Zealand to Contact Energy a State Owned Enterprise which subsequently passed into private ownership in 1999. With the separation of Transpower a new control room was constructed on the former carpark to house the Transpower equipment needed to operate the transmission equipment. The original air blast circuit breakers were replaced with Sprecher & Schuh SF6 circuit breakers in the late 1980s. Generating units The original generator design supplemented the fan pole-generated air flow with through-rotor air flow. During the factory acceptance tests one generator had been subjected to a heat run but in order to keep the windage and friction losses within the allowable 10 percent over the guaranteed value the manufacturer had blocked off the through-rotor flow which reduced air flow in the generator, with the coolers in the closed circuit, to 19.5 m3/s, which was about 90 percent of the design flow. This modification was applied to all of the generators. The limited time taken to commission the generating units had meant that no heat runs were performed, which would have identified the impact of this modification on the stator winding temperatures. As a result, the Roxburgh generators always operated at higher temperatures than most other hydro generators in New Zealand. The traditional practice over summer to manage the generator temperatures was to open the generator air vents and use modified ducting to discharge the hot air outside the building, while also to open the main powerhouse door and start the extraction fans installed high in the wall at the other end of the machine hall. In 1995, it was becoming apparent that it was becoming difficult to maintain the stator winding temperatures within their rated 65 to 75 °C operating range when operating at their maximum output over the summer months from January to April. As a result, it was necessary to de-rate the generators from 40 MW to 35 MW. This de-rating limited the station's operational flexibility. Investigations found that the over-heating of the stator windings was due to a breakdown of the stator winding insulation, thus reducing the heat transfer from the conductors, accumulation of dust and oil on stator winding and heat exchanger surfaces reducing their heat transfer as well as sustained high ambient air and river water temperatures over the summer, compartmentalization of the powerhouse to manage the fire risk, which reduced airflow through the powerhouse, all compounded by an inefficient generator ventilation