An Advanced Series of Wind Turbines Now Feature Power Electronics Capable of Controlling and regulating system voltage. This technology provides further justification for utilities’ investments in wind power, particularly in some rural areas with weak grids. These state-of-the-art wind turbine generators do more than generate power. They provide utility managers with a new tool to support their transmission and distribution systems.
These new wind turbines control and regulate voltage levels through sophisticated power electronics. Until recently, most large wind turbines used induction generators with low voltage capacitors to correct the power factor to near unity during operation. The capacitors would come on in stages, as needed, based on the kW output of the turbines. They kept the power factor close to unity, rather than regulating voltage. The voltage was largely determined by the grid and the turbine output, not by control of the generator’s excitation. Newer wind turbines now can control voltage like conventional utility generators.
One of these state-of-the-art wind turbines is manufactured in the United States, the Zond Z750, which has a nameplate capacity of 750 kW. The unit offers variable speed operation, reducing torque transients and increasing the rotor blades’ ability to capture the kinetic energy available in the wind. The power electronics, which are a prerequisite for variable speed operation, can be programmed to regulate the turbine’s output voltage or to maintain a specified power factor. The electronics package also reduces the inrush current to about 75 percent of full load current during the wind turbine’s startup. Another advantage of the variable speed design is the dramatic reduction in power output fluctuations, which minimizes the potential for the wind turbine to cause voltage flicker during operation.
The Z750 uses a double wound induction generator. In this design, the rotor windings are connected to a 150 kVA bi-directional power converter while the stator windings are connected to the electric system. Variable frequency power is either injected into the rotor or extracted from the rotor, depending upon the total power generated. The voltage and phase angles of the rotor currents are precisely controlled to deliver the desired output voltage and power factor.
These improvements allow advanced wind turbines to be connected to more grids including some of the weaker, rural distribution systems.
Phase 3 of the DOE/EPRI Turbine Verification Project, initiated in January 1997, demonstrated the expanded reach of these new design features for distributed generation. Seven Iowa municipal utilities received federal support for this program. They jointly installed three Z750 units on a windy site typical of North Central Iowa and tied them into an existing 13.8 kV feeder owned by Algona Municipal Utilities, one of the partners in the project.
The three turbines, totaling 2.25 MW, were installed 6.5 line miles from a 69/13.8 kV 10 MVA substation. The site is the highest elevation and thus the windiest location in the county, plus it is adjacent to Algona’s 13.8 kV line. Placing this amount at a relatively long distance from the nearest substation was intended to demonstrate the flexibility of advanced wind generation technology (Figure 1).
In the past, 2.25 MW of wind generation could not be connected a distance of 6.5 miles from the substation without voltage rise and flicker concerns. For example, at full load and unity power factor, the turbines would have increased the feeder voltage at the wind farm about seven percent depending upon ambient temperature. At such a distance, the 1/0 wire (0.4-inch diameter) would need to be replaced with 4/0 (0.56-inch diameter) or larger, and at considerable expense. Even with this wire upgrade, the turbines would have to be planted about 2 miles closer to the substation to prevent excessive voltage rise. A closer location would have resulted in a reduction in wind generation output due to the lower elevation and wind speed.
None of this occurred in the DOE/EPRI-sponsored demonstration. The three turbines operated at a fixed lagging power factor (absorbing VARs) which keeps the voltage rise at manageable levels while minimizing voltage fluctuations from changing wind generator output. Since the turbines are absorbing VARs, some additional capacitors will be added near the substation to provide those consumed by the turbines. This constant power factor mode of operation keeps the voltage variations on the line within acceptable limits as the wind farm output varies. Since the wind farm went into operation in October of 1998 through May of 1999, the wind turbines have generated 4.7 million kWh. The annual generation is expected to be 6 million kWh.
As predicted, there have been no complaints from other customers on the distribution line due to voltage variations. In fact, local reaction to the project has been overwhelmingly positive. Startup of the project was somewhat lengthy since these were the first Z750 units with a 164-foot rotor and the first with this power factor control. However, Zond is resolving the initial problems and the turbines are expected to achieve the target availability of 97 to 98 percent by the end of this year.
Figure 2 illustrates the total output of the three turbines during a 5-minute period when the wind was changing. Even though the output fell by about half, the power factor stayed within 0.5 percentage points of the set point at the interconnection point.
The turbines can be ordered to regulate voltage instead of the power factor. This might be valuable if they were connected to a weak transmission system that needed additional voltage support, especially during critical contingencies.
Another feature of the power electronics is the significant reduction of inrush starting currents. The wind turbine generator is essentially a 1,000 HP induction motor with a wound rotor. When a large motor is first connected to the line, there is a large surge of inrush current that is many times larger than the normal full load current. Such inrush currents would often cause the lights to blink. The power electronics in the advanced wind turbines reduces the inrush current to about 75 percent of normal full-load current. This reduction in the inrush current allows these large wind turbines to be placed on much weaker distribution lines without causing the lights to blink every time the wind turbine starts up. Low inrush starting current also controls voltage dips during startup.
Another advantage of the variable speed design with the wound rotor generator is the elimination of power fluctuations caused by the 3P effect. This effect is caused by a reduction in power generated by each blade as it passes in front of the tubular tower. Even though the blade is upwind, the change in air flow behind the blade as it passes around the tower still reduces the power generated by the blade. Since the turbine has three blades, there are three dips in the power for each revolution of the rotor. With this variable speed design, the dip in power results in a slight change in RPM, rather than a sudden dip in generator power output.
The implications of these innovations are significant. The new power electronics make it much easier to connect large wind turbines directly to existing distribution grids without incurring the expense of building separate collection feeders or new substations. Wind turbines can now become more of an asset to a rural system. In the Midwest, it is likely that the turbines will produce some power but not be at full output during critical summer peak periods. Even if the generation is only at 10 to 20 percent of full load, it will have a beneficial impact on the system by regulating voltage. In this way, wind generation can potentially defer transmission and distribution capital improvements.
The voltage control feature also can be used to advantage in larger wind farms to provide active support to the transmission grid. A 103 MW wind farm for Northern States Power (NSP Phase 3) in Southwestern Minnesota has been designed to provide extra voltage support to the grid as a contingency in the event of the loss of a major transmission line running through the area. If transmission voltage drops by a specified amount for a certain number of cycles, the wind farm’s SCADA system sends out new voltage set points to all 138 wind turbines. Additional voltage support is provided within one second.
Tom Wind was the principal consulting engineer for the Iowa Distributed Wind Generation Project and has been working with other wind generation projects in the Midwest. Wind is an electrical engineer.