When fault occurred in power cable like short circuit,ground and phase break in any reason required to repair by the process of jointing. In faulty power cable outer sheath and core insulation get damage.For jointing these power cable it is essential to adopt special technique and good quality insulating materials.Jointing also done when tap from main cable for service purpose and termination from transformers and other electrical equipments.The use of proper insulating materials depends on voltage rating of the power cable.For repairing of PVC insulated cable fabric and PVC tape used rated up to 6600 volts and impregnated paper tape suitable for 11kv and above.For repairing lead sheth lead tape and lead sleeve is suitable and armour is repair by using of copper flexible wire. Epoxy resin and bitumen used as a sealing compound protect from moisture.Bitumen compound is suitable up to 11kv and Oil resin compound suitable for 22 and 33Kv.The power cable mainly made up of aluminium and copper conductor, special technique used to jointing these cable. Aluminium conductor quite difficult to joint compared to copper conductor because presence of highly tenacious film of aluminium oxide is insulating in character and impairs the conductivity at joint. This film reforms in presence of air as soon as it is removed.We can joint the aluminium conductor by welding,soldering,compression and crimping of solderless lugs and in line connector. Welding is the best method of jointing the aluminium to aluminium conductor because electrically and mechanically stronger then the solder joints.In this method strip the insulation carefully without the scratches and cut in the conductor provide similar heat resistant such as asbestos,loose the conductor strand and clean the properly.Place the carbon mould around the conductor and heat until it became red than weld the conductor strand together by puddling with an iron wire add more aluminium.Such type of method used to aluminium to aluminium conductor joint,straight joint and tee joints.Soldering method special types of solders fluxes used like tin,lead Zinc and Cadmium.Due to presence of highly tenacious film of aluminium oxide which prevents flow of solder between conductor strands by capillary action, while jointing the aluminium conductor by soldering method conductor should be proper cleaned for proper operation of fluxes.Temperature should be maintained around 300-350℃ at the time of jointing by using thermometer.All the accessories and conductor kept the same temperature and removed the dross formed on the surface of the solder should be removed.Straight through joints and tee joint should be done from this method.Compression and crimping of solderless lugs is the simplest method of jointing aluminium conductor.In this method suitable size lugs and In line connector used and crimp with manual crimping tools,hydraulic compressor tool with suitable die depend on the size of the cable.Uniform joint and no any special technique required in this method to prevents the chances of Bi-metallic corrosion special metal alloy like zinc,tin,and silver coated sleeve is used. The advantage of jointing the conductor through this method having high electrical and mechanical strength. If aluminum conductor joint with other materials like copper due to presence of moisture corrosive chemical action takes place, So only tinned copper/brass connector should be used for bimetallic connections or joints and oxide inhibiting compounds should be used.Aluminium has also lower bending strength than copper therefore ,the conductor should be taken straight in to the connector or connected directly to the terminal lugs in the switchgear.The small aluminium conductors up to 4 sq mm in size may be formed in to an eye and connected to the post. The jointing of copper conductor is much easier than aluminum discussed previously.Soldering technique same as aluminum conductor for jointing copper conductor ordinary tin and lead solders are used and soldering flux removes oxide and prevents its formation during soldering ,so that the solder may flow readily and result in proper jointing. The composition of tin and lead varies for the different application and temperature required accordingly.Example 60%(tin) and 40%(lead) is used for electronics applications at temperature 182-184℃.45%(tin) and 55%(lead) is used for sheet metal work at temperature 182-225℃ and 40%(tin) and 60%(lead) is used for plant applications at temperature 182-235℃. The outer insulation or protective sheathing of a cables to be repaired by insulating tape and vulcanizing process especially trailing cable used for give supply for portable and transportable electrical equipments.It is must to use same insulating materials tape same as the core insulation.Such as paper insulated cable a impregnated paper tape is suitable to repair, similarly PVC tape for PVC insulated cable and rubber tape for rubber insulated tape.While performing repairing work the surface should be properly clean on which the tape is to be applied tightly to exclude any air or moisture.First applied insulation tape then few layers of waterproof empire tape finally friction adhesive tape as the finishing tape is called temporary repairing. In vulcanizing process we can repaired the rubber insulated cable joints are completely waterproof ,neat and good looking is called permanent repairing. Hot and cold vulcanizing process used the repairing of outer sheathing of cable.In hot vulcanizing process vulcanizing machine used it is must to maintained the correct water level and 2 bar steam pressure otherwise machine will burn out for completing a joint it will take half or one hour depends on weather conditions. Jacket rubber and insulating rubber is suitable for repairing of outer sheath and core respectively.In cold vulcanizing process self vulcanizing tape used to repair outer insulation of the cable. Outer sheathing repair by using with patches.Patches should be six times larger than damaged rubber covering bevel rubber cover towards center of damage and brush of rubber scrapings after that apply vulcanizing fluid with clean brush and let it dry.Patches and stitch on the outer sheathing firmly from center towards edges with rollers.Other method of repairing outer rubber insulation by bandaging for rubber covering up to 2 mm used 35mm cable wrapping strip if rubber covering more thicker than 2 mm used 50 mm cable wrapping strip.In cold process bevel both ends of cut out approximate 30 mm inward with knife and brush of rubber scrapings thoroughly.Apply vulcanizing fluid with clean brush roughened area bandage inner rubber covering 25mm insulating rubber tape allowing no gapes.
Sunday, December 29, 2019
Wednesday, December 25, 2019
More about cables part-2
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| Vulcanized India Rubber |
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| Polyvinyl chloride |
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| Mineral insulated cable |
Mineral insulated cable copper/aluminium magnesium oxide used in their manufacture.Conductor is inclosed in an outer tube of the same metal (copper or aluminium) and the annular space is filled with magnesium oxide.Such type of cable have high operating temperature overload capacity and resistance to mechanical damaged.Special seal pots and watertight gland fittings are used for terminating these cables.
Silicone-Rubber cables consist of a seathing of silicone rubber protected with braiding of varnished glass thread,operating temperature is 180℃ for continuous and 250 ℃ for intermittent operation.Insulated winding wire known as magnet wire. Many types of winding or magnet wire for use for rewinding of electrical equipment such motor, alternator etc.Oil immersed appliances used in their manufacturing are paper,cotton,glass,PVC and synthetic enamels.Enamels insulated wire is used to form a magnet coil possesses excellent resistance to moisture ,abrasion,chemical and solvents.
The types of enamel wire are Annealed copper wire insulated by a film of natural resin and drying oil suitability to class A thermal classification called plain enamelled.Polyvinyl formal enamelled wire used up to 120℃ temperature and suitable for rewinding of motor,fan,transformer and high speed winding machine.Polyvinyl formal nylon enamelled wire having high resistance to abrasion not suitable for humid conditions because nylon affected by moisture suitability to class A thermal classification.Polyester enamelled wire is excellent thermal stability and flexibility temperature up to 155℃ suitability to class B thermal classification.Epoxy modified polyester wire is suitable to class F thermal classification available in golden colour used for rewinding of motor,transformer in damp location and chemical plants.
Terephthalate polyester enamelled wire is suitable to class F thermal classification range up to 180℃ available in black colour.Polyurethane enamelled wire is suitable to class E thermal classification available in transparent yellowish colour wide application in the field of electronics and telecommunication.Self bonding enamelled wire such type of wire thermoplastic poly vinyl top coat over a polyvinyl-formal or polyurethane as a base coat is suitable for class A thermal classification.The cable used to providing the supply for portable and transportable equipments known as trailing cable are greater flexibility.The installation of power cable either underground,open and over head.Advantages of installation power cable in underground protect from rain storms and freedom from hazards of traffic mostly in substation and urban area but some disadvantage difficult to repair and cost is higher.Before the laying the power cable it is necessary to planning of soil condition such as temperature, thermal resistivity and presence of corrosive elements , moisture etc.The soil contains ashes,chloride salts,nitrates and alkaline materials should avoided for laying the cable.The cable not laid sites adjacent to the dc traction system.Now a days trench is dug in the ground and cable are laid on a bedding of sand and again covered with 10 to 12 cm of sand and protect with bricks or concrete slabs cable from mechanical damaged. Generally depth of trench depends on the voltage rating of the cable for 1.1 kv-0.45m,3.3kv-0.75m and 22 to 33kv- 1m and maintain the space between the cable 0.25 0.40 m in the same trench in order to reduced the effect of mutual heating.In road crossing underground cable cast-iron, galvanize-iron, stoneware or reinforced concrete pipe used to protect from mechanical damage and easier installation. For railway crossing cast-iron or spun-concrete pipes are used. It is necessary to maintain the clearance between the power and communication cable is 0.6m to avoid interference.It is necessary to record the value of resistance,capacitance and inductance and check the continuity, insulation resistance before laying the power cable underground.Mostly the fault occurred in underground cable short circuit,ground fault between phases and any phase break due to any conditions.Current capacity of the cable depends on the size of the cable and type of installation air,duct trench and underground,some other factor like ambient temperature of air and soil.Thermal resistivity of soil for cables laid in ground also load factor and load cycle.Fuse and circuit breakers are device used for protection against short circuits operates after lapse of certain time. Under short circuit condition conductor temperature rise for 11kv up to 160℃ 22 and 33kv up to 120℃.The FT,FTD,P,PL and PC types of cable used in coal mines.FT type cable the tinned copper braiding used instead of wire armour for sake of extra flexibility. FTD type cable not present protective armoured suitable for electric drill.
P type cable stranded steel wire armour is provided for protection against mechanical damaged suitable for conveyor,loaders etc.PL type cable wire armoured provided for mechanical protection suitable for face lighting up to 250 Volts.PC type cable protected with armoured and sheathed with neoprene suitable for remote control equipment.The short time loading of the power cable is the time that cable utilize maximum current rating without exceeding the maximum permissible temperature of the conductor.The XLPE (cross linked polythene cable being used both HT and LT power distribution . The XLPE cable having higher power rating,emergency overload rating,short circuit rating,insulation resistance and moisture resistance.Lower installation cost due to less weight,longer life and resistance chemical and corrosive gasses.During a fire XLPE cable emits less smoke and corrosive gasses.
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| Silicone-Rubber cables |
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| Magnet wire |
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| XLPE |
Wednesday, December 18, 2019
Details to the power,energy and introduction of cable
POWER: Power in electrical circuit work is done at the rate of one watt when an emf of one volt maintain a current of one ampere through a circuit of one Ohm resistance expressed in watt,kilo watt or mega watt.In direct current circuits power is P= I2 X R=V2 /R, Single phase AC circuit P=VI cosφ where cosφ is the power factor,Three phase circuit the power is P=3 Vph Iph cosφ previously discussed. Watt is the electrical unit of power and mechanical unit of the power is "Horse Power" 1 Horse power (HP)=746 watt = 0.746 watt. Three types of power flowing AC circuits Apparent power known as kilo volt ampere(KVA) having two component one is the real power(KW)=apparent power(KVA)* cosφ other is reactive power (KVAR)= apparent power(KVA) * sinφ.Watt-meter as an instrument that used to indicates the power in a circuits directly. The Watt-meter have current coil and potential coil . The current coil made in two part of thick wire connected in series and the potential coil or moving coil made of large number of turn of thin wire in series with a high resistance.The potential coil placed within the two sections of the current coil supported in jewel bearings. A watt-meter should be connected in the main circuit so that the moving coil end of the voltage coil and the series coil are on the same side of the load being measured.
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| Watt-meter |
ENERGY: The total power consumed over a certain period of called energy expressed as "Kilo watt hours" Kwh. We can also say the power utilized at the rate of one kilo watt for one hour known as a unit energy.All tariff of electric energy consumption based of this unit. Energy is measured with the help of watt-hour meter either three phase three wire circuit or three phase four wire circuit for unbalance load.If the current is more than 100 Ampere connected the CT operated KWH meter such that placed around the current carrying conductor in certain ratio 200/5A i.e. 200 ampere flow in main circuits to be measured the 5 ampere in KWH meter.
CABLE: We can define the cable a length one or more than one insulated conductors which are laid up together are one core,two core, three core and multi core.Conductor made up of either aluminium or copper,mainly aluminium conductor used for manufacturing of power cable because light weight lower cost also due to shortage of copper in India.Some advantage to used copper conductor 100 percent conductivity lesser resistance per meter only (0.017241 ohm per meter) but aluminium have 60 percent conductivity and more resistance to copper (0.02245 ohm per meter).The selection of power cable depend upon its applications and insulation used. Different types of insulating materials used for meannufacturig of power cable are Impregnated paper,Rubber,PVC,Cross linked polyethylene(XLPE),Asbestos,Magnesium oxide,Glass,Quartz,Silicon Rubber etc.
Impregnated paper insulated cable have high electrical break down strength 40-50 KV/mtr. and long life but not moisture resistance operated temperature up 100 degree celsius.
Rubber insulated cable have moisture resistance,flexible and easy to handle but less operating temperature 70 degree celsius.
PVC insulated cable dielectric strength and operating temperature same as rubber insulated cable but more tough, longer lasting and greater resistance to moisture acid,alkalies,fuels and oils than rubbers. Cross linked polyethylene (XLPE) insulated cable have greater flexibility and high resistance to moisture operating temperature 90 degree celsius. Asbestos insulated cable suitable for dry location absorbs moisture operating temperature range higher 100-200 degree celsius. Glass,quartz,silicon rubber insulated operating temperature above 150 degree celsius.
Construction of cable: The conductor is covered by layer of semi conducting paper to smooth the outer conductor surface than paper insulation and screen of metallised paper.The cores so formed are stranded together with jute or paper worming filling the interstices and wrapped with a binder of fabric tape interwoven with copper and aluminium strands over the lead seath ,bedding,armouring and serving are provided.
SERVING:The protecting materials over the wire armour of a cable known as serving that will protect the cable against weather and corrosion.The serving made of PVC,neoprene,glass tapes,paper cloths and jute etc.
BEDDING:Layers of impregnated fibrous materials applied to cable beneath the armouring to serve as a cushion like jute paper ,neoprene,glass,fiber tape etc.
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| Cable Construction |
Protect from resistance to decay, abrasion moisture,acids,alkalies,protecting covering provided that depends upon the nature of the insulation and application of the cable.The protecting covering mainly are (A) Metal-Sheathing paper and rubber insulated cable protect against moisture.(B) PVC- Sheathing protect against corrosive,acid and alkalies.(C) General and heavy duty natural rubber.
General and heavy duty polychloroprene protect from heat,flame,oil and moisture.(D) Asbestos yarn protect from strong heat.
Protect from mechanical damaged steel wire and tape armour used applied inn single and double layers.The wire armour should be preferred where cable are subjected to high tensile stress laid on sea-bed and deep shaft.
Friday, December 13, 2019
Details to the capacitor, inductor and magnet.
Capacitor: This is a device that stored electrical energy for
circuits initially power required. Capacitor consists of two conducting
materials are separated by dielectric materials such as mica; paper etc. is the
property of storing electrical energy known as capacitor or condenser but net
charge in a capacitor is zero. When voltage
(V) applied to move electric charge (Q) in dielectric medium then Q ∝
V also written as Q=CV where Q charge in coulombs, V in volt, C in farads is
called capacitance. Other units of capacitance are micro farads, pico farads
and nano farads. We can define the farads when stored electrical energy
increases by one coulombs and voltage changes at the rate of one volt per
second is called one farads. Various type of capacitor based on dielectric
materials paper, mica, ceramic and electrolyte capacitors. Charging the
capacitor by battery connected across it and discharge by shorting the positive
and negative each other. When the voltage applied in two conducting plates (V)
and charged (Q) on the plates then stored energy in a capacitor is E=1/2 Q V or E=1/2
CV2 .DC capacitor used in electronics circuits for small voltages. AC capacitor used
for low voltages applications such as ceiling fan, air cooler and air
conditioner etc. and high voltages capacitors used for power factor improvement
as we know current lead the voltage in
capacitor circuits at 90degree power factor maximum called capacitor banks. The value of
capacitance is directly proportional to the area of conducting materials (i.g.
plate, wire, cylindrical) and inversely proportional to the distance between
conducting materials separated by dielectric materials. (A) Capacitance between
two plates C=K*ε0*A / d Farads where K is permittivity, ε0- epsilon naught the
value is 8.854 X 10-12, A- area of plates sq. cm, d distance between two plates
means thickness of dielectric in cm. (B) Capacitance between more than two plates C=K*ε0*A(N-1) / d Farads.
Permittivity is ability of a dielectric to store energy; we can also define
energy stored per unit volume for unit potential gradient. This is unity for
air and vacuum. The instrument used for measurement multimeter, Ammeter and voltmeter
method and AC bridge method. Separate port provided to measurement of
capacitance in multimeter discharge before used. In ammeter and voltmeter capacitance
calculated C= 2 π f c v if voltage and current measurement through ammeter and
voltmeter. In AC bridge two non inductive resistance, condenser under
measurement and a standard condenser connected in bridge.
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| Construction of capacitor |
Inductor: A coil of any conducting materials such as copper,
aluminum called inductor. When inductor connected a power source due to this
electric current flow a magnetic field developed. The energy is stored in a
magnetic field as long as current flows, this magnetic field helps to store the
electric current for a short time, even if the supply removed. We can say the
capacitor stored electrical energy and inductor stored magnetic energy. When
voltage applied in inductor (coil) due to change in current flux also change,
change in flux EMF induced in it this is called inductance of coil. The
direction of induced e m f opposes the flow of current, inductance is denoted
by (L) and unit is Henry. One henry is define when 1volt is induced in a coil
and current changes at the rate of 1amp
per second. Various types of inductor are Fixed inductor, Iron core inductor,
Air core inductor, Toroidal core inductor and Laminated core inductor. Fixed
type inductor number of turns of the fixed coil remains the same used in
electronics equipment such as radios. In iron core inductor conducting coil
wound on iron core called ferromagnetic materials such as ferrite used higher
frequency tuning .In Air core inductor conducting coil wound on non magnetic
materials such as ceramic and plastic consider as air cored used as radios
frequency. In toroidal core inductor conducting coil wound on magnetic core
circular ring like iron powder used in AC electronics circuits application
where high frequency and low power required. In Laminated core inductor conducting
coil wound on thin steel sheet called soft iron piece used a low frequency
suitable for transformer applications.
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| Construction of inductor |
Magnet: In electrical magnet play an important role. Magnet
is a body which attracts a piece of iron (magnetic material) place near it;
have two poles one is the north another south pole. Opposite pole attracts each
other and similar pole repel each other. There are two types of magnet
permanent and electromagnet; permanent magnet also called natural used in alternator
and other industrial applications but in case of electromagnet magnate produced when current flowing in wingdings of
motors placed around a core of soft iron
as long as supply given. Magnetic field is the area around a magnet within
which it can attract a magnetic substance. Magnetic flux (φ) is a measurement
of the total magnetic which passes through a given area. If a quantity soft
iron materials placed around the current carrying conductor are arranged
themselves in a definite patterns because of lines of force known as magnetic
flux. This line of force travels from north to south outside the magnet and south
to north within the magnet expressed as Maxwell. Flux density (B) is the flux (φ)
per unit area Maxwell/sq.cm i.e. B= φ/A where φ=magnetic flux in Maxwell, A= area
in sq.cm. Magneto motive Force (MMF) is the force which produces magnetic flux in
magnetic circuits close similarity to electromotive force (EMF).MMF expressed as
Gilberts and Ampere turns. MMF=0.4πNI. NI=Ampere turns. Magnetizing Force (H)
is defined as MMF per unit length of path of the magnetic flux expressed as
ampere turn per meter. Permeability is the ratio of flux density to the
corresponding magnetizing force μ=B/H is unity for air and non magnetic materials. There are rules for electromagnets flaming right hand rule finger pointing in the direction of current flow and the thumb will point the direction of North pole. The screw rule turn in the direction of current flow and travels in the direction of north pole. In case of AC generator Fleming right hand rules is applicable thumb point direction of conductor motion forefinger direction of the flux and middle finger will point direction of induced EMF.
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| Construction of magnet |
Tuesday, December 3, 2019
Details to the current and resistance and measurement.
Current: The
flow of electrons in conducting loop called a current. Current denote as (I)
and unit is ampere. Ampere is equal to the current that flow through a circuit
of one ohm when one volt is applied across it. As we know the electrons are negatively
charged and direction flow from negatively point to positively point relatively
and value is -1.6X10-19 coulomb(C). Coulomb(C) is equal to quantity
of electricity transferred in one second by a current of one ampere, It the
unit for measurement of electricity quantity.
Two type current in circuits DC and AC. The flow of Direct current same
at all points in time although an alternating current flow of charged electrons
reverse direction periodically. The current free flow when the cross sectional
area of the conductor more but less dense means current density inversely
proportional to the area of conductor. When more area of the conductors current
density less vice versa. We can define the current density the amount of
electric current flow per unit cross sectional area of material.
Current density is the ratio of the current (I) flowing in a conductor to the cross sectional area (A) of that conductor expressed as J=I/A unit is Ampere/meter2.
The value of current density at
high frequency higher the outer surface
of the conductor and lower at the center of the conductor called the skin effect. Due skin effect transmission losses increased in high
voltage transmission lines can be reduced by increase the voltage and reduce
the current.
Ammeter is an instrument to measure the same current through a
circuit. Ammeter is always connected series with the circuit if it is connected
to parallel due to low resistance (few milli ohm) of ammeter cause short
circuit and burn the ammeter.
Moving
coil ammeter a movable coil placed in permanent magnate coil moved where
current passing through it and pointer moved connected with moving coil used
for DC current measurements.Moving iron
ammeter a soft iron piece moves by the electromagnetic force of a fixed coil of
wire used both AC and DC current.
Moving
magnet type meter a permanent magnet moves the pointer and coil is mounted
on the meter. Electrodynamics type
meter an electromagnet is used instead of permanent magnate to move the
pointer. Hot wire type ammeter a current pass through a wire which expands as
it heat cause move pointer. For
measuring of AC high current cannot pass through ammeter directly used current
transformer (CT) or shunt.
Current
transformer has primary and secondary windings, the secondary winding of CT
always short circuited otherwise core saturation due to high voltage which can
destroy the CT will be induced. CT will
reduce the current for measurement given to meter.
Current can also measure by
using clamp meter in this meter no
any test lid needed. Such type of meter has a split soft iron core which placed
around the conductor by pressing a trigger system and the change in magnetic
flux which proportional value of current read directly on the digital scale.
Such type of meter used both AC and DC measurement. In case of shunt ammeter high resistance alloy made
up of manganin actually a milivoltmeter which is calibrated in ampere.
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| Current density |
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| Skin effect |
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| Ammeter |
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| Moving coil ammeter |
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| Moving iron ammeter |
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Current transformer
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| Clamp meter |
Resistance: The flow of electrons
resists by resistance it is different for different materials. The unit of
resistance is ohm (Ω) and denoted by (R). Ohm is equal to the resistance
offered to unvarying electric current by a column of mercury 106.3cm long
having a cross sectional area of one sq.mm at the temperature of melting ice.
We can also say an e m f of one volt will cause a current of one ampere through
a resistance of one ohm. In any circuit value of resistance higher than then
the voltage and power losses more. Conductance is opposite of resistance the
unit of conductance is mhos (ʊ) and denoted by (Y).The resistance of conductor
directly proportional to the length of the conductor and inversely proportional
to the area of conductor. We can say when the length of conductor increases
resistance should increase vice versa if area of conductor increases resistance
should be decreases vice versa. In mathematical resistance is R=ρL/AL-length of the conductor in cm, R-resistance of the conductor, A- area
of conductor sq. cm,ρ-resisitivity in cm cube. Conductance I=VY where I-current in ampere,
V-voltage in volt, Y-Conductance mhos. We can define the resistivity with
resistance formula ρ=R A / L i.e. resistance offered by any
substance to flow of current per unit area per unit length known as
resistivity. The symbol is rho ρ and unit ohms/cm. The resisitivity of aluminium conductor per sqr mm per meter length is 0.028 ohms and copper 0.018 ohms. Resistance can be measured by Ohmmeter, Comparison between two resistances,Wheatstone bridge,Post office Bridge, Voltmeter method etc.
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| Ohmmeter |
Ohmmeter is an instrument used for measuring resistance directly a dry cell fitted as an energy source and galvanometer number of range with a suitable selector switch.Galvanometer calibrated to register ohms and before using the instrument checked resistance should be zero by touching two test prongs together.Start the measurement of resistance high range to right range reached otherwise instrument will be damaged.
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| Comparison of two resistance |
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Wheatstone bridge
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| Voltmeter |
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