Circuit Breakers Hydro Circuit Protection

Circuit Breaker Ratings

1. Rated Voltage highest rms voltage for which the circuit breaker is designed and is the upper limit for continuous operation.

2. Rated Current the maximum rms current, which the breaker is capable of carrying continuously without exceeding the given temperature, rise at the given ambient temperature.

3. Rated Frequency frequency at which the breaker is designed to operate (60 Hz in North America)

4. Rated Interrupting current current at instant of contact separation. The interrupting current rating can be given as one of the following values

5. Symmetrical Interrupting Current rms value of the a.c. component of the short circuit current the breaker is capable to interrupt

6. Asymmetrical Interrupting Current rms value of the total short circuit current the breaker is capable to interrupt. This includes the dc and ac components.

7. Rated Making Current rms value of the short circuit current on which the breaker can safely close at the rated voltage.

8. Rated Making Current rms value of the short circuit current on which the breaker can safely close at the rated voltage.

9. Rated Short Time Current rms value of current that the circuit breaker can carry in a fully closed position without damage for a specified short time interval. Normally given for 1s or 4s. These ratings are based on thermal limitations.

10. Rated Interrupting Time maximum interval from the time the trip coil is energized until the arc is extinguished

11. Rated Impulse Withstand Voltage (Basic Insulation Level) maximum short duration impulse voltage that the breaker can withstand. BIL is tested with a prescribed shape and duration of the test impulse voltage. 12. K Factor (Voltage Range Factor) For most circuit breakers the rated interrupting current is independent of the operating voltage. For some breakers, mostly oil breakers, the rated interrupting current increases if the operating voltage is lowered down to a certain limit that is given by the K factor. This adjusted rated interrupting current is called Current Interrupting Capability (CIC).

CIC = Rated Interrupting Current x KProvided that

If

then

MEDIUM AND HIGH VOLTAGE CIRCUIT BREAKERS

Medium and High Voltage circuit breakers are used to control the flow of power in power systems and also as the disconnecting equipment when high faults occur on power systems. Circuit breakers then must be capable of performing switching operations on power systems under both, normal and short-circuit conditions. These are the requirements put on every circuit breaker:

it must be a perfect conductor in the closed position (Z = 0)

it must be a perfect insulator in the open position (Z = infinity)

it must be fast when closing. Current starts flowing before the contacts actually touch and slow closures could damage the contacts

it must be fast when opening but it must not extinguish current before its zero crossing and it must not produce overvoltages.

The most difficult operation that high voltage circuit breakers must perform is interruption of high short circuit currents. To illustrate the amount of energy that must be dissipated during breaker operation, let's consider breaker on 230 kV system with possible symmetrical fault current of 10 000 A rms. If the resistance of the arc is 1 , then the power dissipated by the arc is RI2 = 100 MW per phase. 230 kV breakers usually open in 3 cycles (50 ms), so the energy dissipated by the arc during opening is W = Pt = 5 MJ per phase. Compare this to energy of a moving object with a speed of 36 km/hr (10 m/s). If the object would have the same amount of energy, it would have to have a mass of m = 2W/v2 = 100 000 kg. A loaded 7 axle semi-trailer weighs about 50 000 kg. The per phase amount of energy dissipated by a breaker interrupting 10 kA of short circuit current is equivalent to stopping two loaded semi-trailers moving with a speed of 36 km/hr in 50 ms.

A Bit of History

The early distribution systems used low voltages. Power was generated by only several relatively small generators, and therefore the systems had a low short circuit capacity. Interruption of the circuit by using separation of contacts in air was sufficient, although this process drew arc and was damaging to the contacts of the switches. As the capacity of power systems grew, immediate and automatic interruption of short circuit current became necessary. Around 1900 first oil circuit breakers were built by simply immersing a switch in mineral oil. The mineral oil was held by a steel tank. Around 1925, there were made improvements to the plain break circuit breakers by providing arc and pressure control by enclosing the arcs inside arc pots. This method of arc control and interruption was widely employed until 1980's. and there are still many bulk oil breakers installed on North American power systems, although they are no longer manufactured in North America.

During 1920's air blast breakers were also developed. The design followed two diverging paths. One was to design a single break breaker for a high voltage (up to 110 kV); the other was to connect several lower voltage (about 35 kV) interrupters in series. Air blast breakers were installed on North American systems up to 1980's. There are still many existing breakers, but they are no longer manufactured in North America.

In early 1950's sulphur Hexafluoride (SF6) was introduced as an interrupting medium. The initial tendency was to use the design of air blast breakers and the SF6 gas was blown under high pressure into the arc. The latest design is towards lower pressure SF6 breakers (these are called puffer type).

Vacuum was seen as the ideal environment for extinguishing arc since around 1925, but at that time the technology was not sufficiently developed to manufacture reliable vacuum breakers. The main problem was in joining the metal bellows enabling motion of the moving contact, and the ceramic container enclosing the contacts and the arc during breaker opening. The loss of vacuum resulted in explosions, and then in a great reluctance to accept the improved vacuum breakers. The reliability problems and the acceptance of the breakers by utilities were finally resolved during 1980's. Vacuum breakers are now extensively used up to voltages of about 33 kV. They can be designed for high voltages as well, with several interrupters in series.

All commercially produced breakers share the following design features:

in the closed position the current flows through two metal contacts very tightly pressed together

in the open position there is a gap between the contacts. The gap must be sufficiently large to prevent any flashovers.

during the opening and closing operations, one contact is fixed, the other contact moves.

Classification of Circuit Breakers

Classification of circuit breakers in common use is done according to the medium that is used to interrupt the arc. Thus the breakers are classified as:

1. Air a) Plain break

b) air blast

2. Oil a) Bulk oil - plain break

- Arc controlled

b) Minimum oil

3. SF6 (sulphur hexafluoride)

a) Double pressure

b) Single pressure (puffer)

c) Thermal assist or Selfblast

4. Vacuum Air Breakers

HYPERLINK "oil_breakers.htm"Oil Breakers

HYPERLINK "SF6_breakers.htm"SF6 Breakers

HYPERLINK "vacuum_breakers.htm"Vacuum Breakers CB RatingsSF6 Circuit Breakers

Sulphur hexafluoride (SF6) gas is an alternative to air as an interrupting medium. SF6 is a colourless, non-toxic gas, with good thermal conductivity and density approximately five times that of air. SF6 is chemically inert up to temperature of 150 C and will not react with metals, plastics, and other materials commonly used in the construction of high voltage circuit breakers.

The principle of operation is similar to the air blast breakers, except that the SF6 gas is not discharged into the atmosphere. A closed circuit completely sealed and self-contained construction is used. The equipment consists of a compressor, a storage container, a blast valve that admits gas to the interrupting chamber, and a filter through which the exhaust gas is returned to the compressor. This is called the double pressure breaker design.

Figure 15 230 kV, 15 GVA, SF6 Double Pressure Breaker (Westinghouse)Improvement on the double pressure design is the puffer design, also sometimes called the single pressure design. SF6 gas is normally under constant pressure of about 5 ATM. During the opening operation the gas contained inside a part of the breaker is compressed by moving cylinder that supports the contacts or by a piston. This forces the SF6 through the interrupting nozzle.

An example of a SF6 puffer breaker is in Figure 16. When the contacts separate, an arc is established. If the current is not very high, it is extinguished at its first zero crossing by deionizing effects of SF6 and by the pushing the SF6 through the arc by the piston. The contact distance at this point is small and the pressure of the gas that goes through the arc is low. This feature is important because it prevents current chopping when interrupting small inductive currents. In the case of small capacitive currents, the maximum recovery voltage appears cycle after the arc extinction. This give the contacts sufficient time to reach a separation that will be able to withstand the voltage. 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.