In this article we will discuss about:- 1. Construction of Vacuum Circuit Breaker 2. Operating Mechanism of Vacuum Circuit Breaker 3. Contact Materials 4. Properties 5. Applications 6. Advantages 7. Disadvantages.
Contents:
- Construction of Vacuum Circuit Breaker
- Operating Mechanism of Vacuum Circuit Breaker
- Contact Materials for Vacuum Circuit Breakers
- Properties of Vacuum Circuit Breakers
- Applications of Vacuum Circuit Breakers
- Advantages of Vacuum Circuit Breakers
- Disadvantages of Vacuum Circuit Breakers
1. Construction of Vacuum Circuit Breaker:
As far as construction is concerned vacuum circuit breaker is very simple in comparison to an air or oil circuit breaker. A schematic diagram of such a circuit breaker is shown is Fig. 10.19. The outer envelope is normally made of glass due to the ease of jointing it to the metallic end-caps and also because the glass envelope facilitates the examination of the breaker from outside after the operation.
If it becomes milky white from its original finish of silvery mirror then it indicates that the baffle is losing its vacuum. A sputter shield, usually, made of stainless steel, is placed between the contacts and the envelope in order to prevent the metal vapour reaching the envelope as it reduces the breakdown strength between the contacts. Inside the sputter shield the breaker has two contacts, one fixed and the other moving contact.
The moving contact moves through a short distance of 5 to 10 mm depending upon the operating voltage. The metallic bellows made of stainless steel is used to move the lower contact. The design of the bellows is very important because the life of the vacuum breaker depends upon the ability of this component to perform repeated operations satisfactorily. The periphery of the end-cap is sealed to the envelope and the fixed contact stem is an integral part of one end-cap. One end of the fixed as well as moving contact is brought out of the chamber for external connections.
2. Operating Mechanism of Vacuum Circuit Breaker:
The lower end of the breaker is fixed to a spring-operated or solenoid operated mechanism so that the metallic bellows inside the chamber are moved upward and downward during closing and opening operations respectively. The contact movement should be such as to avoid bounce. It is noteworthy that the operating mechanism should provide sufficient pressure for a good connection between the contacts.
ADVERTISEMENTS:
The pressure in a vacuum interrupter at the time of sealing off is kept about 10–6 torr. The interrupting rating is between 250 and 1,000 MVA. The normal current carrying capacity for a single interrupter is 800 – 3,000 A; 4.2 kV – 7.6 kV.
Vacuum Medium:
During contact separation of circuit breaker arc is formed due to ionization of particles in the medium between the contacts. Idea behind the vacuum circuit breaker is to eliminate the medium between the contacts i.e., vacuum. Every medium that has a pressure below atmospheric, which is 760 mm of Hg, is known as vacuum. Low pressures are measured in torr, where 1 torr = 1 mm of Hg. The pressures below about 10–5 torr are considered to be high vacuum.
The breakdown voltage of certain contact gap varies with the absolute pressure in the vacuum interrupters. When vacuum is used to serve the purpose of insulation, it implies that pressure of gas at which breakdown voltage becomes independent of pressure. Such a condition is believed to be realized at values of pressures from 10–4 to 10–6 torr.
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Vacuum Arc:
If an arc is to be formed in vacuum it is essential that the pressure should be low because only then it approaches the ideal arc. The ideal arc should have a low pressure, when it is initiated during its burning period and after its extinction.
Arc formed in vacuum is of different type from those formed in other mediums as it is formed by the neutral atoms, ions and electrons emitted from the electrodes themselves. The cathode surface normally, is not perfectly smooth but have many micro-projections. During the separation of contacts the current will be concentrated in these micro-projections as they are last points of contact. Due to their small area of cross section the projections will suffer explosive evaporation by resistive heating and supply sufficient quantity of vapour for the arc formation.
In vacuum arc the electron emission occurs only at the cathode spots and not from the entire surface of the cathode. For this reason the vacuum arc is also known as the cold cathode arc.
ADVERTISEMENTS:
In cold cathode the emission of electrons could be due to any of the combinations of the following mechanisms:
(i) Field emission;
(ii) Thermionic emission;
(iii) Field and thermionic emission;
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(iv) Secondary emission by positive ion bombardment;
(v) Secondary emission by photons, and
(vi) Pinch effect emission.
At high currents the ionized metal vapour spreads through quite a large volume surrounding the electrodes. At low currents the quantity of vapour produced is greatly reduced. Due to rapid expansion of vapour evolved from the cathode spots in vacuum, the probability of maintaining a charge carrier density sufficient to retain adequate conductivity in the column and sustain the emission process becomes increasingly small. At current zero the cathode spots extinguish in times of the order of 10–8 second.
Vacuum Arc Stability:
In a 50 Hz ac circuit the current passes through zero value 100 times in a second (i.e., once every 10 ms). It is desirable that the current is interrupted when it is passing through zero value, otherwise overvoltage will be induced due to current chopping. Therefore, for successful arc interruption it is necessary that the arc be stable for half-cycle duration and particularly that it continues to exist at currents approaching zero.
The arc stability is found to depend upon:
(i) The contact material and its vapour pressure and
(ii) Circuit parameters such as voltage, current, capacitance and inductance.
In low current circuits most of the evaporation takes place at discrete points, called the cathode spots, while at higher currents the evaporation takes place from cathode and anode spots. In addition to these sources gas is added to the enclosure of the contacts when it is stripped from other parts of the enclosure because of high temperature and impinging metal vapour.
Vapour pressure and arc stability are found to be interrelated. The higher the vapour pressure at lower temperature, the better is the stability of the arc i.e., the longer the life time of the arc. There are certain metals like Zn, Bi, which provide such characteristics and are better electrode materials for vacuum circuit breakers. Besides the vapour pressure, the thermal conductivity of the metal also affects the current chopping level.
A good heat conducting metal will cool very rapidly and its contact surface temperature will fall. This will reduce the evaporation rate and the arc will be chopped because of insufficient vapour. On the other hand, a bad heat conductor will maintain its temperature and vaporization for a longer time and the arc will be more stable.
Shunting the contacts with different values of capacitance reduces the average life time of the arc. The higher the value of the capacitance, the lower is the average life time of the arc. An inductance in series, on the other hand increases the lifetime of the arc. Similarly, higher the system operating voltage, longer is the duration of the arc. This is because more restoring voltage is available to keep the arc alive.
Vacuum Breakdown:
The major insulating media are atmospheric air, oil, paper and porcelain. These can withstand appreciable voltage but they are very small as compared to the withstand voltage of a gap in vacuum.
In the vacuum arc the neutral atoms, ions and electrons do not come from the medium in which the arc is drawn but they are obtained from the electrodes themselves by evaporating its surface material. Because of the large mean free path for the electrons, the dielectric strength of the vacuum is a thousand times more than when the gas is employed for arc interruption.
In the vacuum of range from 10–4 to 10–6 torr, the breakdown strength is independent of the gas density, but varies with gap length only. The actual value of breakdown voltage for a given gap depends upon the condition of electrode surfaces. Highly polished and thoroughly degassed electrodes have shown particularly high breakdown voltages. Contacts get roughened after use and thus the dielectric strength decreases which can be improved by applying successive high voltage impulse sparks.
This does not alter the arc roughened surfaces but removes the loosely adhering metal particles from the electrodes deposited there by the vapour blast during arcing. It has been found that for a vacuum of 10–6 torr some of the metals like silver, bismuth, copper etc. attain their maximum breakdown strength when the gap is slightly less than 3 mm. This property permits the use of short gaps in vacuum switches which results in simplifying the operating mechanism and increasing the speed of operation over conventional switches.
Arc Extinction in Vacuum Interrupters:
The arc interruption process in vacuum interrupters is quite different from that in other types of circuit breakers. The vacuum as such is a dielectric medium and arc cannot persist in ideal vacuum.
However, separation of current carrying contacts causes the vapour to be released from the contacts giving rise to plasma. Thus on separation of current carrying contacts, the contact space is filled with vapour of positive ions liberated from the contact material; the vapour density depending on the current in the arcing. During the decreasing mode of current wave the rate of release of the vapour falls and after the current zero, the medium regains its dielectric strength, provided vapour density around the contacts has substantially reduced.
While interrupting a current of the order of a few hundred amperes by separating the flat contacts under high vacuum, the arc usually has several parallel paths and each arc path originates and sinks in a hot spot of current. Thus the total current is divided in several parallel arcs. The parallel arcs repel each other so that the arc tends to spread over the contact surface. Such an arc is called the diffused arc. The diffused arc can get interrupted easily.
At higher values of currents of the order of a few thousand amperes, the arc gets concentrated on a small region and becomes self-sustained arc. The concentrated arc around a small area causes rapid vaporisation of the contact surface. The transition from diffused arc to the concentrated arc depends upon the material and shape of contact, the value of current and the condition of electrodes. The interruption of arc is possible when vapour density varies in phase with the current and the arc remains in the diffused state. The arc does not restrike again if the metal vapour is quickly removed from the contact zone.
Thus the arc-extinction process in a vacuum circuit breaker is related to a great extent to the material and shape of the contacts and the technique used in condensing the metal vapour. The contact geometry is so designed that the arc root keeps on moving so that the temperature at one point on the contact does not attain a very high value.
The rapid building up of dielectric strength after final arc extinction is a unique advantage of vacuum circuit breaker. There are limitations on oil circuit breakers in switching capacitive currents. In oil circuit breakers after an interruption takes place, the dielectric strength builds up so slowly that delayed restrikes take place often as much as half a cycle later. The vacuum switch is free from this trouble since after half a cycle, corresponding to 2.5 mm contact separation, a dielectric strength of the order of 100 kV (rms) is available. This type of switch has the advantage of restrike-free performance and is ideally suited to capacitor switching. Its operation is also practically independent of contact parting time.
Current Chopping in Vacuum Circuit Breakers:
We have seen that current chopping in air and oil circuit breakers occurs because of instability in the arc column whereas in case of vacuum circuit breakers it depends upon vapour pressure and the electron emission properties of the contact material. The chopping level is also influenced by the thermal conductivity-lower the thermal conductivity, lower is the chopping level. It is possible to reduce the current level at which chopping occurs by selecting a contact material which gives out sufficient metal vapour to allow the current to come to a very low value or zero value, but this is rarely done as it affects the dielectric strength adversely.
Vacuum Arc Recovery Phenomenon:
High vacuum possesses extremely high dialectic strength. At current zero the cathode spot extinguishes within 10–8 second and after this the original dielectric strength is established very soon. This quick return of high dielectric strength is, of course, due to the fact that the vaporized metal which is localized between the contacts, diffuses rapidly due to absence of gas molecules. The metal molecules are blown at high speeds to the glass walls and condense there. After arc interruption the recovery strength during the first few microseconds is 1 kV/µs for an arc current of 100 A, as compared with 50 V/µs in case of air gap.
Because of the attributes of vacuum interrupters, vacuum circuit breakers can be employed without reservations for fault clearing in any location on a system. They are capable of handling the severe recovery transients associated with short line faults or faults close to a transformer without any difficulty. All forms of load switching can be performed with the same ease.
3. Contact Materials for Vacuum Circuit Breakers:
The contact material for vacuum circuit breakers should have the following properties:
1. High electrical conductivity so as to pass normal load currents without overheating.
2. Low contact resistance.
3. High thermal conductivity so as to dissipate rapidly the large heat generated during arcing.
4. High cold and hot hardness to prevent wear and tear during normal opening and closing operations.
5. High density.
6. Sufficiently low vapour pressure so as to reduce the amount of inseparable metal vapour in the chamber.
7. High thermionic function to enable early arc extinction.
8. Low tendency to weld.
9. High boiling point to reduce arc erosion.
10. Low gas content (1 part in 107 or lesser) to ensure longer service life.
11. Low current chopping level.
12. High arc withstand ability.
13. Sufficient mechanical strength to retain structural integrity under high voltage gradients at the surface.
14. The material should preferably not have a surface film. If the film cannot be avoided then it should be conducting.
15. The heat of vapourization should be such that:
(i) Arc energy generates enough metal vapour to sustain arcing till the first natural current zero, and
(ii) There is insufficient vapour present to cause re-ignition after first current zero.
16. Easy to manufacture and economical.
From the above it becomes clear that a number of conflicting requirements have to be satisfied by a material to be suitable for electrodes. No single metal gives all the desirable properties. A high vapour pressure and low conductivity metal is more desirable to limit the current chopping whereas low vapour pressure metals are more desirable from the arc extinction point of view.
Materials having high boiling and melting points have low vapour pressure at high temperatures but are poor conductors whereas metals having low boiling and melting points have high vapour pressure at high temperatures, low electron functions and have good thermal and electrical conductivities.
Therefore, to combine these contradictory properties in one single material composite of two or more metals or a metal and a nonmetal have to be made. Copper-bismuth, copper-lead, copper-tellurium, copper-thallium, silver-bismuth, silver-lead, and silver-tellurium are some of the alloys used as contact materials.
4. Properties of Vacuum Circuit Breakers:
High vacuum has two outstanding properties:
1. Highest Insulating Strength:
In comparison to various other insulating media in use in circuit breakers, vacuum is a superior dielectric medium. It is better than all other media except air and SF6 gas which are generally employed at high pressure.
2. When an ac circuit is opened by separating the contacts in a vacuum, interruption occurs at the first current zero with the dielectric strength across the contacts building up at a rate thousands of times higher than that obtained with other circuit breakers. This is because with the increase in gap due to separation of contacts and movement of breaker contacts, the breakdown kV peak increases. The breakdown voltage of vacuum compared with air for one pair of 9.4 mm diameter tungsten contacts are given in Table 10.1.
The above two properties obviously make the vacuum circuit breakers more efficient, less bulky and cheaper in cost. The service life also is much greater than that of conventional circuit breakers and almost no maintenance is required. Vacuum breakers are ideally suited for most duties encountered in typical electric utility and industrial applications. Their voltage and interrupting rating is such that with little modifications, they can be made to perform specific switching duties on hv ac systems.
5. Applications of Vacuum Circuit Breakers:
1. Because of the short gap and excellent recovery characteristics of vacuum breakers, they are very useful as very high speed making switches in many industrial applications.
2. There are many applications where a simple load-break switch is not enough and at the same time the devices employed should not be costly. They include shunt reactor switching, transformer switching, line dropping, capacitor bank switching. These applications give a fast RRRV and vacuum circuit breakers are the best solutions.
3. Where voltages are high and current to be interrupted is low, these breakers have definite superiority over the oil or air circuit breakers.
4. For low fault interrupting capacities the cost is low in comparison to other interrupting devices.
5. The vacuum switches can be employed for capacitor switching which is a very difficult task for oil circuit breakers.
6. These can be used along with static overcurrent relays and given an overall clearance time of less than 40 ms on phase-to-phase faults.
7. Because of least requirements of maintenance these breakers are very suitable in a country like India, with 11 to 33 kV network extending into vast rural complex. Even with limited MVA rating of say 60 to 100 MVA, it should be suitable for a majority of applications in rural areas.
6. Advantages of Vacuum Circuit Breakers:
1. Vacuum circuit breaker is a self-contained and does not require filling of oil or gas. They do not need auxiliary air system, oil handling system etc. No need of periodic refilling.
2. Rapid recovery of very high dielectric strength on current interruption so that only half cycle or less arcing occurs after proper contact separation.
3. Current interruption occurs at the first current zero after contact separation with no re-striking, making it exceptionally good for capacitor and cable switching and long line dropping.
4. Very high power frequency and impulse withstand voltages with small contact spacing, allowing ease of actuation and timing.
5. No emission of gases—pollution free.
6. Non-explosive and silent operation.
7. Breaker unit is compact and self-contained. It can be installed at any required orientation.
8. Larger number of operations on load; or short circuit is suitable for repeated operating duty and long life.
9. Usable on any voltage up to 230 kV and higher where exceptionally long life and maintenance free operation is desired.
10. There are no gas decomposition products in vacuum and hermetically sealed vacuum interrupter keeps out all environmental effects.
11. Constant contact resistance. In vacuum the contacts cannot oxidise, therefore, their very small resistance is maintained throughout their life.
12. High total current switched. Because of small erosion of contact piece, rated normal current can be interrupted up to 30,000 times and rated short-circuit rupturing current on average a hundred times.
Because of the above reasons together with the economic advantages offered, vacuum circuit breakers have high acceptance.
7. Disadvantages of Vacuum Circuit Breakers:
1. Requirement of high technology for production of vacuum interrupters.
2. The vacuum interrupter is costlier than the interrupting devices in other types of circuit breakers and its cost is affected by production volume. It is uneconomical to manufacture vacuum interrupters in small quantities.
3. Rated voltage of single interrupter is limited to about 36/√3 i.e., 20 kV. Above 36 kV, two interrupters are required to be connected in series. Thus the vacuum circuit breaker becomes uneconomical for rated voltages exceeding 36 kV.
4. Loss of vacuum, due to transit damage or failure, makes the entire interrupter useless and it cannot be repaired at site.
5. It needs additional surge suppressors in parallel with each phase for interruption of low magnetising currents in a certain range.