Page 320 - Industrial Power Engineering and Applications Handbook

Protection of electric motors 12/299
that the relay blocks and prevents the next switching, ing conditions. See also Section 3.6 where we have
unless the fault is acknowledged, the reason for the provided a brief procedure to reproduce the motor thermal
trip is analysed and normal operating conditions are curves 8 versus I and vice versa, for the relay to provide
restored. The settings of the relay for all such unfavour- a replica protection to a motor and to have closer settings
able operating or fault conditions are made depending of its various protective features. The following is a brief
upon the functions and the setting ranges available summary of our discussions so far.
with the relay. Since all the protections are based on As discussed in Section 12.2 and explained in Figure
r.m.s. values, a sensitive relay will also detect the 12.3, an ordinary overcurrent relay will detect between
harmonic quantities present in the system and provide curves 1 and 2, as against the average heating curve 3
more accurate protection for the machine. and is thus oversensitive to insignificant amounts of
unbalance, not actually causing harm to the motor. For
It may often be asked why separate settings are instance, an unbalance in voltage of, say. 3% may cause
necessary for different types of protection when it is an unbalance in the current (i.e. an apparent overloading)
possible to set the relay for a replica protection. It is true of, say, 18% in one particular phase and may be detected
that the relay will monitor thermally what is occurring by an ordinary hi-metallic thermal overload relay, while
within the machine as long as there are only normal to the stator does not heat up accordingly. Even the rotor,
unfavourable operating conditions. But this is not so, which is more sensitive to an unbalance is heated corres-
~~~~
when a fault condition occurs. It is possible that on a ponding to a current of ,I; + 3(0.181, )? or 1.047/,,
fault the temperature rise is not consistent with the assumed i.e. an overloading of nearly 5%. Similarly, during singlc
thermal replica due to high time constants. For instance, phasing the relay should be able to detect twice the full
for a fault of a few seconds the temperature rise of the load current in a star-connected winding, or as shown in
whole machine will almost be negligible (equation (3.2)). curve X of Figurel2.10, for a delta-connected winding.
This would delay the trip, whereas the heat may be and not corresponding to the stator currcnt of 43 . I, for
localized and very high at the affected parts and may a star-connected winding and curve RIY of Figure 12. IO.
escape undetected as well as electrodynamic forces, which for a delta-connected winding. An ordinary relay will
may also cause damage to the machine. Similarly, on a take longer to operate corresponding to the line current,
single phasing, monitoring of the stator temperature alone whereas some of the internal circuits will be subject to
is not sufficient, as the rotor would heat up much faster much higher currents during this period.
(in Y-connected stators) due to double-frequency eddy For meticulous protection, therefore, it is advisable to
current heating and less weight compared to the stator. use a motor protection relay for large LT and all HT
Similarly, it may take much longer to trip under a stalled motors. The following motor details and working con-
condition. A severe unbalance, as may be caused by an ditions are essential to know before making a proper
internal fault, may also result in heavy negative phase selection of a protective scheme:
sequence rotor currents and require protection similar to
short-circuit protection. A short-circuit protection will 1 Type of motor - squirrel cage or slip-ring
not detect a single phasing. Hence the necessity to provide 2 Rating - kW (CMR)
separate protection for different operating and fault 3 Rated voltage and current
conditions to achieve optimum utilization of the machine, 4 Type of starting
with the least risk of damage. The relay must discriminate 5 Starting current versus time characteristics
between an unfavourable operating condition and a fault 6 Locked rotor current and corresponding ‘hot’ thermal
condition. While the former may permit a delay tripping, withstand time
the latter will need a more discrete and quick tripping to 7 Motor thermal withstand characteristic curves
save the machine from more severe damage.
8 Number of starts or reversals, if required, and their
frequency
Underload protection Y Ambient temperature and
It is also possible to provide this protection in such relays. 10 Maximum fault current
This will provide very vital system process information.
A sudden drop in load may be the result of a fallout of Having discussed the effect of the above parameters
the load due to disengagement of the coupling, breakage on the motor’s performance. we will now illustrate by
of a belt or a tool, etc. It can therefore help monitor the way of an example a general case to broadly suggest a
system process line more accurately. procedure that can be followed to select the protective
scheme for a motor. For more detailed selection of the
motor protection relay, reference may be made to the
12.6 Summary of total motor relay manufacturer.
protection
Example 12.7
For the purpose of protection, consider the squirrel cage
In view of the effect of various unfavourable operating motor of Example 7.1 for one hot start for which this motor is
conditions on a motor’s performance, one should be suitable.
meticulous when selecting the most appropriate protection Step I: Assume the motor starting current versus time
to safeguard a motor under the most unfavourable operat- characteristics to be shown in Figure 12 32, and divide the