The wind turbine generator converts mechanical energy to electrical energy. Wind turbine generators are a bit unusual, compared to other generating units you ordinarily find attached to the electrical grid. One reason is that the generator has to work with a power source (the wind turbine rotor) which supplies very fluctuating mechanical power (torque).
Generating Voltage (tension) – On large wind turbines (above 100-150 kW) the voltage (tension) generated by the turbine is usually 690 V three-phase alternating current (AC). The current is subsequently sent through a transformer next to the wind turbine (or inside the tower) to raise the voltage to somewhere between 10,000 and 30,000 volts, depending on the standard in the local electrical grid.
Large manufacturers will supply both 50 Hz wind turbine models (for the electrical grids in most of the world) and 60 Hz models (for the electrical grid in America).
Cooling System – Generators need cooling while they work. On most turbines this is accomplished by encapsulating the generator in a duct, using a large fan for air cooling, but a few manufacturers use water cooled generators. Water cooled generators may be built more compactly, which also gives some electrical efficiency advantages, but they require a radiator in the nacelle to get rid of the heat from the liquid cooling system.
Wind Energy Generator (WEG) Selection
Wind turbines may be designed with either synchronous or asynchronous generators, and with various forms of direct or indirect grid connection of the generator.
The turbine you decide to purchase must fit your needs for size, wind resource, availability, reliability, warranty, spare parts availability, and proximity of operation and maintenance teams.
Size – In general, wind projects are modular energy facilities and can consist of one to one hundred turbines or more. The overall size of a wind project is a function of many variables, including the amount of land available, the number of investors and size of each investor’s contribution, the financing available to the project, the ability of the transmission or distribution grid to handle the additional energy from the project without substantial system upgrades, and the number of turbines available to the project. Often one or several of these factors combined will determine the size of the project.
For instance, a project might be developed initially at 10 MW. After going through part of the interconnection process, you may find out that a project larger than 8 MW will cause the project to incur significant interconnection costs. In this case, it might be prudent to only develop an 8 MW project. In other instances, there might be significant interconnection costs regardless of the size of the project. In these cases, it may be advantageous to construct a much larger project to help spread out the interconnection and other associated costs over as many turbines as possible. The key factor here is economics. The size and number of turbines should be based on obtaining the best possible return while taking into account constraining factors.
The size of the turbine model to be used at a project will be based on available models, the wind resource at a site, and the ability to perform maintenance. Larger machines on taller towers can cause added expense and delays in replacing major components because there are relatively few cranes that have the ability to lift heavy loads to the top of tall towers. Smaller turbine models may make maintenance easier, but could provide lower production revenue because of a shorter tower or a less efficient machine.
Keep in mind that determination of project size and turbine size should be based on the options that will provide the best economic return for investors and the practicality of acquiring equipment and maintaining it.
Wind Resource and Climate – Wind turbines are designed for specific wind resource and environmental criteria, summarized by four International Electrotechnical Commission (IEC) classes (I-IV). IEC class I is the highest wind and turbulence criteria and class IV describes the lowest wind speeds and turbulence criteria. In general, turbines designed for high energy capture in low wind regimes will have larger rotors. On the other hand, turbines designed for high wind regions tend to have larger nameplate generator ratings and smaller rotors. You might need to obtain gust data in order to determine which turbine is right for your site. Consider a wind turbine model with a good track record in areas with a wind resource and climate similar to those at your site.
Availability for Purchase – The turbine you select must not only fit your project and site requirements, it also must be available for purchase within your time frame. Many large wind turbine manufacturers are based in Europe, making transportation and timing important considerations. Also, it is more cost effective for manufacturers to serve customers seeking large numbers of turbines, which can make it difficult for the customer looking for a single turbine or small numbers of turbines.
Reliability – Turbines that are not producing energy are losing money, and a machine that breaks down regularly will quickly eat away at your bottom line. Consider that most projects are designed to be operational 98 percent of the time (“98% availability”). This only allows approximately 7 days per year that each turbine can be shut down for regular and non-scheduled maintenance.
Picking a machine that has a good track record in the field as well as a manufacturer with a reputation for quality equipment and quick response when problems do arise will help to keep your project in the black. Talk to as many different developers, maintenance company representatives, people who are investors in projects, consultants, and others in the industry as you can to learn who is selling the best machines and which models to avoid. If you have to wait a little longer for a quality machine and can build this into your project timeline, it may be a worthwhile consideration.
Warranty – Most wind turbine manufacturers offer a standard 2-year parts and labor warranty, which may include a power curve and availability warranty. It is also common for turbine manufacturers to offer to extend the warranty to 5 years at an additional cost. These warranties will address design and manufacturing flaws and provide replacement parts and labor. Extending the warranty on the machines for smaller projects is generally a good idea, and financing institutions may require this before lending money to the project. An extended warranty will provide insurance against major failure while the project accrues a contingency fund for any major equipment failures after the warranty period expires.
The turbine sales agreement will cover delivery schedule of the machine (important due to tight construction and interconnection schedules), parts, and labor. It may also cover the power curve, ensuring that the machine will generate as much electricity (and revenue) as you had planned for in your business prospectus, although the technical requirements for measuring the power curve can prove very expensive and are not generally affordable for smaller projects.
