Showing posts with label Switchgear and Protection. Show all posts
Showing posts with label Switchgear and Protection. Show all posts

Sunday, June 25, 2017

June 25, 2017

Miniature/Low Oil Circuit Breaker

Miniature Oil Circuit Breaker
Miniature Oil Circuit Breaker
Photo Credit - ABB Group
In the bulk oil circuit breaker, the oil has to perform two functions, first of all it act as an arc quenching medium and secondly it insulate the live part from earth. It has been found that only small percentage of oil is actually used for arc extinction while the major part is utilized for insulation purposes. For this reason the quantity of oil in bulk oil circuit breaker reaches a very high as the system voltage increases. This not only increases. This not only increases the expenses, tank size and weight of the breaker but it also increases the fire risk and maintenance problems.
A low oil circuit breaker employs solid materials for insulation purposes and uses a small quantity of oil which just sufficient for arc extinction. Regarding quenching the arc, the oil behaves identically in bulk as well as low oil circuit breaker. By using suitable arc control devices the arc extinction can be further facilitated in an low oil circuit breaker.

Construction

Miniature/Low oil circuit breaker
Fig. Miniature/Low oil circuit breaker
Photo Credit - Circuit Globe
Above figure shows the cross section of a single phase low oil circuit breaker. There are two compartment separated from each other but both filled with oil. The upper chamber is the circuit breaking chamber while the lower one is to prevent from mixing with the other chamber. This arrangement permit two advantages. First of all, the circuit breaking chamber requires a small volume of oil which is just enough for arc extinction. Secondly, the amount of oil to be replaced is reduced as the oil in the supporting chamber does not get contaminated by the arc.
  1. Supporting chamber
    2. It is a porcelain chamber mounted on a metal chamber. It is filled with oil which is physically separated from the oil in the circuit breaking chamber. The oil inside the supporting chamber and annular space formed both the porcelain insulation and bakelised paper is employed for insulation purposes only.
  2. Circuit breaking chamber
    3. It is a porcelain enclosure mounted on the top of the supporting compartment. It is filled with oil and has the following parts:-
      Upper and lower fixed contact.
      Moving contact
      Turbulator
    The moving contact is hollow and includes a cylinder which moves down over a fixed piston. The turbulator is an arc control device and has both axial and radial vents. The axial venting ensures the interruption of low currents whereas radial venting vents help in the interruption of heavy current.

  3. Top Chamber
    4. It is metal chamber and is mounted on the circuit breaking chamber. It provides expansion space for the oil in the circuit breaking compartment. The top chamber is also provided with a separator which prevent any loss of oil by centrifugal action caused by circuit breaker operation during fault condition.

Operation

Under normal operating conditions the moving contact remains engaged with the upper fixed contact. When fault occurs the moving contact is pulled down by tripping springs and an arc is struck. The arc energy vaporizes the oil and produces gases under high pressure. This action contains the oil to pass through a central hole in the moving contact and results in forcing series oil oil through the respective passage of turbulation. The process of turbulation is ordinary one in which the section of arc are successively quenched by the effect of separate streams of oil moving across each section in turns and bearing away in gases.

Advantages

  1. It requires lesser quantity of oil.
  2. The space required is small.
  3. There is reduced risk of fire.

Disadvantages

  1. Due to smaller quantity of oil the degree of carbonisation is increased.
  2. There is difficulty of removing the gases from contact space in time.
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Wednesday, February 15, 2017

February 15, 2017

Vacuum Circuit Breaker

Vacuum Circuit Breaker
Vacuum Circuit Breaker
Image Credit- Siemens Energy
In Vacuum circuit breakers, vacuum (degree of vacuum being in the range 10-7 to 10-5 torr) is used as the arc quenching medium. Since vacuum offers the highest insulating strength it has far superior arc quenching properties than any other medium. For example, when contacts of a breaker are opened in vacuum, the interruption occurs at first current zero with dielectric strength between the contacts building up at a rate thousands of times higher than obtained with other circuit breaker.

Principle:-

The production of a arc in vacuum circuit breaker and its extinction can be explained as follows;-
When the contacts of the breaker are opened in vacuum (10-7 to 10-5 torr), an arc is quickly extinguished because the metallic vapor, electrons and ions produced during arc rapidly condense on the surface of the circuit breaker contacts resulting in quick recovery of dielectric strength. As soon as the arc is produced in vacuum, it is quickly extinguished due to the fast rate of recovery of dielectric strength in vacuum.

Construction:-

Above figure shows the parts of a typical vacuum circuit breaker. It consist of fixed contact and arc shield mounted inside a vacuum chamber. The movable member is connected to the control mechanism by stainless steel bellows. This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of leak.  A glass vessel or ceramic vessel is used as the outer insulating body. The arc shield prevents the deterioration of the internal dielectric  strength  by preventing metallic vapors falling on the surface of the outer insulating cover.

Working:-

When the breaker operates, the moving contact separates from the fixed contact and an arc is struck between the contacts. The production of arc is quickly extinguished because the metallic vapors, electrons and ions produced during arc are diffused in a short time and seized by the surface of moving and fixed members and shield since vacuum has very fast rate of recovery dielectric strength. The arc extinction in a vacuum breaker occurs with a short contact separation say 0.625 cm.

Advantages:-

  1. They are compact, reliable and have longer life.
  2. There are no fire hazards.
  3. There is no generation of gas during and after operation.
  4. They require little maintenance and arc quiet in operation.
  5. They can successfully withstand lightning surges.
  6. They have low arc energy.


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Location: Chandrapur, Maharashtra, India
February 15, 2017

SF6 Circuit Breaker

SF6 Circuit Breaker
SF6 circuit breaker
In such circuit breaker SF6 gas is used as the arc quenching medium. SF6 is an electro-negative gas and has strong tendency to absorb free electrons.The contacts of the breaker are open in the high pressure. Flow of SF6 gas and arc is struck between them. The conducting free electrons in the arc are rapidly captured by the gap to form relatively immobile ions.The loss of conducting electron in the arc quickly builds up enough insulating strength to extinguish the arc. SF6 circuit breaker has been found to be very effective for high power and high voltage service.

Construction:-

Constructional details of SF6 circuit breaker
Constructional details of SF6 circuit breaker
Above figure shows the parts of typical SF6 circuit breaker. It consists of fixed and moving contacts enclosed in a chamber(called arc interruption chamber) containing SF6 gas. This chamber is connected to SF6 gas reservoir. When the contact of breaker are opened, the valve mechanism permits a high pressure SF6 gas from the reservoir to flow towards the arc interruption chamber. The moving contact is a hollow cylindrical current carrying contact fitted with an arc horn. The moving contact is also a hollow cylinder with rectangular holes in the sides to permit the SF6 gas to let out through these holes after flowing along and across the arc. The tips of the fixed contact and arcing horn are coated with copper-tungsten arc resistant material. Since SF6 gas is costly, it is reconditioned and reclaimed by suitable auxiliary system after each operation of the breaker.

Working:-

In the closed position of the breaker, the contacts remain surrounded by SF6 gas at a pressure of about 2.8 Kg/cm2. When the breaker operates, the moving contact is pulled apart and an arc is struck between the contacts. The movement of the moving contact is synchronized with the operating of a valve which permits SF6 gas at 14 Kg/cm2 pressure from the reservoir to the arc interruption chamber. The high pressure flow of SF6 rapidly absorb the free electrons to form immobile negative ions which are ineffective as charge carriers. The result is that the medium between the contacts quickly build up high dielectric strength and causes the extinction of the arc. After the breaker operation i.e., after arc extinction valve is closed by the action of a set of spring.

Advantages :-

  1. Due to the superior quenching property of SF6 such circuit breaker have very short arcing time.
  2. Since the dielectric strength of SF6 gas is 2 to 3 times that of air, such breakers can interrupt much larger currents.
  3. The SF6 circuit breaker gives noiseless operation due to its closed gas circuit and no exhaust to atmosphere unlike air blast circuit breaker.
  4. The closed gas enclosure keep the interior dry so that there is no moisture problem.

Disadvantages:-

  1. SF6 circuit breaker are costly due to high cost of SF6 gas.
  2. SF6 gas has to be reconditioned after every operation of the breaker. Hence additional equipment is required for this purpose.
There are many advantages of using SF6 circuit breaker, only few have listed above.
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Location: Chandrapur, Maharashtra, India