“STUDY ELECTRICAL DESIGN OF A 220 KV SUBSTATION”

PROJECT WORKUNDER THE ESTEEMED GUIDANCE OF

Mr. S.D. GuatamAsst. Manager (technical) 220 KV P.P.K. I

“Delhi Transco Limited”

SUBMITTED BY:Ishank bounthiyal

C.R.R.INSTITUTE OF TECNOLOGYKANJHAWALA DELHI

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CERTIFICATE

This is to certify that Mr. ISHANK BOUNTHIYAL (roll no. 305119), students of diploma in (instrumentation & control) of

C.R.R.INSTITUTE OF TECNOLOGY has successfully completed their project work at DELHI TRANCO LIMITED on “Study Electrical

Design Of a 220 KV Substation” under the guidance of Mr. S.D. Guatam.The students have performed all the related activities during 02/07/2009 to 18/07/2009 i.e of their project duration.

Mr. S.D. GuatamAsst. Manager (technical)(DELHI TRANSCO LIMITED)

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ACKNOWLEDGEMENT

I express my gratitude to the management of “Delhi Transco Limited” for providing me with this opportunity to undergo training in this esteemed organization.I take the prerogative to express my gratitude to Mr.A.Guruswami , Asst. manager(technical), for his valuable suggestions and guidance throughout my training period.

I also like to thank the entire staff of “Delhi Transco Limited” for making my brief stay in Substation a memorable one.

Ishank bounthiya ( roll no 305119)Chotu ram rural

Institute of Engineering

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Delhi Transco Limited is the State Transmission Utility of the National Capital Territory of Delhi. It is responsible for transmission of power at 220KV and 400KV level, besides upgradation operation and maintenance of EHV Network as per system requirements.

After the enactment of Electricity Act 2003, a new department under the name and style of State Load Dispatch Center (SLDC) under Delhi Transco Limited was created, as an Apex body to ensure integrated operation of the power system in Delhi. SLDC Delhi started its function on the First of January 2004. SLDC is responsible for the real time Load Dispatch function, O&M of SCADA System and Energy Accounting.

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Electricity in India

Electric power generation in India is done predominantly by government sector entities. These are controlled by various central public sector corporations, such as: National Hydroelectric Power Corporation, National Thermal Power Corporation and various state level corporations (state electricity boards - SEBs). The transmission and distribution is managed by the State Electricity Boards (SEBs) or private companies.

The current per capita power consumption is about 612 KWH per year while the world average is 2596 KWH.

Generation

Grand Total Installed Capacity is 1,44,564.97 MW

Thermal Power

Current installed base of Thermal Power is 92,216.64 MW which comes to 64.6% of total installed base.

• Current installed base of Coal Based Thermal Power is 76,298.88 MW which comes to 53.3% of total installed base.

• Current installed base of Gas Based Thermal Power is 14,716.01 MW which comes to 10.5% of total installed base.

• Current installed base of Oil Based Thermal Power is 1,201.75 MW which comes to 0.9% of total installed base.

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Hydro Power

India was one of the pioneering states in establishing hydro-electric power plants, The power plant at Darjeeling and Shimsa was established in 1898 and 1902 respectively and is one of the first in Asia. Current installed base of Hydro Power is 36,033.76 MW which comes to 24.7% of total installed base. Today Hydro sector has turbines as large as 250 MW and single stage projects as big as 1500 MW .

Nuclear Power

Currently, seventeen nuclear power reactors produce 4,120.00 MW (2.9% of total installed base).

Renewable Power

Current installed base of Renewable Power is 12194.57 MW which comes to 7.7% of total installed base.

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Transmission

Transmission of electricity is defined as bulk transfer of power over a long distance at high voltage, generally of 132kV and above. In

India bulk transmission has increased form 3708ckm in 1950 to more than 265,000ckm today. The entire country has been divided into five regions for transmission systems, namely, Northern Region, North Eastern Region, Eastern Region, Southern Region and Western Region. The Interconnected transmission system within each region is also called the regional grid.

The transmission system planning in the country, in the past, had traditionally been linked to generation projects as part of the evacuation system. Ability of the power system to safely withstand a contingency without generation rescheduling or load-shedding was the main criteria for planning the transmission system. However, due to various reasons such as spatial development of load in the network, non-commissioning of load centre generating units originally planned and deficit in reactive compensation, certain pockets in the power system could not safely operate even under normal conditions. This had necessitated backing down of generation and operating at a lower load generation balance in the past. Transmission planning has therefore moved away from the earlier generation evacuation system planning to integrated system planning.

While the predominant technology for electricity transmission and distribution has been Alternating Current (AC) technology, High Voltage Direct Current (HVDC) technology has also been used for interconnection of all regional grids across the country and for bulk transmission of power over long distances.

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Distribution

The total installed generating capacity in the country is over 135000MW and the total number of consumers is over 144 million. Apart from an extensive transmission system network at 500kV HVDC, 400kV, 220kV, 132kV and 66kV which has developed to transmit the power from generating station to the grid substations, a vast network of sub transmission in distribution system has also come up for utilization of the power by the ultimate consumers.

However, due to lack of adequate investment on T&D works, the T&D losses have been consistently on higher side, and reached to the level of 32.86% in the year 2000-01.The reduction of these losses was essential to bring economic viability to the State Utilities.

As the T&D loss was not able to capture all the losses in the net work, concept of Aggregate Technical and Commercial (AT&C) loss was introduced. AT&C loss captures technical as well as commercial losses in the network and is a true indicator of total losses in the system.

High technical losses in the system are primarily due to inadequate investments over the years for system improvement works, which has resulted in unplanned extensions of the distribution lines, overloading of the system elements like transformers and conductors, and lack of adequate reactive power support.

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220 KV Substation

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BUS BAR

A bus bar in electrical power distribution refers to thick strips of copper or aluminium that conduct electricity within a switchboard, distribution board, substation, or other electrical apparatus.

The size of the busbar is important in determining the maximum amount of current that can be safely carried. Busbars can have a cross-sectional area of as little as 10 mm² but electrical substations may use metal tubes of 50 mm in diameter (1,000 mm²) or more as busbars.

Busbars are typically either flat strips or hollow tubes as these shapes allow heat to dissipate more efficiently due to their high surface area to cross-sectional area ratio. The skin effect makes 50-60 Hz AC busbars more than about 8 mm (1/3 in) thick inefficient, so hollow or flat shapes are prevalent in higher current applications. A hollow section has higher stiffness than a solid rod, which allows a greater span between busbar supports in outdoor switchyards.

A busbar may either be supported on insulators, or else insulation may completely surround it. Busbars are protected from accidental contact either by a metal enclosure or by elevation out of normal reach. Neutral busbars may also be insulated. Earth busbars are typically bolted directly onto any metal chassis of their enclosure. Busbars may be enclosed in a metal housing, in the form of bus duct or busway, segregated-phase bus, or isolated-phase bus.

Busbars may be connected to each other and to electrical apparatus by bolted or clamp connections. Often joints between high-current bus sections have matching surfaces that are silver-plated to reduce the contact resistance. At extra-high voltages (more than 300 kV) in outdoor buses, corona around the connections becomes a source of

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radio-frequency interference and power loss, so connection fittings designed for these voltages are used.

Protection

Bus bars are vital parts of a power system and so a fault should be cleared as fast as possible. A busbar must have its own protection although their high degrees of reliability bearing in mind the risk of unnecessary trips, so the protection should be dependable, selective and should be stable for external faults, called through faults.

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BUS BAR SCHEMES

Bus scheme selection criteria

System reliability Possibility of major shut-down Continuity of supply in the event of bus fault Availability of bus in the event of stuck of circuit

breaker Redundancy

Bus switching scheme

Operation flexibility Taking a line / equipment in or out Taking a bus bar in or out Taking a circuit breaker in or out Ease of maintenance Taking out of components for maintenance without loss

of feeder and with ease of changeover Limitation of short circuit level Simplicity of protection arrangement Ease of extension Availability / requirement of land Cost

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Double bus scheme

Most commonly used bus scheme. Normally load will be distributed on both the buses and the

bus coupler will kept closed. For maintenance & extension of any one of the buses the

entire load will be transferred to the other bus. On load transfer of a circuit from one bus to the other bus is

possible through bus isolators provided the bus coupler is closed and thereby two buses are at the same potential.

On load bypassing of any circuit for breaker maintenance is not possible.

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TRANSFORMER

Definition :Transformer is defined as A static piece of apparatus with two or more windings which, by electromagnetic induction, transforms a system of alternating voltage and current into another system of voltage and current usually of different

values and at the same frequency for the purpose of transmitting electrical power.

PARTS OF A POWER TRANSFORMER :

ACTIVE PART:

1) core2) windings (LV,HV,regualting, tertiery)3)tap changer4)cleats and leads5)tank

ACCESSORIES:

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1) radiators

AUXILLARIES:

1)bushings2)buchholz relay/oil surg relay3)temprature indicators4)oil level indicator5)pressure relief device6)marshalling box/control cubical7)oil preservation system8)conservators(gas sealed ,bellows/membrane sealed)9)silica gel breather

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POWER TRANSFORMER (100MVA)

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BASIC PRINCIPLE

The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism) and, second, that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). By changing the current in the primary coil, one changes the strength of its magnetic field; since the secondary coil is wrapped around the same magnetic field, a voltage is induced across the secondaryA current passing through the primary coil creates an electromagnet; the current and its magnetic field are proportional to one another, so that if the current changes, so does the magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron; this ensures that the magnetic field lines produced by the primary current stay within the iron — instead of "leaking" out into the surrounding air — and pass intact through the secondary coil.

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• INDUCTION LAWS

The amount of voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that,

where VS is the instantaneous voltage, NS is the number of turns in the secondary coil and Φ equals the total magnetic flux through one turn of the coil. If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is defined as the product of the magnetic field strength B and the area A through which it cuts

Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer, the instantaneous voltage across the primary winding equals

Taking the ratio of the two equations for VS and VP gives the basic equation for stepping up or stepping down the voltage

• ENERGY LOSSES IN TRANSFORMERTransformer losses are attributable to several causes and may be differentiated between those originating in the windings, sometimes termed copper loss, and those arising from the magnetic circuit, sometimes termed iron loss. The losses vary with load current, and may furthermore be expressed as "no-

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load" or "full-load" loss, or at an intermediate loading. Winding resistance dominates load losses, whereas hysteresis and eddy currents losses contribute to over 99% of the no-load loss. The no-load loss can be significant, meaning that even an idle transformer constitutes a drain on an electrical supply, and lending impetus to development of low-loss transformers

Losses in transformer arise due to:I) Winding Resistance(copper loss)ii) Hysteresis losses (core loss)iii) Eddy current loss

iv)Magnetostriction V) Mechanical lossesvi) Stray losses

• CONSTRUCTION Laminated steel core

Transformers for use at power or audio frequencies typically have cores made of high permeability silicon steel. The steel has a permeability many times that of free space, and the core thus serves to greatly reduce the magnetising current, and confine the flux to a path which closely couples the windings..Later designs

constructed the core by stacking layers of thin steel laminations, a principle that has remained in use. Each lamination is insulated from its neighbors by a thin non-conducting layer of insulation. The universal transformer equation indicates a minimum cross-sectional area for the core to avoid saturation

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The core loss in transformer caused due to eddy current and hysterisis can be delt with by utilizing the following:

• For the hysterisis

The transformer cores are made up of COLD ROLLED GRAIN OREINTED STEEL(CRGO) .the main advantages of using CRGO is that it has a smaller hysterisis loop.hence low core loss and also high permeability.the addition of silicon in iron increases its magnetic properties.

• For eddy current

A small addition of silicon upto 3% increases the resistivity of iron core by 3-4 times.hence provides a more resistive path for eddy current.also, the magnetic cores are made of thin isulated iron sheets.using this technique the the magnetic core is equivalent to many small magnetic circuits,each one reciving a small fraction of the magnetic flux.furthermore these circuits have a resistance higher than the non magnetic core because of their reduced section.

The effect of laminations is to confine eddy currents to highly elliptical paths that enclose little flux, and so reduce their magnitud

• WINDINGSThe conducting material used for the windings depends upon the application, but in all cases the individual turns must be electrically insulated from

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each other to ensure that the current travels throughout every turn. For small power and signal transformers, in which currents are low and the potential difference between adjacent turns is small, the coils are often wound from enamelled magnet wire. Larger power transformers operating at high voltages may be wound with copper rectangular strip conductors insulated by oil-impregnated paper and blocks of pressboard.The primary and secondary windings are arraged in such a way as to reduce the flux lekage.

windings are arraged concentrically to minimize flux lekage.For signal transformers, the windings may be arranged in a way to minimise leakage inductance and stray capacitance to improve high-frequency response. This can be done by splitting up each coil into sections, and those sections placed in layers between the sections of the other winding. This is known as a stacked type or interleaved winding.

• TRANSFORMER OIL

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Transformer oil is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties

Following are the functions of transformer oil:-

i) to insulate the windings

ii) to supress corona

iii) to supress arching

iv) to serve as coolant

The oil helps cool the transformer. Because it also provides part of the electrical insulation between internal live parts, transformer oil must remain stable at high temperatures over an extended period. To improve cooling of large power transformers, the oil-filled tank may have external radiators through which the oil circulates by natural convection. Very large or high-power transformers (with capacities of millions of watts) may also have cooling fans, oil pumps, and even oil-to-water heat exchangers.

Large, high-voltage transformers undergo prolonged drying processes, using electrical self-heating, the application of a vacuum, or both to ensure that the transformer is completely free of water vapor before the cooling oil is introduced. This helps prevent corona formation and subsequent electrical breakdown under load.

Oil filled transformers with conservators (an oil tank above the transformer) tend to be equipped with Buchholz relays. These are safety devices that can sense gas buildup inside the transformer (a side effect of corona or an electric arc inside the windings) and then switch off the transformer. Transformers without conservators are usually equipped with sudden pressure relays, which perform a similar function as the Buchholz relay.

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Oil alternatives :

Today, nontoxic, stable silicone-based or fluorinated hydrocarbons are used, where the added expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Natural or synthetic Esters are becoming increasingly common as alternative, to Naphthenic mineral oil too. Esters are non toxic, readily biodegradable, and have higher flash points than mineral oil. Prior to about 1970, polychlorinated biphenyl (PCB) was often used as a dielectric fluid since it was not flammable

• TRANSFORMER OIL SAMPLING AND TESTING

The life of a transformer is dependent upon three parametes :

1. temprature

2.oxygen

3.moisture

it is necessary to remove the moisture form the transformer after regular interval of time there are a few tests to determine the quality of oil .Generally we use minral oil for transfomer winding insulation and cooling .the test given below are :

1.BDV(break down voltage test)

2.DGA(dissolved gas analysis)

3.dielectric strength of oil

EFFECT OF MOISTURE ON TRANSFORMER OIL

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Most of the power transformwers use paper and oil as the main form of insulation and during manufacture stringent efforts are made to ensure that both are as dry as possible when the new plant is set up.once in service the moisture content begins to increase.excessive can put the life of the transformer at risk .it is important to understand the source of this moisture .its effects and the preventive measures.

SOURCE OF MOISTURE

Once in service the transformer is subjected to the following source of moisture

External form the atmosphere

Internal form manufacture

Internal form cellulose ageing

1)BDV(breakdown voltage )TESTING:

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after taking the sample of transformer oil with suitable equipment,we test the property in the BDV,DIELECTRIC STRENGTH OF MINRAL OIL using as a coolant and insulator for transformer winding.the BDV should be approximate around 60 kv for transformer oil.the procedure of this experiment is given s followes:-

• This test is performed by advanced test kit specially design fopr this test

• In this kit we use special bucket which contains electrode, these electrode can be one of the type as:SPHERICAL,CYLINDRICAL,OR MUSHROOM.we use spherical electrode in this test as per IEC STANDARDS.

• First of all wash every active component of the kit with the transformer oil for removing moisture and dust contents from the elctrodes and buckets .

• Set the gap between the elctrode at exactly 2.5mm for the special electrode .

• In this kit we have special facility to steering the oil automatically, it is based on magnetic effect.

• Set 5 min time duration for steering.it is necessary to mixed up the oil suitably.

• After 5 min stat applying voltage between electrodes .it increases gradually.

• At a perticular value of voltage oil break down.there will be a spark flashing at this breakdown

• Note down the reading of BDV .take 5 more reading• This advanced kit is well programmed to give printed

average value of all 6 observation.

Value by this experiment for UAT(unit auxillary transformer) -2 is found -52.8kv.

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BUCHHOLZ RELAY

In the field of electric power distribution and transmission, a Buchholz relay, also called a gas relay or a sudden pressure relay, is a safety device mounted on some oil-filled power transformers and reactors, equipped with an external overhead oil reservoir called a conservator. The Buchholz Relay is used on conservator type oil preservation systems as a protective device sensitive to events which occurs during dielectric failure inside the equipment.

When an electric arc or overheating develops inside the coils, gas is generated. The relay has two different detection modes. On a slow accumulation of gas, due perhaps to slight overload, gas accumulates in the top of the relay and forces the oil level down. A float operated switch in the relay is used to initiate an alarm signal. This same switch will also operate on low oil level, such as a slow oil leak. If an arc forms, gas accumulation is rapid, and oil flows rapidly into the conservator. This flow of oil operates a switch attached to a vane located in the path of the moving oil. This switch normally will operate a circuit breaker to shut down (isolate) the apparatus before the fault causes additional damage. Buchholz relays have a test port to allow accumulated gas to be withdrawn for testing. Flammable gas found in the relay indicates some internal fault such as

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overheating or arcing, whereas air found in the relay may only indicate low oil level or a leak. Buchholz relays have been applied to large power transformers at least since the 1940.

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SILICA GEL BREATHER

breather provide an economic and efficient means of controlling the level of moisture entering electrical equipment during the change in volume of the cooling medium and/or airspace caused by temperature gradients.

when the volume of oil in the transformer increases the oil expansds and moves into the consevators .hence it pushes the air inside the consevator outwards.as the level of moisture is to be controlled inside the transformer .therefore the excess of air is absorbed by the breather at the time of expansion and gives of the air when the oil contracts.

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BUSHINGS

A bushing is a transformer component that insulates a high voltage conductor passing through a metal enclosure. Bushings appear on switchgear, transformers, circuit breakers and other high voltage equipment.

Description

The bushing is hollow, allowing a conductor to pass along its centre and connect at both ends to other equipment. Bushings are often made of wet-process fired porcelain, and may be coated with a semi-conducting glaze to assist in equalizing the electrical stress along the length of the bushing.

The inside of the bushing may contain paper insulation and the bushing is often filled with oil to provide additional insulation. Bushings for medium-voltage and low-voltage apparatus may be made of resins reinforced with paper. The use of polymer bushings for high voltage applications is becoming more common..

Different types of bushings:

i) oil impregnated paper(OIP)

ii)epoxy resin impregnated paper(ERIP)

Bushings with varity of external insulations are:

i)porceline in brown and grey

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SWITCHYARD EQUIPMENTS:

The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream.

Typically switchgear in substations is located on both the high voltage and the low voltage side of large power transformers. The switchgear located on the low voltage side of the transformers in distribution type substations, now are typically located in what is called a Power Distribution Center (PDC). Inside this building are typically smaller, medium-voltage (~15kV) circuit breakers feeding the distribution system. Also contained inside these Power Control Centers are various relays, meters, and other communication equipment allowing for intelligent control of the substation.

A piece of switchgear may be a simple open air isolator or it may be insulated by some other substance. An effective although more costly form of switchgear is "gas insulated switchgear" (GIS), where the conductors and contacts are insulated by pressurized (SF6)sulfur hexafluoride gas. Another common type is oil insulated switchgear.

Circuit breakers are a special type of switchgear that are able to interrupt fault currents. Their construction allows them to interrupt fault currents of many hundreds or thousands of amps.

• Lightning arrester

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• Wire end cables• Capacitive voltage transformer(CVT)• isolators • current transformers(CT)• circuit breakers• bus-bar lines• wave trap• Battery Box

LIGHTNING ARRESTER

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Lightning arrester is a device which is provided in the switch yard to protect the equipments from lightning.

Lightning arresters are designed to safely channel a lightning strike to ground without damaging equipment. Inside the porcelain housing of an arrester, are a series of spark gaps plus one or more silicon carbide blocks. Silicone carbide has an unusual electrical characteristic. It has a very high resistance to comparatively low-voltage, but a very low resistance to extremely high voltage. When lightning strikes there is a sudden rapid rise in voltage. The silicon carbide resistance breaks down, allowing the current to be conducted to ground. After the surge is passed, the resistance of the blocks increases back to normal levels.

Hence the LA provides a low resistance path in case of high voltage surge.Material used: - silicon carbideSilicon carbide has variable resistance at. It has very high resistance at low voltage and very low resistance at very high voltage.

Lightning arresters are generally located on both the high and low side of a substation transformer to protect it from strikes coming in either direction.

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Outer insulation of LA is made up of porcelain.

WIRE CABLES

Wire cables are use to transport the high voltage transmission lines underground in case of some physical obstruction.

CAPACITIVE VOLTAGE TRANSFORMER

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A capacitor voltage transformer (CVT) is a transformer used in power systems to step-down extra high voltage signals and provide low voltage signals either for measurement or to operate a protective relay.

DESIGN:

In its most basic form the device consists of three parts:

• two capacitors across which the voltage signal is split• an inductive element used to tune the device to the supply

frequency• a transformer used to isolate and further step-down the

voltage for the instrumentation or protective relay

The device has at least four terminals, a high-voltage terminal for connection to the high voltage signal, a ground terminal and at least one set of secondary terminals for connection to the instrumentation or protective relay.

CVTs are typically single-phase devices used for measuring voltages in excess of one hundred kilovolts where the use of voltage transducers would be uneconomical. In practice the first capacitor,

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C1, is often replaced by a stack of capacitors connected in series. This results in a large voltage drop across the stack of capacitors that replaced the first capacitor and a comparatively small voltage drop across the second capacitor, C2, and hence the secondary terminals.

PROTECTION :

A protective surge arrester/spark gap shall preferably be provided to prevent break down of insulation by incoming surges and to limit abnormal rise of terminal voltage of shunt capacitor, tuning reactor, RF choke, etc. due to short circuit in transformer secondary. The details of this arrangement (or alternative arrangement) shall be furnished by Contractor for Employer's review.

HERMETIC SEALING SYSTEM:

Each capacitor unit is hermetically sealed .a stainless steal diaphragm (expansion bellows) is installed to preserve the integrity of oil by maintaining the hermetic seal while allowing the thermal expansion and contraction of the oil. The capacitor units operate is a complete pressure free mode over a very wide ambient temperature range. The base tank is filled with degassed mineral oil hermetically sealed from the environment and from the synthetic oil in the capacitor unit.

CVT SPECIFICATIONS:

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Total burden - 100 MVA

Thermal burden – 750 VA

Rated voltage – 220/ 3^.5 kV

Highest system voltage – 245 kV

Insulation level – 460 kv/1050 kV

Rated freq - 50 Hz

Primary capacitance (c1) – 4881 pf

Secondary capacitance (c2) – 47186 pf

CURRENT TRANSFORMER

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A current transformer (CT) is a type of instrument transformer designed to provide a current in its secondary winding proportional to the current flowing in its primary. They are commonly used in metering and protective relaying in the electrical power industry where they facilitate the safe measurement of large currents, often in the presence of high voltages. The current transformer safely isolates measurement and control circuitry from the high voltages typically present on the circuit being measured.

DESIGN:

The most common design of CT consists of a length of wire wrapped many times around an annular silicon steel ring passed over the circuit being measured. The CT's primary circuit therefore consists of a single 'turn' of conductor, with a secondary of many hundreds of turns.

The CT acts as a constant-current series device with an apparent power burden a fraction of that of the high voltage primary circuit. Hence the primary circuit is largely unaffected by the insertion of the CT.

Common secondaries are 1 or 5 amperes. For example, a 4000:5 CT would provide an output current of 5 amperes when the primary was passing 4000 amperes. The secondary winding can be single ratio or multi ratio, with five taps being common for multi ratio CTs

Current transformers can be used to supply information for measuring power flows and the electrical inputs for the operation of protective relays associated with the transmission and distribution circuits or for power transformers. These current transformers have the primary winding connected in series with the

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conductor carrying the current to be measured or controlled. The secondary winding is insulate from the high voltage and can then be connected to low-voltage metering circuits.

CT SPECIFICATIONS:Highest system: 245vInsulation level: 460/1050 kVRated primary current: 600 ARated STC: 27 Ka for 1 secFreq: 50 HzCT ratio: 600-300/1/1/1/1

CIRCUIT BREAKERS

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A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

TYPES OF CIRCUIT BREAKERS :

There are many different technologies used in circuit breakers and they do not always fall into distinct categories. Types that are common in domestic,commercial and light industrial applications at low voltage

1. MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation

2. MCCB (Moulded Case Circuit Breaker)—rated current up to 1000 A. Thermal or thermal-magnetic operation. Trip current may be adjustable.

Electric power systems require the breaking of higher currents at higher voltages. Examples of high-voltage AC circuit breakers are:

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1. Oil circuit breaker2. water type circuit breakers3. air circuit breakers4. vaccum circuit breakers5. SF6 circuit breakers

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Operating Range Of Different Circuit Breaker

DIELECTRIC STERNGTH OF CIRCUIT BREAKERS

VACUUM CIRCUIT BREAKERS:

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The main idea behind the VCB is to eliminate the medium of contact s. vacume of the order of 10^-5 Hg is maintained .in such a low pressure the elctron crosses the gap without any collision .arc is formed by neutral atom,ions and electron emitted from the electrode themselves.

Vacuum circuit breaker—With rated current up to 3000 A, these breakers interrupt the current by creating and extinguishing the arc in a vacuum container. These can only be practically applied for voltages up to about 35,000 V, which corresponds roughly to the medium-voltage range of power systems. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers.

CONSTRUCTION:

The outer envelope is made up of glass which is joined with metallic end caps .glass envelope fecilitates the examining of the breakers from outside after operation.If it becomes milky white from orignal finish of silver thenit is a sign of loosing vacuum.after the envelope there is a sputter sheild made up of stainless steel to

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prevent trhe metal vapour reaching the envelope .In side there is a moving and fixed contact the metalic bellows are made up of stainless steel.

SF6 CIRCUIT BREAKERS:

Current interruption in a high-voltage circuit-breaker is obtained by separating two contacts in a medium, such as sulfur hexafluoride (SF6), having excellent dielectrical and arc quenching properties. After contact separation, current is carried through an arc and is interrupted when this arc is cooled by a gas blast of sufficient intensity.

.

Gas blast applied on the arc must be able to cool it rapidly so that gas temperature between the contacts is reduced from 20,000 K to less than 2000 K in a few hundred microseconds, so that it is able to withstand the transient recovery voltage that is applied across the contacts after current interruption. Sulfur hexafluoride is

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generally used in present high-voltage circuit-breakers (of rated voltage higher than 52 kV).

Characteristics of SF6 circuit breakers can explain their success:

1. Simplicity of the interrupting chamber which does not need an auxiliary breaking chamber;

2. Autonomy provided by the puffer technique; 3. The possibility to obtain the highest performance, up to 63

kA, with a reduced number of interrupting chambers; 4. Short break time of 2 to 2.5 cycles; 5. High electrical endurance, allowing at least 25 years of

operation without reconditioning; 6. Possible compact solutions when used for GIS or hybrid

switchgear; 7. Integrated closing resistors or synchronised operations to

reduce switching overvoltages; 8. Reliability and availability; 9. Low noise levels.

SF6 specifications:

Rated lighting impulse-1050 kvp(peak voltage)

Rated short circuit current- 40 kA

Rated oprating pressure- 15 kg/cm2

Rated duration of short circuit current- 40kA, 3 sec(fault level)

Gas weight- 21 kg

Rated voltage-245 kv

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Rated freq-50 Hz

Rated normal current – 3150 A

Rated closing voltage – 220 V DC

Rated opening voltage – 220 V DC

Gas pressure – 6 kg/cm2

Relays:

Automotive style miniature relay

A relay is an electrical switch that opens and closes under the control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. It was invented by Joseph Henry in 1835. Because a relay is able to control an output circuit of higher power than the input circuit, it can be considered to be, in a broad sense, a form of an electrical amplifier.

A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off

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so relays have two switch positions and they are double throw (changeover) switches.

Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical.

The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification.

Relays are usuallly SPDT or DPDT but they can have many more sets of switch contacts, for example relays with 4 sets of changeover contacts are readily available. For further information about switch contacts and the terms used to describe them please see the page on switches.

Most relays are designed for PCB mounting but you can solder wires directly to the pins providing you take care to avoid melting the plastic case of the relay.

The supplier's catalogue should show you the relay's connections. The coil will be obvious and it may be connected either way round. Relay coils produce brief high voltage 'spikes' when they are switched off and this can destroy transistors and ICs in the circuit. To prevent damage you must connect a protection diode across the relay coil.

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Operation

When a current flows through the coil, the resulting magnetic field attracts an armature that is mechanically linked to a moving contact. The movement either makes or breaks a connection with a fixed contact. When the current to the coil is switched off, the armature is returned by a force approximately half as strong as the magnetic force to its relaxed position. Usually this is a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing.

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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.[1]

Applications

Relays are used:

• to control a high-voltage circuit with a low-voltage signal, as in some types of modems or audio amplifiers,

• to control a high-current circuit with a low-current signal, as in the starter solenoid of an automobile,

• to detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers (protection relays),

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A DPDT AC coil relay with "ice cube" packaging

• to isolate the controlling circuit from the controlled circuit when the two are at different potentials, for example when controlling a mains-powered device from a low-voltage switch. The latter is often applied to control office lighting as the low voltage wires are easily installed in partitions, which may be often moved as needs change. They may also be controlled by room occupancy detectors in an effort to conserve energy,

• to perform time delay functions. Relays can be modified to delay opening or delay closing a set of contacts. A very short (a fraction of a second) delay would use a copper disk between the armature and moving blade assembly. Current flowing in the disk maintains magnetic field for a short time, lengthening release time. For a slightly longer (up to a minute) delay, a dashpot is used. A dashpot is a piston filled with fluid that is allowed to escape slowly. The time period can be varied by increasing or decreasing the flow rate. For longer time periods, a mechanical clockwork timer is installed.

Overcurrent relay

An "Overcurrent Relay" is a type of protective relay which operates when the load current exceeds a preset value. The ANSI Device Designation Number is 50 for an Instantaneous OverCurrent (IOC), 51 for a Time OverCurrent (TOC). In a typical application the overcurrent relay is used for overcurrent protection, connected to a current transformer and calibrated to operate at or above a specific current level. When the relay operates, one or more contacts will operate and energize a trip coil in a Circuit Breaker and trip (open) the Circuit Breaker.

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Protective relay

A protective relay is a complex electromechanical apparatus, often with more than one coil, designed to calculate operating conditions on an electrical circuit and trip circuit breakers when a fault was found. Unlike switching type relays with fixed and usually ill-defined operating voltage thresholds and operating times, protective relays had well-established, selectable, time/current (or other operating parameter) curves. Such relays were very elaborate, using arrays of induction disks, shaded-pole magnets, operating and restraint coils, solenoid-type operators, telephone-relay style contacts, and phase-shifting networks to allow the relay to respond to such conditions as over-current, over-voltage, reverse power flow, over- and under- frequency, and even distance relays that would trip for faults up to a certain distance away from a substation but not beyond that point. An important transmission line or generator unit would have had cubicles dedicated to protection, with a score of individual electromechanical devices. The various protective functions available on a given relay are denoted by standard ANSI Device Numbers. For example, a relay including function 51 would be a timed overcurrent protective relay.

These protective relays provide various types of electrical protection by detecting abnormal conditions and isolating them from the rest of the electrical system by circuit breaker operation. Such relays may be located at the service entrance or at major load centers.

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Design and theory of these protective devices is an important part of the education of an electrical engineer who specializes in power systems. Today these devices are nearly entirely replaced (in new designs) with microprocessor-based instruments (numerical relays) that emulate their electromechanical ancestors with great precision and convenience in application. By combining several functions in one case, numerical relays also save capital cost and maintenance cost over electromechanical relays. However, due to their very long life span, tens of thousands of these "silent sentinels" are still protecting transmission lines and electrical apparatus all over the world.

Distance relay

The most common form of feeder protection on high voltage transmission systems is distance relay protection. Power lines have set impedance per kilometre and using this value and comparing voltage and current the distance to a fault can be determined. The main types of distance relay protection schemes are

• Three step distance protection• Switched distance protection• Accelerated or permissive intertrip protection• Blocked distance protection

In three step distance protection, the relays are separated into three separate zones of impedance measurement to accommodate for over reach and under reach conditions. Zone 1 is instantaneous in operation and has a purposely set under reach of 80% of the total line length to avoid operation for the next line. This is due to measurements of impedance of lines not being entirely accurate, errors in voltage and current transformers and relay tolerances. These errors can be up to ±20% of the line impedance, hence the zones 80% reach. Zone 2 covers the last 20% of the feeder line length and provides backup to the next line by having a slight over reach. To prevent mal-operation the zone has a 0.5 second time delay. Zone 3 provides backup for the next line and has a time delay of 1 second to grade with zone 2 protection of the next line.

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Ground (electricity)

In electrical engineering, the term ground or earth has several meanings depending on the specific application areas. Ground is the reference point in an electrical circuit from which other voltages are measured, a common return path for electric current (earth return or ground return), or a direct physical connection to the Earth.

A typical earthing electrode (left of gray conduit) .Note the green and yellow marked earth wire.

Electrical circuits may be connected to ground (earth) for several reasons. In power circuits, a connection to ground is done for safety purposes to protect people from the effects of faulty insulation on electrically powered equipment. A connection to ground helps limit the voltage built up between power circuits and the earth, protecting circuit insulation from damage due to excessive voltage. Connections to ground may be used to limit the build-up of static electricity when handling flammable products or when repairing electronic devices. In some types of telegraph and power transmission circuits, the earth itself can be used as one conductor

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of the circuit, saving the cost of installing a separate run of wire as a return conductor. For measurement purposes, the Earth serves as a (reasonably) constant potential reference against which other potentials can be measured. An electrical ground system should have an appropriate current-carrying capability in order to serve as an adequate zero-voltage reference level. In electronic circuit theory, a 'ground' is usually idealized as an infinite source or sink for charge, which can absorb an unlimited amount of current without changing its potential.

The use of the term ground (or earth) is so common in electrical and electronics applications that circuits in vehicles such as ships, aircraft, and spacecraft may be spoken of having a "ground" connection without any actual connection to the Earth

AC power wiring installations

In a mains electricity (AC power) wiring installation, the ground is a wire with an electrical connection to earth. By connecting the cases of electrical equipment to earth, any insulation failure will result in current flowing to ground that would otherwise energize the case of the equipment. A proper bonding to earth will result in the circuit overcurrent protection operating to de-energize the faulty circuit. By bonding (interconnecting) all exposed non-current carrying metal objects together, any fault currents in the system will not produce dangerous voltages which could cause electric shock.

The power ground grounding wire is (directly or indirectly) connected to one or more earth electrodes. These may be located locally, be far away in the suppliers network or in many cases both. This grounding wire is usually but not always connected to the neutral wire at some point and they may even share a cable for part of the system under some conditions. The ground wire is also usually bonded to pipework to keep it at the same potential as the electrical ground during a fault. Water supply pipes often used to be used as ground electrodes but this was banned in some countries when plastic pipe such as PVC

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became popular. This type of ground applies to radio antennas and to lightning protection systems.

A power ground serves to provide a return path for fault currents and therefore allows the fuse or breaker to disconnect the circuit. The power ground is also often bonded to the house's incoming pipework, and pipes and cables entering the bathroom are sometimes cross-bonded. This is done to try to reduce the potential difference between objects that can be touched simultaneously. Filters also connect to the power ground, but this is mainly to stop the power ground carrying noise into the systems which the filters protect, rather than as a direct use of the power ground.

Permanently installed electrical equipment usually also has permanently connected grounding conductors. Portable electrical devices with metal cases may have them connected to earth ground by a pin in the interconnecting plug. (see Domestic AC power plugs and sockets). The size of power ground conductors is usually regulated by local or national wiring regulations.

Power transmission

Some HVDC power transmission systems use the ground as second conductor. This is especially common in schemes with submarine cables as sea water is a good conductor. Buried grounding electrodes are used to make the connection to the earth. The site of these electrodes must be chosen very carefully in order to prevent electrochemical corrosion on underground structures.

In Single Wire Earth Return (SWER) AC electrical distribution systems, costs are saved by using just a single high voltage conductor for the power grid, while routing the AC return current through the earth. This system is mostly used in rural areas where large earth currents will not otherwise cause hazards.

A particular concern in design of electrical substations is earth potential rise. When very large fault currents are injected into the earth, the area around the point of injection may rise to a high potential with respect to distant points. This is due to the limited finite conductivity of the layers of soil in the earth. The gradient of the

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voltage (changing voltage within a distance) may be so high that two points on the ground may be at significantly different potentials, creating a hazard to anyone standing on the ground in the area. Pipes, rails, or communication wires entering a substation may see different ground potentials inside and outside the substation, creating a dangerous touch voltage

Circuit ground versus earth

Voltage is a differential quantity, which appears between two points having some electrical potentials. To measure the voltage of a single point, a reference point must be selected to measure against. This common reference point is called ground and considered to have zero voltage. This signal ground may or may not actually be connected to a power ground. A system where the system ground is not actually connected to another circuit or to earth (though there may still be AC coupling) is often referred to as a floating ground.

Lightning protection systems

Lightning protection systems form a very specialised application of grounding used in an attempt to lessen damage to man-made structures caused by lightning strikes. The concept and goal of lightning protection systems is to mitigate the extreme fire hazard which lightning poses to some types of man-made structures, especially those which are built of flammable materials, such as wood, or electrically resistant materials, such as brick, stone, or concrete. A lightning protection system is an attempt to provide a preferred, low-resistance path for the lightning circuit to follow, in order to lessen the heating effects of lightning's current flowing through or around flammable structural materials, or through porous materials which can contain water, such as brick, stone, or concrete, as the water contained in these rain-soaked masonry elements may explode when flashed to steam by lightning's heat.

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To appreciate the limitations of lightning protection systems, it is important to understand the magnitude of lightning's energy. Because of the incredibly high electrical potential of lightning (oen exceeding 100 million volts and 40,000 amperes), no lightning protection system can guarantee absolute safety from lightning to a structure, its contents, or its occupants. While lightning (as all electrical current) will tend to follow the path of least resistance, lightning will often follow many distinct paths, and secondary side-flashes can be enough to ignite a fire, blow apart brick, stone, or concrete, or injure occupants within a structure or building. Nonetheless, scientists, electrical engineers, and property insurers have accepted and relied upon the benefits of basic lightning protection systems for well over a century.[2]

The components of a basic lightning protection system are air terminals (i.e. lightning rods or strike termination devices), bonding conductors (usually heavy stranded copper or aluminum wires or thick braided or solid copper or aluminum straps), ground terminals (i.e. electrodes, ground or earthing rods, plates, or mesh), and all of the proper connectors and supports to complete the system. The air terminals are typically arranged at or along the upper points of a roof structure, and are electrically bonded together by bonding conductors (sometimes called "down conductors" or misleadingly called "downloads"), which are connected by the most direct route possible to one or more grounding or earthing terminals installed into the earth or ground.[3]

In overhead transmission lines, a ground conductor may also be the top most wire on pylons, poles, or towers. This ground conductor is intended to protect the power conductors from lightning strikes. These conductors are connected to earth either through the metal structure of a pole or tower, or by additional ground electrodes installed at regular intervals along the line. As a general rule, overhead power lines with voltages below 50 kV do not have a ground conductor, but most lines carrying more than 50 kV do. Depending on local conditions and reliability requirements, an over head transmission line may have two overhead ground conductors.

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Ground mat

A ground mat or grounding mat is a flat, flexible pad used for working on electrostatic sensitive devices. It is generally made of a conductive plastic or metal mesh covered substrate which is electrically attached to ground. This helps discharge any static which a worker has built up, as well as any static on tools or exposed components laid on the mat. It is used most commonly in computer repair. Ground mats are also found on fuel trucks, which are otherwise insulated from ground as they make physical contact only with their (rubber and air) tires; obviously static discharge is undesirable during fuel-transfer operations. Similarly, in aircraft refueling, a ground cable connects the tanker (truck or airplane) to the fuel-seeking craft to eliminate charge differences before fuel is transferred.

In an electrical substation a ground mat is a mesh of conductive material installed at places where a person would stand to operate a switch or other apparatus; it is bonded to the local supporting metal structure and to the handle of the switchgear, so that the operator will not be exposed to a high differential voltage due to a fault in the substation.

BATTERY BOX

Batteries are installed in substation to provide power to switching components and to power the substation control equipment in times of AC power loss. They require regular maintenance for proper working .

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Lead Acid Batteries are widely used for years, other alternative is Nickel-cadmium battery

LEAD Acid Battery

Electrochemistry

Each cell contains (in the charged state) electrodes of lead metal (Pb) and lead (IV) dioxide (PbO2) in an electrolyte of about 33.5% w/w (6 Molar) sulfuric acid (H2SO4). In the discharged state both electrodes turn into lead(II) sulfate (PbSO4) and the electrolyte loses its dissolved sulfuric acid and becomes primarily water. Due to the freezing-point depression of water, as the battery discharges and the concentration of sulfuric acid decreases, the electrolyte is more likely to freeze.

The chemical reactions are (charged to discharged):

Anode (oxidation):

Cathode (reduction):

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Because of the open cells with liquid electrolyte in most lead-acid batteries, overcharging with excessive charging voltages will generate oxygen and hydrogen gas by electrolysis of water, forming an explosive mix. The acid electrolyte is also corrosive.

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