Inverse Time Characteristics - ABB RET650 Technical Manual

1MRK 504 135-UEN A
21.3
Technical manual
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
earth-fault current protection, with ability to detect high-resistive earth faults. The
curve is described by equation 114:
æ
= - ×
t s [ ] 5.8 1.35 ln ç
è
k in
EQUATION1195 V1 EN
where:
in> is the set start current for step n,
k 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 start 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.
Table 559: ANSI Inverse time characteristics
Function
Operating characteristic:
æ ö
A
ç ÷
= + × +
t B k tDef
( )
ç ÷
P
ç - ÷
I 1
è ø
EQUATION1249-SMALL V2 EN
I = I /I
measured set
ANSI Extremely Inverse
ANSI Very inverse
ANSI Normal Inverse
ANSI Moderately Inverse
ANSI Long Time Extremely Inverse
ANSI Long Time Very Inverse
ANSI Long Time Inverse
Time inverse characteristics
ö
i
÷
× > ø
Range or value
k = (0.05-999) in steps of 0.01
A=28.2, B=0.1217, P=2.0
A=19.61, B=0.491, P=2.0
A=0.0086, B=0.0185,
P=0.02, tr=0.46
A=0.0515, B=0.1140, P=0.02
A=64.07, B=0.250, P=2.0
A=28.55, B=0.712, P=2.0
A=0.086, B=0.185, P=0.02
Section 21
(Equation 114)
Accuracy
-
631