Generator Protection
The Protection of The generator are the most complex and time-consuming for the following reasons:
- The generator is a large machine connected to buses. These include block transformers, auxiliary transformers and a busbar system.
- It has an excitation system, a driving machine, a voltage generator , controller, cooling system, etc., so it is not a single device. The generator protection must be adapted to the associated devices.
- It is an expensive and important device. If possible, it should not be turned off, as this would cause a power outage and emergency.
In the unitary generator connection system, the generator is connected to the low voltage side of the main grid. increase transformer (GT) and the AGO of auxiliary transformer unit (UAT). The high voltage side of the main transformer is connected to the bus via a distribution board, from where the electricity is fed into the grid. The UAT powers auxiliary equipment that is directly connected to the unit. The generator and main generator transformer (GT) form a team, and each unit has a boiler, turbine, condenser and other auxiliary systems.
When choosing the scheme for Generator Protection In addition to the safety of the generator, the safety of the entire system and the stability of the system in the event of a generator failure must also be taken into account.
Therefore, a generator is an expensive device that requires protection against a variety of possible failures.
Generator error
Some of the important generator errors are:
Abnormal operating conditions
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Main engine failure
If the input to the main engine fails, the generator works as a synchronous engine and draws electricity from the supply network. This engine condition is called “reverse running”.
With a turbogenerator, loss of steam supply can cause reverse operation. As the steam supply is gradually restored, the generator takes over the load without disturbing the system. If the steam supply failure is prolonged, the generator will shut down safely and this condition is relatively harmless. Therefore, no automatic protection is required.
The hydrosets are protected against reversal by mechanical devices in the water wheel . The generator is disconnected from the system when the water flow drops to a low level to maintain electrical production. Therefore, no electrical protection is required in this case.
Field failure
The probability of generator field failure is very low. In modern machines there is a Field Failure Relay is employed.
Opening the field switch or field circuit breaker may result in field failure or loss. In these cases, the generator loses field excitation, accelerates slightly and acts as an induction generator where excitation is derived from the system and power is supplied at a leading power factor. In this condition, stator currents may rise above the machine's average rating and overheat the stator winding.
Overcurrent protection
Overcurrent occurs mainly due to partial failure of the winding insulation or overload of the supply system. In electrical systems where currents increase and current flows, overcurrent emerges as a potential threat. Overcurrent, characterized by excessive discharge of electrical current beyond safe limits, can destroy equipment, disrupt operations, and even pose significant risks to life and property. To tame this formidable enemy, electrical engineering has developed a robust defense mechanism known as overcurrent protection.
Overspeed protection
Overspeed occurs when all or most of the load is suddenly lost. In these cases, the valve that controls the entry of steam or water is turned off by mechanical devices; This prevents dangerous speeding.
Overvoltage protection
An overvoltage in a generator can occur when the main engine speed increases Very sensitive to speed fluctuations due to a sudden loss of load on the generator or due to defects. The inspector continuously checks for excess speed and thus prevents overvoltage on the generator set.
For hydroelectric generators, when the generated voltage rises 20% above the nominal/normal value, the overvoltage relay activates the generator main switch and the generator field switch.
Protection against incorrect charging
Unbalanced charging means that there are different phase currents in the generator. This phenomenon occurs from Earth faults or phase faults in the circuit outside the generator. If unbalanced currents persist, they can seriously burn the rotor core or damage the field winding.
Unbalance conditions result in negative sequence currents that create an armature reaction field that rotates in the opposite direction to the rotor and creates a flux that flows through the rotor at twice the rotational speed. Therefore, interference currents with twice the machine frequency are contained in the rotor body. Field winding and damper winding These fluxes lead to overheating of the stator .
vibration
Vibration is caused by rotor overheating or a mechanical defect or abnormality. Rotor overheating is caused by unbalanced stator currents or rotor ground faults . Overheating deforms the rotor, causing eccentricity. Eccentric running creates vibrations. Protection against unbalanced stator currents and rotor ground faults minimizes vibrations. A vibration meter is also used to detect the impulse caused by electrical or mechanical causes. If there is excessive vibration, an alarm will be triggered.
Bearing overheating
The bearing temperature is adjusted by inserting a Temperature Sensor into a hole in that direction. In large machines where lubricating oil circulates through the path, an oil flow device is used to detect the failure of the oil cooling system. An alarm is triggered if the stroke overheats or there is a failure in the circulation of lubricating oil.
Voltage regulator failure
Modern, fast-reacting automatic voltage regulators are very complex. They are susceptible to component failures. Suitable protective devices are provided to prevent its failure. It is a DD.C time overcurrent relay. which is activated when there is overcurrent in the rotor circuit for a period of time that exceeds a prescribed limit. In such a situation, the excitation is switched to a predetermined value for manual control.
Voltage regulator voltage is supplied through a separate voltage converter . Protection against failure of the controller reference voltage is ensured by the use of a voltage compensation relay that compares the voltage derived from the measuring transformer with the voltage from the Voltage Regulating Transformer When the voltage regulator is operated manually based on the voltage reference, the relay is activated and switches the excitation for manual control to a preset value.
Stator winding failure
- Phase-to-ground error
Rotor error
- Stored Earth Faults
- Loss of excitement
- Rotor overheating due to unbalanced three-phase stator currents
Protection systems
A modern generator is usually equipped with the following protection mechanisms.
Stator protection
- Percentage differential protection (for phase-to-phase faults, three-phase faults and phase-to-ground faults up to approximately 85% of the windings).
- the degree of protection against earth fault
- Short circuit protection in the stator winding
- Stator overheating protection
Rotor protection
- Ground fault protection in the field
- Loss of excitation protection
- Protection against rotor overheating due to unbalanced three-phase stator currents.
Conclusion
In summary, the role of “Generator Guardians” in generator safety is fundamental to ensuring uninterrupted power supply and protecting critical infrastructure. These surveillance systems use advanced technologies such as remote monitoring, automatic shutdown mechanisms and predictive maintenance to mitigate potential risks and optimize generator performance. By proactively identifying faults, malfunctions or anomalies, Generator Guardians prevent costly failures and reduce downtime, ultimately contributing to improved operational efficiency and reduced maintenance costs. Generator protection is essential in various sectors, from hospitals and data centers to manufacturing facilities and residential areas, as it directly impacts daily activities and safety. Adopting and investing in these advanced protection systems is a prudent approach to ensuring a reliable and sustainable energy production scenario for the future.
