Contrl System

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POWER SYSTEM PROTECTION [CONTROL SYSTEM]

Protection systems usually comprise five components: Currentandvoltage transformersto step down the high

voltages and currents of the electrical power system to convenient

levels for the relays to deal.

Protective relaysto sense the fault and initiate a trip, or

disconnection, order;

Circuit breakersto open/close the system based on relay and

auto recloser commands.

Batteriesto provide power in case of power disconnection in the

system.

Communication channels to allow analysis of current and

voltage at remote terminals of a line and to allow remote tripping

of equipment.

PROTECTIVE RELAYS:

A relay is an electrically operated switch.

Definition:-

In electrical engineering, 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 breakerswhen a fault is detected.

Application:-

1. over-current

2. over-voltage


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3. reverse power flow

4. Over- and under- frequency.

RELAY CONSTRUCTION:-

A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core,

an iron yoke which provides a low reluctance path for magnetic flux, a movable

iron armature, and one or more sets of contacts (there are two in the relay pictured). The

armature is hinged to the yoke and mechanically linked to one or more sets of moving

contacts. It is held in place by a spring so that when the relay is de-energized there is an

air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the

relay pictured is closed, and the other set is open. Other relays may have more or fewer

sets of contacts depending on their function. The relay in the picture also has a wire

connecting the armature to the yoke. This ensures continuity of the circuit between the

moving contacts on the armature, and the circuit track on the printed circuit board (PCB)

via the yoke, which is soldered to the PCB.

When an electric current is passed through the coil it generates a magnetic field that

activates the armature and the consequent movement of the movable contact either

makes or breaks (depending upon construction) a connection with a fixed contact. If the

set of contacts was closed when the relay was de-energized, then the movement opens

the contacts and breaks the connection, and vice versa if the contacts were open. 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 force is provided

by 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 reduces noise; in a

high voltage or current application it reduces arcing.When the coil is energized with direct current, a diode is often placed across the coil to

dissipate the energy from the collapsing magnetic field at deactivation, which would

otherwise generate a voltage spike dangerous to semiconductor circuit components.

Some automotive relays include a diode inside the relay case. Alternatively, a contact

protection network consisting of a capacitor and resistor in series (snubber circuit) may

absorb the surge. If the coil is designed to be energized with alternating current (AC), a

small copper "shading ring" can be crimped to the end of the solenoid, creating a small

out-of-phase current which increases the minimum pull on the armature during the AC

cycle.

A solid-state relay uses a thyristor or other solid-state switching device, activated by thecontrol signal, to switch the controlled load, instead of a solenoid.

An optocoupler (a light-emitting diode (LED) coupled with a photo transistor) can be

used to isolate control and controlled circuits.


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By Electromechanical protective Operation:-

Electromechanical relays are used in power system, specifically in its

protection. These are switching devices used to control high power devices.

They have earned a well-deserved reputation for accuracy, dependability, and

reliability.

Their Types Are:-

1. Electro-Magnetic attraction relay.

2. Electro -Magnetic induction relay.

ELECTRO-MAGNETIC ATTRACTION RELAY:-

Magnetic-attraction relays have either a solenoid that pulls in a plunger, or one

or more electromagnets that attract a hinged armature. When the magnetic

force is sufficient to overcome the restraining spring, the movable elementbegins to travel, and continues until the contact close or the magnetic force is

removed. The pickup point is the current or voltage at which the plunger or

armature begins to move and, in a switchgear relay, the pickup value can be

set very precisely.
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Attraction-type relays can operate with either AC or DC on the coils;

therefore, relays using this principle are affected by the DC component

of an asymmetrical fault and must be set to allow for this.

Electro Magnetic Induction Relay:-

A disk is mounted on bearings and is put in motion by the induced field from

an electromagnet. The speed of rotation is controlled by a fixed magnetic field

from a permanent magnet.

The moving element, or rotor, is usually a metal disk, although it sometimes

may be a metal cylinder or cup. The stator is one or more electromagnets with

current or potential coils that induce currents in the disk, causing it to rotate.

The disk motion is restrained by a spring until the rotational forces are

sufficient to turn the disk and bring its moving contact against the stationary

contact, thus closing the circuit the relay is controlling. The greater the fault

being sensed, the greater the current in the coils, and the faster the disk

rotates.

Advantages of using electromechanical relays:-

Retrofitting- Situations where aging electromechanical equipment needs

replacing, many users prefer to work with the existing technology rather than

going to the trouble and expense of redesigning their drawings or facilities toincorporate digital technology. Retrofits in fact account for the largest portion

of the electromechanical relay business.

Harsh Environments- Under conditions of extreme heat for example,electromechanical relays are more reliable, easy to repair, simple to
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understand, simple to test, and simple to maintain. By way of example, any

applications where the ambient temperature is over 80C pretty well rule out

the use of digital relays (e.g. oil fields in hotter climates).

Electromechanical relays perform well in areas prone to electromagnetic

interference (EMI) and radio frequency interference (RFI), they are a popularchoice in harsh EMI and RFI environments. Surges, transients and noise do not

affect an electromechanical protective relay.

Self-powered- Digital relays require the presence of a secure power supply.When dealing with central power facilities, where a large battery is always

present, this is essentially a non-issue. However, in areas where it is

inconvenient to provide a battery, such as a smaller industrial environment or

mining operation, thats a supreme justification for going with electro-magnetic

relays.

TYPES OF RELAYS:-

OVER CURRENT RELAY

An "over current relay" is a type of protective relay which operates when the load currentexceeds a preset value. The ANSI device number is 50 for an instantaneous overcurrent (IOC), 51 for a time over current (TOC). In a typical application the over current

relay is connected to a current transformer and calibrated to operate at or above aspecific current level. When the relay operates, one or more contacts will operate andenergize to trip (open) a circuit breaker.

INDUCTION DISC OVERCORRECT RELAY
http://www.google.co.in/imgres?imgurl=http://static.tecquipment.com/Products/PS251_OVERCURRENT-AND-EARTH-FAULT-RELAY.jpg&imgrefurl=http://www.tecquipment.com/prod/PS251.aspx&h=570&w=600&sz=33&tbnid=-Rk92v6z2GjjGM:&tbnh=90&tbnw=95&prev=/search%3Fq%3Dearth%2Bfault%2Brelay%26tbm%3Disch%26tbo%3Du&zoom=1&q=earth+fault+relay&docid=fOZOgCRbeR1uIM&hl=en&sa=X&ei=O-jqTqT2HYLPrQfMkfyPCQ&ved=0CFsQ9QEwBA&dur=1143


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These robust and reliable electromagnetic relays use the induction principle discoveredby Galileo Ferraris in the late 19th century. The magnetic system in induction disc overcurrent relays is designed to detect over currents in a power system and operate with apre-determined time delay when certain over current limits have been reached. In orderto operate, the magnetic system in the relays produces torque that acts on a metal discto make contact,

According to the following basic current/torque equation:

Where;

K Is a constant

1 and2 are the two fluxes

is the phase angle between the fluxes

The relay's primary winding is supplied from the power systems current transformer via a

plug bridge, which is called the plug setting multiplier (psm). Usually seven equallyspaced tappings or operating bands determine the relays sensitivity. The primarywinding is located on the upper electromagnet. The secondary winding has connectionson the upper electromagnet that are energized from the primary winding and connectedto the lower electromagnet. Once the upper and lower electromagnets are energizedthey produce eddy currents that are induced onto the metal disc and flow through theflux paths. This relationship of eddy currents and fluxes creates torque proportional tothe input current of the primary winding, due to the two flux paths been out of phase by90.

In an over current condition, a value of current will be reached that overcomes thecontrol spring pressure on the spindle and the braking magnet, causing the metal disc to

rotate towards the fixed contact. This initial movement of the disc is also held off to acritical positive value of current by small slots that are often cut into the side of the disc.The time taken for rotation to make the contacts is not only dependent on current butalso the spindle backstop position, known as the time multiplier (tm). The time multiplieris divided into 10 linear divisions of the full rotation time.

Providing the relay is free from dirt, the metal disc and the spindle with its contact willreach the fixed contact, thus sending a signal to trip and isolate the circuit, within itsdesigned time and current specifications. Drop off current of the relay is much lower thanits operating value, and once reached the relay will be reset in a reverse motion by thepressure of the control spring governed by the braking magnet.

DISTANCE RELAY

The most common form of protection on high voltage transmission systems is distancerelay protection. Power lines have set impedance per kilometer and using this value andcomparing voltage and current the distance to a fault can be determined. The ANSIstandard device number for a distance relay is 21.


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CURRENT DIFFERENTIAL PROTECTION

Another common form of protection for apparatus such as transformers, busses andpower lines is current differential. This type of protection works on the basic theory ofKirchhoff's current law which states that the sum of the currents entering and exiting anode will equal zero. It is important to note the direction of the currents as well as themagnitude, as they are vectors. It requires a set of current transformers (smallertransformers that transform currents down to a level which can be measured) at eachend of the power line, or each side of the transformer. The current protection relay thencompares the currents and calculates the difference between the two. As an example, apower line from one substation to another will have a current differential relay at bothsubstations which communicate with each other. In a healthy condition, the relay atsubstation A may read 500 amps (power exporting) and substation B will read 500 amps

(power importing). If a path to earth or ground develops there will be a surge of current.As supply grids are generally well interconnected the fault in the previous example willbe fed from both ends of the power line. The relay at substation A will see a massiveincrease in current and will continue to export. Substation B will also see a massiveincrease in current; however it will now start to export as well. In turn the protection relaywill see the currents traveling in opposite directions (180 degrees phase shift) andinstead of canceling each other out to give a summation of zero it will see a large valueof current. The relays will trip the associated circuit breakers. This type of protection iscalled unit protection, as it only protects what is between the current transformers. It isimportant to note that generally the higher the currents in the lines the larger thedifferential current required for the relay to see it as a fault. This is basically done due tosmall mismatches in current transformers. Small errors will increase as current increases

to the point where the error could cause a false trip if the current differential relay onlyhad an upper limit instead of the rising differential characteristic. It is also important tonote that CTs have a point where the core saturates and the current in the CT is nolonger proportional to the current in the line. A CT can become inaccurate or evensaturate because of a fault outside of its protected zone (through fault) where the CTssee a large magnitude but still in the same direction.

A very common example of a differential current relay is the ground fault currentinterrupter (GFCI), or Residual-current device.


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Earth Fault Relay:-

In the case of a grounded neutral system the vector sum of the current flowing in thethree different phases will flow through the neutral conductor. Under healthy conditionthe resultant current flowing through the secondary grounded neutral current transformerwill be zero. In such a three phase system if one of the three phases gets grounded or

earthed then unbalancing of current occurs, the vector sum of all the three phases willhave certain value. If this value exceeds the set value indicated on the relay, then therelay actuates and trips the main circuit. Further the protection can be achieved usingthree individual CTs for the three phases, the secondary terminals of the CTs have tobe connected in parallel to the sensing circuit of the Earth Fault Relay. In case ofunbalancing of current or earth fault the vector sum of these currents in the three phaseswill have certain value. If the fault level exceeds the set value on the relay the relay tripsthe main breaker. Pressing the RESET button after the clearance of the fault can resetthe relay. The red LED provided on the front panel will glow in case of a fault. Pressingthe TEST button the healthiness of the unit gets checked. The green LED indicates thehealthiness of the unit.

STATIC RELAYS

Application of electronic amplifiers to protective relays was described as early as 1928,using vacuum tube amplifiers. Devices using electron tubes were studied but neverapplied as commercial products, because of the limitations of vacuum tube amplifiers. Arelatively large standby current is required to maintain the tube filament temperature;inconvenient high voltages are required for the circuits, and vacuum tube amplifiers haddifficulty with incorrect operation due to noise disturbances.

Static relays with no or few moving parts became practical with the introduction ofthe transistor. Static relays offer the advantage of higher sensitivity than purelyelectromechanical relays, because power to operate output contacts is derived from aseparate supply, not from the signal circuits. Static relays eliminated or reduced contactbounce, and could provide fast operation, long life and low maintenance.
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DIGITAL PROTECTIVE RELAYS

The functions of electromechanical protection systems are now being replaced bymicroprocessor-based digital protective relays, sometimes called "numeric relays".

A Digital Relay with Micro Processor

CIRCUIT BREAKERS:-

Definition:-

A circuit breakeris an automatically operated electrical switch designed to protectan electrical circuit from damage caused by overload or short circuit. Its basic function isto detect a fault condition and, by interrupting continuity, to immediately discontinueelectrical flow.
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A 2 pole miniature circuit breaker

Four 1 pole circuit breakers

ORIGINS

Inspired by the works of American scientist Joseph Henry and English scientist MichaelFaraday, the circuit breaker was invented in 1836 by an American, Charles GraftonPage.

OPERATION

All circuit breakers have common features in their operation, although details varysubstantially depending on the voltage class, current rating and type of the circuitbreaker.

The circuit breaker must detect a fault condition; in low-voltage circuit breakers this isusually done within the breaker enclosure. Circuit breakers for large currents or highvoltages are usually arranged with pilot devices to sense a fault current and to operatethe trip opening mechanism. The trip solenoid that releases the latch is usuallyenergized by a separate battery, although some high-voltage circuit breakers are self-
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contained with current transformers, protection relays, and an internal control powersource.

Once a fault is detected, contacts within the circuit breaker must open to interrupt thecircuit; some mechanically-stored energy (using something such as springs orcompressed 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 circuitbreakers 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, andmust 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 conductivematerials. Service life of the contacts is limited by the erosion of contact material due toarcing while interrupting the current. Miniature and molded case circuit breakers areusually 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 againwithstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulatinggas, or oil as the medium in which the arc forms. Different techniques are used toextinguish the arc including:

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 ACwaveform, effectively breaking no load current at the time of opening. The zerocrossing occurs at twice the line frequency i.e. 100 times per second for 50 Hz and120 times per second for 60 Hz AC)

Connecting capacitors in parallel with contacts in DC circuits

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

ARC INTERRUPTION

Miniature low-voltage circuit breakers use air alone to extinguish the arc. Largerratings will have metal plates or non-metallic arc chutes to divide and cool thearc. Magnetic blowout coils or permanent magnets deflect the arc into the arc chute.

In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet

of oil through the arc.

Gas (usually sulfur hexafluoride) circuit breakers sometimes stretch the arc using amagnetic field, and then rely upon the strength of the sulfur hexafluoride (SF6) to quenchthe stretched arc.

Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than thecontact material), so the arc quenches when it is stretched a very small amount (


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3 mm). Vacuum circuit breakers are frequently used in modern medium-voltageswitchgear to 35,000 volts.

Air circuit breakers may use compressed air to blow out the arc, or alternatively, thecontacts are rapidly swung into a small sealed chamber, the escaping of the displacedair thus blowing out the arc.

Circuit breakers are usually able to terminate all current very quickly: typically the arc isextinguished between 30 ms and 150 ms after the mechanism has been tripped,depending upon age and construction of the device.

An air circuit breaker for low voltage (less than 1000 volts) power distributionswitchgear

SHORT-CIRCUIT CURRENT

Circuit breakers are rated both by the normal current that they are expected to carry, andthe maximum short-circuit current that they can safely interrupt.

Under short-circuit conditions, a current many times greater than normal can exist. Whenelectrical contacts open to interrupt a large current, there is a tendency for an arc to formbetween the opened contacts, which would allow the current to continue. This conditioncan create conductive ionized gases and molten or vaporized metal which can causefurther continuation of the arc, or creation of additional short circuits, potentially resultingin the explosion of the circuit breaker and the equipment that it is installed in. Therefore,circuit breakers must incorporate various features to divide and extinguish the arc.
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In air-insulated and miniature breakers an arc chutes structure consisting (often) of metalplates or ceramic ridges cools the arc, and magnetic blowout coils deflect the arc into thearc chute. Larger circuit breakers such as those used in electrical power distribution mayuse vacuum, an inert gas such as sulphur hexafluoride or have contacts immersedin oil to suppress the arc.

The maximum short-circuit current that a breaker can interrupt is determined by testing.Application of a breaker in a circuit with a prospective short-circuit current higher thanthe breaker's interrupting capacity rating may result in failure of the breaker to safelyinterrupt a fault. In a worst-case scenario the breaker may successfully interrupt thefault, only to explode when reset.

Miniature circuit breakers used to protect control circuits or small appliances may nothave sufficient interrupting capacity to use at a panel board; these circuit breakers arecalled "supplemental circuit protectors" to distinguish them from distribution-type circuitbreakers.

TYPES OF CIRCUIT BREAKERS

LOW VOLTAGE CIRCUIT BREAKERS

Low voltage (less than 1000 VAC) types are common in domestic, commercial and

industrial application, and include:

MCB (Miniature Circuit Breaker)rated current not more than 100 A. Trip

characteristics normally not adjustable. Thermal or thermal-magnetic operation.

Breakers illustrated above are in this category.

MCCB (Molded Case Circuit Breaker)rated current up to 2500 A. Thermal or

thermal-magnetic operation. Trip current may be adjustable in larger ratings.

Low voltage power circuit breakers can be mounted in multi-tiers in low-voltage

switchboards or switchgear cabinets.

The characteristics of Low Voltage circuit breakers are given by international standards

such as IEC 947. These circuit breakers are often installed in draw-out enclosures that

allow removal and interchange without dismantling the switchgear.

Large low-voltage molded case and power circuit breakers may have electrical motor

operators, allowing them to be tripped (opened) and closed under remote control. Thesemay form part of an automatic transfer switch system for standby power.

Low-voltage circuit breakers are also made for direct-current (DC) applications, for

example DC supplied for subway lines. Special breakers are required for direct current

because the arc does not have a natural tendency to go out on each half cycle as for

alternating current. A direct current circuit breaker will have blow-out coils which

generate a magnetic field that rapidly stretches the arc when interrupting direct current.


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Small circuit breakers are either installed directly in equipment, or are arranged in

a breaker panel.

The 10 ampere DIN rail-mounted thermal-magnetic miniature circuit breaker is the most

common style in modern domestic consumer units and commercial electrical distributionboards throughout Europe. The design includes the following components:

1. Actuator lever - used to manually trip and reset the circuit breaker. Also

indicates the status of the circuit breaker (On or Off/tripped). Most breakers are

designed so they can still trip even if the lever is held or locked in the "on"

position. This is sometimes referred to as "free trip" or "positive trip" operation.

2. Actuator mechanism - forces the contacts together or apart.

3. Contacts - Allow current when touching and break the current when

moved apart.

4. Terminals

5. Bimetallic strip.

6. Calibration screw - allows the manufacturer to precisely adjust the trip

current of the device after assembly.

7. Solenoid

8. Arc divider/extinguisher

This low voltage power circuit breaker can be withdrawn from its housing for

servicing. Trip characteristics are configurable via DIP switches on the front panel.

MAGNETIC CIRCUIT BREAKERS

Magnetic circuit breakers use a solenoid (electromagnet) thats pulling force increases

with the current. Certain designs utilize electromagnetic forces in addition to those of the

solenoid. The circuit breaker contacts are held closed by a latch. As the current in the
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solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases

the latch which then allows the contacts to open by spring action Some types of

magnetic breakers incorporate a hydraulic time delay feature using a viscous fluid. The

core is restrained by a spring until the current exceeds the breaker rating. During an

overload, the speed of the solenoid motion is restricted by the fluid. The delay permits

brief current surges beyond normal running current for motor starting, energizingequipment, etc. Short circuit currents provide sufficient solenoid force to release the

latch regardless of core position thus bypassing the delay feature. Ambient temperature

affects the time delay but does not affect the current rating of a magnetic breaker

THERMAL MAGNETIC CIRCUIT BREAKERS

Thermal magnetic circuit breakers, which are the type found in most distribution boards,

incorporate both techniques with the electromagnet responding instantaneously to large

surges in current (short circuits) and the bimetallic strip responding to less extreme but

longer-term over-current conditions. The thermal portion of the circuit breaker providesan "inverse time" response feature which provides faster or slower response for larger or

smaller over currents respectively.

COMMON TRIP BREAKERS

When supplying a branch circuit with more than one live conductor, each live conductor

must be protected by a breaker pole. To ensure that all live conductors are interruptedwhen any pole trips, a "common trip" breaker must be used. These may either contain

two or three tripping mechanisms within one case, or for small breakers, may externally

tie the poles together via their operating handles. Two pole common trip breakers are

common on 120/240 volt systems where 240 volt loads (including major appliances or

further distribution boards) span the two live wires. Three-pole common trip breakers are

typically used to supply three-phase electric power to large motors or further distribution

boards.

Two and four pole breakers are used when there is a need to disconnect the neutral

wire, to be sure that no current can flow back through the neutral wire from other loads

connected to the same network when people need to touch the wires for maintenance.Separate circuit breakers must never be used for disconnecting live and neutral,

because if the neutral gets disconnected while the live conductor stays connected, a

dangerous condition arises: the circuit will appear de-energized (appliances will not

work), but wires will stay live and RCDs will not trip if someone touches the live wire

(because RCDs need power to trip). This is why only common trip breakers must be

used when switching of the neutral wire is needed


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MEDIUM-VOLTAGE CIRCUIT BREAKERS

Medium-voltage circuit breakers rated between 1 and 72 kV may be assembled into

metal-enclosed switchgear line ups for indoor use, or may be individual components

installed outdoors in a substation. Air-break circuit breakers replaced oil-filled units for

indoor applications, but are now themselves being replaced by vacuum circuit breakers

(up to about 35 kV). Like the high voltage circuit breakers described below, these are

also operated by current sensing protective relays operated through current

transformers. The characteristics of MV breakers are given by international standards

such as IEC 62271. Medium-voltage circuit breakers nearly always use separate current

sensors and protective relays, instead of relying on built-in thermal or magnetic over

current sensors.

Medium-voltage circuit breakers can be classified by the medium used to extinguish the

arc:

Vacuum circuit breakerswith rated current up to 3000 A, these breakers

interrupt the current by creating and extinguishing the arc in a vacuum container.These are generally 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.

Air circuit breakersrated current up to 10,000 A. Trip characteristics are often

fully adjustable including configurable trip thresholds and delays. Usually

electronically controlled, though some models are microprocessor controlled via an

integral electronic trip unit. Often used for main power distribution in large industrial

plant, where the breakers are arranged in draw-out enclosures for ease of

maintenance.

SF6 circuit breakers extinguish the arc in a chamber filled with sulfur hexafluoride

gas.

Medium-voltage circuit breakers may be connected into the circuit by bolted connections

to bus bars or wires, especially in outdoor switchyards. Medium-voltage circuit breakers

in switchgear line-ups are often built with draw-out construction, allowing the breaker to

be removed without disturbing the power circuit connections, using a motor-operated or

hand-cranked mechanism to separate the breaker from its enclosure.

HIGH-VOLTAGE CIRCUIT BREAKERS

Electrical power transmission networks are protected and controlled by high-voltage

breakers. The definition ofhigh voltage varies but in power transmission work is usually

thought to be 72.5 kV or higher, according to a recent definition by the International

Electrotechnical Commission (IEC). High-voltage breakers are nearly


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always solenoid-operated, with current sensing protective relays operated

through current transformers. In substations the protective relay scheme can be

complex, protecting equipment and buses from various types of overload or

ground/earth fault.

High-voltage breakers are broadly classified by the medium used to extinguish the arc.

Bulk oil

Minimum oil

Air blast

Vacuum

SF6

Some of the manufacturers are ABB, GE (General Electric), Tavrida Electric, Alstom,

Mitsubishi Electric, Pennsylvania Breaker, Siemens, Toshiba, Konar HVS, BHEL, CGL,

Square D (Schneider Electric).

Due to environmental and cost concerns over insulating oil spills, most new breakers

use SF6 gas to quench the arc.

Circuit breakers can be classified as live tank, where the enclosure that contains the

breaking mechanism is at line potential, ordead tankwith the enclosure at earth

potential. High-voltage AC circuit breakers are routinely available with ratings up to

765 kV. 1200kV breakers were launched by Siemens in November 2011.

High-voltage circuit breakers used on transmission systems may be arranged to allow a

single pole of a three-phase line to trip, instead of tripping all three poles; for some

classes of faults this improves the system stability and availability.

115 kV bulk oil circuit breaker

SULFUR HEXAFLUORIDE (SF6) HIGH-VOLTAGE CIRCUIT-BREAKERS
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A sulfur hexafluoride circuit breaker uses contacts surrounded by sulfur 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 theSF6 gas.

400 kV SF6 live tank circuit breakers

Batteries:-

They are used to provide dc supply for relays and circuit breakers ,these are

done by the float and boost chargers during power cuts.

FLOAT CUM BOOST CHARGERS:

Normally the DC Power is supplied to the load by the Float Charger. It also

supplies trickle current to the battery to keep it healthy. If the charging

current under Float Mode exceeds a set level. Boost charger is switched ON.

It supplies Quick charging current to the battery. On battery reaching the set

value the Boost Charger is switched OFF.
http://en.wikipedia.org/wiki/File:400kv.jpg


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Power House FCBC is designed to supply continuous power to the DC load

and simultaneously charge the batteries connected. Input supply form 415V.

AC 3 Phases or 220V. AC 1 Ph. is converted to regulate DC. The charger has

two independent systems.

Application:

Float / Boost chargers are must in Power Substations. Generating Stations,

Telephone Exchanges etc. for control / monitoring systems, tripping circuits

and supplying DC Power source.

Trickle current- means charging a battery at a similar rate as its Self-discharging

rate, thus maintaining a full capacity battery.

E.g.: nickel-cadmium batteries.

Batteries arranged for charging during boost mode.


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Float cum Boost charger.

COMMUNICATION CHANNELS:

PLCC [POWER LINE CARRIER COMMUNICATION]:-

PLCC, Power Line Carrier Communication, is an approach to utilize the existing

power lines for the transmission of information. In todays world every house and

building has properly installed electricity lines. By using the existing AC power lines as a

medium to transfer the information, it becomes easy to connect the houses with a highspeed network access point without installing new wirings.


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This technology has been in wide use since 1950 and was mainly used by the grid

stations to transmit information at high speed. Now a days this technology is finding wide

use in building/home automation as it avoids the need of extra wiring. The data collected

from different sensors is transmitted on these power lines thereby also reducing the

maintenance cost of the additional wiring. In some countries this technology is also used

to provide Internet connection.

400kv Electric Power Lines


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PLCC Manufactured by ABB

OPERATING PRINCIPLE

The communication device used for the communication over the power lines is a

MODEM, commonly known as Power Line MODEM (PLM). It works as both transmitter

and receiver, i.e., it transmits and receives data over the power lines. A power line

modem not only modulates the data to transmit it over the power lines and but also

demodulates the data it receives from the power lines. By using modulation techniques,

binary data stream is keyed on to a carrier signal and then coupled on to the power lines

by PLM. At the receiver end another PLM detects the signal and extracts the

corresponding bit stream.


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Data is processed before transmission on power lines according to the above figure.

First data is modulated & filtered and then by using couplers, it is sent over the power

lines.

PLC Modems/Transceivers

PLC Transceiver is the key component of a PLCC system. It is the device which

transmits & receives data to & from the power lines and acts as a hub between the

power stations and our Computers/Network utilization devices. They are wired with the

electrical voltage lines at home or business and work on two modes transmit mode

and receive mode. In transmit mode, they simply receive data from receiver end installed

on the same network and further transmit them. In receive mode, they work the opposite

way.

A number of companies provide PLC transceivers and other networking devices for

PLCC communication. A PLC transceiver is shown in the following image.


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Modulation Techniques

As mentioned earlier, characteristics of the power line channel continuously vary with

time and load. So conventional modulation techniques like ASK, FSK or PSK cannot be


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employed with them. PLCC needs a technique that can deal with the unpredictable

attenuation and phase shifts.

Modulation techniques that opt lower frequency ranges of 35 KHz to 95 KHz can perform

better as compared to the ones using the whole available frequency band. OFDM

(Orthogonal Frequency Division Multiplexing) is the modulation technique that is

used in Home Plug specification network appliances. In OFDM, information is

modulated on to multiple carriers, where each carrier occupies its own frequency in

the range of 4.3 to 20.9 MHz. Incoming bit stream is demultiplexed into N number of

parallel bit streams each with 1/N of original bit rate which are then modulated on N

orthogonal carriers. By using multiple carriers at a time, the modulation technique uses

the available spectrum most efficiently. During the transmission, each frequency ismonitored and if any interference, noise or data loss occurs, the responsible frequency is

removed. However this technique does not perform well when a large attenuation and

jamming occurs in the communication channel, but still it can be very efficient

comparatively.

How signals are superimposed on Power Lines?

There are two different ways by which we can connect a PLC unit with the power lines

capacitive coupling and inductive coupling. In capacitive coupling, a capacitor is

used to superimpose the modulated signal on to the networks voltage waveform.

Another way is inductive coupling which employs an inductor to couple the signal with

the networks waveform.

No physical connection is required to establish inductive coupling. This makes it safer as

compared to capacitive coupling. However this method has higher tendency to lose the

signal during coupling.


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Important Technical Parameters in PLC Communication

Noise on Residential Power Circuit (RPC):

A variety of noises may occur during the communication caused by the home

appliances. Following are some of the types:

1. Noise synchronous to the power system frequency (50Hz or 60 Hz) This type of

noise is generated because of different kind of switching devices.

2. Noise with a smooth spectrum The sources of such type of noise are the

appliances that are not operating synchronously with the power line frequency. For

example the small motors with several windings can generate such type of noise.

3. Single Event Impulse Noise Switching of devices, that contain a capacitor,

generates such type of noise. The reason is sudden discharge of the capacitor in the

RPC.

4. Periodic Noise The type of noise is generated by fluorescent lights, television

receivers etc.

These are some ways to reduce the noise in between the communication over power

lines:

a. Implementation of Forward Error Correction (FEC) codes with interleaving can

reduce the noise in category 1, 2 and 3.

b. Frequency Hopping with the FEC coding can be implemented to deal with the

unknown frequencies.

c. While modulating the signal on to the power lines, television line frequencies

should be avoided.

Signal to Noise Ratio:


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Signal to Noise Ratio (SNR) is a measurement of quality of the signal. It indicates the

amount of the noise in a signal. SNR can be formulated in the following way:

SNR = RECEIVED POWER / NOISE POWER

Increasing SNR means increasing the performance of the communication system. By

applying noise filters on household appliances, the noise entering into the power system

can be reduced. However it will increase the cost of the appliances but is a better

solution to improve overall performance.

Signal Attenuation:

Signal attenuation is basically the reduction in strength of the signal. A signal attenuation

of about 100dB/Km occurs for low voltage power lines and 10dB/km for high voltage

lines. It creates a need of continuous repeaters over a fixed distance. A number of

factors that are responsible for signal attenuation include distance, time, frequency of the

signal, etc.

Applications of PLCC

PLCC technology can be deployed into different types of applications in order to provide

economic networking solutions. Hence merging with other technologies it proves useful

in different areas. These are few key areas where PLC communications are utilized:

a. Transmission & Distribution Network: PLCC was first adopted in the electrical

transmission and distribution system to transmit information at a fast rate.

b. Home control and Automation: PLCC technology is used in home control and

automation. This technology can reduce the resources as well as efforts for activities likepower management, energy conservation, etc.

c. Entertainment: PLCC is used to distribute the multimedia content throughout the

home.


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d. Telecommunication: Data transmission for different types of communications like

telephonic communication, audio, video communication can be made with the use of

PLCC technology.

e. Security Systems: In monitoring houses or businesses through surveillancecameras, PLCC technology is far useful.

f. Automatic Meter Reading Automatic Meter reading applications use the PLCC

technology to send the data from home meters to Host Central Station.

Advantage and Disadvantage:

In order to completely analyze the advantages and disadvantages of PLCC technology,

we look into its basic application that is access to telecommunication networks. From the

economic standpoint, it is very reasonable to use a pre installed wired network instead ofrunning new wires. It certainly reduces a lot of time & money and so is the biggest

advantage of the technology.

In many countries, PLCC is becoming a reliable high speed source to get Internet. And

in some places, especially in remote areas, PLCC technology thankfully made it possible

to avail internet connections.

Power line communication is quite different in characteristics than the conventional

dedicated wirings. Comparatively, it is a harsh medium and data transfer through it can

create a lot of problems. Household appliances like halogen tubes, washing machines,

televisions, etc. can become prone to an unpredictable noise and interference in the

transmission. Continuous plugging and unplugging of electronic devices makes power

line characteristics vary constantly.

IED [INTELLIGENT ELECTRONIC DEVICE]:-

An Intelligent Electronic Device (IED) is a term used in the electric power industry to

describe microprocessor-based controllers of power system equipment, such as circuit

breakers, transformers, and capacitor banks.


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Description:-

IEDs receive data from sensors and power equipment, and can issue control

commands, such as tripping circuit breakers if they sense voltage, current,

or frequency anomalies, or raise/lower voltage levels in order to maintain the desired

level. Common types of IEDs include protective relaying devices, load tap changer

controllers, circuit breaker controllers, capacitor bank switches, recloser controllers,

voltage regulators, etc.

Digital protective relays are primarily IEDs, using a microprocessor to perform several

protective, control, and similar functions. A typical IED can contain around 5-12

protection functions, 5-8 control functions controlling separate devices, an auto reclose

function, self monitoring function, communication functions etc. Hence, they are aptly

named as Intelligent Electronic Devices.

IEC61850 standard for substation automation.

IEDS used in power station manufactured by ABB

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