Friday, June 28, 2013

Protecting Wind Turbines in Extreme Temperatures

By Michael A. Stout, Vice President Engineering, Falcon Electric 
As renewable energy sources continue to gain momentum and drive the electrical grid to a more intelligent and reliable source of power, integrating the right components and peripheral systems are essential to reliability. As most renewable systems are located in outdoor and potentially harsh environments, critical attention must be paid to the suitability and robustness of the equipment to be installed in these environments. Since this equipment is only as reliable as its power source, backup power is often required in addition to the utility power source.
As an example, wind turbine control systems are essential to their operation and safety. The function of these systems provide vital control of the turbine and incorporates power sensitive state-of-the-art electronics that demand absolute reliability. Yet, they are installed in locations subject to every form of weather conditions – from the extreme lows of the artic to excessive heat of the desert.
The most essential function of a wind turbine control system is the continuous control of wind turbine blade speed and braking. In most new turbines, the pitch of the blades control the output frequency of the AC power being generated in addition to bringing the blades to a complete stop in high wind conditions. An electronically controlled braking system assures the blades are locked in stopped state. Should the brakes be applied before the rotor speed is below allowed braking speed, the brakes would be damaged. Without these vital controls, the wind turbine blade rotational speed could reach a runaway condition, causing the complete destruction of the entire turbine. Adjacent property damage and loss of life could also result. A power backup system is demanded to assure these vital control systems remain viable in the event of the loss of the primary utility power source.
To achieve the full functionality of the wind turbine there are a large number of electrical and electronic equipment elements required to ensure the safe, reliable generation of power. These include:
Main control computer, I/O modules, relays and components for monitoring and control of the wind turbine
Equipment dedicated to the continuous remote monitoring of wind turbine operation
The Hub computer to control pitching of blades
Frequency converter, yaw motor protection systems
Power converter (full or dual-fed), filters, phase compensation electronics
UL Listed, wide-temperature range online uninterruptible power supplies (UPS)
Communications computer, network and SCADA-monitoring and control equipment
High voltage, medium voltage and low voltage distribution control equipment
As stated prior, due to the wind turbine locations they are subjected to extreme temperatures swings, typically from -30ºC (-22ºF) to 55ºC (131ºF). All of the electronic equipment and circuits installed in the turbine must be designed to operate reliably over the entire temperature range. As a source of backup power is essential, the online UPS designed into the system must be rated for these harsh temperature environments. Further, due to its key function, it must be specifically designed to provide years of operation while operating over the entire temperature range. As the UPS is located in the Nacelle located at the top of the turbine tower with the rest of the equipment, humidity and condensation can be another factor affecting reliability. The conformal coating of circuit boards is required to protect circuitry on the board from becoming shorted out by moisture condensing on the board. These environmental factors demand the use of a special UPS.
Batteries: The Weakest Link
A standard off-the-shelf UPS is designed for 0ºC to 40ºC office or computer room environments, therefore their reliability is suspect and their ability to survive in a high humidity environment is non-existent. These batteries are typically not rated for the higher temperatures that are often encountered. At these high temperatures, its life can be reduced by as much as 90 percent.
The majority of UPS products rated under 10-kVA use valve regulated lead-acid (VRLA) batteries to provide backup energy. Due to identical battery chemistries and like construction, most battery manufacturers’ specifications are very similar. Battery manufacturers rate their projected battery service life with the battery operating in a 25ºC environment.
However, newer technology VRLA batteries can now yield a four year life at 50°C and up to 12 years at a 25ºC room temperature per the battery manufacturers stated service life projections.
Higher temperatures cause the acid-based battery chemistry to become more active, accelerating destructive factors inside each battery cell. However, very low temperatures slow down the chemical reactions and impair the batteries ability to deliver current. This results in a substantially shortened UPS battery runtime when compared with the stated battery runtimes, which are typically stated with the UPS operating at 25ºC.
To meet the demand for wide temperature range UPS and power conversion products, a few manufacturers are designing products that not only survive in these difficult environments, but offer superior performance. Their cost of ownership is reduced through a robust design and by reducing the number of battery replacements over the UPS’s typical 12-15 year service life. Robust industrial-grade, wider-temperature range products may be found in standalone UPS units or in prepackaged turnkey NEMA rated enclosure systems. Direct from the manufacturer, these systems are ready for immediate installation and operation, reducing the associated project engineering and installation costs.
Word to the Wise
Never use a UPS that is not rated for the temperatures to be encountered in the installed environment. If you require a UPS that will be used in an extreme operating temperature environment and are confused by some manufacturer’s specifications, verify the UPS has a UL Listing stating the wide temperature range desired. Using a UPS outside the stated UL Listed temperature range can invalidate the UL Listing status. This can lead to reliability, code enforcement and product liability problems.