High Voltage Connection To A 1MW Solar PV Array

This job was a 1MW ground mounted solar PV array in a field approximately half a mile from the nearest road. The grid connection was at 11,000V and required us to provide an incoming HV Ring Main Unit, HV/LV transformer and LV switching to G59/2 regulations. We were also required to provide the "accommodation" for the DNO's "edge of park" HV switch which would be some 20 metres from the base of their pole and 50 metres from our transformer and switch room. As this job was in Mid Wales the DNO was Scottish Power Energy Networks who are possibly one of the few remaining DNO's who insist on a brick built sub-station enclosure with a cast concrete roof over. (Nick-named the Mausoleum). Had we been "over the border" in Western Power's area then we would have been able to use a pre-fabricated double walled insulated fibreglass sub-station which is considerably easier to install and lower cost to our client. The first job was to mark out the site in accordance with the Planning Consent before excavating the base for the sub-station which is 1 metre deep. Casting the base in situ with substantial steel reinforcement followed before we could start to bring up the walls and cabling channels.

The roof of the sub-station has to be a minimum of 150mm thick reinforced concrete which is usually cast in situ over the walls when they are complete. This requires a considerable amount of formwork and support and would add a further five weeks to our schedule to allow for the concrete to cure sufficiently before the support could be removed. Our timescale didn't allow for this method so we elected to cast the roof on the ground and use a crane to lift it onto the sub-station later. We constructed a timber mould which contained two layers of steel reinforcement and a lifting frame with four detachable lifting eyes. We cast the roof at the same time as we cast the base for the sub-station.

With the walls completed and the door openings for the main switch room and the meter room in place we could crane the roof weighing an estimated 8 tons out of the mould and onto the sub station, setting it over a mortar grout and silicon weather seal. The SPEN specification for the doors and hardware is very detailed and locating a company who could supply the hardware took some time. With the roof on, the doors were fitted and then a specialist contractor was required to cover the concrete roof with a polymer glass reinforced double coating. Whilst SPEN have their own single phase supply to their battery charging unit on their HV switch our client has to provide a single phase supply to the same enclosure for the lighting and the wall mounted heaters which provide frost protection.

Using a very large canopy tent to protect us from the weather we completed the sub-station build in just over four weeks after which it was "inspected" by the SPEN sub-station inspector who passed it for their switch occupation a few days later. The construction of the brick built sub-station to SPEN's specification cost our client in the region of £25k.  A double walled heavily insulated fibreglass enclosure of the same size over a cast in situ concrete sub base would have cost in the region of £12k.