Time based availability only accounts for downtime. The definition of Contractual Time Based Availability is
Contractual TBA = T_Oper_Avail/T_Total
where T_Oper_Avail = Time a turbine was operating (excluding alarms related to owner/weather/grid category)
T_Total = Total time
Technical Time Based Availability = T_Oper/T_Total
T_Oper = Time turbine was operating and does not exclude any alarms where turbine could not produce power.
Energy based availability (EBA) measures the true impact of plant unavailability for variable power resources such as wind and solar farms. An hour of downtime at high winds costs more revenue than the same hour of downtime at low wind speeds. Calculation of energy based availability requires the knowledge of amount of downtime and an accurate estimate of the energy that particular device would have produced, typically called, Reference Energy. Calculation of reference energy needs to be accurate to calculate EBA accurately.
Reference Energy: The estimate of energy that a turbine would have produced over given time period if it was continuously operating. If there is no fault during a given time period, Reference Energy is equal to Actual Production (or very close to it).
Energy Loss: Energy loss is the amount of lost energy because of downtime and is calculated based on Reference Energy that is calculated for every 10-minute period. If the turbine was no-operating for the full 10-mins, the energy loss will be equal to reference energy for those 10-minutes. If the turbine does operates for part of 10-minutes, energy loss is calculated based on the fraction of time that the turbine is not operating.
Renewables Suite uses multiple methods to calculate reference energy.
A. Neighbor Comparison Model
If historical data for the asset is not available, average energy generated by the neighbors of the target turbine (either geographically or statically provided in the OEM contracts) is used as reference energy.
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If the neighboring “N” turbines are operating (chosen based on distance and may include wind direction), average power output from these turbines is used as reference energy. Only turbines that did not have any fault are used in this calculation.
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If all the neighboring turbines are faulted too, a reference power curve (could be manufacturer power curve or historical power curve is used based on nacelle wind speed)
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If the target turbine does not report the wind speed because of communication loss or some other reason, average wind speed from neighboring turbines is used to estimate the wind speed at the target turbine.
B. Machine Learning Based Model
If historical data is available, Renewables Suite uses an ML based algorithm to calculate the estimate of Reference Energy. This is much more accurate as the method can take into account how the current operating conditions would have affected the target turbine. Neighbor analysis is relatively accurate if the turbines are located in flat topography and are far apart not to cause wake affects. This is typically not the case. Renewables Suite uses wind speed, wind direction, ambient temperature etc. from the target turbine and the neighboring turbines to calculate the 10-min average power and hence the estimate of energy production for the non-operating turbine.
Energy Based Availability (EBA): Contractual energy based availability can be calculated in two ways:
Reference Energy Method
EBA = 1 - (Contractual Energy Loss) / (Reference Energy)
Production
EBA = 1 - (Contractual Energy Loss) / (Actual Production + Contractual Energy Loss)
Typically, OEM contracts use Production based method for actual availability. The choice of the calculation method is determined based on the contractual requirements or customer preference during onboarding.
Typically downtime is classified as a responsibility of OEM or Operator or owner. If downtime occurs because of weather (high wind, icing), grid (requirement by grid to stop the turbine) or owner (customer stop, included maintenance hours) etc. Contractual Energy Based Availability only accounts for energy loss during downtime that is responsibility of OEM/Operator. Technical Energy Based Availability is described as
Technical EBA = 1 - Total Energy Loss/Reference Energy
The energy loss included here is irrespective of the fault being a manufacturer category or owner/weather/grid.
Production based availability focuses on the production lost when the turbine is offline, not the time when a turbine is offline. The lost production is calculated for every 10-minute period related to the average energy production of operating turbines.
The general process for calculating lost production for a 10 minute period is as follows.
- Calculate the amount of time for the associated time categories for each turbine
- Add up the Turbine Unavailable Hours (TUH) for the entire site
- Subtract Turbine Unavailable Hours (TUH) from the total possible period time at the site (0.167*number of turbines) to get Equivalent Operating Hours (EOH)
- Take the total production at the site and divide by Equivalent Operating Hours (EOH) to get Equivalent Production Rate (EPR)
- For each turbine, multiply the turbine's total down time by EPR to get the turbine's lost production (LP)
- Sum up each turbine's lost production to get Operating Period Lost Production (OPLP)
Consider the following example for a 10 minute period at a 10 turbine wind site.
In this example
| Counter | Counter Description | Details | Availability Impact |
|---|---|---|---|
| DT | Down Time | Time the turbine is not available to produce power due to its own internal problems such as equipment | Yes |
| RT | Repair Time | Turbine removed from service for unscheduled repair. These conidtions affect the status of the RT status time for e.g: Repair Mode | Yes |
| MT | Maintenance Time | Turbine removed from service for maintenance purposes. These conditions affect the status of the MT status timer for e.g.: Service switch hydraulics in top cabinet, locked rotor, generator fan service switch, maintenance mode | Yes > x -hrs based on contract |
| SOT | System OK Time | Time the turbine is able to produce power. The turbine may or may not be actually producing power due to some other condition such as low wind | No |
| LOT | Line Out Time | Utility line tripped or in limit. These conditions affect the status of the LOT status timer for e.g.: line overvoltage, line undervoltage Line over frequency, line under frequency |
No |
| WOT | Weather Out Time | Weather-induced condition that inhibited turbine operation such as high Wind or extreme temperature. These conditions affect the status of the WOT status timer for e.g.: Storm cut off (high wind), Low wind, Low ambient temperature, Low gear oil temperature (cold soak) | No |
| EST | External Stop Time | External trip due to operator manual intervention | No |
| EECT | External Energy Curtailment Time | External trip due to external energy curtailment | No |
| CST | Customer Stop Time | Time that the customer has stopped the turbine | No |
| WDCT | Wind Direction Curtailment Time | Time that the turbine was stopped due to wind direction curtailment | No |
| ST | Survey Time | Total period hours | No |
First, count the amount of time in each turbine's event time category.
The possible survey time for this time 10-minute time period is 0.167*(10 turbines) = 1.67 hours.
Then Turbine Unavailable Hours (TUH) must be added up for the entire site (DT, RT, MT).
In this instance Turbine Unavailable Hours (TUH) is 0.167 + 0.067 = 0.23 hours.
After TUH have been calculated, they must then be subtracted from the total possible survey time to get Equivalent Operating Hours (EOH).
Equivalent Operating Hours therefore equal 1.67 - 0.23 = 1.44 hrs
Equivalent Production Rate (EPR) must then be calculated by taking the total production at the wind site and dividing by the Equivalent Operating Hours (EOH).
Equivalent Production Rate (EPR) is 790/1.44 = 550.14 kW.
Lost Production for each turbine is next to be calculated. This is done by multiplying the turbine's total down time by the EPR.
Lost Production Turbine 4 is 0.167 x 550.14 = 91.87kW
Lost Production Turbine 5 is 0.067 x 550.14 = 36.86kW
Each turbine's lost production must then be summed up to get Operating Period Lost Production (OPLP)
Operating Period Lost Production: 91.87 + 36.86 = 128.73 kW
After this process has been completed for all individual 10 minute periods throughout the month, the total actual production and lost production is then aggregated.
Using the aggregated Actual Production and Lost Production values in the Production Based Availability formula yields a PBA of 99.1%.
Data quality is an important metric for calculating energy loss for the downtime. There are many situations where all data signals may not be reporting with data compression, communication error, sensor error etc. being a few of them. Renewables Suite calculates data quality for every 10-min data duration and assign a 0 if data quality is poor and 1 if data quality is good. Energy based availability is reported as energy based availability weighted by data quality. For availability purposes, Renewables Suite uses the following definition of data quality.
Data quality is 0 if
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Target Turbine is not reporting wind speed and active power (NULL values) but other turbines are reporting data. This covers a case where turbine may be operating but not communicating.
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Target Turbine is reporting only “Active Power” but no wind speed and it is not possible to calculate reference energy e.g. nearby turbines (neighbor algorithm) on the site are also not reporting.
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The whole site has lost communication and none of the devices are reporting data for the particular time period.
If data quality is 0 for a 10-min time period, the time period is not used in calculation of aggregate energy based availability.