ABB REG650 Technical Manual page 111

1MRK 502 048-UEN A
Technical manual
The following currents are inputs to the power transformer differential protection
function. They must all be expressed in power system (primary) A.
1. Instantaneous values of currents (samples) from the HV, and LV sides for two-
winding power transformers, and from the HV, the first LV, and the second LV
side for three-winding power transformers.
2. Currents from all power transformer sides expressed as fundamental frequency
phasors with their real and imaginary parts. These currents are calculated within
the protection function by the fundamental frequency Fourier filters.
3. Negative sequence currents from all power transformer sides expressed as
phasors. These currents are calculated within the protection function by the
symmetrical components module.
The power transformer differential protection:
1. Calculates three fundamental frequency differential currents and one common
bias current. The zero-sequence component can optionally be eliminated from
each of the three fundamental frequency differential currents and at the same time
from the common bias current.
2. Calculates three instantaneous differential currents. They are used for harmonic,
and waveform analysis. Instantaneous differential currents are useful for post-
fault analysis using disturbance recording
3. Calculates negative-sequence differential current. Contributions to it from all
power transformer sides are used by the internal/external fault discriminator to
detect and classify a fault as internal or external.
Section 6
Differential protection
105