Office of Energy Efficiency and Renewable Energy [Public domain]
◊ Part of the ‘wind energy’ series of articles ◊
I’ll admit up front that I am not a wind generator expert. Being a retired Electrical Engineer with over 35 years’ experience in the utility industry I have no problem describing the technical aspects of wind turbines and how they impact the electrical grid. I’ll keep things high level and try not to get bogged down in the details.
There are two types of wind generators classified by the orientation of the rotating axis. The vertical axis ‘Darrieus’ or ‘eggbeater’ style are not very common and account for a tiny percentage of commercial installations. The horizontal axis style is the most common type on the market and the type you will see in Ontario. These will be three-bladed turbines mounted on pedestals up to 100 meters tall to provide ground clearance for blades 50 meters in length (GE 1.6-100 Series).
Wind turbines can be further broken down by capacity and generator type. The turbine may be induction, Doubly Fed Induction Generator (DFIG), synchronous or Permanent Magnet Synchronous Generator (PMSG). The different types can operate at either fixed speed or variable depending on the electronic controls they have.
I can spare any more details by saying that the various types of wind turbines have different options, characteristics, features and cost that have important implications for the grid on which they are connected. It is up to various entities to specify grid connection requirements which will help determine the type of wind turbine that is acceptable for implementation. The ‘entities’ will include government, local authorities, the independent system operator (IESO in Ontario) and the licensed delivery company that connects the wind generator to the grid.
Typically generators can turn themselves to face the wind by yaw adjustment and change blade pitch to maintain speed and power output. They rotate at a slow speed (18 rpm) and are typically equipped with a transmission to increase the rotational speed of the generator. Direct drive turbines are becoming available which utilize special generators designed to rotate at very low-speed. Direct drive turbines are more expensive than the conventional ones but they require less maintenance and have higher availability without the mechanical transmission.
Wind generators require a specific range of wind speeds for operation. When the wind is too light, the turbine will not generate sufficient energy to connect to the grid. High winds can increase the blade stresses, forcing the generator to reduce the pitch and shut down.
Weather extremes resulting in high winds, extremely low temperatures or ice may also force the generator to shut down to avoid equipment damage.
Generator may connect to either the transmission or distribution grid. Large wind farms will need to connect to the transmission system due to capacity requirements. Smaller facilities may connect to the distribution system where there is sufficient infrastructure capacity to permit operation. In any case there will be cost and performance implications to existing infrastructure. The IESO and grid authority perform connection assessments during the conceptual phase of projects to make the necessary determination and recommendation for necessary upgrades.
Turbines can have different capabilities depending on what options they are equipped with.
In the most basic form an induction-type will operate at a fixed speed and power factor (unity). It will be able to start, synchronize, connect to the grid and follow its voltage but will not be able to supply load in isolation. The basic configuration will be able to protect itself and other grid-connected customers from electrical faults for a single specific grid configuration. It will be stand alone, self-sufficient without any form of remote monitoring, control, dispatch capability or grid-initiated protection interface. These are the lowest cost and least grid-friendly wind turbines.
Additional features can be added to make the wind turbine installation more grid friendly. Not all features may be available from every manufacturer or every generator type.
These include:
- Variable speed
- Grid voltage support (variable power factor)
- Remote monitoring and control
- Multiple protection settings to accommodate different grid conditions
- Operation without grid connection (islanding)
- Grid protection interface (remote tripping)
- Output level (dispatch) control
A jurisdiction intending to increase its share of wind generation much above 10% should be installing grid friendly devices or will risk serious operational and reliability problems.
Facilities installed in the early phases of the renewable generation programs in Ontario were not particularly grid friendly. Distribution connected generation is unlikely to have remote monitoring by grid operators, grid voltage support, a grid protection interface or dispatch control.
Transmission connected generation has more stringent connection criteria and has most, if not all of the functionality required for grid support.
The takeaway
Most wind farms utilize the horizontal axis design.
Not all wind generators have the same performance or features. It is up to individual jurisdictions to specify the requirements for the wind turbines to achieve the desired outcome.
Increasing the share of wind energy contribution to the grid will require turbines with advanced functionality to ensure power quality and reliability.
Derek
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