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Different errors of CT

Short answer

CT errors are ratio error (difference between actual and nominal ratio), phase angle error (angle between primary and reversed secondary current), and composite error (rms difference including harmonics). They arise from the exciting current needed for core magnetization, and are increased by high burden, saturation, and remanent flux.

What are the different errors of CT? Explain briefly.

A practical CT does not give a secondary current exactly equal to I_p/K_n, because part of the primary ampere-turns is used to set up flux in the core (exciting current I₀ = magnetising component I_m + loss component I_w). This causes the following errors:

1. Ratio (current) error

The actual ratio R = I_p / I_s differs from the nominal ratio K_n.

 Current error (%) = (K_n·I_s − I_p) / I_p × 100
 Approximately   R ≈ n + (I₀ sin(δ+α)) / I_s

where n is the turns ratio N_s/N_p, δ is the burden phase angle and α the angle by which I₀ leads the flux. It is mainly caused by the loss (in-phase) component of I₀.

2. Phase angle error (phase displacement)

The angle β between the primary current phasor and the reversed secondary current phasor.

 β ≈ (180/π) · I₀ cos(δ+α) / (n·I_s)   degrees

It is mainly caused by the magnetising component of I₀. It matters for wattmeters, energy meters and directional/distance relays.

3. Composite error

The r.m.s. difference between (K_n × instantaneous i_s) and instantaneous i_p over one cycle, as a percentage of the r.m.s. value of I_p. It includes ratio error, phase error and harmonic distortion due to saturation. It is the main error specified for protective CTs (5P, 10P).

Factors affecting errors

  • Exciting current (core material, flux density, core size).
  • Burden magnitude and power factor.
  • Saturation due to fault current or DC offset; remanent flux.
  • Supply frequency.

Errors are reduced by low-loss, high-permeability cores, low flux density, small burden and turns compensation (one or two secondary turns fewer than nominal).

✓ Answer checked by Claude Opus 5.5 (high effort)

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