There are several methods of classification of circuit breakers. On the basis of type of current they may be classified as ac circuit breakers and dc circuit breakers.
AC circuit breakers may be classified on the basis of rated voltages. Circuit breakers below rated voltage of 1,000 V are known as the low-voltage circuit breakers and above 1,000 V are called the high-voltage circuit breakers. However, the most general way of classification of the circuit breakers is on the basis of medium of arc extinction such as air-break circuit breakers/miniature circuit breakers, oil circuit breakers (tank type or bulk oil), minimum oil circuit breakers, air-blast circuit breakers, sulphur hexafluoride circuit breakers and vacuum circuit breakers. All high voltage circuit breakers may be classified under two main categories viz., oil circuit breakers and oil-less circuit breakers.
Oil circuit breakers utilize dielectric oil (transformer oil) for arc extinction. Oil circuit breakers can further be subdivided into two classes viz., bulk oil circuit breakers and low-oil content circuit breakers or minimum oil circuit breakers.
In bulk oil circuit breakers transformer oil, with which they are filled, is used to extinguish the arc during opening of the breaker contacts. The oil also serves as an insulator for the current conducting parts from one another and from the earthed tank. The ratings range from 25 MVA at 2.5 kV to 5,000 MVA at 230 kV.
ADVERTISEMENTS:
Various types of arrangements are available for indoor as well as outdoor working at different voltage levels. In low-oil content or minimum oil circuit breakers, oil is used for arc extinction by blast action, and is used chiefly for this function and not so much for insulating live parts from earth. In such breakers, the oil-filled arc enclosing chamber is arranged within the hollow of the porcelain insulator which insulates the live parts of the breaker from earth.
These circuit breakers can be employed for a voltage range of 33 kV to 220 kV and breaking capacities of 1,500 MVA to 7,500 MVA. Another type of oil circuit breaker is the oil impulse breaker. In this breaker, an arc-extinguishing oil jet is produced by a piston pump which is externally energised by means of a spring or compressed air.
The jet of oil is aimed at the gap formed between the separating contacts of the breaker to extinguish the arc. Oil- impulse breaker has many features and characteristics similar to those of the air-blast circuit breakers.
The main types of oil-less circuit breakers are:
ADVERTISEMENTS:
1. Water circuit breakers in which water is used for arc extinction.
2. Air circuit breakers in which the arc is initiated and extinguished in substantially static air in which the arc moves. Such breakers are used for low voltages, generally up to voltages of 15 kV and rupturing capacities of 500 MVA.
3. Air-blast circuit breakers in which a blast of air is used to blow out the arc. In modern air-blast circuit breakers, compressed air is stored in a tank and released through a nozzle to produce a high velocity jet, and this is used to extinguish the arc. Air blast circuit breakers are used for indoor services in the medium high voltage field and for medium rupturing capacity—generally up to voltages of 15 kV and capacities of 2,500 MVA. The air-blast circuit breakers are now employed in high voltage circuits in outdoor switchyards for 220 kV lines.
4. Sulphur hexafluoride circuit breakers in which SF6 under pressure is used to extinguish the arc. SF6 gas have excellent dielectric, arc quenching, chemical and other physical properties and has proved its superiority over other arc quenching mediums such as oil or air.
ADVERTISEMENTS:
5. Vacuum circuit breakers in which the fixed and moving contacts are housed inside a permanently sealed vacuum interrupter. The arc is quenched as the contacts are separated in high vacuum. Vacuum circuit breakers are more efficient, less bulky, cheaper in cost, negligible maintenance and longer life.
1. Oil Circuit Breakers:
These are the oldest type of circuit breakers. The separating contacts of the breakers are made to separate within an insulating oil, which has better insulating properties than air. On occurrence of fault as the breaker contacts open under oil, an arc is struck between them and the heat of the arc evaporates the surrounding oil and dissociates it into a substantial volume of gaseous hydrogen (hydrogen gas along with a small percentage of methane, ethylene and acetylene) at high pressure.
The pressure built-up and flow of gases is influenced by the design of arc-control device, speed of contact travel, the energy liberated by the arc etc. The oil is pushed away from the arc and an expanding hydrogen gas bubble surrounds the arc region and adjacent portion of the contacts.
The arc extinction is facilitated chiefly by two processes:
ADVERTISEMENTS:
Firstly, the hydrogen gas has high heat conductivity and cools the arc.
Secondly, the gas sets up turbulence in oil and forces it into the space between contacts after the final arc interruption at a current zero and thus arcing products from the arc path are eliminated. The result is that arc is extinguished and the circuit current is interrupted.
Oil circuit breakers have the virtues of reliability, simplicity and relative cheapness.
The oil circuit breakers can be divided into:
ADVERTISEMENTS:
1. Bulk oil circuit breakers using a large quantity of oil, also called the dead tank type, because the tank is held at ground potential. Such breakers are available in all classifications of voltage and interrupting rating for indoor and outdoor applications.
2. Low-oil circuit breakers which operate with a minimum amount of oil, so that is why sometimes called minimum oil circuit breakers or small-oil circuit breakers. These circuit breakers are also sometimes called the live tank circuit breakers because the oil tank is insulated from the ground.
The oil can be moved into arc zone after the current reaches zero by the following actions:
(i) By the pressure caused by the natural head of the oil,
(ii) By the pressure generated by the action of the arc itself,
(iii) By the pressure caused by external means.
Thus the oil circuit breakers may be classified as:
(i) Plain break oil circuit breakers.
(ii) Self blast or self-generated oil circuit breakers
(iii) Externally generated pressure oil circuit breakers or forced blast oil circuit breakers or impulse oil circuit breakers.
Oil, as an arc quenching medium, has the following advantages and disadvantages:
Advantages:
(i) Arc energy is absorbed in decomposing of oil.
(ii) The gas formed which is mainly hydrogen, has a high diffusion rate and high heat absorption in changing from the diatomic to monotomic state and thus provides good cooling properties.
(iii) The oil has a high dielectric strength and provides insulation between the contacts after the arc has been finally extinguished and there has been time for the oil to flow into the gap between contacts.
(iv) Cooling oil presents the cooling surface in close proximity to the arc.
(v) The oil used (such as transformer oil) is a very good insulator and allows smaller clearance between line conductors and earth components.
Disadvantages:
(i) Oil is inflammable and may cause fire hazards, if a defective oil circuit breaker should fail under pressure and cause an explosion.
(ii) There is a risk of formation of explosive mixture with air.
(iii) Due to decomposition of oil in the arc, the oil becomes polluted by carbon particles, which reduce its dielectric strength. Hence periodical maintenance and replacement are required.
Maintenance of Oil Circuit Breakers:
After a circuit breaker has interrupted short-circuit currents a few times or load currents several times, the contacts may get burnt due to arcing. In addition the dielectric oil gets carbonized in the vicinity of the contacts, thereby losing some of its dielectric strength. This results in the reduced breaking capacity of the breaker.
The maintenance work of oil circuit breaker therefore requires checking and replacement of contacts and oil. It is good practice to inspect the circuit breaker at regular intervals of 3 or 6 months. According to ISS 335-1963 oil in good condition should withstand 40 kV for one minute in a standard oil testing cup with 4 mm gap between the spherical electrodes.
During inspection of the oil circuit breaker it is recommended to check the following:
1. All current carrying parts be checked and arcing contacts be attended if necessary.
2. Dielectric strength, condition and level of oil should be checked. In case the dielectric strength is low or the oil is badly discoloured, replace it.
3. Inspect the insulation for possible damage. Clean the surface and remove deposits of carbon. Never use loose cotton waste for this purpose.
4. Check closing, tripping and interlock mechanism.
5. Check indicating devices and lamps.
6. Ensure, before closing the tank, that no tool have been left behind, that the tank linings and barriers are in position and secure, and that the tank gasket is in good condition.
2. Air-Break Circuit Breakers:
The arc interruption in oil is due to generation of hydrogen gas owing to oil decomposition. This fact led to study the arc interruption in air. No doubt, arc interruption properties of hydrogen are much superior to air, but air has several advantages, over the oil, as quenching medium.
These are:
1. Elimination of fire risk and maintenance associated with the use of oil.
2. Absence of mechanical stresses that are set up by gas pressure and oil movement.
3. Elimination of cost of regular oil replacement that arises due to deterioration of oil with successive breaking operation.
Relatively inferior arc extinguishing properties of air may be offset by using various principles of arc control and operating air at high pressures.
In the air-break circuit breaker the contact separation and arc extinction takes place in air at atmospheric pressure. High resistance principle is employed in such circuit breakers. The arc is rapidly lengthened by means of the arc runners and arc chutes and arc resistance is increased by cooling, lengthening and splitting the arc. The arc resistance is increased to such an extent that the voltage drop across the arc becomes more than the system voltage and the arc gets extinguished at current zero of ac wave.
Air-break circuit breakers are employed in dc circuits and ac circuits up to 12,000 volts. Such breakers are usually of indoor type and installed on vertical panels or indoor draw-out switchgear. AC circuit breakers are widely employed in indoor medium voltage and low voltage switchgear.
3. Air-Blast Circuit Breakers:
The drawbacks of the oil circuit breakers are the fire risk due to inflammable oil, the deterioration of the oil, necessitating periodic replacement, and the difficulty of reaching the contacts for maintenance purposes. This led to the development of circuit breakers using compressed air or gas as the interrupting medium. Though gases such as nitrogen, carbon dioxide, hydrogen or Freon can be used as the arc interrupting medium but compressed air is the accepted circuit-breaking medium for gas blast circuit breakers.
The reasons are given below:
Nitrogen has circuit breaking properties similar to compressed air and there is no added advantage of using it. Carbon dioxide has the drawback of its being difficult to control owing to freezing at valves and other restricted passages. No doubt hydrogen has increased breaking capacity but it is costlier and needs ancillary apparatus. Freon has high dielectric strength abd good arc extinguishing properties, but it is expensive and it is decomposed by the arc into acid forming elements.
4. Sulphur Hexafluoride (SF6) Circuit Breakers:
In circuit breakers (oil circuit breakers, air-break circuit breakers and air-blast circuit breakers) the extinguishing force builds up relatively slowly after the moment of contact separation, and hence the arc is usually extinguished after a few half cycles of current have passed zero. The prevention arc re- ignition needs a high dielectric strength of the arc path and its fast recovery after current zero.
In case of hv circuit breakers, these properties are particularly required to have quick arc extinction and have less time for quick recovery voltage build up. Vacuum circuit breakers and SF6 circuit breakers have better properties in this regard compared to conventional bulk oil, minimum oil as well as air-blast circuit breakers. Hence modern trend is to employ vacuum circuit breakers and SF6 circuit breakers in hv systems.
Oil, an obviously inflammable substance for extinguishing the hot arc, is a well proven medium because it releases hydrogen which by virtue of its low mass and high velocity is an excellent cooling medium. But modern circuit breakers employ heavy gas SF6 as the medium for-quenching the arc.
SF6 gas, because of its excellent dielectric, arc quenching, chemical and other physical properties, has proved its superiority over other mediums such as oil, or air for use in circuit breakers. Several types of SF6 circuit breakers have been developed by different manufacturers during last two decades for rated voltages 3.6 to 760 kV.
Before 1970’s in medium and high voltage range, air-break, bulk-oil, minimum-oil, air-blast circuit breakers were in use. During 1970’s vacuum circuit breakers were introduced for applications up to rated voltage of 36 kV. Single pressure puffer type SF6 breakers were introduced for rated voltages from 3.3 to 760 kV. Fault levels and rated voltages in the power system have increased. The bulk-oil breakers, minimum-oil breakers, air-blast circuit breakers are becoming obsolete now.
5. HVDC Circuit Breakers:
Light duty dc circuit breakers have been in use since long. However, there is lack of suitable circuit breakers for HVDC systems. At present most of the HVDC systems are with two terminals and in a two terminal HVDC system, HVDC circuit breakers are not required because fault current can be controlled or eliminated by controlling the firing angle of the converters. In multi-terminal HVDC systems, the need of HVDC circuit breakers will arise.
Problems of Direct Current Interruption:
AC circuit breaker easily interrupts the arc at natural current zero in the ac wave. At current zero, the energy (½Li2) to be interrupted is also zero. The contact gap has to cool and recover the dielectric strength to withstand natural transient recovery voltage. With dc circuit breakers, the problem is more complex as the dc waveform does not have natural current zeros. Forced arc interruption would produce high transient recovery voltage and restrike without arc interruption and ultimate destruction of the breaker contacts.
In designing of HVDC circuit breakers, there are three main problems to be overcome.
These are:
(i) Creation of artificial current zero
(ii) Prevention of restrikes and
(iii) Dissipation of stored energy.
The artificial current zero principle is used in HVDC circuit breakers for arc quenching. By introducing a parallel L-C circuit, the arc currents are subjected to oscillations. These oscillations are severe and have several artificial current zeros. The breaker extinguishes the arc at one of the artificial current zeros. The crest currents of the oscillation must be greater than the direct current to be interrupted. Figure 10.20 shows the schematic diagram of such a scheme.
A series resonant circuit with L and C is connected across the main contacts M of a conventional ac circuit breaker through an auxiliary contacts S1 and resistor R is connected through contacts Under normal operating conditions, main contact M and charging contacts St remain closed and the capacitor C is charged to line voltage through the high resistance R. Contacts S1 are open and have line voltage across them.
For interrupting main circuit current Id, the operating mechanism opens contacts S2 and closes contacts S1. This initiates discharge of capacitor C through inductance L, main contacts M and auxiliary contacts S, setting up an oscillatory current shown in Fig. 10.20 (b). Thus artificial current zeros are created and the circuit breaker main contacts M are opened at a current zero Z. Thereafter, contacts S1 are opened and contacts S2 are closed.
Another way of interrupting main circuit direct current is by its diversion to the capacitor so that the magnitude of current to be interrupted by the circuit breaker becomes smaller. This is illustrated in Fig. 10.21. The capacitor C is initially uncharged. When the main contacts M open, the main circuit current is diverted to the capacitor C. Thus the current to be interrupted by the main contacts M becomes smaller. The rate of rise of the recovery voltage across M is dVc/dt = Id/C. The nonlinear resistor R absorbs energy without greatly adding to the voltage across the main contacts M.
The problem of prevention of restrikes is more acute in oscillating current dc circuit breakers, where the time in which the current is chopped is very small (of the order 100 µs). Thus a steep surge of re-striking voltage across the breaker terminals is produced and the circuit breaker must be capable of withstanding this voltage.
For producing a good deionizing arc, the space between two walls of arc chute can be narrowed for restricting the arc and simultaneously it may be splitted into a number of smaller arcs by inserting a grating of vertical metal plates.
A large amount of energy stored in the circuit inductance at the start of the interruption and that supplied by the rectifier during the interruption duration has to be dissipated, otherwise it will be transferred to the system capacitance and set up over-voltages.
A protective spark gap may be connected across the circuit breaker so as to reduce the commutating capacitor size. It will also keep the abnormal voltage caused at the switching instant at desired level. By means of high frequency currents the spark gap acts as an energy dissipating device. Alternatively a Zno arrester may be connected across the breaker which will limit the transient recovery voltage and absorb associated energy.