Inverse Time Characteristics - ABB REB650 Technical Manual

1MRK 505 277-UUS C
19.3
Technical Manual
æ
ç
td
[ ]
ç
s
=
t
Pickupn
ç
ç
- ×
0.339 0.235
è
EQUATION1647 V1 EN
where:
Pickupn is the set pickup current for step n
td is set time multiplier for step n
i is the measured current
The RD inverse curve gives a logarithmic delay, as used in the Combiflex protection
RXIDG. The curve enables a high degree of selectivity required for sensitive residual
ground-fault current protection, with ability to detect high-resistive ground faults. The
curve is described by equation 78:
æ
i
[ ]
s = - ×
t 5.8 1.35 ln ç
è ×
td Pickupn
EQUATION1648 V1 EN
where:
Pickupn is the set pickup current for step n,
td is set time multiplier for step n and
i is the measured current
The timer will be reset directly when the current drops below the set pickup current
level minus the hysteresis.

Inverse time characteristics

When inverse time overcurrent characteristic is selected, the operate
time of the stage will be the sum of the inverse time delay and the set
definite time delay. Thus, if only the inverse time delay is required, it is
of utmost importance to set the definite time delay for that stage to zero.
Time inverse characteristics
ö
÷
÷
÷
÷
ø
i
ö
÷
ø
Section 19
(Equation 77)
(Equation 78)
465