ABB RED615 Technical Manual page 383

1YHT530004D05 D
615 series
Technical Manual
S
n
I
=
n Τ
3 U
×
n
GUID-0024F590-C940-4636-A56B-A8C13A5F1BBA V1 EN
The rated load current of the transformer on the HV side is 209.9 A (40 MW / (√3
× 110 kV)) and the rated load current of the transformer on the LV side is 1154.7 A
(40 MW / (√3 × 20 kV)). This means that the CT ratio corrections for the HV and
LV sides are:
CT ratio correction (HV) = 1.429 (300 A / 209.9 A)
CT ratio correction (LV) = 1.299 (1500 A / 1154.7 A)
Small power transformers in a tap
With a relatively small power transformer in a line tap, the line differential
protection can be applied without the need of current measurement from the tap. In
such cases, the line differential function is time delayed for low differential
currents below the high set limit and LNPLDF coordinates with the downstream
IEDs in the relevant tap. For differential currents above the set limit, the operation
is instantaneous. As a consequence, when the load current of the tap is negligible,
the low resistive line faults are cleared instantaneously at the same time as
maximum sensitivity for the high resistive faults are maintained but with a time
delayed operation. The maximum sensitivity for high resistive faults is maintained
at the same time, but with a time delayed operation.
GUID-F1B36FF9-7463-4D8D-8EDC-70A09B52CAE9 V1 EN
Figure 201: Influence of the tapped transformer load current to the stabilized
low stage setting
The stabilized stage provides both DT and IDMT characteristics that are used for
time selective protection against faults which are not covered by the instantaneous
stage. The impedance of the line is typically an order of magnitude lower than the
Section 4
Protection functions
(Equation 43)
377