ABB REU 523 Technical Reference Manual page 18

REU 523
18
Combined Overvoltage and Undervoltage Relay
Technical Reference Manual
Characteristic of the undervoltage stages
The IDMT characteristic curve group C is designed for undervoltage stages U< and
U<<. Stages U< and U<< can be configured to use different characteristics. The
relationship between time and voltage at IDMT characteristic can be expressed as
follows:
×
k a
t s [ ]
= ------------------------------------------------- -

U< U –
×
b ----------------- - 0.5 –

U<
where
t = operate time
k = time multiplier k< or k<<
U = measured voltage
U< = set start value for U< or U<<
a = constant 480
b = constant 32
c = constant 0.055
p = constant (see table 4.1.4.5-1)
The C-type characteristic is illustrated in Fig. 4.1.4.5.-3.
Table 4.1.4.5-1 Values of constant p
Time/voltage
characteristic
p
+ c
p


A B
2 3
1MRS 750942-MUM
C
2