Defensive Mechanism of S.s


Defensive Mechanism of S.S


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/ 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.


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.


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

  •    Latching relay
  •    Reed relay
  •    Mercury-wetted relay
  •    Polarized relay
  •       Machine tool relay

  •       Contactor relay
  •       Solid state contactor relay
  •       Buchholz relay
  •       Forced-guided contacts relay
  •       Solid-state relay
  •      Overload protection relay
  •       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, such a relay has six terminals in total. It is ambiguous whether the poles are normally open, normally closed, or one of each.

DPDT – Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to two SPDT switches or relays actuated by a single coil. Such a relay has eight terminals, including the coil.

QPDT – Quadruple Pole Double Throw. Often referred to as Quad Pole Double Throw, or 4PDT. These have four rows of change-over terminals. Equivalent to four SPDT switches or relays actuated by a single coil, or two DPDT relays. In total, fourteen terminals including the coil.

  •       Protective relay
  •       Overcurrent rela
  •       Distance relay



            Electrical systems by nature involve two 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. Both types of protection equally necessary for safe system operation.

            The importance of over voltage protection for a power system can not be over emphasized. Major equipment failures, expensive repairs, personnel safety and plant down time are certain consequences of inadequate protection from voltage surges.

            Surge arresters are designed to limit dangerous system over voltages. Whether lighting-or System- produced-to safe values when they occur on power systems. An arresters is a voltage limiting device. The functions are to discharge energy associated with a system over voltage condition, limit and interruption the power 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, certain protective characteristics of the arrester must be coordinated with the prevailing insulation levels on the system being protected. Insulation is a basic factor that must be considered in the application of arresters on a system. Insulation co-ordination is only a small part of the over all subject of arrester application. Several other factors must also be considered by the engineer when selecting surge protection. The location of the arresters, the inter-connection of ground leads, the insulation level of the protected equipment and the rating of the surge arresters are important in protecting equipment from harmful over voltage.

II.Surge Arrester operation:

            The basic operation of a surge arrester is single. In its noffi1al state, an arrester must act as an insulator. When a high voltage surge occurs. The arrester must cease to be an insulator and must turn into a short to-ground-in million thus of a second. The operation of the most widely used type of surge arresters the value, type of arrester is dealt with. Other types of arresters, such as expulsion arresters and line Oxide arresters (Gapless arresters) are either on the decline or too new for a general discussion at this time. The active elements of a valve type 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 number of in-series air gaps with sufficient dielectric strength to withstand the highest power frequency on the system. During severe over voltage conditions, the gap must always, breakdown at a voltage level some what below the insulation withstand voltage level of the equipment it is protecting, other wise equipment damage and or plant down time will result. the gap therefore serves as the switch which turns on the arrester. the voltage level at which the arrester goes from the passive (insulating) to the active (conducting) state, is called the spark over voltage.

            The valve block controls what happens after the arrester has been turned on. If only a gap is used, once a surge has been diverted to ground, a dead short circuit exists between line and ground and the 50 hertz-system energy 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 able to do this because It is made of a non-linear resistance material, silicon carbide. The valve block offers a very high resistance to 50 Hz current while displaying a low resistance to surge current. In addition, it also consumes the surge energy passes through it.

            Spark over and discharge voltage are the two 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.

III. Arrester Classification :

            There are three classifications of surge arresters used for over voltage protection in a system.

1.Distribution Type:

            The arresters are generally used in distribution system for equipment 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 are the lowest in cost.

2.Intermediate Type :

            These units cost approximately two or three times as much as equivalent distribution units. For this, the arrester offers lower maximum spark over and discharge voltage characteristics that afford a greater margin of protection plus the capability of discharging large surge levels. These arresters also have a pressure relief system to safely vent internal pressure if the unit falls before the porcelains shell has a chance to rupture. These arresters are used for the L.V. protection of Power transformers in sub-transmission sub-station i.e.110/33/22/11KV and 66/22/11KV sub-station.

3.Station Type:

            These arresters offer the best protective characteristics and the highest thermal capability but they cost about twice as much as equivalent intermediate units. Like intermediate arresters, station arresters have a pressure-relief system to safely vent internal pressure if the unit fails before a porcelain shell has a chance to rupture. These arresters are generally used in 230KV, 110KV and 66KV systems.

4.Basic insulation level:

            Basic Impulse Insulation Level (BIL) is the voltage level that equipment insulation is capable of withstanding without sustaining damage. The voltage withstand of insulation is function of time. Inorder to establish volt-time impulse insulation levels of transformers standard impulse tests standard voltage withstand tests are conducted on selected units as type 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) is also performed on some units. A curve plotted through these three points defines the minimum insulation withstand curve for insulation co-ordination (Fig.3) The true withstand level for the transformer lies above the plotted curve.

5. Surge arrester application:

            With an understanding of how an arrester performs its functions and a knowledge of equipment insulation, we can now move into the application area and consider the several factors that comprise surge arrester application as it relates to over voltage protection of transformers, The selection of surge arresters merit are carefully considered. Various factors have to be taken into account in order to arrive at a reliable and at the same time economical means of protection. The important points are:

            i)Selection of rated voltage.

            ii)Selection according to the 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  designed to operate and reseal effectively after a surge has passed. Because of the system grounding and connection, this, voltage is typically higher than the phase to ground voltage / on the healthy phases will 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 across an arrester during a surge discharge is directly proportional to the 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  point of equipment protection to select the lowest voltage rating for the application.

ii)Arrester location:

            Surge arresters should always be located as close as possible to the terminals of the equipment protected. In the case of transformer protection, mounting the arresters directly on the transformer is the best of insurance. An appreciable distance between the surge arrester, and the protected equipment reduces protection, afforded by the arresters and also increases the voltage impressed upon the transformer at time of surge discharge. Also because of the extra travel distance between the equipment and its arrester, surge wave could rise above the equipment damage point before the arrester comes to its rescue.

            n addition, the arrester connecting leads should be kept as short as possible because of their voltage contribution to discharge the voltage. During current flow to ground through an arrester, the interconnecting leads provide a voltage contribution because of current passing through an impedance. Depending on surge magnitude, rate of rise type of conductor, a typical value of voltage contribution to discharge voltage by interconnecting leads is i.e. 1.6 KV / foot.

            In practice, the protection range is given by the following simple formula.

                        L          =          U – Ua  x V     Where

                                                2 X S

                        L          =          Protection range of arrester in meters

                                                (measured along the line)

                        U         =          Impulse withstand voltage of protected equipment in KV.                                                   (BIL of equipment)

                        Ua       =          Spark over voltage of an arrester in K. V. (Peak) of the system.                                                       During earth fault conditions, the voltage

                        V         =          Velocity of wave progression with

                                                V line              =          300 meters /micro  sec.

                                                V cable            =          150 meters /micro  sec.

                        S          =          Steepness of incoming wave front in KV /  sec.


            (The protection range of an arrester increases with the difference between the impulse voltage IV’ and the spark over voltage Va. Therefore, an arrester with protective level tends to extend the protective range)

iii)Interconnection of Grounds:

            It is essential that the arrester ground terminal be interconnected with the transformer tank and secondary neutral to provide reliable surge protection for the transformers.

Iv)Insulation coordination: .

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

            After determining the rated voltage of an arrester, the protective level has to be carefully selected. For complete protection of the equipment, the “protective level” viz. the level to which the over voltages are omitted by the arrester, must be lower than the withstand level by a factor of at least 1.2 for lightning surges and 15 for switching surges. The value thus selected must be checked against that given in I.S.S. 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 two margins of protection calculations:

                                    CWW -FOW SO                    BIL -DV + IX)

            MP1 =                         CWW      x 100% MP2           =           BIL      x 100%


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                 = Basic Impulse Insulation level of the transformer.

DV                  = Discharge voltage of the arrester at 10 KA surge.

IX                    =  Voltage contribution of connecting leads at the rate of 1.6 KV / ft.

MP                  =  Margin of Protection

            Insulation co-ordination in an important aspect to be considered when surge protective is to be afforded to transformers with reduced BILS

vi Protection against direct strokes:

i)          Protection against direct strokes can be handled by shielding the station equipment’s by                the provision of either

            a)         Mast or rods or

            b)         a net work of overhead ground wires in such a way that equipment’s and switches                                     of all lie in the protected zone.

ii)         The protected zone for a rod mast is generally assumed as a cone with a base radius                       equal to the height of the rod or mast above ground.

iii)        For small sub-stations it may be sufficient to run one or GI wires across the station                        from adjacent line towers. Extra wires may be run from the tower to the structure and                over the station.

iv)        The grounds of the station shield should be solidly tied to the station ground bus to                       prevent difference of surge potential between the shield and other g-rounded parts of                    the 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 which is an unforeseen one is more essential aspect of any establishment / organisation.

            As accidents occur mainly due to unsafe execution, actions and circumstances, these accidents can be avoided by adopting safety precautions, implementing safety procedures and following safety rules.

General safety methods:

I.          While execution of any work, that part of equipment or line is to be isolated from the                    supply.

2.         Using discharge rods, charging, current if any is to be discharged.

3.         Using Earth rods, all phases/conducting path are to be property earthed by securing                       good Earthing.

4.         When even opening an AB switch or removing of fuse, it is also advisable and                   preferable to wear rubber gloves.

5.         Use of belt rope is another safety method to be adopted to work on elevated places.

Safety 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 get proper approval from the competent controlling authority for execution of the work specifying the date, time, duration,  place of work, affected parties etc. .

            For Grid feeders and Stations, the authorized officer for issue of approval is S.E.               (L.D. Centre), Madras, For 110 KV, 66 KV, radial feeders Superintending Engineer /                        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 proper approval from the competent authority, no L.C. should be issued nor availed by anybody. If the above procedure is not followed, it is nothing but a suicidal. Further it also amounts to murder of others.

            So, after getting proper approval, line clear is to be  issued to the requested party. But the issue and receiver should be aware/have full knowledge about the SS equipment’s, control room panel details etc.,

The line clear issuing person should clearly record the following:

            a) Which breaker have been tripped

            b) Which A.B. switches were opened

            c) Where Earthing was done

            d) What is the Safer place / Line to carry on the execution of work

Safety arrangements in control room:

1)         Key Board should be in open condition so that the keys could be taken out quickly                       during any urgency.

            Line clear keyboard should be in locked up condition to prevent other persons from           using the keys inside, before the cancellation of the Line clear permit.

The keys should be placed in the key board in an orderly manner according to their numbers. Otherwise, the required lock could not be opened in time and the possibility of opening a wrong lock may happen.

2)         Rubber mat should be provided on the floor in front of the panel board.

3)         The following details should be clearly displayed in the control room.

                        Approved operating instructions for all equipment’s.

                        Break down instructions.

Operating instructions including for the emergency operations to be carried out in the event of operation of buchholz relay. Differential relay, Group control trip, total supply failure, grid failure. The operator should be fully conversant with the above instructions and   the must be able to act quickly and effectively.

4)         The Board containing D.C. cable layout. A cable layout panel wiring diagram and Earthing layout should be displayed in the control room. This is necessary to attend the faults immediately after their occurrence.

5)         D.C. Earth leakage test system should be available.

6)         There should not be any defective power plugs, switches and bulb holders in the    control room wiring.

7)         One artificial respirator should be available in ready condition.

8)         Stools made of insulating material should be used for operating high tension          communication equipment’s (Telephones).

9)         Adequate number of rubber gloves, belt ropes, discharge rods, and earth rods in     good condition should be available in the control room.

Battery room:

1.         Battery room should be in locked up condition.

“Naked flame is prohibited inside of the battery room” and “Smoking prohibited” warnings should be kept written on the battery room door.

2.         One exhaust fan should be functioning.

3.         Accurate D.C. cell testing volt meters, hydro meters and thermometers should be               available in the battery room.

4.         Pilot cell voltage, specific gravity and temperature should be taken every week.

5.         The specific gravity should not be maintained below 1195 at 15.6°C and below 1183                    at 32. 20°C. The battery should not be allowed to discharge below 1160.

6.         Cell voltage should be maintained between 1.95 V to 2.05 V. The battery should not                     be allowed to discharge below 1.85 V.

7.         Battery should be allowed neither to over charge not to undercharge. It should not also                 be kept idle.

8.         Electrolyte level must be checked in every shift. It must be ensured that the level is                       10mm above the top of the plates.

9.         Weak cells should be rectified then and there.

10.       While taking specific gravity readings, care must be taken not to allow the acid to                         come in contact with the eyes.

Safety adopted for transformers:

1.         Transformers are to be maintained periodically as per schedule. Switches on HV side                     and LV side are to be isolated after reducing the Load by tripping the breakers.

2.         Kiosks and OCB : All the Live parts of the kiosk should have H. T. insulation tape. To be protected by wiremesh. It should be vermin proof. Keys are to be kept with interlock. When ever to open the door of the kiosk, kiosk should be tripped link should be opened by the interlock key. The opening of the links are to be verified physically. After doing all the above precautions, the tank should be lowered down. Proper care is to be taken and it should be kept in mind that supply is available at the roofing.

            Oil leak should be arrested. Back feeding is avoided.

            Cotton waste should not be used for cleaning purpose.

3.         AB switches:

Handle of the AB switch is to be earthed properly. Blades should be kept at opening position. It should not be closed automatically, proper maintenance is to be done for this. AB switch blades are to be opened fully. AB switches are to be kept locked on both            conditions. AB switches are to be opened only after tripping the breakers.

4.         Lightning arresters :

            Lightning arresters are used to bypass the sudden lightning surges and thereby to protect the equipment’s.Only after proper 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.

 5.        Current transformers:

            Current transformer secondary side is to be short circuited during maintenance and testing. Before doing any testing, the current transformers are to be discharged.

6.         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.

7.         Capacitors and H. T. Coupling capacitor:

            Capacitors should be provided inside fencing. Before attempting to do any work, proper discharging is to be done. They only it should be attempted for maintenance work. Proper Earthing should be provided during the execution of the work. After completion of the work, Earthing is to be removed.

8.         Earth pits:

            Sub-station earth connections should be properly maintained so that the earth            resistance is minimum. Water should be poured in the earth pits daily. Earth connections, must be capable of protecting the persons working in the electrical equipment’s and protect in the equipment’s during heavy fault current. Earth resistance should not exceed the following limits.

            Grid stations: I Ohm Other sub-stations ..2 Ohm.

            Distribution transformers ..5 Ohm.

            They must be a clearance of 5 feet, between the sub-station fence and the electrical equipment’s / 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 less than 5 ft. feet fence Earthing must be linked with the sub-stations Earthing. The iron gates in the sub-station fence should also be earthed separately.

9.         Fire fighting equipments:

            These equipment’s are to be kept on good and working condition. Proper schedule of maintenance is to be done for keeping them in good conditions. These equipment’s should be kept at an easily accessible place so as to use them immediately under emergency. Dry sand heaps are to be available wherever necessary. Empty buckets are to be provided.

 10.      S.S. Yard:

            1.         S.S. yard should be provided with fencing.

            2.         Unauthorised persons should not enter into the yard

            3.         Cable ducks are to be provided with slabs.

            4.         Best illumination is to be provided for the yard.

            5.         A warning board with a display that “Umbrella” stick Dogs should not be brought                         inside the  yard” is to be provided at the entrance of the yard.

            6.         A separate room is to- be provided for keeping the empty drums. At the                                         entrance of the room “No smoking” Board is to be provided.


1.         The territory of the work spot which was declared safety to work is to be clearly identified by tying a rope. Inside this boundary is to be further identified by hanging a green flag. Outside this boundary where it is unsafe to work is to be identified by a red flag.

2.         Wherever necessary caution boards like “Men on working” “Don’t Switch on” Safe for work” etc., are to be provided.

3.         If any unauthorized, unskilled staff happen to go near the equipment’s he can do so with the assistance and under the vigil of an experienced, authorised staff.

4.         Conversation is strictly prohibited wile execution of any work. It should be                        totally avoided especially when work is being carried out on any bus bars.

5.         Placing the materials, tools and plants and men are to be at a safety clearance from the Live. parts.

            6.         T & Ps like spanners etc. are to be lifted and brought down only by means of                                 ropes and not by throwing and catching.

7.         Study and safe ladder with steps at convenient intervals is to be used. To avoid slippage of the ladder, necessary precaution is to be taken at the bottom of the ladder by providing empty gunnies.

            8.         Lifting of any ladder or rods (Earth) are to be done only horizontally. Vertical

                         lifting may 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


Source by N.Sankari