The Sandia Inverter Model provides a means to predict AC output power () from DC input power (). The form of the model is as follows: where , , and Parameters: : DC input voltage (V). This is typically assumed to be the array’s maximum power voltage. : DC voltage level (V) at which the…

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Inverter saturation, commonly referred to as “clipping”, occurs when the DC power from the PV array exceeds the maximum input level for the inverter. In response to this condition, the inverter typically adjusts DC voltage to reduce the DC power. This is done by increasing voltage above the MPP voltage, thus reducing DC current. Most,…

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Grid connected inverters must be able to reliably detect a loss of grid condition and rapidly disconnect from the grid system. This behavior prevents the formation of an unintentional island (a stand-alone power system with its own generation and loads operating in balance). Current standards (e.g., IEEE 1547 and IEC 62116) specify that the inverter…

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As more PV systems are added to the electrical grid, the requirements for interconnection are changing. New PV inverters are being required to provide advanced functions to help support the robust operation of the electrical system. These functions include: Volt-VAr (voltage regulation) Commanded Power Factor (voltage regulation) Frequency-watt (frequency regulation) Commanded Maximum Power (frequency regulation)…

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Inverter efficiency is the ratio of the usable AC output power to the sum of the DC input power and any AC input power. Typical grid-tied inverter efficiencies exceed 95% under most operating conditions. Efficiency changes as a function of AC output power, DC voltage, and sometimes inverter temperature. Sandia National Laboratories and BEW have…

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Conversion from DC to AC power allows this power to be tied to the AC grid. This conversion can be accomplished with high efficiencies but there are energy losses that need to be estimated. There are a number of model algorithms that are used to estimate this conversion efficiency. They will be described and compared…

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The performance of an illuminated PV array or system is characterized by the system’s I-V curve. To harvest power from the system the voltage must be adjusted in order to maximize the power. This function is called Maximum Power Point Tracking (MPPT). MPPTY is typically performed by the inverter or a DC-DC converter. A control…

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Gantner Instruments (Juergen Sutterlueti, http://www.gantner-webportal.com) and Steve Ransome (www.steveransome.com), using outdoor IV measurements on a wide variety of module technologies have developed a PV module performance model based on defining loss factors that affect the shape of the IV curve. Details of this model can be found on publications listed on Ransome’s website. We will include a summary of…

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The PVWatts model is provided by the National Renewable Energy Laboratory (NREL) as an online PV performance modeling application. The equations that underlie PVWatts are really quite simple. There are two primary input variables: Effective irradiance () PV cell temperature () PVWatts relates these two variables to the maximum power point () with two equations…

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DC wiring losses are mainly caused by the ohmic resistance of the cabling that interconnects PV devices and strings, although losses can also occur in connections and fuses. The power loss varies as a function of the array current squared. Differences in cable length or size among parallel strings can introduce differences in voltage drop,…

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