Page 312 - Industrial Power Engineering and Applications Handbook

Protection of electric motors 12/291
have the required fault level, may be chosen with a lower setting a built-in single-phasing protection or the tripping circuit
of the OCR. This protection may, however, be applied in an is provided with a single-phasing preventor. If a separate
MCB or an MCCB, when they possess a rupturing capacity less protection for this is considered desirahle, one may use
than required. Since the MCB and the MCCB can both be
current limiting the characteristics of the fuses and the breakers a negative phase sequence relay like the one shown in
can be coordinated such that faults that are in excess of the Figure 12.23(b) or similar static or PLC-based relays.
rupturing capacity of the breakers alone are handled by the For larger motors, however, one should employ a relay
fuses. like a motor protection relay. which covers in one unit
In such cases it may often be possible to meet the requirement all the protection as described in Section 12.5.
by selecting a higher frame size of MCB or MCCB, which may
possess a higher rupturing capacity also. If not, and to save on Note One should employ only current \enring relays ar far a\
cost, one may provide HRC fuses for back-up protection. possible for such applications as a negative sequence current has a
7 - To make a proper selection of HRC fuses it is essential that the severe effect on motor windings due to a much lower negatiLe
current-time characteristic curves for the releases of the breaker qequence impedance of the motor (Section 1?.2(v)) than ;I
and the fuses are available from their manufacturers. corresponding negative sequence voltage.
Such a relay is connected on the supply side a\ shown in Figure
Coordination of fuses with a switch or a contactor 12.23(a). The arrangement is such that unless the relay closes, the
Since both these devices possess a certain level of making motor switching circuit will not energize and the motor will not
and breaking capacities, the same criteria will apply as start. The contact closes only when the supply voltage i\ normal
and the phase sequence positive. Even for undervoltage conditions.
for the breaker noted earlier. Rating of fuses shall not be the torque developed by the relay may not be adequate to close the
more than the switch or contactor rating. circuit. Such relays are, therefore, effective against
Coordination of fuses with a transformer (i) Negative sequence voltages when torque developed bq it\
Consider a distribution HV/LV transformer. If the fuses coil is negative.
are provided on both HV and LV sides, the fuses on the (ii) Voltages far too low to produce an adequate torque to closc
the relay. This is possible during a start only. as during a run
HV side must protect a fault within the transformer while the relay contacts are already established and the coil doer
the fuses on the LV side must clear overcurrent and fault not detect a fall in the voltage.
conditions on the LV side. Thus, for a fault on the LV (iii) Single phasing during a start. a% this will also produce a negative
side, only the LV side fuses must operate and not the HV torque to close the relay.
side, similar to the requirements discussed above.
If the transformer is HV/HV, the same requirement 12.4.5 Protection against single phasing (SPP)
must prevail, i.e. for a fault on the downstream (secondary
side) only the downstream fuses must operate and not An ordinary thermal relay senses only the line currents
the fuses on the upstream (primary side). and is not suitable for detecting a single-phasing condition.
Referring to the curves of Figure 12.10, an ordinary relay
set at 110% of FLC will not trip in the event of a single
,Nore It is. however, possible to eliminate the use of HRC fuses in phasing when the motor is operating underloaded. say,
LT hl~tems at least. with the availability of more advanced technology
in an MCCB or an MPCB (motor protection circuit breaker). See at only 60% or less of the rated current, while in the lone
Section 12.1 I for D fuse-free system. phase X, the current would be as high as 135% of FLC.
For single-phasing protection, ‘single-phasing pre-
12.4.3 Protection against stalling and ventors’ are available. Although it is essential to protect
locked rotor even a small motor against single phasing. there is little
point in employing these preventors unless they form
Motors which do not possess a sufficient gap between part of the basic starter (OCR with built-in SPP feature)
their hot withstand and starting curves generally call for as an economic consideration.
such a protection. Large LT motors and all HT motors As discussed above, most of the leading manufacturers
are recommended to have a separate protection against of switchgear components produce thermal overcurrent
such a condition. A locked rotor protection relay basically relays with a built-in feature of single-phasing prevention.
is an overcurrent relay, having an adjustable definite time The use of a separate single-phasing preventor device is
delay to trip the motor when it exceeds its permissible thus not necessary up to medium-sized motors, where
starting time, but before the safe stall withstand time. this feature is available in the relays. For large-motors
This feature is available in a motor protection relay and for critical installations, a separate unit for single-
discussed later. Where, however, such a relay (MPR) is phase protection may be required for prompt single-
not provided, a high-set IDMT overcurrent relay can be phasing alarm and/or trip. Interestingly, where star delta
chosen to match the upper range of the motor thermal switching is employed, the overload relays, which are
withstand curve (Figure 12.16). now connected in phases of the motor windings,
automatically sense an abnormal condition in any 01- all
12.4.4 Protection against voltage unbalance or the phases, and provide a single-phasing protection. See
negative phase sequence the power circuit diagram in Figure 13.56 for more clarity.
A separate single-phasing protection device is available
Such a condition also generates negative sequence in two versions:
components. For small and medium-sized motors, say,
up to 100 h.p., no separate protection for such a condition 1 Voltage sensing, and
is normally essential, when the overcurrent relay possesses 2 Current sensing