In a power system, a generator or alternator is the most important and costly equipment. As it is accompanied by a prime mover, excitation system, voltage regulator, cooling system, etc., its protection becomes very complex and elaborate. It is subjected to more types of trouble than any other equipment, so it must be protected from faults. The various faults that occur on a generator are classified as,
- Stator winding faults
- Rotor circuit faults
- Faults due to abnormal operation of the generator.
Stator Winding Faults :
Stator winding faults are related to three-phase armature winding that includes phase to phase, phase to ground, and inter-turn faults. Due to the failure of insulation in armature windings these faults occur. Among the three stator winding faults, the most common and dangerous fault is the phase-to-earth fault, while the other two faults occur rarely. Inter turn faults are more difficult to be detected.
Depending upon the severity of the fault, stator faults may result in the burning of stator conductors, welding of the stator core laminations (due to heat generated at the point of fault), and complete shut down of the system.
Phase to Earth Fault :
The phase-to-earth fault is the short circuit that occurs between any phase conductor and the earth. This type of fault occurs mainly due to insulation breakdown between a conductor and earth or due to breakdown of phase conductor and falling on the ground. Such a fault mainly occurs in armature slots.
Depending upon the fault current the stator core burning can take place. If the fault current is more than 20A then the cause will be severe i.e., the fault can cause severe damage to the generator. An earth fault protection with earthing resistance is provided for the generators to protect them against phase-to-earth faults.
Phase to Phase Fault :
A short circuit between any two phases is called a phase to phase fault or line to line fault. At the end of the armature windings i.e., in the overheating parts outside the slot, this fault occurs. But this fault occurs very rarely since the insulation between the coils of different phases in a slot is large.
Inter-turn Faults :
The inter-turn fault i.e., the fault between the turns of the same winding of stator causes the flow of circulating currents through the winding turns undergoing the fault. The protection scheme against inter-turn fault is split-phase relaying protection.
Rotor Circuit Faults :
Rotor earth fault means rotor's field winding to ground fault. It should be noted that a single line to ground fault of rotor winding does not affect the operation of the alternator. Since the rotor field circuit is ungrounded, it causes severe damage when a second line to ground fault occurs.
The second fault causes a dead short circuit across a part of the rotor's winding. So, the flux produced in that part of winding will be zero. Thereby, causing a non-uniform flux distribution. This results in unbalanced magnetic forces which may damage the bearings and shaft depending upon the severity of fault i.e., upon the part of field winding being short-circuited.
Rotor earth fault protection provides an indication for the first fault itself. So that, preventive measures can be taken before a second fault occurs.
Abnormal Operation of Generator :
Other than stator and rotor faults, there are some other situations that occur in the generator due to which it is subjected to some abnormal running conditions. The following are the abnormal conditions to which an alternator may be subjected in service.
Unbalanced Loading :
The unbalanced loading or unbalanced currents on a generator can be due to unsymmetrical faults (both internal and external) or due to improper operation of the circuit breaker (not breaking all three phases) or due to open circuiting of a phase or due to failure of one contact of the circuit breaker.
The unbalanced loading causes circulation of negative sequence currents and if allowed to persist, it causes tremendous heating of the rotor winding and rotor stampings or damages the field winding. A negative sequence relay is used to protect the generator from negative sequence currents that occurred due to unbalanced loading.
Thermal Overloading :
The thermal overloading of an alternator occurs due to any of the following reasons. They are,- Its overloaded operation for a prolonged period.
- Failure of the cooling system.
- Core faults (i.e., short circuit between the laminations or failure of core bolt insulation).
The first two abnormal conditions result in the over-heating of stator winding (indirectly the stator body) and so insulation failure may occur. Due to core faulting directly, the stator body gets heated. Two protection schemes are employed to detect the overheating of the stator. They are,- By measuring the temperature of the coolant at the inlet and outlet.
- By measuring the temperature of the stator core at various places. This is possible by embedding the temperature sensing elements (like RTDs, thermistors, and thermocouples) in the stator slots.
Over Speed :
The over-speeding of alternators occurs due to the sudden loss of electrical loads which may be due to tripping of the main circuit breaker. As the mechanical input to large alternators cannot be stopped instantaneously, the rotor is accelerated to a very high speed. Hence, the output frequency increases.
In the case of turbo-alternators, the speed governor controls the speed. Also, they are provided with mechanical over-speed devices and over-speed or over-frequency relays. In the case of hydro-alternators, as the flow of water to the turbine cannot be stopped instantaneously due to high mechanical inertia, the over-speed relays are provided with more settings.
Overvoltage :
Over-voltages are due to the over-speeding of the alternator or due to the mal-operation of the voltage regulator or due to lightning surges. The over-voltages result in insulation failure. In the case of turbo-alternators, AVR controls the overvoltages due to over speeds whereas, in the case of hydro and gas turbine alternators, over-voltage relays serve the purpose. Protection against over-voltages due to lightning surges can be provided by connecting lightning arresters and surge capacitors at the output terminals.
Failure of Prime Mover :
The failure of the prime mover is said to be taken place when the driving torque of the prime mover falls below the total losses of the alternator. When the failure of the prime mover takes place, as the alternator still be working in synchronism with the interconnected power system, it continues to maintain its synchronism with the power system and starts running as a synchronous motor in the same direction.
Although the power drawn from the system will be below (which will be equal to the sum of losses in the machine and mechanical load on the machine i.e., turbine), the turbine may be damaged severely.
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. The protection scheme employs a sensitive directional watt-metric (power) relay and the scheme is known as reverse power protection.
Loss of Excitation :
There are a number of possibilities for the loss of excitation. It may be caused due to the mal-operation of the field circuit breaker or loss of field to the main exciter or loose contact of main exciter brushes etc.
Due to the loss of excitation, the alternator speed increases slightly (since the mechanical input continues to remain unchanged). So, the alternator acts as an induction generator that takes the magnetizing current from the connected power system.
If the alternator is a cylindrical rotor type, then the rotor body is overheated. Since heavy rotor currents flow through it due to slip speed. If it is a salient pole-type alternator, then the rotor will not be heated up. Since large synchronous generators (salient pole type only) are provided with damper windings.
In either of the cases, the stator gets heated up and the system stability falls due to the heavy magnetizing current being drawn. The field failure or loss of field protection scheme employing offset mho or directional impedance relay is used for the protection against loss of excitation.
Various Protection Schemes Employed for Generator :
