Testing Solar Panel Inverters

A solar panel inverter must meet a large number of requirements before it is permitted to supply power. This applies to both grid-connected inverters and inverters operating within a microgrid configuration. These requirements cover both technical and safety aspects. In the case of a connection to the public power grid, the relevant grid code requirements also apply. Larger installations (>3×80A) are subject to stricter requirements than smaller systems.

For this reason, inverters are generally divided into four categories:

These power classes may vary by country, depending on local regulations. This is separate from the grid code requirements, which also vary by country.

In addition, there is a separate category of inverters that can be connected to a battery. These systems allow for the temporary storage of energy that is not being consumed at that moment, or can provide additional power to support consumption.

In these pages, we focus specifically on the safety requirements and grid codes of various countries. We do, however, discuss the functional tests performed on inverters and the equipment required for those tests.

A solar inverter system consists of several components

Solar panels
The Solar Panel Inverter
Optionally a battery storage system
A grid or microgrid connection

We have specific solutions for all components in this system, but here we’ll focus specifically on testing the inverter.
(See related topics at the bottom of this page for references to solar panel testing, battery testing, and AC microgrid simulation.)

To easily test the solar panel inverter under repeatable, constant conditions—such as light intensity and temperature—we use simulation equipment for solar panels, batteries, and the grid or microgrid.
Optionally, a temperature chamber can also be used to simulate extreme temperature conditions.

A grid or a microgrid installation?

Just to clarify: with a grid-tied connection, the inverter is connected to the electrical grid. The voltage and frequency of the inverter depend on the grid voltage and frequency. The inverter must synchronize with the grid before it can supply power.
In a microgrid, the system is completely disconnected from the grid. The voltage and frequency are generated within the inverter itself and, of course, depend on the equipment to be connected in the country where it is installed.

As a result, we do have two different test setups, depending on whether it’s a grid connection or a microgrid application. We’ll go over both of these in the following pages.

To test a solar panel inverter, we will need the following:

Solar Panel Simulation
Optional battery simulation
An AC grid emulator or an AC load (microgrid)
And for accurate measurement results, a good multi-channel power meter.

Solar Panel Simulation

For solar panel simulation, we use a high-speed DC power supply with specialized software to easily generate the IV curves of various photovoltaic materials. For low power levels, these are primarily high-speed DC power supplies so that we can later perform Maximum Power Point Tracking tests effectively.

TTMS’s product portfolio includes solar panel simulators ranging from a few hundred watts up to modular systems of as much as 10 MW. The various models will be discussed in more detail by power category at a later stage.

The DUT

We then divide the inverters into four categories.
– Micro inverters ranging from mW to approximately 1,500 W
– Small single-phase and three-phase inverters ranging from 1,000 W to 11 kW
– Larger three-phase inverters ranging from 10 kW to about 55 kW
– Large three-phase inverters of 55 kW and above

Each category is broken down further below.

Optional battery simulation

For battery simulation, we use a bidirectional power supply. TTMS’s product portfolio includes bidirectional power supplies from three suppliers, ranging in power capacity from a few hundred watts up to modular systems of as much as 10 MW. The various models will be discussed in more detail later, organized by inverter power category.

An AC grid or an AC load (microgrid)

An AC grid emulator must be able to simulate both the voltage and the frequency of the grid. While this can be done with an AC power supply alone, an AC power supply cannot absorb power. Therefore, in addition to the AC power supply, you will also need an AC load. However, AC grid emulators are now available that allow you to easily set a negative current (current drawn from the DUT).

In a microgrid application, the voltage and frequency are determined by the inverter itself. This means you do not need an AC grid emulator, but rather a (regenerative) AC load. Depending on the various tests, there are also some specifications that can help you with this. These include an adjustable power factor and an adjustable output impedance for tests such as anti-islanding tests.

TTMS offers grid emulators from two suppliers in its product portfolio, ranging from 2 kW to modular systems of > 1 MW. Specific grid emulators with a high voltage range (0–1050 V rms) are also available for extreme applications.

For the microgrid application, TTMS supports AC loads from three different suppliers, many of which also feature a regenerative power function (power is fed back into the grid). The various models will be discussed in more detail at a later stage, organized by inverter power category.

A good multi-channel power meter

For testing, you need to measure voltage, current, and frequency in detail. In most DC power supplies and AC grid emulators/AC loads, this data is also available on the unit itself. But if you want accurate measurements—which are absolutely essential if, for example, you also want to perform efficiency measurements—you’ll need a high-quality multichannel power meter. This is also useful for power factor measurements and measuring harmonics.

TTMS also offers a wide range of power meters and current sensors to perform these tests on power levels ranging from small to large.