Electrical Tester online
December 2008
Vacuum bottles in vacuum breakers do not last forever

Vacuum bottles in vacuum breakers do not last forever

01 December 2008

Linus Claesson and Klas Björck - Product managers

More and more companies are moving towards the vacuum technology for different reasons. Vacuum is the ideal dielectric, contact travel is very short (about 12.5 mm) and a small contact travel requires a very short breaking time. This means that there is less required stored energy to break the circuit. Less stored energy and short breaking time simplifies the operating mechanism. A vacuum breaker has approximately one third as many parts as a comparable SF6 breaker.

The Mean Time To Failure (MTTF) of a vacuum breaker mechanism is many times longer than for non-vacuum breakers. Improvements in vacuum technology over the past several years have resulted in most of the world’s medium voltage switchgear manu-facturers adopting the vacuum option. More than half of the world’s current annual productions of medium voltage circuit breakers are vacuum circuit breakers.

However the vacuum bottles in vacuum breakers do not last forever. Leakage starts after years or decades and the bottles fill with air making the breaker unreliable. In most cases the leakage process is rapid once it has started. In addition to leakage, dirt on the poles and on the exterior surface of the bottle can make it unsafe during operation. The mechanics of the breaker can become misaligned so that the distance between the poles is no longer adequate. Vidar uses high voltage DC to test the integrity of vacuum breakers. There are some 2000 units of Vidar in field service all over the world that perform daily tests on the integrity of vacuum breakers.

The Programma VIDAR is the ideal tool for testing vacuum breakers due to its wide coverage range and its ease of use. A green lamp indicates approval of the breaking chamber and a red lamp if it is defective.
One question raised is why there is no built-in ammeter in VIDAR. The idea behind this is that it should be possible to analyse a trend over the years to determine the time of replacement of the breaker. Answer: It is possible to use an ammeter and make the measurements but the result will not add any information.

The reason is that there are too many parameters that cannot be controlled. One day you get the value 5 micro amps and the next day 10 micro amps depending on the conditions at the test, e.g. humidity, degree of pollution, temperature. These measuring values have nothing to do with the condition of the breaker.

As an alternative to the high DC voltage the VIDAR utilises, a high AC voltage can also be used. The electrical resistance of the vacuum in a breaker is identical in behaviour for AC and DC. The main difference in using DC vs. AC is that AC also is sensitive to the capacitance of the breaker. The DC (resistive) current component is 100 to 1000 times lower in magnitude than the AC (capacitive) current component, depending on the individual bottle capacitance and therefore difficult to distinguish when testing using AC. As a result AC requires heavier equipment for testing compared to the 8 kg VIDAR.

When the VIDAR product was introduced, a rigorous study was undertaken at the facility of one of the major vacuum bottle manufacturers. The study concluded that VIDAR was suitable for testing all potential sources of dielectric malfunction of a vacuum breaker in the field and that using AC would not add any additional information.

Both the DC and the AC methods are detailed in standards. The DC method is described in the ANSI/IEEE 37.20.2-1987 standard.

At the National Institute of Radiation Protection in Stockholm, VIDAR, together with a Siemens N 677 have been analysed for x-ray radiation. The outcome proved safe for anyone operating the combination of Vidar and a vacuum breaker.