By default, 51NHA is used as a standalone mode. Substation-wide application can be
achieved using horizontal communication where the detection of a faulty feeder is done
by comparing the harmonics ground-fault current measurements.
The function picks up when the harmonics content of the ground-fault current exceeds the
set limit. The operation time characteristic is either definite time (DT) or inverse definite
minimum time (IDMT). If the horizontal communication is used for the exchange of
current values between the protection relays, the function operates according to the DT
characteristic.
The function contains a blocking functionality. It is possible to block function outputs,
timer or the function itself, if desired.
4.2.5.4
Operation principle
The function can be enabled and disabled with the
Operation
setting. The corresponding
parameter values are “Enable” and “Disable”.
The operation of 51NHA can be described using a module diagram. All the modules in the
diagram are explained in the next sections.
PICKUP
Harmonics
calculation
BLOCK
TRIP
IG
I_REF_RES
Current
comparison
Level
detector
Blocking
logic
t
Timer
t
GUID-D9185AD6-2682-46EE-B6A8-5DB16FB2AC9A V1 EN
Figure 193:
Functional module diagram
Harmonics calculation
This module feeds the measured residual current to the high-pass filter, where the
frequency range is limited to start from two times the fundamental frequency of the
network (for example, in a 50 Hz network the cutoff frequency is 100 Hz), that is,
summing the harmonic components of the network from the second harmonic. The output
of the filter, later referred to as the harmonics current, is fed to the Level detector and
Current comparison modules.
Section 4
1MAC059074-MB A
Protection functions
402
615 series ANSI
Technical Manual
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