Inrush current in high efficiency solar PV modules

The inrush current is significantly higher than the operational current of the PV module, but the actual value is typically not communicated by the module manufacturers. Unsurprisingly, this can lead to frustration, when the module string inrush current by far exceed the PV test equipment maximum current specification.

This blog post is a theoretical summary of the capacitance, and inrush current effect, seen in particular in HE module strings. Further we present our test results, when testing said modules with the emazys Z300 PV Tester & Troubleshooter.

High efficiency (HE) solar modules can store significant electronic energy as excess charge i.e. capacitance. This charge is released instantaneously, as an inrush current when the module string is “shorted” during field testing. The capacitance can even cause measurement errors, when it is not taken into account.

Photovoltaic (PV) modules with more than 19% conversion efficiency are high efficiency (HE) modules. HE modules have become standard for utility scale solar power plants, and we often see them as bifacial modules i.e. with solar cells mounted on both sides.

HE-bifacial modules have a significantly higher energy output, than traditional PV modules with less efficiency. Such modern PV modules present both advantages and challenges for service technicians!

First of all, personel safety immediately becomes even more critical. Secondly, tools and test equipment must be able to handle the high energy output, when used with HE modules. The development within utility scale PV modules indicates that we can expect even more powerful modules in the future. As of August 2024, the most powerful commercial PV modules are rated at 720 W with +23% efficiency.

Inrush current in high efficiency solar PV modules has become a challenge, in the solar operations and maintenance industry. The Z300 PVT (orange instrument in the photo) was made to quickly test and troubleshoot even high efficiency module strings. Furthermore the instrument is digitally connected. The controller app for the Z300 PVT will send your test results directly to the cloud for later access. In this way that data is secured and the reporting work and document handling is made highly convenient with less room for human error

The effect of capacitance in high-efficiency PV modules

HE solar cells are characterised by the persistence (long life-time) of the charge carriers. For this reason a HE solar cell will have slower time response to changes in voltage or current i.e. a higher electronic capacitance. This capacitance is know as “diffusion capacitance” and it is present in all solar cells, but it is notably larger in high efficiency solar cells.

HE and HE-bifacial modules have an innately high diffusion capacitance, so when we assemble strings of these modules we must be prepared to observe extreme levels of inrush current, when we carry out an electrical test. The stationary Short Circuit Current (Isc) is typically in the 12 – 18 Ampere range.

The inrush can many times higher, but the expected value is not written anywhere on the modules. The technician must hence be aware of effect of capacitance in some photovoltaic modules, and act accordingly when doing field work. If a PV test equipment is not made for handling the initial inrush peak, it can heat up significantly. In a worst case the equipment may be damaged. The capacitance can even cause measurement errors, when it is not taken into account.

Flash testing versus field testing

When “flash” testing of PV modules individually, the module is irradiated with a short flash of light, while the Current-Voltage (IV) characteristic curve is measured. Because of the capacitance effect, the module react slowly with more impedance, and the flash must be carefully synchronised to the measurement. This has caused quite some challenges for the manufacturers of test equipment, that is aimed at module fabrication and factory quality control. It would be fair to say, that these challenges have now reached the field test regime. While measurement accuracy may be a minor problem, the challenges with inrush current has the potential to become a real problem.

Field test with the Z300 photovoltaic tester & troubleshooter – Z300 PVT

Using the emazys Z300 PV Tester & Troubleshooter, we tested 2 different solar power plants in Denmark on bright summerdays in August 2024. Both systems are comprised of HE bifacial modules on single-axis trackers. In what follows we share the test results as an examples of the application for the emazys Z300 PVT.

Field test 1) Longi LR5-72HBD 550M – 550 W

The first field test was done on a 32 MWp solar power plant in Denmark. The power plant is build entirely with Longi LR5-72HBD 550M modules on single axis trackers.

The Open Circuit Voltage (Voc) is 49,8 V and the Short Circuit Current (Isc) is 13.57 A.
All strings have 27 modules, so we expect a string Voc of 27 x 49,8 V = 1344,6V and a string Isc of 13.57 A at Standard Test conditions (STC).

To properly test the both the Z300 PVT and the string we conducted 7 string tests in about 11 minutes, at different levels of irradiation. The result can be seen in the table below.

Field test 2) EGING PV EG-605M60-HU/BF-DG – 605 W

The second field test was carried out at a 6-hectare solar PV power plant in Denmark. The plant is built entirely with EGING PV EG-605M60-HU/BF-DG modules, mounted on single-axis trackers.

Each module has an open-circuit voltage (Voc) of 41.9 V and a short-circuit current (Isc) of 18.44 A. With 32 modules per string, this gives an expected string Voc of:

32 × 41.9 V = 1,340.8 V

and a string Isc of 18.44 A under Standard Test Conditions (STC). Notably, the expected string voltage is almost the same as for the Longi module strings tested earlier, despite having five additional modules. However, the EGING modules are expected to deliver a significantly higher current.

During the test, we performed nine measurements 11 minutes. Weather conditions were stable, with an irradiation of ~670 W/m² on the module face. The results are shown in the table below.

The Z300 PVT is controlled via an app, ensuring fully connected data capture. Every test result is automatically stored in the emazys cloud database, making it quick and convenient to access results and generate reports in any standard spreadsheet format.

No adverse effects of inrush current were observed in these high-efficiency PV modules.

Conclusion

High-efficiency (HE) solar modules have higher capacitance, which can lead to measurement errors and large inrush currents. These inrush currents can be particularly challenging for compact test equipment designs.

The Z300 PVT is specifically engineered to handle high-efficiency modules and manage the inrush currents often observed at string level in the field. Inrush current magnitude is influenced by module efficiency, operating current, voltage, and irradiance. Bifacial modules can further increase this effect.

If your test equipment was not designed for HE modules, it is worth reconsidering your setup. Testing HE or bifacial modules with inadequate equipment can lead to incorrect results — and in some cases, irreversible damage.

Ways to reduce inrush current risk:

• Break long strings into shorter strings to lower voltage.
• Separate parallel strings into single strings to lower current.
• Temporarily orient trackers away from the sun.
• Test later in the day under lower irradiance (simple and effective).
• Reduce overall string power (P = I × V) by any practical means.
• Be mindful of cell technology — some materials have higher capacitance than others.

For advice on this complex topic, or to discuss whether the Z300 PVT is the right fit for your needs, feel free to contact Emazys.