ABB RELION 620 Series Technical Manual page 620

Section 6
Supervision functions
614
this mode of operation, the value of the setting is in the range of around 100
milliseconds.
• In the "IIt, CB" or "IIt & stall, CB" modes, the purpose of this setting is to check for
the life of the protection scheme after the CB_CLOSED input has been activated.
Based on the values of the phase currents, the completion of the start-up period cannot
be judged. So in this mode of operation, the value of the time delay setting can even
be as high as within the range of seconds, for example around 30 seconds.
The activation of the BLOCK input signal deactivates the MOT_START output.
Thermal stress calculator
Because of the high current surges during the start-up period, a thermal stress is imposed
on the rotor. With less air circulation in the ventilation of the rotor before it reaches its full
speed, the situation becomes even worse. Consequently, a long start-up causes a rapid
heating of the rotor.
This module calculates the thermal stress developed in the motor during start-up. The heat
developed during the starting can be calculated with the equation.
t
2
( )
W = R i ∫ t dt
s s
0
GUID-7A92209F-7451-415B-8C3D-276A6ED4A44B V1 EN
R combined rotor and stator resistance
s
i starting current of the motor
s
t starting time of the motor
This equation is normally represented as the integral of I²t. It is a commonly used method
in protective protection relays to protect the motor from thermal stress during starting. The
advantage of this method over the traditional definite time overcurrent protection is that
when the motor is started with a reduced voltage as in the star-delta starting method, the
starting current is lower. This allows more starting time for the motor since the module is
monitoring the integral of I²t.
The module calculates the accumulated heat continuously and compares it to the limiting
value obtained from the product of the square of the values of the Motor start-up A and
Motor start-up time settings. When the calculated value of the thermal stress exceeds this
limit, the OPR_IIT output is activated.
The module also measures the time START_TIME required by the motor to attain the
rated speed and the relative thermal stress IIT_RL. The values are available in the
Monitored data view.
1MAC504801-IB E
(Equation 92)
620 series ANSI
Technical Manual