Fundamental Frequency Differential Currents - ABB Relion Technical Manual

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Differential protection

Fundamental frequency differential currents

The fundamental frequency differential current is a vectorial sum (sum of fundamental

frequency phasors) of the individual phase currents from the different sides of the

protected power transformer.

Before any differential current can be calculated, the power transformer phase shift, and

its transformation ratio, must be accounted for. Conversion of all currents to a common

reference is performed in two steps:

all current phasors are phase-shifted to (referred to) the phase-reference side,

(whenever possible the first winding with wye connection)

all currents magnitudes are always referred to the first winding of the power

transformer (typically transformer high-voltage side)

The two steps of conversion are made simultaneously on-line by the pre-programmed

coefficient matrices, as shown in equation

for a three-winding power transformer.

These are the internal compensation algorithms within the differential

function. The protected power transformer data is always entered per its

nameplate. The Differential function will adapt nameplate data and select

proper reference windings.

EQUATION1879 V1 EN

is the resulting Differential Currents

is the current contribution from the W1 side

is the current contribution from the W2 side

for a two-winding power transformer, and in

1MRK 502 048-UUS A

(Equation 1)

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