A relatively new fire protection methodology for hydroelectric generators is gaining traction to replace traditional carbon dioxide and water-based fire suppression systems. This alternative scheme may not be the correct solution for every generator and should be evaluated on a case-by-case basis. The author provides guidance and clarifies a number of questions, issues and ambiguities.
By Dominique Dieken, PE, CFPS
Traditionally, fire protection for hydroelectric generators has consisted of total-flooding carbon dioxide (CO2) or water spray fire suppression/extinguishing systems. Recently, water mist and hybrid mist/inert gas agent extinguishing systems added to the choices of fire protection options. These are reactive, meaning they only respond after fire has occurred, with the goal of reducing the damage potential and preventing total destruction of the generator.1 This article presents an alternative fire protection approach that prevents a fire from developing to the self-sustaining stage, rather than suppressing an already established fire.
Before delving into the specifics, the background of fires in hydro generators must be understood. Decades ago, generator winding insulation materials comprised combustible materials, such as asphalt, cloth ribbon and polyester. These “thermoplastic” materials resulted in fires when overheated. Electrical faults result in a sharp winding temperature rise and, when the autoignition temperature is reached, the combustible materials burn. Over time, insulation materials were improved to withstand higher winding temperatures and transitioned to epoxy resins and fiberglass. These newer materials are known as “thermoset” plastics. With their proliferation in generators, fires became less frequent. Although thermosets have a lower fire risk than thermoplastics, they are still combustible, and laboratory testing confirmed that epoxy used on generator windings can burn.2 Instances exist where self-sustaining fires occurred in generators with thermoset insulation materials when the windings were not immediately deenergized. Once flaming occurs, even thermosets can result in self-sustaining fire that can no longer be averted without suppression.
However, if a fire is detected in its early stages and the generator is deenergized before reaching the critical temperature of the thermoset insulation, the fire will self-extinguish. Indeed, this was exactly the experience of one major public utility in the Pacific Northwest. The utility’s generators had been modernized with state-of-the-art materials, but a design flaw resulted in overheating of the end turns, which started to smoke. In two instances, operators smelled smoke. In two more instances, an air-sampling smoke detection system in the generator housing initiated an alarm. In all cases, operators promptly tripped and deenergized the affected unit and all cases resulted in charred insulation but no self-sustaining fire.
Says who?
FM Global, a worldwide standards-making organization and industrial insurer, took the first step toward accepting the omission of fire suppression systems under certain conditions in its 2016 edition of Data Sheet (DS) 5-3/13-2, Hydroelectric Power. These conditions comprise:
- Thermoset winding insulation materials.
- A smoke detection system must be provided within the generator housing.
- This system must automatically trip and deenergize the generator.
FM Global only implicitly accepts this protection methodology and does not explicitly state that this method is an acceptable substitute for traditional suppression/extinguishing systems. Rather, DS 5-3 requires suppression/extinguishing systems for thermoplastic insulation materials and only infers that thermoset materials do not need suppression if the three conditions are met. The current (2020) edition of NFPA 850, Fire Protection for Electric Generating Plants and High Voltage Direct Current Converter Stations, is vaguer and requires fire suppression/extinguishing systems for windings “consisting of materials that will not self-extinguish when de-energized.” NFPA 850 indirectly accepts the absence of fire suppression/extinguishing systems on other insulation materials by remaining silent on the matter.
The problem with the NFPA statement is that no other insulation materials currently exist. If taken literally, all generators require fire suppression/extinguishing systems. A paragraph in the appendix even states that if a generator with thermoset insulation is not electrically isolated, self-sustaining fire can occur and a suppression system should be provided.
Although, on the surface, the three conditions FM Global delineated seem fairly straight-forward, the fire protection scheme’s failure to function as intended puts tens of millions of dollars (or more) at stake from the loss of a generator and the associated downtime. Therefore, decision-makers, engineers and project managers should exercise the highest level of diligence to ensure that these three conditions are met correctly and thoroughly. Before deciding to omit or decommission a fire suppression system in favor of an alternative fire protection scheme, owners and operators of hydroelectric facilities should obtain the concurrence of all stakeholders.
Thermoset materials
Both the stator and rotor winding insulation materials must be made throughout of thermoset materials. Because this is the key provision of the alternative protection scheme, its proper determination and certification is paramount. A guess is not good enough. Neither is a Class F rating. The class corresponds to a maximum temperature the windings are able to withstand and is not a guarantee of low fire-risk winding insulation materials. Only qualified persons thoroughly familiar with the design and construction of the stator and rotor are in a position to determine whether this criterion is met. Engineers in responsible charge and/or engineering managers employed by either the generator manufacturer or the generator user may be considered qualified to render such conclusions. These findings should be properly documented in a credible format, such as a written and signed letter on company stationery, that certifies that the stator and rotor insulation systems consist of resin, wedges, insulation tape, end cap, baffle, etc., that conform to the type of insulation known as “thermoset.”
Smoke detection
The success of the alternative fire protection scheme depends on prompt detection of fire in its early stages. Therefore, detecting smoke as soon as possible and before flaming is paramount. FMDS 5-3 permits either FM-approved photoelectric spot-type smoke detectors or a very early warning fire detection (VEWFD) system. Spot-type smoke detectors comprise regularly spaced individual sensing units. Per FMDS 5-3, these must be installed in the generator housing above and below the stator windings or in the exhaust louver areas. The closer the distance between detectors, the quicker the response.
VEWFD systems comprise a central detector unit, an air pump and a network of perforated tubing. The pump continuously draws air into the sampling pipe, which contains regularly-spaced holes. The air sample is continuously analyzed by the detector. When the detector senses obscuration caused by the presence of smoke particles, it initiates an alarm. Detectors are provided with multiple thresholds that provide progressive warning levels. VEWFD systems are considered the industry standard in early smoke detection and are preferable to spot-type detectors, in the author’s opinion.
Because the alternative protection scheme is based on an FM standard, the detection system should also be FM-approved and designed and installed in accordance with its FM approval and the system manufacturer’s instructions. Unlike conventional fire detection, for which clear requirements are addressed in codes, the selection, design and testing of VEWFD is largely driven by the systems’ listing and approvals and the manufacturers’ guides. Current industry practice is established on the design-build principle, where fire protection contractors act as the manufacturers’ authorized distributor, designer, installer and tester. To protect this practice, system manufacturers are typically reluctant to support and work with organizations other than their authorized distributors. This situation hampers oversight and leads to an over-reliance on the installing contractor’s ability to deliver a correctly selected, designed and tested system. Owners seeking to install a VEWFD system should therefore have an owner’s engineering representative familiar with these systems and with hydroelectric generators to ensure that the system is specified, designed, installed and tested correctly. For example, air-sampling smoke detection systems are available in both early warning fire detection (EWFD) and VEWFD modes. The difference is the performance criteria, with VEWFD requiring a tighter alarm threshold of 1% obscuration/ft with a maximum transport time of 60 seconds.
After the installation, the system must be properly acceptance-tested. The correct test method of VEWFD includes both a transport time test (aka “canned smoke test”) and an obscuration test (aka “wire burn test”). These confirm that the VEWFD system is able to detect the minimum smoke threshold within a maximum allowable time.
Generator trip
To ensure the self-extinguishing property of thermoset materials, the heat source (i.e. the electrical power source in the windings) must be removed before the self-sustaining fire stage is reached. Although the prompt operator action of the Pacific Northwest utility in manually tripping the units precluded fire damage, depending solely on the correct human response is not a reliable protection scheme. Not all operators have a keen sense of smell, nor may operators be in the immediate area. The tripping action must therefore be automatic and prompt. This is achieved by interlocking the alarm setpoint of the fire detection system with an emergency generator trip. DS 5-3 does not address permissible time delays in tripping the unit and only requires to “trip the generator upon activation [of the fire detection system],” meaning that no delay is acceptable in principle. Built-in protection layers of VEWFD systems should alleviate concerns about the unit instantly tripping without warning. VEWFD systems generally have multiple pre-alarm and alarm settings. Each has a progressively higher obscuration threshold. This results in multiple warnings before the last alarm level trips the unit.
Conclusion
One major advantage of the alternative fire protection scheme over traditional suppression/extinguishing systems is its prevention of a self-sustaining fire, rather than extinguishment of a fire in progress. While this alternative fire protection scheme does not provide an absolute guarantee against a fire loss, neither do traditional suppression/extinguishing systems. All engineered systems have an inherent failure rate. The objective is to minimize the failure rate through correct implementation of the steps necessary for any protection system’s success, along with the proof in the form of proper documentation. This will also maximize the comfort level of stakeholders that the alternative protection provides an equivalent, if not superior, level of protection versus a traditional system.
As a final note, the proper fire prevention and protection of a generator also includes good maintenance practices. Dirt, blocked or dirty air filters, high ambient temperatures, grease and dirt blocking vent holes, and clogged heat exchangers can all result in overheating. Grease, rags, oil leaks and oil accumulations on the winding surfaces provide additional fuel for a fire and should be removed and their causes corrected.
Notes
- Dieken, D., Fire Protection Options for Air-Cooled Hydroelectric Generators, Power Magazine, April 2011.
- Test for the Flammability of Stator Windings, CEATI, 2018.
Dominique Dieken, PE, CFPS, is a consulting fire protection engineer with over 30 years’ experience in fire protection engineering consulting, working with electric power producer clients domestically and internationally. Dieken does not sell fire protection systems and is not affiliated with any manufacturers or vendors.
The views expressed in this article are those of its author and in no way represent the positions and views of past, present and future employers or clients. The information, recommendations or conclusions in this article should not be interpreted as any guarantee that the reader will achieve the same results.
