In this article we will discuss about:- 1. Protection of Low-Voltage (Below 1,000 V) 3-Phase Induction Motors 2. Protection of Large Sized Motors 3. Protection for Exclusively Synchronous Motors.
Protection of Low-Voltage (Below 1,000 V) 3-Phase Induction Motors:
These are the most widely used industrial motors. Circuit diagram for a magnetic contactor starter for a low-voltage 3-phase induction motor is shown in Fig. 8.26. Magnetic contactor starter essentially consists of a set of ‘start’ and ‘stop push buttons with associated contacts, overload and under-voltage protective devices. The start push button (usually green coloured) is a momentary contact switch that is held normally open by a spring. The stop push button (usually red coloured) is held normally closed by a spring.
The operation is as follows:
When the start push button is pressed, the operating coil (or the main contactor) gets energized through the overload relay contacts OL (normally closed). This closes the three main contacts M that connect the motor to the supply. At the same time a set of auxiliary or maintaining contacts MC are closed. When the maintaining contacts MC are closed, a new circuit is established through stop pushbutton, maintaining contacts MC and operating coil (or main contactor). Since operating coil circuit is now maintained by the auxiliary contacts MC, the starter button is released. Now the motor starts.
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For stopping a motor, the stop push button is pressed, the operation coil gets de-energized, thereby opening all the main contacts and auxiliary contacts.
If the supply fails or line voltage drops below a certain value, the main contacts and the maintaining contacts are opened. Upon return of the supply, the contactors cannot close until the start button is again pushed. Because a contactor that is controlled by a three-wire control circuit maintains the interruption of the circuit even after the supply is restored, it is said to provide under-voltage protection for the motor. Such a protection is provided when it is desired to prevent the unexpected starting of a motor.
Thermal overload relays (bimetallic strip type or solder film type) are commonly used for motor overload protection. Both act to open the motor control circuit and, therefore, to disconnect the motor from the source of supply. HRC fuses provide very rapid short-circuit protection. Current is cut off by HRC fuse even before it attains prospective peak. The selection of thermal relay is such that for normal starting conditions, the relay does not operate. A setting range is provided for adjustment for different variations in load conditions.
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Protection chart for 3-phase induction motors is given below:
Protection of Large Sized Motors:
Several types of protective relays are available for different applications. Such relays sense the abnormal operating condition and make the motor circuit breaker to trip. Large sized motors are provided protection against faults in windings and associated circuits, excessive overloads and short circuits, under-voltage, phase unbalance and single phasing, phase reversal and switching over-voltages.
Characteristics of relays are such that the operating time reduces with the increase in the magnitude of fault current.
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1. Stator Winding Protection:
(a) Overcurrent Protection:
This is the basic type of protection that is employed for short-circuit protection of stator windings. The devices for such protection range from fuses for motor voltages of 600 V and lower, through direct-acting overcurrent tripping elements on circuit breakers, to separate overcurrent relays and circuit breakers for voltages of 2,200 V and higher. Where fuses or direct-acting devices are employed, there must be one protective element in each ungrounded conductor.
Where relays and CTs are employed with ac-tripping from the output of the CTs, a CT and relay are required for each ungrounded conductor. If battery or capacitor tripping is employed, 3 CTs with two phase relays and one ground relay will do for a 3-phase circuit whether or not the source neutral is grounded.
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Motor may be grouped in two categories viz., non-essential service motors and essential service motors.
For non-essential service (other than essential services) motors it is the practice to provide both inverse-time and instantaneous phase and ground overcurrent relays for automatic tripping. The inverse-time phase relays are adjusted to pick up at about 3.5 to 4 times rated motor current, but to have enough time delay so as not to operate during the motor starting period. The instantaneous phase relays are adjusted to pick up a little above the locked-rotor current. The inverse-time ground relays are adjusted to pick up at about 20% of the rated motor current or about 10% of the maximum available ground-fault current, whichever is smaller.
Percentage differential relaying is provided for large motors [2,200 V to 4,999 V inclusive 1,000 kW and above and 5,000 V and higher of capacity 400 kW and above]. The percentage differential relaying provides faster and more sensitive protection than overcurrent relaying but at the same time it does not operate on starting or other transient over-currents.
For essential-service motors the inverse-time phase overcurrent relays are usually not employed. The reason for this omission is to trip the motor breaker automatically only for short circuits and not to trip for any other reason. This is because the tripping of such a motor may force a partial or complete shutdown of a generator or any other device with which the motor is associated, any unnecessary tripping should be avoided.
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(b) Stator Overheating of Motors:
Overheating of motors is due to prolonged overloading, stalled rotors or unbalanced stator currents. For complete protection, three-phase motors should have an overload element in each phase as an open circuit in the supply to the power transformer feeding a motor will cause twice as much current to flow in one phase of the motor as in either of the remaining two phases. In spite of the desirability of overload element in all the three phases, motors of rating of about 1,000 kW and lower are generally provided with overload elements in only two phases, on the assumption that the open phase condition will be detected and corrected before any motor can overheat.
For non-essential service motors it is the practice for motors of less than 1,000 kW rating to provide either replica-type thermal overload replays or long-time inverse-time over current relays or direct-acting tripping devices to disconnect a motor from its source of supply in the event of overload.
Other things being equal, the replica type relay will generally provide the best protection as its time-current characteristic nearly matches the heating characteristic of a motor over the full range of overcurrent. Also it, take into account the heating effect of the load on the motor before the overload condition occurs. The inverse-time overcurrent relay will tend to overprotect at low currents and to under-protect at high currents. However, the overcurrent relay is very easy to adjust and test, and it is self-reset. The overload relays will also provide protection in the event of phase-to-phase faults.
The tripping of relays may be adjusted as follows:
1. For continuous-rated motors without service factor or short-time overload ratings—not more than about 115% of rated current of motor.
2. For motors with 115% service factor—about 125% of rated current of motor.
3. For motors with special short-time overload ratings, or with other service factors, the motor characteristic will determine the required tripping characteristic but the tripping current should not exceed about 140% of rated motor current.
Motors rated higher than 1,000 kW are generally provided with resistance temperature detectors embedded in the stator slots between the windings. In such cases, a single relay operating from these detectors is used instead of the replica type or inverse-time overcurrent relays. Also, current-balance relays capable of operating at about 25% or less unbalance between the phases currents should be provided. If the resistance temperature detectors are not provided but current-balance relays are provided, a single replica type thermal overload relay may be substituted for the resistance-temperature-detector relay.
The protection recommended for some essential-service motors is based on minimizing the possibility of unnecessarily tripping the motor, even though such practice may sometimes endanger the motor. So long-time inverse-time-overcurrent relays are used for motors of all ratings, but they merely control an alarm and leave tripping in the operator control. For motors that are likely to suffer locked rotor, supplementary instant aneous overcurrent relays are provided and their contacts are connected in series with those of the inverse-time overcurrent relays to trip the motor circuit breaker automatically.
Supplementary instantaneous overcurrent relays are adjusted to pick up at about 2 to 3 times of the rated motor current.
For essential-service motors for which automatic tripping is desired in addition to alarm for overloads between about 1.15 times of rated current and the pick-up of the instantaneous over-current relays, thermal relays of either replica type or the temperature-detector type should be employed depending on the size of the motor. Such relays permit operation for overloads as far as possible beyond the point where the alarm will be sounded but without damaging the motor to the extent that it must be repaired before it can be used again.
(c) Rotor Overheating Protection:
Rotor faults are more likely to occur in wound rotor motors. The increase in rotor current is reflected on stator current and the stator overcurrent protections thereby act. The setting of stator overcurrent relay is generally of the order of 1.6 times full-load current. This is enough to detect the rotor faults.
(d) Under-Voltage Protection:
Motor draws excessive current when operated on under-voltage and so under-voltage protection can be provided by overload devices or temperature-sensitive devices. However, a separate single element under-voltage relay energized with phase-earth or phase-to-phase voltage can be provided for protection against a three phase drop in voltage or an attempt to start with low voltage on all phases. A time delay feature is usually incorporated to avoid tripping by a transient voltage drop.
(e) Unbalance and Single Phasing Protection:
The unbalanced 3-phase supply causes negative sequence current to flow in the motor which may cause overheating of the stator and rotor windings of the motor. The unbalanced protection provided to a motor should be such as to avoid prolonged unbalanced condition, but should not disconnect the motor for permissible unbalance of short duration.
The unbalance voltage protection can be based upon the following methods:
i. Bimetallic relays arranged to trip faster for unbalanced currents.
ii. Single phase relays sensing overcurrent in heavily loaded phases.
iii. Phase unbalance relays.
For small motors separate phase unbalance relay is not justified because of its cost. Additional phase failure relay (single phasing preventer) is provided where essential. For larger motors, additional unbalanced current relays are provided. The secondary currents of CTs are fed to a negative phase sequence filter. The output of the negative sequence filter is given to overcurrent unit or static level detector which communicate tripping command to the starter or circuit breaker when the negative sequence current exceeds a preset limit.
(f) Reverse Phase Protection:
In some applications, phase reversal is dangerous such as in case of elevators, cranes, hoists, trams etc. In such applications reverse phase protection must be provided.
The phase reversal relay based on electromagnetic principle consists of a disc motor driven by magnetic system actuated by secondaries of two line CTs and PTs. For correct phase sequence the disc experiences a torque in positive direction and, therefore, keeps the auxiliary contacts closed. In the event of occurrence of phase reversal, the torque acting on the disc reverses and the disc starts rotating in opposite direction and so open the auxiliary contacts. Thus the magnetic coil of a starter can be de-energized or circuit breaker can be tripped.
The solid-state phase-reversal relays and phase-failure relay senses the phase reversal or phase failure. Under abnormal operating condition it sends tripping command to output stage, which is an auxiliary relay or static device.
Protection of Exclusively Synchronous Motors:
Synchronous motor may be either a loaded-start motor or an unloaded-start motor. A loaded-start motor is any motor other than either a synchronous condenser, or a motor driving a generator, it includes any motor driving a mechanical load even though automatic unloading means may be used.
1. Amortisseur-Overheating Protection:
Such a protection during starting or loss of synchronism should be provided for all loaded-start motors. This protection is provided by a time-delay thermal overload relay connected in the field discharge circuit. This protection is not required in case of unloaded-start motors.
2. Field Winding Overheating Protection:
Protection against field winding overheating because of prolonged over-excitation is provided for synchronous motors or condensers with automatic voltage regulators without automatic field-current limiting features. A thermal overload relay with time delay or a relay that responds to an increase in field winding resistance with increasing temperature may be employed. In an attended station, the relay would merely control an alarm.
3. Loss of Synchronism Protection:
A synchronous motor may pull out of step either due to excessive load or due to reduction in supply voltage. All loaded-start synchronous motors should be provided protection against loss of synchronism.
The condition of loss of synchronism can be detected by a relay that responds to the change in power factor that occurs when pole slipping occurs. One such typical circuit is illustrated in Fig. 8.29. In this arrangement the voltage between two phases is compared with the current in the third phase; an attracted armature relay energized from a full-wave bridge rectifier is differentially connected and is in the operated state so long as the motor is in synchronism. A non-linear resistor is provided for the protection of rectifiers and for extending the operating range of the relay.
4. Loss of Excitation Protection:
All unloaded-start synchronous motors without automatic voltage regulators should be provided with loss of excitation protection. This is a low-set, time- delay-reset undercurrent relay whose coil is connected in series with the field winding.