1. • Ensure a representative, impartial Facility Power Curve is produced that accurately captures the performance of the wind farm being analysed.
As part of the Power Purchase Agreement (PPA) for projects submitted under the Renewable Energy Independent Power Producers Procurement Programme (REIPPPP) in South
Africa, owners of wind farms must, at the end of the first year of operation, report a Facility Power Curve (FPC) for the asset. This power curve is used in order to calculate Deemed
Energy Payments from the network operator during a system event (for example, grid downtime or curtailment) and must be checked by an Independent Engineer in order to
confirm its compliance with the PPA.
A set of rules have been outlined within the PPA for each project, however, these are not rigidly defined and there is scope for large deviations in the FPC produced. This variation
can lead to un-representative FPCs and therefore the compensation mechanism has the potential to fail to capture accurately the impact of availability on this key metric of wind
farm performance.
Based on Wind Prospect’s experience as Independent Engineer on operational projects in South Africa a new methodology is presented. This method avoids many of the potential
pitfalls of the PPA requirements whilst ensuring compliance with the rules and also remaining true to the intention of the metric. We also address the often limited data available for
an FPC after only one year of operation – for example how to treat wind speed and direction bins with minimal data coverage. The benefit of the proposed new approach is that a
true representation of the asset’s energy production can be obtained without biases introduced due to poor turbine availability. The new method requires detailed understanding
and more intensive data analysis; however it is considered that this yields benefit for both parties (buyer and seller of energy) as more accurate Deemed Energy Payments will result.
Abstract
Background
Optimising Facility Power Curve (FPC) Calculations
in South Africa
Kamil Beker/David Pullinger/Matthew Behrens
Wind Prospect Ltd.
PO.023
EWEA Analysis of Operating Wind Farms 2016– Bilbao– 14-15 April 2016
QR
code
Initially, wind data provided by the owner/operator is checked and cleaned to remove
erroneous values. Any time offsets are identified and corrected for. Typically, not all
wind speed and direction bins are populated but this is expected due to the variability
of wind throughout the year. Wind Prospect’s approach involves creating 5 FPCs and
these unpopulated bins are approximated according to a prioritised set of rules. For
each FPC produced, the data availability is determined. The process can be summarised
as illustrated below.
• FPC-A – All Data;
• FPC-B – All WTGs operational;
• FPC-C – No more than 2 WTGs not operational;
• FPC-D – Same as FPC-C, scaled (Total WTGs/No. of operating WTGs);
• FPC-E – Omni-directional.
The final FPC is produced as a combination of the above power curves (FPC-A to FPC-E,
excluding FPC-C) in order to fill gaps in the data and provide representative values in all
valid wind speed bins.
Once these 5 tables have been produced, the following prioritised set of rules is applied.
Objective
According to the FPC procedure outlined in South African PPAs [1], wind farm data must be recorded for 1 year from the Commercial Operation Date (COD) of the project. This data
must consist of wind speed (m/s), wind direction (degrees) and power output of the facility. Wind data may be measured by either a meteorological mast, turbine or other
appropriate self-supporting measurement station used to monitor conditions experienced on-site. The PPA requires that this data be recorded in the following manner:
• Average values taken over 10 minute intervals
• Logged by a data logger on the project site and timestamped
• If the power output of the facility cannot be measured directly at the delivery point, then it can be calculated by summing the energy produced by each turbine and multiplying by
0.975 to account for electrical losses
As part of the facility power curve, which is produced by the wind farm owner/operator, the wind data must be binned into speed intervals of 0.5m/s and direction intervals of 15
degrees. For each wind direction bin, the FPC must show the relationship between wind speed and capacity of the wind farm.
Data
cleaning &
validation
Produce
FPC A-E
Apply
Prioritised
Set of Rules
Produce
Final FPC
Criteria Rule / Order of Priority
Wind speeds
< 6 m/s
FPC-A is used.
Wind speeds
≥ 6 m/s
1. FPC-B
2. FPC-D
3. FPC-A
4. Otherwise bin left empty.
Empty data
bins
1. If an empty data bin has 2 adjacent values, then the average of the
adjacent bins is used.
2. If an empty data bin only has 1 adjacent value in the wind direction
bin to its immediate left (West) or right (East), then this value is used.
3. If there are still data gaps between wind speed intervals in a direction
bin, then linear interpolation is carried out (between lower and
higher wind speed values within the same bin).
4. The last value (i.e. highest wind speed value in each direction bin) is
used to fill the remaining gaps up to 20.5 m/s.
Wind speeds
>20.5 m/s
1. FPC-E is used to fill any remaining empty data bins. Any empty data
bins remaining after this process are left blank as the value either did
not occur or there were not at least 3 data samples for that bin.
1. Power Purchase Agreement IPPIDXXX – Wind Projects, between XXX and Eskom Holdings SOC Limited pursuant to the Renewable Energy Independent Power Producer
Procurement Programme Round 1.
2. International Electro-technical Commission, “IEC 61400-12-1:2005 Wind Turbines - Part 12-1: Power performance measurements of electricity producing wind turbines,” IEC,
2005Journal Article, Name of Journal.
References
• FPC-A – Periods when not all turbines are operating have been included.
• FPC-B – There is significantly less data included within the calculation than for FPC-A. This method has been chosen in order to avoid double-counting of the impact of wind farm
availability within the Deemed Energy Payment calculation.
• FPC-C – There is an inherent negative bias due to time periods when not all turbines are operating. However, this FPC is not included in the calculations and is used merely to
demonstrate sensitivity to the no. of WTGs operational.
All negative values are excluded. All data bins are required to have a sample count of 3
or more to be considered complete [2] and if this requirement is not satisfied, then the
next FPC is used according to the order of priority.
Limitations
Proposed Method