ABB REC650 ANSI, Technical Manual

Page: 664 / 700

• If there is a risk of intermittent faults. If the current relay, close to the faults, picks up
and resets there is a risk of unselective trip from other protections in the system.
• Delayed resetting could give accelerated fault clearance in case of automatic
reclosing to a permanent fault.
• Overcurrent protection functions are sometimes used as release criterion for other
protection functions. It can often be valuable to have a reset delay to assure the release
function.
19.2 Operation principle
19.2.1 Mode of operation
The function can operate in a definite time-lag mode or in a current definite inverse time
mode. For the inverse time characteristic both ANSI and IEC based standard curves are
available.
If current in any phase exceeds the set pickup current value , a timer, according to the
selected operating mode, is started. The component always uses the maximum of the three
phase current values as the current level used in timing calculations.
In case of definite time-lag mode the timer will run constantly until the time is reached or
until the current drops below the reset value (pickup value minus the hysteresis) and the
reset time has elapsed.
The general expression for inverse time curves is according to equation 75 .
[ ]
= ×
-
+
æ ö
ç ÷
ç ÷
ç ÷
æ ö
ç ÷
ç ÷
ç ÷
è ø
è ø
P
A
t td
i
C
Pickupn
s B
EQUATION1640 V1 EN
(Equation 75)
where:
p, A, B, C are constants defined for each curve type,
Pickupn is the set pickup current for step n,
td is set time multiplier for step n and
i is the measured current.
Section 19
1MRK 511 287-UUS A
Time inverse characteristics
658
Technical manual