Page 218 - Wind Energy Handbook

192 WIND-TURBINE PERFORMANCE

described by the probability distribution (usually annual) of 1 h (sometimes,
10 min) mean wind speeds. To calculate the average energy production for a given
probability distribution of wind speeds a relationship between wind speed and
wind power is needed—this is the power curve of the wind turbine. As dynamic
effects are not of interest for long-term performance, averaging of the measured
wind speed and wind turbine power is carried out which improves the correlation
between them and attenuates the effects of wind turbulence. This is not to say that
site turbulence is irrelevant—a point which will be dealt with later.
It is also important to note the power from the wind turbine which is of relevance
here is the net power, defined as the power available from the wind turbine less
power needed for control, monitoring, display or maintaining operation. In other
words it is the power available to the user and is measured at the point of
connection to the network.

4.7.1 Field testing methodology

Although a few years ago, the International Energy Agency (IEA) recommended
practices for wind turbine testing which were the nearest thing to an agreed
procedure for wind turbine evaluation. Now, as mentioned, an IEC standard is
available: IEC 61400-12: Wind-turbine Generator Systems, Part 12:- Wind-turbine
Power Performance Testing (1998). It is interesting to contrast it with Volume 1 of
the IEA recommendations (1982) and subsequent editions (1990), which deal with
power performance testing. These notes will draw on both the IEC standard, which
forms the basis of European and national standards, and its precursor IEA
documents. It is essential that the IEC standard, and any amendments which may
have been agreed, be obtained before contemplating power performance measure-
ments.
The wind turbine should ideally be located on a site free from obstructions and
local topographic features which could affect the measurements; the IEC standard
specifies relevant criteria. Site calibration is needed for non-ideal sites, and the IEC
document also includes an approach, included as Annex B (designated ‘informa-
tive’ in contrast to the compulsory, ‘normative’ content of the Standard). This is to
facilitate the evaluation of machine performance in situ, within a commercial wind
farm, which is unlikely to conform to the test site requirements. However, MEAS-
NET find that this calibration procedure is not rigorous enough, and have recom-
mended an alternative method. Testing should be conducted under ‘natural’
conditions which excludes towing tests, testing while it is raining (or other
precipitation) and when ice accumulates on the turbine, all of which affect wind
turbine performance and should be avoided. If conditions like rain do occur during
the tests the data affected should be appropriately demarcated. Clearly, modifica-
tions or adjustments should not be made to the wind turbine during the period of
data collection.
The test procedure consists of taking a series of measurements of wind speed,
wind direction, atmospheric pressure and temperature, net power and in some
circumstances rotor speed. These measurements should be taken over as wide a
range of wind speeds as possible. All data should be checked for accuracy and