Innovative solutions help wind farms meet grid interconnection standards.
As wind-generated electricity continues to play a larger role in the worldwide power supply, electric utilities increasingly grapple with the challenges of connecting that power to the grid while maintaining system reliability.
Wind offers an environmentally friendly, utility-scale renewable resource to meet the ever-increasing demand for power, but its interaction with the grid is unique. Some countries have adopted interconnection standards for new wind farms, and in some cases, utilities are requiring that wind farms provide dynamic reactive compensation, similar to what a fossil fuel synchronous-type generator would provide. This enables wind-based resources to provide equivalent voltage stability support to the grid at the point of interconnection.
Reactive power and fault ride-through
Unlike the dynamic voltage support provided by conventional power sources such as coal-fired plants, reactive power derived from wind farms in some cases cannot be provided dynamically or in continuously variable amounts. Accordingly, a growing number of grid operators are requiring wind farms to possess the same range of power factor compensation and dynamic reactive power capability as conventional generation sources. To meet this requirement, many wind farms today are employing innovative systems that regulate grid voltages and provide dynamic reactive power support, thereby enhancing the stability of the greater power grid.
Low voltage ride-through (LVRT) is another requirement for wind farms—the ability of generators to remain stable and connected during normally cleared electrical faults on a transmission grid. These faults, which often result from natural causes such as lightning strikes, in some cases can cause a large transient voltage depression across wide network areas. Typically, conventional synchronous generators are permitted to trip off line only in the case of a permanent fault on a directly connected circuit. Fault ride-through requirements are currently defined in most regions of the world where a large number of utility-scale wind farms are being installed.
The voltage regulation system at Kettles Hill wind farm in the Canadian province of Alberta. Kettles Hill utilizes the AMSC D-VAR system. Photo courtesy of American Superconductor.
Other common interconnection requirements for wind turbines include operating continuously up to rated output within normal grid voltage ranges, maintaining a constant terminal voltage, and remaining connected during small step voltage changes.
To protect the integrity and smooth operation of the transmission grid, a handful of countries around the world have adopted interconnection standards for new wind farms that explicitly require the wind farm to provide certain amounts of dynamic reactive compensation—Australia, New Zealand, Canada, Spain and the United Kingdom among them. Some of the strictest regulations are found in the U.K. and some provinces in Canada. Of course, interconnection standards vary from country to country (and between individual provinces or states) depending on local grid characteristics and utility-specific requirements.
For instance, (see chart, p. 39) Canadian utility SaskPower, the principal supplier of electricity for the province of Saskatchewan, has adopted wind farm power factor capability requirements of 90 percent leading (supplying VARs to the system) to 95 percent lagging (absorbing VARs from the system) at the high voltage point of interconnection (POI) to the grid, as well as wind turbine fault ride-through and in some cases post-fault system voltage recovery requirements.
Ontario’s IESO (Independent Electricity System Operator), responsible for the day-to-day operation of the province of Ontario’s electrical system, has taken similar steps by enacting wind farm power factor capability requirements of 90 percent leading to 95 percent lagging at the generator terminals and wind turbine fault ride-through requirements for worst-case contingencies affecting the wind farm.
Current state of the U.S. grid connection standards
While U.S. regulatory authorities have yet to adopt specific dynamic reactive compensation requirements, existing interconnection requirements for U.S. wind farm installations are outlined under the Federal Energy Regulatory Commission’s Order 661-A. Finalized in May 2005, the order focuses on voltage regulation and LVRT but includes several unique characteristics.
Regarding LVRT, the order requires wind farms to remain in service during any three-phase fault resulting in transmission voltage as low as zero volts, as measured at the high voltage point of interconnection (POI) to the grid, and that is normally cleared (4–9 cycles) without separating the wind farm from the transmission system. Wind farms installed prior to Dec. 31, 2007 are allowed to trip off line in the case of a fault depressing the voltage at the POI to below 0.15 p.u., or 15 percent of nominal voltage.
Wind turbines. Photo courtesy of American Superconductor.
The order does not associate a specific timeframe or speed of response requirement for supplying reactive power on a post-contingency basis, providing wind farm developers and owners with significant latitude in addressing these requirements. The interconnecting utilities, however, on a case-by-case basis, are allowed to enforce more stringent rules if they can demonstrate through power system simulations that enhanced capability is needed to support the reliability of the grid.
Reactive power solutions
Fortunately, there are innovative solutions readily available that allow wind farms to successfully comply with varying global grid interconnection requirements. Classified as Flexible AC Transmission System (FACTS) devices, static compensators (STATCOMS) or static VAR compensators (SVC), these solutions provide dynamic reactive power capability that allows them to meet grid interconnection standards. For example, American Superconductor’s D-VAR system is a proprietary STATCOM system that monitors and regulates wind farm voltages and power factor, and instantaneously stabilizes voltage levels by injecting dynamic reactive power into the grid.
Among the ranks of U.S. wind farms to have adopted reactive compensation solutions are the 80 MW Caprock Wind Ranch, located 20 miles southeast of Tucumcari, N.M., and the 10 MW HRD Hawi Wind Farm, located near Upolo Point on the Island of Hawaii. These wind farms are utilizing AMSC’s D-VAR devices to provide both LVRT and steady state power factor correction. Caprock, which was connected onto a major Southwestern U.S. transmission grid in 2004, was the first wind farm to utilize a FACTS device to provide LVRT capability. Many wind farms in the U.K., New Zealand and Australia have also deployed reactive power solutions.
More wind to come
The rapid acceleration of wind energy adoption is expected to continue for many years to come. According to the American Wind Energy Association, approximately 31 billion kilowatt-hours (kWh) were generated by wind power in the U.S. in 2007 and the Global Wind Energy Council projects that wind power capacity will double from 74,223 MW in 2006 to 149,500 MW in 2010.
Given the unique characteristics of wind generation and the trend toward larger wind parks, it is likely that more projects will require dynamic reactive compensation as a condition of interconnection. Centralized FACTS systems provide an effective means of addressing these requirements.
Tim Poor is vice president and deputy general manager for AMSC (American Superconductor) Power Systems. He joined AMSC in September 2001 as director of sales and business development for power electronic systems. In May 2007, he was appointed to his current position.