ABB RELION REX640 - Page 673

In-zone transformer
20kV
300A/11500A/1
PROTECTED
ZONE
110kV
Yd11
40MW
HVLV
L ~
RED 615
RED 615
Figure 382: In-zone transformer example about CT ratio correction calculation
The CT ratio correction calculation starts with the rated load current calculation
for HV and LV sides. The rated load current is defined as the rated power of the
transformer divided by the square root of three times the nominal phase-to-phase
voltage at the HV or LV side.
I
S
U
nT
n
n
=
×3
(Equation 156)
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.
1MRS759142 F
Protection functions
REX640
Technical Manual
673