ABB REG650 Technical Manual page 97

1MRK 502 034-UEN -
Technical Manual
Figure 38 shows how internal treatment of measured currents is done in case of two-
winding transformer.
The following currents are inputs to the power transformer differential protection
function. They must all be expressed in true power system (primary) A, that is, as
measured.
1. Instantaneous values of currents (samples) from HV, and LV sides for two-
winding power transformers, and from the HV, the first LV, and the second
LV sides 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 both
(all three) power transformer sides are used by the internal/external fault
discriminator to detect and classify a fault as internal or external.
Section 5
Differential protection
91