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When phase-to-ground voltage measurement is selected the function
automatically introduces division of the base value by the square root of
three.
9.2.7.1
Measurement principle
All the three voltages are measured continuously, and compared with the set values,
Pickup1
for Step 1 and
Pickup2
for Step 2. The parameters
OpMode1
and
OpMode2
influence the requirements to activate the PICKUP outputs. Either
1 out of 3
,
2 out of 3
or
3 out of 3
measured voltages have to be higher than the corresponding set point to issue the
corresponding PICKUP signal.
To avoid oscillations of the output PICKUP signal, a hysteresis is included.
9.2.7.2
Time delay
The time delay for step 1 can be either definite time delay (DT) or inverse time overvoltage
(TOV). Step 2 is always definite time delay (DT). For the inverse time delay three
different modes are available:
•
inverse curve A
•
inverse curve B
•
inverse curve C
The type A curve is described as:
t
TD
V
Vpickup
Vpickup
=
−
>
>
EQUATION1625 V2 EN
(Equation 81)
The type B curve is described as:
t
TD
V
Vpickup
Vpickup
=
⋅
⋅
−
>
>
−
−
480
32
0 5
0 035
.
.
ANSIEQUATION2287 V2 EN
(Equation 82)
The type C curve is described as:
t
TD
V
Vpickup
Vpickup
=
⋅
⋅
−
>
>
−
+
480
32
0 5
0 035
.
.
ANSIEQUATION2288 V2 EN
(Equation 83)
1MRK 502 048-UUS A
Section 9
Voltage protection
291
Technical manual
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