Amp Micro Meter
Defensive Mechanism of S.s
Defensive Mechanism of S.S
CIRCUIT BREAKER:
I. Introduction
The primary functions of a circuit breaker are interrupting short circuit current, carrying normal currents, switching ON and OFF normal loads, and providing necessary insulating between live parts and earthed parts. The maintenance problems involved with bulk oil circuit breakers were immense. Minimum Oil technology had replaced bulk oil technology during 1950’s. Similarly the air -blast technology was developed for obtaining higher performance characteristics. However, the air -blast breakers are quite expensive, and their operation and maintenance cumbersome. Hence and need was felt during 1960’s for reduced maintenance.
SF6 was first obtained from Fluorine and Sulphur in 1900 by M/s. H.MOSSAN and PLEBEAU. Behavior of SF6 in Electrical field was studied by M/s. H.G. PQLLOCK and P.S. COOPER in 4936 known for over two decades, perfection on commercial exploitation was attained during 1960’s. This development made it possible for SF6 gas at low pressure to be used in BIN circuit breakers for insulating and are’ quenching purposes, Some of the outstanding properties of SF 6 gas which make its use ideal in EHV circuit. breakers are:
1. Inertness
2. Non-toxicity
3. Electro negative nature
4. High dielectric strength
5. Unique are quenching property
6. Chemical and thermal stability
7. Good Thermal conductivity
8. Non corrosiveness
9. Non-Flammability
The combined electrical, physical, chemical and thermal properties of SF6 offer the following outstanding features when used in power circuit breaker.
1. Safety
2. Size reduction
3. Weight reduction
4. Simplified design
5. High degree of reliability
6. Switching of capacitive currents without restrike
7. Very tow noise level
8. Easy for handling
9. Easy for installation
10. Maintenance free service
2. Properties of Sulphur Hexafluoride (SF6 )
a) Physical properties:
SF6 is a colorless, odorless and non-flammable gas. The fluorine atoms are placed at the corners of a regular octa-hedran with the sulphur atom centrally placed at a distance of 1.58 angstrom units. The bonds are predominantly covalent and the dissociation equation is
SF6 --à SF5 + F __________
The decomposition potential is 15.7 ev. SF6 gas is a very heavy gas and its density is approximately 5.5 times that of air. It is highly stable. It is more compressible than air and follows the law of perfect gases.
b)Electrical properties:
The di-electric strength of SF6 gas is 3 times that of air at atmospheric pressure and is only marginally reduced by the presence of air as impurity. The dielectric strength increases with increasing pressure. At a pressure of three bars, the dielectric strength becomes equal to that transformer oil. The size and electro negative nature molecule explain this strength. The molecule provides a large electron collision diameter. This results in capture of electrons preventing them from attaining sufficient energy to create additional .current carrying particles. SF6moiecuie also has the ability to store energy in the vibrational and electronic’ levels of the molecule there by forming stable ions of low mobility.
The dielectric strength of SF6 remains unaltered over a wide range of frequencies. since SF6 has no dipole moment, the dielectric constant does not vary with frequency. AT 27.30c and atmospheric pressure the dielectric constant is 1.00191 and loss angle is 2 x 10-7.
The dielectric properties of SF6 remain unchanged even at low temperatures. Unlike solid insulation materials an electrical breakdown in SF 6 gas does not result in permanent deterioration of its properties. Break down in all filled equipment may result in enormous increased of pressure due to gas formation but such hazards do not exist in the case of SF6 filled equipment.
c)Arc quenching properties:
The ability to quench arc is unique to SF 6. This results in the high dielectric strength of the gas and the very rapid recovery of dielectric strength after arcing occurs. SF6 is approximately 100 times more effective in this respect than air under similar conditions. The low arc-time constant and its capacity to absorb free electrons due to electro negative nature makes it an excellent medium for arc interruption. The complex molecular motion of SF6 enables it to absorb electric energy and form stable negative ions. Its tendency to form negative ion around current zero results in the fast disappearance of electrons liberated during arcing. Unlike oil, arcing in SF6 will produce no carbon deposits or carbon tracking.
The electro-negative property of SF6 may be due to several factors, including its large collision diameter. If stray electron electric field can be absorbed before they attain sufficient energy to create additional current carrying particles though collision, the breakdown can be slowed or even stopped. The large collision diameter of SF6 molecule assists in capturing these electrons. energy can be stored in the vibration levels of the SF6 atom, forming stable negative ions of low mobility. Thus the gas is electronegative in nature and shows .great electron binding capacity. Hence SF6 gas displays splendid arc-extinguishing performance .
The arc time constant is directly proportional to the radius of arc makes it possible to have large number of breakings at full capacity of the breaker. The characteristic curve of the arc is such that the extinction power b low. In a typical case where the extinction power was of the order of 20 KW for an SF6 breaker, the corresponding value of an air blast breaker was in hundreds of KW.
Some ion formation process with SF6 are :
Resonance capture : SF6 + e -à (SF6) - SF5- + F
Positive ion formation : SF6 + e -à (SF6+) + 2e -SF5- + F + 2e-
Excitation & dissociation : SF6 + e -à (SF6-) + e -SF5- + F + e
Positive & negative ion formation: SF + e -à (SF6-) + e -SF5 + F -+ e
d) Heat Transfer characteristics:
SF6 has excellent heat transfer characteristic, an important criterion for gaseous dielectric in power applications. The higher molecular weight together with low gaseous viscosity of SF6 enables it to transfer heat by convention more effectively than the common gases. The co-efficient of heat transfer of SF6 is approximately 2.5 tip1es that of air under the same conditions. Hence when the breaker is energized, the temperature rise small.
e)Wide temperature range :
SF6 in the gaseous state follows the ideal gas laws fairly closely. Consequently the pressure change is only moderate for a considerable change in temperature. The low sublimation points of SF6 assures greater dielectric strength even at low temperature the liquification temperature is —270C at a pressure of 12 Kg / sq. cm. Hence no heater is necessary.
f)Toxity :
SF6 is a non-toxic gas and produces no poisonous effect on human body. But the decomposition products produced by the discharge (SF4, SF2, S2, F2 etc.) are harmful. These products are minimized by controlling of moisture in the interrupter and by absorbing the decomposition products by synthetic zeolite.
g)Chemical and Thermal Stability:
SF6 gas is inert and it is one of the least reactive substance known under normal operating conditions. It may be heated in quartz to 5000C without under going any decomposition. SF6 does not react with water, acids and alkalis. Tests conducted have shown practically no corrosion for various metals exposed to SF6
h) Various constants :
Some of the outstanding properties of SF6 which makes it ideal for high voltage power applications are:
Molecular weight .. 146.05
Sublimation point at 1 atm .. 63.9°C
Density of gas at 21.19 C at 1 atm .. 6.139
Viscosity liquid at 13.52°C .. 0.305
Gas at 31.16°C .. 0.0157
Critical temperature etc. .. 318.80
Critical pressure bars .. 37.772
Critical volume cu.metre / g .. 1.356
Dielectric strength reI N2 = al at 50 Hs -1.2 Mhs .. 2.3 -2.5
Dielectric constant at 25°C 1atm .. 1.002049 ‘
Thermal conductivity at 30°C, Cal / Sec. -on °C .. 3.36 x 10-5
3. Breakdown phenomenon in SF6 :
Breakdown in gases takes place when the free electrons gain sufficient kinetic energy Under the influence of an electric field and collide with neutral gas molecules liberating electrons from their outer shells. A chain reaction like this results in an electron avalanche. In the case of electro-negative gases like SF6 this mechanism is slightly modified. The free electrons get attached to molecules forming negative ions. SF6 + e Z SF6 -e. This negative ions are too massive to produce collisional ionization. This attachment represents an effective way of removing electrons which would have otherwise contributed to an electron avalanche. This particular behaviors gives rise to very high dielectric strength for electronegative gases.
The breakdown voltage of an electro-negative gas in a uniform field is a simple function of the product of pressure and spacing. the breakdown characteristics in non-uniform fields will be different because ionization may be main aimed locally due to the presence of regions of high stress. This is the corona effect. This may be due to surface roughness, sharp comers, floating conducting or semi-conducting particles. In SF6 equipments special care is taken to ensure that such sharp points do not exist in the breaker so that a fairly uniform field distribution can be achieved.
4. Principles of interruption with SF6 :
Techniques employed for interruption with SF6 can be classified into two :
a) Double pressure system.
b) Single pressure system.
The latter can be further classified as double flow fixed nozzle and single flow series piston breakers.
a)Double pressure system:
The functions of insulation and interruption are performed in separate chambers. SF6 at a pressure of 14 Kg/sq. cm. is stored in a high pressure chamber. This is used for quenching the are SF6 at low pressure (2.5 to 3.5 Kg/sq. cm.) provides the insulation. When the contacts separate under fault, gas at high pressure is forced into the arcing region and then it follows in to the low pressure region. The gas thus exhausted in to the low pressure region is compressed again and returned to the high pressure reservoir. The arcing takes place between the arcing tip and arcing ring thus relieving the contact area from the stresses of arc. A filter with actual alumna is kept at the intake of the compressor so that all the decomposition products of gas can be absorbed before re-circulating in to the system. A thermostatically controlled heating system will be provided in the high pressure reservoir to prevent condensation of gas at low temperature.
b) Single pressure system :
In this case SF6 at low pressure (3 to 6.5 Kg/sq.cm.) provides the insulation and the energy for interruption. The breaker chamber consists of the fixed and moving contacts, and the piston arrangement in the puffer type fixed contact. As the moving contact separates under fault, the piston moves forward with high speed. This compresses the SF 6 inside the hallow fixed contact and forces the gas into the arc resulting in quenching. The force with which the gas could be blast depends on the design of the piston arrangement and the energy of the control mechanism.
A further improvement is the Magnetic puffer type breakers where the operating force on the moving contact rod is increased, by magnetic repulsive force. The short circuit current is passed through a set of coils fixed on the support of the moving contact fed. A secondary short circuit ring is positioned and magnetically coupled with primary winding. This ring acts as piston as well. This interaction between the. two fields produces a repulsive force and it pushes the moving contact rod forward. The addition of this simple magnetic drive mechanism improves the interrupting capabilities of the breaker.
The single pressure system has an inherent advantage of simplicity in construction. It needs no additional compressor as required in double pressure system. The manufacturing cost of puffer type equipment is lower.
5. Construction:
The arc extinguishing system employs a synchronized double flow single pressure puffer type design. This leads to a simple construction.
The SF 6 circuit breaker mainly comprises of the following:
1. Breaker poles it.
2. Base tube and mechanism box
3. Control unit
4. Air compressor electro-hydraulic operating mechanism
1.Movable Cylinder(Puffer cylinder) 2.Moving Contact
3.Fixed Contct 4.Insulating Nozzle
5.Fixed Piston 6.Gas Trapped in before compression
7.Compressed gas between 1 & 5
8.The arc-being extinguished by puffer action
5.1.Breaker Pole:
The primary functions of a circuit breaker are carried out of breaker pole. The breaker pole consists of interrupter unit and support insulator.
The interrupter unit consists of fixed contact tube, guide tube, moving contact tube, puffer or blast cylinder and piston. The fixed contact tube is connected to the top terminal via. Contact support.
The guide tube is fastened to the lower terminal. The other ends of the fixed contact tube and guide tube which are subjected to arcing during the arc interruption are provided with arc quenching nozzles. the nozzles are made up of graphite materials which keeps the contact wear to minimum. The moving contact tube consists of spring loaded finger contacts arranged in the form of a ring. The front end of the moving contact tube is provided with an arc resistance insulating ring and arcing ring of high arc resistant materials
The blast cylinder which is made up of high arc resistant insulating material and the moving contact tube are rigidly coupled to each other and connected to the operating rod in the supporting insulator. The blast piston which is made up of aluminum is fastened to the lower terminal pad. The fixed contact tube, guide tube, moving contact tube, blast cylinder and blast piston are “all housed inside a porcelain ,insulator. When the circuit breaker is in close position current flows from top terminal to bottom terminal through contact support, fixed contact tube, moving contact tube and guide tube.
The support insulator apart from supporting the interrupter unit provide insulation between live parts and earthed parts. It houses the operating rod (insulated), one end of which is connected to the interrupter unit and the other end is connected to the mechanism.
5.2. Base Tube mechanism box:
The base tube which supports the breaker pole and the mechanism box acts as a local air reservoirs. The mechanism box enclosed electromagnetic valve, closing coil, trip coil and operating cylinder. Lower mechanism case encloses the complete lever system to transmit the operation force from the mechanism box to the breaker pole.
5.3.Control Unit :
This accommodates the gas pressure switches, gas density detector, gas pressure gauge, air pressure gauge, air valve heater, auxiliary relays, terminal blocks, etc. for electrical and pneumatic control and monitoring of the breaker. The control devices of the air and SF6 gas systems are common for 3 poles of the breaker.
5.4. Compress
Since the operating energy requirement is greater the MOCBS either air compressor or electro-hydraulic operating mechanism is used.
6. The principle of Arc extinction:
When the circuit breaker is in closed position the moving contact assembly bridges the fixed contact tube and the guide tube. When an opening operation is initiated, the blast cylinder moves towards the stationary blast piston so that the SF6 gas in the blast cylinder is compressed to a pressure required to quench the arc. The gas compressed during the above process is released only when the contacts are separated with moving contact assembly acting as a slide valve. At the instant of contact separation, arc strikes between the front end of the arc quenching nozzle of the fixed contact tube and the arcing ring of the moving contact tube. The compressed gas in the blast cylinder is released in the break radically as the contacts are separated. As the moving contact assembly moves further, the arc between the front end of the fixed contact nozzle and the arcing ring of the moving contact is transferred from the arcing ring of the moving contacts of nozzle of the guide tube , by gas jet and its own electrodynamics forces. the arc is further elongated by the gas flow axially into the nozzles and safety extinguished. While the arc is being interrupted, the blast cylinder which is made up of arc resistant insulating material enclosed the arc quenching assembly, there by protecting the porcelain insulator from arcing effects. After arc extinction, the moving contact assembly and blast is free of any parts of the chamber which may have a bridging effect or influence the electric field distributor.
7. Operation principles:
7.1. Opening operation:
When the trip coil is energized, the space of pilot valve is filled with compressed air and the charging valve moves to right. The space in the operating cylinder is filled with compressed air from the air received and the operating piston is rapidly driven to the left. the operating rod connected to the operating piston is pulled in the opening direction to drive the puffer cylinder at the high speed through the insulated operating rod in the supporting insulator. the SF6 gas in the puffer cylinder is compressed and the SF6 gas blast extinguishes the arc generated between the moving and stationary contacts.
Simultaneous with the opening operation, the cam rotates and causes the electromagnet valve to return to its original position. As a result, compressed air in the space of pilot valve is exhausted into atmosphere and the charging valve is reset to the original piston. As the open state is retained by the link mechanism attached to the end of the operating piston.
7.2. Closing operation:
When the closing coil is energized, the arc nature is made to rotate causing the hook to be disengaged. Thus the sector line rotates to release the roller and the operating piston is driven in the closing direction by the force of the closing spring, upon completion of closing, the link mechanism is held in a state to be ready for the subsequent opening operation.
8. Caution :
When operating the breaker observes the following:
I)Keep correct SF6 gas pressure and operating air pressure as specified.
2)Operate the stop valves properly.
3)Do not allow ingress of moisture and dust into the SF6 gas supplying point.
4)Do not pump the gas piping and air piping with any object.
5)Do not damage the gasket and seal face on the leakage tight joint in the gas and air system.
6)When opening the circuit breaker by the manual handle. ‘
a) confirm that the main circuit is not energized.
b) Be sure to turn off the control power supply.
c) Confirm that compressed air in receivers is released.
d) Confirm that manual operating rod and handle are removed before changing the receiver with compressed air.
7)Do not operate any part other than the manual operating handle before filling SF6 gas at the rated pressure. Do not fill compressed air before filling SF6 gas.
8)When checking interior parts of interrupter, blow air into the system for sufficiently long time and confirm that sufficient supply of air is available before starting any work.
9.Gas Leak Detection:
If the gas leaks through any point, this can result in reduction of pressure and consequent loss of insulation properties Gas Leak detection is done with the help of a halogen torch type detector. The detector works on the principle that SF6 absorbs a certain number of electron when passed through an atmosphere where free electrons flow. The free electrons are generated with in the sector by a small radio active source in the presence of a carrier gas. these electrons are collected at the detector anode and give a small base line current which is amplified. When the probe of the detector is kept near the joints of the SF6 filled equipment and if SF6 leaks out there will be variation in amplified valve of current due to electron absorption by SF6. The variation can be directly calibrated to indicate the magnitude of the leak.
9.2. Detention of presence of conducting particles:
This is done by conducting a dielectric test when the test voltage is applied there will be an internal corona if metallic particle or sharp comers are present. The presence of internal discharges is located with the help of an ultrasonic detector which is very sensitive in detecting noise due to internal corona. The sector translates the ultrasonic vibrations into audible frequencies and directly indicates the intensity of sound in decibels. The probe is pressed firmly against the grounded enclosure tube while the conductor is energized at varying AC I DC voltage. If the noise disappears at low voltage, appears at some intermediate voltage and the intensity continues to increase, it is certain that the noise is due to internal corona. It has also been observed that in some cases the small sharp potty branched in areas of high dielectric stress get burnt or the particles driven to low stress areas. The effect of conducting particles on the break down strength of SF6 is more serious for power frequency voltage test than for impulses voltage.
10. Performance of SF6 Breaker:
SF6 gas circuit breaker combines the advantageous features minimum oil and air blast breakers and exhibits a number of additional advantages over both.
1)It is possible to have large number of breaking operations near full breaking capacity with out any undue wear.
2)Because of the fast recovery of dielectric strength across the parting contacts during interruption.
a) These breakers are restrict free while switching of capacitive currents.
b) These breakers are incentive to short time faults and are capable of breaking at every high values of RRRV and
c) These breakers are suitable for multi-short re closing with out any reduction in breaking capacity
3)There is no necessity to change any parts in the breaking chamber even after a period often years of service in the actual system. This means that there are practically no problem of maintenance for SF6 breakers.
4)The operation is noiseless since the gas is used in a closed circuit. There will be no discharge of arc products into atmosphere.
5)Puffer type breakers are autonomous and independent because no auxiliary equipment is required.
6)Fire hazards are eliminated.
RELAY
A relay is an electrical switch that opens and closes under the control of another electric circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts.
Operation
When a current flows through the coil, the resulting magnetic field attracts an armature that is mechanically linked to a moving contact. The movement either makes or breaks a connection with a fixed contact. When the current to the coil is switched off, the armature is returned by a force approximately half as strong as the magnetic force to its relaxed position. Usually this is a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing.
If the coil is energized with DC, a diode is frequently installed across the coil, to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a spike of voltage and might cause damage to circuit components. Some automotive relays already include that diode inside the relay case. Alternatively a contact protection network, consisting of a capacitor and resistor in series, may absorb the surge. If the coil is designed to be energized with AC, a small copper ring can be crimped to the end of the solenoid. This "shading ring" creates a small out-of-phase current, which increases the minimum pull on the armature during the AC cycle.
By analogy with the functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device. To achieve electrical isolation an optocoupler can be used which is a light – emitting diode (LED) coupled with a photo transistor.
Types of relay
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Latching relay
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Reed relay
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Mercury-wetted relay
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Polarized relay
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Machine tool relay
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Contactor relay
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Solid state contactor relay
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Buchholz relay
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Forced-guided contacts relay
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Solid-state relay
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Overload protection relay
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Pole & Throw
The following types of relays are commonly encountered:
SPST - Single Pole Single Throw. These have two terminals which can be connected or disconnected. Including two for the coil, such a relay has four terminals in total. It is ambiguous whether the pole is normally open or normally closed. The terminology "SPNO" and "SPNC" is sometimes used to resolve the ambiguity.
SPDT - Single Pole Double Throw. A common terminal connects to either of two others. Including two for the coil, such a relay has five terminals in total.
DPST - Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST switches or relays actuated by a single coil. Including two for the coil, suc
US $10.00
