ABB RED615 Technical Manual page 430

Section 4
Protection functions
424
Earth-fault detector
The operation is based on comparing the amplitude and the phase difference
between the sum of the fundamental frequency component of the phase currents
(ΣI, residual current) and the fundamental frequency component of the neutral
current (Io) flowing in the conductor between the transformer or generator's neutral
point and earth. The differential current is calculated as the absolute value of the
difference between the residual current, that is, the sum of the fundamental
frequency components of the phase currents I_A, I_B and I_C, and the neutral
current. The directional differential current ID_COSPHI is the product of the
differential current and cosφ. The value is available in the monitored data view.
ID COSPHI _ = ( Σ I Io − ) cos
GUID-46782962-D465-47D2-8ECE-3FF0B87B324F V3 EN
ΣI
GUID-87E4DEDD-9288-41D9-B608-714CF3CC7A04 V1 EN
ϕ
GUID-C4F98C50-7279-4DAA-8C77-5C761572F4B4 V1 EN
Io
GUID-2D713C98-4F81-4DF4-8193-C47120A65489 V1 EN
An earth fault occurring in the protected area, that is, between the phase CTs and
the neutral connection CT, causes a differential current. The directions, that is, the
phase difference of the residual current and the neutral current, are considered in
the operation criteria to maintain selectivity. A correct value for CT connection
type is determined by the connection polarities of the current transformer.
The current transformer ratio mismatch between the phase current
transformer and neutral current transformer (residual current in the
analog input settings) is taken into account by the function with the
properly set analog input setting values.
During an earth fault in the protected area, the currents ΣI and Io are directed
towards the protected area. The factor cosφ is 1 when the phase difference of the
residual current and the neutral current is 180 degrees, that is, when the currents are
in opposite direction at the earth faults within the protected area. Similarly,
ID_COSPHI is specified to be 0 when the phase difference between the residual
current and the neutral current is less than 90 degrees in situations where there is
no earth fault in the protected area. Thus tripping is possible only when the phase
difference between the residual current and the neutral current is above 90 degrees.
The stabilizing current IB used by the stabilizing current principle is calculated as
an average of the phase currents in the windings to be protected. The value is
available in the monitored data view.
×
ϕ
Residual current
Phase difference between the residual and neutral currents
Neutral current
1YHT530004D05 D
(Equation 56)
615 series
Technical Manual