to the 220 kV system and units 6 to 8 to the 110 kV system. The transformers are located on a platform above the draft tube. Each transformer weighs 59 tons when fill of oil. From the transformers overhead conductors carried the power across the tailrace to an outdoor switchyard. The generating units were delivered with guaranteed efficiencies of 92.2% at three quarters load turbines, 97.36% at three quarters load and 97.67% at full load with a combined efficiency of 89.77% at three quarters load. The 110 kV and 220 kV systems were connected by a 50 MVA 220/110 kV interconnecting transformer supplied by Brown Boveri. The outdoor 220 kV and 110 kV circuit breakers were also supplied by Brown Boveri and were of the air blast type. Auxiliary power supply To ensure a reliable auxiliary to the power station two auxiliary generating units were installed below the unloading bay and supplied from a shared 3 ft (0.27 m) diameter 243 ft (22.6 m) long penstock which ran from the top of the dam. Each unit has a horizontal Francis 765 hp turbine supplied by Drees & Co of West Germany which drove via flywheel a 625 kVA 400 V generator supplied by General Electric. At full load each unit consumes of water. The auxiliary generating units were upgraded at a cost of NZ$2.5 to $3 million in 2017. Lake Roxburgh Lake Roxburgh, the lake formed behind the dam, extends for nearly towards the town of Alexandra. Operation Operation of the power station is covered by the requirements of six resource consents that expire in 2042. These require a minimum discharge of from the power station. With the commissioning of Roxburgh, the sediment which had previously flowed down the Clutha river became trapped behind the dam. Regular surveys commenced in 1961 to monitor this sediment. By 1979 the average river bed level downstream of the Alexandra bridge has increased by 3.6 metres since the lake was created in 1956. The completion of the Clyde Power Station in 1992 reduced the sediment inflows from the Clutha River, leaving the Manuherikia River as the principal source. Floods in 1979, 1987, 1994 and 1995 have led many residents of Alexandra to put pressure on the owners of Roxburgh power station to better manage the sediment build-up. A major flood in 1999 caused flooding of large parts of the main business area of Alexandra.