Air circuit breaker working principle; Air circuit breaker

Air circuit breakers as clear from the name that these circuit breakers operate at atmospheric pressure. ACBs are still preferable choice up to voltage 15 KV. Even ACB’s can be used for LT also . MCCB’s can be used upto 630/ 800A above that limit air circuit breakers are used.

Working Principle of Air Circuit Breaker

Main aim for every circuit breaker is to prevent rebuilt of arcing after electric current goes zero by creating a situation where in the contact gap will withstand the system recovery voltage. For interrupting arc it creates an arc voltage in excess of the supply voltage. Arc voltage is defined as the minimum voltage required maintaining the arc.

This circuit breaker increases the arc voltage by mainly three different ways,

1. Air circuit breaker increases the arc voltage by cooling the arc plasma. As the temperature of arc plasma is decreased, the mobility of the particle in arc plasma is reduced; hence more voltage gradient is required to maintain the arc.
2. It may increase the arc voltage by lengthening the arc path. As the length of arc path is increased, the resistance of the path is increased, and hence to maintain the same arc electric current more voltage is required to be applied across the arc path. That means arc voltage is increased.
3. Splitting up the arc into a number of series arcs also increases the arc voltage.

OPERATIONAL PROCEDURE OF ACB

 Every circuit breaker objective is usually to increase the arc area this can be achieved by forcing the arc into contact with as large an area as possible of insulating material. There are arc chutes in every circuit breaker usually arc is driven into it. Arc chute is suitably shaped in such a way to help to achieve cooling. This type of arc chute should be made from some kind of refractory material. High temperature plastics reinforced with glass fiber and ceramics are preferable materials for making arc chute.

 There is further arc slitter inside the arc chute. The main arc chute is divided into numbers of small compartments by using metallic separation plates. These metallic separation plates are actually the arc splitters and each of the small compartments behaves as individual mini arc chute. In this system the initial arc is split into a number of series arcs, each of which will have its own mini arc chute. So each of the split arcs has its own cooling and lengthening effect due to its own mini arc chute and hence individual split arc voltage becomes high. These collectively, make the overall arc voltage, much higher than the system voltage.

This was working principle of air circuit breaker now we will discuss in details the operation of ACB in practice.

The air circuit breaker, operated within the voltage level 1 KV, does not require any arc control device.

During breaking of a circuit breaker arc is produced which needed to be quenched as earlier as possible so that arc doesn’t re-strike. Circuit breakers usually named after the Medium in which circuit breaker breaks or medium used to quench the arc during breaking of circuit breaker. In Air circuit breaker air serves the same purpose.

 Mainly for heavy fault electric current on low voltages ACBs with appropriate arc control device, are good choice. These breakers normally have two pairs of contacts. The main pair of contacts carries the electric current at normal load and these contacts are made of copper. The additional pair is the arcing contact and is made of carbon. When circuit breaker is being opened, the main contacts open first and during opening of main contacts the arcing contacts are still in touch with each other. As the electric current gets, a parallel low resistive path through the arcing contact during opening of main contacts, there will not be any arcing in the main contact. The arcing is only initiated when finally the arcing contacts are separated. The each of the arc contacts is fitted with an arc runner which helps, the arc discharge to move upward due to both thermal and electromagnetic effects as shown in the figure. As the arc is driven upward it enters in the arc chute, consisting of splitters. The arc in chute will become colder, lengthen and split hence arc voltage becomes much larger than system voltage at the time of operation of air circuit breaker, and therefore the arc is quenched finally during the electric current zero.

Air circuit breakers working is shown above which self explanatory. 

Although this type of circuit breakers have become obsolete for medium voltage application, but they are still preferable choice for high electric current rating in low voltage application.

Advantages of air circuit breakers

1. There is no chance of fire hazard which usually occur in Oil circuit breaker.
2. The breaking speed of Air circuit breaker is much higher than oil circuit breaker.
3. Arc quenching is much faster in case of air circuit breaker than oil circuit breaker.
4. As the duration of arc is smaller, so lesser amount of heat realized from arc to electric current carrying contacts hence the service life of the contacts becomes longer.
5. The stability of the system can be well maintained as it depends on the speed of operation of circuit breaker.
6. Requires much less maintenance compared to oil circuit breaker.

There are also some disadvantages of air blast circuit breakers-

1. In order to have frequent operations, it is necessary to have sufficiently high capacity air compressor.
2. Frequent maintenance of compressor, associated air pipes and automatic control equipments is also required.
3. Due to high speed electric current interruption there is always a chance of high rate of rise of re-striking voltage and electric current chopping.
4. There also a chance of air pressure leakage from air pipes junctions.

Suspension Type Insulators; Suspension Insulators Advantages and Disadvantages

Suspension Insulator

As discussed earlier Pin type insulators are useful for using at 33KV and above 33 KV it becomes uneconomical to use Pin type insulators. This is mainly due to increased size, Weight of pin type insulator. Thus it become very difficult for handling and replacement of Such insulators. Due to above stated problem Suspension type insulators are developed.

It is quite interesting to know about suspension insulators, Suspension type insulators consists of  numbers of insulators are connected in series to form a string and the line conductor is carried by the bottom most insulator.

Each insulator of a suspension string is called disc insulator because of their disc like shape. Suspension type insulators are very economical to use and advantages and disadvantages for the suspension type insulators are as below:-

Suspension Insulator Advantages:-

1. In suspension type insulators each disc is designed for normal voltage rating 11KV and 15KV on the higher voltage side. So by using different numbers of discs, a suspension string can be made suitable for any voltage level. E.g. if there is transmission voltage of 66KV then there will be 6 discs in suspension type insulator.


2. It becomes easy to replace any disc insulator of suspension type insulator as every disc is of small size.

3. Suspension type insulators provide more flexibility to the line. The connections at the cross arm is such that insulator string is free to swing in any direction and will take position where mechanical stress is minimum.

4. As the electric current carrying conductors are suspended from supporting structure by suspension string, the height of the conductor position is always less than the total height of the supporting structure. Therefore, the conductors may be safe from lightening.

5. It become easier with suspension type insulators to meet demand as additional disc can be added with rising voltage.

6. The suspension type insulators are generally used with steel towers. As the conductors run below the earthed cross-arm of the tower, therefore, this arrangement provides partial protection from lightning.

Disadvantages of Suspension Insulator

1. One of the main disadvantage of Suspension insulator string is  the every disc of suspension type insulator is costlier than pin and post type insulator

2. Suspension string requires more height of supporting structure than that for pin or post insulator to maintain same ground clearance of electric current conductor.

3. The amplitude of free swing of conductors is larger in suspension insulator system, hence, more spacing between conductors should be provided.

SF6 Circuit breaker; SF6 Circuit breaker Applications

SF6 Circuit Breaker

As clear from the name circuit breaker which operates in Sulphur Hexfluoride gas is known as SF6 circuit breaker. It is most widely used in HT Breakers.

SF6 has excellent insulating property.

SF6 Gas has high electro-negativity. Which means that they have ver high affinity to absorb electrons. Whenever a free electron collides with the SF6 gas molecule, it is absorbed by that gas molecule and forms a negative ion.

 SF6 gas can absorb electron in two different ways

 SF6  + e ----->     SF6-

SF6  + e ----->     SF5-  + F

You can see from above that SF6 when comes in contact with free electrons then they absorb the electrons and these –ve ions are much heavier then free electrons which will leads to lower mobility of charged particle in SF6 gas as compared to other gases. As everyone knows that this charged particle mobility is majorly responsible for conducting electric current through a gas. Hence, for heavier and less mobile charged particles in SF6 gas, it acquires very high dielectric strength.

There are various other properties other then higher dielectric strength such has this gas has also unique property of fast recombination after the source energizing the spark is removed. The gas has also very good heat transfer property as this gas has lower gaseous viscosity due to lesser molecular mobility.

So due to its high dielectric strength and high cooling effect SF6 gas is approximately 100 times more effective arc quenching media than air.

Due to these above properties of this gas SF6 circuit breaker is used in complete range of medium voltage and high voltage electrical power system. These circuit breakers are available for the voltage ranges from 33KV to 800KV and even more.

Properties Of SF6 (Sulfur Hexafuoride) Gas

a) Toxicity

SF6 is odorless, colorless, tasteless, and nontoxic in its pure state. It can, however, exclude oxy­gen and cause suffocation. If the normal oxygen content of air is re­duced from 21 percent to less than 13 percent, suffocation can occur without warning. Therefore, circuit breaker tanks should be purged out after opening.

b) Toxicity Of Arc Products

Toxic decomposition products are formed when SF6 gas is subjected to an elec­tric arc. The decomposition products are metal fluorides and form a white or tan powder. Toxic gases are also formed which have the characteristic odor of rotten eggs. Do not breathe the vapors remaining in a circuit breaker where arcing or corona dis­charges have occurred in the gas.

Evacuate the faulted SF6 gas from the circuit breaker and flush with fresh air before working on the circuit breaker.

c) Physical Properties

SF6 is one of the heaviest known gases with a den­sity about five times the density of air under similar conditions. SF6 shows little change in vapor pressure over a wide temperature range and is a soft gas in that it is more compressible dynamically than air.

The heat trans­fer coefficient of SF6 is greater than air and its cooling characteristics by convection are about 1.6 times air.

d) Dielectric Strength

SF6 has a di­electric strength about three times that of air at one atmosphere pressure for a given electrode spacing. The dielectric strength increases with increasing pressure; and at three atmospheres, the dielectric strength is roughly equivalent to transformer oil. The heaters for SF6 in circuit breakers are required to keep the gas from liquefying because, as the gas liquifies, the pressure drops, lowering the dielectric strength.
.
The exact dielectric strength, as compared to air, varies with electrical configuration, electrode spacing, and electrode configuration.

e) Arc Quenching

SF6 is approxi­mately 100 times more effective than air in quenching spurious arcing. SF6 also has a high thermal heat capacity that can absorb the energy of the arc without much of a temperature rise.

f) Electrical Arc Breakdown

Because of the arc-quenching ability of SF6, corona and arcing in SF6 does not occur until way past the voltage level of onset of corona and arcing in air. SF6 will slowly decompose when ex­posed to continuous corona.

All SF6 breakdown or arc products are toxic. Normal circuit breaker operation produces small quantities of arc products during current interruption which normally recombine to SF6.

Arc products which do not recombine, or which combine with any oxygen or moisture present, are normally re­moved by the molecular sieve filter material within the circuit breaker.

Disadvantages of SF6 CB

There is main disadvantage associated with SF6 Gas is that SF6 gas is a Greenhouse gas.  There are so many safety regulation has been introduced in many countries in order to prevent its release into atmosphere. Puffer type design of SF6 CB needs a high mechanical energy which is almost five times greater than that of OCB.

SF6 is the most potent greenhouse gas with a global warming potential that is 23,900 times greater than that of carbon dioxide (CO2); it is also very persistent in the atmosphere with a lifetime of 3,200 years

There are mainly three types of SF6 CB depending upon the Voltage level of application-

1.         Single interrupter SF6 CB applied for up to 245 KV(220 KV) system.

2.         Two interrupter SF6 CB applied for up to 420 KV(400 KV) system.

3.         Four interrupter SF6 CB applied for up to 800 KV(715 KV) system.

Working of SF6 Circuit Breaker

As all circuit breakers will work on the same principle there is only difference in arc quenching medium. In Air circuit breakers air is used as quenching medium in the same way in SF6 circuit breakers SF6 gas is used as quenching medium.

Here SF6 gas was compressed and stored in a high pressure reservoir. During operation of SF6 circuit breaker this highly compressed gas is released through the arc in breaker and collected to relatively low pressure reservoir and then it pumped back to the high pressure reservoir for re utilize.

The working of SF6 circuit breaker is little bit different in modern time. Innovation of puffer type design makes operation of SF6 CB much easier. In buffer type design, the arc energy is utilized to develop pressure in the arcing chamber for arc quenching.

Here the breaker is filled with SF6 gas at rated pressure. There are two fixed contact fitted with a specific contact gap. A sliding cylinder bridges these to fixed contacts. The cylinder can axially slide upward and downward along the contacts. There is one stationary piston inside the cylinder which is fixed with other stationary parts of the SF6 circuit breaker, in such a way that it can not change its position during the movement of the cylinder. As the piston is fixed and cylinder is movable or sliding, the internal volume of the cylinder changes when the cylinder slides.

During opening of the breaker the cylinder moves downwards against position of the fixed piston hence the volume inside the cylinder is reduced which produces compressed SF6 gas inside the cylinder. The cylinder has numbers of side vents which were blocked by upper fixed contact body during closed position. As the cylinder move further downwards, these vent openings cross the upper fixed contact, and become unblocked and then compressed SF6 gas inside the cylinder will come out through this vents in high speed towards the arc and passes through the axial hole of the both fixed contacts. The arc is quenched during this flow of SF6 gas.

During closing of the circuit breaker, the sliding cylinder moves upwards and as the position of piston remains at fixed height, the volume of the cylinder increases which introduces low pressure inside the cylinder compared to the surrounding. Due to this pressure difference SF6 gas from surrounding will try to enter in the cylinder. The higher pressure gas will come through the axial hole of both fixed contact and enters into cylinder via vent and during this flow; the gas will quench the arc.

Caution:-
Always keep an eye on SF6 Levels in circuit breakers, don't operate them if SF6 level indicates in red zone. If you try to operate the breaker in that position then there are very chances that explosion may takes place.

Comparison between SF6 and Vacuum Circuit Breakers

SF6 Circuit Breaker			Vacuum Circuit Breaker
Criteria	Puffer Circuit Breaker	Self-pressuring circuit-breaker	Contact material-Chrome-Copper
Energy requirements for Opeartion	Energy requirement for the operation is high as the mechanism needed to supply the energy for compressing the gas.	Operating Energy requirements are low, because the mechanism must move only relatively small masses at moderate speed, over short distances. The mechanism does not have to provide the energy to create the gas flow	Operating energy requirements are low, because the mechanism must move only relatively small masses at moderate speed, over very short distances.
Arc Energy	Because of the high conductivity of the arc in the SF6 gas, the arc energy is low. (arc voltage is between 150 and 200V.)		Because of the very low voltage across the metal vapour arc, energy is very low. (Arc voltage is between 50 and 100V.)
Contact Erosion	Due to the low energy the contact erosion is small.		Due to the very low arc energy, the rapid movement of the arc root over the contact and to the fact that most of the metal vapour re-condenses on the contact, contact erosion is extremely small.
Arc extinguishing media	The gaseous medium SF6 possesses excellent dielectric and arc quenching properties. After arc extinction, the dissociated gas molecules recombine almost completely to reform SF6. This means that practically no loss/consumption of the quenching medium occurs. The gas pressure can be very simply and permanently supervised. This function is not needed where the interrupters are sealed for life.		No additional extinguishing medium is required. A vacuum at a pressure of 10-7 bar or less is an almost ideal extinguishing medium. The interrupters are ‘sealed for life’ so that supervision of the vacuum is not required.
Switching behavior in relation to current chopping	The pressure build-up and therefore the flow of gas is independent of the value of the current. Large or small currents are cooled with the same intensity. Only small values of high frequency, transient currents, if any, will be interrupted. The de-ionization of the contact gap proceeds very rapidly, due to the electro-negative characteristic of the SF6 gas and the arc products.	The pressure build-up and therefore the flow of gas is dependent upon the value of the current to be interrupted. Large currents are cooled intensely, small currents gently. High frequency transient currents will not, in general, be interrupted. The de-ionization of the contact gap proceeds very rapidly due to the electro-negative characteristic of the SF6 gas and the products.	No flow of an ‘extinguishing’ medium needed to extinguish the vacuum arc. An extremely rapid de-ionization of the contact gap, ensures the interruption of all currents whether large or small. High frequency transient currents can be interrupted. The value of the chopped current is determined by the type of contact material used. The presence of chrome in the contact alloy with vacuum also.
No. of short-circuit operation	10—50	10—50	30—100
No. full load operation	5000—10000	5000—10000	10000—20000
No. of mechanical operation	5000—20000	5000—20000	10000—30000

Above Table will differentiate between SF6 and Vacuum Circuit breakers

How to Repair tube light; Repairing tube light

Tube light circuit is shown above which consists of following components:-

1. Tube Rod

2. Starter

3. Ballast (Choke)

4. Power supply

There are following problems that occur in tube lights and how to repair them are also discussed below:-

Faults in Tube Lights

Tube Light Starting Problem

While switching on the Tube light if light doesn't flicks and even not turned ON then there is very chances that supply isn't coming in tube light so check for the supply at supply end.If this does not help, try wiggling the tube gently in its sockets by rocking it back and forth and from side to side. This will scour away minute deposits of corrosion or dust that can sometimes hinder the flow of electricity. Be sure to do this when the light switch is turned off.

It is normal for the light in new fluorescent tubes to flicker or appear to swirl in the first 100 hours of operation.

If an older tube exhibits these symptoms, turn off the switch, remove the tube, then clean the ends thoroughly. Socket shapes vary, but the tube-removal process is the same.

Replacing Tube Light

To remove a straight fluorescent tube, rotate it a quarter-turn in either direction and pull the tube straight down out of the sockets. If the tube is circular, simply unsnap it from the brackets supporting it and pull it free from the single socket.

To clean the ends of a tube, scrub the pins projecting from them with fine-grit sandpaper, then wipe away all dust with a cloth or paper towel. If any pins are bent, squeeze them gently with needle-nose pliers to straighten them.

When inspecting the ends, examine the glass portions. A brownish tint is normal on tubes that have been in use for some time. Tubes whose ends are blackened usually are wearing out.

If only one end of a tube appears blackened, reverse the tube end-for-end and reinstall it after cleaning the pins. If the tube is blackened only along one side, rotate it after cleaning and reinstall it so that the blackened portion is turned 180 degrees from its former position.

Tubes that are blackened at both ends still may last a considerable time. So, if a tube still malfunctions after you have cleaned and repositioned it, check the condition of other components of the light before shopping for a replacement tube.

Starter Problem

The first component to check is the starter. This is a small cylinder, approximately 2 inches long and usually silver colored. Its purpose is to accumulate current briefly when the light is switched on and then release it after the tube is lighted.

The starter is responsible for the momentary delay in lighting when some fluorescent tubes are switched on. If it is faulty, it can also be the cause of initial flickering as the tube warms up, or of failure to light at all.

Not all fluorescent lights have starters, but if yours does, it will usually be located near a tube socket.

Lights with more than one tube have a separate starter for each. If you do not find a starter, unplug the light or shut off the power, then remove the deflector above the tube and look there, or disassemble the base if the light is a desk or floor model.

Starter Replacement

To remove a starter, press it inward and twist it counterclockwise a quarter turn; it should pop out.

There is no way to tell if it is malfunctioning except by replacing it, and since starters are available at hardware stores for less than a dollar, it is worth the gamble. Take the old part with you to obtain a duplicate.

Fluorescent lights designed without starters are called rapid-starting lights, and this designation is usually printed or stamped on them. With these, dirt on the tube can sometimes prevent lighting or can cause flickering.

The cure is to remove the tube and clean it by wiping it first with a cloth dipped in dish detergent, and then with a cloth dipped in plain water. Be careful when handling tubes; they are fragile and if smashed may explode into shards.

If a fluorescent tube blinks on and off-a slower and more distinct process than flickering-the fault may lie in loose wiring or in another component, called the ballast. The ballast is almost always to blame if the fixture hums during operation.

To inspect the wiring and ballast, remove the deflector or disassemble the base, as well as any other parts necessary to expose the tube sockets and wiring.

Check that the plastic twist-on connectors joining the wires are firmly tightened and that the ground wire (usually green) is fastened tightly to the metal body of the fixture. No exposed wires should be present.

A wire that appears disconnected probably is, and should be reconnected. The sockets, as well as other components, should also be firmly fixed in place.

Ballast replacement

The ballast is a rectangular metal or plastic component resembling a small box with wires issuing from both ends. To test it, install a working fluorescent tube and a new starter. If the light malfunctions, the ballast is at fault and must be replaced.

Mark the ballast wires and the wires leading to the socket with pieces of tape so that they are paired to simplify reinstallation, then unfasten the wires from their connectors and unscrew the ballast from the fixture.

Take the ballast to a hardware or electrical supply store when you need a replacement.

If replacing both the starter and the tube did not make the light work and the wiring was intact, then the problem is definitely in the ballast. If noise is the only problem, get a low-noise ballast, which is clearly marked as such. If the light is operated in temperatures below 50 degrees Fahrenehit

(another cause of blinking and flickering), obtain a low-temperature ballast.

When shopping for a new fluorescent tube, compare lumens (brightness), wattage and life expectancy. Most manufacturers print this information on the cartons. Bulbs typically last for at least a year, often much longer.

It is worth noting that the life expectancy of tubes is based on the number of times the tube is started. Because tubes consume greater amounts of electricity during starting, it is actually best to leave fluorescent lights burning rather than turn them on and off at frequent intervals.