A generator is a piece of apparatus or equipment which converts energy from the chemical form to electrical form. Generators are subjected to electrical stress on the machine insulation, mechanical forces working on different variable components of the machine and rise in temperature. A machine in good condition doesn’t only keep up a trifling performance for a couple of years, it will also stand up to far more than machine overload. With this in mind, preventive measures should be undertaken to keep off overloads and other abnormal conditions concerning the machine so it serves to perfection. All things must be taken into account, example construction, economic style, operation, sound etc nothing should be eliminated.
In an electrical generator if there are two faults to look out for: Internal fault and External fault. The electrical generator can be made subject to either or both faults. These generators are usually linked to a power system (or systems) therefore any fault occurring in the power grid should be cleared from the generator quickly as potential otherwise he could develop into a permanent injury in the generator.
There are many schemes designed to protect the generator as the faults are numerous and large in size. Generator protection can be broken down into parts. Care should be taken in synchronising systems used and by extension, the settings obtained so all sensitive, discriminative and selective generator protection themes are achieved. The three types of generator protection are:
1. Protective relays for fault detection within generator
2. Protective relays for fault detection outside the generator.
Apart from these relays, there are oil flow devices, lightning arresters, strator coil winding shaft bearing devices for temperature, overspeed safeguards, electrical devices, oil and winding etc. Some of these protecting devices are units of non-trip or stop this means these devices only generate alarm throughout all abnormalities.
protecting skins basically operate the master tripping relay of the generator. Note that protective relays do not delay fault, they only indicate and minimise its time period of fault to prevent extreme temperature rise within generators else permanent injury will be seen. It is important to avoid unnecessary knots within the generator and to this end, one must put in surge capacitance or a combination of surge capacitance and surge diverter. This helps to reduce the consequences of alternative and lightening surgeries in the machine. Below are some protection schemes usually applicable in generators: Stator Earth Fault Protection, Protection against Insulation failure, Rotor failure Fault Protection, Protection against Stator Overheating, Low Vacm Protection, Protection from unbalanced stator, Protection against Loss of boiler firing, Protection against Prime Mover failure, Protection against Lubrication failure, Overspeed protection, Protection against Rotor Distortion, Protection against vibration, Protection against difference in expansion between rotating and stationary parts and Back up Protection of Generators.
1. Stator Earth Fault Protection
when the device neutral line is past a resistance in current electrical device is set within the neutral to earth affiliations. When relay operation time is inversely proportional to the fault current it is known as Inverse Time Relay. This phenomenon is applied across the CT secondary section once the generator is connected on to the bus bar; in this case power is fed from the generator via delta star electrical device and instant relay is applied for constant purposes. In the former situation, the planet faults relay is needed to be sequentially arranged with various fault relays within the system. This is possibly the explanation as to why Inverse Time Relay is used in this situation. In the latter situation, the planet fault loop is limited to the mechanical device winding and the first winding of the electrical device therefore there is no segregation with different earth fault relays in the system. For this reason, Inverse Relay is preferred in this case.
2. Protection against Insulation Failure
Longitudinal differential protection is the paramount protection within the winding of a generator.
Repose Flip Fault Protection is another protection for the winding of a generator. In previous days, the protection type was thought to be unfounded as a result of insulation breakdown between points in the same winding section.
Ideally, a generator should provide high voltage as compared to its output and that involves an outsized variety of conductors per slot. As generator size and voltage increases, this protection type is becoming terribly essential for massive units of generation.
3. Rotor Earth Fault Protection
A single Earth fault doesn’t produce any serious disadvantage in the generator but if the second Earth fault occurs, a part of the winding sector can short-circuit and ensue therefore unbalancing magnetic flux in the system and consequently there could be serious mechanical injury to the generator bearings. There are three working strategies offered in fault groups in the rotor. These are:
a. AC injection method
b. DC injection method
c. Potentiometer method
4. Protection against Stator Overheating
Overloading will cause slow and steady warming in the stator coil winding of the generator. Apart from overloading, insulation and cooling system failures in stator oil laminations also cause overheating of the stator coil winding. The heating is discovered by deeply lodged temperature detectors at different points in the stator coil winding. Usually, temperature detector coils are designed to be highly resistant. They serve as an arm of the bridge circuit. Smaller generators do not usually have these temperature coils. Instead, they have a thermal relay which tracks the flow of current in the stator coil. By this arrangement, only detection of overheating is possible, there is no protection from overheating (caused by cooling system failure and or short-circuiting stator oil laminations).
5. Low vacuum Protection
Now, this type of protection is within a kind of regulator that compares the vacuum against gas pressure. As a general rule, it is fitted to the generator and its power set higher than thirty (30) MW. The more appealing thing is for the regulator to discharge the set via the secondary governor until the usual vacuum conditions are restored.
6. Unbalanced Stator Loading Protection
An imbalance in load produces negative currents in the starter coil circuit. The negativity of this current produces a reaction field which rotates at double synchronized speed with its rotor and induces a higher magnitude of current in the rotor. This is a giant current which causes heating in the rotor circuit, specifically inside the generator Also, imbalance that occurs as a result of external faults or unbalanced loading in the system can be resolved by not observing said issue. If it continues, these faults can be cleared by putting in a negative section sequence relay with traits to match with the curve of the machine.
7. Protection against Loss of Boiling firing
Two techniques are double in investigating the loss of boiler firing. In the first technique non-distinguishable opened contact area unit given the fan motors that can trip the generator of two motors fail. The second technique uses boiler pressure contacts that unburden the generator if the pressure in the boiler sinks below ninety (90).
8. Protection against the failure of a Prime Mover
It is still possible for the generator to rotate even if energy is not a consequence of the first cause. Driving mode basically means it takes current from the system rather than send it to the system. In a rotary engine set, the steam acts as a fluid maintaining oil turbine blades at an unyielding temperature. Availability failure can lead to heating due to friction with distorted rotary engine blades. The steam failure of our could cause critical mechanical damage plus enforcing an impressive driving load on the generator. Reverse power relay is used for this reason. Now because the generator starts to rotate in driving mode (look up the meaning above) the reverse power relay could possibly trip the generator.
9. Protection against Lubrication failure
This protection type is not thought of as necessary since lubrication oil is often gained from an identical pump as the failure of governor or oil and we create a stop to shut the valve.
10. Overspeed Protection
developing mechanical overspeed devices on every Steam and hydro rotary engine produced that operate on the steam throttle or main step valve is the general way of things. Though not the norm to back up these devices by degree overspeed relay on steam driven sets, it is seen as smart and sensible to lead on electricity units and because of the response of the governor it is sort of slow and so is set liable to over-speed. Once fixed perfectly, the relay is at times provided from the static magnet generator used for governor management.
11. Protection against Rotor Distortion
Following termination, the cooling rates at highest and lowest of the rotary engine casing certain measures (examples: Square) are totally different and this irregular temperature distribution causes destruction of the rotor and thus reduces the disruption.it is also commonly applied to show the rotor at a low speed though its cooling down within the study of the forces concerned with the massive trendy rotor. It is currently at a requirement to apply shaft eccentricity detectors.
12. Protection against vibration
Vibration detectors are at times set up as pedestals. These detectors are made up of a coil set on springs and sandwiched between some permanent magnets. The voltage output from the coil is proportional to the degree of vibration and is passed from the coil to group action circuits and into interval indicating instrument.
13. Protection against expansive difference between stationary parts and rotating parts of the generator.
Due to a difference in mass, the rate at which the rotor gets heated is different from that of the casing. The result of this is the expansion of the rotor at a significantly to the casing and it is mandatory that the irregular growth be beat. Propositions are created on the bigger machine for freelance which makes provision for steam to be set to bind joints on the casing. In this, a strategy is produced for axial growth measure to assist in feeding him to all right points and jointly to produce signals of any risky or perilous growth. Shaft axle growth detector is mainly the exact same as portrayed or rooted rotor distortion except that the detector magnets are set to the rotary engine casing.
14. Back up Protection of Generators
Backup protection is good for strongly rated machines like synchronized generators. If faults were to occur and perchance had not been taken care of by the adequate protection theme then identical protection relays should be made and used to erase the fault. Overcurrent relays are mostly used for this reason. As the end product of the synchronous electrical phenomenon of modern machines is a lot larger than hundred percent (100%), the maintained fault current fed from the machine into external fault is invariably below the usual full load current. the typical idmt relays would not prove satisfying as a result of their current settings. The overcurrent relay would furthermore probably work for loss of field on the machine disconnecting it’s finally the only way to beat this is to allow it to be mandatory to associate over current relay together.
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