FUNCTION CHARACTERISTICS
305
— Underfrequency - 81U
Preface
The protection is available in the XMR-V, XMR-P and XMR-T versions.
Four operation thresholds, independently adjustable with adjustable delay are provided.
Each threshold may be separately enabled or disabled.
Operation and settings
The frequency, acquired from U
12
,
U
23
,
U
31
phase-to-phase input voltages
[1]
is compared with the
setting values; a start is issued when the frequency goes down the adjustable threshold (START);
after expiry of the associated operate time a trip command is issued; if instead the frequency exce-
eds the threshold, the element is restored.
When the frequency value is out of the lock range, it is fixed to the upper or lower limit (20 Hz or 60 Hz).
All elements operate with definite time characteristic.
Each element can be enabled or disabled by setting ON or OFF the State parameter inside the Set
\ Profile A (or B) \ Underfrequency-81U \ f< Element (f<< Element, f<<< Element, f<<<< Element) \ De-
finite time menu.
The underfrequency protection is enabled only when the maximum of the phase-to-neutral input
voltages U
LMAX
=max(U
L1
, U
L2
, U
L3
) overcomes 0.2 U
n
for a tfENadjustable time.
Setting of the tfEN value is available inside the Set \ Base menu with “level 1” session.
The trip of f< element may be inhibited by the start of the second, third and/or fourth element (f<<,
f<<<, f<<<<) by setting ON the Disable f< by start f<<, Disable f< by start f<<<, Disable f< by start
f<<<< (
f<disbyf<<,f<disbyf<<<,f<disbyf<<<<) parameters available inside the Set \ Pro-
file A (or B) \ Underfrequency-81U \ f<< Element (f<<< Element,f<<<< Element) \ Setpoints menus.
Similarly the trip of the:
• f<< element may be inhibited by start of the third and/or fourth element (f<<< and/or f<<<<) by setting
ON the Disable f<< by start f<<<, start f<<<< (
f<<disbyf<<<,f<<disbyf<<<<) parameter avai-
lable inside the Set \ Profile A (or B) \ Underfrequency-81U \ f<<< Element, (f<<<< Element) \ Setpoints
menus.
• f<<< element may be inhibited by start of the fourth element (f<<<<) by setting ON the Disable f<<<
by start f<<<< (f<<<disbyf<<<<) parameter available inside the Set \ Profile A (or B) \ Under-
frequency-81U \ f<<<< Element \ Setpoints menu.
Breaker failure (BF)
Each thresholds (f<, f<<,f<<<,f<<<<) can be associated to BF (H) and BF (L) protection by activating
the relative parameter in the matrices “Selection of function tripping for BF (H)” or “Selection of
function tripping for BF (L)” in relevant BF menus
[2]
:
• Set \ Profile A (or B) \ Breaker failure - BF side H
• Set \ Profile A (or B) \ Breaker failure - BF side L
Note 1 The frequency can also be measured on the three phase-to-phase voltages (calculated vectorially)
Note 2 The common settings concerning the Breaker failure protection are adjustable inside the Breaker Failure - BF menu.
f
f<<<<
def
f<<<
def
f
<<
def
f<
def
t
t
f
def
<<
t
f
def
<<<
t
f
def
<<<<
t
f
def
<
General operation time characteristic for the underfrequency elements - 81U
TRIP
Timers-F81.ai
t
fEN
81O-81U Enable
81O-81U disabled 81O-81U disabled81O-81U enabled
0.20 U
n
Max (U
L1
,U
L2
,U
L3
)
t
Overfrequency & underfrequency enable timer
XMR-D EQUIPMENT MANUAL
Ed. 2.9 - 02/2021
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