Until recently, many medium voltage switch gears used large numbers of oil powered circuit breakers for medium voltage distribution systems.
Using oil to quench a circuit presented many problems. Flammability and high maintenance costs being two of the biggest disadvantages.
This prompted manufacturers to look for a different means of powering their high voltage circuit breaker. Magnetic and air blast circuit breakers were developed, but these also had several disadvantages.
The big problem with these was their bulkiness, making them cumbersome to use.
Further research led to the simultaneous development of two types of circuit breakers. One used SF6 gas as the quenching medium and the other used a vacuum.
In a vacuum circuit breaker, the load and fault currents are broken with vacuum interrupters. When the contacts in the vacuum interrupters separate, the overloaded current initiates the discharge of a metal vapour arc which flows through the plasma.
Within microseconds, the arc is extinguished and the conductive metal vapour condenses. This results in the dielectric strength in the breaker to build up.
The vacuum circuit breaker depends largely on the materials and the forms of contact. During development many different materials were tried. It was decided that the best material to use in the vacuum high voltage circuit breaker was chromium alloy copper.
The chromium in this alloy is distributed through the copper as fine grains. This special material in the vacuum circuit breaker limits current chopping to about 4 or 5 Amps.
At currents less than 10KA, the vacuum arc will burn as a diffuse discharge. At higher values the arc changes into a constricted form with an anode spot. If the constricted arc remains in one spot for too long the contacts can become overstressed to the point where deionization at the contact point is not guaranteed.
This problem is overcome by shaping the contacts so that the current flowing through will result in a magnetic field, at right angles to the axis of the arc.
This will cause the root of the arc to rotate around the contact rapidly, resulting in uniform heat distribution over the surface. This type of contact is called a radial magnetic electrode field and is used in most circuit breakers.
A new version of the vacuum interrupter switches the arc from a diffusion state to constricted by subjecting the arc to an axial magnetic field. This is accomplished by guiding the current of the arc through a coil fashioned outside of the vacuum chamber.
As an alternative, the field can be created by directing the contact along the required path. These contacts are most advantageous when the short circuit current exceeds 31 KA.
In a SF6 high voltage circuit breaker, the current keeps flowing through plasma ionized with SF6 gas after contact separation. As long as it keeps burning, the arc is fed by a constant flow of gas.
At current zero, the arc is extinguished. The continuous flow of gas is able to de-ionize the contact gap and establish the dielectric strength needed to prevent a re-strike.
Whether the gas flows across the arc axis or parallel to the axis influences the efficiency of interruption of the arc. Research suggests that an axial flow of gas will create turbulence causing an intense and continuing interaction between the gas and the plasma as the current nears zero.
In practice, cooling the arc cross-gas-flow is normally achieved by causing the arc to move within the stationary gas. However, this interruption process may lead the arc to become unstable, resulting in major fluctuations in its ability to interrupt the circuit.
A pressure differential along the arc is needed to achieve axially of gas flow. First generation SF6 circuit breakers employed a two-pressure principle like the air-blast circuit-breaker. It required a certain amount of gas to be stored at high pressure and then pushed out into the arcing chamber.
The high pressure gas and the accompanying compressor were eliminated in the design of the second generation. In this design, the differential pressure was created with a piston moving the contacts and compressing the gas inside a small cylinder as the contact opens.
The major disadvantage of this puffer system is that it requires a hefty operating mechanism.
Price-wise, these high voltage circuit breakers couldn’t compete with the oil circuit breakers. This led to further development with the goal of reducing the cost.
The type of high voltage circuit breaker you come across in your career as an electrician will largely depend on the company you work for. You may even be inspired to develop a brand new style altogether.
Most manufacturers recommend a complete internal and external inspection of high voltage circuit breakers about every six months – and no less than every twelve months.
However, this involves considerable labour costs and delays in production. Proper external inspections should be adequate, without sacrificing dependability.