Generator protection relays

A generator is subjected to electrical stresses obligatory on the insulation of the machine, mechanical forces functioning on the varied elements of the machine, and temperature rise. These are the most factors that build protection necessary for the generator or generator. Even once properly used, a machine in its good running condition doesn’t solely maintain its nominal rated performance for several years, however it will conjointly repeatedly stand up to bound more than over load.

Hence, preventive measures should be taken against overloads and abnormal conditions of the machine in order that it will serve safely. Despite of sound, economical style, construction, operation, and preventive means that of protection, the chance of that fault can’t be fully eliminated from any machine.The devices employed in generator protection, make sure the fault, created dead as quickly as attainable.

An electrical generator is subjected to either internal fault or external fault or each. The generators ar usually connected to associate power system, thus any fault occurred within the power grid ought to even be cleared from the generator as shortly as potential otherwise it should produce permanent injury within the generator.The number and type of faults occur in generator, are huge. that’s why generator or generator is protected with many protecting schemes. The generator protection is of each discriminative and non-discriminative kind. care is to be taken in coordinating the systems used and therefore the settings adopted, so the sensitive, selective and discriminative generator protection theme is achieved.

Types of Generator Protection

The various sorts of protection applied to the generator may be categorised into 2 manners,

1.Protective relays to sight faults occurring outside the generator.
2.Protective relays to find faults occurring within the generator.
Other than protecting relays, associated directly with the generator and its associated electrical device, there are lightning arrestors, over speed safe guards, oil flow devises and temperature activity devises for shaft bearing, stator coil winding, electrical device winding and electrical device oil etc. a number of these protecting arrangement area unit of non-trip sort i.e. they solely generate alarm throughout abnormalities.
But the opposite protecting schemes ultimately operate master tripping relay of the generator. this could be noted that no protecting relay will forestall fault, it solely indicates and minimizes the period of the fault to forestall extreme temperature rise within the generator otherwise there is also permanent injury in it. it’s fascinating to avoid any undue tresses within the generator, and for that it’s usual follow to put in surge capacitance or surge diverter or each to scale back the consequences of lightning and alternative voltage surges on the machine. The protection schemes sometimes applied to the generator square measure mentioned here below in short.

Protection against Insulation Failure

The main protection provided within the mechanical device winding against section to section or section to earth fault, is longitudinal differential protection of generator. Second most vital protection theme for mechanical device winding is repose flip fault protection. this kind of protection was thought of gratuitous in previous days as a result of breakdown of insulation between points within the same section winding, contained within the same slot, and between that a possible distinction exists, terribly quickly changes into AN earth fault, then it’s detected by either the mechanical device differential protection or the mechanical device earth fault protection. A generator is meant to provide comparatively high voltage as compared to its output and that thus contains an outsized variety of conductors per slot. With increasing size and voltage of the generator, this type of protection is turning into essential for all massive generating units.

Stator Earth Fault Protection

When the mechanical device neutral is earthed through a resistance, a current electrical device is mounted within the neutral to earth affiliation. Inverse Time Relay is employed across the CT secondary once the generator is connected on to the bus bar. just in case of generator feeds power via a delta star electrical device, an instant Relay is employed for constant purpose. within the former case, the planet faults relay is needed to be stratified with different fault relays within the system. this can be the explanation why Inverse Time Relay is employed during this case. however within the latter case, the planet fault loop is restricted to the mechanical device winding and first winding of the electrical device, hence, there’s no want of grading or discrimination with different earth fault relays within the system. that’s why instant Relay is preferred within the case.

Rotor Earth Fault Protection

A single earth fault doesn’t produce any major drawback within the generator however if the second earth fault is occurred, however, a part of the sector winding can become short-circuited and ensuing and unbalanced magnetic flux within the system and consequently there could also be major mechanical injury to the bearings of the generator. There square measure 3 strategies offered to sight the categories of fault within the rotor. The strategies are

  1. Potentiometer method
  2. AC injection method
  3. DC injection method

Unbalanced Stator Loading Protection

Unbalancing in loading produces negative sequence currents within the stator coil circuit. This negative sequence current produces a reaction field rotating at double of synchronous speed with relevancy the rotor and thus induce double frequency current within the rotor. This current is sort of giant and causes heating within the rotor circuit, particularly within the generator. If any unbalancing occurred because of fault within the stator coil winding itself, that may be cleared outright by the differential protection provided within the generator. If the unbalancing is occurred because of any external fault or unbalanced loading within the system, could|it’s going to|it should} stay unobserved or may persist for a big amount of your time counting on the protection coordination of the system. These faults then be cleared by putting in a negative section sequence relay with the characteristics to match the face up to curve of the machine

Protection against Stator Overheating

Overloading will causes warming within the stator coil winding of the generator. Not solely overloading, failure of cooling systems and insulation failure of stator coil laminations conjointly cause warming of the stator coil winding. The warming is detected by embedded temperature detectors at varied points within the stator coil winding. The temperature detector coils ar ordinarily resistance components that kind one arm of the bridge circuit circuit. within the case of smaller generator ordinarily below thirty MW, the generators don’t seem to be equipped with embedded temperature coil however ar sometimes fitted with thermal relay and that they ar organized to live the present flowing within the stator coil winding. This arrangement solely detects warming caused by overloading and doesn’t offer any protection against warming because of failure of cooling systems or short circuited stator coil laminations. though over current relays, negative section sequence relays, and devises for observance constant flow are wont to offer a definite degree of thermal overload protection.

Low Vacuum Protection

This protection, typically is within the type of a regulator that compares the vacuum against gas pressure, it’s ordinarily fitted to the generator set higher than thirty MW. the fashionable apply is for the regulator to unload the set via the secondary governor till traditional vacuum conditions ar restored. If the vacuum conditions don’t improve below twenty one in. the stop valves ar closed and therefore the main fuse is tripped.

Protection against Lubrication Oil Failure

This protection isn’t thought-about essential since the lubrication oil is often obtained from identical pump as governor oil and a failure of the governor oil can mechanically create stop valve to shut.

Protection against Loss of Boiler Firing

Two strategies are accessible for sleuthing the loss of boiler firing. within the 1st methodology, unremarkably opened (NO) contacts area unit given the fan motors which can trip the generator if quite 2 motors fail. The second strategies use a boiler pressure contacts that unload the generator if boiler pressure falls below around ninetieth.

Protection against Prime Mover Failure

If the first cause fails to provide energy to the generator, the generator can still rotate in driving mode meaning it takes current from the system rather than activity it to the system. In rotary engine|turbine} set the steam acts as a fluid maintaining the turbine blades at a relentless temperature. Failure of the availability can thus lead to heating because of friction, with ensuant distortion of rotary engine blades. The, failure of steam offer will cause severe mechanical harm additionally of imposing a significant driving load on the generator. Reverse power relay is employed for this purpose. As shortly because the generator starts rotating in driving mode, the reverse power relay can trip the generator set.

Over Speed Protection

While it’s the overall follow to produce mechanical over speed devices on each steam and hydro rotary engine, that operate directly on the steam throttle or main step valve, it’s not usual to backup this devises by associate degree over speed relay on steam driven sets. It is, however, thought of sensible follow on electricity units, because the response of the governor is relatively slow and therefore the set is additional liable to over-speed. The relay once fitted is sometimes provided from the static magnet generator used for the management of governor.

Protection against Rotor Distortion

The cooling rates following termination, at the highest and bottom of the rotary engine casing, square measure totally different and this uneven temperature distribution tends to cause destruction of the rotor. to reduce the disruption, it’s common apply to show the rotor at low speed throughout the cooling down amount. within the read of the forces involved massive trendy rotor, it’s currently customary apply to suit shaft eccentricity detectors.

Protection against Difference in Expansion between Rotating and Stationary parts

During the running up amount, the speed of heating of the rotor differs from that of the casing, thanks to the distinction in mass. As a result, the rotor expands at a unique rate to the casing and it’s necessary to beat this unequal growth. to the present finish, proposition is created on the larger machine for freelance provides of steam to be set to bound joints on the casing. it’s fascinating thus to produce a way of measure the axial growth to help the operator to feed the steam to the right points and conjointly to produce indication of any dangerous growth. The shaft axial growth detector is largely the same as the instrumentality represented for rotor distortion instrumentality, except that the detector magnets square measure mounted to the rotary engine casing.

Protection against Vibration

Vibration detectors ar sometimes mounted on the bearing pedestals. The detector consists of a coil mounted on springs between U formed permanent magnets. The voltage output from the coil, that is proportional to the degree of vibration, is passed from the coil into group action circuits so into interval indicating instrument.

Back up Protection of Generator

Back up protection should tend in extremely rated machine like synchronous generator or generator. If faults occurred had not been cleared by the suitable protection theme then duplicate protection relays ought to be operated to clear the fault. Over current relays ar usually used for this purpose. as a result of the synchronous electrical phenomenon of contemporary machine is usually bigger than hundred %, the sustained fault current fed from the machine into associate external fault is invariably below the conventional full load current. the conventional IDMT relays wouldn’t prove satisfactory as a result of their current settings should be about to the total load and their time sitting short if operation is to be obtained, leading to probable lack of discrimination with different over current relays within the system. Further, the over current relay would likely operate for loss of field on the machine, disconnecting it untimely. to beat this drawback is it’s become customary to use associate over current relay together with beneath voltage relay.

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