ABB RED650, Technical Manual, Page: 881 / 926

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
98 :
[ ] 5.8 1.35 ln
= - ×
× >
æ ö
ç ÷
è ø
i
t s
k in
EQUATION1195 V1 EN-US
(Equation 98)
where:
in>
is the set start current for step n,
k is set time multiplier for step n and
i is the measured current
If the curve type programmable is chosen, the user can make a tailor made inverse
time curve according to the general equation
99 .
[ ]
= + ×
-
>
æ ö
ç ÷
ç ÷
ç ÷
æ ö
ç ÷
ç ÷
è ø
è ø
p
A
t s B k
i
C
in
EQUATION1196 V1 EN-US
(Equation 99)
Also the reset time of the delayed function can be controlled. There is the
possibility to choose between three different reset time-lags.
• Instantaneous Reset
• IEC Reset
• ANSI Reset.
If instantaneous reset is chosen the timer will be reset directly when the current
drops below the set start current level minus the hysteresis.
If IEC reset is chosen the timer will be reset after a set constant time when the
current drops below the set start current level minus the hysteresis.
If ANSI reset time is chosen the reset time will be dependent of the current after
fault clearance (when the current drops below the start current level minus the
hysteresis). The timer will reset according to equation
100 .
1MRK 505 394-UEN A
Section 25
Inverse time characteristics
Line differential protection RED650 2.2 IEC 875
Technical manual