A generator or alternator in a power system is a critical and expensive piece of equipment. It is typically accompanied by a prime mover, such as a steam turbine or gas turbine, which provides the mechanical energy to turn the generator's rotor. The generator also has an excitation system, which is responsible for creating the magnetic field that induces an electrical current in the stator windings. Additionally, it may have a voltage regulator, which controls the output voltage of the generator to ensure that it stays within a certain range. Cooling systems, such as air or water-cooled systems, are also used to keep the generator operating within safe temperature limits.
Because of the complexity and expense of generators, protecting them from faults is essential. Generators are subjected to a wide range of potential problems, such as overloading, short-circuiting, and loss of excitation. In order to protect the generator from these types of faults, a variety of protection devices and systems are used. These may include overcurrent relays, differential relays, and voltage relays, as well as more advanced systems such as microprocessor-based protection devices.
The various faults that occur on a generator are classified as,
1. Stator winding faults.
- Phase to Earth Fault
- Phase to Phase Fault
- Inter-turn Faults
2. Rotor circuit faults
3. Faults due to abnormal operation of the generator.
- Unbalanced Loading
- Thermal Overloading
- Over Speed
- Overvoltage
- Failure of Prime Mover
- Loss of Excitation
1. Stator winding faults:
In a generator, the stator winding is the stationary part of the machine that houses the armature winding, which is connected to the power source and generates electricity. Stator winding faults can occur due to a failure of insulation in the armature windings, which can lead to various types of faults such as phase-to-phase, phase-to-ground, and inter-turn faults.
The most common and dangerous stator winding fault in a generator is the phase-to-earth fault. This type of fault occurs when there is a connection between one of the phases of the armature winding and the ground, which can cause a significant amount of current to flow through the stator conductors. This can result in the burning or melting of the conductors and can also generate a lot of heat at the point of the fault, which can cause the stator core laminations to weld together. This can lead to a complete shut down of the generator.
Phase-to-phase and inter-turn faults are less common and occur less frequently than phase-to-earth faults. Phase-to-phase faults occur when there is a connection between two different phases of the armature winding, while inter-turn faults occur when there is a connection between different turns of the same phase. These faults can also cause damage to the stator conductors and core laminations, but typically to a lesser extent than phase-to-earth faults. Inter-turn faults are also more difficult to detect.
Phase to Earth Fault:
A phase-to-earth fault is a type of short circuit that occurs when there is a connection between one of the phase conductors of the generator and the earth. This type of fault can occur due to a breakdown in insulation between a conductor and the earth, or when a phase conductor breaks and falls onto the ground. These types of faults are most common in armature slots, where the conductors are located.
When a phase-to-earth fault occurs, it can cause a significant amount of current to flow through the stator conductors. Depending on the amount of current that flows, the stator core may be damaged. If the fault current is more than 20A, then the damage caused by the fault can be severe and can cause significant damage to the generator.
To protect the generator from phase-to-earth faults, an earth fault protection with earthing resistance is provided. This protection system is designed to detect the phase-to-earth fault and to trip the generator's breaker, which will disconnect the generator from the power source and prevent further damage. The earthing resistance is used to limit the fault current that can flow through the generator, in case of earth fault.
Phase to Phase Fault:
A phase-to-phase fault, also known as a line-to-line fault, is a type of short circuit that occurs when there is a connection between any two phases of the generator. This type of fault occurs when the insulation between the coils of different phases in a slot breaks down. This fault is typically found at the end of the armature windings, in the overheating parts outside the slot.
This type of fault is relatively rare since the insulation between the coils of different phases in a slot is relatively large. The insulation is designed to prevent the conductors of different phases from coming into contact with each other, which reduces the likelihood of a phase-to-phase fault occurring. However, if a phase-to-phase fault does occur, it can cause a significant amount of current to flow through the stator conductors, which can cause damage to the generator.
A phase to phase fault can cause the stator core burning, damage to the winding and generator tripping. The fault current in this case can be several times the full load current of the generator.
To detect and mitigate this type of faults, generators are usually equipped with protection devices such as overcurrent protection and differential protection. These devices are designed to detect abnormal current levels and to trip the generator's breaker, which will disconnect the generator from the power source and prevent further damage.
Inter-turn Faults:
An inter-turn fault, also known as a turn-to-turn fault, is a type of short circuit that occurs when there is a connection between different turns of the same winding of the generator stator. This type of fault can occur due to a breakdown in insulation between the turns of the winding.
When an inter-turn fault occurs, it can cause circulating currents to flow through the winding turns that are undergoing the fault. These circulating currents can cause significant damage to the generator over time, such as overheating and burning of the stator winding, if not detected and mitigated in time.
The protection scheme against inter-turn faults is typically split-phase relaying protection. This type of protection uses a relay that is connected to the stator winding and is designed to detect abnormal current levels. The relay compares the current flowing in each phase of the stator winding and, if it detects an imbalance, it will trip the generator's breaker, disconnecting the generator from the power source and preventing further damage.
Split-phase relaying protection is a sensitive and reliable protection scheme against inter-turn faults, as it can detect even small inter-turn faults that may occur in the stator winding. However, this protection scheme may not detect turn-to-turn faults that occur in the rotor winding.
2. Rotor circuit faults:
Rotor earth fault, also known as rotor winding to ground fault, occurs when the field winding of the rotor in a generator experiences a connection to the ground. This type of fault is generally caused by a breakdown in insulation or a damaged winding.
It is important to note that a single line-to-ground fault of the rotor winding does not affect the operation of the generator. This is because the rotor field circuit is ungrounded, so a single fault will not cause a complete short circuit. However, a second line-to-ground fault can occur and this will cause severe damage to the generator.
When a second fault occurs, it causes a dead short circuit across a part of the rotor's winding. This results in the flux produced in that part of the winding to be zero, which causes a non-uniform flux distribution. This, in turn, causes unbalanced magnetic forces which can damage the bearings and shaft, depending on the severity of the fault and the part of the field winding that is short-circuited.
To prevent this type of damage, rotor earth fault protection is provided. This protection system is designed to detect the first fault and provide an indication, so that preventive measures can be taken before a second fault occurs. This protection system can detect even small faults and provide a warning, which can help prevent serious damage to the generator.
3. Faults due to abnormal operation of the generator:
Unbalanced Loading:
Unbalanced loading or unbalanced currents on a generator can occur due to a variety of factors, such as unsymmetrical faults, internal and external, improper operation of the circuit breaker, open circuiting of a phase or failure of one contact of the circuit breaker.
Unbalanced loading causes circulation of negative sequence currents, which are currents that are out of phase with each other. These negative sequence currents can cause significant heating of the rotor winding and rotor stampings, which can lead to damage to the field winding. This heating can occur quickly and can cause damage to the generator if it is not detected and mitigated in time.
To protect the generator from negative sequence currents caused by unbalanced loading, a negative sequence relay is used. This relay is designed to detect abnormal current levels and, if it detects negative sequence currents, it will trip the generator's breaker, disconnecting the generator from the power source and preventing further damage.
The negative sequence relay is a device that compares the current in each phase of the generator and if it detects an imbalance, it will operate the breaker to trip the generator and disconnect it from the power source. This protection is necessary to prevent damage to the generator and its components due to the unbalanced loading caused by negative sequence currents.
Thermal Overloading:
Thermal overloading in a generator occurs when the temperature of the stator winding or stator core exceeds its maximum allowable limit. This can happen due to a variety of reasons, such as overloaded operation of the generator for a prolonged period, failure of the cooling system, or core faults such as a short circuit between the laminations or failure of core bolt insulation.
When the stator winding or core overheats, the insulation around the winding can fail, which can cause damage to the generator and lead to a reduction in performance.
To detect thermal overloading, two protection schemes are typically used:
- Measuring the temperature of the coolant at the inlet and outlet: This method involves measuring the temperature of the coolant that is used to cool the generator. If the temperature at the inlet is significantly higher than the temperature at the outlet, it indicates that the coolant is not effectively dissipating the heat generated by the generator, which can be a sign of thermal overloading.
- Measuring the temperature of the stator core at various places: This method involves embedding temperature sensing elements such as RTDs, thermistors, and thermocouples in the stator slots. By measuring the temperature at various places in the stator core, it is possible to detect any localized overheating, which can indicate a core fault or other problem.
Over Speed:
Over-speeding in a generator refers to when the rotor of the generator is spinning at a speed that exceeds its normal operating range. This can occur due to a sudden loss of electrical load on the generator, such as when the main circuit breaker trips.
In large alternators, the mechanical input to the generator cannot be stopped instantaneously. So, when the load on the generator is suddenly reduced, the rotor can be accelerated to a very high speed, which increases the output frequency.
In the case of turbo-alternators, the speed is controlled by a governor, which regulates the speed of the rotor by adjusting the mechanical input. These types of generators are also provided with mechanical over-speed devices and over-speed or over-frequency relays, which are designed to detect when the rotor is spinning at an excessive speed and to take action to reduce the speed and prevent damage to the generator.
In the case of hydro-alternators, the flow of water to the turbine cannot be stopped instantaneously due to high mechanical inertia. So, the over-speed relays are provided with more settings to protect the generator from over-speeding.
Overvoltage:
Overvoltage in a generator refers to when the voltage generated by the generator exceeds its normal operating range. This can happen due to a variety of reasons, such as over-speeding of the alternator, mal-operation of the voltage regulator, or lightning surges.
Overvoltages can cause insulation failure, which can result in damage to the generator and other components in the electrical system. In the case of turbo-alternators, the Automatic Voltage Regulator (AVR) controls the overvoltages due to overspeeding. The AVR is a device that monitors and regulates the voltage generated by the generator to keep it within a safe operating range.
In the case of hydro and gas turbine alternators, over-voltage relays are used to protect the generator from overvoltages. These relays detect when the voltage generated by the generator exceeds its normal range and take action to reduce the voltage and prevent damage.
Protection against overvoltages due to lightning surges can be provided by connecting lightning arresters and surge capacitors at the output terminals of the generator. Lightning arresters are devices that limit the voltage that can reach the generator and surge capacitors are devices that can absorb the energy from a lightning surge, diverting it away from the generator.
Failure of Prime Mover:
The failure of the prime mover in a generator refers to when the driving torque of the prime mover falls below the total losses of the alternator. The prime mover is the mechanical device that drives the generator, such as a steam turbine, gas turbine or hydro turbine.
When the failure of the prime mover occurs, the alternator will continue to maintain synchronism with the interconnected power system and will start running as a synchronous motor in the same direction. However, the power drawn from the system will be lower than normal, which will be equal to the sum of the losses in the machine and the mechanical load on the machine, such as the turbine.
This can cause damage to the turbine, as it will be operating under abnormal conditions. In the case of a steam turbine, the blades may get overheated due to insufficient cooling provided by the reduced amount of steam against the heat produced due to windage loss. In the case of hydro-turbines, cavitation problem arises due to low water flow.
To protect the generator from damage due to the failure of the prime mover, a sensitive directional watt-metric (power) relay is used. This protection scheme is known as reverse power protection. It monitors the power flowing in the generator and trips the generator if the power flow is in the opposite direction to the normal power flow, which indicates a problem with the prime mover.
Loss of Excitation:
Loss of excitation in a generator refers to the loss of the electrical current that is used to create the magnetic field that drives the generator. This can happen due to a variety of reasons, such as a mal-operation of the field circuit breaker, loss of field to the main exciter, or loose contact of main exciter brushes.
When the excitation is lost, the alternator speed increases slightly (since the mechanical input continues to remain unchanged) and the alternator starts to act as an induction generator that takes the magnetizing current from the connected power system. This can cause the alternator to overheat.
If the alternator is a cylindrical rotor type, the rotor body will be overheated due to the heavy rotor currents that flow through it due to slip speed. However, if it is a salient pole-type alternator, the rotor will not be heated up because large synchronous generators (of this type) are provided with damper windings.
In either case, the stator will get heated up and the system stability will fall due to the heavy magnetizing current being drawn. To protect the generator from loss of excitation, a field failure or loss of field protection scheme that employs offset mho or directional impedance relay is used. This protection scheme monitors the electrical current flowing in the generator and trips the generator if the current flow is abnormal which indicates a problem with the excitation.
Various Protection Schemes Employed for Generator:
- Percentage Differential Protection: It is used to protect the stator winding against internal phase and ground faults. This protection scheme compares the current flowing in the different phases of the generator and trips the generator if there is a significant difference between the current flowing in the different phases, indicating a fault in the stator winding.
- Stator Inter-turn Faults Protection: It is used to protect the stator winding against faults between the turns of the same phase in the case of generators having parallel windings or multi-turn per phase per slot. This protection scheme uses split-phase relaying to detect and isolate faults in the stator winding.
- Stator Over-heating Protection: It protects the stator from excess heating due to overloads or due to failure of the excitation system. This protection scheme uses temperature sensing elements embedded in the stator slots to detect overheating and trip the generator to prevent damage.
- Field Failure Protection: It protects the stator and rotor from excess heating due to induction generator action caused during the loss of excitation. This protection scheme uses offset mho or directional impedance relaying to detect a loss of excitation and trip the generator to prevent damage.
- Protection Against Unbalanced Loading: It protects the rotor from excess heating due to negative sequence field caused by unbalanced 3-phase loads. This protection scheme uses a negative sequence relay to detect unbalanced loading and trip the generator to prevent damage.
- Rotor Earth-fault Protection: It indicates the occurrence of the first earth-fault in the rotor circuit, so that preventive measures can be taken before the occurrence of a second earth fault.
- Reverse Power Protection: It protects the alternator against motoring due to failure of prime-movers. This protection scheme uses a sensitive directional watt-metric (power) relay to detect a reverse power flow, indicating a problem with the prime mover, and trip the generator to prevent damage.
- Over-speed Protection: It protects the alternator against over-speeding due to sudden loss of electrical load. This protection scheme uses an over-speed or over-frequency relay to detect when the alternator is running at an abnormal speed and trip the generator to prevent damage.
- Over-voltage Protection: It protects the alternator against insulation failure caused due to over-voltages. This protection scheme uses over-voltage relays to detect when the voltage generated by the alternator is abnormal and trip the generator to prevent damage. In case of lightning surges, protection can be provided by connecting lightning arresters and surge capacitors at the output terminals.
- Bearing-over-heating Protection: It protects the shaft bearings from overheating. This protection scheme uses temperature sensors to monitor the temperature of the bearings and trip the generator if the temperature exceeds a certain threshold to prevent damage.
- Protection Against Vibration: It protects the rotor against distortion due to vibrations caused at the time of abnormal conditions. This protection scheme uses vibration sensors to monitor the vibration levels of the rotor and trip the generator if the vibration exceeds a certain threshold to prevent damage.
- Protection Against Voltage Regulator Failure: It protects the machine against mal-operation of the voltage regulator. This protection scheme monitors the voltage output of the alternator and trip the generator if the voltage is not within a specified range, indicating a problem with the voltage regulator.
- Protection Against Auxiliary Failure: It provides protection against the failure of power plant auxiliaries. This protection scheme monitors the status of various power plant auxiliaries such as cooling systems, lubrication systems, etc. and trip the generator if any auxiliary system fails to prevent damage.