UK Power Networks - Fault Level Rating Analysis

Analysis of four primary substations within the UK Power Networks region to ascertain whether the original installed switchgear was capable of an increased fault level rating

Most of UK Power Networks’ (and most other Distribution Network Operator equipment installed in between the 1950s and the 1970s) primary switchboards are rated at 250MVA, 11kV. Following the network growth, the company has found that some of its originally installed switchboards have a need for checking of the original rating plate parameters of that switchgear (unless operated in non-ideal mode such as bus sections open).

As with most DNOs, UK Power Networks has a variety of solutions at its disposal to help address this current and future potential issue.  Where original switchboards are in a healthy condition (proven via means such as partial discharge monitoring), one such option is to consider whether the original switchgear can be safely re-rated to an increased fault level.

On this particular occasion, UK Power Networks requested Switchgear Support Ltd (SSL) to investigate the feasibility of increasing the rating on the following switchboards:

• 2 x double busbar Brush Switchgear VBAD switchboards (27 panels and 29 panels)
• 1 x double busbar low type South Wales Switchgear C4X/D20X switchboard (23 panels)
• 1 x single busbar high type South Wales Switchgear C4X/D20X switchboard (12 panels)

In order to ascertain whether a particular switchboard is suitable for an increased fault level rating, there are numerous factors to be considered, including :

• Circuit breaker rating
• Isolating contact rating (if contacts not on CB)
• CT ratings (if wound type)
• Busbar rating
• Earthing bar ratings (including earthing attachments if non-integrally earthed units or earthing trucks if double busbar equipment)
• Bus section/coupler ratings

SSL is able to analyse the Brush Switchgear or South Wales Switchgear originally-installed equipment and in almost every case prove categorically whether the original equipment can be upgraded and re-rated.  The reports produced are detailed enough to satisfy any Health and Safety Executive requirements.

The outcome :

SSL was able to demonstrate to UK Power Networks that the switchgear could be increased to a minimum 350MVA, 11kV rating provided some individual circuits (not all) had the original OCBs retrofitted.

Dave Johnson, Director of SSL, said “For this type of study, we have to assume from the onset that the switchgear maximum rating is as stated on the originally supplied nameplates.  We have to positively identify (via numerous methods) that the switchgear has been certified at a higher fault level than that stated.  One thing is certain – NEVER rely on copies of original sales catalogues (of which there are many copies still in existence) as they often state maximum ratings, but do not indicate there are often an enormous amount of switchgear variants – many of which are NOT rated at the maximum stated levels”

Photos courtesy of UK Power Networks