To help respond to increasing demand for wind-based power, last summer the International Electrotechnical Commission (IEC) released the second edition of its standard for measuring the power quality of grid-connected wind turbines. According to the introduction to the document, the purpose of the standard, IEC 61400-21, “is to provide a uniform methodology that will ensure consistency and accuracy in the presentation, testing and assessment of power quality characteristics of grid connected wind turbines.”
The new edition is designed to meet needs that have evolved as the use of wind turbines has increased. Once only used in single installations, wind turbines today are typically found on large wind farms. Their output also has increased dramatically in recent years.
“The additions to the standard are taking into account recent experiences and technical needs, in particular much related to the recent development of grid code requirements to large wind farms. The revised standard will still be for testing of wind turbines, though it may contain information that will be useful for testing of wind farms,” says John Olav Tande, convener of maintenance team 21, a subset of IEC technical committee 88, which focuses on developing wind turbine standards.
To meet this need, the following items have been incorporated into the new edition of the standard:
- Interharmonics and current distortions (<9 kHz)
- Response to voltage dips
- Active power ramp rate limitation and set-point control
- Reactive power capabilities and set-point control
- Grid protection and reconnection time after grid faults
Developed to be used by wind turbine manufacturers, purchasers, operators, planners, regulators, certification authorities, testing organizations, and planners and regulators of electric networks, IEC 61400-21 focuses on standardizing the measurement of power quality characteristics for single wind turbines with a three-phase grid connection. The power quality characteristics include wind turbine specifications; voltage quality, such as emissions of flicker and harmonics; voltage drop response; active and reactive power control; and grid protection and reconnection time. The standard also will help in the estimation of the power quality expected from different wind turbines when they are installed at specific sites.