Toshiba GRT100 Series Instruction Manual page 18


CT accuracy error (generally considered as 5%)

Tap error: Error between maximum/minimum tap and the middle tap when taking the middle
tap of the tap changer as a reference.

Matching error: The error due to CT mismatch may be small enough to be neglected in the
setting.

Relay calculation error, and others (5%)
The recommended setting is "Sum of above"  1.5 (margin).
p2 is the percentage restraining factor which defines the restraining characteristic in the large
current region. The setting is determined from the maximum erroneous differential current which
is generated when a large through fault current flows.
kp is the break point of the dual percentage restraining characteristics. It is set above the
maximum operating current level of the transformer between the maximum forced-cooled rated
current and the maximum emergency overload current level, as a ratio to the CT secondary rated
current.
Setting of k2f
k2f is set to detect the second harmonic content in the inrush current during transformer
energization and blocks GRT100 to prevent incorrect operation due to the inrush current. A
setting of 15% is suggested if there is no data on the minimum second harmonic content.
Setting of k5f
k5f is set to detect the fifth harmonic content during transformer over-excitation and blocks
GRT100 to prevent incorrect operation due to transient over-excitation conditions.
A setting of 30% is suggested if there is no data on the minimum fifth harmonic content.
Setting of k
h
Set above the estimated maximum inrush current.
Setting for CT ratio matching
Taking the transformer shown in Figure 2.3.4 as an example, the CT ratio matching settings kct1
to kct3 can be calculated as follows. For transformer capacity, take the maximum one from the
rated capacity of the three windings.
Calculation steps
(1) Transformer capacity (kVA)
(2) Voltage(kV)
(3) Rated line current(A)
=(1)/( 3  (2))
(4) CT ratio
(5) Secondary rated line current(A) =(3)/(4)
(6) CT secondary rating(A)
(7) Setting =(6)/(5)
Note: Using the ratio of "CT rated-current (IN)" to "Transformer rated-current (IT)", the user can
obtain a kct value (=IN/IT). We recommend the user chooses the CT whose rated-current is not
higher than the transformer rated-current multiplied by 2 so that the DIFT function can obtain the
Primary
154
150
60
2.50
5
Kct1=2.00
 17 
6 F 2 S 0 7 8 9
Secondary Tertiary
40  10
3
66 11
350 2100
120 240
2.92 8.75
5 5
Kct2=1.71 Kct3=0.57