ABB RES670 Technical Manual page 892

Section 26
Inverse time characteristics
886
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 92:
æ
i
[ ]
s = - ×
t 5.8 1.35 ln ç
è
×
td Pickupn
EQUATION1648 V1 EN-US
where:
Pickupn is the set pickup current for step n,
td 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 93.
æ
ç
ç A
[ ]
+
s =
t
ç
P
æ ö
i
ç
-
ç ÷
C
ç
è ø
è
Pickupn
EQUATION1640 V1 EN-US
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 pickup 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 pickup 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 pickup current level minus the hysteresis).
The timer will reset according to equation 94.
ö
÷
ø
ö
÷
÷
B ×
td
÷
÷
÷
ø
1MRK 511 408-UUS A
Phasor measurement unit RES670 2.2 ANSI
Technical manual
(Equation 92)
(Equation 93)