Wind turbines offer new voltage control feature

An advanced series of wind turbines now feature power electronics capable of controlling and regulating system voltage. This technology provides further justification for utility investment in wind power, particularly in rural areas with weak grids. These wind turbines do more than generate power: they provide utility managers with a new tool to support their transmission and distribution systems.

These new wind turbines now control and regulate voltage levels through their 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. Their function was to keep the power factor close to unity rather than try to regulate the voltage. The voltage was largely determined by the grid and the turbine output, not by control of the generator's excitation. These new wind turbines can now control voltage like conventional utility generators.

One such turbine is the Zond Z750, used in the two projects described in the main article. This has a nominal capacity of 750 kWe. The turbine employs variable speed operation to reduce torque transients and increase the blades’ ability to capture more of the kinetic energy available in the wind. The power electronics, a prerequisite for variable speed operation, can be programmed to regulate the turbine’s output voltage or maintain a specified power factor. The electronics also reduces the inrush current to 75 per cent 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 minimises the potential for the wind turbine to cause voltage flicker during operation.

These performance improvements allow advanced wind turbines such as the Z750 to be connected to more grids, including some of the weakest rural distribution systems.

The demonstration of the expanded reach of these new design features for distributed generation was the primary focus of Phase 3 of the DoE/EPRI Turbine Verification Programme initiated in January 1997. Seven Iowa municipal utilities received federal support for this programme. They jointly installed three Z750 units at 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 were installed 6.5 line miles from a 69/138 kV 10 MVA substation. Placing this amount of generation at a relatively long distance from the nearest substation was intended as a demonstration of the flexibility of this technology.

In the past, 2.25 MWe 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 by about six per cent depending upon conductor temperature. At such a distance, the standard wire would need replacing with one of a larger size at considerable expense. Even with this wire upgrade, the turbines would have to be planted 3 miles closer to the substation to prevent excessive voltage rise. At this closer location, the turbines would have generated less power because of a slightly slower wind speed due to a lower elevation.

None of these problems occurred in the DOE/EPRI-sponsored demonstration. The three wind turbines operated at a fixed lagging power factor (absorbing reactive power) which keeps the voltage rise at manageable levels while minimising voltage fluctuations from changing wind generator output. The low inrush starting current also controls voltage dips during start-up. The variable speed design essentially eliminates power fluctuations found with other constant speed induction generators when the blade passes the tubular tower.

The implications of these innovations are significant. They make it easier to connect large wind turbines to the existing distribution system, which is particularly useful for rural electrical systems.

By directly connecting to the exisiting distribution grids, there is no expense for building collection feeders or new substations. Connecting wind generation to existing distribution cicruits can provide some degree of support to both the distribution and transmission systems even when the turbines are generating below full capacity.

In the MidWest USA, it is likely that the turbines will produce some power, but not at full output during the critical summer peak periods. Even if generation is only at 10-20 per cent, 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 can also be used with advantage in larger wind farms to provide active support to the transmission grid. The Lake Benton II 104 MWe wind farm for Northern States Power in southwest 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 the transmission voltage drops by a specified amount for a certain number of cycles, then the wind farm’s SCADA system will send out a new voltage signal to all of the 138 wind turbines. Additional voltage support is provided within one second.

A future application for these advanced wind turbines is for continuous self-regulation of the voltage, whether they are generating power or not. This technology could be developed relatively quickly and easily, and could defer significant investment in system improvements that may otherwise be required.