By Hilmar Maas, Fire Safety Solutions Expert
Siemens Building Technologies Division
It is a rare occurrence but one that turbine owners aren’t willing to risk at a wind farm: a fire. The remote location of most wind projects and the sheer height of most turbines mean fire-fighting efforts if needed are too little too late.
At 80-plus meters high (and growing), today’s turbines are outside the range of the fire department. When it comes to offshore turbines, which are soon to hit U.S. waters, human intervention in the event of fire is virtually impossible.
Of course, guidelines exist for turbine fire prevention systems. NFPA 850 is the U.S. National Fire Protection Association’s recommended practice for protection of electric generating plants and high-voltage direct current converter stations. It also identifies hazards and protections for wind-power facilities. But these are just recommended practices and not the law (although individual states and counties might have additional bylaws).
In Europe, manufacturers must comply with section 1.5.6 of the 2006/42/EC machinery directive, which states: “Machinery must be designed and constructed in such a way as to avoid any risk of fire or overheating posed by the machinery itself or by gases, liquids, dust, vapors, or other substances produced or used by the machinery.”
However, even with stringent manufacturing safeguards, a turbine is made of various mechanical and electrical components where failures can occur and potentially lead to fire. Lightning provides yet another source of concern outside the control of manufacturers and wind-farm operators.
Once ignited, it is tough to prevent a fire from burning through an entire turbine, especially in the nacelle. Repairs are costly and often the entire generator system, nacelle, or rotor blades need replacing. Risks to wind technicians make fire-prevention measures even more critical, especially out at sea where high waves or gusty winds can make rescue efforts next to impossible.
That’s why a comprehensive fire-safety plan is important because it is the only way to effectively avoid risk and serious losses.
Should a fire develop, automatic fire alarm and extinguishing systems can minimize damage and reduce downtimes — and should be a must for every turbine regardless of state standards or bylaws. Sensitive smoke and heat detectors, and fire alarms that detect thermal and optical signals, can detect a fire at an early stage and forward the information to a central alarm system that will initiate a complete shutdown.
As a measure of protection before a spark ignites, graduated protection concepts offer the highest available level of fire protection. A graduated protection structure means defective system parts in a turbine are selectively disconnected from the grid before potentially causing a surge or risk of fire. For example, a proper arc-fault-detection system for switchgear will detect a fault and immediately open the medium-voltage circuit breaker on the high-voltage side of the transformer.
Whether onshore or offshore, proper configuration in a turbine must also account for internal and external risks — from electrical components inside a nacelle and lightning due to an unexpected weather system.
- For electrical systems: Protection systems, including fuses and circuit breakers, should selectively detect faults and promptly disconnect defective parts of a turbine’s network or individual electrical equipment (such as transformers, cables, and generators). When necessary, protection systems must also ensure immediate and controlled shutdown of a turbine with all-pole disconnection from the grid. If protection devices are triggered, a fault signal should automatically alert personnel at a remote-monitoring center.
- For lightning strikes: Lightning current and over-voltage surge arresters are used to protect electrical equipment from high currents resulting from direct lightning strikes. Proper protection must take into account each specific turbine type at a wind farm. To guarantee optimal protection when performing an initial risk assessment, it’s necessary to consider the potential paths of a lightning current (for instance, from the rotor blade via the hub, the nacelle, and tower to the foundation).
A graduated protection structure is ideal for onshore and offshore turbines. For the latter, there is also another available level of defense. Siemens Building Technologies Division has created an Active Fire-Fighting System (AFFS) that detects fires in offshore turbines and automatically extinguishes them. This lets project owners avoid the hassle of launching a costly and time-consuming offshore fire-fighting effort.
AFFS combines intelligent fire detectors with integrated ASAtechnology or Advanced Signal Analysis (ASA). The sensors in this system record incoming data or signals, which are converted into mathematical components using algorithms and compared to pre-programmed values. By selecting an ASA parameter, the algorithms and fire detector are set to monitor expected local environmental influences. The optimal parameter is then adjusted to observe potential risks or differences in the existing environment and trigger a signal if atypical.
A Sinorix fire-extinguishing system is included that uses nitrogen, an inert gas, to fight fires. This extinguishing system operates on the principle of inertization, or oxygen displacement. During a fire, the oxygen content in the area is diluted by nitrogen to create a non-explosive, non-flammable environment. Nitrogen is inexpensive and obtained quickly and easily by removing it from the ambient atmosphere.
The fire-detection system monitors pain points and system-relevant components of the wind turbines. If AFFS detects a fire in the nacelle or in the tower caused by a short circuit in a control cabinet, the detector system transmits this data to a turbine’s control unit.
If the fire detector is triggered, the system immediately activates the gas-extinguishing system to put out the fire, leaving behind no residue. The wind turbine is automatically shutdown by its controller and the system is shutdown under “no-load” conditions.
When necessary, operators at an offsite control station can take further steps through remote access. The fire-detection system continuously sends status messages and system data to the control station. Subsequently, the cause of the fire can be identified and the turbine returned to operation in the shortest time possible.
To enhance safety, one system is installed in the nacelle and one in the tower base. Both systems are interconnected but operate autonomously in the event of a network outage or power failure. The AFFS systems also have a modular design, which makes replacement quick and easy by the installed support-system of the nacelle.
The AFFS system was the first to gain certification by third-party validation organizations, VdS Schadenverhütung GmbH and Germanischer Lloyd, a part of DNV, for the protection of offshore wind turbines. It is the first system to receive recognition by both testing and approval bodies for the combined system of fire detection and extinguishing systems for wind turbine equipment.
Although a rare event, it is nearly impossible to extinguish a fire in a wind turbine using conventional fire-fighting methods, and especially in offshore waters. Complete fire prevention will always rank top in the industry, and should be top of mind for offshore wind developers now working on projects in U.S. waters. As a back-up defense system, Advanced Signal Analysis and an AFFS system is a worthy safeguard against serious turbine damage and repair costs.
A comprehensive fire-protection plan for wind turbines ideally includes more than one safeguard. An extensive portfolio of monitoring and protection devices is more likely to ensure the safe and high availability of a turbine and a wind farm. Products in this portfolio may include:
- Circuit breakers. In wind turbines, they have the job of switching and protecting the main circuit, and disconnecting it from the network during maintenance work. In their fire-protection function, circuit breakers prevent fires that are triggered by overload and short-circuit currents, and protect against non-permissible heating of cables from overloading. Open circuit breakers can also provide alarm signals for integration in the communication or network system of a turbine.
- Semiconductor protection fuses prevent fires caused by uncontrolled failure of power semiconductors, such as IGBTs. They also protect high-quality devices and system components (like thyristors) in converters and soft starters from the impact of a short circuit.
- Differential current-monitoring devices reveal undetected ground faults in electrical installation by indicating the presence of differential currents. By sending an immediate signal, these devices enable preventive maintenance before a fault current potentially causes a fire.
- Measuring instruments for power monitoring make it possible to measure the quality of the in-feed from a wind turbine and monitor the electrical quantities of the main circuit. This provides early detection of overloads and operational faults, preventing associated damage.
- Residual-current devices are important for the safety of maintenance personnel because they protect against dangerous shock currents in the event of direct or indirect contact. These protection devices detect fault currents caused by insulation faults. They initiate the disconnection of the affected circuit and, therefore, also prevent fires.
- Busbars. Installing busbars instead of cables in turbines can effectively contribute to fire safety. Unlike cables insulated with PVC, the sheet metal housing of busbars has a considerably lower fire load. Epoxy coatings that are resistant to aging also offer a high degree of surface protection for the conductors. Additional benefits of busbars include the high short-circuit strength of the tap-off units and their thermal loading capacity in the event of lightning strikes.