Chandu

A project report

ON

‘at’

220/132kV

Transmission Substation

Mulugu Road, Warangal(T.G)

Contents 1.Company Profile

2.Electrical Substation

2.1.Types Of Substation

2.1.1.Transmission Substation

2.1.2.Distribution Substation

2.1.3.Collector Substation

2.2.Components Of Substation

3.Conductors

3.1.Aluminum In Place Of Conductors

3.2.Types Of Conductors

3.2.1.AAC

3.2.2.AAAC

3.2.3.ACSR

3.2.4.ACAR

4.Transformers

4.1.Instrument Transformers

4.1.1.Current Transformer

4.1.2.Potential Transformer

4.1.2.1. Capacitor Voltage Transformer

4.2.Auto Transformer

5.Capacitor Bank

6.Isolators

7.Circuit Breakers

7.1.Types Of Circuit Breakers

7.1.1.Sulfur Hexafluoride H V Circuit Breaker

7.1.2.Carbon Di Oxide H V Circuit Breaker

8.Lightning Arresters

9.Description Of Substation

9.1.Panel Section 9.1.1.Control Panel Section

9.1.2.Relay And Protection Panel Section 9.2.Yard

10.Some Full Forms Related To Substation

11.Components Used In Yard(220kV Substation ,Naubasta)

12.Conclusion

Company Profile

Telangana Power Transmission Corporation Limited,

incorporated under the Companies Act 1956, was incorporated in

2006 with the main objective to acquire, establish, construct, take

over, erect, lay, operate, run, manage, hire, lease, buy, sell, maintain,

enlarge, alter, renovate, modernize, work and use electrical

transmission lines and/or network through extra high voltage, high

voltage and associated sub-stations, cables, wires, connected with

transmission ancillary services, telecommunication and telemetering

equipment in the State of Uttar Pradesh, India and elsewhere.

Electricity

Saved

is

Electricity

Produced..! Page | 1

Electrical Substation

A substation is a part of an electrical generation,

transmission, and distribution system. Substations transform voltage

from high to low, or the reverse, or perform any of several other

important functions. Between the generating station and consumer,

electric power may flow through several substations at different

voltage levels.

Substations may be owned and operated by an electrical

utility, or may be owned by a large industrial or commercial

customer. Generally substations are unattended, relying on SCADA for

remote supervision and control.

A substation may include transformers to change voltage

levels between high transmission voltages and lower distribution

voltages, or at the interconnection of two different transmission

voltages. The word substation comes from the days before the

distribution system became a grid. As central generation stations

became larger, smaller generating plants were converted to

distribution stations, receiving their energy supply from a larger plant

instead of using their own generators. The first substations were

connected to only one power station, where the generators were

housed, and were subsidiaries of that power ratio.

As this project report is based on 220kV Grid Transmission

Substation, Naubasta, Kanpur; so the components used there are

described below. Page | 2

2.1 Types of substation

1. Transmission substation A transmission substation connects two or more

transmission lines. The simplest case is where all transmission

lines have the same voltage. In such cases, substation contains

high-voltage switches that allow lines to be connected or isolated for

fault clearance or maintenance. A transmission station may have

transformers to convert between two transmission voltages, voltage

control/power factor correction devices such as capacitors,

reactors or static VAR compensators and equipment such as

phase shifting transformers to control power flow between

two adjacent power systems.

2. Distribution substation

A distribution substation transfers power from the

transmission system to the distribution system of an area. It is

uneconomical to directly connect electricity consumers to the main

transmission network, unless they use large amounts of power, so

the distribution station reduces voltage to a level suitable for local

distribution.

The input for a distribution substation is typically at least

two transmission or sub transmission lines. Input voltage may be, for

example, 115 kV, or whatever is common in the area. The output is a

number of feeders. Distribution voltages are typically medium

voltage, between 2.4 kV and 33 kV depending on the size of the area

served and the practices of the local utility. The feeders run along

streets overhead (or underground, in some cases) and power the

distribution transformers at or near the customer premises.

Page | 3

3. Collector substation

In distributed generation projects such as a wind farm, a

collector substation may be required. It resembles a distribution

substation although power flow is in the opposite direction, from

many wind turbines up into the transmission grid. Usually for

economy of construction the collector system operates around 35 kV

and the collector substation steps up voltage to a transmission

voltage for the grid. The collector substation can also provide power

factor correction if it is needed, metering and control of the wind

farm. In some special cases a collector substation can also contain an

HVDC converter station.

2.2 Components of Substation Various components are used at grid transmission

substations. These are as follows :-

(i) Conductors

(ii) Current Transformers

(iii) Potential Transformers

(iv) Power Transformers (Auto Transformer)

(v) Capacitive Voltage Transformers

(vi) Line Isolators

(vii) Bus Isolators

(viii) Lightning Arresters

(ix) Capacitor Bank

(x) Circuit Breakers

Page | 4

Conductors

In physics and electrical engineering, a conductor is an

object or type of material which permits the flow of electric charges in

one or more directions. For example, a wire is an electrical

conductor that can carry electricity along its length.

In metals such as copper or aluminium, the movable

charged particles are electrons. Positive charges may also be mobile,

such as the cationic electrolyte(s) of a battery, or the mobile protons of

the proton conductor of a fuel cell. Insulators are non-conducting

materials with few mobile charges and which support only

insignificant electric currents.

3.1 Aluminium in place of Copper:

a) Much lower cost

b) Lighter weight

c) Larger diameter

d) Lower voltage gradient less ionization/corona

3.2 Types of conductors used in 220kV substations are:-

a) AAC -> All Aluminium Conductors

b) AAAC -> All Aluminium Alloy Conductors

c) ACSR -> Aluminium Conductor Steel Reinforced

d) ACAR -> Aluminium Conductor Alloy Reinforced

Page | 5

All Aluminium Conductors (AAC)

APPLICATIONS

AAC are used primarily for overhead transmission and

primary and secondary distribution, where ampacity must be

maintained and a lighter conductor (compared to ACSR) is desired,

and when conductor strength is not a critical factor. Classes B and C

are used primarily as bus, apparatus connectors and jumpers, where

additional flexibility is required.

CONSTRUCTION

Aluminium 1350-H19 wires, concentrically stranded. Page | 6

All Aluminium-Alloy Conductor (AAAC)

APPLICATIONS

Used as bare overhead conductor for primary and

secondary distribution. Designed utilizing a high-strength aluminiumalloy

to achieve a high strength-to-weight ratio; affords good sag

characteristics. Aluminium-alloy gives 6201-T81 gives AAAC higher

resistance to corrosion than ACSR.

CONSTRUCTION

Aluminium-alloy 6201-T81 wires, concentrically stranded.

Page | 7

Aluminium Conductor Steel Reinforced

(ACSR)

APPLICATIONS

Used as bare overhead transmission conductor and as

primary and secondary distribution conductor and messenger

support. ACSR offers optimal strength for line design. Variable steel

core stranding enables desired strength to be achieved without

sacrificing ampacity.

CONSTRUCTION

• Aluminium 1350-H19 wires, concentrically stranded about a steel

core. Standard core wire for ACSR is class A galvanized.

• Class A core stranding is also available in zinc-5% aluminium -

mischmetal alloy coating.

• Additional corrosion protection is available through the application of

grease to the core or infusion of the complete cable

with grease.

• ACSR conductor is also available in non-specular. Page | 8

Names of ASCRs

S.No. Names Size(mm)*

6/1/3.55 1. Rabbit

30/7/3.00 2. Panther

54/7/3.00 3. Zebra

54/7/3.53 4. Moose

*No. Of strands / No. Of steel Strands/diameter of strands Page | 9

Aluminium Conductor Aluminium Alloy

Reinforced (ACAR)

APPLICATIONS

Used as bare overhead transmission cable and as primary

and secondary distribution cable. A good strength-to-weight ratio

makes ACAR applicable where both ampacity and strength are prime

considerations in line design; for equal weight, ACAR offers higher

strength and ampacity than ACSR.

CONSTRUCTION

Aluminium 1350-H19 wires, concentrically stranded about

an aluminium-alloy 6201-T81 core. Although the alloy strands

generally comprise the core of the conductor, in some constructions

they are distributed in layers throughout the aluminium 1350-H19

strands.

Page | 10

Transformers

A transformer is a static electrical device that transfers

energy by inductive coupling between its winding circuits. A varying

current in the primary winding creates a varying magnetic flux in the

transformer's core and thus a varying magnetic flux through the

secondary winding. This varying magnetic flux induces a varying

electromotive force (emf) or voltage in the secondary winding.

Transformers range in size from thumbnail-sized used in

microphones to units weighing hundreds of tons interconnecting the

power grid. A wide range of transformer designs are used in

electronic and electric power applications. Transformers are

essential for the transmission, distribution, and utilization of

electrical energy.

Fig. Equivalent Circuit Diagram of an Ideal Transformer Page | 11

Instrument transformer

Instrument transformers are high accuracy class electrical

devices used to isolate or transform voltage or current levels. The

most common usage of instrument transformers is to operate

instruments or metering from high voltage or high current circuits,

safely isolating secondary control circuitry from the high voltages or

currents. The primary winding of the transformer is connected to the

high voltage or high current circuit, and the meter or relay is

connected to the secondary circuit.

Instrument transformers may also be used as an isolation

transformer so that secondary quantities may be used in phase

shifting without affecting other primary connected devices.

Types of Instrument Transformers:-

Current Transformers

Potential Transformers Page | 12

Current transformers

Current transformers (CT) are a series connected type of

instrument transformer. They are designed to present negligible load

to the supply being measured and have an accurate current ratio and

phase relationship to enable accurate secondary connected

metering.

Current transformers are often constructed by passing a

single primary turn (either an insulated cable or an uninsulated bus

bar) through a well-insulated toroidal core wrapped with many turns of

wire. This affords easy implementation on high voltage bushings of

grid transformers and other devices by installing the secondary turn

core inside high-voltage bushing insulators and using the passthrough

conductor as a single turn primary.

A current clamp uses a current transformer with a split

core that can be easily wrapped around a conductor in a circuit. This is

a common method used in portable current measuring

instruments but permanent installations use more economical types of

current transformer.

Specially constructed wideband CTs are also used, usually

with an oscilloscope, to measure high frequency waveforms or

pulsed currents within pulsed power systems. One type provides an IR

voltage output that is proportional to the measured current;

another, called a Rogowski coil, requires an external integrator in

order to provide a proportional output.

Ratio

The CT is typically described by its current ratio from

primary to secondary. A 1000:5 CT would provide an output current

of 5 amperes when 1000 amperes are passing through its primary Page | 13

winding. Standard secondary current ratings are 5 amperes or 1

ampere, compatible with standard measuring instruments.

Burden and accuracy

Burden and accuracy are usually stated as a combined

parameter due to being dependent on each other.

Metering style CTs are designed with smaller cores and VA

capacities. This causes metering CTs to saturate at lower secondary

voltages saving sensitive connected metering devices from damaging

large fault currents in the event of a primary electrical fault. A CT

with a rating of 0.3B0.6 would indicate with up to 0.6 ohms of

secondary burden the secondary current will be within a 0.3 percent

error parallelogram on an accuracy diagram incorporating both

phase angle and ratio errors.

Relaying CTs used for protective circuits are designed with

larger cores and higher VA capacities to insure secondary measuring

devices have true representations with massive grid fault currents on

primary circuits. A CT with a rating of 2.5L400 would indicate it can

produce a secondary voltage to 400 volts with a secondary current of

100 amperes (20 times its rated 5 ampere rating) and still be within

2.5 amperes of true accuracy.

Care must be taken that the secondary winding of a CT is

not disconnected from its low-impedance load while current flows in the

primary, as this may produce a dangerously high voltage across the

open secondary (especially in a relaying type CT) and could

permanently affect the accuracy of the transformer.

Page | 14

High Voltage Types

Current transformers are used for protection,

measurement and control in high voltage electrical substations and

the electrical grid. Current transformers may be installed inside

switchgear or in apparatus bushings, but very often free-standing

outdoor current transformers are used.

In a switchyard, live tank current transformers have a

substantial part of their enclosure energized at the line voltage and

must be mounted on insulators. Dead tank current transformers

isolate the measured circuit from the enclosure. Live tank CTs are

useful because the primary conductor is short, which gives better

stability and a higher short-circuit current withstand rating. The

primary of the winding can be evenly distributed around the

magnetic core, which gives better performance for overloads and

transients. Since the major insulation of a live-tank current

transformer is not exposed to the heat of the primary conductors,

insulation life and thermal stability is improved.

A high-voltage current transformer may contain several

cores, each with a secondary winding, for different purposes (such as

metering circuits, control, or protection).

Fig. Equivalent Circuit Diagram of a Current Transformer

Page | 15

Potential transformers

Potential Transformer or Voltage Transformer are used in electrical power system for stepping down the system voltage to a safe value which can be fed to low ratings meters and relays. Commercially available relays and meters used for protection and metering, are designed for low voltage.

Potential transformers (PT) (also called voltage transformers (VT)) are a parallel connected type of instrument transformer. They are designed to present negligible load to the supply being measured and have an accurate voltage ratio and phase relationship to enable accurate secondary connected metering.

Ratio The PT is typically described by its voltage ratio from

primary to secondary. A 600:120 PT would provide an output voltage of 120 volts when 600 volts are impressed across its primary winding. Standard secondary voltage ratings are 120 volts and 70 volts, compatible with standard measuring instruments.

Burden and accuracy

Burden and accuracy are usually stated as a combined parameter due to being dependent on each other.

Metering style PTs are designed with smaller cores and VA capacities than power transformers. This causes metering PTs to saturate at lower secondary voltage outputs saving sensitive connected metering devices from damaging large voltage spikes found in grid disturbances.

A small PT (see nameplate in photo) with a rating of 0.3W, 0.6X would indicate with up to W load (12.5 watts ) of secondary burden the secondary current will be within a 0.3 percent error parallelogram on an accuracy diagram incorporating both phase angle and ratio errors. The same technique applies for the X load (25 watts) rating except inside a 0.6% accuracy parallelogram.

Page | 16

Markings

Some transformer winding primary (usually high-voltage) connection points may be labelled as H1, H2 (sometimes H0 if it is internally designed to be grounded) and X1, X2 and sometimes an X3 tap may be present. Sometimes a second isolated winding (Y1, Y2, Y3) (and third (Z1, Z2, Z3) may also be available on the same voltage transformer. The primary may be connected phase to ground or phase to phase. The secondary is usually grounded on one terminal to avoid capacitive induction from damaging low-voltage equipment and for human safety.

Types of PTs

There are three primary types of potential transformers (PT): electromagnetic, capacitor, and optical. The electromagnetic potential transformer is a wire-wound transformer. The capacitor voltage transformer (CVT) uses a capacitance potential divider and is used at higher voltages due to a lower cost than an electromagnetic PT. An optical voltage transformer exploits the electrical properties of optical materials.

Capacitor Voltage Transformer

A capacitor voltage transformer (CVT), or capacitance coupled voltage transformer (CCVT) is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal, for measurement or to operate a protective relay. In its most basic form the device consists of three parts: two capacitors across which the transmission line signal is split, an inductive element to tune the device to the line frequency, and a transformer to isolate and further step down the voltage for the instrumentation or protective relay.

The tuning of the divider to the line frequency makes the overall division ratio less sensitive to changes in the burden of the connected metering or protection devices.

Page | 17

The device has at least four terminals: a terminal for connection to the high voltage signal, a ground terminal, and two secondary terminals which connect 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 wound primary voltage transformers would be uneconomical. In practice, capacitor C1 is often constructed as a stack of smaller capacitors connected in series. This provides a large voltage drop across C1 and a relatively small voltage drop across C2.

The CVT is also useful in communication systems. CVTs in combination with wave traps are used for filtering high frequency communication signals from power frequency. This forms a carrier communication network throughout the transmission network.

Fig. Equivalent Circuit Diagram of CVT Page | 18

Auto transformer

An autotransformer (sometimes called autostep down transformer) is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting on itself and not to any kind of automatic mechanism.

In an autotransformer portions of the same winding act as both the primary and secondary transformer. The winding has at least three taps where electrical connections are made. Autotransformers have the advantages of often being smaller, lighter, and cheaper than typical dual-winding transformers, but autotransformers have the disadvantage of not providing electrical isolation.

Working The primary voltage is applied across two of the

terminals, and the secondary voltage taken from two terminals, almost always having one terminal in common with the primary voltage. The primary and secondary circuits therefore have a number of windings turns in common. Since the volts-per-turn is the same in both windings, each develops a voltage in proportion to its number of turns. In an autotransformer part of the current flows directly from the input to the output, and only part is transferred inductively, allowing a smaller, lighter, cheaper core to be used as well as requiring only a single winding.

One end of the winding is usually connected in common to both the voltage source and the electrical load. The other end of the source and load are connected to taps along the winding. Different taps on the winding correspond to different voltages, measured from the common end. In a step-down transformer the source is usually connected across the entire winding while the load is connected by a tap across only a portion of the winding. In a step- up transformer, conversely, the load is attached across the full winding while the source is connected to a tap across a portion of the winding.

Page | 19

Limitations

An autotransformer does not provide electrical isolation between its windings as an ordinary transformer does; if the neutral side of the input is not at ground voltage, the neutral side of the output will not be either. A failure of the insulation of the windings of an autotransformer can result in full input voltage applied to the output. Also, a break in the part of the winding that is used as both primary and secondary will result in the transformer acting as an inductor in series with the load (which under light load conditions may result in near full input voltage being applied to the output) . These are important safety considerations when deciding to use an autotransformer in a given application.

Because it requires both fewer windings and a smaller core, an autotransformer for power applications is typically lighter and less costly than a two-winding transformer, up to a voltage ratio of about 3:1; beyond that range, a two-winding transformer is usually more economical.

In three phase power transmission applications, autotransformers have the limitations of not suppressing harmonic currents and as acting as another source of ground fault currents. A large three-phase autotransformer may have a "buried" delta winding, not connected to the outside of the tank, to absorb some harmonic currents.

In practice, losses mean that both standard transformers and autotransformers are not perfectly reversible; one designed for stepping down a voltage will deliver slightly less voltage than required if it is used to step up. The difference is usually slight enough to allow reversal where the actual voltage level is not critical.

Like multiple-winding transformers, autotransformers operate on time-varying magnetic fields and so will not function with DC.

Page | 20

Applications

Autotransformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 138 kV to 66 kV for transmission. Another application is in industry to adapt machinery built (for example) for 480 V supplies to operate on a 600 V supply. They are also often used for providing conversions between the two common domestic mains voltage bands in the world (100-130 and 200-250).

On long rural power distribution lines, special autotransformers with automatic tap-changing equipment are inserted as voltage regulators, so that customers at the far end of the line receive the same average voltage as those closer to the source. The variable ratio of the autotransformer compensates for the voltage drop along the line.

A special form of autotransformer called a zig zag is used to provide grounding (earthing) on three-phase systems that otherwise have no connection to ground (earth). A zig-zag transformer provides a path for current that is common to all three

phases (so-called zero sequence current).

Fig. Equivalent Circuit Diagram of an Auto-Transformer

Page | 21

Capacitor Bank

A capacitor bank is a grouping of several identical

capacitors interconnected in parallel or in series with one another.

These groups of capacitors are typically used to correct or counteract

undesirable characteristics, such as power factor lag or phase shifts

inherent in alternating current (AC) electrical power supplies.

Capacitor banks may also be used in direct current (DC) power

supplies to increase stored energy and improve the ripple current

capacity of the power supply.

Single capacitors are electrical or electronic components

which store electrical energy. Capacitors consist of two conductors

that are separated by an insulating material or dielectric. When an

electrical current is passed through the conductor pair, a static

electric field develops in the dielectric which represents the stored

energy. Unlike batteries, this stored energy is not maintained

indefinitely, as the dielectric allows for a certain amount of current

leakage which results in the gradual dissipation of the stored energy.

The energy storing characteristic of capacitors is known as

capacitance and is expressed or measured by the unit farads. This is

usually a known, fixed value for each individual capacitor which

allows for considerable flexibility in a wide range of uses such as

restricting DC current while allowing AC current to pass, output

smoothing in DC power supplies, and in the construction of resonant

circuits used in radio tuning. These characteristics also allow

capacitors to be used in a group or capacitor bank to absorb and

correct AC power supply faults.

The use of a capacitor bank to correct AC power supply

anomalies is typically found in heavy industrial environments that

feature working loads made up of electric motors and transformers.

This type of working load is problematic from a power supply

Page | 22

perspective as electric motors and transformers represent inductive

loads, which cause a phenomenon known as phase shift or power

factor lag in the power supply. The presence of this undesirable

phenomenon can cause serious losses in terms of overall system

efficiency with an associated increase in the cost of supplying the

power.

The use of a capacitor bank in the power supply system

effectively cancels out or counteracts these phase shift issues,

making the power supply far more efficient and cost effective. The

installation of a capacitor bank is also one of the cheapest methods of

correcting power lag problems and maintaining a power factor

capacitor bank is simple and cost effective.

One thing that should always be kept in mind when

working with any capacitor or capacitor bank is the fact that the

stored energy, if incorrectly discharged, can cause serious burns or

electric shocks. The incorrect handling or disposal of capacitors may

also lead to explosions, so care should always be exercised when

dealing with capacitors of any sort.

Fig. Capacitor Bank Installed In a Substation Page | 23

Isolators

In electrical engineering, a disconnector or isolator switch or disconnect switch is used to make sure that an electrical circuit can be completely de-energised for service or maintenance. Such switches are often found in electrical distribution and industrial applications where machinery must have its source of driving power removed for adjustment or repair.

High-voltage isolation switches are used in electrical substations to allow isolation of apparatus such as circuit breakers and transformers, and transmission lines, for maintenance.

Often the isolation switch is not intended for normal

control of the circuit and is used only for isolation; in such a case, it functions as a second, usually physically distant master switch (wired in series with the primary one) that can independently disable the circuit even if the master switch used in everyday operation is turned on. Isolator switches have provisions for a padlock so that inadvertent operation is not possible. In high voltage or complex systems, these padlocks may be part of a trapped-key interlock system to ensure proper sequence of operation.

In some designs the isolator switch has the additional ability to earth the isolated circuit thereby providing additional safety. Such an arrangement would apply to circuits which interconnect power distribution systems where both end of the circuit need to be isolated.

The major difference between an isolator and a circuit breaker is that an isolator is an off-load device intended to be opened only after current has been interrupted by some other control device. Safety regulations of the utility must prevent any attempt to open the disconnector while it supplies a circuit.

Page | 24

Fig. An Isolator Used In 33 kV Substations

F ig. A High Voltage Isolator

Page | 25

Circuit Breakers A circuit breaker is an automatically operated electrical

switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and interrupt current flow. Unlike a fuse, which operates once and then must 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.

Operation The circuit breaker must detect a fault condition; in low-

voltage circuit breakers this is usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism. The trip solenoid that releases the latch is usually energized by a separate battery, although some high-voltage circuit breakers are self-contained with current transformers, protection relays, and an internal control power source.

Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using something such as springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself.

Small circuit breakers may be manually operated, larger units have solenoids to trip the mechanism, and electric motors to restore energy to the springs.

The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting (opening) the circuit. Contacts are made of copper or copper alloys, silver alloys, and other highly conductive materials. Service life of the contacts is limited by the

Page | 26

erosion of contact material due to arcing while interrupting the current. Miniature and molded case circuit breakers are usually discarded when the contacts have worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts.

When a current is interrupted, an arc is generated. This arc must be contained, cooled, and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium the arc forms in.

Different techniques are used to extinguish the arc are :-

Lengthening / deflection of the arc Intensive cooling (in jet chambers) Division into partial arcs Zero point quenching (Contacts open at the zero current time crossing of the AC waveform, effectively breaking no load current at the time of opening. The zero crossing occurs at twice the line frequency i.e. 100 times per second for 50 Hz . Connecting capacitors in parallel with contacts in DC circuits.

Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit.

Types of circuit breakers

1. Sulphur hexafluoride (SF6) high-voltage circuit breakers

A sulphur hexafluoride circuit breaker uses contacts surrounded by sulphur hexafluoride gas to quench the arc. They are most often used for transmission-level voltages and may be incorporated into compact gas-insulated switchgear. In cold climates, supplemental heating or de-rating of the circuit breakers may be required due to liquefaction of the SF6 gas. Issues related to SF6

circuit breakers.

Page | 27

Issue Related To SF6

The following issues are associated with SF6 circuit breakers:-

(a)Toxic lower order gases

When an arc is formed in SF6 gas small quantities of lower order gases are formed. Some of these by-products are toxic and can cause irritation to eyes and respiratory systems.

(b)Oxygen displacement

SF6 is heavier than air, so care must be taken when entering low confined spaces due to the risk of oxygen displacement.

(c)Greenhouse gas

SF6 is the most potent greenhouse gas that the

Intergovernmental Panel on Climate Change has evaluated. It has a

global warming potential that is 23,900 times worse than CO2.

Alternatives to SF6 circuit breakers

Circuit breakers are usually classed on their insulating medium. The follow types of circuit breakers may be an alternative to SF6 types.

Air blast

Oil

Vacuum

CO2

Fig. High Voltage SF6 Circuit Breaker Page | 28

2. Carbon Dioxide (CO2) High-Voltage Circuit Breakers

In 2012 ABB presented a 72.5 kV high-voltage breaker that uses carbon dioxide as the medium to extinguish the arc. The carbon dioxide breaker works on the same principles as an SF6

breaker and can also be produced as a disconnecting circuit breaker. By switching from SF6 to CO2 it is possible to reduce the CO2

emissions by 10 tons during the product’s life cycle.

Fig. High Voltage CO2 Circuit Breaker (maker ABB)

Page | 29

Lightning Arresters Lightning arresters are protective devices for limiting

surge voltages due to lightning strikes or equipment faults or other events, to prevent damage to equipment and disruption of service. Also called surge arresters.

Lightning arresters are installed on many different pieces of equipment such as power poles and towers, power transformers, circuit breakers, bus structures, and steel superstructures in substations.

Lightning, is a form of visible discharge of electricity between rain clouds or between a rain cloud and the earth. The electric discharge is seen in the form of a brilliant arc, sometimes several kilometres long, stretching between the discharge points. How thunderclouds become charged is not fully understood, but most thunderclouds are negatively charged at the base and positively charged at the top. However formed, the negative charge at the base of the cloud induces a positive charge on the earth beneath it, which acts as the second plate of a huge capacitor.

When the electrical potential between two clouds or between a cloud and the earth reaches a sufficiently high value (about 10,000 V per cm or about 25,000 V per in), the air becomes ionized along a narrow path and a lightning flash results.

The conductor has a pointed edge on one side and the other side is connected to a long thick copper strip which runs down the building. The lower end of the strip is properly earthed. When lightning strikes it hits the rod and current flows down through the copper strip. These rods form a low-resistance path for the lightning discharge and prevent it from travelling through the structure itself.

The lightning arrestor protects the structure from damage by intercepting flashes of lightning and transmitting their current to the ground. Since lightning strikes tend to strike the highest object in the vicinity, the rod is placed at the apex of a tall structure. It is connected to the ground by low-resistance cables. In the case of a building, the soil is used as the ground, and on a ship, water is used. A lightning rod provides a cone of protection, which has a ground

Page | 30

radius approximately, equal to its height above the ground. Surges due to lightning are mostly injected into the power

system through long cross-country transmission lines. Substation apparatus is always well shielded against direct lightning strokes. The protection of transmission lines against direct strokes requires a shield to prevent lightning from striking the electrical conductors.

Terminal equipment at the substation is protected against by surge diverters, also called surge arrester or lightning arresters. A diverter is connected in parallel or shunt with the equipment to be protected at the substation between the line and ground. Ideally, it should • become conducting at voltage above diverter rating • become non conducting again when the line-to-neutral voltage becomes lower than the design value. In other words, it should not permit any power follow-on current;

• not conduct any current at normal or somewhat above normal power frequency voltages.

Earthing screen and ground wires can well protect the electrical system against direct lightning strokes but they fail to provide protection against travelling waves, which may reach the

terminal apparatus.

A lightning arrester or a surge diverter is a protective device, which conducts the high voltage surges on the power system to the ground.

Fig. Lightning Arresters Used In Substations

Page | 31

Description of a Substation

It is divided into two parts:-

1) Panel Section

(a) Control Panel Section

(b) Relay & Protection Panel Section

2) Yard

(a) 220 kV Section

(b) 132 kV Section

(c) 33 kV Section

3) Battery Room(Extra)

Fig. A 220kV Transmission Substation

Page | 32

1. Panel Section

It is a room which contains all types of panels:-

(a)Control Panel Section

(i) Control And Relay Panel

(ii) Feeder

(iii) Bus Coupler Control Panel

(iv) Distribution Bus

(v) Remote Tap Changer

(vi) Auto Transformer On Load Tap Changing Control Panel

(vii) Direct Current Distribution Box

(viii) Float And Boost Charger

(ix) Capacitor Bank Panel

(x) Transformer H.V. and L.V. Side Control Panel

(xi) Triple Feeder

(xii) L.T. Distribution Board

(xiii) 40MVA Transformer

(b) Relay And Protection Panel Section

(i) Relay Panel

(ii) Protection Panel

(iii) Rotational Load Shedding

(iv) Line Protection Panel

(v) Transformer Control Panel

(vi) Apex Metering Panel

(vii) Auto Transformer PROIN Page | 33

Some Full Forms Related To Substations S.No. Short Forms Full Forms

1. PLCC Power Line Carrier Communication

2. LA Lightning Arresters

3. CBT Capacitor Bank Transformer

4. CT Current Transformer

5. PT Potential Transformer

6. CVT Capacitive Voltage Transformer

7. LV Low Voltage

8. HV High Voltage

9. DCDB Direct Current Distribution Board

10. CTR Current Transfer Ratio

11. VTR Voltage Transfer Ratio

12. LSI Line Side Isolator

13. BSI Bus Side Isolator

14. CB Circuit Breaker

15. TI Tendom Isolator

16. BCT Base Current Transformer

17. MRI Meter Reading Instrument

18. OTI All Temp. Indicator

19. WTI Winding Temp. Indicator

20. kV Kilo Voltage

Page | 35

Components Used In Yard

(220kV Substation Mulugu Road, Warangal)

(1) Wave Trap (A Type Of Inductor)

(2) Two Transformer (160MVA)(220/132kV)

(3)One Auto Power Transformer (63MVA)(132/33kV)

(4)Two Auto Power Transformer (44MVA)(132/33kV)

(5)Two Auto Power Transformer (250kVA)(33/0.4kV)

(6)Two Capacitor Bank (5MVAR)

(7)Junction Box

(8)Insulator Disc (To Isolate Pillar And Power Line Wire)

(9)Jumpher (A Small Piece Of Power Line Wire)

(10)Panther Wire(Used In 33kV And 132kV Power Line)

(11)Zebra, Moose And Dear Wires(Used In 220kV Power Line)

(12)Transformer Cooling By Mulsifier System Page | 36

Conclusion

Now I have studied a lot about the electrical transmission

system. One must have never thought that so many things are

required for just switching on a television or a refrigerator or say an

electric trimmer. The three wing of electrical system viz.

Generation, transmission and distribution are connected to each

other and that too very perfectly. Here man and electricity work as

if they are a family. Lots of labour, capital and infrastructure is

involved in the system just to have a single phase,220V,50Hz

power supply at our houses.

At last I would say...

Energy Saved Is Energy Produced Page | 37