Toshiba GRT100 Series, Instruction Manual

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6 F 2 S 0 7 8 9
2.3.2 Matching of CT Secondary Currents
In order to restrain erroneous differential currents, the currents supplied to the differential
elements must be matched in phase and amplitude under through-load and through-fault
conditions.
In GRT100, the matching is performed through the settings.
2.3.2.1 Matching of Phase Angle
It is necessary to compensate for phase angle difference among line currents on each side of the
transformer when the transformer windings have both star- and delta-connections.
GRT100 can compensate for the phase angle difference by the setting and does not require CT
secondary circuit arrangement such as delta-connection on the star-connected side of the power
transformer which was common for the former transformer protection.
The phase angle matching is performed by inputting the phase angle of each winding according
to the hands of a clock. For details of the setting, refer to 2.3.4.
2.3.2.2 Matching of CT Ratio
When I
1
to I
3
relevant to 1CT to 3CT secondary currents are supplied, the differential current I
d
is
calculated employing 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.
The setting kct1 is obtained by using the following equation.
kct1 = I
n
/I
base1
= I
n
/( 3

I
base1
) if the 1CT is delta-connected.
where
I
n
= rated secondary current of the 1CT.
I
base1
= secondary current of the 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.
The 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.