Fires and explosions in substations

As the age of transformers in substations increases, and as transformers are required to carry heavier overloads, the incidence of fires and explosions with oil-filled equipment has increased. Even 10 years ago most large users anticipated only one major fire every 5 years; current experience suggests that many users may suffer from more than one fire every year. Substations are normally equipped with installations to combat the effects of fire. Water spray or water deluge systems are common and Halon fire extinguishing equipment was used until Halon gases were banned. Fire extinguishing plant cannot prevent a fire, but can mitigate the damage and reduce the loss resulting from fires. Other methods suggested to reduce the affects of fires include evacuation of the oil and replacement of it by an inert gas, or blowing a cooling gas through the transformer to cool the oil. These methods have not been demonstrated to be successful. More effective means of controlling fires involve installations that prevent contact between burning oil and atmospheric oxygen. Where transformers are installed underground, or in a cavern, successful designs have involved chambers where the transformers are surrounded by a nitrogen or CO/sub 2/ atmosphere, and where the oil is cooled in a remote location. Research into transformer explosions has identified two possible models for the development of the explosion, based on the length and energy of the initiating arc. If the rate of increase of pressure is low, then conventional pressure relief devices will operate to reduce the pressure and guide the expelled oil into a safe area. However, if the rate of increase of pressure is high, the effect is similar to a 'detonation' used to initiate an explosion, and the resultant rapid increase in pressure generally leads to tank rupture before the pressure relief device can operate. Oil will spill out from the tank, allowing intimate contact between hot oil and oxygen, and a fire usually results from the explosion. The most effective way of eliminating fires is to use a nonflammable insulating material. PCBs were effective in preventing fires in transformers but are now banned. Insulating fluids with higher fire points have been used, but they will still burn. The use of SF/sub 6/ gas as both an insulating and a cooling medium has been successfully used in transformers. The initial designs used SF/sub 6/ gas at a relatively low pressure (below 3 bar) for distribution and small power transformers. Medium and high power transformers were trialled with a two-phase insulating system using SF/sub 6/ gas as the insulating medium, supported by a fluid such as perchlorethylene to remove the heat. Although successful as an interim step there is some doubt about the long-term viability of chlorinated fluids used in this two-phase system. High voltage, high power transformers that are both insulated and cooled by SF/sub 6/ gas at high pressure (6 bar) have been successfully deployed in a number of indoor and outdoor substations. The use of a non-burning gas for both insulating and cooling the transformers addresses the high risk perceived to be involved in the installation of transformers in substations and eliminates the need for fire extinguishing equipment.