WORLD OF ELECTRIC MOTOR (Types Classification and History of Motor)

The motor or an electrical motor is a device that has brought about one of the biggest advancements in the fields of engineering and technology ever since the invention of electricity. A motor is nothing but an electro-mechanical device that converts electrical energy to mechanical energy. Its because of motors, life is what it is today in the 21st century. Without motor we had still been living in Sir Thomas Edison’s Era where the only purpose of electricity would have been to glow bulbs. There are different types of motor have been developed for different specific purposes.

In simple words we can say a device that produces rotational force is a motor. The very basic principal of functioning of an electrical motor lies on the fact that force is experienced in the direction perpendicular to magnetic field and the current, when field and electric current are made to interact with each other. Ever since the invention of motors, a lot of advancements has taken place in this field of engineering and it has become a subject of extreme importance for modern engineers. This particular webpage takes into consideration, the above mentioned fact and provides a detailed description on all major electrical motors and motoring parts being used in the present era.

Classification or Types of Motor

The primary classification of motor or types of motor can be tabulated as shown below,

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In the year 1821 British scientist Michael Faraday explained the conversion of electrical energy into mechanical energy by placing a current carrying conductor in a magnetic field which resulted in the rotation of the conductor due to torque produced by the mutual action of electrical current and field. Based on his principal the most primitive of machines a D.C.(direct current) machine was designed by another British scientist William Sturgeon in the year 1832. But his model was overly expensive and wasn’t used for any practical purpose. Later in the year 1886 the first electrical motor was invented by scientist Frank Julian Sprague. That was capable of rotating at a constant speed under a varied range of load, and thus derived motoring action.

MAIN CLASSIFICATION 

1) DC Motor

2) Synchronous Motor

3) 3 Phase Induction Motor

4) 1 Phase Induction Motor

5) Special Types of Motor

Among the four basic classification of motors mentioned above the DC motor as the name suggests, is the only one that is driven by direct current. It’s the most primitive version of the electric motor where rotating torque is produced due to flow of electric current through the conductor inside a magnetic field.

Rest all are A.C. electrical motors, and are driven by alternating current, for e.g. the synchronous motor, which always runs at synchronous speed. Here the rotor is an electro – magnet which is magnetically locked with stator rotating magnetic field and rotates with it. The speed of these machines are varied by varying the frequency (f) and number of poles (P), as Ns = 120 f/P.

In another type of AC motor where rotating magnetic field cuts the rotor conductors, hence circulating current induced in these short circuited rotor conductors. Due to interaction of the magnetic field and these circulating currents the rotor starts rotates and continues its rotation. This is induction motor which is also known as asynchronous motor runs at a speed lesser than synchronous speed, and the rotating torque, and speed is governed by varying the slip which gives the difference between synchronous speed Ns , and rotor speed speed Nr,

It runs governing the principal of EMF induction due to varying flux density, hence the name induction machine comes. Single phase induction motor like a 3 phase, runs by the principal of emf induction due to flux, but the only difference is, it runs on single phase supply and its starting methods are governed by two well established theories, namely the Double Revolving field theory and the Cross field theory.

Apart from the four basic types of motor mentioned above, there are several types Of special electrical motors like Linear Induction motor(LIM),Stepper motor, Servo motor etc with special features that has been developed according to the needs of the industry or for a particular particular gadget like the use of hysteresis motor in hand watches because of its compactness.

Synchronous Motor

Electrical Motor in general is an electromechanical device that converts energy from electrical domain to mechanical domain. Based on the type of input we have classified it into single phase and 3 phase motors. Among 3 phase motors Induction and synchronous motors are more widely used.

When a 3 phase electric conductors are placed in a certain geometrical positions (In certain angle from one another) there is an electrical field generate. Now the rotating magnetic field rotates at a certain speed, that speed is called synchronous speed. Now if an electromagnet is present in this rotating magnetic field, the electromagnet is magnetically locked with this rotating magnetic field and rotates with same speed of rotating field. Synchronous motors is called so because the speed of the rotor of this motor is same as the rotating magnetic field. It is basically a fixed speed motor because it has only one speed, which is synchronous speed and therefore no intermediate speed is there or in other words it’s in synchronism with the supply frequency. Synchronous speed is given by

Construction of synchronous motor

Normally it’s construction is almost similar to that of a 3 phase induction motor, except the fact that the rotor is given dc supply, the reason of which is explained later. Now, let us first go through the basic construction of this type of motor

From the above picture, it is clear that how this type of motors are designed. The stator is given is given three phase supply and the rotor is given dc supply

Main features of synchronous motors are

• Synchronous motors are inherently not self starting. They require some external means to bring their speed close to synchronous speed to before they are synchronized.

• The speed of operation of is in synchronism with the supply frequency and hence for constant supply frequency they behave as constant speed motor irrespective of load condition

• This motor has the unique characteristics of operating under any power factor. This makes it being used in power factor improvement.

Principle of Operation Synchronous Motor

Synchronous motor is a doubly excited machine i.e two electrical inputs are provided to it. It’s stator winding which consists of a 3 phase winding is provided with 3 phase supply and rotor is provided with DC supply. The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating magnetic flux. The rotor carrying DC supply also produces a constant flux. Considering the frequency to be 50 Hz, from the above relation we can see that the 3 phase rotating flux rotates about 3000 revolution in 1 min or 50 revolutions in 1 sec. At a particular instant rotor and stator poles might be of same polarity (N-N or S-S) causing repulsive force on rotor and the very next second it will be N-S causing attractive force. But due to inertia of the rotor, it is unable to rotate in any direction due to attractive or repulsive force and remain in standstill condition. Hence it is not self starting.

To overcome this inertia, rotor is initially fed some mechanical input which rotates it in same direction as magnetic field to a speed very close to synchronous speed. After some time magnetic locking occurs and the synchronous motor rotates in synchronism with the frequency.

Methods of starting of Synchronous Motor

• Synchronous motors are mechanically coupled with another motor. It could be either 3 phase induction motor or DC shunt motor. DC excitation is not fed initially. It is rotated at speed very close to its synchronous speed and after that DC excitation is given. After some time when magnetic locking takes place supply to the external motor is cut off.

• Damper winding : In case, synchronous motor is of salient pole type, additional winding is placed in rotor pole face. Initially when rotor is standstill, relative speed between damper winding and rotating air gap flux in large and an emf is induced in it which produces the required starting torque. As speed approaches synchronous speed , emf and torque is reduced and finally when magnetic locking takes place, torque also reduces to zero. Hence in this case synchronous is first run as induction motor using additional winding and finally it is synchronized with the frequency.

Application of Synchronous Motor

• Synchronous motor having no load connected to its shaft is used for power factor improvement. Owing to its characteristics to behave at any power factor, it is used in power system in situations where static capacitors are expensive.

• Synchronous motor finds application where operating speed is less (around 500 rpm) and high power is required. For power requirement from 35 kW to 2500KW, the size, weight and cost of the corresponding induction motor is very high. Hence these motors are preferably used. Ex- Reciprocating pump, compressor, rolling mills etc

Working Principle of Three Phase Induction Motor

An electrical motor is such an electromechanical device which converts electrical energy into a mechanical energy. In case of three phase AC operation, most widely used motor is Three phase induction motor as this type of motor does not require any starting device or we can say they are self starting induction motor.

For better understanding the principle of three phase induction motor, the basic constructional feature of this motor must be known to us. This Motor consists of two major parts:

Stator: Stator of three phase induction motor is made up of numbers of slots to construct a 3 phase winding circuit which is connected to 3 phase AC source. The three phase windings are arranged in such a manner in the slots that they produce a rotating magnetic field after AC is given to them.

Rotor: Rotor of three phase induction motor consists of cylindrical laminated core with parallel slots that can carry conductors. Conductors are heavy copper or aluminum bars which fits in each slots & they are short circuited by the end rings. The slots are not exactly made parallel to the axis of the shaft but are slotted a little skewed because this arrangement reduces magnetic humming noise & can avoid stalling of motor.

Working of Three Phase Induction Motor
Production Of Rotating Magnetic field

The stator of the motor consists of overlapping windings offset by an electrical angle of 120°. When the primary winding or the stator is connected to a 3 phase AC source, it establishes a rotating magnetic field which rotates at the synchronous speed.

Secrets behind the rotation:

According to Faraday’s law an emf induced in any circuit is due to the rate of change of magnetic flux linkage through the circuit. As the rotor windings in an induction motor are either closed through an external resistance or directly shorted by end ring, and cut the stator rotating magnetic field, an emf is induced in the rotor copper bar and due to this emf a current flows through the rotor conductor.

Here the relative velocity between the rotating flux and static rotor conductor is the cause of electric current generation; hence as per Lenz’s law the rotor will rotate in the same direction to reduce the cause i.e. the relative velocity.

Thus from the working principle of three phase induction motor it may observed that the rotor speed should not reach the synchronous speed produced by the stator. If the speeds equals, there would be no such relative velocity, so no emf induction in the rotor, & no current would be flowing, and therefore no torque would be generated. Consequently the rotor can not reach at the synchronous speed. The difference between the stator (synchronous speed) and rotor speeds is called the slip. The rotation of the magnetic field in an induction motor has the advantage that no electrical connections need to be made to the rotor.

Thus the Three Phase Induction Motor is:
• Self-starting.

• Less armature reaction and brush sparking because of the absence of commutators and brushes that may cause sparks.

• Robust in construction.

• Economical.

• Easier to maintain.

Miniature Circuit Breaker or MCB

What is MCB ?

Nowadays we use more commonly Miniature Circuit Breaker or MCB in low voltage electrical network instead offuse.

The MCB has some advantages compared to fuse.

1. It automatically switches off the electrical circuit during abnormal condition of the network means in over load condition as well as faulty condition. The fuse does not sense but Miniature Circuit Breaker does it in more reliable way. MCB is much more sensitive to over current than fuse.

2. Another advantage is, as the switch operating knob comes at its off position during tripping, the faulty zone of the electrical circuit can easily be identified. But in case of fuse, fuse wire should be checked by opening fuse grip or cutout from fuse base, for confirming the blow of fuse wire.

3. Quick restoration of supply can not be possible in case of fuse as because fuses have to be rewirable or replaced for restoring the supply. But in the case of MCB, quick restoration is possible by just switching on operation.

4. Handling MCB is more electrically safe than fuse.

Because of to many advantages of MCB over fuse units, in modern low voltage electrical network, Miniature Circuit Breaker is mostly used instead of backdated fuse unit.

Only one disadvantage of MCB over fuse is that this system is more costlier than fuse unit system.

Miniature Circuit Breaker Working Principle

There are two arrangement of operation of miniature circuit breaker. One due to thermal effect of over current and other due to electromagnetic effect of over current. The thermal operation of miniature circuit breaker is achieved with a bimetallic strip whenever continuous over current flows through MCB, the bimetallic strip is heated and deflects by bending. This deflection of bimetallic strip releases mechanical latch. As this mechanical latch is attached with operating mechanism, it causes to open the miniature circuit breaker contacts. But during short circuit condition, sudden rising of electric current, causes electromechanical displacement of plunger associated with tripping coil or solenoid of MCB. The plunger strikes the trip lever causing immediate release of latch mechanism consequently open the circuit breaker contacts. This was a simple explanation of miniature circuit breaker working principle.

Miniature Circuit Breaker Construction

Miniature circuit breaker construction is very simple, robust and maintenance free. Generally an MCB is not repaired or maintained, it just replaced by new one when required. A miniature circuit breaker has normally three main constructional parts. These are:

Frame of Miniature Circuit Breaker

The Frame of Miniature Circuit Breaker is a molded case. This is a rigid, strong, insulated housing in which the other components are mounted.

Operating Mechanism of Miniature Circuit Breaker

The Operating Mechanism of Miniature Circuit Breaker provides the means of manual opening and closing operation of miniature circuit breaker. It has three-positions “ON,” “OFF,” and “TRIPPED”. The external switching latch can be in the “TRIPPED” position, if the MCB is tripped due to over-current. When manually switch off the MCB, the switching latch will be in “OFF” position. In close condition of MCB, the switch is positioned at “ON”. By observing the positions of the switching latch one can determine the condition of MCB whether it is closed, tripped or manually switched off.

Trip Unit of Miniature Circuit Breaker

The Trip Unit is the main part, responsible for proper working of miniature circuit breaker. Two main types of trip mechanism are provided in MCB. A bimetal provides protection against over load current and an electromagnet provides protection against short-circuit current.

miniature circuit breaker working principle

There are three mechanisms provided in a single miniature circuit breaker to make it switched off. If we carefully observe the picture beside, we will find there are mainly one bi – metallic strip, one trip coil and one hand operated on – off lever.Electric current carrying path of a miniature circuit breaker shown in the picture is like follows. First left hand side power terminal – then bimetallic strip – then current coil or trip coil – then moving contact – then fixed contact and – lastly right had side power terminal. All are arranged in series.

Miniature Circuit Breaker

If circuit is overloaded for long time, the bi – metallic strip becomes over heated and deformed. This deformation of bi metallic strip causes, displacement of latch point. The moving contact of the MCB is so arranged by means of spring pressure, with this latch point, that a little displacement of latch causes, release of spring and makes the moving contact to move for opening the MCB. The current coil or trip coil is placed such a manner, that during short circuit fault the mmf of that coil causes its plunger to hit the same latch point and make the latch to be displaced. Hence the MCB will open in same manner. Again when operating lever of the miniature circuit breaker is operated by hand, that means when we make the MCB at off position manually, the same latch point is displaced as a result moving contact separated from fixed contact in same manner. So, whatever may be the operating mechanism, that means, may be due to deformation of bi – metallic strip , due to increased mmf of trip coil or may due to manual operation, actually the same latch point is displaced and same deformed spring is released, which ultimately responsible for movement of the moving contact. When the the moving contact separated from fixed contact, there may be a high chance of arc. This arc then goes up through the arc runner and enters into arc splitters and is finally quenched. When we switch on an MCB, we actually reset the displaced operating latch to its previous on position and make the MCB ready for another switch off or trip operation.

Types of Diodes

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Generator Protection Types and an Example Settings

The following are the main protection schemes adopted for our generator.1. Generator Differential Protection
2. Generator & Transformer Differential Protection
3. Loss of Field or Loss of Excitation Protection
4. Negative Sequence or Current Unbalance Protection
5. Over Fluxing or Over Excitation Protection
6. Over Current Protection
7. Stator Earth Fault Protection
8. Rotor Earth Fault Protection (64R)
9. Restricted Earth Fault Protection
10. Backup Impedance Protection
11. Low Forward Power Protection
12. Reverse Power Protection
13. Pole Slip Protection
14. Pole Discrepancy Protection
15. Local Breaker Back Protection
16. Bus Bar Protection
17. Over Frequency Protection 18. Under Frequency Protection 
19. Over Voltage Protection 
1.GENERATOR DIFFERENTIAL PROTECTION:Setting : 0.5 Amp Time : InstantaneousIt is one of the important protections to protect generator winding against internal faults such as phase-to-phase and three phase-to-ground faults. This type of fault is very serious because very large current can flow and produce large amounts of damage to the winding if it is allowed to persist. One set current transformers of the generator on neutral and phase side, is exclusively used for this protection. The differential protection can not detect turn-to-turn fault and phase to ground within one winding for high impedance neutral grounding generator such as ours. Upon the detection of a phase-to-phase fault in the winding, the unit is tripped with out time delay.Relays acted : a. Flag operation at Protection panel.b. Acting of Master relayc. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerb. Tripping of Field breakerc. Stop command to Turbine thro’ Mark-IVStatus : a. Unit is at coasting down.Once the differential protection operated, the unit can not be taken into service unless the generator winding is thoroughly examined by the maintenance staff of any internal faults2.GENERATOR-TRANSFORMER DIFFERENTIAL PROTECTION :Setting : 0.75 Amp Time : InstantaneousIt protects 11KV bus duct, 11/0.440KV unit auxiliary transformer, 11/20KV step-up transformer against internal faults such as phase-to-phase and three phase-to-ground faults. This type of fault is very serious because very large current can flow and produce large amounts of damage to the winding if it is allowed to persist. One set current transformers of the generator on neutral side and another set current transformer on 220KV side after transformer, is exclusively used for the protection. Upon the detection of difference in current between these current transformers, the unit is tripped with out time delay.One the generator-transformer differential protection operated, the unit can not be taken into service unless the 11KV bus duct, unit auxiliary transformer, power transformer are thoroughly examined by the maintenance staff for any internal faults.Relays acted : a. Flag operation at Protection panel.b. Acting of Master relay c. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerb. Tripping of Field breakerc. Stop command to Turbine thro’ Mark-IVStatus : a. Unit is at coasting down.
3.LOSS OF FIELD OR EXCITATION PROTECTION :Setting : K1-2, K2-1, K3-2 Trip after 2 Sec.When the synchronous machine with excitation, is connected to the grid, it generates reactive power along with active power to the grid and the rotor speed is same as that of grid frequency. Loss of field or loss of excitation results in loss of synchronism between rotor flux & stator flux. The synchronous machine operates as an induction machine at higher speed and draws reactive power from the grid. This will result in the flow of slip frequency currents in the rotor body as well as severe torque oscillations in the rotor shaft. As the rotor is not designed to sustain such currents or to withstand the high alternating torques which results in rotor overheating, coupling slippage and even rotor failure. A loss of excitation normally indicates a problem with the excitation system. Some times it may be due to inadvertent tripping of filed breaker, open or short circuit of field winding or loss of source to the exciter. If the generator is not disconnected immediately when it loses excitation wide spread instability may very quickly develop and major system shutdown may occur. When loss of excitation alarm annunciates at annunciation panel, the machine may probably be running with less excitation at leading MVAR power. Increase the excitation on the machine until it reaches on lagging MVAR power. The machine trips on the same protection along with alarm resynchronize the machine and try to stabilize at required MVAR power. If not possible, trip the machine immediately and inform to the maintenance staff for thorough checking of the Automatic Voltage Regulator (AVR) and its associated parts.Relays acted : a. Flag operation at Protection panel.b. Acting of Master relayc. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerb. Tripping of Field breakerc. Stop command to Turbine thro’ Mark-IVStatus : a. Unit is at coasting down.4.NEGATIVE SEQUENCE OR CURRENT UNBALANCE PROTECTION :Setting : Alarm – 75% of 12s Time - 5 Sec.Trip – 75% of 12s Time - 300 Sec.When the machine delivering the equal currents in three phases, no unbalance or negative phase sequence current is produced as the vector sum of these currents is zero, when the generator is supplying an unbalanced load to a system, a negative phase sequence current is imposed on the generator. The system unbalance may be due to opening of lines, breaker failures or system faults. The negative sequence current in the stator winding creates a magnetic flux wave in the air gap which rotates in opposite direction to that of rotor synchronous speed. This flux induces currents in the rotor body, wedges, retaining rings at twice the line frequency. Heating occurs in these areas and the resulting temperatures depend upon the level and duration of the unbalanced currents. Under these conditions it is possible to reach temperatures at which the rotor material no longer contain the centrifugal forces imposed on them resulting in serious damage to the turbine-generator set. Any machine as per design data will permit some level of negative sequence currents for continuous period. An alarm will annunciate at annunciation panel if negative sequence currents exceeds a normal level. Reduce the MVAR power on the machine if necessary load also and keep the machine for some time till the alarm vanishes at annunciation panel. If the machine trips on the Negative sequence protection never take the machine into service until the temperatures on the rotor parts settle down to its lower value. Resynchronize the machine to the grid after considerable time under grid & feeder parameters are within limits. If the unit trips again on the same protection, stop the machine after consideration time so as to cool down the rotor parts and inform to the maintenance staff for thorough examination of the system. Relays acted : a. Flag operation at Protection panel.b. Acting of Master relayc. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerb. Tripping of Field breakerStatus : a. Unit is at FSNL.5. OVER FLUXING OR EXCITATION OR VOLTS PER HERTZ PROTECTION: Setting : Alarm – 1.17 Time - 10 Sec.Trip – 1.17 Time - 30 Sec.Per unit voltage divided by per unit frequency commonly called Volts/Hertz is a measurable quantity that is proportional to flux in the generator or step-up transformer cores. Moderate over fluxing (105-110%) increases core loss resulting in increase of core temperatures due to hysterics & eddy currents loss. Long term operation at elevated temperatures can shorten the life of the stator insulation. Severe over fluxing can breakdown inter-laminar insulation followed by rapid local core melting. Over fluxing normally can be caused by over speed of the turbine or over excitation during Off-line condition, and load rejection or AVR mal-functioning during On-line condition. If alarm annunciation panel, Increase/Reduce the speed of the turbine to rated generator speed (3000RPM) and reduce the generator voltage to rated during Off-line condition. Reduce the MVAR power on the generator during On-line condition. If the machine trips on over fluxing protection during On-line, Keep the machine at FSNL till the grid parameters stabilize and resins. Again the machine trips on the same stop the machine for examination of the AVR & Governor systems by maintenance staff. Relays acted : a. Flag operation at Protection panel.b. Acting of Master relay c. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerb. Tripping of Field breakerc. Stop command to Turbine thro’ Mark-IVStatus : a. Unit is at coasting down.6.OVER CURRENT WITH VOLTAGE RESTRAINT PROTECTION :Setting : Alarm – 85% Time - 10 Sec.Trip – 100% Time - 0.5 Sec.Normally generators are designed to operate continuously at rated MVA, frequency and power factor over a range of 95 to 105% rated voltage. Operating the generator at rated MVA with 95% voltage, 105% stator current is permissible. Operating of the generator beyond rated KVA may result in harmful stator over current. A consequence of over current in winding is stator core over heating and leads to failure of insulation. If alarm annunciates at annunciation panel, Reduce the stator current to the below the rated by reducing the MVAR power on the machine. When the trips on the same protection, Resins the machine after keeping the machine at FSNL for some time, and keep the stator current below the rated. Relays acted : a. Flag operation at Protection panel.b. Acting of Master relayc. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerb. Tripping of Field breakerStatus : a. Unit is at FSNL.7 STATOR EARTH FAULT PROTECTION :Setting : 70% Time - 5 Sec.Normally the generator stator neutral operates at a potential close to ground. If a faulty phase winding connected to ground, the normal low neutral voltage could rise as high as line-to-neutral voltage depending on the fault location. Although a single ground fault will not necessarily cause immediate damage, the presence of one increases the probability of a second. A second fault even if detected by differential relay, may cause serious damage. The usual method of detection fault is by measuring the voltage across the secondary of neutral grounding transformer (NGT). Here are two over lapping zones to detect stator ground faults in a high impedance grounded generator system, the two zones are put together cover 100% stator winding for earth faults. A fundamental frequency neutral over voltage relay covers about 0-95% of the stator zonal winding for all faults except those near the neutral. Another third harmonic neutral under voltage relay covers remaining 96-100% of the stator zone 2 winding on neutral side. Relays acted : a. Flag operation at Protection panel.b. Acting of Master relay c. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerb. Tripping of Field breakerc. Stop command to Turbine thro’ Mark-IVStatus : a. Unit is at coasting down.8.ROTOR EARTH FAULT PROTECTION (64R) :Settings : Less than 80K ohmAny rotor field winding of the generator is electrically isolated from the ground. Therefore the existence of one ground fault in the field winding will usually not damage the rotor. However the presence of two or more ground faults in the winding will cause magnetic and thermal imbalance plus localized heating and damage to the rotor metallic parts. The rotor earth fault may be caused due to insulation failure of winding or inter-turn fault followed by localized heat.Relays acted : a. Flag operation at Protection panel.b. Acting of Master relay c. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerb. Tripping of Field breakerc. Stop command to Turbine thro’ Mark-IVStatus : a. Unit is at coasting down.9.RESTRICTED EARTH FAULT PROTECTION:Settings : 0.1 Amp. Time : InstantaneousIt is similar to generator differential protection in working. It protects the high voltage winding of 11/220KV power transformer against internal faults. One set current transformers of the power transformer on neutral and phase side, is exclusively used for this protection. The protection can not detect turn-to-turn fault within one winding. Upon the detection of a phase-to-phase or phase-to-ground fault in the winding, the unit to be tripped without time delay.Relays acted : a. Flag operation at Protection panel.b. Acting of Master relayc. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerb. Tripping of Field breakerc. Stop command to Turbine thro’ Mark-IVStatus : a. Unit is at coasting down.Once the restricted earth fault protection operated, the unit can not be taken into service unless the transformer winding is thoroughly examined by the maintenance staff for any internals faults. 10.BACKUP IMPEDANCE PROTECTION:Settings ; K1-3, K2-0.71 Time – 1.5 Sec.As in name implies, it is used to protect the generator from supplying the over loaded or faulty system. It is backup protection of the generator over current protection. In measures ratio of the voltage and current supplied by the generator and initiates trip signal when the measured impedance is less than the preset value.If the machine trips on the Backup protection, never take the machine into service until the temperatures of the generator settle down to its lower value. Resynchronize the machine to the grid after considerable time when grid & feeder parameters are within limits.Relays acted : a. Flag operation at Protection panel.b. Acting of Master relay c. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerb. Tripping of Field breakerc. Stop command to Turbine thro’ Mark-IVStatus : a. Unit is at coasting down.11.LOW FORWARD POWER PROTECTION:Setting : 0.5% Time : 1 Sec.The generator will not develop output power when turbine input is less than the no load losses and motoring action develops on the turbine. The generator is able to generate power, usually 55 to 10% of generator capacity, within pre-determined time after closing of 220KV breaker.Relays acted : a. Flag operation at Protection panel.b. Acting of Master relayc. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerStatus : a. Unit is at FSNL with potential. The unit trips on the low forward protection, Resins the machine and increase input power to the turbine as quickly as possible within low forward power time setting. Even after two to three attempts, the machine is tripping on the same protection; probably the governor of turbine is faulty. Inform to maintenance staff for rectification of the same.12.REVERSE POWER PROTECTION:Setting : 0.5% Time - 2.0 Sec.It is backup protection to the low forward protection. Motoring of a generator will occur when turbine output is reduced such that it develops less than no-load losses while the generator is still on-line, the generator will operate as a synchronous motor and driving the turbine. The generator will not be harmed by synchronous motoring and a steam turbine can be harmed through over heating during synchronous motoring if continued long enough. The motoring of the turbine output can be detected by reverse power protection. The avoid false tripping due to power swings a time delay is incorporated before tripping signal is generated.Relays acted : a. Flag operation at Protection panel.b. Acting of Master relayc. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerb. Tripping of Field breakerc. Stop command to Turbine thro’ Mark-IVStatus : a. Unit is at coasting down.The unit trips on the reverse power protection. Resins the machine and increase the input power to the turbine as quickly as possible within low forward power time setting. Even after two to three attempts, the machine is tripping on the same protection; probably the governor of turbine is faulty. Inform to maintenance staff for rectification of the same. 13.POLE SLIP OR OUT-OF-STEP PROTECTION:Setting : 6.9 ohm.When a generator loses synchronism, the resulting high current peaks and off-frequency operation may cause winding stresses, pulsation torques and mechanical resonances that have the potential danger to turbine generator. Therefore, to minimize the possibility of damage, it is generally accepted that the machine should be tripped without time delay preferably during the first half-slip cycle of the loss of synchronism condition. The electrical center during loss-of-synchronous conditions can occur in the generator as a result of increased impedance of the generator while decrease system impedance. The protections normally applied in the generator zone such as back-up impedance, loss of excitation etc., will not protect a generator during loss of synchronism under normal generator conditions.Relays acted : a. Flag operation at Protection panel.b. Acting of Master relayc. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerb. Tripping of Field breakerc. Stop command to Turbine thro’ Mark-IVStatus : a. Unit is at coasting down.The unit trips on the Pole slip protection, Resynch the machine after stabilization of the grid parameters14.POLE DISCREPANCY PROTECITON:Setting : 0.5 Sec.If One or two poles of generator breaker fail to close during synchronization, all poles of the breaker trip on this protection. It may be due to mechanical failure of the breaker un equal distribution of closing signal to the breaker from protection system.Relays acted : a. Flag operation at 220KV Breaker panel.b. Indication at Annunciation Panel.Consequences : a. tripping of 220KV breakerStatus : a. Unit is at FSNL with potential.The generator breaker trips on the pole discrepancy protection, Resynch the generator. Even after two to three attempts, the machine is tripping on the same protection, probably the generator breaker is faulty. Inform to maintenance staff for rectification of the same. 15.LOCAL BREAKER BACKUP PROTECTION:Setting : 25% Time : 0.8 Sec.For most of the faults, the generator breaker involves tripping the generator from the system. Failure of the breaker to open probably results in loss of protection and other problems such as motoring action or single phasing, If one or two poles of the generator breaker fail to open due to mechanical failure in breaker mechanism, the result can be a single phasing and negative phase sequence currents inducted on the rotor. The LBB protection is energized when the breaker trip is initiated after a suitable time interval if confirmation of the confirmation of breaker tripping from three poles is not received. The energized tripping signal from LBB protection will trip all 220KV generator breakers and all 220KV feeder breakers through Bus-bar protection.Relays acted : a. Flag operation at Protection panel.b. Acting of Master relay for all units. c. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerb. Tripping of Field breaker of all units. Status : a. all Units are at FSNL.Once the LBB protection operated, the entire station is in dark. First restore all essential services to all units such as lube oil system and turning gear etc., from battery backup and. Checkup the faulty 220KV breaker and isolate the breaker from the system by opening the both side of the isolators.After restoring all services from station supply, Close 220KV feeder breakers first and take all units into service one after the other duly co-coordinating with the DE/LD.Since it involves complex operation, it is necessary to get help from maintenance staff for restoring the normally in the station. Never attempt to close the faulty 220KV generator in panic as it causes permanent damage to the generator and transformer.16.BUS BAR PROTECTION:Setting : 0.8 Amp. There are mainly three protection zones namely called generator zone, bus duct transformer zone, 220KV breakers zone. The protection of generator zone and bus duct & transformer zone are covered in previous schemes. All 220KV breakers at switchyard will come under Bus-Bar protection. Functioning of this scheme is similar to the generator differential protection or generator-transformer differential protection. It measures all incoming currents from the generators at 220KV side and all outgoing currents in 220KV feeders, and initiates trip signal if it detects any deviation more than the preset value as the algebraic sum of all currents at 220KV bus must be less than the preset value. It isolates all 220KV generator breakers and all 220KV feeder breakers connected to 220KV bus.Relays acted : a. Flag operation at Protection panel.b. Acting of Master relay for all units. c. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerb. Tripping of Field breaker of all units. Status : a. all Units are at FSNL.Once the Bus-Bar protection operated, the entire station is in dark. First restore all essential services to all units such as lube oil system and turning gear etc., from battery backup and 6.6/0.44KV Stage – II reserve power supply. Checkup the entire 220KV switch yard for any wire snapping or equipment damage. After restoring all services from station supply, Close 220KV feeder breakers first and take all units into service one after the other duly co-ordinating with the DE/LD.Since it involves complex operation, it is necessary to get help from maintenance staff for restoring the normalcy in the station. Never attempt to restore the 220KV supply at switch yard in panic unless the entire system is thoroughly examined and satisfy yourself as it causes permanent damage to the equipment or injury/death to the person working at switch yard. 17.OVER FREQUENCY PROTECTION:Setting : 52 Hz Time - 2 Sec.For a generator connected to a system, abnormal frequency operation is a result of a severe system disturbance. The generator can tolerate moderate over frequency operation provided voltage is within an acceptable limits. The machine operated at higher speeds at which the rotor material no longer contain the centrifugal forces imposed on them resulting in serious damage to the turbine-generator set. The abnormal over frequency on the machine may be due to improper speed control adjustment or disoperation of the speed controller or severe grid disturbance or sudden load through off. Relays acted : a. Flag operation at Protection panel.b. Acting of Master relay c. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerb. Tripping of Field breakerc. Stop command to Turbine thro’ Mark-IVStatus : a. Unit is at coasting down.The unit trips on the over frequency protection, Resins the machine. Even after two to three attempts, the machine is tripping on the same protection; probably the governor of turbine is faulty. Inform to maintenance staff for rectification of the same.18.UNDER FREQUENCY PROTECTION:Setting : 48 Hz Time : 2.0 Sec.For a generator connected to a system, under frequency operation is a result of a severe system disturbance. The generator can tolerate moderate under frequency operation provided voltage is within an acceptable limits. The machine operated at lower higher speeds causes severe over fluxing in the generator-transformer. The abnormal under frequency on the machine may be due to improper speed control adjustment or disoperation of the speed controller. Relays acted : a. Flag operation at Protection panel.b. Indication at Annunciation PanelConsequences : a. NILStatus : a. Unit is at lower speed with potential. Increase governor speed until machine reaches full speed. Even after two to three attempts, the machine are running at lower speed, probably the governor of turbine is faulty. Inform to maintenance staff for rectification of the same. 19.OVER VOLTAGE PROTECTION :Setting : a. 110% Time - 2.0 Sec.b. 120% Time - 0.3 Sec.Generator voltage is at present value under normal operating conditions as selected by operator in AVR. If it parts from preset value, May be due to AVR mal-functioning or a system disturbance. Severe over voltage can cause over fluxing and winding insulation failure. The over voltage protection can be considered as a backup to the Volts-per-Hertz protection.Relays acted : a. Flag operation at Protection panel.b. Acting of Master relayc. Indication at Annunciation Panel.Consequences : a. Tripping of 220KV breakerb. Tripping of Field breakerStatus : a. Unit is at FSNL without potential.

Raise the generator voltage slowly with manual mode in AVR and keep generator voltage within the limits of normal voltage. If it is unable to control the generator voltage, trip the field breaker and inform to the maintenance staff for rectification of the AVR.