The circuit breaker in which the contacts open and close in the SF6 medium is called the SF6 circuit breaker. These circuit breakers are available for all levels of voltages. Sf6 is also a suitable medium for metal-clad High Voltage substation called gas insulated substation. Single interruption sf6 circuit breaker is suitable for 245KV systems. Double interruption sf6 circuit breaker is suitable for 420KV systems. Four-interruption sf6 circuit breaker is suitable for 800KV systems.
Because the sf6 gas is a Greenhouse gas, it is strictly prohibited to use in many developed countries. But in developing countries like India, we can still use sf6 gas for insulation and arc quenching medium in the system. But it is mandatory to take serious care for minimum leakage of the gas from the devices during their service life. It is also strictly prohibited to release sf6 gas to the air due to any reason. The operating energy required for a puffer type designed sf6 circuit breaker is almost five times higher than the minimum oil circuit breaker of equivalent rating. This is the reason most of the high voltage sf6 circuit breakers use a hydraulic or pneumatic operating system.
In puffer type interrupter the arc energy itself develops the required pressure in the arc chamber. Since the puffer mechanism uses the energy developed from the arcing, the requirement of the overall operating energy of the circuit breaker gets reduced up to a significant extent. This is the reason we can use the spring operating mechanism for circuit breakers up to the 245KV system. After the introduction of vacuum circuit breakers, the use of sf6 circuit breakers in the medium voltage range is gradually decreasing.
First Generation SF6 Circuit Breaker
In this breaker, the sf6 gas is stored in a high-pressure stationary chamber. During the opening of the contacts of the circuit breaker, the compressed sf6 gas is allowed to blow to the interruption chamber for quenching the arc. Then it is collected into a low-pressure reservoir. Finally, it is pumped back to the high-pressure chamber for the next operation. The main disadvantage of this method is that there is always a chance of liquefication of SF6 gas in the high-pressure chamber at low temperatures. So, additional heating arrangement or heaters had to be provided for preventing the liquefication of the gas.
Modern SF6 Circuit Breaker
This is the single pressure principle. The interruption chamber is filled with sf6 gas at the rated pressure. The differential pressure for extinguishing the arc is generated during the movement of moving parts of the introduction mechanism. In the simplest form of the breaker both contacts are fixed. A hollow metallic cylinder bridges the contacts during the closed position of the sf6 circuit breaker. The cylindrical chamber filled with sf6 gas moves against a mechanically fixed piston during the opening of the contacts. As a result, the space inside the chamber is being reduced during the movement of the cylinder. Therefore the pressure of the sf6 gas inside increases. As soon as the metallic cylinder breaks the bridge between the contacts the arcing starts. At the same instant, the nozzles (holes) in the inner wall of the cylinder get free from obstruction hence get open and release sf6 gas at high pressure in between the contacts. Therefore it prevents the ionization of the arc. As a result of that ultimately the arc is quenched.
The arc is a very essential part of the operation of circuit breakers in a power system. The main design of the circuit breaker focuses on the phenomenon of arc quenching.
When two current-carrying contacts are physically separated from each other, there may be an arc appeared between them. The arc is an ionized medium or plasma through which the current continues to flow. Therefore after separation of the contacts in a circuit breaker, the current continues to flow to prevent current chopping phenomena in the system. The current chopping causes transient overvoltage in the system. The overvoltages stress the insulation, therefore, there would be every chance of insulation failure in the system. Therefore we can say the arc is not an undesirable thing for the operation of the system. In other words, arc plays an important role in the operations of the system.
An alternating current momentarily crosses zero at certain intervals depending on its frequency. During zero crossing of the current arc is squinched automatically. But after zero-crossing there is a regeneration of arc due to reionization of the medium between the contacts of the circuit breaker. The function of circuit breaker interrupters is mainly to prevent the regeneration of the arc. That means the current is naturally quenched during the zero-crossing across the open circuit breaker contacts, circuit breaker technology prevent the current to re-establish after zero-crossing by providing sufficient dielectric strength between the contacts.
When the current-carrying contacts are being separated inside a circuit breaker interrupter, the arcing may take place even at a lower voltage level. This is because the ions neutralize the space charge thus it allows a large current to flow even at a relatively low voltage gradient.
The phenomenon of arcing depends on the following factors.
The pressure of the medium in which the arcking is taking place.
The ionization and deionization agents present in the medium.
The potential difference across the electrodes between which arcing is taking place.
The variation of voltage with respect to time across the electrodes between which the arcing is taking place.
The basic design and the materials of electrodes.
The basic design and shape of the arcing chamber of the circuit breaker.
Puffer System of SF6 Circuit Breaker
In the first one, nozzles are made up of insulating materials and fixed with the moving contact. On the opening of the circuit breaker, the gas-filled cylinder moves downwards along with the moving contact. In this design, the piston inside the gas-filled cylinder is stationery. Therefore the moving cylinder along with the moving contact moves downward against the stationary piston. As a result, the pressure inside the cylinder increases. Then the compressed sf6 gas releases through the nozzle and extinguishes the arc between fixed and moving contacts.
Operating Mechanisms of Circuit Breakers
There are three types of operating mechanisms available for operating a circuit breaker. These are spring mechanisms, pneumatic mechanisms, and hydraulic mechanisms. For medium-voltage sf6 circuit breakers, the spring mechanism is quite suitable. For extra-high voltage applications the interruption needs to be quite fast it is within 2 to 3 cycles. For achieving such high-speed operation we require pneumatic or hydraulic mechanisms for operating the circuit breaker.
Spring Mechanism of Circuit Breaker Operation
In this mechanism, there are two springs or better to say two sets of springs one for closing and the other is for tripping. The closing spring is first deformed stores required energy for closing operation as well as the tripping operation. Whenever a closing pulse is applied the closing coil gets energized and displaces the closing spring latch. As a result, the closing spring gets relaxed. During this relaxation, the spring releases its potential energy to move the moving contacts toward the fixed contacts. At the same time, the rest portion of the stored potential energy of the closing spring is utilized for deforming the tripping spring of the mechanism. Once the tripping spring is deformed, it stores the energy required for the opening movement of the contacts as well as for supplying the pressure in the blast chamber of the puffer system. After a closing operation, immediately the closing spring stars charging for the next cycle of operation.
Closing Spring Charging – Closing Operation – Closing Spring Charging – Tripping Spring Charging – Tripping Operation.