82
FUNCTION CHARACTERISTICS
Both thresholds have a operating characteristic circular in plan R, X with definite time curve.
The operating characteristic is a circle with its center located in the source impedance of the plan.
The element trips when the representative point of the calculated impedance is inside the circle
corresponding to the threshold set.
Both underimpedance elements may be enabled or disabled by setting ON or OFF the State para-
meter inside the Set \ Profile A or B \ Underimpedance-21 \ Z< (Z<<) Element \Definite time).
The first threshold trip may be inhibited by start of the second threshold by setting ON the
Z<disby
Z<< parameter available inside the Set \ Profile A or B \ Underimpedance-21 \ Z< (Z<<) Element \
Setpoints menu.
The elements are enabled in the 20 Hz ... 70 frequency range for each phase, if the corresponding
phase voltage is greater than 1% U
n
and the current is greater than 5% I
n
.
Since the impedance calculation for the three independent phases, a possible inhibition on stage
has no effect on the rest.
t-int-F21.ai
Z
Z<< Z<
t
Z
<<
t
t
Z
<
TRIP
General operation time characteristic curve for the underimpedance element - 21
F21_char.ai
X (p.u. Z
N
)
R (p.u. Z
N
)
0.5
1.0
Z <
Z <<
2.0 3.0
TRIP
TRIP
NO TRIP
R,X operatingcharacteristic of the impedance protection - 21
General logic diagram of the underimpedance elements - 21
all-F21.ai
Z
ON=inhibit
2nd Pickup Element 1st Pickup Element
Block1
Block1
Z
Block2 Block2
Block3
Block1
Block2
Block3
&
Block1
Block2
State State
Z<<
def
t
Z
<<
def
t
Z<<RES
Z<disbyZ<<
Z<
def
t
Z
<
def
t
Z<RES
Z< inhibition
Z< Start
Z< Trip
Z<< Start
Z<< Start
Z<< Trip
XMR-D EQUIPMENT MANUAL
Ed. 2.9 - 02/2021
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