Functionality
22 USER MANUAL FPC 200 - 3/2017
Earth fault overcurrent protection - ANSI code 50/51|N/G
Earth fault overcurrent protection is one of the basic functions of FPC 200 numerical relay. It comprises of various time-
delayed characteristics. Protective function includes CLP and inhibition by Inrush restraint [2.2.5] function.
2.2.2.1 Functionality
Earth fault overcurrent function is used as non-
directional earth fault protection. Pickup value of this
function is normally set lower than pickup value of
phase overcurrent protection. Protection picks up when
the earth fault current exceeds the set threshold value.
The function can be enabled or disabled through
corresponding menu. Trip time characteristics can be
selected to be Instantaneous, Definite time (DT) or
Inverse definite minimum time (IDMT). When
instantaneous operation mode is selected the trip signal
is stated as the DT characteristic.
Several earth fault protection instances with different
settings can run independently at the same time.
The selection of pickup value, pickup delay as well as
drop-out ratio and drop-out delay helps the user to fine
tune the protection according to the project
specifications.
50N/51N Ground prot
Mode:
Enable:
IDMT coeficient:
0,02
ON
Pickup value: 1,2xIn
A
Trip delay: 0,4s
Ext. inverse
1A 1B 2A 2B 3A 3B 4A
Figure 2.18: Earth fault overcurrent setting.
2.2.2.2 Measurements
Earth fault current is acquired directly through analog
measurement or calculated.
Sensitive current input can be selected for direct earth
fault measurement. When sensitive analog input I
e
is
not present earth current 3I
0
is calculated using phase
currents analog measurements.
2.2.2.3 Delays
Function includes following delays explained in Chapter
2.1:
Pickup delay
Trip delay
Drop-out delay
2.2.2.4 Cold load pickup
A temporary increased starting current can appear
when energizing feeder loads that had a long zero
voltage period (e.g. air-conditioning systems, heating
installations, motors...). Its value can be up to several
times higher than the nominal current. To avoid
unwanted protection operation the pickup limit has to
be raised temporarily. The function is set with
parameter Enable. It triggers only when CB Close
command is executed manually.
The dynamic pickup value changeover is common to all
overcurrent elements. With inverse characteristics, the
CLP influences only the fault detection limit, whereas
the time calculation of the selected inverse
characteristic is not affected.
Pickup value
Drop-out value
CLP activated
Monitored value
Fault area
Fault detection
Figure 2.19: Operation with CLP activated for a shorter than
fault time duration.
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