|Vacuum circuit breaker|
|Sunday, 03 December 2006|
Vacuum circuit breakers are devices which open and close high voltage and large electric current by opening and closing the path between a movable electrode and a fixed electrode which are placed in a vacuum container. A vacuum circuit breaker connects or interrupts current between a power source and a load of high voltage and current, for example, over thousands of voltages and of hundreds amperes. A vacuum circuit breaker recovers the electrical insulation between its main electrodes at the zero point of current and cut s off the current, thereby protecting a circuit from any overcurrent. Vacuum circuit breakers are generally used in electric power systems having low surge impedances, in which the chopping level does not present any serious problem so that above described copper rich contact materials could be used. Vacuum interrupters are typically used to reliably interrupt medium voltage alternating current (AC) currents and high voltage AC currents of several thousands of amperes or more. During the past several decades, vacuum circuit interrupters have gained world-wide acceptance over other technologies. Depending on vacuum circuit breaker construction, vacuum circuit breakers offer many advantages as compared to other types of circuit interrupters. Those advantages include relatively long life due to controlled contact erosion; maintenance-free operation provided by enclosure of the contacts within the hermetically sealed housing; excellent sticking resistance due to the use of hard contact materials; little or no atmospheric contact contamination, which contamination can detrimentally form oxides and corrosion layers on the contacts; little or no noise during operation due to containment of arcing within the hermetically sealed housing; relatively few environmental effects as compared to those interrupters where current interruption does not occur in a vacuum and hence, where greenhouse or toxic gases can be freely emitted into the operating environment; and very low current chop, resulting in a minimal induced transient voltage spike during circuit interruption so that surge suppressors are not required. Because the vacuum circuit breaker has various advantages, demands for higher voltage withstand property and larger current breaking capability of the vacuum circuit breaker has become increasingly high.
A vacuum circuit breaker generally comprises a vacuum vessel or container made up of an insulating casing and end plates closing the opposite ends thereof, and a pair of separable electrodes disposed in the vacuum vessel. Vacuum circuit breakers typically include separable main contacts which are disposed within an insulating housing. One of the contacts is fixed relative to the housing and to an external electrical conductor which is interconnected with the circuit to be controlled by the circuit breaker. The other separable main contact is movable. An electrode arrangement in a vacuum circuit-breaker comprises a pair of a stationary or fixed electrode and a movable electrode. The moveable contact assembly usually comprises a stem of circular cross-section having the contact at one end enclosed within a vacuum chamber and a driving mechanism at the other end which is external to the vacuum chamber. An operating rod assembly comprising a push rod, which is fastened to the end of the stem opposite the moveable contact, and a driving mechanism provide the motive force to move the moveable contact into or out of engagement with the fixed contact. Compression springs are provided in connection with the operating rod assembly in order to be able to separate the moveable contact from the fixed contact and to assure the necessary force so that the contacts will not accidentally open under inappropriate conditions. Each of fixed and movable electrodes comprises four individual elements of an arc electrode, an arc electrode support which supports the arc electrode, a coil electrode material which extends from the arc supporter, and an electrode rod at the end of the coil electrode. A vacuum circuit-breaker for a large current is constructed so that a pair of separable electrodes are disposed in a vacuum vessel, and rods connected to the rear surfaces of these electrodes extend to the outside of the vacuum vessel. Each pair of the above electrodes is composed of an electrode portion on the front surface side and a coil electrode portion on the rear surface side which are opposed to each other. A current flows from one rod to the other rod by way of the coil electrode portion and the arc electrode portion of one electrode, and the arc electrode portion and the coil electrode portion of the other electrode. In order to effectively shield an arc which occurs during a switching operation, the vacuum circuit breaker is provided with a vacuum chamber having a switching mechanism therein.
Generally, vacuum circuit breakers must be capable of momentarily carrying current and closing against momentary current loads substantially in excess of the rated current capacity of the breaker without producing objectionable weld spots between the contacts of the breaker and without otherwise damaging the contacts of the breaker. The breaker must be capable of breaking a current when overloaded. Vacuum circuit breakers must be capable of withstanding, without damage or a disruptive discharge, an impulse crest voltage and a continuous AC voltage at the rated voltage of the device. The electrodes of the vacuum circuit breakers are required to have performances such that they can handle a large circuit-break current. In general, an arc electrode is required to satisfy basic characteristics such as a large breaking capacity, high withstand voltage, small contact resistance (high electric conductivity), high fusion resistance, small wastage of a contact, and small chopped current. The breaking performance of vacuum circuit breakers is known to be greatly influenced by the material properties of arc electrode portions at the part facing the arc running faces of electrodes. The material of these movable and fixed electrodes must provide a large breaking current, a small chopping current, a high dielectric breakdown voltage between electrodes, a difficulty in welding and only a small amount of heat during current carrying. A large number of alloys have been researched and developed for such electrode materials, and melting and casting of alloys such as Cu-Bi (bismuth) and Cu-Te (tellurium), or sintering alloys such as Cu-W (tungsten) and Cu-Mo (molybdenum) have been used practically.