protection is subject to the electrical stresses imposed on the insulation of the machine, the mechanical forces acting on the different parts of the machine and the increase in temperature. These are the main factors that make generator or alternator protection necessary. Even if used correctly, a perfectly functioning machine not only maintains the specified rated performance for many years, but also repeatedly resists some overload.
Preventive measures must be taken against overloads and abnormal conditions of the machine so that it can operate safely. Also ensuring efficient design, construction, operation and preventive means of protection: the risk of breakdowns cannot be completely eliminated by any machine. The devices used to protect the generator ensure that, in the event of a fault, it is removed as quickly as possible.
An electric generator can be subject to internal or external faults or both. Generators are normally connected to a power supply system, therefore any failure that occurs in the power supply system must be removed from the generator as soon as possible, otherwise it could cause permanent damage to the generator.
The number and variety of failures that occur in a generator are enormous. That’s why the generator or alternator is protected with various protection schemes. Generator protection is discriminatory and non-discriminatory. Great attention must be paid to the coordination of the systems used and the configurations adopted to ensure the achievement of a sensitive, selective and discriminatory generator protection scheme.
Types of generator protection
The various forms of protection applied to the generator can be classified in two ways,
Protection relay to detect faults that occur outside the generator.
Protection relay to detect faults that occur inside the generator.
In addition to the protection relays, associated directly with the generator and its transformer, there are lightning rods, overspeed protections, oil flow devices and temperature measuring devices for shaft bearings, stator winding, transformer and oil for transformers etc. Some of these protective devices are not provisions on the type of trip, that is, they generate an alarm only during anomalies.
But the other protection schemes eventually operate the generator’s main trip relay. It should be noted that no protection relay can prevent faults, it only indicates and minimizes the duration of the fault to avoid the increase in the high temperature in the generator; otherwise, there may be permanent damage.
It is desirable to avoid undue stress on the generator and for this it is normal practice to install an overvoltage capacitor or an overvoltage diverter or both to reduce the effects of lightning and other voltage peaks on the machine. The protection schemes generally applied to the generator are discussed briefly below.
Protection against insulation faults
The main protection provided in the stator winding against phase-phase or phase-earth faults is the longitudinal differential protection of the generator. The second most important protection scheme for stator winding is protection from shift failures.
This type of protection was considered unnecessary in the previous days because the breakdown of the insulation between points in the same phase winding, contained in the same slot, and between which there is a potential difference, turns very quickly into a ground fault, and therefore it is detected by the stator differential protection or the stator earth fault protection.
A generator is designed to produce a relatively high voltage relative to its output and therefore contains a large number of conductors per slot. With the increase in size and voltage of the generator, this form of protection is becoming essential for all large generation units.
Stator earth fault protection
When the stator neutral is grounded through a resistor, a current transformer is mounted on the neutral-ground connection. The reverse time relay is used on the CT secondary when the generator is connected directly to the bus bar. In the event that the generator supplies energy through a delta star transformer, an instantaneous relay is used for the same purpose.
In the first case, the earth fault relay must be evaluated with other fault relays in the system. This is why the reverse time relay is used in this case. But in the latter case, the earth fault circuit is limited to the stator winding and the transformer primary winding, so there is no need to classify or discriminate with other earth fault relays in the system. That’s why in the case the instantaneous relay is preferable.
This type of protection was considered superfluous in the previous days because the breakdown of the insulation between points in the same phase winding, contained in the same slot, and between which there is a potential difference, turns very quickly into a ground fault and is therefore detected by the stator differential protection or stator earth fault protection.
A generator is designed to produce a relatively high voltage relative to its output and therefore contains a large number of conductors per slot. With the increase in size and voltage of the generator, this form of protection is becoming essential for all high generation units.
Stator earth fault protection
When the neutral stator is grounded through a resistance, a current transformer is mounted on the neutral ground connection. The reverse time relay is used on the CT secondary when the generator is connected directly to the bus bar. In the event that the generator supplies energy through a delta star transformer, an instantaneous relay is used for the same purpose.
In the first case, the earth fault relay must be evaluated with other fault relays in the system. This is why the reverse time relay is used in this case. But in the latter case, the earth fault circuit is limited to the stator winding and the transformer primary winding, so there is no need to classify or discriminate with other earth fault relays in the system. That’s why in case the instantaneous relay is preferable.
Stator overheating protection
Overloading can cause the generator stator winding to overheat. Not only overload, cooling system failure and stator blade insulation failure also cause the stator winding to overheat.
Overheating is detected by temperature detectors integrated in various points of the stator winding. The coils of the temperature detector are normally resistance elements that form an arm of the Wheatstone bridge circuit. In the case of a smaller generator, generally less than 30 MW, the generators are not equipped with an integrated temperature coil, but are generally equipped with a thermal relay and are designed to measure the current flowing in the stator winding.
This arrangement only detects overheating caused by overload and does not provide any protection against overheating due to cooling system failures or short stator laminations. Although overcurrent relays, negative phase sequence relays and devices for controlling constant flow are also used to provide some degree of thermal overload protection.
Low vacuum protection
This protection, generally in the form of a regulator that compares the vacuum with the atmospheric pressure, normally adapts to the generator set above 30 MW. Modern practice requires that the regulator discharges the group through the secondary regulator until normal vacuum conditions are restored. If the vacuum conditions do not improve below 21 inches, the shut-off valves close and the main switch trips.
Protection against breakage of lubrication oil
This protection is not considered essential as the lubricating oil is normally obtained from the regulator oil pump itself and a regulator oil failure causes the shut-off valve to close automatically.
Protection against boiler leakage
Two methods are available to detect boiler combustion loss. In the first method, normally open (NO) contacts are provided with the fan motors which can trip the generator in the event of failure of more than two motors. The second methods use boiler pressure contacts which discharge the generator if the boiler pressure drops below approximately 90%.
Main motor failure protection
If the primary engine is unable to supply mechanical power to the generator, the generator will continue to rotate in engine mode, which means it absorbs electricity from the system rather than supplying it to the system.
In a steam turbine, the steam acts as a coolant by keeping the turbine blades at a constant temperature. Therefore, interrupting the power supply will cause overheating due to friction, with consequent distortion of the turbine blades.
Although it is common practice to provide overspeed mechanical devices on both the steam turbine and hydraulic turbine, which operate directly on the steam throttle valve or main shutoff valve, it is not common to back up these overspeed relay devices to steam powered appliances.
However, it is considered a good practice in hydroelectric units, since the regulator response is relatively slow and the whole is more subject to excessive speed. The installed relay is generally supplied by the permanent magnet generator used to control the regulator.
Protection against rotor distortion
The cooling rates after stopping at the top and bottom of the turbine housing are different and this uneven temperature distribution tends to cause destruction of the rotor. To minimize interruption, it is common practice to rotate the rotor at low speed during the cooling period. In view of the forces involved in a large modern rotor, it is now standard practice to install eccentric shaft detectors.