High voltage vacuum circuit breakers are essentially a switching device which are mostly used for frequent switching of medium voltage level alternating current circuits with vacuum as the dielectric and interrupting medium. The advantage of vacuum in the interrupter is basically because of its high dielectric strength ranging from 106 -108 Volts/meter and impressive arc recovery characteristics. At a very high vacuum, few molecules are present for the ionization process to continue. The arc is mainly supported by the metal vapour from the contacts.
Table of Contents
Construction:
The Vacuum circuit breaker consists of three vacuum switches and an operating mechanism. The Vacuum switch has a glass or ceramic envelope containing a pair of contacts which are made up of alloy. The fixed contact is brazed to the top flange of the breaker and the moving contact is connected to the bottom flange with stainless steel bellows. These bellows provide the sealing against the exterior also aiding in the movement of the moving contact.
The entire switch is sealed at a pressure of 10-7 mm of Hg or less after the essential processing of degassing the copper contacts and other parts. For the prevention of the metal vapour formed during the arcing reaching the envelope, which will result in degradation of breakdown voltage level between the contacts, a sputter shield is provided which collects these particles ensuring controlled depositions and efficient thermal management.
Gettering, a process to remove the impurities from the vacuum system to improve the vacuum performance is done inside the switch, which absorbs the gas molecules coming out of the contacts which may otherwise remain inside the switch. All the live parts inside the switch are insulated with resin materials. A properly processed vacuum circuit breaker has a shelf life of more than 20 years.
The vacuum switch has a natural tendency to close the contacts because of differential pressure acting in the metal bellow. To counter the effect of atmospheric pressure two springs are attached. When the armature is closed electro-magnetically, it compresses the reaction spring allowing the contacts to close with an adequate pressure for holding its position. A mechanical latch is also used to hold the device’s closed position, which is detached by a shunt trip coil. This design optimizes the operating power of the coil.
Arc-interruption in Vacuum
The arc interruption in a vacuum differs from other mediums in many respects, however, the major difference is in the nature of the arc. In other media, the arc exists through a gaseous ionization process, whereas in a vacuum, the metal vapour arc carries the current between the two contacts of the interrupter. The main requirement of the interruption of the arc is achieved in vacuum via the proper contact separation of 10-20mm depending on voltage rating and current zero of the cycle. The current is interrupted at the current zero of the cycle because of the lack of metal vapour and the presence of a vacuum between the open contacts.
The reignition of the arc is a negligible occurrence because of the absence of gaseous contaminants and products of ionization. However, if the contact separation occurs at the current zero, the arc continues for the next half cycle only.
Operating Mechanism:
Because of the high dielectric strength and arc interrupting ability of the vacuum circuit breaker, the contact travel is usually low about 1/10th of what it is in oil circuit breakers. Contacts of vacuum circuit breakers are of butt type.
To avoid contact lift-off caused by the electrodynamic force during a short circuit, the contact pressure spring is incorporated on each pole, which limits this force to a permissible level for each interrupter and ensures accurate contact-making. Closing springs also provide the contact force towards the end of the closing stroke.
The closing spring in the mechanism is utilised to accelerate all of the mechanical parts and charge contact pressure springs and tripping springs. The atmospheric pressure acting on the moving contact stem also provides the force required for accelerating the contact during closing operation.
The force to be exerted by the tripping spring is small but high acceleration is to be achieved in the short travel. The damping of moving contact at the end of the opening stroke is to be carefully coordinated to prevent damage to the interrupter due to overtravel.
Presently the motor-operated spring closing and solenoid mechanism are used for operating the vacuum circuit breakers.
Solenoid-operated mechanism:
In this mechanism, a solenoid, which is a coil turned a number of times, produces a magnetic field on the application of electric current, which pulls apart the moving contact of the circuit breaker into the open position. The problem with this system is that the proper utilization of the solenoid’s potential does not take place as the plunger starts its movement before the rated force in the solenoid is built up fully.
Because of this mechanical plunger holding arrangements have to be made with a collapsible lever and spring-loaded balls mounted on the plunger to nullify the initial movement of the plunger until the rated force is developed by the solenoid.
The closing coil consumes high power for operation and requires an expensive battery attached to the system. This along with the holding arrangement makes the structure bulky.
Spring operated mechanism:
The springs store all the energy required for the separation and closure of the vacuum circuit breaker. The operation of the charged spring latch releases the spring power which is unleashed into the moving action of the contact inside the interrupter through a cam and four linked mechanisms. The rate of opening and closing is not affected by varied electrical parameters like voltage and current fluctuations, which makes it robust and stable. However, the speed of operation can be adjusted by altering the spring pressure.
Contact bounce:
The fact that the butt-type contacts are likely to bounce at contact makes the manufacturers of vacuum circuit breakers specify the limit of bounce duration of around 2 milliseconds to maintain long contact life.
This phenomenon is affected by many parameters.
These are: –
- Contact geometry and contact material
- Elasticity and inertia of moving contact
- Capacity to absorb the shock of moving system and interrupter mountings
- The energy of moving contact
- The natural frequency of mountings and the moving system
The above parameters can normally be dealt with by selecting the proper material, ingenious mounting designs, damping characteristics during closing, pre-compression and arrangement of contact force springs.
Current carrying components:
In addition to the aforementioned mechanical considerations, the vacuum circuit breaker is designed to carry a normal rated current. The normal current rating generally gets assigned by its contact system which is governed by the short circuit interrupting ability e.g. 40 kA vacuum circuit breaker will have a normal current rating of 2 to 3 kA. These ratings of interrupters are for open-air operation. The heat produced in vacuum interrupter though is small, must be conducted through the contact system only because of the heat insulating effect of the medium.
This calls for the judicious design of air volume ratio, convection, radiation, and the adoption of proper heat sinks to achieve the specified rating of the interrupter inside the cubicle.
Choice of material for contact tip:
As the contacts are separated in a vacuum, the current converges on the tip and because of the joule heating produces a molten metal bridge that vaporises instantaneously. This vapour so produced is enough to sustain the arc that develops during separation. The contact only supplies the arcing medium, the metal vapour in which the arc exists.
One might think that the choice supports high vapour pressure material capable of producing copious vapour with relatively low energy input to the contact. Refractory materials like tungsten and molybdenum produce requisite vapour at high temperatures. However, at current zero when the contact separation occurs, points on the contact surface remain incandescent because of the material’s low vapour pressure as the cathode has to reach a high temperature to facilitate the vapour, in relatively low thermal conductivity, which results in slow cooling process.
These points in the contact become points of thermionically emitted electrons, which prevents the contact gap from recovering dielectric strength. For this reason, these materials have never been used satisfactorily in power circuit breakers with high fault current ratings however, they are used successfully in low-current vacuum circuit breakers.
On the other hand, a very low vapour pressure material would also be unsatisfactory as it will result in excessive erosion of the contact and also an abundance of metal vapour making it impossible for the vacuum to get restored quickly enough after current zero.
Current chopping
It is a phenomenon of the current becoming zero abruptly in a premature state before the power frequency current-zero. Chopping is not particular to vacuum, it is also observed in most of the interrupting media. But in vacuum circuit breakers the mechanism causing the current chopping is different from other media.
In Air circuit breakers current chopping occurs because of the instability of the arc column but in the case of vacuum circuit breakers, the current chopping depends on the contact material and specifically on the material of the cathode.
The current chopping gives rise to high transient voltages which potentially can hamper the insulation of the equipment by inducing more fatigue and electrodynamic stress. However current chopping in practical applications of vacuum circuit breakers is not a serious problem because the vacuum circuit breaker tends to be self-protecting.
On the interruption of a current of a few amperes, the vacuum circuit breaker does so by means of current chop almost immediately after the contacts have separated. When this occurs, the voltage commences to rise across the contact gap. Before it reaches a high value, a reignition occurs because the very small contact separation is unable to support the high voltage. This reignition involves the discharging of the local capacitance, which almost invariably occurs through a higher frequency oscillation.
Being a good interrupting device, the vacuum circuit breaker interrupts this current towards the end of the oscillation, and the voltage across the contacts begins to rise again. This sequence of reignition, clearing and recovery may repeat itself a number of times before the switching process is completed. Each time some of the energy trapped in the system is dissipated in the arc and other extraneous loss mechanisms. Though the voltage may rise progressively to higher values, after each sequential clearing because of the widening contact gap, the highest voltage attained is usually much lower than it would have been if no reignition had occurred.
Advantages of Vacuum Circuit breakers
Because of the various advantages, vacuum circuit breakers are becoming increasingly popular and are utilised in various applications. The salient features of vacuum contactors are listed below: –
- High Interrupting speed
- Extremely low contact erosion
- Stable contact resistance
- Low operating power
- Long life
- Less maintenance
- Reduction in size and weight compared to other breakers of similar rating.
- Economical in the long run
- Non-inflammable and non-toxic
- Environmentally safe.
