By Jack Smith
September 1, 2011
The use of wind farms as a form of commercial electric power is on the rise. According to the American Wind Energy Association (AWEA), more than 1,100 megawatts (MW) of new electrical power generation capacity was installed in the US during the first quarter of 2011. Another 5,600 MW of capacity was under construction as the US began the second quarter, the AWEA said.
Wind turbines convert kinetic energy from the wind into mechanical energy, which, in turn, produces electricity. A wind farm is a large array of wind turbines connected to the electrical power grid.
Wind turbines can be oriented horizontally or vertically. With horizontal-axis wind turbines, the main rotor shaft is arranged horizontally; with vertical-axis wind turbines, the main rotor shaft is arranged vertically. This column will focus on horizontal-axis wind turbines because most commercial wind turbines are of this type.
Wind turbines: More complex than they appear
A typical horizontal wind turbine includes rotor, generator, and structural support systems. The rotor consists of the blades and the hub. The generator system includes the electrical generator, gearbox, power conversion components, and controls. The structural support system includes the tower and rotor yaw mechanism, which positions the rotor into the wind to extract the maximum amount of power from the wind.
Wind turbines are much more complex than first impressions may imply. For example, the nacelle, which is located at the top of the tower and behind the blades of a horizontal wind turbine, contains the gearbox and shaft mechanisms, generator, power conversion components, controller, and the brake.
Another way to view wind turbine complexity is in the technologies that must come together to make them work - and make them work efficiently enough to make them economically and environmentally viable. Wind turbines are associated with aerodynamics, electrical and electronic technologies, hydraulics, and mechanics. All of these technologies and systems must be optimized to ensure the turbine operates as efficiently as possible.
Basic wind turbine operation
Because of the length required to explain detailed operation, this column covers only the basics of wind turbine operation.
Most horizontal-axis wind turbines have three blades, but some have two. Wind blowing past the blades causes them to lift and rotate. This rotation spins the rotor, which consequently turns the central drive shaft. A gearbox, located inside the nacelle, converts the low-speed central drive shaft rotation into a higher speed rotation sufficient to turn the generator.
The generator transforms the rotational energy of the higher speed shaft into electrical energy. In some systems, rotational speed is governed so that the generator turns at constant speed, thereby producing the appropriate frequency. Other systems convert varying frequency ac into dc, and then convert this dc back into 60 Hz ac using inverters. For purposes of this column, we’ll focus on the latter.
Electrical energy from the 60 Hz ac inverters flows through cables inside the wind turbine tower. Transformers located in the tower base step up the 460 voltage in alternating currents (VAC) from the inverter and/or the generator to about 34.5 kV. If the wind turbine is part of a wind farm, a collector further steps up the power from multiple turbines (which must be synchronized) to typical grid voltage levels of 138 kV or more.
Most utility-grade wind turbines have the capability to automatically control yaw angle, which is the misalignment between the wind direction and the direction in which the turbine is pointed. Anemometers and wind vanes on the back of the nacelle provide wind speed and direction measurements to the yaw control. The yaw control operates the yaw motor, which rotates the top part of the turbine (rotor and nacelle) into the wind to extract the maximum amount of power from it.
Modern wind turbine controls can measure and/or control as many as 500 parameters. Some of these parameters include generator voltage and current; frequency: shaft rotational speed; nacelle, rotor, and bearing vibration; hydraulic power unit pressure; blade pitch; and component temperatures.
Many of the systems and components associated with wind turbines are similar to those found in typical industrial facilities. Although wind turbines and industrial facilities have components such as generators, inverters, circuit breakers, and transformers in common, components used in wind turbine installations must withstand temperature and humidity extremes, mechanical vibration from rotating components, and unwanted electromagnetic interference.
Some wind turbine components must have designs that are different from their industrial facility counterparts. For example, average wind turbine transformer loading is typically around 30 % to 35 %, which is light compared to industrial transformers. Core losses are more significant in lightly loaded transformers.
Inverters used in some wind turbine systems use some of the same electronic power circuits used in variable frequency drives. Consequently, there may be harmonics present on ac coming from the wind turbine inverter. These harmonics - along with potentially non-sinusoidal wave forms from the turbines - can cause excessive transformer heating.
Working on wind turbines
When working on wind turbine electrical systems, use the appropriate tools for the task, and follow all applicable safety requirements. Wind turbine electrical system installation is covered in Article 705 of ANSI/NFPA 70, National Electrical Code.
Typically, many electrical measurements associated with wind turbines can be made with a high-quality digital multimeter (DMM) with an amp clamp or clamp meter with voltage measurement capabilities. Although a high-quality DMM with an internal low-pass filter can be used for many inverter measurements, some measurements require an oscilloscope or a high-quality scope-meter combination. A high-quality power quality analyzer can help troubleshoot harmonics issues as well as voltage and current phase imbalance, transients, and inverter component failures.
As the number of wind turbine installations increase, so should our knowledge of their operation.
Until next time, keep standing on “Solid Ground.”