- Circuit breakers are critical to the safe operation of an electrical grid.They are needed in electricity generators, where the full power of an entire power plant (gigawatts of electricity) must be switched on and off, and on transmission lines in substations to direct the power flow at voltages in excess of 1500 kV. Circuit breakers are also critical components in distribution grids, where very high currents need to be managed at moderate voltage levels.
- A circuit breaker, irrespective of its position in a grid has two tasks: it is responsible for the daily switching of lines during normal operation, and for the disconnection of the power supply in case of overload or short circuit. Several GVA of power can be tamed by a circuit breaker within fractions of a second.
A circuit breaker should be capable of opening on the occurrence of a fault and clearing the fault. It should be closed on to a fault. It should be capable of carrying fault current for a short time while another circuit breaker is clearing the fault.
A circuit-breaker rated at In = 125 A for an ambient temperature of 40°C will be equipped with a suitably calibrated overcurrent tripping relay (set at 125 A). The same circuit-breaker can be used at higher values of ambient temperature, however, if suitably “derated”. Thus, the circuit-breaker in an ambient temperature of 50°C could carry only 117 A indefinitely, or again, only 109 A at 60°C, while complying with the specified temperature limit.
Derating a circuit-breaker is achieved, therefore, by reducing the trip-current setting of its overload relay, and marking the CB accordingly. The use of an electronic-type of tripping unit, designed to withstand high temperatures, allows circuit-breakers (derated as described) to operate at 60°C (or even at 70°C) ambient.
Restriking voltage :
Where V = Restriking voltage
Vm = Peak voltage at the instant of are interruption
L = Inductance per phase of the system
C = Capacitance per phase of the system
Natural frequency of oscillation:
Rate of Rising of Restriking Voltage (RRRV)
RRRV is maximum
Frequency of damped oscillation,
If is known as critical resistance
If there will be oscillation
If there will be no transient oscillation
The symmetrical breaking current of a circuit breaker is the current which it will interrupt at a power factor of 0.15 for railing up to 500 MVA and a power factor of 0.3 for 750 MVA with a recovery voltage of 95% of normal voltage.
Asymmetrical breaking current of a circuit breaker is the current which it will interrupt when there is symmetrical component, its peak value let then asymmetrical breaking current (I) may be calculated as
The (peak) making current of a circuit breaker is the peak value of maximum current in the first cycle of current after the current is closed by the current breaker.
Rated (peak) making current = 2.55 × rated asymmetrical breaking capacity.
Short Time Operated Duty
Any Circuit breaker must be capable of the following duty
B – 3’ – MB – 3’ – MB
Where B = breaking operations
MB = making operations
3’ = 3 = min time interval
Types of Circuit Breaker
- Oil circuit breaker
- Minimum oil circuit breaker
- Bulk oil circuit breaker
- Air blast circuit breaker
- Sulphur hexa fluoride circuit breaker
- Vacuum circuit breaker
Electric Arc Model for SF6 Circuit Breaker
The characteristics of vacuum as medium and cost of the vacuum CB does not makes it suitable for voltage exceeding 38 kV.These days for higher transmission voltage levels SF6 Circuit Breakers are largely used. OCB and ABCB have almost become obsolete. In fact, in many installations, SF6 CB is used for lower voltages like 11 kV, 6 kV etc..
Sulphur Hexafluoride symbolically written as SF6 is a gas which satisfies the requirements of an ideal arc interrupting medium. So SF6 is extensively used these days as an arc interrupting medium in circuit breakers ranging from 3 kV upto 765 kV class. In addition to this SF6 is used in many electrical equipments for insulation. Here first we discuss in brief, some of the essential properties of SF6 which is the reason of its extensive use in circuit breakers
- SF6 gas has high dielectric strength which is the most important quality of a material for use in electrical equipments and in particular for breaker it is one of the most desired properties. Moreover, it has high Rate of Rising of dielectric strength after arc extinction. This characteristic is very much sought for a circuit breaker to avoid Restriking.
- SF6 is colorless, odorless and non-toxic gas.
- SF6 is an inert gas. So in normal operating condition, the metallic parts in contact with the gas are not corroded. This ensures the life of the breaker and reduces the need for maintenance.
- The characteristics of vacuum as medium and cost of the vacuum CB does not makes it suitable for voltage exceeding 38 kV.
- SF6 has high thermal conductivity which means the heat dissipation capacity is more. This implies greater current carrying capacity when surrounded by SF6 .
- The gas is quite stable. However, it disintegrates to other fluorides of Sulphur in the presence of arc. but after the extinction of the arc, the SF6 gas is reformed from the decomposition.
- SF6 being non-flammable so there is no risk of fire hazard and explosion.
(Transient Recovery Voltage) TRV is the voltage across the opening contacts of a fault-interrupting circuit breaker (CB) immediately after the electric arc is extinguished For calculation of the initial part of TRV, the modeling of the arc resistance in a SF6 CB is important, because it has a significant impact on the TRV. Black box models use a mathematical description of the electrical behavior of an electrical arc. These types of models do not give full representation of the physical processes taking place inside the CB. Recorded voltage and current traces during the "thermal period" are used to obtain the CB parameters that are later on substituted in differential equations.
Vacuum Circuit Breaker
- The vacuum circuit breaker is mainly restricted to system voltage below 38 kV.
- In a Vacuum circuit breaker, vacuum interrupters are used for breaking and making load and fault currents. When the contacts in vacuum interrupter separate, the current to be interrupted initiate a metal vapour arc discharge and flows through the plasma until the next current zero.
- The arc is then extinguished and the conductive metal vapour condenses on the metal surfaces within a matter of microseconds. As a result, the dielectric strength in the breaker builds up very rapidly.
- The properties of a vacuum interrupter depend largely on the material and form of the contacts. Over the period of their development, various types of contact material have been used. At the moment it is accepted that an oxygen-free copper chromium alloy is the best material for High voltage circuit breaker.
- In this alloy, chromium is distributed through copper in the form of fine grains. This material combines good arc extinguishing characteristic with a reduced tendency to contact welding and low chopping current when switching inductive current. The use of this special material is that the current chopping is limited to 4 to 5 Amps.
Medium Voltage Oil Circuit Breakers
- The following suggestions are for use in conjunction with the manufacturer’s instruction books for the maintenance of medium-voltage oil circuit breakers:
- Check the condition, alignment, and adjustment of the contacts.Thoroughly clean the tank and other parts which have been in con tact with the oil.
- Test the dielectric strength of the oil and filter or replace the oil if the dielectric strength is less than 22 kV. The oil should be filtered or replaced whenever a visual inspection shows an excessive amount of carbon, even if the dielectric strength is satisfactory.
- Check breaker and operating mechanisms for loose hardware and missing or broken cotter pins, retain ing rings, etc.
- Before replacing the tank, check to see there is no friction or binding that would hinder the breaker’s operation. Also, check the electrical operation.
- Avoid operating the breaker any more than necessary without oil in the tank as it is designed to operate in oil and mechanical damage can result from excessive operation without it.When replacing the tank and refilling it with oil, be sure the gaskets are undamaged and all nuts and valves are tightened properly to prevent leak age.
Air Blast Circuit Breaker
The other type of circuit breaker that we discuss here is Air Blast Circuit Breaker(ABCB). This type of breakers is also becoming obsolete. Once Air Blast type of breakers was preferred in Extra High Voltage substations. Now it is difficult to find new HV/EHV substations equipped with Air Blast Circuit Breakers.
Note: One should not be confused between Air Circuit Breaker and Air Blast Circuit Breaker.
- Air Circuit Breakers are usually used in low voltage applications below 450 volts. You can today find these in Distribution Panels (below 450 volts).
- Air Blast Circuit Breakers are high capacity breakers and can be seen in old substations mainly above 132 kV. The working principle of these two circuit breakers is quite different. Here we will only discuss the working of ABCB.
- In Air Blast Circuit Breaker, air at high pressure is blast upon the arc formed between the contacts. The air blast blows away the ionized air between the contacts.
See the Sketches below illustrating the arc extinction process of the axial blast type breaker.
Advantages of the Air Blast Circuit Breaker(ABCB)
- Arc extinction is very fast. Hence it is suitable for frequent opening and closing operation.
- Due to refilling of separated contacts space by fresh air at high pressure, the separation requirement between the contacts is quite less in comparison to OCB. This makes the size of the breaker smaller.
- The ionized gas flushed out to the atmosphere. Hence unlike OCB here the arc quenching medium does not deteriorate with time. This eliminates some maintenance burden.
- It is non-inflammable.
- Finally, one important advantage is that in ABCB the arc quenching depends on the high-pressure air which is obtained from a compressor, an external source. So in case of ABCB, the arc extinction or arcing time does not depend upon the arc current. (In case of OCB the arcing time depends on the current to be interrupted).
- The breaker breaking capacity depends upon the external source, the high-pressure air.
Minimum Oil Circuit Breaker
The simplified constructional diagram of a Minimum Oil Circuit Breaker (MOCB) is shown in the figure. It consists of two oil-filled chambers namely upper chamber and lower chamber, which are separated from each other.
The arc extinction process is carried out in the upper chamber. So, it is called as an arc extinction chamber or current interruption chamber of Minimum Oil Circuit Breaker (MOCB).This chamber houses an arc control device, an upper fixed contact, and a ring-shaped lower fixed contact.
The arc control device is fitted to the upper to the upper fixed contact.The moving contact slides through the lower fixed contact such that a physical (or electrical) maintained between them. The entire assembly of upper fixed contact. lower fixed contact and arc control device is enclosed in a glass fiber enclosure which is surrounded by oil.
Applications of Minimum Oil Circuit Breaker
- For indoor applications, minimum oil circuit breakers can be used up to 36 kV.
- For outdoor applications, minimum oil circuit breakers can be employed upto the line voltages of 220 kV.