
•
If there is a risk of intermittent faults. If the current IED, 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.
26.2
Principle of operation
IP15777-1 v2
26.2.1
Mode of operation
M16687-3 v7
The function can trip 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. Also programmable curve types are supported via the component inputs: p,
A, B, C pr, tr, and cr.
Different characteristics for reset delay can also be chosen.
If current in any phase exceeds the set pickup current value (here internal signal
pickupValue), 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.
For definite time delay curve ANSI/IEEE Definite time or IEC Definite time are
chosen.
The general expression for inverse time curves is according to equation
Section 26
1MRK 511 408-UUS A
Inverse time characteristics
882
Phasor measurement unit RES670 2.2 ANSI
Technical manual
Содержание Relion RES670
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