Putnam divides grid problems that have arisen with wind into interface (or engineering) issues, operational issues, and planning issues. On the problem of harmonics -- undesirable distortions of the utility AC sinusoidal voltage and current waveforms which can damage both utility distribution and customer load equipment -- the fact sheet points out that systems available today produce output with very little harmonic distortion, unlike some of the first generation wind plant of the 1980s. With the addition of harmonic correction devices and the trend towards the use of advanced power electronics in variable speed wind turbines, harmonics are no longer a significant utility concern, claims Putnam.
A similar picture emerges with reactive power. Early wind plants using induction generators were installed with inadequate hardware for reactive power compensation, leading to increased line losses and difficulty controlling system voltage. Now utilities with considerable experience of wind, such as Southern California Edison (SCE) and Pacific Gas & Electric (PG&E) in California, require small power producers using induction generators to provide near unity power factor at the point of interconnection. Power electronics technology used with modern, variable speed wind turbines has demonstrated a full range of power factor control under all operating conditions.
Difficulty in controlling voltage is accentuated when the wind plant is located in a remote area and connected to the utility through transmission lines originally designed to service only a light load. In some locations SCE has determined that the least cost option is to curtail wind plant production and to compensate plant operators accordingly. Similarly, frequency control can be a problem when a wind plant feeds into a small network. System frequency varies when gusting winds cause rapid changes in power output. While maintaining normal system frequency has not been a problem in the wind farm areas of California, it has on the Hawaii Electric Light Company (HELCO) system. A study by the US Electric Power Research Institute concluded that in order to accommodate more wind, the HELCO system would require variable speed turbines with power electronic control.
With current wind plant penetration levels in California, the variability of wind plant output has not required any change in operating reserve requirements. The exact point at which the integration of intermittent generation such as wind begins to degrade system economics is unclear, but may be at penetration levels in excess of 5%. As markets for electricity become more competitive, the ability to forecast and control the wind resource will increase the value of wind to utilities.
Unit commitment is the scheduling of specific power plants on the utility system to meet expected demand. The most conservative approach to unit commitment and economic dispatch is to discount any contribution from interconnected wind resources. But these days wind plant output can be fairly predictable, such as in California's Altamont Pass. Further research is needed to develop the capability to accurately forecast this output, says Putnam.
Large wind turbines typically have low speed, large diameter blades coupled to an electric generator by a high ratio gear box. This feature results in inertia and low mechanical stiffness between turbine and generator which gives large wind turbines excellent transient stability properties. Operating experience with wind power plants in California confirms that wind turbine transients due to speed fluctuations or network disturbances have not resulted in system stability problems. Furthermore, system protection or safety has not become an issue in California, despite the large amount of wind power on the grid, states the fact sheet.
Putnam concludes his report: "The positive integration experience with wind energy in California . . . can provide valuable insights to utilities planning new projects and needs to become more widely understood" .
Putnam's work was funded by the US Department of Energy and is based on interviews with system operators and dispatchers from PG&E and SCE. Both utilities have had extensive experience with the integration of wind energy on their systems since the early 1980s.
