In this article we will discuss about:- 1. Stator Protection of Generators 2. Overload or Overcurrent Protection of Generators 3. Over-Voltage Protection 4. Over-Speed Protection 5. Protection against Motoring 6. Negative Sequence Protection 7. Protection against Vibration 8. Bearing Overheating Protection 9. External Fault Backup Protection 10. Direct Connected Protection.
Stator Protection of Generators:
The earth fault current is usually limited by the resistance of neutral earthing resistor. When the fault current is less than 20 A, negligible burning of the iron core will occur if the machine is tripped within some seconds. The repair work then amounts to replacement of the damaged coil without restacking of core laminations.
If, however, the earthing resistor is selected to pass a much larger earth fault current (greater than 200 A) severe burning of the stator core will take place, necessitating restacking of laminations. Even with a high speed earth fault differential protection severe damage may be caused owing to the large time constant of the field circuit and the relatively long time required to completely suppressing the field flux. In the case of high earth-fault currents it is therefore normal practice to install a circuit breaker in the neutral of the generator in order to reduce the total fault clearance time.
Circulating current biased differential protection provides the earth fault protection but the sensitivity of such a protection for earth fault depends upon the resistance in neutral to earth connection and the position of earth fault in the winding.
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A separate and sensitive earth fault protection is generally necessary for generators with resistance earthing.
The occurrence of short circuiting between the stator windings is quite rare because the insulation in a slot between coils of different phases is at least twice as thick as the insulation between one coil and the iron core. However, a phase-to-earth fault may result in a phase-to-phase fault within the slots. If a phase-to-phase fault should occur, this is most likely to occur at the end connections of the stator winding i.e., in the overhanging parts outside the slots.
Such a fault causes severe arcing with high temperatures, melting of copper and risk of fire if the insulation is not made of fire-resistant, non-inflammable material. In this case there will be no damage to the core laminations, and the repair work will, therefore, be limited to replacement of damaged coils and mechanical parts of the end structure.
Circulating current biased differential protection provides adequate and speedy protection against phase-to-phase faults in the generator zone.
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Short-circuits between the turns of one coil may occur in case the stator winding makes use of multi-turn coils. Such faults may develop due to incoming current surges with a steep wave-front that may causes a high voltage [L(di/dt)] across the turns at the entrance of the stator winding. Differential protection and overcurrent protection does not sense the inter-turn faults. Stator inter-turn fault protection is separately provided.
Overload or Overcurrent Protection of Generators:
Overloading of the generator may be caused either due to partial breakdown of winding insulation or due to excessive load on the power supply system. Overcurrent protection for alternators is not considered necessary, since modern alternators are capable of withstanding complete short circuit at their terminals for sufficient time without much overheating and damage. On occurrence of such faults, the generator can be disconnected from the system manually.
In case an overload protection is provided for generators such a protection might disconnect the generator from the system due to momentary troubles outside the power station or temporary overload on the system and thus interfere with the continuity of the supply.
Over-Voltage Protection of Generators:
The field excitation system of modern generators is so designed that under normal operating conditions overvoltage conditions cannot occur. However, overvoltage in a generator occurs when speed of the prime mover increases due to sudden loss of the load on the generator. If there is an overvoltage in a generator, and if it persists, the generator main circuit breaker and the generator or exciter field breaker should be tripped, as it is not safe for the generator to continue to operate under such condition.
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Generated overvoltage of significant duration or magnitude does not generally occur in turbo-generators because the control governors are very sensitive to speed variations. So overvoltage protection is not generally required with turbo-generators. Overvoltage protection is recommended for all hydroelectric or gas-turbine generators that are subject to over-speed and consequent overvoltage on loss of load.
The over-voltage protection is provided with an overvoltage relay which has two units—one instantaneous relay set for pick up at about 130-150 % of the rated voltage and another IDMT relay set for pick up at about 110% of the rated voltage. Both relay units should be compensated against the effect of variations in frequency.
The relay should be energized from a PT (potential transformer) other than the one used for the automatic voltage regulator. The operation of the relay should first insert additional resistance in the field circuit of the generator. Then if overvoltage persists the main generator breaker and the generator and the generator or exciter field breaker should be tripped.
Over-Speed Protection of Generators:
The main cause of the over-speed is the sudden loss of all or major part of electrical load on the generator. The over-speed of a generator would result in over-frequency operation of the generator itself and of the system supplied by it. In certain circumstances the rise in speed may be so considerable that the action of the governors or closing of the emergency valve will not prevent rise in speed. Hence over-speed protection is recommended for all prime-mover driven generators.
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The over-speed element may be furnished as part of the prime mover, or of its speed governor, or of the generator, it should operate the speed governor, or whatever other shut-down means is provided, to shut-down the prime mover. It should also trip the generator circuit breaker; this is to prevent over-frequency operation of equipment and machinery connected to the system supplied by the generator, and also to prevent possible over-frequency operation of the generator itself from the ac system.
The over-speed device should also trip the auxiliary circuit breaker where auxiliary power is drawn from the generator leads. In certain cases, an over-frequency relay may be suitable for providing both of these types of protection. However, a direct-connected centrifugal switch is preferred.
The over-speed element should usually be adjusted to operate at about 3 to 5% above the full-load rejection speed. Supplementary over-speed protection is required for some forms of gas turbines.
Protection against Motoring of Generators:
In the event of prime-mover failure the generator continues to rotate as a synchronous motor drawing electrical power from the system and driving the prime mover. This motoring action of the machine is known as inverted operation. This operation is not desirable, as firstly, the machine ceases generation and no purpose is served by its motoring operation.
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The mechanical faults such as loss of vacuum, failure of bearings, or lubricating troubles etc. are protected by the special indicating instruments/protective devices which may stop the prime mover automatically but such faults cannot be detected on the electrical side. However, the inverted operation of the machine can be protected by employing reverse power relay.
Protection against motoring is beneficial to the prime movers and to the system as a whole, but not to the generator.
During the motoring action of the generator, the power flows from the bus-bars to a machine and the conditions in the three phases are balanced. Hence a single-element directional power relay (reverse power relay) sensing the direction of power flow in any one phase in sufficient. The CTs for reverse power protection may be either at the neutral end or the bus-bar end of the generator stator windings. The setting depends on the type of the prime mover. Intentional time lag is provided in the reverse power protection to prevent operation by synchronising surges and power oscillations following system disturbances.
A steam turbine needs protection against overheating when its steam supply is cut off and the generator coupled to it acts as- a motor. Such overheating occurs because there is not enough steam in the turbine to carry away the heat generated owing to windage losses. Modern condensing turbines will even overheat at outputs of less than approximately 10% of rated load. A condensing turbine that operates normally at high vacuum will withstand motoring much longer than a topping turbine that operates normally at high back pressure. A sensitive power directional equipment is generally used, and it should operate at less than 3% of the power rating of the generator.
In case of hydro-generator sets mechanical protection is provided on the water wheel to disconnect the generator from the system when water flow drops to a value likely to causes cavitation. Hence electrical protection is not required. However, motoring protection may occasionally be desirable for unattended hydraulic turbines for their protection against cavitation of the turbine blades. Cavitation of blade occurs on lean water flow that might occur, for example, from blocking of the trash gates. Power-directional-relaying equipment operating on a motoring current of somewhat less than 2.5% of the full-load rating of the generator can be provided for such protection.
The diesel-engine driven generators would draw about 15% of its rated power or more from the system when acting as motor, and the motoring action should be avoided by suitable protective relaying. This is because the power drawn by the generator during motoring action constitutes an undesirably high load on the system. Also, there may be danger of fire or explosion from unburned fuel.
The power required to drive a gas turbine with the generator coupled to it working as a motor varies from 10 to 50 of its full-load rating and such heavy load on the system must be avoided by providing protective relaying. There is usually no turbine requirement for protection against motoring.
Negative Sequence Protection of Generators against Unbalanced Loads:
If, owing to a fault, there is an imbalance in the three-phase stator currents, double frequency currents are induced in the rotor core. This causes overheating of rotor and possible damage to the rotor. Unbalanced stator currents also cause severe vibrations and heating of stator. Current balance relaying equipment would not be very effective for protection against such faults because such equipment energized by phase currents would operate quickly for small imbalances and too slowly for large imbalances.
Negative sequence current filter with overcurrent relay is used to provide protection against unbalanced loading.
From the theory of symmetrical components we know that unbalance three-phase currents have a negative sequence component. The negative phase sequence current causes heating of the stator. In case of high-speed turbo-generators, the continuous current which can be carried is usually in the range of 10 and 15 per cent of the positive sequence continuous rating. The negative sequence heating follows a normal resistance law and so it is proportional to the square of the current. The heating time constant of the machine largely depends upon the cooling system used and is expressed as I22 t = K where I2 is the negative sequence current expressed on the per-unit basis of the continuous maximum rating, t is the current duration in seconds, and K is a constant which for turbo-generators usually lies between 3 and 20.
It is generally necessary to install negative sequence relays that match the above heating characteristic of the machine. The arrangement is shown in Fig. 8.23. Three CTs are connected in the three phases and the output from their secondaries is fed to the coil of the overcurrent relay through a negative phase sequence filter. Negative sequence filter circuit comprises resistors and inductors connected in such a way that negative sequence component flows through the relay coil.
The overcurrent relay used is with inverse characteristics matching with I22 t rating curve of the machine. The relay can be set to operate at a particular value of the imbalanced current or the negative sequence component current.
Protection against Vibration of Generators:
Rotor earth fault protection and negative sequence protection of generator against unbalanced loads, respectively prevent or reduce vibration under those circumstances. A vibration detector may be mounted on one of the bearing pedestals in the case of a horizontal shaft generating set, or on the upper guide-bearing in case of a vertical shaft generating set. It may be set to trip the machine or initiate an alarm when the radial deflections of certain duration exceed a per-determined value.
Bearing Overheating Protection of Generators:
Bearing overheating can be detected by a relay actuated by a thermometer-type bulb inserted in a hole in the bearing, or by a resistance-temperature-detector relay, such as used for stator overheating protection, with the detector embedded in the bearing. In case lubricating oil is circulated through the bearing under pressure, the oil temperature may be monitored if the system has provision for giving an alarm if the oil circulation is stopped.
Such protection is provided for all unattended generators where the size or importance of the generator warrants it. Such protection for attended generators is generally limited only to sound an alarm.
External Fault Backup Protection of Generators:
Overcurrent and earth-fault protection is provided for backup protection of large sized (above 1 MVA) generators protected by differential protection against external phase-to-phase faults and earth-faults. The schematic arrangement is illustrated in Fig. 8.24 (a). Induction type IDMT relays may be employed for this purpose.
Since the faults in stator winding are fed by the stator winding itself, their influence on current in the outgoing terminals of generator is governed by the fault level of the main bus. Thus overcurrent and earth-faults relays do not provide satisfactory protection against internal faults.
Sequence of operation is illustrated in Fig. 8.24 (b). The relays setting is selected that the generator overcurrent and earth-fault protection does not normally respond to external faults such as at F.
However, overcurrent and earth-fault protection of generator (3) may be set to operate with due time lag for larger values of external fault currents so that the fault F is not supplied by the generator in case it persists for long time duration.
Hence high set, definite minimum time, induction type, inverse overcurrent, earth-fault relays are required for backup protection of generators.
Direct Connected Generator Protection:
Though generators are usually connected to the grid through step-up power transformers, but if the generation and transmission/distribution voltages are the same (as in case of captive power plants), the generator may be connected directly to the load line. Direct-connected generators are usually of small ratings (of the order of tens of MW).
A basic protection scheme for a direct-connected 30 MVA generator is given in Fig. 8.25.
The scheme may consist of the following protective schemes:
1. Biased differential protection.
2. Negative phase sequence protection.
3. Standby earth-fault protection.
4. Backup overcurrent and earth-fault protection.
In addition to above protective schemes the following may also be provided:
1. Rotor earth-fault protection.
2. Field-failure protection.
3. Reverse power protection in case of back pressure turbine sets and engine driven sets.