Fires in substations
The 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.
This article provides an overview to help substation designers identify fire hazards within a substation, identify appropriate fire protection measures, and evaluate the benefit of incorporating these measures. It is only an overview and is not intended to be all-inclusive or to provide all the necessary details to carry out a project.
For further details on this topic, it’s recommended for designers to refer to IEEE 979.
The physical objects or conditions that create latent (undeveloped) demands for fire protection are called hazards. Every fire hazard has the following attributes:
TABLE 1 – Types and Origins of Substation Fires as Reported by a Major Utility, 1971–1994
|Types and Origins of Fires||Percentages|
|Oil-insulated circuit breakers||14.0|
|Hot work procedures (welding, cutting, and grinding)||9.3|
|Flammable liquid storage or handling||3.1|
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, “Recommended Practice for Fire Protection for Electric Generating Plants and Current Converter Stations” ; and CIGRE TF 14.01.04, “Report on Fire Aspects of HVDC Valves and Valve Halls”  — 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.
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.
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. Table 14.1 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.
Some of the specific components encountered in substation switchyards that are fire hazards are:
- Oil-insulated transformers and breakers
- Oil-insulated potheads
- Hydrogen-cooled synchronous condensers
- Gasoline storage or dispensing facilities
- Combustible service building
- Storage of pesticides or dangerous goods
- Storage warehouses
- Standby diesel-generator buildings
The failure of some of the critical components such as transformers and breakers can directly result in losses of revenue or assets.
Other switchyard components could create a fire exposure hazard to critical operational components (i.e., combustible service buildings located close to bus support structures or transmission lines).
Control and Relay-Building Hazards
A control or relay building can include the following potential hazards:
- Exposed combustible construction
- Combustible finishes
- Emergency generators, shops, offices, and other noncritical facilities in the control buildings
- Batteries and charger systems
- Switchyard cable openings that have not been fire-stopped
- Adjacent oil-insulated transformers and breakers
- High-voltage equipment
- Dry transformers
A fire in any of these components could damage or destroy critical control or protection equipment. Damages could result in a long outage to customers as well as significant revenue losses.
Indoor Station Hazards
Fires in indoor stations are caused by some of the same substation-related hazards as switchyards and control rooms. The impacts of any fires involving oil-insulated equipment, oil-insulated cable, and HVDC (high-voltage dc) valves in an indoor station can result in major fires, with accompanying large asset losses and service disruptions.
The basic problems with major fires in indoor stations is that the building will contain the blast pressure, heat, and smoke, and which can result in:
- Blast damage to the building structure (structural failure)
- Thermal damage to the building structure (structural failure)
- Smoke damage to other equipment (corrosion damage)
Reference: Don Delcourt, BC Hydro