test at declared burdenSecondary resistance, loop burden and excitation test

For a CT secondary circuit, the connected burden is the sum of relay, meter, terminal and lead resistance. A first engineering check is:

Sburden ≈ Isec2 · (Rrelay + Rmeter + Rleads)

The actual burden must stay within the selected core rating, including the one-way lead length and temperature-dependent conductor resistance.

3. VT, Digital Output and Insulation Coordination

Voltage transformers require ratio, accuracy, burden, ferroresonance behaviour, earthing arrangement and the correct phase-to-earth or phase-to-phase insulation level. Where a process bus is used, IEC 61869-9 and IEC 61850 engineering replace analogue copper assumptions with a defined digital interface, time source, merging-unit performance and network redundancy.

4. Factory and Site Verification

  • Confirm the highest voltage for equipment, power-frequency withstand and lightning impulse withstand.
  • Check short-time thermal current, dynamic current, secondary terminal safety and earthing.
  • Measure ratio, polarity, winding resistance, insulation and excitation; document the complete protection loop burden.
  • Never copy a CT class from a relay datasheet without calculating the installed cable loop and fault duty.

*Reference: IEC 61869-1:2023; IEC 61869-2:2012; IEC 61869-3; IEC 61869-9.*