ABB RELION REX640 Technical Manual page 591

1MRS759142 C
REX640
Technical Manual
S
n
I
=
n
Τ
U
⋅ 3
n
GUID-BAB5307B-F247-4420-B98E-DDCCFD2E80FD 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 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
 

400 A
 
GUID-7240593E-F265-46A0-B991-CD77E12ADD41 V1 EN
Figure 324: 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
transformer impedance providing significantly higher fault currents when the fault is
located on the line.
500kVA

02 A 2 %
Section 4
Protection functions
(Equation 103)
RED 615
400A/1
585