Circuit Breakers

From Open Electrical
Jump to: navigation, search

Circuit breakers

IEC Circuit Breaker Symbol

Electrical circuit breaker is a switching device which can be operated both manually and automatically for controlling and protection of any electrical power system. As the modern power system deals with huge currents, the spacial attention should be given during designing of circuit breaker to safe interruption of arc produced during the opening/closing operation of circuit breaker.

According to their arc quenching (rapid cooling) media the circuit breaker can be divided as:

  • 1) Air circuit breaker
  • 2) Oil circuit breaker
  • 3) Vacuum circuit breaker
  • 4) SF6 circuit breaker

According to their services the circuit breaker can be divided as:

  • 1) Outdoor circuit breaker
  • 2) Indoor circuit breaker

According to the operating mechanism of circuit breaker they can be divided as:

  • 1) Spring operated circuit breaker
  • 2) Pneumatic circuit breaker
  • 3) Hydraulic circuit breaker

According to the voltage level of installation types of circuit breaker are referred as:

  • 1) High voltage circuit breaker (> 72 kV)
  • 2) Medium voltage circuit breaker (1-72 kV)
  • 3) Low voltage circuit breaker (< 1 kV)

Circuit breaker ratings according to the IEC 62271-100, IEC 62271-1 and IEC 60947-2 are explained on this page "Circuit Breaker Ratings".

Short summary for breakers:

  • Plain-break air breakers are used in low voltage and medium voltage up to 15 kV
  • For low and medium voltages fuses can be also used, but the main disadvantage is that they must be replaced after fault clearing
  • In medium voltage systems minimum oil, SF6 and vacuum breakers are also being used
  • For high voltages minimum oil, SF6 and blast-air breakers are used, but always with multiple interrupters in series
  • The maximum voltage per interrupter is 100 kV for air-blast and SF6 breakers, 170 kV for minimum oil breakers

Air circuit breakers (ACB)

The circuit breaker which operates in air at atmospheric pressure.

The working principle of this breaker is rather different from those in any other types of circuit breakers. The main aim of all kind of circuit breaker is to prevent the reestablishment of arcing after current zero by creating a situation where the contact gap will withstand the system recovery voltage. The air circuit breaker does the same but in different manner. For interrupting arc it creates an arc voltage in excess of the supply voltage. Arc voltage is defined as the minimum voltage required maintaining the arc. This circuit breaker increases the arc voltage by mainly three different ways:

  • It may increase the arc voltage by cooling the arc plasma. As the temperature of arc plasma is decreased, the mobility of the particle in arc plasma is reduced; hence more voltage gradient is required to maintain the arc.
  • It may increase the arc voltage by lengthening the arc path. As the length of arc path is increased, the resistance of the path is increased, and hence to maintain the same arc current more voltage is required to be applied across the arc path. That means arc voltage is increased.
  • Splitting up the arc into a number of series arcs also increases the arc voltage.

There are mainly two types of ACB available.

  • 1) Plain air circuit breaker
  • 2) Air-blast circuit breaker
Figure 1.a) ABB Low voltage air breaker, 600V/400A
Figure 1.b) WEG Low voltage air breaker, 400V/6300A
Figure 1.c) Air blast circuit breaker rated for 500 kV

Plain break

Air-break circuit breakers extinguish the arc by simply stretching it until the dielectric strength of the gap is larger than the voltage across the gap. The longer arc has a larger cooling surface and higher resistance, which decreases the current flow and the amount of heat created. To stretch the arc, horn gap shaped contacts are used. Due to a natural convection, the arc moves upwards. To further increase the length, the arc is stretched by forcing it into an arc chute made of metal barriers or insulating material. The metal barriers chop the arc into many smaller arcs. Used from 120 V up to 15 kV.

Figure 2.a) Air breaker - plain break
Figure 2.b) Air breaker - plain break, sketch

Air-blast break

In the air-blast breaker the arc is not stretched. To extinguish the arc, a blast of compressed air is directed into the arc path to cool the ionized gas and remove it from the gap between the contacts. The contacts are held closed by a spring. A blast of air into the interrupting head forces the contacts to open. The contacts will close as soon as the air flow stops. The compressed air can be blown into the arc perpendicular to it (cross blast), or along its axis (axial blast). All modern breakers use the axial blast. The air blast circuit breakers are built up to the highest used voltages (765 kV) by connecting several interrupter heads in series.

Figure 3.a) Axial air-blast breaker
Figure 3.b) Axial air-blast breaker
Figure 3.c) Cross air-blast breaker

Miniature circuit breakers

For miniature circuit breakers (or MCB) used in households refer to the Small air breakers for low voltage domestic circuits.

Oil circuit brakers (OCB)

Mineral oil has better insulating property than air. The oil is used to insulate between the phases and between the phases and the ground, and to extinguish the arc. When electric arc is drawn under oil, the arc vaporizes the oil and creates a large bubble of hydrogen that surrounds the arc. The oil surrounding the bubble conducts the heat away from the arc and thus also contributes to deionization and extinction of the arc. Disadvantage of the oil circuit breakers is the flammability of the oil, and the maintenance necessary (i.e. changing and purifying the oil). The oil circuit breaker is the one of the oldest type of circuit breakers.

Bulk oil circuit breakers (BOCB)

Bulk oil circuit breaker (or BOCB) is a such type of the circuit breakers where oil is used as arc quenching media as well as insulating media between current carrying contacts and earthed parts of the breaker. The oil used here is same as transformer insulating oil. These types of breakers are designed in all voltage ranges from 1 kV up to 330 kV.

Figure 4.a) 66 kV Oil Circuit Breaker
Figure 4.b) Bulk oil circuit breaker

Modern arc-controlled oil breakers have an arc control device surrounding the breaker contacts to improve extinction of arc. In cross blast interrupters, the arc is drawn in front of several lateral vents. The gas formed by the arc causes high pressure inside the arc control device. The arc is forced to bow into the lateral vents in the pot, which increases the length of the arc and shortens the interruption time. The axial blast interrupters use similar principle. Oil breakers are design for both three-phase and single-phase circuit brakers.

At voltages higher than 115 kV, separate tanks for each phase are used. The practical limit for the bulk oil breakers is 275 kV.

Minimum oil circuit brakers (MOCB)

In minimum oil breakers the oil is used only for extinguishing of the arc. The arc control devices are the same as for the bulk-oil breakers. However, unlike bulk oil circuit breakers, these designs place the interrupting units in insulating chambers at live potential. To improve breaker performance, oil is injected into the arc. The interrupter containers of the minimum oil breakers are made of insulating material and are insulated from the ground. This is usually referred to as live tank construction. For high voltages (above 132 kV), the interrupters are arranged in series. It is essential to ensure that each interrupter carries its share of the duty. Care must be taken that all breaks occur simultaneously, and that the restriking voltage is divided equally across the breaks during the interrupting process. The features of designing MOCB is to reduce requirement of oil, and hence these breaker are called minimum oil circuit breaker. These designs are available in voltages ranging from 1 kV to 765 kV using the multi-break technique.

Figure 5. 36 kV MOCB a) Typical design, b) Cross-section of the interrupting chamber

Vacuum circuit brakers (VCB)

Vacuum circuit breakers are used mostly for low and medium voltages. Vacuum interrupters are developed for up to 36 kV and can be connected in series for higher voltages. The interrupting chambers are made of porcelain and sealed. They cannot be open for maintenance, but life is expected to be about 20 years, provided that the vacuum is maintained. Because of the high dielectric strength of vacuum, the interrupters are small. The gap between the contacts is about 1 cm for 15 kV interrupters, 2 mm for 3 kV interrupters.

Figure 6.a) Vacuum interrupter
Figure 6.b) 12 kV, 40 kA Indoor vacuum circuit breaker

Service life of the VCB is much longer than other types of circuit breakers. There is no chance of fire hazard as oil circuit breaker. It is much environment friendly than SF6 circuit breaker.

Sulfur-hexafluoride (SF6) circuit breakers

Gas properties

Figure 7. SF6 gas molecule

Sulfur-hexafluoride (SF6) is an excellent gaseous dielectric for high voltage power applications. SF6 is a colorless non-toxic gas, with good thermal conductivity and density approximately five times that of air (6.14 kg/m3.). It does not react with materials commonly used in high voltage circuit breakers. It has been used extensively in high voltage circuit breakers and other switchgear employed by the power industry. Applications for SF6 include gas insulated transmission lines and gas insulated power distribution substations. The combined electrical, physical, chemical and thermal properties offer many advantages when used in power switchgear. Some of the outstanding properties of SF6 which make its use in power applications desirable are:

  • high dielectric strength
  • unique arc-quenching ability
  • excellent thermal stability
  • good thermal conductivity

The SF6 gas is identified as a greenhouse gas, safety regulation are being introduced in many countries in order to prevent its release into atmosphere.

Breaker properties

The principle of operation is similar to the air blast breakers, except that SF6 is not discharged in the atmosphere. A closed-circuit, sealed construction is used.

There are mainly three types of SF6 CB depending upon the voltage level of application:

  • 1) Single interrupter SF6 CB applied for up to 245 kV (220 kV) system
  • 2) Two interrupter SF6 CB applied for up to 420 kV (400 kV) system
  • 3) Four interrupter SF6 CB applied for up to 800 kV (715 kV) system

During the opening operation the gas contained inside a part of the breaker is compressed by a moving cylinder that supports the contacts or by a piston. This forces the SF6 through the interrupting nozzle. When the contacts separate, an arc is established. If the current is not very high, it is extinguished at the first zero crossing by the pushing the SF6 through the arc by the piston. If the short circuit current is high, the arc extinction may not occur at the first zero crossing, but the gas pressure will increase sufficiently to blow the arc out. By connecting several interrupting heads in series, SF6 breakers can be constructed for voltages of up to 765 kV.

Figure 8.a) SF6 CB scheme
Figure 8.b) 40.5 kV, SF6 circuit breaker

Gas Insulated Substation (GIS)

In GIS the high-voltage conductors, circuit breaker interrupters, switches, current transformers, voltage transformers and lightning arresters are encapsulated in SF6 gas inside grounded metal enclosures. Locations where gas insulated substation is preferred:

Large cities and towns
Under ground stations
Highly polluted and saline environment Indoor GIS occupies very little space
Substations and power stations located Off shore
Mountains and valley regions

Gas insulated substation has gas monitoring system. Gas inside each compartment should have a pressure of about 3 kg/cm2.The gas density in each compartment is monitored. If the pressure drops slightly, the gas is automatically trapped up. With further gas leakage, the low pressure alarm is sounded or automatic tripping or lock-out occurs.

Trivia