Operation Manual 37107F
MFR 3 - Multi Function Relay
© Woodward
Page 103/165
Inverse Time-Overcurrent Monitoring with Voltage Restraint
NOTE
This monitoring function is an additional functionality for the inverse time overcurrent monitoring func-
tion. If the inverse time overcurrent monitoring is disabled (Parameter 140), time-overcurrent monitor-
ing with voltage restraint is disabled too.
All percentage indications of the current are in relation to the generator rated current (Parameter 29).
Function:
This function is recommended for a generator that must be monitored with droop excitation and pre-
cautions for short-circuit excitation (e.g. supplementary components) are not available. A short-circuit close to
the terminal may be caused due to the low voltage excitation cannot be maintained. As a result, the unit cannot
maintain power in order to initiate a voltage independent overcurrent delay. The voltage restraint functionality
reduces the overcurrent threshold of the inverse time overcurrent monitoring function proportionally with the
monitored voltage if this function is enabled. If the monitored voltage falls below the threshold defined by the
knee curve setting (Parameter 146), the overcurrent threshold remains at the value of the knee curve setting. The
reduction of the inverse time threshold occurs according to Figure 6-5.
Current L1: corresponds to voltage L1-L2
Current L2: corresponds to voltage L2-L3
Current L3: corresponds to voltage L3-L1
20
20
90
I
[%]
a
p
I
90
V [%]
100
n
V
Figure 6-5: Characteristic of the inverse time-overcurrent monitoring with voltage restraint (knee curve setting 20 %)
Legend: I
a
Adjusted current threshold value
I
p
Configured value (Parameter 143)
V
n
Rated
voltage
V
Monitored
voltage
