Critical Limitations of Wind Turbine Power Curve Warranties
Critical Limitations of Wind Turbine Power Curve Warranties
Critical Limitations of Wind Turbine Power Curve Warranties
A. Albers
Deutsche WindGuard Consulting GmbH, Oldenburger Straße 65, D-26316 Varel, Germany
E-mail: a.albers@windguard.de, Tel: (++49) (0)4451/9515-15, Fax: (++49) (0)4451/9515-29
Most power curve warranties require verifi- A clear trend has been observed over the
cation tests being performed on the basis of past few years that power curve verification
the standard IEC 61400-12-1 [1]. This procedures as defined in standard warran-
seems being justified as IEC 61400-12-1 is ties require more and more special test con-
so far the only international standard on ditions, often far beyond the requirements
power curve tests and as the general proce- defined in IEC 61400-12-1 and sometimes in
dure outlined in this standard is well ac- contradiction to some requirements defined
cepted. in IEC 61400-12-1.
However, the standard IEC 61400-12-1 has Many standard warranties require additional
practical shortcomings: Tests according to data filtering on atmospheric conditions, like
this standard are expensive as met masts on turbulence intensity, wind shear, vertical
reaching hub height are required. In com- flow inclination, air density, air temperature
plex terrain, the method is applicable only if and wind direction. This seems justified from
a site calibration with additional masts has the viewpoint of the turbine supplier, who
been performed prior to the installation of may intend to warrant the power curve only
the turbines. Furthermore, only turbines at under well defined environmental conditions
the boarder of wind farms can be tested. (warranty of generic power curve). However,
These shortcomings are a true burden for the consequence for the turbine owner can
turbine owners to perform verifications of the be that the power curve tested under such
warranted power curve at all, and some- special atmospheric conditions may not be
times power curve tests are just impossible. representative for the wind farm site any-
Thus, alternative procedures for the verifica- more and may thus be of limited value for
tion of power curves should be seriously economic considerations (e.g. wind re-
considered for power curve warranties, at source assessments). Sometimes, the addi-
least for cases, where an application of the tional data filtering leads to very idealised
standard IEC 61400-12-1 is difficult. In many measured power curves, which are just too
optimistic for economic considerations, i.e.
the warranty level is indirectly lowered by Other special testing conditions defined in
the filtering. In other cases, the definition of some standard power curve warranties are
special test conditions seems being quite related to certain conditions of the tested
arbitrary, and the intention of the turbine turbines. The justification of such turbine
supplier to well define the testing conditions conditions must be checked case by case: It
is clearly not met. Figure 1 illustrates the is mostly plausible to exclude special turbine
effect of special data filters as defined in conditions from power curve tests, for which
power curve warranties on the measurement the turbine supplier is not responsible, e.g.
results in terms of the annual energy pro- situations with improper grid conditions or
duction (AEP). The influence on the AEP noise reduced operation. Other special con-
varies approximately from -3 % to +3 %, but ditions, like e.g. automatic load reduction,
in the majority of the cases, the AEP is im- may be considered as the turbine condition
proved by the special data filtering, i.e. the representative for the wind farm site and
verified power curve tends being overopti- should thus not be excluded from power
mistic for the wind farm site. curve tests. A few standard warranties re-
In order to overcome the conflict of warrant- quire even the cut-in hysteresis to be ex-
ing a power curve for well defined conditions cluded from power curve verifications. This
(intention of turbine supplier) and getting a is completely unjustified, as the cut-in hys-
warranted power curve representative for teresis can be well measured and forms a
the wind farm site (intention of owner), war- normal, although turbulence dependent, part
ranted power curves should be defined site of the power curve. Excluding the cut-in
specific rather than generic. The new draft hysteresis leads to a systematic improve-
revision of the standard IEC 61400-12-1 [4] ment of measured power curves in favour of
provides methodologies to normalise power turbine suppliers. Many standard warranties
curve data in terms of the turbulence inten- require the application of database B ac-
sity and wind shear to pre-defined site spe- cording to IEC 61400-12-1 instead of data-
cific conditions [5]. In terms of the air den- base A (main result according to current
sity, such data normalisation is common version of IEC 61400-12-1). Database A
practice since a long time. Potential short- includes the high wind cut-off hysteresis,
comings of the normalisation procedures are while database B excludes the cut-off hys-
covered by additional uncertainties, which teresis from power curve tests. The use of
again reduce the warranty level. This should database B is supported by the fact that
be acceptable for the turbine supplier and often not much data is present in the wind
for the owner. Another advantage of data speed range where the high wind hysteresis
normalisation about data filtering is that the takes place, what can lead to quite arbitrary
extension of the measurement period, as measurement results. On the other hand,
sometimes caused by data filtering, is database B does not cover potential prob-
avoided. lems of a turbine to keep rated power at high
wind speeds and can lead to overoptimistic
4 AEP calculations. Furthermore, the arbitrari-
AEPspecial filter - AEPIEC filter [%]
avail. warranty
Furthermore, quite often problems with the
covered by
0.6
acceptance and interpretation of measure- 0.4