SHORTCOMING OF AEP
CALCULATION AS DEFINED IN
IEC61400-12-1
MEASNET_PP_Shortcoming of AEP Calculation as Defined in IEC61400-12-1.doc
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MEASNET Procedure:
Shortcoming of AEP Calculation as Defined in IEC61400-12-1
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1. Description of Problem
The MEASNET Power Curve Experts Group has recently identified a possible source of
discrepancies in the standard IEC 61400-12-1. It is regarding the calculation of the AEP as
defined in chapter 8.3 of the standard.
It is said there: “AEP-measured shall be obtained from the measured power curve by
assuming zero power for all wind speeds above and below the range of the measured power
curve.”
This formulation did lead to confusion in a MEASNT round robin test. There is a small
difference in AEP-measured when the power is set to zero for all wind speeds above the bin
averaged wind speed of the highest covered bin and when power is set to zero only for the bin
centres of the bins not covered by the power curve test (see illustration in Figure 1).
This methodology for the calculation of the AEP should be corrected in the frame of the ongoing revision of the standard.
P set to zero outside bin averaged wind speed of highest covered bin
P set to zero in bin centres of not covered bins
1.2
1.0
P/P-­‐rated [-­‐]
0.8
0.6
0.4
0.2
0.0
0
5
10
15
20
25
v [m/s]
Figure 1: Difference in assumed power curve for calculation of AEP-measured when setting power values
to zero in not covered bins (red) and when considering only covered bins (blue). The highest covered bin is
here 16,0 m/s.
2. Proposal
The MEASNET Power Curve Experts Group suggests to perform the integration of the
measured power curve only up to the bin averaged wind speed of the highest wind speed bin
(blue curve in Figure 1), because the extension of the measured power curve by linear
interpolation to zero in the next higher bin is not backed by the measurement.
For the same reason, the MEASNET Power Curve Experts Group suggests removing the
extension of the measured power curve to zero power at a wind speed 0.5 m/s below the
lowest covered wind speed bin as defined in the standard by the sentence in chapter 8.3: “The
summation is initiated by setting Vi-1 equal to Vi-0,5 m/s and Pi-1 equal to 0,0 kW.”
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Both of them can be achieved by applying the following changes to chapter 8.3:
1. Start of the summation in equation (6) with i=2 instead of with i=1
2. Change of the description of N in the legend of equation (6) to: “N is the number of
bins covered by the power curve test”
3. Deletion of the sentence: “The summation is initiated by setting Vi-1 equal to Vi0,5 m/s and Pi-1 equal to 0,0 kW.”
4. Change of the 2 sentences: “AEP-measured shall be obtained from the measured
power curve by assuming zero power for all wind speeds above and below the range
of the measured power curve. AEP-extrapolated shall be obtained from the measured
power curve by assuming zero power for all wind speeds below the lowest wind speed
bin in the measured power curve and constant power for wind between the highest
wind speed in the measured power curve and the cut-out wind speed.”
by: “AEP-measured shall be obtained from the measured power curve by performing
the summation according to equation (6) only for the bins covered by the power curve
test. AEP-extrapolated shall be obtained from the measured power curve by extending
the summation according to equation (6) with constant power for wind between the
highest wind speed in the measured power curve and the cut-out wind speed.”
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