In the generator mainly faults are the earth faults and the inter turn faults they are caused by the thermal and mechanical stresses.The field system is usually ungrounded so when fault between field winding and rotor body exist does not give rise fault current. But when the second earth fault exist it short circuit the rotor winding and then produce the unsymmetrical field system and unbalance the force on rotor.This causes vibration of the rotor and damage the bearings.So rotor earth fault protection is to provided to restrict the fault spreading on ward.
Because of that fault, unbalanceing in three phase stator currents exists.As the unbalance three phase currents have the negative sequence component, it rotates in a opposite direction at the synchronous speed giving rise to double frequency currents.This results in to the overheating of the rotor and also damage to the rotor.temperature indicators are used for detecting the rotor overheating.
Rotor open circuit faults are less to exist causes arcing problems and reduced excitation. Loss of the field failure occurs due to the short circuit or open circuit in field winding. In the case of the generator start running as induction generator, supplying power as the leading power factor.Due to the loss of excitationand loss of synchronism and system stability occur.rotor protection can also be done by using the tripping scheme which opens the field circuit breaker which will trip the generator unit breaker.
Rotor Protection:
Different schemes are used for protection of faults occurring in rotor. These schemes are of the following:
Rotor earth fault protection:
As the field circuit are operating unearthed a single earth fault does not affect the operation of the generator.But this fault increases the stress to the ground because stator transients induce an extra voltage in the field winding.If there is only the single earth fault but the relay should be provided to give the knowledge that fault has to occurred so that the generator may take out of the service until the second fault occurs and become the cause of serious damage for the rotor. There are two basic methods used for the rotor earth fault protection of the generator.
Method I:
In this method a high resistance is connected across the rotor circuit and its mid point is grounded through a sensitive relay. This relay detects the earth fault for whole circuit except the rotor center point
Method II:
In this method dc injection or ac injection method is used. .In it either dc or ac voltage is connected between the field circuit and ground through a sensitive over voltage relay and current limiting resistor or capacitor. A single earth fault in the rotor circuit will complete the circuit including voltage source, sensitive over voltage relay and earth fault. DC injection method is simple and has no problems of leakage currents. If we use dc the over voltage relay will be more sensitive than if we use ac because in case when we use ac the relay not picking up the current that flows normally through capacitance to ground and also care should be taken to avoid resonance between capacitance and inductance.
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Rotor overheating Protection:
Negative sequence component of the unbalanced currents of the stator winding causes double frequency current to be induced in the rotor winding due to this component overheating of the rotor take place. In case of over current due to over excitation in the rotor circuit, a dc relay is used. This relay senses and initiates alarm. Application of such relay is limited because relaying of dc quantities is relatively uncommon
Rotor Temperature Alarm:
This kind of protection is only provided in case of large generators. It gives the level of temperature. In it resistance is measured by comparing voltage and current by a double actuating quantity moving coil relay. The operating coil being used as voltage coil and restraining coil used as current coil. The relay measures the ratio of voltage and current because resistance gives the measure of rotor temperature.
Automatic Field Suppression and Use of Neutral circuit Breaker:
When a fault on the generator winding exist even through the generator circuit breaker is tripped, the fault continues to be fed as long as the excitation will exist. For the quick removal of the fault, it is necessary to disconnect the field simultaneously with disconnection of the generator. So it is very necessary to discharge its magnetic field as soon as possible in short duration. Hence it should be ensured that all protection system not only the trip the generator circuit breaker but also trip the automatic field discharge switch.
Loss of Field Protection:
Loss of field occurs due to tripping of the supply of the field current which occurs because of the reasons.
Loss of field to the main exciter.
Accidental tripping of the field breaker.
Short circuit in the field circuit.
Poor brush contact in exciter.
Loss of AC supply to the excitation system.
Field Protection Phenomena:
when the field supply is tripped, it speed increased and it start behaving as induction generator so heavy currents are produced in the teeth and wedges of the rotor. Because of the drop in excitation voltage the generator output voltage drops slowly to compensate this voltage current start increasing then generator become under excited and start drawing reactive power 2 to 4 times the generator load. Before losing excitation, the generator is delivering power to the system. But when loss of field occur this large reactive load thrown on the system abruptly with loss of generator‘s reactive power and it further causes voltage reduction and extensive instability
Protection against Loss of field:If the system has capability to tolerate the difference of reactive power then automatic protection is not required but if the system will be instable in this condition and has not capability to tolerate then automatic protection is required. Under current Moving coil relay is connected across a shunt in series with field winding. But in case of large generators which operate over a wide range of field excitation then this relay will not work properly because field failure due to the failure of the excitation is not detected by it because it is held in by the ac induced from the stator. The most valid type of protection in this case is by using directional-distance type relay operating by alternating current and voltage at the generator terminals. In offset-mho relay is used and its setting is like that when the excitation goes certain value then this relay start operating because machine start running asynchronously. Its characteristics are shown on R-X diagram. When excitation is lost the generator impedance start a curve from the first quadrant to the fourth quadrant. This region is enclosed in the operating area of the relay so the relay will operate when the generator starts to slip poles and will trip the field breaker and disconnect the generator from the system. The generator may then return to service when the cause of failure is cleared.
Fig: Loss of field protection
Effects produced by loss of field:
It can endanger the generator. Connected system or both.
Loss of synchronism.
Over heating of stator winding.
Increased rotor losses.
Pole Slipping:
When angular displacement of the rotor exceeds the stability limit then rotor slips a pole pitch or we can say rotor flux slips with respect to stator flux. This condition is called pole slipping.
Causes of Pole Slipping
Following are the causes for pole slipping.
Power system fault that persists for long duration
.Connecting line between two systems is open.
Because of insufficient torque that keeps rotor in synchronism.
Faulty excitation system
Operating errors.
Pole Slipping Phenomena:
Pole slipping does not occur very often when faults are cleared very fast. When pole slipping occurs due to this synchronizing power will start flowing in reverse direction twice for every slip cycle. On drawing this synchronizing power on the impedance plane the flow of it characterized by cyclic change in the load impedance and load impedance locus passes between +R and —R quadrants because real power flows in reverse direction. When the load impedance is very reactive in nature then two systems are 180 degree out of phase, this instant is when drawn on the jx axis the point corresponding to this instant is called transition point. At this stage only reactive power flows and system voltage reached to zero at the electrical mid point of the two systems. Mid point is that point where pole slipping take place and its location can be determined from the apparent load impedance to the point where the locus crosses the jx axis. Three parameters magnitude, direction and rate of change of load impedance with respect to the generator terminals tell us about the pole slipping, that is it taking place.
Fig: Offset mho type pole protection relay
Need of Pole Slipping Protection
High current and torque can
Loosen or causes of wear off winding.
Damage shaft and coupling.
Stator and rotor over heating.
Excitation system damage
Protection of Generator due to Unbalanced Loading:
Due to fault there is an imbalance in the three phase stator currents and due to these imbalance currents, double frequency currents are induced in the rotor core. This causes the over heating of the rotor and thus the rotor damage. Unbalanced stator currents also damage the stator.
Negative sequence filter provided with the over current relay is used for the protection against unbalance loading. From the theory of the symmetrical components, we know that an unbalanced three phase currents contain the negative sequence component. This negative phase sequence current causes heating of the stator. The negative heating follows the resistance law so it is proportional to the square of the current. The heating time constant usually depend upon the cooling system used and is equal to I²t=k where I is the negative sequence current and t is the current duration in seconds and k is the constant usually lies between 3 and 20.
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Its general practice to use negative current relays which matches with the above heating characteristics of the generator. In this type of protection three CTs are connected to three phases and the output from the secondaries of the CTs is fed to the coil of over current relay through negative sequence filter. Negative sequence circuit consists of the resistors and capacitors and these are connected in such way that negative sequence currents flows through the relay coil. The relay can be set to operate at any particular value of the unbalance currents or the negative sequence component current.
Under and Over voltage protection:
Over Voltage Protection:
Over voltage occurs because of the increase in the speed of the prime mover due to sudden loss in the load on the generator. Generator over voltage does not occur in the turbo generator because the control governors of the turbo generators are very sensitive to the speed variation. But the over voltage protection is required for the hydro generator or gas turbine generators. The over voltage protection is provided by two over voltage relays have two units — one is the instantaneous relays which is set to pick up at 130 to 150% of the rated voltage and another unit is IDMT which is set to pick up at 110% of rated voltage. Over voltage may occur due to the defective voltage regulator and also due to manual control errors.
Under Voltage Protection:
If more than one generators supply the load and due to some reason one generator is suddenly trip, then another generators try to supply the load. Each of these generators will experience a sudden increase in current and thus decreases the terminal voltage. Automatic voltage regulator connected to the system try to restore the voltage. And under voltage relay type-27 is also used for the under voltage protection.
Under/Over Frequencies Protection:
Over Frequency Operation:
Over frequency results from the excess generation and it can easily be corrected by reduction in the power outputs with the help of the governor or manual control
Under Frequency Operation:
Under frequency occurs due to the excess. During an overload, generation capability of the generator increases and reduction in frequency occurs. The power system survives only if we drop the load so that the generator output becomes equal or greater than the connected load. If the load increases the generation, then frequency will drop and load need to shed down to create the balance between the generator and the connected load. The rate at which frequency drops depend on the time, amount of overload, on the load and generator variations as the frequency changes. Frequency decay occurs within the seconds so we can not correct it manually. Therefore automatic load shedding facility needs to be applied.
These schemes drops load in steps as the frequency decays. Generally load shedding drops 20 to 50% of load in four to six frequency steps. Load shedding scheme works by tripping the substation feeders to decrease the system load. Generally automatic load shedding schemes are designed to maintain the balance between the load connected and the generator.
The present practice is to use the under frequency relays at various load points so as to drop the load in steps until the declined frequency return to normal. Non essential load is removed first when decline in frequency occurs. The setting of the under frequency relays based on the most probable condition occurs and also depend upon the worst case possibilities.During the overload conditions, load shedding must occur before the operation of the under frequency relays. In other words load must be shed before the generators are tripped.
Stator Over Heating:
ProtectionStator over heating is caused due to the overloads and failure in cooling system. It is very difficult to detect the over heating due to the short circuiting of the lamination before any serious damage is caused. Temperature rise depend upon I^2Rt and also on the cooling. Over current relays can not detect the winding temperature because electrical protection can not detect the failure of the cooling systemSo to protect the stator against over heating, embed resistance temperature detector or thermocouples are used in the slots below the stator coils. These detectors are located on the different places in the windings so that to detect the temperature throughout the stator. Detectors which provide the indication of temperature change are arranged to operate the temperature relay to sound an alarm
Stator Ground Fault Protection:
The method of grounding affects the protection which is employed by the differential protection. High impedance reduces the fault currents and thus it is very difficult to detect the high impedance fault, differential protection does not work for the high impedance grounding. The separate relay to the ground neutral provides the sensitive protection. But ground relay can also detect the fault beyond the generator, it the time co-ordination is necessary to over come this difficulty.
If we use the star- delta transformer bank, then it will block the flow of ground currents, thus preventing the occurrence of the fault on other side of the bank from operating ground relays. In unit protection scheme the transformer bank limits the operation of the fault relay to the generator
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