ABB RELION REX640 Technical Manual page 673

Protection and control

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1MRS759142 F

In-zone transformer

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.

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

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.

Technical Manual

Protection functions

(Equation 156)