ABB RELION 620 Series Technical Manual page 766

Section 4
Protection functions
760
The Str over delay time setting has different purposes in different modes of operation.
• In the "IIt" or "IIt & stall" modes, the aim of this setting is to check for the
completion of the motor start-up period. The purpose of this time delay setting is
to allow for short interruptions in the current without changing the state of the
MOT_START output. In 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.
1MRS757644 F
(Equation 143)
620 series
Technical Manual