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Applications of CTs/PTs; CT burden

Short answer

CTs used for metering (ammeters, energy meters), protection (overcurrent, differential, distance relays), SCADA, and core-balance earth-fault detection. PTs used for metering (voltmeters, energy meters), protection (distance, directional, voltage/frequency relays), synchronising, open-delta residual voltage, and CVTs for carrier coupling. CT burden is the total secondary impedance (relays, meters, leads) expressed in VA at rated secondary current (usually 0.8 PF lagging); exceeding rated burden increases errors and causes saturation.

What are the applications of CTs and PTs in power system? What is the meaning of burden on CT?

Applications of CTs and PTs in power system

Current transformers (CTs)

  • Metering: ammeters, wattmeters, energy meters (kWh, kVArh) and tariff metering on HV and LV feeders.
  • Protection: overcurrent, earth fault, differential (transformer, generator, busbar), distance and directional relays.
  • Supplying current to power-factor meters, maximum demand indicators and SCADA transducers.
  • Core-balance CTs for sensitive earth-fault detection on cables and motors.

Potential (voltage) transformers (PTs/VTs)

  • Metering: voltmeters, wattmeters, energy meters, frequency and power-factor meters.
  • Protection: distance relays, directional relays, under/over-voltage, under/over-frequency, reverse power relays.
  • Synchronising of generators and synchro-check.
  • Open-delta (broken-delta) connection for residual voltage in earth-fault detection.
  • CVTs at EHV also couple power-line carrier signals.

Both provide isolation from high voltage and standardise signals to 1 A/5 A and 110 V.

Burden on CT

The burden of a CT is the load connected to its secondary: the total impedance of relay coils, meters and connecting leads. It is expressed in volt-amperes (VA) at rated secondary current and specified power factor (usually 0.8 lagging), or in ohms.

 Burden (VA) = I_s² × Z_burden
 e.g. 5 A CT, Z = 0.6 Ω → 5² × 0.6 = 15 VA
  • Standard ratings: 2.5, 5, 10, 15, 30 VA.
  • If the connected burden exceeds the rated burden, the CT needs more e.m.f., flux and exciting current, so ratio and phase errors increase and the CT may saturate during faults.
  • For 1 A CTs the lead burden is 25 times smaller than for 5 A CTs for the same lead resistance, so 1 A CTs are preferred for long leads.

✓ Answer checked by Claude Opus 5.5 (high effort)

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