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ABB Substation Automation
Products and Systems
1MRS750886-MUM
User's Guide
Characteristic for undervoltage stages
The IDMT characteristic curve group C is designed for undervoltage stages U< and
U<<. The stages U< and U<< can be configured to use different characteristics. The
relationship between time and voltage at inverse-time characteristic can be
expressed as follows:
×
k a
t s [ ]
=
------------------------------------------------- -
U< U
–
×
b
----------------- - 0.5
–
U<
where
t = operate time [s]
k = time multiplier k< or k<<
U = measured voltage [V]
U< = set start voltage [V] for U< or U<<
a = constant 480
b = constant 32
c = constant 0.055
p = constant (see table 3)
The C-type characteristic is illustrated in Fig. 2.1.3.5.-3.
Table 3: The values of constant p
Time/current
A
characteristic
p
2
+
c
p
B
3
REU 513
C
2
21
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