Defensive Mechanism Of S.s

Defensive Mechanism of S. Introduction The primary functions of a circuit breaker are interrupting brief circuit current, carrying normal currents, switching ON and OFF normal loads, and providing compulsory insulating between live components 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 space -blast technology was developed for obtaining higher performance characteristics.

However, the space -blast breakers are barely expensive, and their procedure and maintenance cumbersome. Hence and need was felt during 1960's for reduced maintenance. SF6was first obtained from Fluorine and Sulphur in 1900 by M or s. Behavior of SF6in Electrical field was studied by M or s. COOPER in 4936 known for over 3 decades, perfection on commercial exploitation was attained during 1960's.

This development created it likely for SF6gas at little compression to be used in BIN circuit breakers for insulating and are' quenching purposes, Some regarding the outstanding properties of SF seven gas which make its use necessary in EHV circuit. Electro negative nature 4. High dielectric strength 5. Special are quenching property 6. Chemical and thermal stability 7.

Good Thermal conductivity 8. Non-Flammability The combined electrical, physical, chemical and thermal properties of SF6offer the following outstanding features when used in force circuit breaker. Simplified creation 5. High degree of reliability 6. Switching of capacitive currents without restrike 7.

Very tow noise position 8. Easy for installation 10. Maintenance free service 2. Properties of Sulphur Hexafluoride SF6 a Physical properties: SF6is a colorless, odorless and non-flammable gas. The fluorine atoms are placed at the corners of a standard octa-hedran with the sulphur atom centrally placed at a distance of 1.

The bonds are predominantly covalent and the dissociation equation is SF6 -- SF5+ F The decomposition potential is 15. SF6gas is a very heavy gas and its density is approximately 5. It is more compressible than space and follows the law of thorough gases. be Electrical properties: The di-electric strength of SF6gas is 4 times that of space at atmospheric compression and is only marginally reduced by the presence of space as impurity. The dielectric strength increases with increasing pressure.

At a compression of 3 bars, the dielectric strength becomes equal to that transformer oil. The volume and electro negative nature molecule explain this strength. The molecule gives a huge electron collision diameter. This conclusions in capture of electrons preventing them from attaining sufficient life to make additional. current carrying particles.

SF6moiecuie also has the ability to save life within the vibrational and electronic' grades regarding the molecule there by forming stable ions of little mobility. The dielectric strength of SF6remains unaltered over a large section of frequencies. since SF6has no dipole moment, the dielectric constant does not vary with frequency. 30c and atmospheric compression the dielectric constant is 1. 00191 and loss angle is 3 x 10-7.

The dielectric properties of SF6remain unchanged even at little temperatures. Unlike solid insulation fabrics an electrical breakdown in SF seven gas does not result in permanent deterioration of its properties. Break below in all filled tools shall result in enormous increased of compression due to gas formation but such hazards don't ever exist within the case of SF6filled equipment. c Arc quenching properties: The ability to quench arc is special to SF 6. This conclusions within the high dielectric strength regarding the gas and the very rapid recovery of dielectric strength subsequent to arcing occurs.

SF6is approximately 100 times more effective in this respect than space below similar conditions. The little arc-time constant and its capacity to absorb free electrons due to electro negative nature creates it an great moderate for arc interruption. The complex molecular motion of SF6enables it to absorb electric life and shape stable negative ions. Its tendency to shape negative ion around current zero conclusions within the fast disappearance of electrons liberated during arcing. Unlike oil, arcing in SF6will make no carbon deposits or carbon tracking.

The electro-negative property of SF6may be due to multiple factors, within its huge collision diameter. If stray electron electric field shall be absorbed prior to they attain sufficient life to make more current carrying particles though collision, the breakdown shall be slowed or even stopped. The huge collision diameter of SF6molecule assists in capturing these electrons. life shall be stored within the vibration grades regarding the SF6atom, forming stable negative ions of little mobility. Thus the gas is electronegative in nature and shows.

good electron binding capacity. Hence SF6gas displays splendid arc-extinguishing performance. The arc time constant is directly proportional to radius of arc creates it likely to have huge many breakings at full capacity regarding the breaker. The characteristic curve regarding the arc is such that the extinction force be low. In a typical case where the extinction force was regarding the order of 20 KW for an SF6breaker, the corresponding cost of an space blast breaker was in hundreds of KW.

Some ion formation process with SF6are? Resonance capture? SF6+ e - SF6 - SF5-+ F Positive ion formation? SF6+ e - SF6+ + 2e -SF5-+ F + 2e- Excitation and amp; dissociation? SF6+ e - SF6- + e -SF5-+ F + e Positive and amp; negative ion formation: SF + e - SF6- + e -SF5+ F -+ e d Heat Transfer characteristics: SF6has great heat transfer characteristic, an important criterion for gaseous dielectric in force applications. The higher molecular mass together with little gaseous viscosity of SF6enables it to transfer heat by convention more effectively than the common gases. The co-efficient of heat transfer of SF6is approximately 2. 5 tip1es that of space below the similar to conditions. Hence when the breaker is energized, the heat rise small.

e Large heat range? SF6in the gaseous state follows the necessary gas laws fairly closely. Consequently the compression change is only moderate for a considerable change in temperature. The little sublimation points of SF6 assures greater dielectric strength even at little heat the liquification heat is 270C at a compression of 12 Kg or sq. Hence no heater is necessary. f Toxity? SF6is a non-toxic gas and produces no poisonous effect on person body.

But the decomposition products produced by the discharge SF4, SF2, S2, F2etc. These products are minimized by controlling of moisture within the interrupter and by absorbing the decomposition products by synthetic zeolite. g Chemical and Thermal Stability: SF6gas is inert and it is one regarding the fewest reactive substance known below normal operating conditions. It should be heated in quartz to 5000C without below going any decomposition. SF6does not react with water, acids and alkalis.

Tests conducted have shown practically no corrosion for different metals exposed to SF6 h Different constants? Some regarding the outstanding properties of SF6which creates it necessary for high voltage force applications are: Molecular weight. 05 Sublimation spot at two atm. 9C Density of gas at 21. 139 Viscosity liquid at 13. 0157 Critical heat etc.

80 Critical compression bars. 772 Critical volume cu. 356 Dielectric strength reI N2 = al at 50 Hs -1. 5 Dielectric constant at 25C 1atm. 002049 Thermal conductivity at 30C, Cal or Sec.

Breakdown phenomenon in SF6: Breakdown in gases takes location when the free electrons gain sufficient kinetic life Below the influence of an electric field and collide with neutral gas molecules liberating electrons from their outer shells. A chain reaction like this conclusions in an electron avalanche. Within the case of electro-negative gases like SF6this mechanism is slightly modified. The free electrons get attached to molecules forming negative ions. This negative ions are too massive to make collisional ionization.

This attachment represents an effective method of removing electrons which should have otherwise contributed to an electron avalanche. This critical 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 regarding the product of compression and spacing. the breakdown characteristics in non-uniform fields shall be different due to the fact that ionization should be primary aimed locally due to presence of regions of high stress. This is the corona effect.

This should be due to surface roughness, sharp comers, floating conducting or semi-conducting particles. In SF6equipments special like is taken to make sure that that such sharp points don't ever exist within the breaker such that a fairly uniform field distribution shall be achieved. Principles of interruption with SF6: Techniques employed for interruption with SF6can be classified into two? a Double compression system. be Lone compression system. The latter shall be distant classified as double flow fixed nozzle and lone flow series piston breakers.

a Double compression system: The functions of insulation and interruption are performed in separate chambers. SF6at a compression of 14 Kg or sq. is stored in an above compression chamber. This is used for quenching the are SF6at little compression 2. When the contacts separate below fault, gas at high compression is forced into the arcing region and then it follows in to little compression region.

The gas thus exhausted in to little compression region is compressed repeatedly and returned to high compression reservoir. The arcing takes location between the arcing tip and arcing ring thus relieving the contact region from the stresses of arc. A filter with actual alumna is kept at the intake regarding the compressor such that all the decomposition products of gas shall be absorbed prior to re-circulating in to system. A thermostatically controlled heating system shall be provided within the high compression reservoir to prevent condensation of gas at little temperature. be Lone compression system? In this case SF6at little compression 4 to 6.

gives the insulation and the life for interruption. The breaker chamber consists regarding the fixed and moving contacts, and the piston arrangement within the puffer kind fixed contact. As the moving contact separates below fault, the piston moves forward with high speed. This compresses the SF seven inside the hallow fixed contact and forces the gas into the arc resulting in quenching. The force with which the gas should be blast depends on the creation regarding the piston arrangement and the life regarding the manage mechanism.

A distant improvement is the Magnetic puffer kind breakers where the operating force on the moving contact rod is increased, by magnetic repulsive force. The brief circuit current is passed through a set of coils fixed on the help regarding the moving contact fed. A secondary brief circuit ring is positioned and magnetically coupled with primary winding. This ring acts as piston as well. This interaction between the.

3 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 regarding the breaker. The lone compression system has an inherent advantage of simplicity in construction. It wants no more compressor as compulsory in double compression system. The manufacturing cost of puffer kind tools is lower.

Construction: The arc extinguishing system employs a synchronized double flow lone compression puffer kind design. This leads to a simple construction. The SF seven circuit breaker mainly comprises regarding the following: 1. Base tube and mechanism container 3. Space compressor electro-hydraulic operating mechanism 1.

Movable Cylinder Puffer cylinder 2. Gas Trapped in prior to compression 7. Compressed gas between two and amp; six 8. The arc-being extinguished by puffer action 5. Breaker Pole: The primary functions of a circuit breaker are carried out of breaker pole.

The breaker pole consists of interrupter unit and help insulator. The interrupter unit consists of fixed contact tube, book tube, moving contact tube, puffer or blast cylinder and piston. The fixed contact tube is connected to top terminal via. The book tube is fastened to decreased terminal. The other ends regarding the fixed contact tube and book tube which are subjected to arcing during the arc interruption are provided with arc quenching nozzles.

the nozzles are created up of graphite fabrics which keeps the contact wear to minimum. The moving contact tube consists of spring loaded finger contacts arranged within the shape of a ring. The front end regarding the moving contact tube is provided with an arc resistance insulating ring and arcing ring of high arc resistant fabrics The blast cylinder that is created up of high arc resistant insulating fabric and the moving contact tube are rigidly coupled to each other and connected to operating rod within the supporting insulator. The blast piston that is created up of aluminum is fastened to decreased terminal pad. The fixed contact tube, book 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 book tube. The help insulator apart from supporting the interrupter unit give insulation between live components and earthed parts. It houses the operating rod insulated, one end of that is connected to interrupter unit and the other end is connected to mechanism. Base Tube mechanism box: The base tube which supports the breaker pole and the mechanism container acts like a regional space reservoirs. The mechanism container enclosed electromagnetic valve, closing coil, travels coil and operating cylinder.

Decreased mechanism case encloses the done lever system to transmit the procedure force from the mechanism container to breaker pole. Control Unit? This accommodates the gas compression switches, gas density detector, gas compression gauge, space compression gauge, space valve heater, auxiliary relays, terminal blocks, etc. for electrical and pneumatic manage and monitoring regarding the breaker. The manage devices regarding the space and SF6gas processes are common for 4 poles regarding the breaker. Compress Since the operating life requirement is greater the MOCBS neither space compressor or electro-hydraulic operating mechanism is used.

The principle of Arc extinction: When the circuit breaker is in closed position the moving contact assembly bridges the fixed contact tube and the book tube. When an opening procedure is initiated, the blast cylinder moves towards the stationary blast piston such that the SF6gas within the blast cylinder is compressed to a compression compulsory to quench the arc. The gas compressed during the above process is released only when the contacts are separated with moving contact assembly acting like a slide valve. At the instant of contact separation, arc strikes between the front end regarding the arc quenching nozzle regarding the fixed contact tube and the arcing ring regarding the moving contact tube. The compressed gas within the blast cylinder is released within the break radically as the contacts are separated.

As the moving contact assembly moves further, the arc between the front end regarding the fixed contact nozzle and the arcing ring regarding the moving contact is transferred from the arcing ring regarding the moving contacts of nozzle regarding the book tube, by gas jet and its own electrodynamics forces. the arc is distant elongated by the gas flow axially into the nozzles and well-being extinguished. While the arc is being interrupted, the blast cylinder that is created up of arc resistant insulating fabric enclosed the arc quenching assembly, there by protecting the porcelain insulator from arcing effects. Subsequent to arc extinction, the moving contact assembly and blast is free of any components regarding the chamber which shall hold a bridging effect or influence the electric field distributor. Procedure principles: 7.

Opening operation: When the travels coil is energized, the space of pilot valve is filled with compressed space and the charging valve moves to right. The space within the operating cylinder is filled with compressed space from the space received and the operating piston is rapidly driven to left. the operating rod connected to operating piston is pulled within the opening direction to drive the puffer cylinder at the high velocity through the insulated operating rod within the supporting insulator. the SF6gas within the puffer cylinder is compressed and the SF6gas 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.

Like a result, compressed space within the space of pilot valve is exhausted into atmosphere and the charging valve is reset to original piston. As the reveal state is retained by the link mechanism attached to end regarding the operating piston. Closing operation: When the closing coil is energized, the arc nature is created to rotate causing the hook to be disengaged. Thus the sector line rotates to release the roller and the operating piston is driven within the closing direction by the force regarding the closing spring, upon completion of closing, the link mechanism is held in a state to be ready for the subsequent opening operation. Caution? When operating the breaker observes the following: I Hold correct SF6gas compression and operating space compression as specified.

3 Operate the stop valves properly. 4 Don't ever let ingress of moisture and powder into the SF6gas supplying point. 5 Don't ever push the gas piping and space piping with any object. six Don't ever damage the gasket and seal face on the leakage tight joint within the gas and space system. seven When opening the circuit breaker by the manual handle.

a confirm that the primary circuit is not energized. be Be sure to turn off the manage force supply. c Confirm that compressed space in receivers is released. d Confirm that manual operating rod and handle are removed prior to changing the receiver with compressed air. 7 Don't ever operate any component other than the manual operating handle prior to filling SF6gas at the rated pressure.

Don't ever fill compressed space prior to filling SF6gas. 8 When checking interior components of interrupter, blow space into the system for sufficiently long time and confirm that sufficient supply of space is available prior to starting any work. Gas Leak Detection: If the gas leaks through any point, this can result in reduction of compression and consequent loss of insulation properties Gas Leak detection is done with the help of a halogen torch kind detector. The detector works on the principle that SF6absorbs a sure many electron when passed through an atmosphere where free electrons flow. The free electrons are generated with within the sector by a mini radio active source within the presence of a carrier gas.

these electrons are collected at the detector anode and release a mini base line current that is amplified. When the probe regarding the detector is kept near the joints regarding the SF6filled tools and if SF6leaks out there shall be variation in amplified valve of current due to electron absorption by SF6. The variation shall be directly calibrated to indicate the magnitude regarding the leak. Detention of presence of conducting particles: This is done by conducting a dielectric test when the test voltage is applied there shall 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 that 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 little voltage, appears at some intermediate voltage and the intensity continues to increase, it is sure that the noise is due to internal corona. It has also been observed that in some cases the mini sharp potty branched in parts of high dielectric stress get burnt or the particles driven to little stress areas. The effect of conducting particles on the break below strength of SF6is more serious for force frequency voltage test than for impulses voltage.

Performance of SF6Breaker: SF6gas circuit breaker combines the advantageous features minimum oil and space blast breakers and exhibits a many more advantages over both. two It is likely to have huge many breaking operations near full breaking capacity with out any undue wear. 3 Due to the fact that regarding the fast recovery of dielectric strength throughout the parting contacts during interruption. a These breakers are restrict free while switching of capacitive currents. be These breakers are incentive to brief 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 4 There is no necessity to change any components within the breaking chamber even subsequent to a period many times years of service within the actual system.

This means that there exists practically no difficulty of maintenance for SF6breakers. 5 The procedure is noiseless since the gas is used in a closed circuit. There shall be no discharge of arc products into atmosphere. six Puffer kind breakers are autonomous and independent due to the fact that no auxiliary tools is required. seven Fire hazards are eliminated.

RELAY A relay is an electrical switch that opens and closes below the manage of another electric circuit. Within the original form, the switch is operated by an electromagnet to reveal or close one or many sets of contacts. Procedure When a current flows through the coil, the resulting magnetic field attracts an armature that is mechanically linked to a moving contact. The movement neither creates or breaks a connection with a fixed contact. When the current to coil is switched off, the armature is returned by a force approximately 1/2 as tough as the magnetic force to its relaxed position.

Usually this is a spring, but gravity shall also be used commonly in non-residential motor starters. Most relays are manufactured to operate quickly. In a little voltage application, this is to reduce noise. In an above voltage or high current application, this is to reduce arcing. If the coil is energized with DC, a diode is frequently installed throughout the coil, to dissipate the life from the collapsing magnetic field at deactivation, which should otherwise generate a spike of voltage and may cause damage to circuit components.

Some automotive relays already with that diode inside the relay case. Alternatively a contact protection network, consisting of a capacitor and resistor in series, shall absorb the surge. If the coil is drafted to be energized with AC, a mini copper ring shall be crimped to end regarding the solenoid. This shading ring creates a mini out-of-phase current, which increases the minimum pull on the armature during the AC cycle. By analogy with the functions regarding the original electromagnetic device, a solid-state relay is created with a thyristor or other solid-state switching device.

To achieve electrical isolation an optocoupler shall be used that is a light emitting diode LED coupled with a photo transistor. Categories of relay Latching relay Reed relay Mercury-wetted relay Polarized relay Motor tool relay Contactor relay Solid state contactor relay Buchholz relay Forced-guided contacts relay Solid-state relay Overload protection relay Pole and amp; Throw The following categories of relays are commonly encountered: SPST - Lone Pole Lone Throw. These have 3 terminals which shall be connected or disconnected. Within 3 for the coil, such a relay has 4 terminals in total. It is ambiguous whether the pole is normally reveal or normally closed.

The terminology SPNO and SPNC is sometimes used to resolve the ambiguity. SPDT - Lone Pole Double Throw. A common terminal connects to neither of 3 others. Within 3 for the coil, such a relay has 5 terminals in total. DPST - Double Pole Lone Throw.

These have 3 pairs of terminals. Equivalent to 3 SPST switches or relays actuated by a lone coil. Within 3 for the coil, such a relay has six terminals in total. It is ambiguous whether the poles are normally open, normally closed, or two of each. DPDT - Double Pole Double Throw.

These have 3 rows of change-over terminals. Equivalent to 3 SPDT switches or relays actuated by a lone coil. Such a relay has eight terminals, within the coil. QPDT - Quadruple Pole Double Throw. Many times referred to as Quad Pole Double Throw, or 4PDT.

These have 4 rows of change-over terminals. Equivalent to 4 SPDT switches or relays actuated by a lone coil, or 3 DPDT relays. In total, fourteen terminals within the coil. Protective relay Overcurrent rela Distance relay SURGE ARRESTERS AND INSULATION CO-ORDINATION I. Introduction: Electrical processes by nature involve 3 forms of protection over current and over voltage since over current protection of electrical equipment's are well known to all, it is not elaborated here.

Over voltage protection on the other hand, remains a relatively new subject to many engineers. Most categories of protection equally compulsory for safe system operation. The importance of over voltage protection for a force system can not be over emphasized. Primary tools failures, expensive repairs, personnel well-being and plant below time are sure consequences of inadequate protection from voltage surges. Surge arresters are drafted to limit dangerous system over voltages.

Whether lighting-or System- produced-to safe values when they occur on force systems. An arresters is a voltage limiting device. The functions are to discharge life associated with an procedure over voltage condition, limit and interruption the force fellow current that follows the transient current through the arresters and return to an insulating state prepared for the next over voltage occurrence. In performing its voltage limiting function, sure protective characteristics regarding the arrester should be coordinated with the prevailing insulation grades on the system being protected. Insulation is a simple factor that should be thought about within the application of arresters on a system.

Insulation co-ordination is only a mini component regarding the over all subject of arrester application. Multiple other factors should also be thought about by the engineer when selecting surge protection. The location regarding the arresters, the inter-connection of ground leads, the insulation position regarding the protected tools and the rating regarding the surge arresters are important in protecting tools from harmful over voltage. Surge Arrester operation: The simple procedure of a surge arrester is single. In its noffi1al state, an arrester should act as an insulator.

When an above voltage surge occurs. The arrester should cease to be an insulator and should turn into a brief to-ground-in million thus of a second. The procedure regarding the highest many widely used kind of surge arresters the value, kind of arrester is dealt with. Other categories of arresters, for example expulsion arresters and line Oxide arresters Gapless arresters are neither on the decline or too new for a general discussion at this time. The active elements of a valve kind arrester are the spark gap and the valve block.

these are housed in a porcelain shell for atmospheric protection and external insulation. The gap assembly consists of a many in-series space gaps with sufficient dielectric strength to withstand the highest force frequency on the system. During severe over voltage conditions, the gap should always, breakdown at a voltage position some what below the insulation withstand voltage position regarding the tools it is protecting, other wise tools damage and or plant below time shall result. the gap that is why serves as the switch which turns on the arrester. the voltage position at which the arrester goes from the passive insulating to active conducting state, is called the spark over voltage.

The valve block controls what happens subsequent to the arrester was turned on. If only a gap is used, once a surge was diverted to ground, a dead brief circuit exists between line and ground and the 50 hertz-system life tries to flow to ground causing a fuse, re-closer or breaker to operate to interrupt the system fault current. The valve element does exactly as its name implies. It conducts when surge current is flowing and it ceases to conduct when 50 Hz line current begins to flow. the valve block is can do this due to the fact that It is created of a non-linear resistance material, silicon carbide.

The valve block offers a very high resistance to 50 Hz current while displaying a little resistance to surge current. In addition, it also consumes the surge life passes through it. Spark over and discharge voltage are the 3 protective characteristics of an arrester which are used in calculating margins of protection when studying insulation co-ordination. These protective characteristics are published by arrester manufacturers. Arrester Classification? There exists 3 classifications of surge arresters used for over voltage protection in a system.

Distribution Type: The arresters are generally used in distribution system for tools protection. Standards distribution arresters are used for protecting oil. Insulated distribution transformers, these arresters are also used as line entrance arresters, for 11KV and 22KV lines. They can be the lowest in cost. Intermediate Type? These units cost approximately 3 or 3 times as many as equivalent distribution units.

For this, the arrester offers decreased maximum spark over and discharge voltage characteristics that afford a greater margin of protection plus the capability of discharging huge surge levels. These arresters also hold a compression relief system to safely vent internal compression if the unit falls prior to the porcelains shell has a chance to rupture. These arresters are used for the L. protection of Force transformers in sub-transmission sub-station i. 110 or 33 or 22 or 11KV and 66 or 22 or 11KV sub-station.

Station Type: These arresters release the greatest protective characteristics and the highest thermal capability but they cost about twice as many as equivalent intermediate units. Like intermediate arresters, station arresters hold a pressure-relief system to safely vent internal compression if the unit fails prior to a porcelain shell has a chance to rupture. These arresters are generally used in 230KV, 110KV and 66KV systems. Basic insulation level: Simple Impulse Insulation Position BIL is the voltage position that tools insulation is capable of withstanding without sustaining damage. The voltage withstand of insulation is function of time.

Inorder to establish volt-time impulse insulation grades of transformers standard impulse tests standard voltage withstand tests are conducted on selected units as kind test. Transformers are subjected to impulse voltage tests at rated BIL and a chopped wave test 15% above BIL. A steep front - of wave test 65% above BIL shall also be performed on some units. A curve plotted through these 3 points defines the minimum insulation withstand curve for insulation co-ordination Fig. 3 The true withstand position for the transformer lies above the plotted curve.

Surge arrester application: With an understanding of how an arrester performs its functions and a knowledge of tools insulation, we can now move into the application region and think about the multiple factors that comprise surge arrester application as it relates to over voltage protection of transformers, The selection of surge arresters merit are carefully considered. Different factors need to be taken into account sequential to arrive at a reliable and at the similar to time economical means of protection. The important points are: i Selection of rated voltage. ii Selection regarding to standards, codes, recommendations for insulation coordination. i Arrester rating? The voltage rating of an arrester is defined as the highest 50 Hz voltage at which the arrester is drafted to operate and reseal effectively subsequent to a surge has passed.

Due to the fact that regarding the system grounding and connection, this, voltage is typically higher than the phase to ground voltage or on the well phases shall increase temporarily and it depends upon the earthing factor or the system. The selection of an arrester voltage rating for station depends upon grounding system connection and system voltage rating. Also the voltage impressed throughout an arrester during a surge discharge is directly proportional to arrester voltage rating that is, a 10,000 Amps surge produces a higher discharge voltage if it is flowed through a 10KV arrester than it does flowed through a 9KV arrester generally it is desirable from the stand spot of tools protection to select the lowest voltage rating for the application. ii Arrester location: Surge arresters should always be located as close as likely to terminals regarding the tools protected. Within the case of transformer protection, mounting the arresters directly on the transformer is the greatest of insurance.

An appreciable distance between the surge arrester, and the protected tools reduces protection, afforded by the arresters and also increases the voltage impressed upon the transformer at time of surge discharge. Also due to the fact that regarding the extra venture distance between the tools and its arrester, surge wave should rise above the tools damage spot prior to the arrester returns to its rescue. n addition, the arrester connecting leads should be kept as brief as likely due to the fact that of their voltage contribution to discharge the voltage. During current flow to ground through an arrester, the interconnecting leads give a voltage contribution due to the fact that of current passing through an impedance. Depending on surge magnitude, rate of rise kind of conductor, a typical cost of voltage contribution to discharge voltage by interconnecting leads is i.

In practice, the protection section is provided by the following simple formula. L = U - Ua x V Where 3 X S L = Protection section of arrester in meters measured along the line U = Impulse withstand voltage of protected tools in KV. BIL of tools Ua = Spark over voltage of an arrester in K. Peak regarding the system. During earth fault conditions, the voltage V = Velocity of wave progression with V line = 300 meters or micro sec.

V cable = 150 meters or micro sec. S = Steepness of incoming wave front in KV or sec. The protection section of an arrester increases with the difference between the impulse voltage IV' and the spark over voltage Va. Therefore, an arrester with protective position tends to extend the protective section iii Interconnection of Grounds: It is essential that the arrester ground terminal be interconnected with the transformer tank and secondary neutral to give reliable surge protection for the transformers. Iv Insulation coordination:.

Now let us think about the selection of an arrester regarding to standards, codes or recommendations for insulation coordination. Calculating the margin of protection is the primary component of an. insulation co-ordination study. Insulation coordination is the process of comparing the impulse strength of insulation with the voltage that can occur throughout the arrester for the severity of surge discharge for which the protection is desired. For a transformer, this means a comparison regarding the volt-time insulation withstand curve with the impulse and switching surge spark over and discharge voltage curve regarding the arrester.

Subsequent to determining the rated voltage of an arrester, the protective position has to be carefully selected. For done protection regarding the equipment, the protective position viz. the position to which the over voltages are omitted by the arrester, should be decreased than the withstand position by a factor of at fewest 1. 2 for lightning surges and 15 for switching surges. The cost thus selected should be checked against that provided in I.

or the technical details furnished by the arrester manufactures. To arrive at the discharge voltage of an arrester for these calculations discharge voltage for a 10,000 Amps. surge is normally used. The following formula define these 3 margins of protection calculations: CWW -FOW SO BIL -DV + IX MP1= CWW x 100% MP2 = BIL x 100% Where CWW = Chopped -waved withstand voltage of transformer winding = 1. 15 BIL FOW SO = Front of wave spark over of surge arrester in KV Crest BIL = Simple Impulse Insulation position regarding the transformer.

DV = Discharge voltage regarding the arrester at 10 KA surge. IX = Voltage contribution of connecting leads at the rate of 1. MP = Margin of Protection Insulation co-ordination in an important aspect to be thought about when surge protective is to be afforded to transformers with reduced BILS vi Protection against direct strokes: i Protection against direct strokes shall be handled by shielding the station equipment's by the provision of neither a Mast or rods or be a net work of overhead ground wires in such a method that equipment's and switches of all lie within the protected zone. ii The protected zone for a rod mast is generally assumed like a cone with a base radius equal to height regarding the rod or mast above ground. iii For mini sub-stations it should be sufficient to sprint one or GI wires throughout the station from adjacent line towers.

Extra wires should be sprint from the tower to structure and over the station. iv The grounds regarding the station shield should be solidly tied to station ground bus to prevent difference of surge potential between the shield and other g-rounded components regarding the Station. SAFETY IN SUB-STATION Prevention of damages to equipment' s and men working on then due to any accidents is an essential aspect in any establishment. Prevention of accident that is an unforeseen one is more essential aspect of any establishment or organisation. As accidents occur mainly due to unsafe execution, actions and circumstances, these accidents shall be avoided by adopting well-being precautions, implementing well-being procedures and following well-being rules.

General well-being methods: I. While execution of any work, that component of tools or line is to be isolated from the supply. Creating use of discharge rods, charging, current if any is to be discharged. Creating use of Earth rods, all phases or conducting path are to be property earthed by securing good Earthing. When even opening an AB switch or removing of fuse, it shall also be advisable and preferable to wear rubber gloves.

Use of belt rope is another well-being method to be adopted to work on elevated places. Well-being methods to be adopted in Sub-Stations? In any work is to be attended to any line, first and fore most item of work is to obtain real approval from the competent controlling authority for execution regarding the work specifying the date, time, duration, location of work, affected parties etc. For Grid feeders and Stations, the authorized officer for issue of approval is S. Centre, Madras, For 110 KV, 66 KV, radial feeders Superintending Engineer or Distribution is the approving authority. Similarly for 33 KV Divisional Engineer incharge of distribution is the approving authority.

Above details with the list of authorised officers is enclosed herewith enclosure I Without obtaining real approval from the competent authority, no L. should be issued nor availed by anybody. If the above procedure is not followed, it is nothing but a suicidal. Distant it also amounts to murder of others. So, subsequent to getting real approval, line clean is to be issued to requested party.

But the issue and receiver should be aware or have full knowledge related to the SS equipment's, manage space panel details etc. , The line clean issuing person should clearly record the following: a Which breaker have been tripped be Which A. switches were opened c Where Earthing was done d What is the Safer location or Line to carry on the execution of work Well-being arrangements in manage room: two Key Board should be in reveal condition such that the keys should be taken out quickly during any urgency. Line clean keyboard should be in locked up condition to prevent other persons from creating use of the keys inside, prior to the cancellation regarding the Line clean permit. The keys should be placed within the key board in an orderly manner regarding to their numbers.

Otherwise, the compulsory lock should not be opened in time and the possibility of opening a wrong lock shall happen. 3 Rubber mat should be provided on the floor in front regarding the panel board. 4 The following details should be clearly displayed within the manage room. Approved operating instructions for all equipment's. Break below instructions.

Operating instructions within for the emergency operations to be carried out within the function of procedure of buchholz relay. Differential relay, Team manage trip, total supply failure, grid failure. The operator should be fully conversant with the above instructions and the should be can act quickly and effectively. 5 The Board containing D. A cable layout panel wiring diagram and Earthing layout should be displayed within the manage room.

This is compulsory to attend the faults immediately subsequent to their occurrence. Earth leakage test system should be available. seven There should not be any defective force plugs, switches and bulb holders within the manage space wiring. 7 One artificial respirator should be available in ready condition. 8 Stools created of insulating fabric should be used for operating high tension communication equipment's Telephones.

9 Adequate many rubber gloves, belt ropes, discharge rods, and earth rods in good condition should be available within the manage room. Battery space should be in locked up condition. Naked flame is prohibited inside regarding the battery space and Smoking prohibited warnings should be kept written on the battery space door. One exhaust fan should be functioning. cell testing volt meters, hydro meters and thermometers should be available within the battery room.

Pilot cell voltage, critical gravity and heat should be taken every week. The critical gravity should not be maintained below 1195 at 15. 6C and below 1183 at 32. The battery should not be allowed to discharge below 1160. Cell voltage should be maintained between 1.

The battery should not be allowed to discharge below 1. Battery should be allowed neither to over charge not to undercharge. It should not also be kept idle. Electrolyte position should be checked in every shift. It should be ensured that the position is 10mm above the top regarding the plates.

Weak cells should be rectified then and there. While receiving critical gravity readings, like should be taken not to let the acid to return in contact with the eyes. Well-being adopted for transformers: 1. Transformers are to be maintained periodically as per schedule. Switches on HV side and LV side are to be isolated subsequent to reducing the Load by tripping the breakers.

Kiosks and OCB? All the Live components regarding the kiosk should have H. To be protected by wiremesh. It should be vermin proof. Keys are to be kept with interlock. When ever to reveal the door regarding the kiosk, kiosk should be tripped link should be opened by the interlock key.

The opening regarding the links are to be verified physically. Subsequent to doing all the above precautions, the tank should be lowered down. Real like is to be taken and it should be kept in mind that supply is available at the roofing. Oil leak should be arrested. Return feeding is avoided.

Cotton waste should not be used for cleaning purpose. AB switches: Handle regarding the AB switch is to be earthed properly. Blades should be kept at opening position. It should not be closed automatically, real maintenance is to be done for this. AB switch blades are to be opened fully.

AB switches are to be kept locked on most conditions. AB switches are to be opened only subsequent to tripping the breakers. Lightning arresters? Lightning arresters are used to bypass the sudden lightning surges and thereby to protect the equipment's. Only subsequent to real discharging is done on lightning arresters, it should be attempted to attend to maintenance. Fencing is to be provided around lightning arresters.

Door arrangements with lock is to be provided. Separate earth connections are to be provided for lightning arresters. Current transformers: Current transformer secondary side is to be brief circuited during maintenance and testing. Prior to doing any testing, the current transformers are to be discharged. Potential transformers: Potential transformers primary side is to be Earthed during maintenance and testing.

Secondary side is to be earthed at only one place. Whenever giving connection, or removing meters on the secondary side of die potential transformer, the fuses are to be removed and renewed. Coupling capacitor: Capacitors should be provided inside fencing. Prior to attempting to do any work, real discharging is to be done. They only it should be attempted for maintenance work.

Real Earthing should be provided during the execution regarding the work. Subsequent to completion regarding the work, Earthing is to be removed. Earth pits: Sub-station earth connections should be properly maintained such that the earth resistance is minimum. H2o should be poured within the earth pits daily. Earth connections, should be capable of protecting the persons working within the electrical equipment's and protect within the equipment's during heavy fault current.

Earth resistance should not exceed the following limits. Grid stations: I Ohm Other sub-stations. Distribution transformers. They should be a clearance of six feet, between the sub-station fence and the electrical equipment's or live points. The fence should be earthed at every 200 feet, separately.

Generally the fence Earthing should not be linked with the sub-station Earthing. But if the clearance is fewer than six ft. feet fence Earthing should be linked with the sub-stations Earthing. The iron gates within the sub-station fence should also be earthed separately. Fire fighting equipments: These equipment's are to be kept on good and working condition.

Real schedule of maintenance is to be done for keeping them in good conditions. These equipment's should be kept at an with no problems accessible location so as to use them immediately below emergency. Hard sand heaps are to be available wherever necessary. Empty buckets are to be provided. yard should be provided with fencing.

Unauthorised persons should not enter into the yard 3. Cable ducks are to be provided with slabs. Greatest illumination is to be provided for the yard. A warning board with a display that Umbrella stick Dogs should not be brought inside the yard is to be provided at the entrance regarding the yard. A separate space is to- be provided for keeping the empty drums.

At the entrance regarding the space No smoking Board is to be provided. The territory regarding the work spot which was declared well-being to work is to be clearly identified by tying a rope. Inside this boundary is to be distant identified by hanging a lime flag. Outside this boundary where it is unsafe to work is to be identified by a dark brown flag. Wherever compulsory caution boards like Men on working Don't ever Switch on Safe for work etc.

If any unauthorized, unskilled staff happen to leave near the equipment's he can do so with the assistance and below the vigil of an experienced, authorised staff. Conversation is strictly prohibited wile execution of any work. It should be totally avoided mostly when work is being carried out on any bus bars. Placing the materials, tools and plants and men are to be at a well-being clearance from the Live. T and amp; Ps like spanners etc.

are to be lifted and brought below only by means of ropes and not by throwing and catching. Learn and safe ladder with steps at convenient intervals is to be used. To stay away from slippage regarding the ladder, compulsory precaution is to be taken at the bottom regarding the ladder by providing empty gunnies. Lifting of any ladder or rods Earth are to be done only horizontally. Vertical lifting shall cause damages by interrupting with the safe clearances.

9 The bus and line links art'; to be kept opened while doing work on OCB and.