Plumes of smoke from Belco’s North Power Plant were caused by tests run on its new generators, the company said yesterday.A spokesman for Ascendant Group, the parent company of the electricity producer, said that tests had twice caused “brief discharges of smoke from the exhaust stack”.He added the smoke was caused by start-stop tests on the plant’s new heavy fuel oil engines, which had identified that their “combustion parameters” needed to be fine-tuned. The spokesman said the main contractor, Burmeister & Wain Scandinavian Contractor, would work with the engine manufacturers to “avoid such discharges in the future”.He added: “Once the new, efficient engines have been brought online, there will be far less smoke, noise and vibrations felt by our neighbours compared to the old engines that are due to be decommissioned.”Belco apologised for any inconvenience to householders in the area around its Serpentine Road, Pembroke, plant.