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The Jebel Ali power and desalination station M is designed to provide power and distilled water in a cogeneration process by using the following design scheme:
Combined cycle gas turbine with its own HRSG and steam turbine for power and steam generation.
The whole power system is split into 3 blocks. Each block consists of 2 Gas Turbines (Siemens SGT5-400F) with their own dedicated HRSG and condensing extraction type Alstom Steam Turbine (HDC200/2AS-NE33AU) with an adaptive stage.
HRSG is of type Dual Pressure and is accompanied by a duct burner for secondary firing this firing is only available for Natural Gas firing.
Each block also contains 2 medium pressure Boilers.
Multi stage flash (MSF) evaporation purification technology is used for seawater desalination.
8 MSF desalination units combined have a capacity of around 140MIGD.
Under the normal operating conditions the gross output of the power plant and desalination system is 2000MW and 105 MIGD (with 6 MSF units running).
This plant is run on the principle of "Combined Cycle"
1.2 Plant Performance Data
Total Power Generated: 2000MW
Gross Power output at generator terminals:
Sum of all Gas Turbines : 1404.62MW (70.231%)
Sum of all Steam Turbines : 595.38MW (29.769%)
1.2.2 Auxiliary Electric Power Consumption
All GT packages including HRSG: 17064kW (0.86%)
All ST packages: 212.25kW (0.01%)
All generator transformers and unit auxiliary transformers: 16487.3kw (0.83%)
Balance of power plant: 3796.45kW (0.25%)
Heating, ventilation and air conditioning: 7000kW (0.35%)
Total electrical power consumed by the Power plant: 44.56MW (2.3%)
Total electrical power consumed by the Desalination Plant: 94MW (4.7%)
Net Power Consumed at Jebel Ali "M" Plant: 138.56MW (7%)
1.2.3 Net Power Output
We now know that even though this is a 2000MW power plant, 7% of that power is used to run the auxiliary units around the plant (which includes both power generation and desalination)
This leaves us with 93%
Net Power Output Jebel Ali "M" plant: 1861.44MW
1.3 Type of Fuel
This power plant uses natural gas as its main fuel and the backup fuel used is diesel oil. The Gas Turbine is a double fuel fired but on the other hand in the HRSG supplementary firing is available for natural gas only. The main gas fuel supply line is for the M station is connected to the existing main supply line installed along Jebel Ali power station complex. The supply for diesel fuel is taken from unloading bays or from the existing DFO 24" pipeline. Header fuel oil will be used to supply oil to the six DFO storage tanks. The substation voltage is 400kV and is gas insulated, which is connected to DEWA grid.
The table below gives us the typical natural gas composition, which is the primary fuel at Jebel Ali M station.
Table 1.1: Typical Natural gas composition
The most likely H2S content is likely to be less than 60ppm. But in some operation condition the H2S content may increase to 100-150ppm. H2S content of 250ppm is highly unlikely and is considered mainly for the DUSUP pipeline network design.
The usual CO2 content is highly less than 2.5-3.0 mole %. But the upper limit of 5% is not reached during normal operation.
Chapter 2: Electrical Relays
2.1 Basic Ideas of relay protection
An electrical power system should always ensure the supply of electrical energy without interruption to every load connected to the system. There are usually several thousands of kilometers of transmission lines running when the consumers are located far away from the power generating site. These are HV lines are usually in the open and are overhead transmission lines; there are many chances of their damage due to storms and damage to the insulations etc. A fault in any part of this line would not only cause electrical damage such as over current due to a surge but also mechanical damage which means snapping of these cable causing damage to life and property. A major problem which occurs due to electrical damage is the short circuit of these lines, which has to be dealt with as quickly as possible.
Any kind of fault is detected in the power system using relays and not only faults even abnormal conditions are recognized in the electrical system and it becomes necessary to operate the switchgear in a manner that will isolate the fault. By performing the action mentioned before we not only save the faulty equipment from further damage but also a system crash is prevented and the abnormality is not allowed to spread in the other surrounding regions. This is one of the major responsibilities of a relay i.e. to prevent our switchgear from further damage by coordinating with it in the process of isolation of faulty equipment.
The protective relay must be capable of not only regaining control of a system when a fault is detected but also to withstand the fault conditions for some time before the relay picks up the fault and isolates it. This is usually done without troubling any normal operation of surrounding equipment.
Protective relays don't prevent faults, they only take action once the fault is detected, and this is a point that should be noted. But a technology that could anticipate the faults and prevent that from happening would really be a breakthrough discovery.
An example of a device which can anticipate and prevent major faults is the Buchholz relay; it is capable of detecting faults cause by gas accumulation in a transformer
2.2 Nature and Causes of Faults
A fault simply implies any abnormality which causes depreciation in the basic insulation strength between phase conductors and earth; or between phase conductors. Whenever a large system with several components simultaneously running, like in a power plant we have transformers, switchgears, transmission lines also generators and distribution circuits, it is inevitable that a failure will in the system somewhere take place.
Due to larger length and more exposure to atmosphere probability of failure is respectively higher on power lines than any other part of the system.
The several causes of failure are mentioned below;
Due to deteriorating quality of the insulation and also because of unpredictable factors like perching of birds, accidental SC by snakes etc.; breakdown may occur at normal voltage.
On the opposite end, because of switching surges or surges caused by lightning or switching surges an abnormal voltage may be created in the system and this would damage insulation.
Nowadays it has become a common practice to insulate more than 3 or 5 times required value as a precaution of 3-5 times value of voltage. But even then, erosion of insulation occurs due to many environmental factors like deposited cement dust in industrial areas, or salt depositions on insulation of equipment in open coastal area, like in the Jebel Ali "M" Station power plant. The greatest concern for any insulation is erosion due to ageing also unpredictable and not equal rise and fall of temperature throughout the year cause irregular contractions and expansions which is also a major cause of insulation failure in the region.
Another major concern is that when lightning strikes it produces millions of volts and it is not logical to provide this abnormality with decent insulation. Surge impedance and line resistance are the limiting factors, when surges due to lightning travel in power circuits with the speed of light.
2.3 Consequences of a fault
Total failure of a system might occur when a fire starts due to an occurrence of a fault, this not only happens at the point where the fault has occurred. Short Circuit is the most common and also one of the most life threating faults which occur in a large system, this fault also causes severe damage to the system.
Some consequences are given below:
Under voltage occurs in a major part of the power system, this in turn causes loss of electrical supply to our consumer and wastage of production occurs.
A major problem which goes hand in hand with short circuit is the damage of elements in the system caused by the electrical arc.
Overheating and irregular mechanical forces could cause harm to important apparatus.
Stability of our system is a very important concern which gets shattered when a fault occurs and due to this instable part of our network, we may have to resort to complete system shutdown (rare cases).
Relays which work on mechanisms of pressure coils fail when a severe under voltage fault occurs.
Due to under voltage high current is produced in the no voltage coils of motors this causes definitive loss of production as the equipment has to be restarted and started from step one.
The table below gives us a breakdown of the areas where faults related to voltage, current or surges usually occur;
Percentage of total
Over Head lines
CT's and PT's
Table 2.1 Frequency of fault occurrence in different links of a power system
2.4 Zones of protection
This is a very important step in creating a design of a system, zone overlapping is the set law given to us. How to implement this law depends on the creative thinking of the designing committee. One or more components are included in the protection and that part of our power system is more commonly known as the protected zone.
The next figure shows us an example of arrangement for this method of protection;
C:\Users\Mohmed Talha\Desktop\doosan\project report (mid sem)\IMAGES\Zone protection.jpg
Fig 2.1 Overlapping zones of protection
2.5 Essential qualities of protection
There are 4 basic necessities that every protection system needs to have in order to pass by the current quality standards,
Fastness or speed of operation
The first 2 factors i.e. reliability and selectivity are very important because without these two the protection scheme would be very ineffective and also could cause more harm than good, i.e. become liable.
Probability of failure occurring in the protection scheme can be termed as reliability.
Breaker defects can also cause the reliability of a system to largely drop. Therefore every component is regarded as a probable source of failure in a protection scheme. It is not always necessary that the most expensive equipment would be the most ideal solution for our scheme but even economical equipment when maintained properly and regularly would perform a lot better in a fault situation than any other one, that performs that the same function. Another major factor which contributes to the reliability of the system is the knowledge and skill of the personnel involved in the maintenance of various circuit breakers, relays, switchgears and other components of our power system. Impregnated coils, braced joints, high contact pressures and dust free enclosures are some factors which make our relays today immensely reliable.
A study of power systems shows that order of probability of failure is (from high to low): relays, breakers, wiring, CT's, PT's and batteries. The failure rate increases more when the relays we use contain transistor components.
This is a factor which implies that only the fault is isolated and the surrounding equipment of our power system is kept in a healthy condition. If a protection responds to only surges in its own zone then this is known to be absolute protection; and if it uses the settings of several other zones to work out if a fault has occurred or no, is known as relative protection. In both cases the action taken depends on the equipment and the protection schemes.
2.5.3 Fastness of operation
All protective relays have to be fast in responding to a fault, a few reasons as to why are given below:
Never should the response time exceed CCT (Critical Clearing Time)
If fault currents are carried for too long it could cause heavy damage to any electrical apparatus connected to the system, this could also be life threatening.
If Under voltage is not detected and the equipment or fault not isolated, it would cause overloading of the drive.
Without loss of synchronous running load can be transferred from faulty equipment to a healthy one only if the fault is corrected over a short period of time.
This concept means that a protection scheme must easily differentiate between the overloading occurring in the other zone and reading when there is a fault in its zone, and should properly operate the designed protection in both cases. It should be able to discriminate between these two different fault phenomenon. Most importantly a relay in the system should know when there is an overload and when there is a fault.
This discrimination can be achieved in two easy ways, one if there is an inbuilt scheme in the relay and two, if this property can be added using auxiliaries like a minimum voltage relays.
2.6 Concept of primary and backup protection
So far I have explained what are the basic requirements of a protection scheme, also the concept of zone protection and factors that affect our protection schemes have been explained.
We now move on to the concept of primary and backup protection also known as redundancy, which means having copies of an original or having two piece of equipment for the same function. In this case the secondary can operate the system if the primary is taken out of service or is isolated for maintenance. This is one of the concepts which are highly implemented at Jebel Ali "M" station. Application of this idea is not very economical, but since it is a 2000MW power plant it would be justified to constantly be sure that the equipment and personnel working here are constantly protected. The idea behind this concept is that if a fault cannot be easily cleared by the primary scheme, due to malfunction of the equipment or some other reason; the backup protection can take its place and perform the same operation which would be done by the primary scheme in that zone.
2.7 Basic Principle of operation of a Protective System
It is a given fact that every part of a protection scheme is assigned to perform a specific operation in the case of a fault or surge, the same goes for all the relays in the system. This is explained better by taking the example that a particular relay will operate when it detects an increase in current over the required or set value or another may compare the voltage and current with the ration V/I and if this value is less than the set value a pickup will occur. The relay mentioned in the first example is known as the overcurrent relay and the other relay mentioned is known as an impedance relay. There are so many different types of relays in various protection schemes each of those relays has to perform a specific function and as I mentioned before all these functions are specified by design specialists. They have to make sure that the relay they use recognizes the particular fault and acts a certain way it is designed to, this freedom of weather to trip breakers in its own zone or trip an upstream or downstream breaker is given to the designers.
2.7 Types of Relays
This part of the report includes pictures of various types of relays we see at Jebel Ali "M" Station.
Fig 2.2 Latching relay
Fig 2.3 Reed relay
Fig 2.4 Mercury-wetted relay:
Description: C:\Users\vkash\Desktop\pathak1\polarized relayfor vehicle trackers.jpg
Fig 2.5 Polarized relay
Description: C:\Users\vkash\Desktop\pathak1\machine tool make timer relay.jpg
Fig 2.6 Machine tool relay
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Fig 2.7 Contactor relay
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Fig 2.8 Overload protection relay
Fig 2.9 Buchholz relay
Fig 2.10 Construction of Buchholz relay
Description: http://upload.wikimedia.org/wikipedia/commons/thumb/3/33/Solid_state_relay.jpg/220px-Solid_state_relay.jpg Description: http://upload.wikimedia.org/wikipedia/en/thumb/b/b4/Solid-state-contactor.jpg/220px-Solid-state-contactor.jpg
Fig 2.11 Solid-state relay
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Fig 2.12 Protective relay
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Fig 2.13 Overcurrent relay
Fig 2.14 working of disc overcurrent relay
Fig 2.15 Distance relay
Description: C:\Users\vkash\Desktop\Digital Current Differential Relay for Protecting EHV transmission Line765.jpg
Fig 2.16 Current Differential Protection
Chapter 3: Circuit Breakers
3.1 The Function and principle of a circuit breaker
The main of a circuit breaker is to control electrical power in a system by switching circuits ON, by carrying load and by switching circuits OFF under manual or automatic supervision. Circuit breakers are usually in a closed position while carrying load, or in an open position which provides electrical isolation.
They are summoned on to change from one condition to the other only occasionally, and to perform the special function of closing on to a faulty circuit or blocking short circuit current only on very rare occasions. Therefore the main property of a circuit breaker is that they must be reliable and work instantaneously to operate any switching operation when called upon after long period of time without movement.
During the past 50 years as a result of growth in network size, the severity of duties such as interruption of short circuits has immensely increased. Due to the growing technology in the world, network voltages have raised from 132 to 750kv now in this period experimental network systems of 1000kV are being built. SC ratings have risen from the order of 1x106 kVA on networks with low circuit severity factors and associated with ill- defined proof testing techniques, to 50x 106 kVA on networks that involve very high circuit severity factors, also these are associated with elaborate proof testing.
Earlier plain break oil circuit breaker designs required a rather variable time of 10-20 cycles to operate their switching functions. But the introduction of arc controlled systems quickly reduced it to 6-8 cycles, improving this technology further many designs have been now made that can operate within 2 cycles.
3.2 Requirements of a circuit breaker:
The power dealt by the circuit breakers is quite large and serves as an important link between the consumers and suppliers. The following are the necessary requirements for a circuit breaker or switchgear
It must safely interrupt the normal working current as well as the short circuit current
After occurrence of fault the switchgear must isolate the faulty circuit as quickly as possible i.e. keeping the delay to minimum
It must have a high sense of discrimination i.e. in systems where an alternate arrangements have been made for continuity of supply it should isolate the only faulty circuit without affecting any of the healthy ones.
It should not operate when an over current flows under healthy conditions
Circuit breaker Tripping schemes
Relay with make contact type
Relay with break contact type
The make type contact necessities auxiliary DC supply for operation, while the break type contact relays derive their tripping energy from main supply source, they are discussed as follows;
Relay with make contact type: The relays are connected in star, while their three contacts are connected in parallel and this parallel unit of contacts is connected in series with breaker auxiliary switch and trip coil to battery supply.
When a fault occurs on any of the phase the relay will close the contact this energizes the respective trip coil which opens the CB and along with it auxiliary switch is opened and the trip coil De energized, the supply of current to fault path is stopped and the relay contact comes to normal position. The advantage of the auxiliary switch is that breaking of the tripping circuit takes place only across this switch and arcing, etc. which is harmful to contacts over the relay contacts is avoided.
Relay with break type contact: The tripping circuit derives its energy from the main supply source through CT's or PT. The relay elements and the trip coil of each phase are connected in series and are so connected as to form a star connection. Under the normal working conditions the relay contacts are closed and at the same time the trip coils energized. When a fault occurs, the relay contacts open and CB trip coils are energized to open the CB.
3.3 Oil Circuit Breakers
The most successful of the arc interrupting systems in history was undoubtedly the oil CB which is still used in its principle nature in present day practice. The oil CB uses the properties of the arc by using its energy to crack the oil molecules and generate gas, principally hydrogen, which with properly designed control systems can be used to sweep, cool and compress arc plasma and so de-ionise itself with a self-extinguishing process. But this system was unstable and it became evident that there was a need for circuit breakers which possessed a more positive system of interruption than the fortuitous de-ionisation associated with uncontrolled gas and oil flow. An early and notable step up was the general electric (USA) H Type CB introduced in 1920s, which employed two metal explosion pots per phase, oil filled and with insulation nozzles through which the moving contacts were withdrawn vertically upwards, the explosion pot had been mounted on ceramic insulators within an air- insulated cubicle structure. Later, Slepian (Westinghouse) proposed a 'deion grid' in which the arc was forced to be submerged in the OCB tank, which increased 'effectiveness of the means of preventing the escape of gases generated in the vicinity of the arc without passing through the arc steam (Baker and Wilcox, 1930). Another approach was to use the arc to generate high pressures within a small insulating chamber immersed in the oil, such as that developed by GE (Prince and Skeats, 1931) in USA, which restricted oil and gas escape to an axial flow surrounding the arc plasma in the throat of the interrupter; and later in the cross flow interrupter developed by the British Electrical Research Association (Whitney and Wedmore, 1930), which forms the basis of many present day designs.
The controlled turbulence and high pressure and resultant rapid de-ionization in these systems eliminated the erratic operation of the plain break by virtually eliminating the leakage current, but with this it also eliminated the useful voltage damping and voltage control function this current had performed in previous designs, voltage division then reverting to the capacitance controlled distribution.
A desirable compromise would be to retain the advantages that leakage current can afford but eliminating the erratic nature of this control. No means of achieving this have as yet been suggested and this may remain in soluble, because of the difficulties of the control problem it creates. For this to take place in a surrounding in which dielectric stress imposed by the network is changing at several thousand volts per microsecond and in which arc plasma conductivity changes approximately a billion times as fast as temperature in the critical range of 1000-3000 K associated with thermal ionization.
The idea of a single break carrying out the whole duty however extended too high in voltage in some designs in terms of contemporary techniques at this period, some difficulty was observed in situations such as switching long open ended transmission lines. These limitations were associated with the electrical and mechanical strength of the insulation materials then available, which neither permitted the CN to be designed with the acceleration necessary to ensure restrike free switching, nor to have their jet assemblies restricted sufficiently to prevent the arc, in unfavorable situations, from flashing through the jets and along the outside of the interrupter, thus by-passing the interrupting mechanism provided.
The advances in performance of present day e.h.v. dead tank oil and low oil CB construction have been brought about by using the multibreak designs, but with the added complication of positive voltage control; by reducing the inertia of the moving parts through the use of new high tensile materials or eliminating mechanical linkages by the use of high pressure oil drives; by improved containment of the arc with the interrupter as the result of the grater pressures that can be sustained through the use of materials such as thread wound fiberglass; and by working on techniques for arc control, which include limited forced oil flow pressurizing of the interrupter. The overall complication of low oil circuit isolation switches, made possible by the improved internal dielectric parameters following shorter arc time.
The multibreak (Prince, 1935) impulse CB already referred to was a special case as it relied entirely on oil flow produced by a piston driven by external energy. The best known example of this type is the 8-break 287 kV 2500 MVA General Electric Boulder Dam installation commissioned in 1935, which afforded a 3-cycle interruption under all conditions of switching. These CB were also the first to be proved by means of realistic high power synthetic testing using current and voltage supplied from different circuits and synchronized within a few ÂµSec at current zero, using a system devised by Skeat's(1936). These tests were carried out without any sort of failure to an equivalent SC level in excess of 4000 MVA, and it is of historic interest to the world of synthetic testing, on which modern high power breakers rely largely for proof of rating, to note that these CB were still operating successfully, after 35years of service, in a network with a fault capacity of the order of 7000 MVA.
The high price of powerful equipment needed to drive the oil in both American and British models of this system discouraged future projects in this area, thus hampering development in this field, at a time when the modular construction of the air blast CB made possible began to be apparent. This together with a change away from oil and it should be considered that even engineering is not free from the influence of fashion encouraged a swing to airblast construction. Nevertheless the difficulties inherent in deciding on such long term development policies in switchgear are exemplified by the decade or more which passed before the HV airblast CB matched the best oil CB practice in both their performance and reliability.
3.4 Air Circuit Breakers:
Atmospheric air is used as an interrupting medium in an ACB. The arc is drawn between its contacts and extended via arc runners on to an arc chute where it is presented with a large cooling surface of arc splitter plates. These break the arc into a number of series arcs. The running principle of an ACB is the same as that of an MCB. Free air circuit breakers are often used in LV and MV applications up to approximately 20KV. A rated current of typically 4000A and also work perfectly in case of a SC current of up to 90kA at 12kV. Fault level, number of operations and types of load are applications of LV switchgears where tireless operation is required. Also due to economic considerations molded case CB have replaced many LV applications where previously ACB's were used. But, ACB still dominate in areas where high performance, long term reliability and maintainability are basic requirements. A very typical application to support this statement is in generating station's LV auxiliary supply.
The main application of HV ACB's has been in applications where the exclusion of flammable materials is a fundamental requirement. Again a typical application being in a generating stations HV auxiliary supplies, mainly 11kV.
But such high rated ACB's are very expensive and are not recommended, thus this is diminishing and the scales are tipping over to the more favorable SF6 circuit breakers. A further application of the ACB is for use with DC supplies, this method of interruption still being the most suitable for d.c. circuits. DC circuit breakers are widely used where ratings of up to 3 kV exist.
3.5 AIR BLAST CIRCUIT BREAKERS:
These use a blast of compressed air at a pressure of 25-75 bar which is directed across the arc patch to cool and remove ionized gas. Only when arc lengths are short and at first or zero current the air blast circuit breakers perform fast in interruption. Also in the receiver of the CB compressed air has to be stored locally. This local reserve has to be replenished from a local air compressor. Usually a suitable ring main network is used as a central system to feed the circuit breakers.
2 types of Air Blast Circuit Breaker exist:
Sequentially isolated circuit breaker - recloses after air blast
Pressurized head circuit breaker- remains open after air blast
3.6 SF6 circuit breakers
A circuit breaker in which the current carrying contacts operate in Sulphur Hexafluoride or SF6 gas is known as anÂ SF6 Circuit Breaker.
The main reasons why SF6 stand out is its electro negative and insulation property. Giving it a high affinity when it comes to absorbing an electron. This also means that whenever a free electro collides with SF6 it gets absorbed by it and creates a negative ion.
C:\Users\Mohmed Talha\Desktop\doosan\project report (mid sem)\SF6 CB.jpg
Fig 3. - Working of an SF6 CB
Disadvantages of SF6 breakers
SF6 is considered as a greenhouse gas and though it is very efficient in some circuit breakers, laws are being passed which restrict the emission of this gas into the atmosphere in some countries.
Also the energy requirement of an SF6 breaker is 5 times that of an oil circuit breaker which is not very economical
Fig 3. - One type of SF6 rotating arc principle
Types of SF6 circuit breakers:
Single interrupter- 220kV system
Double interrupter- 400kV system
Four interrupter- 715kV system
Working of the SF6 CB (ref. http://www.electrical4u.com/electrical-switchgear/sf6-circuit-breaker.php)
The working of SF6 CB of first generation was quite simple; it is some extent similar to air blast circuit breaker. Here SF6 gas was compressed and stored in a high pressure reservoir. DuringÂ operation of SF6 circuit breakerÂ this highly compressed gas is released through the arc and collected to relatively low pressure reservoir and then it pumped back to the high pressure reservoir for reutilize, Innovation of puffer type design makes operation of SF6 CB much easier. In buffer type design, the arc energy is utilized to develop pressure in the arcing chamber for arc quenching.Â Here the breaker is filled with SF6 gas at rated pressure. There are two fixed contact fitted with a specific contact gap. A sliding cylinder bridges these to fixed contacts. The cylinder can axially slide upward and downward along the contacts. There is one stationary piston inside the cylinder which is fixed with other stationary parts of the SF6 circuit breaker, in such a way that it cannot change its position during the movement of the cylinder. As the piston is fixed and cylinder is movable or sliding, the internal volume of the cylinder changes when the cylinder slides.
During opening of the breaker the cylinder moves downwards against position of the fixed piston hence the volume inside the cylinder is reduced which produces compressed SF6 gas inside the cylinder. The cylinder has numbers of side vents which were blocked by upper fixed contact body during closed position. As the cylinder move further downwards, these vent openings cross the upper fixed contact, and become unblocked and then compressed SF6 gas inside the cylinder will come out through this vents in high speed towards the arc and passes through the axial hole of the both fixed contacts. The arc is quenched during this flow of SF6 gas.
During closing of the SF6 circuit breaker, the sliding cylinder moves upwards and as the position of piston remains at fixed height, the volume of the cylinder increases which introduces low pressure inside the cylinder compared to the surrounding. Due to this pressure difference SF6 gas from surrounding will try to enter in the cylinder. The higher pressure gas will come through the axial hole of both fixed contact and enters into cylinder via vent and during this flow; the gas will quench the arc.
3.7 Vacuum Circuit Breakers:
Vacuum CB's do not require an interrupting medium or an insulation medium. The interrupters do not contain ionizable material
During the separation of current-carrying contacts, contact pressure reduces real contact surface reduces and the temperature of contacts increases to melting temperature, this produces metal vapors which initiates the vacuum arc, maintaining until the next current zero. Due to the special geometry of spiral contacts, the arc column is kept rotating by the radial magnetic field produced in order to involve a wider surface than that of a fixed contracted arc. Thus, overheating and erosion of the contacts are prevented. So the lifespan of the CB is increased.
Since there is no interruption or insulation material in the medium there is definitely no decomposition of gases or particles.
3.7.1 Advantages of vacuum circuit breakers:
Very long lifetime of the contacts
Less maintenance required
Less moving parts in mechanism
Less force needed to separate the contacts
Environment friendly. Since interruption takes place in a vacuum medium, VCB's do not require gas or liquid addition. This reduces the possibility of leakage of gas that can be harmful to the environment.
3.8 Miniature Circuit Breakers:
Miniature CB is only used at LV, mainly in domestic or light industrial or commercial operations. In general they are used in the same applications as semi-enclosed or cartridge fuses and offers an alternative for protecting radial or ring circuits. They are usually only single phase devices and have a typical rated load current range of up to 100A with a maximum SC rating of 16kA at 240V. Manually operated over center spring operating mechanisms are used. MCB's usually employ a series overload coil for rapid SC tripping and bimetallic element for tripping on overloads. All miniature CB operate on the air- break principle where an arc formed between the main contacts is forced, by means of an arc runner, and the magnetic effects of the SC currents, into metallic arc splitter plates. These cause a no. of series arc to be formed and at the same time extract energy from the arc and cool it to achieve a state called arc extinction.
With some design modifications of the MCB this arc interruption process can be so rapid that current cut-off can be achieved in much the same way as described for a current-limiting fuse.
MCB's do not provide rapid operation for very low values of earth leakage current. In today's world wiring regulations require that a very rapid operation is achieved in the occasion of an earth fault to subsidize the harms of electrocution. This requires operation for earth fault currents as low as 30mA in a time of 2-3ms.
C:\Users\Mohmed Talha\Desktop\doosan\project report (mid sem)\MCB working.jpg
Fig 3.1 - Working principle of a MCB
To achieve this requirement on MCB a variation on the basic construction is done. Such a modified device is known as 'earth leakage CB'. Tripping at such low values of earth leakage current is done by using an internal current transformer to pass feed and return conductors. Resultant flux of the CT core is zero. Under EF condition the feeding and return currents will be of different values, this current difference cause flux to generate with the CT core which produces an output voltage at its secondary terminals. The tripping circuit of the residual current device is energized from the secondary winding terminals.
The contacts of the MCB and residual current devices are not maintainable and have to be replaced after a limited number of operations is necessary. This problem is seldom and eroded contacts can be usually detected by overheating which causes unnecessary tripping of the device.
Chapter 4: Introduction to Protection Schemes
This chapter of the report contains details of various protection schemes used in almost all power systems around the world. Some of the numerical data is specific to Jebel Ali "M" Station Power Plant.
4.1 Different types of protection
4.1.1 Generator Differential Protection, 87G
The main objective of this protection scheme is to protect the generator during short circuit fault conditions.
4.1.2 Overvoltage Protection 59.1 (definitive time o/v)
This is the first stage and its intention is to protect against lower overvoltage (Slow Stage)
E.g.: 132v trip activated within 6s
Overvoltage Protection 59.2 (instantaneous o/v)
This is the second stage and its main function is to protect against higher values of over voltage (Fast stage)
E.g.: 143v trip activated within 0.01s
4.1.3 Under voltage Protection 27.1 (Definite Time Undervoltage)
This is a first stage intended to protect against undervoltage (slow stage)
E.g.: 99v trip activated within 6s
Undervoltage protection 27.2
This is the second stage and its function is protection against undervoltage (fast stage)
E.g.: 93.5v trip activated within 0.01s
Remark: to prevent wrong operation of the undervoltage relay, its function is stopped when the CB is open.
4.1.4 Negative Phase sequence Protection 46
The function of this protection scheme is to detect the danger of the rotor surface overheating due to negative sequence current caused by an unbalanced load.
4.1.5 Voltage controlled overcurrent protection 51V
This scheme is used to protect the zone in the middle of the HV circuit breaker and generator neutral point against service over currents and the zone from the generator's neutral point till the HV power system as a backup protection which is superimposed over the current stage.
4.1.6 Reverse power protection 32
This is used to protect our turbine against 'motoring operation'. Motoring operation means that we will be consuming power rather that generating any which is our main function.
4.1.7 Loss of excitation protection 40.1G
The duty of this is to protect our generator when brought under various potentially dangerous operating schemes; this can give permission to a situation in which loss of excitation occurs. When this occurs mechanical stress is created because of torque surges in one hand and thermal imbalance due to induced currents on the other.
4.1.8 Loss of excitation integrator 40.2G
The function of this is to enable a trip function under repeated power swings. When protection 40 is activated without tripping.
4.1.9 Minimal impedance protection 21
This is a backup protection which deals with short circuits in the following zones;
Parts of Generator stator winding
Isolated phase bus duct
Delta connected USTT windings
4.1.10 Underfrequency protection 81U
Protection scheme which in the case of low frequency protects the equipment from dangerous vibrations
4.1.11 Overfrequency Protection 81O
Protection scheme which in the case of high frequency protects the equipment from dangerous vibrations.
4.1.12 100% stator earth fault protection 64GN
Provides earth fault protection when earth faults are detected close to the neutral point on a generator.
4.1.13 95% stator earth fault protection 59GN
Protection against earth faults for generator stator winding and all galvanic connected parts.
4.1.14 Pole slip protection 78
This function detects the condition of a generator, which is completely out of step with the power system it has been connected to.
4.1.15 Overload (49/50L)
Overcurrent causes inadmissible heating of the generator stator winding, this protection prevents this from happening.
4.1.16 inadvertent energizing protection (50/27)
Severe damage can be caused to the machine when the generator is accidently energized and the machine is not in running condition. When the breaker and closed and machine is at standstill the generator starts to behave in the manner of an induction motor and the rotor winding slot wedges and rotor core behave as rotor current conductors. Arcing during this situation is caused between different components and this whole problem results in damage and rapid overheating.
4.1.17 V/Hz saturation (overfluxing) protection (24)
If the ratio of the frequency exceeds certain limits, overfluxing or overexcitation of a generator or transformer may occur. Excess voltage or lower frequency can cause a rise in the V/Hz ratio; this produces excessive flux densities in the magnetic core of the machine and transformer. This excess flux may be induced into components which are not properly laminated to withstand such flux. This will give rise to eddy currents in solid components and the core laminations can be destroyed causing rapid overheating and damage. Thus this protection is very important and setting for this must be carefully decided and put under routine supervision.
4.1.18 VTS, Voltage transformer supervision
This supervision scheme is used to prevent incorrect voltage measurements by constantly monitoring the voltage inputs of voltage transformers.