Solar microinverter testing in a production environment

Solar panels are growing more efficient over time. According to the Tracking the Sun report from the Lawrence Berkeley National Laboratory, in the 10 years between 2011 and 2021, the median solar panel efficiency grew from about 14.5% to about 20.1% and will reach 25% in a few years. As solar panels increase in power, the inverters that connect to them will need to follow suit.

Unlike a central inverter that connects to multiple solar panels, one microinverter attaches to each solar panel and converts that panel’s DC power into AC power. This panel-level approach allows for more granular control and optimization of the power output, lessening the impact of events such as shading. As demand grows for solar microinverters and as their power capacities increase, engineers are increasingly aware of the challenge in testing them. Unfortunately, the testing is not simple, as solar microinverters must optimize power transfer during fluctuating levels of irradiance, and the test must ensure that a microinverter operates optimally in all possible cases.

As an example of the current challenges in testing microinverters, a leading company making solar microinverters recently developed a new product. The company needed to acquire production test equipment that would allow it to achieve fast time-to-market while meeting several key requirements:

Each test system would require sufficient power to exercise the microinverters beyond their typical operating power and voltage levels.
Each test system would require the ability to simulate the fluctuating output of solar cells subjected to varying levels of irradiance.
Reduced test system cost as it was going into a large production-scale application.

To keep up with orders, factory managers wanted to triple throughput by testing 150 inverters at a time. In addition, the test equipment had to squeeze into limited production-floor space at the company’s new facilities. What’s more, as most forecasters predict double-digit percentage growth in the demand for solar microinverters, managers wanted to reserve space for future production expansion.

Functional microinverter test system requirements

Microinverters are typically tested using programmable DC power supplies that mimic a solar panel’s output, but the test presents two distinct test challenges:

First, solar panels have a nonlinear current-voltage (I-V) curve, along which lies a maximum power point (MPP). A standard programmable DC supply features a hard output regulating characteristic that maintains a programmed voltage regardless of load current (within specified operating limits), and it cannot reproduce a solar panel’s fast I-V curve.

Second, the microinverter must track the MPP using a maximum power-point tracking (MPPT) function and adjust its output characteristic accordingly to always extract maximum power from the panel. MPPT functions typically impose an AC ripple on a panel output, which a standard power supply would regulate out.

Consequently, microinverters are typically tested using a dedicated solar array simulator. However, because prototypes based on the company’s new microinverter design had been fully characterized and validated with a solar array simulator, the design engineers decided that a simulator with the fast I-V curve that mimics an actual PV panel while preserving a microinverter’s MPPT ripple would not be required for production functional test. The lack of a requirement for full solar array simulation simplified the search for a test system, but the application imposed other requirements that proved difficult to meet.

Foremost, the engineers required that the test equipment include a programmable DC supply that could provide an output of 800 W to 1,700 W with a voltage range between 120 VDC and 100 VDC. The output had to exhibit a constant-power characteristic across its output voltage range. This meant that the programmable power supply had to offer an auto-ranging capability that allows the supply to automatically adjust its output voltage and current over the full operating range in order to provide the required power.

The company’s engineers searched online for test equipment that met their needs, and while complete test systems were available that combined a PV panel simulator and grid simulator, they were very expensive. Also, the team identified bi-directional programmable power supplies that could both source and sink sufficient power, but the options they saw would take up too much space. Not finding exactly what they needed, they contacted the applications team at one of their supplier partners, AMETEK Programmable Power, whose test systems they used in other areas of their global business. Applications engineers at AMETEK Programmable Power realized that they had no off-the-shelf solution that would meet the microinverter maker’s needs. The existing programmable DC supply was limited to 600 W, which was short of the 800 W to 1700 W requirement.

A custom solution

With evolving technologies, test requirements present a moving target, and there may not be a ready solution. The team at AMETEK Programmable Power began to consider whether they could modify an existing product to create a custom test solution for this manufacturer. The local applications support team met frequently with engineers at the inverter manufacturer to fully understand the application. They determined that they could adapt an autoranging Asterion DC ASA programmable power supply through a firmware change to increase the current per channel by about one ampere and meet the manufacturer’s 120-V at 14-A to 100-V at 17-A specification, as shown in Figure 1. Testing confirmed that the customized programmable power supply could operate at the higher power without safety or reliability concerns.

The microinverter manufacturer purchased 50 of the custom three-channel devices to meet its 150-channel need. In doing so, it avoided 84% of the cost of a more expensive test system that would have required more rack space and more integration. After the installation, the company added the same test solution at other manufacturing locations.

In the ever-changing field of microinverter testing, there are key requirements such as cost, space, and capabilities that need to be reviewed in order to not only test current requirements but also satisfy future needs.  Additionally, understanding the true requirements of the test can help drastically simplify the test setup and further reduce cost, which was the case for this manufacturer.  Suppliers like AMETEK Programmable Power should not only provide commercial off-the-shelf products for microinverter testing but also have technical staff that can help their customers understand the requirements of the test to determine the appropriate test equipment.