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Case Study

DP Earl & Co Ltd is a specialist electrical engineering company that commissions and maintains HV and LV power systems. A recent protection study was carried out for a blue-chip pharmaceutical company at their research and development site. The existing system comprised of an 11KV ring system that supplied six distribution substations; during routine equipment maintenance a check of relay settings found there to be poor discrimination between protection relays in the scheme.

The site fault levels were calculated using Powernet HV software from the incoming supply to the main LV switchboards at each substation. The electricity supplier provided fault levels for the 11KV supply. These fault levels needed to be put into Powernet HV but it was not possible to achieve the correct earth fault level for the supply, as Powernet HV calculates the earth fault level from the short circuit fault level.

This problem was overcome with assistance from AMTECH Technical Support, who provided a network model that could be adjusted to provide the required earth fault and short circuit fault levels.

The protection device settings for the power system up to the outgoing circuits on the main LV switchboards for each substation were recommended from the discrimination curves produced in Protect HV. The poor discrimination between protective devices on the system has now been resolved.

Says David Earl, 'AMTECH's software has enabled us to offer protection studies as an additional service to compliment our existing services. This new service is now frequently used and has paid back our investment quicker than expected.'

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