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In α-method phase matching, since the DIFT provides a function to eliminate the zero-sequence
current by software, the DIFT is insensitive to the fault described.
In β-method phase matching, however, since the zero-sequence current is not eliminated because
of Star connection on the Delta-winding side, the DIFT may operate unnecessary.
In case the GRT100 is applied to a transformer with in-zone earthing transformer, the [Phase
matching] = “Alpha” setting is recommended.
2.2.3.4 Matching of CT Ratio
If I
1
to I
3
correspond to 1CT to 3CT
secondary currents, differential current I
d
is calculated
according to the following equation,
I
d
= kct1⋅I
1
+ kct2⋅I
2
+ kct3⋅I
3
where kct1 to kct3 are settings corresponding to 1CT to 3CT.
Setting kct1 is obtained by using the following equation.
kct1 = I
n
/I
base1
= I
n
/( 3 × I
base1
) if 1CT is delta-connected.
where
I
n
= rated secondary current of 1CT (1A or 5A)
I
base1
= secondary current of 1CT based on the kVA rating of the power transformer.
= transformer capacity(kVA)/(
3 × rated voltage(kV)) × CT ratio of 1CT
If the 1CT secondary circuit is delta-connected,
3 × I
base1
is used instead of I
base1
in the equation
above.
Settings kct2 and kct3 are obtained in the same way.
The differential current I
d
is zero under through-load and through-fault conditions.
kct1 × I
1
to kct3 × I
3
are equal to the rated secondary current of each CT when the rated line
currents based on the kVA rating of the power transformer flow.