Advantages of Vacuum Interrupter Technology in Medium Voltage Switchgear

Since the company's inception, Electrical Switchgear Engineers NOJA Power has been committed to providing customers reliable and sustainable solutions for medium voltage switchgear products.

To meet the reliability and sustainability standards set out by utilities, NOJA Power’s OSM series Automatic Circuit Recloser (ACR or “auto-recloser”) employs a vacuum interrupter housed in solid dielectric insulation to eliminate the need for environmentally damaging insulating mediums such as Sulfur Hexafluoride (SF6). The practice of using vacuum interrupters in medium voltage switchgear is well established in industry due to several key benefits such as cost reduction and reliability.

A major advantage of vacuum interrupters is that it is an environmentally friendly and easily recyclable technology, unlike other interrupting mediums such as SF6 and oil. SF6 gas is an effective insulating medium for switchgear applications, but is classed as a greenhouse gas and has a global warming potential 23,900 times higher than Carbon Dioxide (CO2) according to the Intergovernmental Panel on Climate Change (IPCC). With growing concern on environmental damage that SF6 causes, it is subject to handling and disposal regulations that increase its life cycle cost. Using a vacuum interrupter housed in solid dielectric insulation ensures that utilities are using a sustainable technology.

The development of the vacuum interrupter, commonly referred to as a ‘VI’, since the first vacuum switch patent in 1890 has provided customers with a reliable solution for their switching needs. The VI has a hermetically sealed ceramic housing that ensures a permanent vacuum over its useful lifetime of 30 years, with no maintenance required. The ceramic body houses a stationary contact and a moving contact that is connected to an actuating mechanism. The contact movement is provided through the bellows of the VI.

When the vacuum interrupter is opened, an electric arc involving hot plasma is formed between the contacts which facilitate the movement of electrons. The plasma is formed from the molten metal of the contacts and is emitted from a local area on the contact called the ‘cathode spot’. When the alternating current passes through zero, the plasma arc is extinguished and the vacuum resists any transient recovery voltages (TRV) for a successful switching operation.

The reliability and the small contact gaps seen in modern VIs were due to two major design elements. Firstly, the contact material, usually a copper alloy, plays an important role in ensuring that contact erosion is virtually eliminated in modern VIs to ensure reliable switching and meeting the OSM Series Recloser’s 30,000 full load operations. Secondly, the contact geometry is carefully designed to induce a magnetic field (axial or transverse) that moves the electric arc and cathode spot during opening to increase its switching capability.

The reliability and small size of the vacuum interrupter, coupled with a solid dielectric insulation design that is validated through extensive finite element analysis and independent type testing, ensures that the OSM Series Recloser is a cost effective and reliable solution to meet electrical utilities distribution requirements.

“We chose to use vacuum interrupters in our products because it is the best interruption medium available today and is also environmentally friendly,” Neil O’Sullivan, Group Managing Director, stated.

NOJA Power's installation of over 90,000 OSM Series in 104 countries is a testament to the technology’s reliability. The OSM Series uses vacuum interrupters enclosed within a solid dielectric pole to achieve rated voltages of 15kV, 27kV and 38kV, which have all been individually type tested to ensure reliability in the harshest environments. With a specified lifetime of 30 years, the NOJA Power’s OSM Recloser provides utilities a cost effective, reliable and environmentally sustainable solution to their switching requirements.