Bus Bars Protection 

1. Single Bus Bar

The schematic chart of a solitary busbar framework is introduced in fig.1. Before examining about this framework, some critical ideas are characterized underneath:

The single busbar framework introduced in fig.1 utilizes two detaches switches, and an electrical switch every feeder. If there should arise an occurrence of a feeder blame, the electrical switch confines the feeder. In the same way, the breaker is utilized to switch on and off the stacked feeder. The distinction switch on both sides of the breaker are utilized to confine the breaker for repair. The distinction switches are correspondingly utilized for bringing as a part of the breaker to the dead feeder. The distinction switch on the feeder side of the breaker may be overlooked, if there is no other wellspring of era, nourishing the heap, which is accepting a parallel supply from the busbar under thought.

The single busbar framework has the accompanying

Focal points 

1. Basic outline.

2. Moderately modest.

3. Effectively transferred (control).

4. Least Cost.

6. Low accessibility.

Weaknesses

a.)The supply from the busbar is hindered amid the routine support of the busbar framework and disengages switches associated with it.

b.)Feeder supply is lost amid the repair and upkeep of the feeder, breaker, and the distinction switches.

c.)Short circuits in the busbar lead to the loss of supply to all the feeders.

d.)Feeder short circuits in the event of electrical switch disappointment lead to loss of supply to all feeders.

2.Main And Transfer Bus Bar

Typically bus bar deficiency is extremely uncommon where as normal upkeep of electrical switch must be completed habitually. Essentially the issue in electrical switch is likewise regular. In this framework, in a bad position in any of the breakers, the standby breaker with mix of exchange bus can be utilized for ordinary operation.

The primary peculiarities of principle and exchange bus are as per the following:

Points of interest

1. Medium expense.

2. Adaptable operation.

3. Breaker or line transfers can be taken out of administration whenever for upkeep and with just slight adjustments to line security.

4. Potential gadget on principle bus can be utilized for handing-off.

Hindrances

1. Inconsistent (bus or breaker flaw close down the substation).

2. Requires an additional breaker for bus tie.

3.bus  sudden death round transferring confused since it may save any line breaker.

4. Exchanging convoluted when evacuating a breaker for upkeep.

5. Obliges separate bus insurance plan.

    3) Double bus single breaker

The plan is costlier yet has higher unwavering quality than other two plans

Assume force stream is currently in course from source-1 to source-2. For this situation bus 1 is charged through CB-2, DS-1 and DS-4. In the event that blame happens in line of source -2, it is detached by opening CB-4

furthermore DS-2. Additionally if there should be an occurrence of support of Bus-1, Bus-2 is charged through bus  coupler by shutting DS-5, DS-10 and CB-5 .Both the buses  now are at same potential. Source-1 can supply Bus-2 through DS-11 furthermore the heap can be joined with bus 2 through DS-12 securely. Presently DS-3, DS-4, DS-5 and CB-5 can be opened. Comparable operations can be performed at source-2 side. Furthermore thus upkeep at Bus-1 can be performed securely.

The fundamental peculiarities of this plan are as per the following:

Focal points

1. Adaptable operation.

2. Probability of keeping up a busbar at once without the loss of supply.

3. Breaker or line transfers can be taken out of administration whenever with just slight alteration of line insurance.

4. Either fundamental bus may be separated for support.

Hindrances 

An additional breaker is needed for the bus tie.

2.Fault in the sudden death round debilitates both bus bars

3.Complicated exchanging obliged when evacuating a breaker for support.

4.Bus sudden death round transferring convoluted since it must have the capacity to extra any breaker.

5.Requires different entangled bus defensive plan.

6.High expense.

7.Loss of supply of comparing feeder amid upkeep of that feeder.

SF6 CIRCUIT BREAKERS

Sulphur-hexaflouride (SF6) is an inert insulating gas, which is becoming increasingly popular in modern switchgear designs both as an insulating as well as an arc-quenching medium. SF6 has the following advantages :

1)Very good insulating strength

2)High affinity for electrons which ultimately give rise to high dielectric strength to SF6

3)Heat Transfer-ability much higher than air

4)Chemically stable without any chance of carbon decomposition
For medium- and low-voltage installations, the SF6 circuit breaker remains the same construction wise as that for oil and air circuit breakers mentioned above, except for the arc interrupting chamber which is of a special design, filled with SF6.

In an SF6 circuit-breaker, the current continues to flow after contact separation through the arc whose plasma consists of ionized SF6 gas. For, as long as it is burning, the arc is subjected to a constant flow of gas which extracts heat from it. The arc is extinguished at a current zero, when the heat is extracted by the falling current. The continuing flow of gas finally de-ionizes the contact gap and establishes the dielectric strength required to prevent a re-strike.

As SF6 has high affinity for electrons whenever a free electron collides with the neutral gas molecule negative ions are formed. The electron is absorbed by the neutral gas molecule. The following reactions are involved:

SF6 + e- === SF6-

SF6 + e- === SF5- + F

The negative ions formed ash shown above will be heavier as compared to free electrons and under a given electric field the ions will not attain sufficient energy to initiate cumulative ionization in the gas. In this way the gas de-ionizes and establishes the dielectric strength required to prevent a re-strike.

ADVANTAGES

•  they have  very short arcing time due to greater arc quenching property of SF6.
• the dielectric strength of SF6 gas is 2 to 3 times greater than that of air thus making they can interrupt larger currents.
• the closed gas circuit makes their operation noiseless and keep the interior dry avoiding any possible moisture problem.
• since SF6 is non inflammable gas there ain't any risk of fire and no carbon deposits 
• they have low maintenance cost, light foundation requirements and minimum requirement auxiliary equipment.
• SF6 breakers are totally enclosed and sealed from atmosphere hence are very suitable in the places where explosion hazard exists.

DISADVANTAGES

•  these breakers  are costly because SF6 is expensive .
• SF6 gas has to be reconditioned after each and every operation of the breaker and for which other additional devices are required.

APPLICATION

SF6 breakers have been used for voltages 115kV to 230 kV, power ratings 10 MVA to 20 MVA and interrupting time less than 3 cycles.

VACUUM CIRCUIT BREAKERS 

Vacuum breakers are additionally comparable in development like alternate sorts of breakers, with the exception of that the breaking medium is vacuum and the medium fixed to guarantee vacuum. The utilization of vacuum for circular segment elimination has the accompanying preferences:

1)Vacuum has high dielectric quality.

2)When contacts get open amid operation in AC frameworks, the current is hindered right away present zero moment which causes the dielectric quality between the contacts to develop at a high rate than that can be accomplished with other circuit breakers.

3)Ideally vacuum is such a dielectric in which curve ought not persevere.

When the vacuum circuit breaker operates, the fixed contact and moving contact shown in the figure separates. Metal Vapor is then released from the contacts which fills the vacuum between the contacts and thus maintains the arc. Metal vapor formed is actually known as plasma which is the combination of electrons and positive metal ions. After the first current zero instant is reached the plasma vanishes and the medium regains its dielectric strength. The arc will now get extinguished as only vacuum is left between the contacts.
Because it is virtually impossible for electricity to flow in a vacuum, the early designs displayed the ability of current chopping i.e. switching off the current at a point on the cycle other than current zero. This sudden instantaneous collapse of the current generated extremely high-voltage spikes and surges into the system, causing failure of equipment.

Another phenomenon was pre-strike at switch on. Due to their superior rate of dielectric recovery, a characteristic of all vacuum switches was the production of a train of pulse during the closing operation.

Although of modest magnitude, the high rate of rise of voltage in pre-strike transients can, under certain conditions produce high-insulation stresses in motor line end coils. Modern designs have all but overcome these problems. In vacuum circuit breakers, operating speeds are also much higher which, together with contact materials that ensure high dielectric strength at a small gap, have ensured that prestrike transients have ceased to become a significant phenomenon. These have led to the use of vacuum breakers more common in modern installations.

ADVANTAGES

•VCB is self contained does not need filling of gas or oil. They do not need auxiliary air system, oil handling system etc. No need of periodic refilling.

•No emission of gas, pollution free

•Non explosive

•Silent operation

•Large number of operation on load or short circuit .suitable for repeated operating duty ,long life

•Suitable for capacitor switching ,cable switching, industrial switching

•Constant contact resistance

•Constant dielectric

DISADVANTAGES

•For interruption of low magnetizing currents in certain range, additional surge suppressors are required in parallel with each phase of a VCB

•It require high technology for production

•It is more expensive.

 MINIMUM OIL CIRCUIT BREAKERS

The amount of oil required in bulk type oil circuit breakers is very high making it uneconomical for use. In fact, only one-tenth of the oil is actually used for arc extinction and remaining for insulation. Minimum Oil Circuit Breakers is thus an alternative to bulk type. Minimum Oil Circuit Breakers are usually employed at increased voltage and fault levels.

ADVANTAGES

A low oil circuit breaker has following advantages compared to bulk oil circuit breaker

1. It requires lesser quantity of oil
2. It requires smaller space
3. There is reduced risk of fire
4. Maintenance problems are reduced

DISADVANTAGES

A low oil circuit breaker has following disadvantages compared to bulk oil circuit breaker.

OIL CIRCUIT BREAKERS

Oil circuit breakers are the oldest type of circuit breakers. Oil circuit breakers are usually employed for high voltage applications up to 11kV with a current rating of up to 2400A. In this design, the main contacts are immersed in oil and the oil acts as the ionizing medium between the contacts. The oil is mineral type, with high dielectric strength to withstand the voltage across the contacts under normal conditions.

Oil has the following advantages:

•  they have ability of cooling oil to flow into the space after current zero and arc goes out
• oil presents the cooling surface.
• energy absorption by decomposition of oil

Oil circuit breakers maybe of various types, but regardless of that the basic arc extinction process remains same in all circuit breakers. When main contacts are separated during the operation of the circuit breakers an arc will be produced. The arc will thus generate heat decomposing the oil around the main contacts and liberate gases (mostly hydrogen). The production of the hydrogen gas in this manner appreciably increases the dielectric strength of the gap between the contacts which aids in the arc extinction process.

Depending upon the amount of oil to be used oil circuit breakers may be classified into the following two types:

1)Bulk Oil Circuit Breaker
2)Minimum Oil Circuit Breakers

In the initial stages, the use of high-volume (bulk) oil circuit breakers was more common. In this type, the whole breaker unit is immersed in the oil. This type had the disadvantage of production
of higher hydrogen quantities during arcing and higher maintenance requirements. Subsequently these were replaced with low oil (minimum oil) types, where the arc and the bubble are confined into a smaller chamber, minimizing the size of the unit.

1 ) BULK TYPE OIL CIRCUIT BREAKERS

In bulk type oil circuit breakers a large amount of oil is used. The oil is required in large amount as it performs the following functions:
a)Production of hydrogen gas for arc extinction
b)Insulation between the open contacts
c)Insulation of line contacts form earthed metal tank.

AIR CIRCUIT BREAKERS

Air Circuit Breakers (ACB’s) are used for medium and low voltages. In air circuit breakers, the interrupting contacts are situated in air instead of any other medium. The arc produced is chopped into number of small arcs by the arc-chute as it rises due to heat and magnetic forces.
In such circuit breakers high resistance principle is employed for effective arc extinction. The arc is lengthened by means of the arc runners and arc chutes thus increasing the resistance. Arc resistance can be increased in the following ways:

1.  Cooling: When the temperature is decreased by cooling the voltage required to maintain the ionization of the air increases which in turn increases the resistance.
2.Lengthening: As the length of the arc increases the resistance also increases.
3.Splitting: An appreciable voltage is absorbed at the two contact surfaces so that on splitting of an arc into a number of small arcs in series the voltage available for the actual arc column is reduced.

Figure shows a typical construction of an air circuit breaker. It consists of two sets of contacts viz. Main Contact and Arcing Contact. The arc chute is used for arc extinction process. When fault occurs in the system, the main contacts are separated initially, the current then shifts towards the arcing contacts. The arcing contacts also separates now drawing an arc between them. This arc is then moved up by arc chutes along the arc runner. The arc moves upwards and is split by the arc splitter plates. The ac is then extinguished by the high resistance method as illustrated above.

Application:
This type of circuit breaker is used for mainly medium and low voltages and low breaking capacity. they become more bulky, the arc chute design more complex and the cost will be much higher as the voltage and breaking capacity increases .

The ACB are usually applied for control of power station auxiliaries and industrial plants switchgears. They are also used for control of motor loads and furnaces where repeated switching are to be performed, where OCBs are not performed.

Circuit Breakers:

These are the devices used in the electrical network used to interrupt the circuit contacts manually or automatically based on the necessary on use. There are different types of circuit breakers. Some of them are discussed below :

Types Of Circuit Breakers (based on medium of Arc Extinction)

1) Air Circuit Breakers
2) Oil Circuit Breakers
3) Vacuum Circuit Breakers
4) SF6 Circuit Breakers