ABB REG670 - Page 1257

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
163:
[ ] 5.8 1.35 ln= - ×
× >
æ ö
ç ÷
è ø
i
t s
k in
EQUATION1195 V1 EN (Equation 163)
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 164.
[ ] = + ×
-
>
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 (Equation 164)
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
165.
1MRK502052-UEN B Section 25
Inverse time characteristics
1251
Technical manual