What could happen to solar inverters in case of high-altitude nuclear weapon testing

High-altitude electromagnetic pulses (HEMPs) generated by high-altitude nuclear explosions caused by nuclear weapon testing can produce damaging current and voltage surges in electrical and electronic systems on Earth with serious consequences for the systems themselves, as well as for power grids and electricity supply.

Both the United States and the Soviet Union conducted their largest nuclear weapon tests at high altitude in 1962. In both cases, significant damages to the power grids were reported. For this reason, several research institutions have investigated in recent years the extent to which ectromagnetic pulses may affect power supply at both national and macro-regional levels in an effort to identify possible options for mitigating impacts.

Following this approach, a group of researchers from the University of Tennessee and the US Department of Energy's Oak Ridge National Laboratory (ORNL) has recently investigated the sensitivity to HEMPs of solar inverters. “Our work provides insight into the protection of solar inverters and defense against HEMPs,” the research's lead author, Qiu Wei, told pv magazine.

The scientists explained that HEMPs propagate in three stages that are commonly classified as E1, E2, and E3, with each component being defined by different spatial and temporal characteristics. “The E1 is the beginning of the explosion and varies quickly in hundreds of nanoseconds,” they highlighted. “The energy of E1 is much higher than E2 and E3, it also has the greatest impact on the grid.”

The group analyzed, in particular, the effect of HEMPs on solar inverters during the E1 stage. It used a pulse current injection (PCI) setup based on charge/discharge capacitors and injection probes to reproduce intense transient electromagnetic disturbances, which generally arise when a source emits a short-duration pulse of energy, and to assess the immunity levels of three different inverters – a 22 kW device manufacturer by Denmark based Danfoss, an 8.2 kW inverter from Austrian producer Fronius, and a customized inverter with an output of 6 kW.

They selected two “typical” immunity levels known as EC5 and EC8, with open voltages of 2,000 V and 8,000 V, respectively.

The researchers also applied a port vulnerability analysis scheme based on three different vector network analyzers (VNAs) with a wide frequency ranging from 50 Hz to 1.3 GHz. VNAs are standard test instruments to measure electrical network parameters to implement wide frequency range measurements. “The port vulnerability is evaluated through its spectrum and cumulative energy, and the advantage of the PCI model is that it can quickly analyze the transient response,” they further explained.

The scientists said their analysis showed that the VNAs agreed with each other, which ensured that measurements were taken properly.

“Under the immunity levels EC5 and EC8, the PCI test results show that the maximum voltage and current under EC5 can reach 1500 V and 40 A, respectively, while EC8 can reach 8000 V and 150 A, respectively, revealing that EC8 can lead to serious vulnerabilities,” they stated. “The port vulnerability analysis results demonstrate that the inverters can be damaged due to higher voltage or lower rise time, and also reveal that EC8 is challenging to defend against due to its wider energy frequency ranging from 0.1 MHz to 100 MHz.”

As mitigating strategies, they proposed reducing the impulse voltage for functional and basic insulation, as well as shielding and grounding. “Proper grounding requires small grounding resistance and more grounding points. Short and flat grounding wires should be buried deep underground to reduce resistance and inductance,” they added. “The space between two adjacent grounding points should be small. And one grounding point should be as close as possible to the protected device.”

Their findings are available in the paper “Assessing the vulnerability of solar inverters to EMPs: Port testing, PCI modeling, and protection strategies,” published in Measurement.

The Oak Ridge National Laboratory conducted a similar study on the effects of HEMPs on solar panels in 2020. These tests showed no observable loss of solar module function due to large electric field transients.