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Setting of ik
ik determines the minimum operation sensitivity of the DIF element. ik is set as a ratio to the CT
secondary rated current.
The minimum sensitivity setting ik is determined from the maximum erroneous differential
current under normal operating conditions.
Setting of p1, p2 and kp
Percentage restraining factor (% slope)
= (Differential current) / (Through current)
= (Differential current) / [{(Incoming current) + (Outgoing current)} /2]
p1 is the percentage restraining factor which defines the DIF restraining characteristic in the small
current region. The setting is determined by the sum of:
• 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
Kh is the HOC setting and should be set above the estimated maximum inrush current.
The recommended setting is more than “Maximum peak value of Inrush current” × kct.
Setting for CT ratio matching
Taking the transformer shown in Figure 2.2.5.1 as an example, the CT ratio matching settings kct1
to kct3
can be calculated as follows. For transformer capacity, take the maximum of the rated
capacites of the three windings.