Iskra FPC 200 - Underfrequency Protection - ANSI Code 81 L

Functionality
44 USER MANUAL FPC 200 - 3/2017
Underfrequency protection - ANSI code 81L
Underfrequency protection is a function that protects the feeder or other elements of the power system from
undesirable frequencies. It comprises of time-delayed characteristic and block on derivative of frequency with respect to
time, which can be used to prevent operation of breaking element in certain frequency transient conditions.
2.4.2.1 Functionality
Underfrequency protection is a function, used as time
delayed protection. It picks up when measured
frequency is lower than the selected threshold. The
function can be enabled or disabled through
corresponding menu.
Several 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.
81L Underfrequency
Enabled:
1A 1B 2A 2B
No
Pickup value:
-0,50Hz
Trip delay:
60,0s
CONTROL SETTINGS
Pickup delay: 5ms
Figure 2.31: Underfrequency protection setting as seen on
HMI.
2.4.2.2 Measurement
Frequency is determined based on healthy analog
acquisition line measurement with priority of phase
voltage measurements first, than phase current
measurements. In addition the healthy line is
considered as a line which value is nearest to U
n
or I
n
.
2.4.2.3 Delays
Function includes following delays explained in Chapter
2.1:
 Pickup delay
 Trip delay
 Drop-out delay
2.4.2.4 Block settings
Pickup of any instance separately of protection can be
individually blocked by:
 Any user defined signal [2.6.5].
 derivative of frequency with respect to time
 minimal voltage level