system. In 1997 the practice was begun of no longer opening the air vents over the summer as doing so effectively took one of the eight air coolers within the generator out of the circuit. Modifications made to address the overheating issue included improving the airflow through the powerhouse, partial return to the original design of through-rotor cooling; changing the way water passes through the cooler and tube fin spacing; changing the core air duct configuration; and making the coolers slightly larger. As a result, the volume of air circulating within the unit has been increased by approximately 28 percent to 25 m3/s. Also steps were taken to improve the airflow though the powerhouse as described below. Beginning in 2002 a major refurbishment was undertaken on all generating units, which among other works consisted of installing of new stator cores and windings, re-insulating the rotor poles, refurbishing the turbine runner and wicket gates, replacing the wearing ring on the turbine shaft, replacing the stator air coolers, as well as refurbishing where necessary any mechanical components. Fire protection When it was owned by the NZED the power station had been self-insured. Once it was transferred to the a state-owned enterprise ECNZ in the late 1980s it became necessary to obtain commercial insurance coverage. To obtain this insurance it became necessary to mitigate the risk of a station fire. As a result, from the mid-1990s onwards the ECNZ upgraded the fire protection at the station, which to reduce the spread of any smoke or fire included compartmentalization of the powerhouse into several separate fire zones. This compartmentalization lead in 1995 to either installing approved fire-stopping, replacing existing doors with fire-rated doors or installing double-sided firewalls with fire-rated doors. Once such barrier was installed between the stator floor and the cable gallery on the downstream side of the powerhouse. All doors were fitted with heavy-duty adjustable closers. Unfortunately this compartmentalization restricted airflow and caused temperatures over the summer to reach the mid-30s °C on the machine hall floor and the mid-40s °C on the generator floor, peaking around 8 p.m. To improve the air flow through the powerhouse temporary wedges were used to hold the fire doors open, but this compromised fire security. A permanent solution was implemented in 1999, when the wedges were replaced with electro-magnetic door retainers which combined with automatic door closers which hold the door open, but which in