Application - ABB RELION REX640 Technical Manual

Section 4
Protection functions
4.1.9.6
396

Application

MPTTR is intended to limit the motor thermal level to predetermined values during
the abnormal motor operating conditions. This prevents a premature motor insulation
failure.
The abnormal conditions result in overheating and include overload, stalling, failure
to start, high ambient temperature, restricted motor ventilation, reduced speed
operation, frequent starting or jogging, high or low line voltage or frequency,
mechanical failure of the driven load, improper installation and unbalanced line
voltage or single phasing. The protection of insulation failure by the implementation
of current sensing cannot detect some of these conditions, such as restricted
ventilation. Similarly, the protection by sensing temperature alone can be inadequate
in cases like frequent starting or jogging. The thermal overload protection addresses
these deficiencies to a larger extent by deploying a motor thermal model based on load
current.
The thermal load is calculated using the true RMS phase value and negative sequence
value of the current. The heating up of the motor is determined by the square value of
the load current. However, while calculating the thermal level, the rated current
should be re-rated or de-rated depending on the value of the ambient temperature.
Apart from current, the rate at which motor heats up or cools is governed by the time
constant of the motor.
Setting the weighting factor
There are two thermal curves: one which characterizes the short-time loads and long-
time overloads and which is also used for tripping and another which is used for
monitoring the thermal condition of the motor. The value of the Weighting factor p
setting determines the ratio of the thermal increase of the two curves.
When the Weighting factor p setting is 100 percent, a pure single time constant
thermal unit is produced which is used for application with the cables. As presented in
Figure 218, the hot curve with the value of Weighting factor p being 100 percent only
allows an operate time which is about 10 percent of that with no prior load. For
example, when the set time constant is 640 seconds, the operate time with the prior
load 1 x FLC (full Load Current) and overload factor 1.05 is only 2 seconds, even if
the motor could withstand at least 5 to 6 seconds. To allow the use of the full capacity
of the motor, a lower value of Weighting factor p should be used.
Normally, an approximate value of half of the thermal capacity is used when the motor
is running at full load. Thus by setting Weighting factor p to 50 percent, the protection
relay notifies a 45 to 50 percent thermal capacity use at full load.
For direct-on-line started motors with hot spot tendencies, the value of Weighting
factor p is typically set to 50 percent, which will properly distinguish between short-
time thermal stress and long-time thermal history. After a short period of thermal
stress, for example a motor start-up, the thermal level starts to decrease quite sharply,
simulating the leveling out of the hot spots. Consequently, the probability of
successive allowed start-ups increases.
1MRS759142 C
REX640
Technical Manual