ABB ret650, Technical Manual

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• If there is a risk of intermittent faults. If the current relay, close to the faults, starts
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.
21.2 Operation principle
21.2.1 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 start 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 (start value minus the hysteresis) and
the reset time has elapsed.
The general expression for inverse time curves is according to equation 108 .
[ ]
= + ×
-
>
æ ö
ç ÷
ç ÷
ç ÷
æ ö
ç ÷
ç ÷
è ø
è ø
p
A
t s B k
i
C
in
EQUATION1189 V1 EN
(Equation 108)
where:
p, A, B, C are constants defined for each curve type,
in> is the set start current for step n,
k is set time multiplier for step n and
i is the measured current.
For inverse time characteristics a time will be initiated when the current reaches the set
start level. From the general expression of the characteristic the following can be seen:
Section 21
1MRK 504 135-UEN A
Time inverse characteristics
628
Technical manual