Functions
2.7 Motor Protection (Motor Starting Protection 48, Motor Restart Inhibit 66, Load Jam Protection)
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A5, Release date 11.2012
136
Total Time T
Reclose
The total waiting time T
Reclose
before the motor can be restarted is therefore composed of the equilibrium time
and the time T
Restart
calculated from the thermal replica, and the value that is needed to drop below the limit for
restarting. If the calculated temperature rise of the rotor is above the restarting limit when the motor is shut
down, the minimum inhibit time will be started together with the equilibrium time.
Thus the total inhibit time T
Reclose
can become equal to the minimum inhibit time if it is longer than the sum of
the two first mentioned times:
T
Reclose
= T
Equal
+ T
Restart
for T
Min Inhibit
< T
Equal
+ T
Restart
T
Reclose
= T
Min Inhibit
for T
Min Inhibit
≥ T
Equal
+ T
Restart
if the calculated excessive temperature > re-
starting limit
The operational measured value 809 T
Reclose
(visible in the thermal measured values) is the remaining time until
the next restart is permissible. When the rotor excessive temperature is below the restarting limit and thus the
next restarting attempt is permitted, the operational measured value for the waiting time has reached zero.
Extension of Cool Down Time Constants
In order to properly account for the reduced heat exchange when a self-ventilated motor is stopped, the cool
down time constants can be increased relative to the time constants for a running machine with the factor Kτ
at STOP (address 4308). The criterion for the motor stop is the undershooting of a set current threshold
BkrClosed I MIN. This understands that the motor idle current is greater than this threshold. The pickup
threshold BkrClosed I MIN affects also the thermal overload protective function (see Section 2.9).
While the motor is running, the heating of the thermal replica is modeled with the time constant τ
R
calculated
from the motor ratings, and the cool down calculated with the time constant τ
R
· Kτ at RUNNING (address
4309). In this way, the protection caters to the requirements in case of a slow cool down (slow temperature
equilibrium).
For calculation of the restarting time T
Restart
the following applies:
with
k
t at STOP
Extension factor for the time constant = Kτ at STOP, address 4308
k
t at RUNNING
Extension factor for the time constant = Kτ at RUNNING, address 4309
Θ
pre
thermal replica at the moment of motor shutdown (depending on operational condition)
τ
R
Rotor time constant, calculated internally
Behavior in Case of Power Supply Failure
Depending on the setting in address 235 ATEX100 of Power System Data 1 (see Section 2.1.3.2) the value of
the thermal replica is either reset to zero (ATEX100 = NO) if the power supply voltage fails, or cyclically buffered
in a non-volatile memory (ATEX100 = YES) so that it is maintained in the event of auxiliary supply voltage fail-
ure. In the latter case, when power supply is restored, the thermal replica uses the stored value for calculation
and matches it to the operating conditions. The first option is the default setting. For further details, see /5/.
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