Introduction of Vacuum Interrupters
Historically, the idea of vacuum interrupters dates back to 1920s when Sorensen and Mendenhall at the California Institute of Technology started research on the concept of short circuit current interruption in a vacuum medium. They conducted an experiment by placing two fixed contacts and a C-shaped moving contact in a glass envelope and used a magnetic actuator to drive the moving contact, tested at ~15kV. But it was in the 1950s that its commercial manufacturing started when General Electricals USA developed the Spiral Petal Arc Control Contact Vacuum Interrupter. Since then a lot of research has been done in the field of vacuum interrupters to produce low cost low maintenance vacuum circuit breakers, switches and reclosers to be employed at medium and high voltages. In the following sections, we will study the basics of vacuum arc formation and its interruption, design, and applications of vacuum interrupter technology and its limitations.
The Phenomenon of Vacuum Arcing and its Interruption
As the contacts separate a sequence of events takes place that eventually builds up the arc between the gaps. We can explain this as follows. As the contacts separate, the actual area of contact between the electrodes decreases so that the contact resistance rises which increases the heating of the contact spot. The spot melts and an unstable molten metal bridge gets formed between the contacts and eventually the bridge ruptures. At this moment an arc is formed with the metal vapor and is maintained between the contacts. The arc modes are of two types:
Low Current Diffuse Arc
High Current Columnar Arc
For currents above 5kA, a single high pressure arc column is formed similar to arcing in air medium.
At the natural current zero the metal vapors condense rapidly and recombine on contacts as well as shield surfaces and start to regain vacuum condition. Full recovery can be obtained within microseconds of current zero. The circuit is successfully interrupted if the dielectric strength exceeds the circuit applied voltage.
Design of Vacuum Interrupter
The cross section of a vacuum interrupter is shown in figure 1.1. It consists of a fixed contact in which a copper electrode is hardened to an end plate while the moving contact is attached to the other end plate by a bellows. The cylindrical vacuum chamber is made of alumina ceramic. The ends of the chamber are metalized. A metal vapor condensation shield around the contacts prevents the surface of ceramic from being coated. The chamber is tightly sealed to maintain the vacuum.
Taking into considerations the current and voltage to be interrupted, to ensure minimum metal erosion and to minimize local heating there are three main designs for making contacts of the vacuum interrupter given below :
- Butt contacts (for low current arc).
- Spiral or transverse magnetic field(TMF) contacts (for high current arc).
- Axial magnetic field(AMF) contacts (for high current arc).
The contacts materials are normally made of copper alloys such as Cr-Cu, Cu-Bi, Cu-Ag, etc. The pictorial representation of the different contact structures is shown in figure 1.2.
Applications of Vacuum Interrupters
Vacuum interrupters are primarily employed in th e medium voltage range(1. 5-38kV) switchgear. In recent years the researchers have been able to successfully implement 145kV vacuum circuit breaker is showing below. Switchgear employing vacuum interrupters can be employed in the following categories :
- Circuit breakers.
- Capacitance bank switching.
- Load switching.
- Inductive current switching.
Limitations to the Use of Vacuum Interrupter Technology
In the recent years, vacuum interrupter technology has become a major choice in power systems. This is because even though SF6 has better arc quenching capability, it is now being considered as an environmental threat as it is identified as one of the major global warming gases. Thus vacuum interrupter technology becomes an environmental friendly alternative. However, cost and technical requirements become driving factors in selecting vacuum technology over the SF6 and air switching counterparts. We can explain this as follows:
- In low voltage applications, the air switching technology becomes more suitable owing to the lower cost of the air switching equipment.
- While in high voltage applications the performance of SF6 becomes superior to vacuum technology.
- The major difficulty faced by the vacuum technology is the physically large interrupters that are heavy and pose handling and maintenance as well as cost issues.
- Also, the contact materials need to be optimized for use at high voltages.
The vacuum interrupter technology is an environment friendly substitute to the SF6 but its use is restricted to the medium voltages due to cost and technological limitations. Recent researches have been able to implement vacuum circuit breakers at high voltages but the growing demand of power requires improved technologies to be able to extensively implement the vacuum interrupter technology.