ABB 615 series Technical Manual page 499

1MAC059074-MB A
615 series ANSI
Technical Manual
divided by the square root of three times the nominal phase-to-phase voltage at the HV or
LV side.
S
n
I
=
nT
⋅ 3 V
n
GUID-FD7DAEE6-80AB-4C5E-98C7-863063099802 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 87L coordinates with the downstream protection relays 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.
RED 615
400A/1
PROTECTED ZONE
33kV
500 kVA
 
I . 8 75 A
L

3 33 kV
 
. 8 75 A
 
I * 1A . 0 02
 

400 A
 
GUID-F1B36FF9-7463-4D8D-8EDC-70A09B52CAE9 V2 EN
Figure 244: Influence of the tapped transformer load current to the stabilized low
stage setting
500kVA

A 2 %
Section 4
Protection functions
(Equation 80)
RED 615
400A/1
493