The High Voltage Threat

To have a reliable and efficient power network, it is necessary to keep the various pieces of power system equipment in healthy working condition. While it's true that a simple error of judgement or the mis-understanding of some subtle power issue can sometimes lead to a dangerous situation, it is not the well trained, experienced and cautious engineers that generally present a hazard in this area. It is more often than not (and perhaps surprisingly) an aspect of the equipment itself: insulation. The reliable operation of any high voltage equipment depends critically on the integrity of its insulation. This equipment during service, is always under the influence of electrical, mechanical, thermal and environmental stresses, which cause the slow degradation of insulation quality.

Compromising HV insulation

Two issues face the manufacturer when deciding which insulation materials to use:

1) Price

Almost all raw materials used in the production of electrical equipment have increased in price. Does the manufacturer stick to their traditional materials? This could mean their production costs rise, which means they either have to charge more (which may make them less competitive, even unpopular with their clients) or they have to absorb the cost and probably reduce their profit margin.

Alternatively they may be forced to use cheaper materials to maintain the lower production costs and the same pricing.

2) Client Requirements

Corporate clients buying HV equipment are increasingly requiring their suppliers to meet their own environmentally friendly buying policy.

Environmentally 'friendlier' materials for electrical equipment -usually based on plastic or synthetic rubbers are produced less widely than traditional more hazardous materials such as oil and porcelain. Partly as they are still fairly rare and partly as the technology is still evolving these materials may not have been tested as widely as the older materials. They may be unproven but manufactures are being compelled to use them.

Both of these issues may compromise safety. By opting to use cheaper components or an insulation material that may not have been proven to be up to the task, a manufacturer could inadvertently compromise the safety of a HV installation.

If the insulation of the high voltage equipment is not reliable and quality checked, there is a risk of partial discharge taking place inside the insulation. These partial discharges can cause the insulation degradation, which in turn can cause even explosion, particularly if the discharge takes place within a panel unit. This could lead to manufacturers being sued - should the worst happen. It makes good economic sense then to guard against any potential risks of litigation by opting to supply equipment that has not only been type tested but that has had it's testing work certified by an independent third party. Such independent verification demonstrates due diligence to produce safe products and offers a robust defence against litigation and accusations of liability.

Some leading manufacturers of high voltage equipment conduct product testing themselves in their own laboratories and clients enjoy the reassurance of this testing expertise. Others let clients witness the tests of the product first hand to give them confidence but a growing number have their equipment type tested and certified by a certification body as they want to take advantage of the extra protection it offers.

Independent certification of type testing work also saves money on retesting a product in front of a client and enables the manufacture to explore many different international markets as a third party certificate will enable them to already meet a specific country's requirements for high voltage equipment. Whichever route the manufacturers take to demonstrate the safety of their products, it is vital that the client seeks such a demonstration. It will enable them to manage any potential risks that the equipment may hold and give them confidence in the safety and quality of the equipment.

Type testing is an exercise that takes one or more random samples of a piece of equipment and tests them in a laboratory against international standards for performance. Should the samples perform consistently and pass the tests, the whole batch from which the samples come, can be certified.

Type Testing: A brief guide

High voltage testing is usually carried out according to international standards such as those proscribed by IEC and ANSI. The origin of these standards lies in the collective experience of operators and manufacturers of electrical equipment over the past 100 years based on the typical worse case scenarios that equipment might encounter. Even today these tests continue to evolve as the design and understanding of electrical systems evolves.

For testing purposes, three types of voltages can be used allowing the tester to determine the fitness for purpose of a given piece of equipment:

AC (Alternating Current) Voltage - Most AC testing comes in 2 categories: voltage-withstand testing where a voltage typically 50% higher than the working voltage is applied to the equipment (which is usually coupled with a partial discharge test - these are very small electrical breakdown events that indicate that the insulation is compromised) and tan delta testing which is a measurement of capacitance to determine if there is contamination of the insulation material that causes it to heat up due to resistive loss in the material. Both these latter tests are used to check if a piece of equipment is likely to fail within the intended lifespan of the equipment.

DC (Direct Current) Voltage - It might be strange to want to test any item of AC equipment under DC conditions, but under certain circumstances AC equipment can experience DC such when a piece of network is suddenly left open circuit and the insulation does not allow trapped charge to leak away. DC is by far the most sensitive form of test for identifying the existence of trapped electrical charge or tiny microscopic particles that may move and build up over time leading to a catastrophic breakdown that may occur days, weeks or years after installation.

Impulse Voltage - these are transient voltages that last less than a second but pose the sternest test of insulation fitness for purpose. There are normally two classes of test. Lightning impulse is a very fast transient lasting just 50 microseconds that demonstrates the ability of the equipment to withstand an over-voltage typically 5 times the steady state rating of the equipment. As the name suggests it originates from lightning striking the electrical system nearby. The other class of transient voltage is switching impulse. These occur when electrical equipment switches at non-zero points on the AC cycle. Switching transients typically last around a quarter of a second and are typically 4 times the rated working voltage, but because the voltage peak exists for longer it can be a more onerous test than lightning impulse.

Most testing is normally done in dry laboratory conditions but, for equipment that is required to worked out-doors, additional electrical testing is often required under artificial rain or salt fog conditions. The characteristics of the precipitation are usually determined by the expected level and type of pollution that will be encountered. Wet tests under impulse conditions are hence the most arduous test of insulation performance and are often user specific.

The type testing has long been the most practical testing solution for large pieces of HV equipment. Using one or two samples of an item makes more sense than testing every piece individually. Partly as they are expensive commodities themselves, partly for the delay it would incur to the manufacturers and partly for the sheer logistical difficulties of moving significant quantities of large equipment to laboratories.

So equipment with questionable insulation may indeed be a threat to the operating personnel, nearby equipment and to the power system network itself. However the users of HV equipment can reduce any potential risks and have peace of mind by specifying equipment whose test work has been thoroughly completed and certified by an independent third party.

Credits: Article produced by ASTA BEAB Certification Services in association with NaREC Clothier Laboratory. Image courtesy of NaREC Clothier Laboratory.


About NaREC Clothier Laboratory:

The NaREC Clothier Laboratory is one of the largest High Voltage testing laboratories in the world. The main laboratory is designed to operate at voltages of up to 2MV AC, 1.2MV DC and 4MV impulse, however the facility has smaller test areas available that are able to test at the more typical voltage levels encountered by mains or distribution level equipment. As a fully independent laboratory the NaREC Clothier Laboratory is able to provide the impartial testing to accredited standards that most end users require when purchasing equipment for the safety critical applications.

NaREC Clothier Laboratory, New and Renewable Energy Centre, Victoria Road West, Hebburn, NE31 1UR, UK
Phone: (44) 191 430 9200
E-mail: info@narec.co.uk
Website: www.narec.co.uk


About NaREC The New and Renewable Energy Centre Ltd:

NaREC is a market-focused economic development company whose mission is to create and enable world-class new and renewable energy provision building upon the North East's industrial and academic expertise.

NaREC has developed through a combination of public and private sector funding which has allowed the growth of a unique combination of technical expertise and physical facilities supporting the development of the new and renewable energy sector. NaREC supports new technology developments in various areas including on and offshore wind, photovoltaics, biomass, distribution network restructuring, domestic scale renewables, marine and tidal energy.