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Air - Blast Circuit Breakers: Types, Advantages, Disadvantages, Application

       Air – Blast Circuit Breakers

Air - Blast circuit breakers

employ high pressure Air – Blast as an arc quenching medium. The contacts are opened in a high flow of air blast established by the opening blast valve. The air blast cools the arc and sweeps away the arcing products to the atmosphere. It rapidly increases the dielectric strength of the medium between contacts and then prevents from re-establishing the arc. Consequently, the arc is extinguished and the flow of current is also interrupted.

Advantages of Air – Blast Circuit Breakers

The air – blast circuit breaker’s advantages are given below: 

(i) The risk of fire is eliminated.

(ii) The growth of dielectric strength is so rapid that final contact needed a very small gap for arc extinction. It reduces the size of the device.

(iii) The arcing time is very small due to the quick build-up of dielectric strength between both contacts. That’s why, the arc energy is only a fraction of that in oil circuit breakers, thus resulting in less burning of contacts.

(iv) Cause of lesser arc energy, the air-blast circuit breakers are very suitable for conditions where frequent operations are required.

(v) The energy supplied for the arc extinction is gained from high-pressure air and is independent of the current interruption.

(vi) Short and consistent arc duration.

(vii) The burning of contacts is less due to less arc energy (since the arc duration is short and consistent).

(viii) Less maintenance is required.

(ix) High-speed operation.

(x) The facility of high-speed reclosure.

Disadvantages of Air – Blast Circuit Breaker

The disadvantages of air – blast circuit breaker are given below :

(i) The air has many properties one is the relatively inferior arc extinguishing property.

(ii) The air-blast circuit breakers are very sensitive circuit breakers than others to the variations in the rate of rise of restriking voltage.

(iii) Maintenance is required for the circuit breaker for compressor plant which supplies the air-blast.

(iv) The air leakage at the pipeline fittings create problems.

(v) Current chopping.

Applications of Air – Circuit Breaker

The air blast circuit breakers are using wide applications in high voltage installations. The majority of these circuit breakers for voltages up to 110 kV are of this type.

Types of Air – Blast Circuit Breakers

The classification of air – blast circuit breakers are

depending upon the direction of air-blast in relation to the arc,

(i)            

Axial – blast type

In which the arc – blast is directed along the arc path.

(ii)          

Cross – blast type

In this air – blast is directed at right angles to the arc path.

(iii)        

Radial – blast type

In this type of circuit breaker air – blast is directed radially.

(i)            

Axial – Blast Air Circuit Breaker

The figure shows the essential components of a typical

Axial-Blast Air Circuit Breaker

. The fixed and moving contacts of the circuit breaker are held in the closed position by spring pressure under normal conditions. The air reservoir of the circuit breaker is connected to the arcing chamber through an air valve. This valve remains closed during normal conditions but when the fault occurs on the system it opens automatically by the tripping.

When a fault occurs, the tripping impulse causes opening of the air valve which connects the air reservoir to the arcing chamber. The high-pressure air entering the arcing chamber which pushes away the moving contact against the spring pressure. The moving contacts are separated and an arc is struck between them. At the same time, the high-pressure air blast flows along the arc and takes away the ionized gases along with it. Consequently, the arc is extinguished and interrupted the flow of the current.It should be noted that in these types of circuit breakers, the contact separation required for interruption is generally small almost (

1-75

cm or so). These small gaps may constitute inadequate clearance for the normal service voltage. Cause of that, an isolating switch is incorporated as a part of such a circuit breaker.  After the fault interruption, this switch opens immediately to provide the necessary clearance for insulation.

(ii)          

Cross – Blast Air Circuit Breaker

In the cross – blast air circuit breaker, an air-blast is directed at right angles (90

o

) to the arc. The cross-blast lengthens and forces the arc into a suitable escarpment for arc extinction. The below Figure shows the essential parts of a typical cross-blast air circuit breaker. When the moving contacts are withdrawn, an arc is struck between both fixed and moving contacts. The high contact pressure cross-blast forces the arc into a chute consisting of arc splitters and baffles. The splitters help to increase the length of the arc and baffles provide improved cooling. The result is that the arc is extinguished and the current flow is interrupted. Because the air blast pressure is similar for all currents, the inefficiency at low currents is eliminated. The final gap for interruption is great enough to give normal insulation clearance therefore, a series isolating switch is not necessary.

SOURCE: https://www.chloshikhe.in/2020/06/air-blast-circuit-breakers.html

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Transformer Cooling, Rating, Conservators & Breathers & Name plate

Transformer Cooling

When a

transformer

is in operation the heat is generated due to I

2

R losses in the windings and core losses. The removal of the heat is called cooling.

Cooling Methods :

As far as

cooling methods

are concerned, the transformers are following two types :

1.) Dry type2.) Oil immersed type

1.) Dry type transformers:

 Small

transformers

upto 25 kVA size are of dry type and have the following cooling arrangements:

·       Air Natural

·       Air Blast

2.) Oil immersed transformers:

 In general most transformers are of oil immersed type. The oil provides better insulation than air as it is the better conductor of heat than air. Mineral oil is used for this purpose.

·       Oil Natural (ON) Cooling

·       Oil Blast (OB) Cooling

·       Forced Oil and Forced Air Flow (OFB) Cooling

·       Forced Oil and Water (OFW) Cooling

1.) Air Nature (AN) Cooling:

In a dry-types self-cooled transformer, the natural circulation of surrounding air is used for its cooling. The windings are protected from mechanical injury by the sheet metal enclosure. This type of cooling is satisfactory for low-voltage small transformers upto a few kVA.

2.) Air Blast (AB) Cooling:

The dry-type forced air-cooled transformer is similar to that of dry-type self -cooled transformer with the addition that continuous blast of filtered cool air is forced through the core and windings for better cooling. The blast is produced by a fan.

What is Transformer: Its Working Principle & Construction

3.) Oil Natural (ON) Cooling:

The majority of transformers of medium and large rating have their windings and core immersed in oil which acts both as a cooling medium and an insulating medium. Oil immersed transformers are enclosed in the sheet-steel tank. The heat produced in the cores and windings is  passed to the oil. Heated oil becomes lighter and rises to the top and its place is taken by cool oil from the bottom of the tank. The heat of the oil is transferred to the walls of the tank by natural circulation of oil. The heat is then transferred to the surrounding atmosphere through natural radiation and convection. The Oil gets cooler and falls to bottom. Thus, a continuous natural circulation of oils takes place.

   Plain tanks are economical upto a rating of 25KVA. Above this rating large cooling surface is generally provided by using corrugations, fins, tubes (circular or elliptical) and radiator tanks.

4.) Oil Blast (OB) Cooling:

In this type of cooling, forced air is directed over cooling elements of transformer immersed in oil.

5.) Forced Oil and Forced Air Flow (OFB) Cooling:

Oil is circulated from top of the transformer tank to a cooling plant. Cool oil is then returned to the bottom of the tank.

6.) Forced Oil and Water (OFW) Cooling:

In this type of cooling forced oil flow with water cooling of the oil in the external water heat exchanger takes place. This type of cooling is similar to OFB cooling except that the heat exchanger uses water instead of air for cooling oil. The water is circulated in cooling tubes placed in the heat exchanger.

Conservators and Breathers

The Oil is not allowed to come in contact with the atmospheric air which may contain moisture. The moisture spoils the insulating properties of oil. Atmospheric air may cause acidity and sludging of oil. A

conservator

is an air-tight metal drum placed above the level of the top of the tank and connected with it by a pipe. It is partially filled with oil.

When the oil expands, or contracts by the change in temperature, there is a displacement of air. When the transformer cools, the oil level goes down and the air is drawn in. This is known as

Breathing

. The air coming in is passed through a device called breather for the purpose of extracting moisture. The

Breather

consists of a small vessel which contains a drying agent like silica gel crystals impregrated with cobalt chloride. Silica gel is checked regularly and dried and replaced when necessary.

What is Instrument Transformer : Types, Construction, Working Principle & Advantage

Rating of the Transformer

It is to be noted that, since the copper loss depends on current and core loss depends on voltage, the total loss in the transformer depends on the volt-ampere product, and not on the phase angle between voltage and current, that is, independent of the load power factor. The rating of the transformer is, therefore, in voltamperes (VA) and not in watts (W). In actual practice, the rating of the transformer is specified in VA, kVA or MVA depending upon its size.

Transformer Name Plate

According to BIS (Bureau of Indian Standard) 2026 every transformer must be provided with a name plate giving the following information:

Type (Power, Auto, Booster, etc.)

Year of manufacture

Number of phases

Rated kVA (for multiwinding transformers rated kVA of each winding)

Rated frequency

Rated voltage of each winding

Connection symbol

Percent impedance voltage at rated current (measured value corrected to 75o

C)

Type of cooling

Mass and volume of insulating oil

As per BIS 2026, the preferred kVA ratings are

6, 3, 10, 16, 25, 40, 63, 100, 160, 250, 400, 630, 1000 kVA, etc.

source : https://www.chloshikhe.in/2020/05/transformer-cooling-rating-conservators-name-plate.html

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What is an Instrument Transformer : Types, Advantages and Their Differences

Instrument Transformer

It is generally not a safe practice to connect instruments, meters, or control apparatus directly to high voltage (hv) circuits. Instrument Transformers ( Current Transformers and Potential Transformers ) are universally used to reduce high voltages and currents to safe and practical values which can be measured by conventional instruments ( the normal range is 1A or 5A for current and 110V for voltage).

Advantages of Instrument Transformers

The most important advantages of instrument transformers are as follows :

1.)  Ammeters and voltmeters for use with these transformers may be standardized at 1A or 5A and 110V respectively.

2.)  The single range instrument may be used to cover a large current or voltage range with Instrument Transformers. In case of wattmeter or watthour meter it may cover both a large current or voltage range.

3.)  The measuring instruments may be located away from the high voltage (hv) circuit for safety to the operator

.4.)  The current flowing in a busbar or any other conductor can be measured by a current transformer (CT) of split-core type without breaking the current circuit.

Types of Instrument Transformers The principle of the instrument transformer is fundamentally the same as the power transformer. The Instrument Transformers are classified as follows: 1.) Potential Transformers.     2.) Current Transformers.

1.) Potential Transformer (P.T.) A potential transformers is a step-down transformer used along with a low range voltmeter for measuring a high voltage.

2.) Current Transformers (C.T.)

A current transformer is a device for the transformation of current from a higher value to a lower value, or for the transformation of current at a high voltage into a proportionate current at a low voltage with respect to the earth potential.

source: https://www.chloshikhe.in/2020/04/instrument-transformer-types-advantages-differences.html

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Universal Motor

Universal  Motor Universal motor is a series wound motor, which operates at approximately the same speed and output on either D.C.  or  A.C. of approximately same voltage For some applications, it is desirable to employ a motor that will operate on either A.C. or D.C. supply. By a compromise design, fractional horse power series motors may be built to operate satisfactorily on either 50 Hz A.C. or direct current at 115 or 230 volts. Such motors are known as Universal Motors. Construction of Universal Motor The armature of universal motors is of the same construction as ordinary series motor. In the same sizes the voltage induced by the transformer action in a coil during its commutation period does not tend to produce sufficient current to cause any serious commutation problem. High resistance brushes are employed to aid the commutation. In large motors compensating winding is providing to improve the commutation. The stator core and yoke are laminated to reduce eddy currents produced by alternating flux. Its speed varies considerably with load. At no load it reaches high value as high as 20000 rpm and drops to 8000-10000 rpm on full load in some designs. Universal Motor  Commutation on A.C.is poor than on D.C. Due to the sparking a universal motor causes much interference on radio. The wear and tear of brushes is also more. Universal motor can be operate on vide variation of voltages. The motors are designed for voltages between 50 to 250 volts, generally motors as built for 200-250 V range. At lower than 50 volts large currents make the commutation power and operation of motor is not satisfactory. The efficiency of universal motor is low compare to those of large motor but is better than other FKW motors. The efficiency varies between 30 to 70 percent depending on size and speed. For small motors and small speeds efficiency is lower , for large motors and for large speeds it is higher. The power factor varies between 0.7 to 0.8 on full load. Since the universal motor has commutator and brushes , a certain amount of servicing is required from time to time. This is the disadvantage in these motors. But with improved manufacturing methods a brush life of 1000 hours is attained. Due to high speed. Commutator and brushes rubbing against it, the universal motor is more noisy. Quiet operation is obtained by good mechanical construction and careful electrical design. Torque / Speed Characteristics of Universal Motor Figure shows Torque / Speed Characteristics of Universal Motor. The motor gives 3 to 5 times the full load torque at the time of starting. This motor gives quite high speed. In general the universal motor has characteristics very similar to a D.C. series motor. At no load speed is extremely high and it falls as the load increases. Universal motor characteristics Reversal of Direction of Rotation. The direction of rotation can be changed by interchanging connections to the field with respect to the armature as in a D.C. series motor. Advantages of Universal Motor 1.) High speed from 3600 rpm to 25000 rpm.2.) High torque at low and intermediate speeds.3.) Higher power output. Disadvantages of Universal Motor 1.) Motor become noisy at high speeds.2.) Requirement for careful balancing to avoid vibration.3.) Poor commutation on A.C. operation.

Source : https://www.chloshikhe.in/2020/04/universal-Motor-construction-working-and-characteristics.html

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CONSTRUCTION AND WORKING PRINCIPLE OF THREE BRUSH GENERATOR

CONSTRUCTION AND WORKING PRINCIPLE OF THREE BRUSH GENERATOR Such generators are used to charge the batteries and lighting systems in automobiles and trains. The main requirement for such a service is that the generator must maintain its load current practically constant regardless of speed variations. Three brush generator is illustrated in Figure. In construction it is a simple shunt generator, with the exception that the shunt field is connected to the positive brush A and an auxiliary brush C about 120 electrical degrees away in the direction of rotation and the negative brush B is usually grounded to the frame of the car or engine. The shunt field current obviously depends upon the voltage between positive and auxiliary brushes, the latter being adjustable in position related to the former and, therefore, charging current can be adjusted to increase the charging current the auxiliary brush C is required to be shifted towards the grounded brush B in the direction of rotation. In this generator advantage is taken of armature reaction to regulate the charging current. The flux due to armature current produces a distorting effect resulting in weakening of field near the leading pole tips and, therefore, the voltage generated by the inductors between the positive brush and auxiliary brush will be reduced by an increase in armature current. This, in turn, reduces the field flux and with in the voltage between the position and grounded brushes. In other words since induced e.m.f. is proportional to product of flux and speed i.e.        , at low speed flux   is comparatively high while at high speed flux   is correspondingly lower, for a wide range in speed, therefore, the terminal voltage remains nearly constant. Third brush generators are generally designed for weak shunt field and a strong armature reaction field so that the latter may have a substantial effect upon the former one. With proper setting of the auxiliary brush C, the current supplied to the battery and lamps can be maintained at or neat a predetermined constant value. At low speeds the generated voltage is less than the battery voltage and the battery would also tend to feed current to the generator in a direction opposite to that of the generated voltage of the machine in addition to supply of power to the train lighting system. Such a current would cause motor action in the generator and would put a heavy strain on the batteries with no useful purpose. To prevent this discharging of the battery through the low resistants of the armature circuit of the generator, there should be a cut out relay which connects the battery to the generator only after the generator has built upto a voltage slightly higher than that of the battery. If the generator slows down, this relay opens when the current reverses and begins to flow from battery to generator.

source : https://www.chloshikhe.in/2020/04/construction-working-principle-three-brush-generator.html

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Buchholz Relay

BUCHHOLZ RELAY

Buchholz relay is a gas actuated relay installed in oil immersed transformers for protection against all kinds of faults. Named after its inventor, Buchholz, it is used to give an alarm in case of incipient faults in the transformer and  to disconnect the transformer from supply in the event of severe internal faults. It is usually installed in the pipe connecting the conservator to the main tank as shown in figure(A). It is a universal practice to use Buchhholz relays on all such oil immersed transformers having ratings in excess of 750 Kva.

 CONSTRUCTION :    Figure (B) shown the constructional detail of a Buchholz relay. It takes the form of domed vessel placed in the connecting pipe between the main tank and the conservator. The device has two elements. The upper element consists of a mercury type switch attached to a float. The lower element contains a mercury switch mounted on a hinged type flap located in the direct path of the flow of oil from the transformer to the conservator. The upper element closes an alarm circuit during incipient faults whereas the lower element is arranged to trip the circuit breaker in case of severe internal fauit.

OPERATION :    The operation of Buchholz relay is as follows:(i) In case of incipient faults within the transformer, the heat due to fault causes the decomposition of some transformer oil in the main tank. The products of decomposition contain more than 70% of hydrogen gas. The hydrogen gas being light tries to go into the conservator and in to process gets entrapped in the upper part of relay chamber. When a predetermined amount of gas gets accumulated, it exerts sufficient pressure on the float to cause it to tilt and close the contacts of mercury switch attached to it. This completes the alarm circuit to sound an alarm.(ii) If a serious fault occurs in the transformer, an enormous amount of gas is generated in the main tank. The oil in the main tank rushes towards the conservator via the Buchholz relay and in doing so tilts the flap to close the contacts of mercury switch. This completes the trip circuit to open the circuit breaker controlling the transformer. ADVANTAGES (i) It is the simplest from of transformer protection.(ii) It detects the incipient faults at a stage much earlier than is possible with other forms of protection. DISADVANTAGES (i) It can only be used with oil immersed transformers equipped with conservator tanks.(ii) The device can detect only faults below oil level in the transformer. Therefore, separate protection is needed for connecting cables.

visit  CHLOSHIKHE.IN  for more information

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Lightning arrester

LIGHTNING ARRESTERS

Lightning Arresters or a Surge Diverter is a protective device which conducts the high voltage surge on the power system to the ground.

ACTION : The action of the lightning arrester or surge diverter is an under :(i) Under normal operation, the lightning arrester is off the line i.e. it conducts no current to earth or the gap is non-conducting.(ii) On the occurrence of overvoltage, the air insulation across the gap breaks down and an arc is formed, providing a low resistance path for the surge to the ground. In this way, the excess charge on the line due to the surge is harmlessly conducted through the arrester to the ground instead of being sent back over the line.(iii) It is worthwhile to mention the function of non-linear resistor in the operation of arrester. As the gap sparks over due to overvoltage, the arc would be a short-circuit on the power system and may cause power-follow current in the arrester. Since the characteristic of the resistor is to offer high resistance to high voltage, it prevents the effect of a short-circuit. After the surge is over, the resistor offers high resistance to make the gap non-conducting. TYPES OF LIGHTNING ARRESTER There are several types of lightning arresters in general use. 1.) Rod Gap Arrester2.) Horn Gap Arrester3.) Multigap Arrester4.) Expulsion type lightning arrester5.) Valve type lightning arrester

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