The Sandia Performance Model for Grid-Connected PV Inverters is an empirically-based performance model that uses parameters from a database of commercially available inverters maintained by Sandia National Laboratory. The parameters are based on manufacturer specifications and laboratory measurements for a range of inverter types.
The Sandia model consists of a set of equations that SAM uses to calculate the inverter's hourly AC output based on the DC input (equivalent to the electrical output of the photovoltaic array) and a set of empirically-determined coefficients that describe the inverter's performance characteristics. The equations involve a set of coefficients that have been empirically determined based on data from manufacturer specification sheets and either field measurements from inverters installed in operating systems, or laboratory measurements using the California Energy Commission (CEC) test protocol.
Because SAM does not track voltage levels in the system, it assumes that for each hour of the simulation, the inverter operates at the photovoltaic array's maximum power point voltage, given the solar resource data in the weather file for that hour.
The inverter single-point efficiency model calculates the inverter's AC output by multiplying the DC input (equivalent to the array's DC electrical output) by a fixed DC-to-AC conversion efficiency that you specify on the Inverter page. Unlike the Sandia inverter model, the single-point efficiency model assumes that the inverter's efficiency does not vary under different operating conditions.
Note. SAM's Sandia inverter library contains parameters for inverters in the List of Eligible Inverters per SB1 Guidelines at http://www.gosolarcalifornia.org/equipment/inverters.php. We try to keep the library as current as possible, but there may be periods when SAM's library is out of date. If the library appears to be out of date, you can check for updates by clicking the link on the Help menu to see if we have prepared a new module library.
If you are an inverter manufacturer and would like to add your inverter to the list, you should contact the California Energy Commission (CEC) or Sandia National Laboratories directly. For information about the Sandia Test and Evaluation program, see http://energy.sandia.gov/?page_id=279. For a list of Sandia contacts, see http://energy.sandia.gov/?page_id=2772. For CEC contacts, see http://www.gosolarcalifornia.ca.gov/equipment/add.php.
The Sandia inverter model is described in King D et al, 2007. Performance Model for Grid-Connected Photovoltaic Inverters. Sandia National Laboratories. SAND2007-5036. http://infoserve.sandia.gov/sand_doc/2007/075036.pdf. Also see the Sandia PV Modeling and Analysis website at http://energy.sandia.gov/?page_id=2493 for more on PV system performance modeling.
The CEC inverter test protocol is described in Bower W et al, 2004. Performance Test Protocol for Evaluating Inverters Used in Grid-Connected Photovoltaic Systems. http://bewengineering.com/docs/index.htm
To use the Sandia inverter model:
1.Choose an inverter from the list of available inverters. SAM displays the inverter's characteristics and model coefficients for your reference.
If you are modeling an inverter not included in the database and want to use the Sandia model, you can try to find an inverter with similar characteristics to your inverter's specifications.
Each inverter listing shows the manufacturer name, model number and AC voltage rating, and information in brackets about the organization responsible for generating the test data and the year the data was generated. "CEC" indicates that test data was generated by the California Energy Commission.
Inverter Characteristics
When you select an inverter from the Sandia database on the Inverter page, SAM displays the inverter's parameters for your reference.
Note. SAM displays a few of the parameters from the library on the Inverter page. If you want to see the complete set of parameters in the Inverter library, you can do so in the library editor.
The following table describes the parameters in the Sandia inverter library, which are explained in more detail in the King 2004 reference cited above.
AC Voltage (Vac)
Rated output AC voltage from manufacturer specifications.
Power ACo (Wac)
Maximum output AC power at reference or nominal operating conditions. Available from manufacturer specifications.
Power DCo (Wdc)
Input DC power level at which the inverter's output is equal to the maximum AC power level. Available from manufacturer specifications.
PowerSo (Wdc)
DC power required for the inverter to start converting DC electricity to AC. Also called the inverter's self-consumption power. Sometimes available from manufacturer specifications, and not to be confused with the nighttime AC power consumption.
PowerNTare (Wac)
AC power consumed by the inverter at night to operate voltage sensing circuitry when the photovoltaic array is not generating power. Available from manufacturer specifications.
Vdcmax (Vdc)
The inverter's maximum DC input voltage.
Idcmax (Adc)
The maximum DC voltage input, typically at or near the photovoltaic array's maximum power point current.
Coefficient 0 (1/V)
Empirically-determined coefficient that defines the relationship between AC and DC power levels at the reference operating condition.
Coefficient 1 (1/V)
Empirically-determined coefficient that defines the value of the maximum DC power level.
Coefficient 2 (1/V)
Empirically-determined coefficient that defines the value of the self-consumption power level.
Coefficient 3 (1/V)
Empirically-determined coefficient that defines the value of Coefficient 0.
MPPT-low (Vdc)
Manufacturer-specified minimum DC operating voltage, as described in CEC test protocol (see reference above).
Vdco (Vdc)
The average of MPPT-low and MPPT-high, as described in the CEC test protocol (see reference above).
MPPT-hi (Vdc)
Manufacturer-specified maximum DC operating voltage, as described in CEC test protocol (see reference above). The test protocol specifies that the inverter's maximum DC voltage should not exceed 80% of the array's maximum allowable open circuit voltage.
