Inverter

We have evaluated the IV curve of each array under analysis, modelled its operation at its maximum power point and estimated the DC power losses and voltage drop in the collection network connecting the array to the inverter. We now consider the behaviour of the inverter, modelling how its thresholds and limits and efficiency curves modify the performance of the array and its power output.

There are broadly two stages to the inverter modelling:

  • Establish whether the array MPP is within the inverter's operational envelope and if it isn't, identify the alternative voltage and current at which the inverter will operate the array (the "Operating Point").

  • Calculate the inverter efficiency at the Operating Point, and hence the AC power produced by the inverter.

SolarFarmer models the impacts of the following thresholds and limits of an inverter's Maximum Power Point Tracking (MPPT) on power output:

  • Minimum MPPT DC voltage: this limit may be encountered under high-temperature conditions

  • Maximum AC Power: the maximum power that an inverter can output. With lower module prices, it is common to oversize arrays relative to the inverter to operate the inverter at its maximum output power for more of the time, so this limit is regularly encountered; the inverter manages DC power by adjusting the voltage across its inputs to move the array away from its MPP.

  • Maximum DC current

  • Maximum MPPT DC voltage: this limit may be encountered under low-temperature conditions, when module voltage increases. If the array has been correctly designed, this should be a rare occurrence.

  • Minimum DC Power

These limits can be thought of as confining the portions of the array IV curve previously derived in which the inverter is able to operate and export AC power as shown in the diagram below:

Inverter operating limits

In a well-designed system, the array absolute Maximum Power Point will fall inside the inverter limits most of the time, so we save time by first testing this single point against all of the inverter limits described in the first five sub-sections of this section to see if the inverter can allow the array to operate at its absolute Maximum Power Point. If this is the case, then the limits described in these sections have no effect on inverter output at the timestamp in question, and the calculation can proceed to Inverter Efficiency as described in Inverter Efficiency directly. If the array absolute Maximum Power Point falls outside the inverter envelope, then it will be necessary to work through these five sub-sections using the full array IV curve rather than just the maximum power point to establish inverter performance at this timestamp.

It is important that all quantities be considered at the DC inputs to the inverter. Thus, the DC voltage and power considered must include the effects of ohmic losses between the array and inverter, and the maximum AC power limit requires special consideration, as described in Maximum AC Power.

Where two points on an IV curve offer the same maximum power, it is assumed that the inverter will select the point with the higher voltage (to reduce ohmic losses).

The user can input the absolute minimum and maximum DC voltages limits (as opposed to minimum and maximum MPPT voltages) but SolarFarmer doesn't currently consider them in its modelling.

How the Effect of each threshold or limit is evaluated is discussed below. If it is established at any stage that there is no combination of voltage and current at which the array can be operated by the inverter, then it should be assumed that the inverter shuts down and that for the time step in question:

$$\text{MPP}\left( \text{IV}\left( t \right) \right) = 0$$

The Tare loss described in Night Time Tare Loss will also apply at such times.