Correction of Errors Introduced by Instrument Transformers - Digital Signal Processing in Power System Protection and Control - page 100

Fig. 7.3 Transient waveform
of the saturated CT secondary
current

i 1
i 2

i 2 e ( n )

i ( n- 1)

i 2 ( n- 2)
i 2 ( n- 3)

i 2 ( n )

i 2 ( n +1)

i 2 ( n +2)

i 2 ( n +3)

i 2 ( m )

i ( m- 1)
2

Time

7.2.2 Detection of the Unsaturated Fragment of the Waveshape

An exemplary detailed waveshape of the CT saturated secondary current is shown
in Fig. 7.3 , that can be used as illustration and basis for the developed saturation
detection methods.
One may see that the CT saturates between the samples (n - 1) and (n). It
means that the core flux reached the saturation level. The saturation ends between
the samples (m - 1) and (m). Therefore, the transformation is correct, which
means that the secondary current is proportional to the primary current ð i 2 ¼ N i i 1 Þ;
until the sample (n), and after the sample (m).
Determination of the sample (n) is based on the comparison of the measured
secondary current i 2 ðÞ and the estimated secondary current i 2e ðÞ :

j i 2e ð n Þ i 2 ð n Þ j D i 2e ð n Þ j j ð 7 : 4 Þ

and

j

i 2e ð n Þ

j i 2 ð n Þ j j 0 ð 7 : 5 Þ

If the difference between the two exceeds a part D of the estimated level, it
means that there is a sudden change of the waveshape and saturation happened
between the samples.
If the condition ( 7.4 ) is satisfied, but

j i 2e ð n Þ j i 2 ð n Þ j j 0 ð 7 : 6 Þ

like between the samples (m) and (m - 1) in Fig. 7.3 , it marks the end of
saturation.
Estimation of i 2e ðÞ may be done with the assumption that the derivatives of the
secondary current between the samples (n - 1) and (n - 2) are the same, as