Risk of fire in substations has been historically low, but the possible impacts of a fire can be catastrophic. Fires in substations can severely impact the supply of power to customers and the utility company’s revenue and assets. These fires can also create a fire hazard to utility personnel, emergency personnel, and the general public. The recognition of the fire hazards, the risks involved, and the appropriate fire-protection mitigation measures are some of the key considerations for the design and operation of new or existing substations.
A probability that a fire will actually occur during a specified time interval
-The magnitude of a possible fire
-The consequence of the potential loss
Energized electrical cables with combustible insulation and jacketing can be a major hazard because they are a combination of fuel supply and ignition source. A cable failure can result in sufficient heat to ignite the cable insulation, which could continue to burn and produce high heat and large quantities of toxic smoke. Oil-insulated cables are an even greater hazard, since the oil increases the fuel load and spill potential.
The hazard created by mineral-oil-insulated equipment such as transformers, reactors, and circuit breakers is that the oil is a significant fuel supply that can be ignited by an electrical failure within the equipment. Infiltration of water, failure of core insulation, exterior fault currents, and tap-changer failures are some of the causes of internal arcing within the mineral insulating oil that can result in fire. This arcing can produce breakdown gases such as acetylene and hydrogen. Depending on the type of failure and its severity, the gases can build up sufficient pressure to cause the external shell of the transformer tank or ceramic bushings to fail or rupture. Once the tank or bushing fails, there is a strong likelihood that a fire or explosion will occur. A possible explosion could cause blast damage. The resulting oil-spill fire could spread to form a large pool of fire, depending on the volume of oil, spill containment, slope of the surrounding area, and the type of the surrounding ground cover (i.e., gravel or soil). Thermal radiation and convective heating from the oil spill fire can also damage surrounding structures and structures above the fire area.
A study was carried on the substation fires reported by a major utility for the period from 1971 to
1994 (below table) shows the types and origins of fires and the percentage for each category. The “miscellaneous fires” category covers a wide range of fires from grass fires to a plastic wall clock failing and catching fire. It is impossible to predict all of the different types of fires that can occur.
One of the key steps in the design of new substations and the assessment of existing substations is to identify conditions that are fire hazards. Once the fire hazards of a planned or existing substation are identified, then fire protection measures can be incorporated to eliminate or lessen the fire hazard.
There are a wide range of types and causes of the fires that can occur in substations. The types of fires depend on the equipment and systems used in the stations. Fires involving dc valves, outdoor or indoor oil-insulated equipment, oil-insulated cable, hydrogen-cooled synchronous condensers, or PCB-insulated equipment are usually well documented, and these types of equipment are easily recognized as a fire hazard. There are a number of other substation-specific types of fires that are not as well documented.
IEEE 979, “Guide for Substation Fire Protection;” Factory Mutual ‘Data Sheets’; NFPA 851, provide guidance on other types of fire hazards and fire protection. Also, the Edison Electric Institute’s ‘Suggested Guidelines for Completing a Fire Hazards Analysis for Electric Utility Facilities (Existing or in Design)’1981 provides reference guidelines for the fire-hazard analysis process.
Fire protection measures can be subdivided into life-safety and investment categories.
Life-safety measures are considered to be mandatory by fire codes, building codes, or safety codes. As such, the codes mandate specific types of fire protection, with very little flexibility in their selection.
Investment-related fire protection is provided to protect assets, conserve revenue, and help maintain service to customers. This type of fire protection is not commonly mandated by legislation but is driven by economic reasons such as asset losses, revenue losses, and the possible loss of customers. Therefore, there is considerable flexibility in the fire risks that are mitigated, the fire protection measures used, whether the risk is offset by purchasing insurance, or whether the risk of a loss is absorbed as a cost of doing business.
The selection of investment-related fire protection can be done based on company policies and standards, insurance engineering recommendations, industry practices, specific codes and standards (IEEE 977 and NFPA 850), or by risk-based economic analysis.