operation of different types of circuit breaker

8/13/2019 Operation of different types of Circuit Breaker

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Circuit Breaker OperationIn addition to the events that cause a trip, a circuit breaker for switchgear

applications must also be selected for the method by which it opens when

tripped. This is important, because when contacts are opened quickly at

high voltage levels, a conductive metallic vapor can form that allows

current to continue to travel between the open contacts. This phenomenon,

known as arcing, creates the greatest obstacle to circuit interruption.

As a result, medium- and high-voltage circuit breakers employ one of four

different arc interrupting technologies. All take advantage of the fact that

even the most powerful AC overcurrent cycles pass the ero-current level

twice in one cycle. !y reducing the amount of conductive gas between the

contacts, the arc cannot be sustained when it passes through a current ero.

"ince the current in #C circuits

does not follow a sine-wave

pattern, circuit interruption is very

difficult. This makes the #C

interrupting rating for most

breakers much lower than the

interrupting rating for AC circuits.

Air magnetic breakersuse the arc

to generate a magnetic field that

forces the arc into arc chutes, which lengthen and cool the arc, allowing it

to be e$tinguished at a current ero.

Oil breakersare of several types,

including bulk oil, but they all work in a

relatively similar way. %ere, the contacts

are immersed in a container of non-

conductive oil. &hen an overcurrent

occurs, the arc heats the surrounding oil

forcing it to flow violently. The rapidly

flowing oil displaces the arcing gases and breaks the arc path.


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'il breakers always carry the haards of handling and disposing of spent oil,

and the potential for oil fire. In (urope, however, special minimum-oil

designs have been developed to reduce these drawbacks, and some

minimum oil breakers have even been approved for limited indoor use.

#ifferent oil breakers are designed for different power levels, with the

highest rated for )*+ k to + k.

Sulfur hexafluoride (SF6)is an

insulating gas used in circuit breakers

in two ways. In puffer designs, it/s

blown across contacts as they open to

displace the arcing gas. In blast

designs, it/s used at high pressures to

open contacts as it simultaneously e$tinguishes the arc. "01breakers are

rated for the highest voltage of all breaker designs.

Vacuum breakersenclose the contacts

within a vacuum chamber, so when the

arc of metallic vapor forms it is

magnetically controlled and thereby

e$tinguished at current ero. acuum breakers are rated up to )*.+ k.

AS! Classes Of "ela#sThere are many types of protective relays. The revision of A2"I )3.4,

5elays and 5elay "ystems Associated with (lectric 6ower Apparatus listsand defines five classes of relays7

8. Auxiliar# rela#- one that operates in response to the opening orclosing of its operating circuit to assist another relay or device inperforming a function.

9. $rotecti%e rela#- one whose function is to detect defective lines,apparatus, or other dangerous or undesired conditions, and to initiateor permit suitable switching, or to give adequate warning.

). "egulating rela#- one that operates because of a departure of a

parameter from predetermined limits and that functions through


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supplementary equipment to restore the quantity to within theselimits.

*. &onitoring rela#- one whose function is to verify that system orcontrol circuit conditions con form to prescribed limits.

+. $rogramming rela#- one whose function is to establish or detect

electrical sequences.

"ome +8 different types of relays and 9* kinds of protection are listed and

defined in the "tandard. The types of relays include alarm, differential,

distance, directional power, timing, voltage, etc., while the kinds are7

differential protection, directional over-current protection, ground

protection, pilot wire protection, etc.

The "tandard defines high speed as a qualifying term applied to a relay,

indicating that the time of its operation usually does not e$ceed l:9th of a

second ;) cycles on 1 cycle base


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'verloads, if continued, cause conductor circuit or motor overheating. They

may not require immediate disconnect from the system only before the

operating temperatures become too high. The overcurrent protective device

used for overloads should have a time-overcurrent characteristic. A short-

circuit should be removed from the system immediately however, a small

amount of time may be allowed to permit selective operation of the device

closest to the short-circuit location.

Arcing ;and other< ground faults may be smaller than the normal full load

current. Therefore, a different scheme must be used if the system is to be

protected from this type of fault. This ground fault current may cause

serious damage at the location of the fault, but still may be too small to trip

the overcurrent device. This ground-fault sensing and protection concepts

are discussed elsewhere in this manual. They involve au$iliary trip devices

operated by e$ternal sensors. The 2(C ;9)-4+< requires that ground fault

protection be provided on certain services rated 8,A or more, where at

least 8,9A of ground fault current might flow. These systems need to be

coordinated with the downstream overload protection devices, to prevent

shutting down the entire system in the event of a down-stream ground fault.

$oints ,o Consider !n ,ransformer SelectionThere are several points to consider when selecting a transformer.

-. kVA "ating

The kA rating must be sufficient to handle the load. Consideration should

also be given to possible future load growth. "ome companies handle growth

using a large number of various sie transformers, installing them with a

rating close to the actual load requirements, and then changing the bank

when the load grows. &hile this is the way electric utilities do it, most

industrial plants do not have the necessary stock of spares to operate in this

manner, and the practice does not lend itself to the use of substations. 'f

course, transformer ratings can be increased by forced cooling.

The transformer kA, impedance, and voltage rating determine the short-

circuit current interruption requirements for the protective device.


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5emember that replacing a transformer with a larger unit to handle a larger

connected load also impacts the interrupting requirements.

/. Voltage "atings and "atios

The transformer should be selected to give the proper voltage at the load

terminals. This voltage is the system voltage desired, not the equipment

utiliation voltage. 0or instance, if motors are rated for *1 olts, the

transformer no-load voltage should be *B olts. This permits a voltage drop

in the feeders to the point of use.

0. Voltage ,aps

ost modern transformers have taps in the windings that make it possible to

slightly change the turns ratio. These taps do not materially affect the

voltage drop through the transformer they merely change the voltage level.

The standard for the taps in transformers used in industrial systems is two 9-

8:9= taps both above and below rated voltage. Tap changing must be

carried out with the transformer disconnected from the circuit. %owever,

load tap changing transformers are available at additional cost.

Taps are intended to ad@ust the transformer to the primary voltage level

actually present. Therefore, if the input voltage is 9-8:9= above the

nominal system voltage, the tap should be set to provide 9-8:9= more

primary turns in order to keep the secondary voltage at the design level

during operation. Taps are not intended to be used to raise or lower the

secondary voltage from the design or rated values.

"ome transformers are supplied with several taps on the secondary

windings. These permit the choice of any one of the secondary terminal

voltages appropriate to that particular tap. %owever, this is not what is

referred to as changing the taps on a transformer. "ome auto transformers

are designed to provide continuous ad@ustment of the secondary voltage by

using wires wound around a toroidal core and providing a sliding contact to

bared regions on these secondary turns. This is equivalent to having a

secondary tap for every turn.

1. ,#pes of Construction


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>iquid-filled transformers may be filled with either transformer oil or an

insulating liquid specifically designed for transformers. These liquids

perform two functions, serving as heat transfer mediums and as insulation.

#ry-type transformers are available in either ventilated or sealed

enclosures.

The physical location of the transformer is of primary importance in

determining the type of transformer to be used for a particular application.

The characteristics of a given location may preclude the use of certain types

or may make one type more desirable than another. In general, liquid-filled

and sealed dry-type transformers are suitable for indoor and outdoor

locations. 'il-filled transformers are suitable only for outdoor locations, and

may not be used indoors because of the flammability of the oil ;unless

located in a suitable fireproof vault


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Transformers may be sub@ect to overvoltages from lightning or switching

surges and should be appropriately protected. These surge handling

capabilities are described by the impulse level the amount of a momentary

voltage surge which the insulation can withstand. #ry-type transformers

with their basic insulation being air, have appro$imately one-half the

impulse level handling capability of similar kA liquid-filled transformers.

Therefore, more thought must be given to dry-type transformer situations.

6ole-top distribution transformers are frequently e$posed to these voltage

surges. That is why these transformers have the insulation on their windings

strengthened to withstand lightning ;and other< surges. The strength of this

insulation is given by the !asic Impulse >evel ;!I>< number. Enits with high

!I> numbers can be obtained at a cost premium. As an e$ample, typical

pole-top insulators for 4-8) k would have a !I> of 889 k since that is their

flash-over voltage. The transformer used on such a line should have a !I>

greater than this value.

Another insulation parameter is the insulation between primary and

secondary windings. This is essential in the application of instrument

transformers, and in applications which require isolation between circuits

;such as the filament transformers for high-voltage F-5ay tubesow-voltage transformers are generally designed to conform with the

impedance/s as shown in the T5A2"0'5(5 I6(#A2C( Table in the

Appendi$. These values are sub@ect to standard tolerances of plus-or-minus

3 percent of the nominal impedance values listed. anufacturers will design

transformers to meet other impedance requirements if the standard values

are not adequate, but this naturally adds to the cost.

Transformers with lower impedance have lower voltage drop, but will also

allow higher available fault currents. "ome devices, such as welders and arc

furnaces, use transformers that are specifically designed to have relatively


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high percent impedance/s. This is done to reduce the disturbances on the

primary power system when these devices are operating.

3. Single+$hase or ,hree+$hase ,ransformers

The recent trend is to use three-phase transformers rather than three

single-phase transformers to make up a three-phase bank. Three-phase

transformers have an e$cellent service record, cost less to install, and

require less space. If three single-phase transformers connected delta-delta

are used, the bank can still operate three-phases in the event one

transformer fails. %owever, the capacity is reduced to about +3 percent of

the original capacity and the voltages are unbalanced. Transformer failures

are rare, and the e$tra space and labor needed to connect a bank of three

instead of one three-phase unit may not be @ustified.

4. 5#e+*elta Arrangements

Another consideration when ordering transformers is the manner in which

they are connected to the system. 0or e$ample, when using three-phase

transformers, specify how the primary and secondary windings are to be

connected, whether they are to be delta or wye, and the polarity.

. !nterrupting "ating

Today/s utility transformers are capable of delivering very large currents to

a building. "hort circuits can cause high currents and electrical arcs that

generate tremendous heat and large mechanical forces in a building, which

in turn could seriously damage or destroy apparatus and conductors in a

very short time if they are not promptly interrupted. This increased short

circuit potential over the values assumed to be adequate a few years ago

should be considered whenever any transformer is changed out for a larger

one.

"ela# ,#pesThis course is not intended to make relay engineers out of youG The

discussion is limited to naming some basic operating principles, and

discussing some of the common relay applications.


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All relays provide an output signal which can be used to initiate some

action. This output signal is often used to close one or more set of contacts,

operate a circuit breaker, or a switch. "ome electronic relays provide an

output current or a voltage signal, the more elaborate programmable

controllers send digital signals which can be read by other devices. The

basic electromechanical relays, however, simply operate to close a set of

contacts when moved by either7

8. (lectromagnetic attraction, or9. (lectromagnetic induction ;turn like a motor


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connections, @oints, and connections to @oints are insulated, and the

stationary structures consist of self-supporting steel frames.

etal-clad switchgear should not be confused with metal enclosed

switchgear where the individual components are not isolated from each

other even though enclosed in a protective metal cabinet.

Single $hase $oer(5atts7 Volt+Amperes and VA"s)

'hm/s law for alternating-current circuits uses the quantity called

impedance, ;represented by the letter H


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This is called the reactive power ;or magnetiing power< and is measured

in olt-Amperes-5eactive ;A5s


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operation of different types of circuit breaker

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