Winter Icing Conditions Test Turbine Controls and Project Reliability  

A turbine control anemometer acts as the eyes and ears of any wind turbine. Turbine control and project SCADA systems rely on this relatively small component to direct the turbine into the
prevailing wind for optimal efficiency. Winter in the northern latitudes is a windy time and can also be a stressful time for many turbine operators. Unfortunately, atmospheric icing conditions can frequently occur in windy locations. A wind turbine anemometer cups or ultrasonic sensors can freeze over, stopping all data transmission to the turbine controls, bringing operation to a halt. It is critical that a wind turbine ceace operation if communication with the anemometer is interrupted. If a turbine continues to operate when it is not pointed into the wind, the shear across the turbine blades can cause damaging mechanical strain on the machine.

Any power that is produced during the time before a turbine shuts down for a met failure is not recorded by the SCADA control system. This means decreased wind farm reliability and significant lost profits for owners and utilities. “Icing events are not only bad for the owner. They are a nightmare for the utility” said one large Colorado Utiliy representative. “With more wind than ever feeding into the grid, this is true now more than ever.” The industry’s eyes are turning to sensor reliablity as linked to project reliability.

The end of 2009 brought increased interest in offshore wind development in the United States. Over 700 people were in attendance at the American Wind Energy Association workshop on offshore development last December in Boston, Massachusets to discuss the exciting challenges ahead for the North American wind industry in offshore wind development Anemometers, both for assessment and turbine control are an essential part of the offshore equation. Anemometers positioned on turbines offshore are exposed not only to extreme icing, but also highly corrosive salt water. For this reason it is critical that anemometers chosen for offshore use are not only heated but are built of a non‐corrosive material and extensively tested to perform in such an environment. Annodized aluminum is an example of a common construction material to find when procuring turbine control anemometers. Leaving a natural finish on an allumium alloy will also prevent corrosion. A common lab test for corrosion, which is said to simulate offshore conditions is the MIL‐STD‐810 Method 509.3 corrosion chamber test.

Many turbine manufacturers offer cold weather packages now to accommodate for wind turbine projects operating in cold weather environments. Offered in either ultrasonic or cup style, there  are a handful of cold weather products on the market today. Wind industry accumulative knowledge and field experience have demonstrated few are without some problems.
Due to increased concern and awareness of icing issues both off‐shore and on‐land, there is some movement away from the traditional cup and vane anemometers over to ultrasonic  technology.

Ultrasonic anemometers are generally preferred for turbine control due maintenance free operation and longer life cycles, but until recently have been a second choice of wind profilers. This is primiarily due to the percieved high cost. Because ultrasonic sensors have no moving parts, they cost less to maintain and have longer life cycles than traditional cup and vane sensors.. Future ultrasonic anemometers with intelligent interfaces will deliver added values such as electronic compasses and barometric pressure sensors. In 2005, all Automated Surface Observing Systems (ASOS) in the United States switched over to Ultrasonic wind speed and directional sensors. ASOS systems serve as the nation’s primary surface weather observing network. Wind developers will often use ASOS wind data as reference data when determining the feasibility of a project site. In today’s uncertain economy and competitive industry climate, downtime due to met failure is not an option for any project owner. To assure customers of reliable project life, turbine manufacturers are requiring rigorous lab field testing prior to accepting a new brand of anemometer into its supply chain. An example of a lab test is a freezing rain simulation test, which is meant to simulate environmental conditions during extreme icing. The MIL‐STD‐810F , Method 521.2 test is the accepted test for ice free operation. This test requires a certain chain of events that is meant to simulate extreme icing conditions in the field.

The test begins with a one hour cold soak followed by thirty minutes of high wind within a sealed chamber. Over the next three hours the chamber temperature is reduced to 1.4⁰ C while the sensor is hit with cold water through a wind tunnel at approximately 14m/s. This ‘icing test’, or any similar testing proceedure, will help identify the most durable turbine anemometers. When asked about the MIL‐STD‐810F test, one anemometer manufacturer said; “This test has really helped us contribute to the industry and improve our manufacturing plan. We came into the test with a certain level of heating and left the test with plans to change to a much higher powered heater. Our entire production line was changed to be able to meet industry standards. You just need a higher heating capacity using a standard 24V power supply if you are going to survive in those type of conditions”

In environments where extreme icing is present, a higher power heater is required. Sources vary on exactly what that power level must be, but “somewhere over 220W would appear to be sufficent”. Most importantly, a sensor must be tested in the field before it is installed for turbine operational control. The MIL‐STD‐810F test is a common and accepted standard for simulation of wind turbine field conditions.

Winter field testing is completed by most turbine manufacturers before they can accept and integrate a new anemometer. The lab can not tell you everything. A winter day in the midwest can
be quite unpredictable. Hoarfrost off the coast of Rhode Island or ice rime near Lake Ontario simply cannot be simulated in the lab. Manufacturers typically spend up to a year reviewing and testing new equipment. Other tests that are common for anemometers are the IEC 60945 vibration analysis test and the MIL‐STD‐810, Method 509.3 corrosion test.

Turbine downtime due to inclement weather need not be a concern on the long worry list of a project owner. The coming offshore market can learn from the experiences of onshore developers as well. An anemometer with a powerful enough heater that has performed and tested well in the lab and field will be an effective eyes and brain of any turbine, no matter if it is installed in Palm Springs or Idaho Falls.