protection of substation

7/29/2019 Protection of Substation

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PROTECTION OF SUBSTATION


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IntroductionA Substation covers various equipment such as Circuit

Breakers, Isolators, Earth Switches, Lightning Arresters,Grounding System and etc.

Protection scheme is designed to limit the effects of

disturbances in the power system which is allowed to persist,

may damage plant and interrupt the supply of electric energy.

The greatest threat to the security of a power supply

station is short circuit which imposes sudden and violent

change in he power system. Rapid isolation of the fault bynearest switchgear will minimize the damage. It covers

various types of protection used in switchyard such as relays,

circuit breakers and etc.


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ELECTRICAL SUBSTATION

An electrical substation is a subsidiary station of electricity generation,transmission and distribution system where voltage is transformed from highto low or low to high using transformers.

Substations generally contains transformers and have switching,protection and control equipment. In large substations, circuit breakers areused to interrupt any short-circuits or overloaded currents that may occur onthe network. Smaller distribution stations may use recloser circuit breakers orfuses for protection of branch circuits.


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DESIGN

The selection of the location a substation must have following

factors:1. Sufficient land area is required for installation of equipment

with necessary clearances for electrical safety and access tomaintain large apparatus such as transformers.

2. The site must have room for expansion due to load growth orplanned transmission additions. Environmental effects must beconsidered, such as drainage, noise and road traffic effects.

3. Grounding and ground potential rise must be calculated toprotect passers by during a shortcircuit in transmission

system.4. The substation site must be reasonably central to the

distribution area to be served.


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NEED FOR PROTECTION

A protective scheme isolates the faulty section of the system from the healthysections. An electric power system consists of generators, transformers,transmission lines and distribution lines etc. Short circuits and other abnormalconditions often occur on the power system. The heavy current associatedwith short circuits is likely to cause damage to the equipment if suitable

protection at each section of the power system is not provided.

A protective scheme includes major circuit breakers and protective relays toisolate the faulty section of a healthy system from healthy sections. A circuit

breaker can disconnect the faulty element of the section when it is called uponto do so by the protective relay. The function of a protective relay is to detectand locate a fault and issue a command to the circuit breaker to disconnect thefaulty element.

Thus if a fault occurs in an element of a power system, an automatic protectivedevice is needed to isolate the faulty element as quickly as possible to keep thesystem in healthy operation.


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NATURE AND CAUSES OF FAULTS

Faults are mainly caused by:

Insulation failure.

Conducting path failure.

Overvoltages due to lightening or switching surges.

Puncturing or breaking of insulators. Accumulation of foreign particles on the surface of string and pin

insulators.

Failure of conducting path due to broken conductors.

Faults on overhead lines due to lightening strokes, ice and snow loading,storms and etc.

Tripping of circuit breakers due to errors in switching operation, testing ormaintenance work, defects in protective devices.

Faults due to poor quality of system components or because of faultysystem design.

Failure of solid insulation due to aging, heat, moisture, overvoltage,accidental contact with earth or earth screens, flash-over voltages and etc.,


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COMPONENTS OF PROTECTIONThe main components used in the protection scheme in a substation

to isolate the faulty section of the system from the healthysection are

FUSES

EARTHING

INSULATION

ISOLATORS

RELAYS

CIRCUIT BREAKERS

LIGHTNING ARRESTERS


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FUSE

A fuse is a protective device used for protecting cables and

electrical equipment against overloads and short circuits. A fusewas patented by Thomas Edison in 1890.

It is a short piece of metal, inserted in the circuit, which meltswhen excessive current flows through it and thus breaks thecircuit. In electrical and electronics engineering, a fuse (short for

fusible link) is a type of over-current protection device.Its typical component is a metal wire or strip (element) that meltswhen too much current flows, which interrupts (disconnects) thecircuit in which it is connected.

Circuit or device failure is often a reason for excessive current. A

fuse blows (interrupts excessive current) so that further damageis prevented. A fuse typically is not intended to protect from theinitial cause of over-current.


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TYPES OF FUSES

In general fuses are classified into two types:

A. HIGH VOLTAGE FUSE

B. LOW VOLTAGE FUSE

The low voltage fuses are subdivided into two types:

Semi-Enclosed Rewireable Fuse

High-Rupturing Capacity Cartridge Fuse

The high voltage fuses are categorised into three types:

Cartidge Type

Liquid type

Metal Type


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EARTHING

In electrical engineering, ground or earth may be the reference point in anelectrical circuit from which other voltages are measured, or a common return

path for electric current, or a direct physical connection to the Earth. The term

ground and grounding are used in electrical engineering to represent electrical

equipment that is securely bonded to the ground for safety reasons.

In electricity supply systems, an earthing (grounding) system defines the electrical

potential of the conductors relative to that of the Earth's conductive surface.

The choice of earthing system has implications for the safety and

electromagnetic compatibility of the power supply. A functional earth

connection serves a purpose other than providing protection against electrical

shock. In contrast to a protective earth connection, a functional earth

connection may carry a current during the normal operation of a device.

Functional earth connections may be required by devices such as surge

suppression and electromagnetic-compatibility filters, some types of antennas

and various measurement instruments. Generally the protective earth is also

used as a functional earth, though this requires care in some situations.


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INSULATOR

An insulator, also called a dielectric, is a material that resists the flow of electriccurrent.

An insulating material has atoms with tightly bonded valence electrons. Thesematerials are used in parts of electrical equipment, also called insulators orinsulation, intended to support or separate electrical conductors without

passing current through themselves.

Insulators used for high-voltage power transmission are made from glass,porcelain, or composite polymer materials.

Porcelain insulators are made from clay, quartz or alumina and feldspar, and arecovered with a smooth glaze to shed dirt. Insulators made from porcelain richin alumina are used where high mechanical strength is a criterion. Porcelainhas a dielectric strength of about 410 kV/mm.

Glass has a higher dielectric strength, but it attracts condensation and the thickirregular shapes needed for insulators are difficult to cast without internalstrains. Some insulator manufacturers stopped making glass insulators in thelate 1960s, switching to ceramic materials.


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TYPES OF INSULATORS

PIN TYPE INSULATORS

SUSPENSION TYPE INSULATORS

STRAIN INSULATORSSHACKLE INSULATORS


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ISOLATOR

The isolators are used to connect and disconnect the high voltagepower systems under no load conditions. Circuit-Isolator disconnect provides three-pole, group-operated, visible-air-gap isolation in distribution substations. The Circuit-

Isolator can be used to interrupt low-level charging currentsassociated with substation bus work and circuit-breaker bushings,as well as other low-voltage currents commonly present insubstations.

Circuit-Isolator is available in ratings from 69 kV through 138 kV,

in 1200 A, 1600 A, and 2000 A versions. Side-break, vertical-break, center-break, and double-break styles can be furnished, tosuit almost any substation layout.


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RELAYS

A Relay is an electrical switch that opens and closes under thecontrol of another electrical circuit. In the original form, theswitch is operated by an electromagnet to open or close one ormany sets of contacts. It was invented by Joseph Henry in 1835.

Because a relay is able to control an output circuit of higherpower than the input circuit, it can be considered to be, in a broadsense, a form of an electrical amplifier.

A relay is a automatic device which detects an abnormal conditionin an abnormal condition in an electrical circuit and causes a

circuit breaker to isolate the faulty element of the system. Insome cases it may give an alarm or visible indication to alertoperator.


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CLASSIFICATION OF RELAYS BASED ON CONSTRUCTION

Relays are broadly classified into following categories depending on the

technology of their construction and operation: Electromagnetic Relay

Static Relay

Microprocessor Relay

CLASSIFICATION OF RELAYS BASED ON THEIR FUNCTION

Protective relays can be classified into following categories, depending on theduty they are required to perform.

OverCurrent Relays

Undervoltage Relays Impedance Relays

Under Frequency Relays

Directional Relays


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CIRCUIT BREAKER

A circuit breaker is an automatically-operated electrical switchdesigned to protect an electrical circuit from damage caused byoverload or short circuit. An early form of circuit breaker wasdescribed by Edison in an 1879 patent application, although hiscommercial power distribution system used fuses. Its purpose

was to protect lighting circuit wiring from accidental short-circuits and overloads.

A protective relay detects abnormal conditions and sends a trippingsignal to the circuit breaker. After receiving the trip commandfrom the relay, the circuit breaker isolates the faulty part of the

power system.A circuit breaker has two contacts- a fixed contact and a moving

contact. Under normal conditions these contacts remain in closedposition. When the circuit breaker is required to isolate the faultypart, the moving contact moves to interrupt the circuit.


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TYPES OF CIRCUIT BREAKERS

The circuit breakers are classified as:

OIL CIRCUIT BREAKER

AIR-BLAST CIRCUIT BREAKER

SULPHUR HEXA FLUROIDE CIRCUIT

BREAKER

VACUUM CIRCUIT BREAKER


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LIGHTNING ARRESTER

A lightning arrester is a protective device which conducts the highvoltage surges on the power system to the ground. LightningArresters are protective devices for limiting surge voltages due tolightning strikes or equipment faults or other events, to preventdamage to equipment and disruption of service. Also called surge

arresters.A lightning arrester is a protective device which conducts the high

voltage surges on the power system to the ground. It consists of aspark gap in series with a linear resistor. One end of the diverteris connected to the terminal of the equipment to be protected and

the other end is effectively grounded.The length of the gap is so set that the normal line voltage is not

enough to cause an arc across the gap but a dangerously highvoltage will break down the air insulation from an arc. The

property of the non-linear resistance is that its resistance

decreases as the voltage or current increases or vice-versa.


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CONCLUSION

A protective scheme includes majorly circuit breakers andprotective relays to isolate the faulty section of a healthy systemfrom healthy sections. A circuit breaker can disconnect the faultyelement of the section when it is called upon to do so by the

protective relay. The function of a protective relay is to detectand locate a fault and issue a command to the circuit breaker todisconnect the faulty element. Protective scheme isolates thefaulty section of the system from the healthy sections.

Thus a healthy and protective substation holds circuit breakers,

protective relays, lightning arresters, isolators and insulation forthe protection of every component of its family.


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QUERIES???


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THANK YOU

protection of substation

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