ABB RELION 620 Series Technical Manual page 554

Section 4
Protection functions
4.3.3.4
548
Operation principle
The function can be enabled and disabled with the Operation setting. The
corresponding parameter values are "On" and "Off".
The operation of LREFPNDF can be described using a module diagram. All the
modules in the diagram are explained in the next sections.
GUID-079EB39A-B62C-47DF-9B53-9432AB24A9CE V2 EN
Figure 276: Functional module diagram
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.
_ ( ) cos
ID COSPHI I Io
= Σ −
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
× ϕ
Residual current
Phase difference between the residual and neutral currents
Neutral current
1MRS757644 F
(Equation 81)
620 series
Technical Manual