Inrush current in high efficiency solar PV modules
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. This capacitance can even cause measurement errors, when it is not taken into account.
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.
Inrush current in high efficiency solar PV modules
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 and installers.
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.
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, while the inrush could go much higher depending on the test conditions and PV technology.
The field technician must hence be aware of effect of capacitance in some photovoltaic modules, and act accordingly when doing field work. Whenever 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 severely damaged.
Flash testing versus field testing
When “flash” testing of PV modules individually right after fabrication, 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 modules, as well as manufacturers of test equipment aimed at factory quality control.
While measurement accuracy may be a minor problem, the challenges associated to inrush current can more complex. Test and commissioning of solar energy assets, is mandatory in most parts of the world now. Technicians must be "armed" with appropriate equipment, that will last while testing hundreds of strings per day during buys periods.
Field test with the Z300 photovoltaic tester & troubleshooter – Z300 PVT
Using the emazys Z300 PV Tester & Troubleshooter, we tested different solar power plants in Denmark on bright summerdays in August 2024. All systems were comprised of HE bifacial modules on single-axis trackers. In the following we share our field test notes from one of the testing sessions.
领英推荐
Longi LR5-72HBD 550M – 550 W
The 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. These modules have a Pmax of 550W.
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 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.
Capture test result, time-stamp, GPS coordinates and photos
Conclusion
High efficiency modules have high capacitance which can cause measurement errors and a large in-rush current. The latter can pose severe challenges to compact architecture test equipment. The Z300 PVT is designed to measure high efficiency modules and handle the in-rush current, that is often observed at the string level in the field. The inrush current is a function of module efficiency, current, voltage, and irradiance. Also, when a module is bifacial, it will typically increase the inrush current.
If you do not have test equipment, that was specifically designed for testing HE modules, the general recommendation would be to consider it for the future. If you test HE and bifacial modules with inadequate equipment, the result may be irreversible damage.
You can however decrease the likelihood of a large inrush current, by doing the following:
Please do not hesitate to reach to emazys for advice on this complex topic.
Literature and background - links:
Business Development Executive, EMEA. New markets development, software and solutions sales.
6 个月Very interesting and insightful. Thank you for sharing.
Assistant Manager Solar at University of Lahore || x Solar Operations Engineer(Project Execution) and O&M Engineer at G-Solar || Renewable Energy || PEC Electrical Engineer
6 个月Well done