ABB RELION REX640 Technical Manual page 749

1MRS759142 F
4.3.4.3 Functionality
The numerical stabilized low-impedance restricted earth-fault protection function
LREFPNDF for a two-winding transformer is based on the numerically stabilized
differential current principle. No external stabilizing resistor or non-linear resistor
are required.
The fundamental components of the currents are used for calculating the residual
current of the phase currents, the neutral current, differential currents and
stabilizing currents. The operating characteristics are according to the definite
time.
LREFPNDF contains a blocking functionality. The neutral current second harmonic is
used for blocking during the transformer inrush situation. It is also possible to block
function outputs, timers or the function itself.
4.3.4.4 Analog channel configuration
LREFPDIF has two analog group inputs which must be properly configured.
Table 678: Analog inputs
Input
I3P
IRES
There are a few special conditions which must be noted with the configuration.
Table 679: Special conditions
Condition
IRES measured
Improper analog channel configuration causes a validation error if the analog
channels are not completely configured or they do not match with certain settings.
For troubleshooting, check the contents of this chapter and also the preprocessing
blocks in this document. The configuration can be written to the protection relay
once the mismatch is corrected.
4.3.4.5 Operation principle
The function can be enabled and disabled with 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.
REX640
Technical Manual
See the preprocessing function blocks in this document for the possible
signal sources.
Protection functions
Description
Three-phase currents
Residual current (measured)
Description
The function requires that the current meas-
ured from the neutral side is connected.
Operation setting. The
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