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Module 2 : Current and Voltage Transformers

Lecture 7 : CT Saturation and DC Offset Current

7.1

Origin of DC Off-set Current

Table 1

Time	t = 0	t =	t = 2	t = 4	t = 6	t = 8	t = 10
	1	0.3678	0.1353	0.0183	0.0024	0.0003	0.00004

It is more or less obvious that, dc offset is not seen in the waveform after 5 time constants. The peak value of dc offset current I₀can be worked out by setting the current i(t₀) to zero.
This implies that

Thus

(1)

Clearly, the peak value of dc offset current depends upon the following parameters:

Time at which fault strikes,

Phase angle

of ac voltage and

|Z_line| and

of transmission line.

Voltage V_m