Circuit breakers

Circuit breakers

Circuit breakers are a switching equipment in the electrical system, with manual trip levers or automatic control. Circuit breakers are designed to switch off the electrical circuits in case of faults and overloads and to switch it back on for normal operation.

The main function of this equipment is arc quenching of alternating current in electric generators with the help of insulating oil.

Circuit breakers have 2 designs: bulk oil (also known as dead tank design) and minimum oil circuit breakers.

The dead tank circuit breakers with voltage range up to 20 kV have a single-tank with three poles; and 35-220 kV have three-tanks – with a pole in each tank. These designs include one or three magnetic- hydraulic drives.

The dead tank design circuit breakers with voltage up to 35 kV are equipped with built-in current transformers, and up to 110 kV are built with voltage transformers.

The oil  in the tanks of circuit breakers extinguishes electric arcs and serves as an insulating material.

Unlike dead tank design, minimum oil circuit breakers use oil only as arc extinguishing medium, while insulation is provided by ceramic insulators, textolite, etc.

Also, minimum oil circuit breakers have smaller dimensions; they are less explosive and fire-resistant compared to dead tank  circuit breakers.

These circuit breakers include 2 wire conductors:  main (more than one current carrying conductors) and arc interrupters.

Advantages and disadvantages

The main advantages of circuit breakers are: their design and high efficiency regardless of weather conditions.

Disadvantages are in large volumes of insulating oil, especially for a dead tank design. It significantly increases the overall dimensions of circuit breakers, increases their explosiveness and fire hazard

Principle of operation

The main contacts of circuit breaker are immersed in insulating oil. The oil acts as an arc quenching media. The arc forms between the contacts when they separate. When the distance between the contacts increases, the arc goes out and the current flow stops. The physics of this phenomenon is as follows.When a certain distance between the contacts is reached, the current goes through the zero value, the voltage decreases and is insufficient for breakdown of the gas gap between the contacts, the electric arc is broken and the shutdown process ends. Also, the electric arc is extinguished by the gases released due to partial decomposition of the oil in the region of arc formation.

Transformer oil for circuit breakers

Preparation of transformer oil for circuit breakers goes in two stages.

Before filling the tank of a circuit breaker, the oil  is tested. It is tested for dielectric strength, mechanical impurities and coal. The follow up tests should be made as part of maintenance, to determine the breakdown voltage, acid number, the presence of mechanical impurities and coal and flash point. The samples are taken  from the valves provided in the lower part of the tank. The sampling container is rinsed and the oil is taken to the laboratory for full inspection of physical and chemical properties.

According to the established standards, the main characteristics of oil filled in circuit breakers:

  • breakdown voltage of oil, depending on the operating voltage of circuit breakers:
  • up to 15 kV – 25 kV;
  • 15 – 35 kV – 30 kV;
  • 60-150 kV – 55 kV;
  • 220 kV-60 kV.
  • acid number mg KOH/oil below 0.2 mg;
  • flash point  more 135 oC;
  • moisture content  below 10 g / t.

Oil purification equipment for circuit breakers

If oil tests showed a discrepancy from the established norms, the used oil could be  utilized or cleaned with the help of special equipment. A CMM-0,6 filter cart for  industrial oil cleaning, filtration and regeneration from GlobeCore – is a multitasking equipment. It has a capacity of 0.6 m3/hour (with a filtration fineness of 5 μm) providing a breakdown voltage  less 60 kV and  ISO 4406 purity class – / 14/12.

CMM-0,6 from GlobeCore  is mobile and compact (length – 780 mm, width – 570 mm, height – 1510 mm) and can be delivered to the site of operation of power equipment.

The GlobeCore design department is always ready to account for customer’s requirements for the hardware components, to commission on site, and also to provide instruction on operation.