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

Inverter modeling is a critical component of PV simulation, as it directly impacts the accuracy of energy output predictions and overall system performance. Through Maximum Power Point (MPP) tracking, the inverter dynamically adjusts to changes in irradiance, temperature, and other environmental factors, ensuring that the PV system operates at its optimal efficiency while adhering to system constraints. This capability is essential for achieving reliable and efficient energy production under real-world conditions.

A comprehensive understanding of the inverter's performance is vital, particularly in the DC-to-AC conversion process. Accurate modeling of this conversion not only predicts the energy output but also evaluates the inverter's ability to manage power quality and reactive power requirements. These aspects are crucial for maintaining grid stability and ensuring compliance with modern energy standards.

Additionally, auxiliary losses—energy consumed by support systems such as cooling, lighting, and motor drives—must be incorporated into performance and efficiency calculations.

Key features of inverters

Power factor (pf) setting: The power factor (cos(φ)) determines the ratio of active to reactive power at the inverter outputs and at the grid injection point. The calculated apparent power at the inverter output will not exceed the maximum apparent AC power specified in the inverter's datasheet.
Power grid limitation: If required by grid operators, a power limit can be set at the inverter to control the injected electrical active power at the grid injection point.
Night power consumption: An important feature of inverters is their night power consumption, usually specified in watts in the technical datasheet. This represents standby mode consumption when GHI = 0 and DNI = 0.

Maximum Power Point (MPP) calculation

The Maximum Power Point (MPP) calculation identifies the optimal operating point on the IV curve on the inverter input. This point is critical for ensuring the PV system operates efficiently, maximizing the energy harvested under varying environmental and operational conditions.

Process: The Maximum Power Point (MPP) calculation utilizes the IV curve at the inverter input, which is derived from the previous simulation steps. Multiple Maximum Power Point Tracking (MPPT) circuits are simulated to continuously adjust the operating points of PV modules, ensuring maximum power delivery.
Inputs: The inputs for MPP calculation include the P-V characteristics, calculated as P=V×I. If the inverter's power output needs to be limited due to its capabilities or grid restrictions, the input circuits regulate power from the DC array accordingly.

DC/AC conversion

Once the Maximum Power Point (MPP) is identified, the inverter converts the DC power generated by the PV modules into usable AC power. This critical step ensures that the energy harvested from the solar panels can be effectively delivered to the grid or used in local applications, maximizing the system's overall efficiency and utility.

To simulate this conversion process, we employ the Sandia Inverter Model, which provides a detailed and accurate framework to determine the efficiency curve under different operating conditions, specifically varying DC input power and voltage levels. This model considers various factors, including the inverter's efficiency and other dynamic operating characteristics, enabling precise predictions of the AC output under diverse environmental and operational conditions.

Following this, we calculate the Complex output working point, which involves determining both the active and reactive components of the electrical power 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).

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.