The generators are costly and difficult to maintain. To increase the system’s reliability, the protection of the generator plays an important role.
The protection of generator is very complex due to the following reasons;
- It is a very large electromechanical device used to generate electrical energy at very high AC voltage and is connected to the bus bar.
- Prime movers, voltage regulators, cooling systems, excitation systems are always connected with the generator. Therefore, the working of these accessories depends on the operation of a generator.
- The breakdown of the generator causes a severe unbalance in a power system network, and it results in power shortage. Hence, the protection scheme for the generator must be adequate that it should not shut off.
In this article, we will discuss various types of generator faults;
The generator faults can be classified into three types;
- Stator Fault
- Rotor Fault
- Abnormal Running Conditions
Stator faults are the faults associated with the generator’s stator. The stator consists of a three-phase armature winding.
This type of fault mainly occurs due to insulation failure of armature winding. The stator faults in the generator are further classified into three types;
- Phase-to-earth Faults
- Phase-to-phase Faults
- Inter turn Faults
The phase-to-earth fault is common in generators. It occurs in the armature slots. These faults are dangerous and can cause severe damage to the expensive equipment.
The fault current less than 20A causes negligible burning of a core if the generator tripped quickly. But in case the fault current is very high, it may burn the stator core. And in the worst case, it may lead to a requirement of replacing lamination.
The laminations are very costly and time-consuming. Therefore, separate and sensitive earth fault protection is necessary for the generators to avoid this kind of situation.
The phase-to-phase faults are very uncommon in generators. This type of fault occurs due to short-circuit between two-phase windings.
Due to the insulation used between coils of different phases in slots being large, this type of fault rarely occurs in generators.
But when a phase-to-earth fault occurs due to overheating of a coil, phase-to-phase fault may occur.
This fault is likely to occur at the end connection of the armature winding, overheating parts outside the slots.
This fault causes a severe arc with a very high temperature. And it may lead to the melting of copper and fire if the insulation is not fire-proof.
Stator Inter-turn Faults
In alternators, multi-turn coils are used. Hence, a short-circuit may occur between the turns of one coil, which is known as an inter-turn fault.
This type of fault may occur due to current surges with a high value of voltage across the turns.
But in case if single turn coils are used, this type of fault never occurs. Therefore, single-turn coils are used in a large machine (more than 50 MVA).
But in some countries, multi-turn coils are used. In this condition, it is necessary to provide protection for inter-turn faults.
The rotor of an alternator contains a field winding as most of the alternators are rotating field types.
Like the stator, the rotor is made up of a number of turns. Therefore, it is common to occur fault between the conductor to earth and short-circuit between turns of the field winding.
This type of fault occurs due to the mechanical and thermal stress acting on the field winding insulations.
Generally, the field winding is not grounded. Therefore, single-line to ground fault does not give any fault current.
A second fault to earth will short circuit a part of the field winding, and it is resulting in an unsymmetrical field system.
This unbalanced system will rise the unbalance force on the rotor. And it led to an increase in the pressure on bearings and shaft distortion if it is not cleared very early.
Hence, it is necessary to know the existence of the first occurrence of the earth fault. So that corrective measures can be taken before a second fault occurs.
The unbalance loading on the generator produces the negative sequence current. The rotating magnetic field produced by the negative sequence current is in the opposite direction to that of the rotor magnetic field.
And it induces an EMF in the rotor winding, resulting in overheating the rotor.
To avoid rotor faults, rotor earth fault protection and rotor temperature indicators are used in the generators.
Abnormal Running Conditions
Several situations in a generator can operate for some time. However, it is not a fault. But, if the generator continuously works in this condition, it may lead to fault. This type of condition is known as abnormal running conditions.
To avoid the malfunction of a generator, protection against abnormal running conditions must be provided.
These conditions are;
- Over speeding
- Unbalanced loading
- Failure of a prime mover
- Loss of excitation
- Cooling system failure
The generator winding is heated by continuous overloading, resulting in temperature rise in the winding.
Suppose the temperature increases above a specific limit, the insulation of a winding may get damaged. The overloading effect and temperature rise depend on the degree of overloading.
Sometimes, by overloading, the overcurrent protection operates. The overcurrent protection is set at a high value to avoid this situation. Hence, the overcurrent protection does not sense the overload.
The over-speed problem may occur in the case of a hydroelectric power plant by a sudden loss of load. Because the water flow to the turbine cannot be stopped or reduced instantly.
To avoid this situation, a turbo-governor system is used. But if there is any fault in a governor system, dangerous over-speeding may occur.
Therefore, it is necessary to continuously supervise the governor system and take corrective measures in a governor system.
Unbalanced loading conditions created by an unsymmetrical fault neat the generator. Due to unbalanced loading, a negative sequence current is produced in the generator.
The negative sequence current produces a rotating magnetic field that rotates at synchronous speed. And the direction of rotation is opposite to the rotor.
Therefore, the effective relative speed between the two is double the synchronous speed. And the EMF get induced, having double the normal frequency in the rotor winding.
Due to the induced EMF, the circulating current is responsible for temperature rise in rotor winding and stamping.
Continuous unbalanced load of more than 10% of rated load cause tremendous temperature rise, especially in case of a cylindrical rotor of turbo alternator.
Negative sequence protection is essential to avoid dangerous situations by unbalanced loading.
In a generator, the overvoltage occurs due to the over speeding of the rotor or malfunction of a voltage regulator.
Not only the internal overvoltage is dangerous. But atmospheric voltages can also reach the generators. The lighting produces these atmospheric surges stroked on a high voltage transmission line.
These surges are transferred to the generator by inductively and capacitively. The surge arrester and surge capacitors are used to avoid the effect of lighting stroke.
Another reason for overvoltage is when the contacts of a circuit breaker open or close, a transient overvoltage surge is produced. This type of surge is known as a switching surge.
The switching surges are eliminated by using a modern circuit breaker, and sometimes, RC surge suppressors are used.
During arcing ground, transient overvoltage is generated. The amplitude of this overvoltage is more than five times of standard line to peak amplitude.
This type of surge is dangerous and can be eliminated by using resistance earthing.
Failure of Prime mover
The failure of a prime move results in the motoring operation of an alternator. The generator draws active power from the network and continues to run at synchronous speed. And it operates as a synchronous motor.
This may lead to dangerous mechanical conditions if it is allowed to operate for more than twenty seconds.
There is seriously overheating in temperature blades. To avoid this situation, reverse power protection is achieved by directional power relays.
Loss of Excitation
The loss of excitation occurs due to;
- Failure in the field winding
- Open-circuit or short-circuit in field winding
- Fault in excitation system
The loss of excitation results in loss of synchronism within a second. And it causes the increase in speed of the rotor.
The generator starts operating as an induction generator as it draws reactive power from the network. And the generator starts drawing excitation current from a network equal to the full load rated value.
It results in temperature rise in a stator winding and overheating in the rotor body due to induced current.
It is also responsible for the pole slipping condition resulting in the voltage reduction for the output above half of the rated load.
The loss of excitation must not persist for a long time and shut off the generator immediately. To avoid this situation, a tripping scheme is used to trip the generator circuit breaker immediately after the failure in the field.
Cooling System Failure
The cooling system plays a vital role to maintain the temperature of different parts of the large generator. If it fails to operate, it causes severe overheating above the safe limit and leads to insulation failure.
The thermocouples or resistance thermometers are used in a large machine to sense the temperature.
When the temperature exceeds the predefined limit, corrective measures must be taken to avoid temperature rise.
Apart from the above abnormal conditions, there are some other conditions that are rarely observed in practice. These conditions are;
- Wrong synchronization
- Bearing current
- Moisture in generator winding
- Leakage in hydrogen circuit
- Local overheating
- Oxygen in the pure water circuit
- Excessive bearing temperature, etc.
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