Inverter model

In this document

The inverter model accurately replicates the physical behavior of an actual inverter, providing insights into its performance and helping optimize the overall efficiency of the solar power system.

Overview

The inverter model simulates the conversion of DC electrical input—derived from PV module IV curves—into AC output suitable for grid connection. It includes the selection of optimal operating points for each inverter input, models conversion efficiency, and incorporates grid and inverter operational constraints to accurately represent inverter behavior within the power system.

Inverter model methodology

The inverter model processes the IV curves produced by the PV conversion model for each input. It determines the optimal operating points, converts the collected DC power into AC output, and applies relevant inverter and grid constraints to ensure correct system operation.

Key features of inverters:

Power factor (pf) setting: The power factor (cos(φ)) defines the ratio of active to reactive power at both the inverter outputs and the grid connection point.
Power grid limitation: Grid operators may impose a limit on the inverter’s active power output to manage the amount of power injected into the grid.
Night power consumption: In standby mode, when both GHI and DNI are zero, the inverter still consumes a small amount of power, as specified in its technical datasheet.

The Inverter model:

Receives IV curves for each input.
Identifies the optimal operating point (Maximum Power Point Tracking) for each input.
Converts these working points into corresponding AC voltage and current outputs.

Figure 1: Inverter model.

Maximum Power Point Tracking

The Maximum Power Point Tracking (MPPT) algorithm determines the optimal operating point on each inverter input IV curve, taking into account both inverter and grid constraints. This point must satisfy the following conditions:

The voltage must be within the allowable input range ⟨V_mppopermin,V_mppopermax⟩.
The current must not exceed the maximum input current (I_max).
The power must not exceed the maximum allowable value (P_max), which is set by the lower limit of the inverter and grid.

The optimal operating point is typically highlighted in the IV curve (Figure 2). The gap between this point and the theoretical maximum—if all constraints were ignored—is reported as clipping losses.

The determined working points for each input are then:

Used to recalculate DC losses on cables
Provided as inputs for the DC/AC power conversion process

Figure 2: Maximum Power Point Tracking.

DC/AC conversion

Once the Maximum Power Point (MPP) is determined, the inverter converts the DC power from the PV modules into AC power suitable for grid integration. This step is simulated using the Sandia Inverter Model, which incorporates inverter efficiency and dynamic performance under various environmental and operational conditions to predict the resulting AC output. The model then determines the complex output working point, specifying both the active and reactive power components at the inverter output.

Complex output working point

In Solargis Evaluate, electrical power within the AC network consists of both active and reactive power. The resulting complex AC output of the inverter can be expressed as

$S = P + i * Q = S * c o s (φ) + i * S * s i n (φ)$

where:

$S$ is the complex value of apparent electrical power,
$P$ is active electrical power,
$Q$ is reactive electrical power,
$φ$ can be calculated as φ=arccos(pf).

Note: The level of reactive power at the injection point depends on electrical components' parameters and user-defined power factor settings.

Comparison with other software

The performance and modeling of inverters are comparable across various solar simulation software:

Software	Parameter name	Notes
Solargis Prospect	Inverters (DC/AC) conversion	SANDIA model for grid-connected PV inverters.
Solargis Evaluate	Inverter losses: Power limitation and DC/AC conversion	SANDIA model for grid-connected PV inverters, checks of output power limitation, clipping losses, and night power consumption.
PVsyst	Inverter loss: During operation (efficiency) Over nominal inv. power/voltage Due to max. input current Due to the power/voltage threshold	Single or three efficiency inverter profiles built from maximum, CEC or EU efficiency, considering operating limits, clipping correction, multi-MPPT.
SAM (NREL)	Inverter: Power clipping Power consumption Nighttime consumption Efficiency	SANDIA inverter model or Inverter part load curve model, checks of MPPT or power clipping, string voltage check (page 72).
SolarFarmer (DNV)	Inverter: Min/Max DC voltage Min DC power Max DC current Efficiency Max AC power Overpower shutdown Inverter tare	Inverter models for Maximum and Weighted Efficiencies or CEC Measured Efficiency Curves, identification of operation outside inverter MPPT tracking area.

Auxiliary losses

Auxiliary losses represent the energy consumed by various support systems required for the operation and maintenance of a PV plant. These include cooling systems, air conditioning, motor drives, and other essential subsystems. While these losses are necessary for ensuring the reliability and functionality of the plant, they reduce the net energy output and must be factored into performance and efficiency calculations to provide an accurate assessment of overall plant productivity. This category also includes outdoor lighting, security systems, and facilities for maintenance staff.

Auxiliary losses are applied during different times of the day:

Night losses: These are continuous constant losses measured in watts (night constant losses). They are considered only when the power on the AC side of the inverter(s) is equal to or less than zero.
Day losses: These can be categorized into two types:
- Continuous constant losses: Measured in watts (day constant losses).
- Proportional losses: These are proportional to the inverter output power expressed in watts per kilowatt (W/kW) (day proportional losses). Users can set a threshold for active power at the AC side of the inverter(s) to define when each type of loss should be considered.