Arc Detection Analysis for Solar Applications

Introduction

Arc detection in photovoltaic (PV) systems is a requirement for future solar designs due to new dangers, especially fires, which can occur in PV solar panel installations. This article describes what has created the need for arc detection, an analysis of detection methods, and a possible solution to integrate arc detection in PV inverter equipment and installations.

Background

There are two types of inverters used in solar PV installations today—microinverters and string inverters. Microinverters convert electricity from one panel, whereas string inverters convert electricity from multiple panels or a string of panels. This article will focus on the string inverter type of installation. The power inverter systems in these installations convert the dc power output by the panels to the ac current that can be utilized directly in the home, stored in a battery system, or sent back to the grid. In a typical residential solar PV installation, individual rooftop PV modules are connected in series to form these strings, which in turn are connected to string inverters that can handle between two and four strings. In addition, maximum power point trackers (MPPTs) inside the inverter optimize the match between the PV panels and the output, whether for home use, battery storage, or the utility grid.

Electrical arcing is a serious condition that can occur in solar PV and other current conversion applications that can result in the potential for fire. The detection and reaction (system shutdown) to potential arcing situations is a critical safety feature required of these systems. Arcs can occur on both dc and ac side of PV inverters.

A disconnection of a cable, for example, may cause a dc arc when high current is flowing. Compounding this problem is the fact that the PV array will supply current continuously while irradiance is occurring on the solar cell. This can lead to continuous arcing and lead to fires. This makes the dc side of the PV inverters highly susceptible to dangers. Although there are requirements to disconnect the solar panels in the inverters, this is just for maintenance and not for normal operation.

On the ac side of the application, the arc may extinguish itself at zero crossover, which makes the ac side of PV inverters somewhat less susceptible to risks associated with arcing, since its crossover occurs every 50 Hz or 60 Hz. Also available on the market are arc fault circuit interrupters (AFCI), used for the detection of arc faults in ac circuits.

Therefore, arc detection is indeed a very important factor for solar PV inverters.

Arc detection should consider detection of faults in a PV inverter and shutting down only that affected area of the inverter to ensure safe operation of the device, while the rest of the inverter operates safely. In addition, the start-up or shutdown operation of a PV inverter should also be taken into account, with regard to its characteristics related to arcing.

DC Arcing Detection—Investigation

The Norwegian University of Science and Technology (NTNU) investigations show voltages of 30 V are enough to start and sustain an arc. Their testing method is focused on the voltage domain to detect an arc. They also observed that while the arc is burning, the voltage across the PV module (typically 60 V) drops. The drop in voltage across the arc and for their test was of the magnitude of 10 V. The main reason for analysis in the voltage domain is that a low cost microcontroller was used in the experiment. Otherwise, they recommend analysis in the power spectral density of the current signal, using a more powerful DSP.

An international workshop in Switzerland in 2007, organized by Swissolar titled Arcing in Photovoltaic DC-Arrays—Potential Dangers and Possible Solutions, presented some interesting facts on the effect of dc arcing on the MPPT tracking, and suggested that this should play an important part in arc detection mechanisms in the future.

Figure 1. Effects of arcing on MPPT (Willi Vaassen, TÜV).

Figure 2 shows the resulting MPPT with various arc gaps of 1 mm, 3 mm, and 6 mm, resulting in a huge reduction in performance, as would be expected.

Figure 2. Effects of arc detection on MPPT working point (Willi Vaassen, TÜV).

Further investigation by TÜV show the working point deviation due to the same gap sizes in the MPPT tracker. The results again show a much reduced MPPT performance.

A suggested solution to the proposed issue of dc arcing is based on current measurement analysis. The detection mechanism monitors the current in the load and the current to ground. The current in the load is passed through a filter which removes all but the arc signature frequency range. This is then signal conditioned and passed through logic to turn off the source of the arcing, either the PV module or the PV inverter.

Arc Detection Simulation

Setup

In Figure 3 is a possible setup for arc generation, in line with UL1699B.

Figure 3. Arc generator. (Photo property of ADI, taken in Solar Lab in Limerick facility.)

A PV power system in series with an arc generator and ballast resistor of 1 Ω, forms the basis of the test system setup. Both the voltage and the current through the system are analyzed for possible mechanisms of detection.

Figure 4. Arc setup.

Analysis of Voltage Waveforms

A first look at the voltage across the arc shows some interesting information. With the arc gap open, the voltage across the gap is 71 V approximately. As the gap is closed, a small arc occurs and can be seen on the plot in Figure 5 as a 20 V drop across the gap. As the gap remains closed, a steady current flows and little voltage is detected across the arc.

Figure 5. DC and ac component of the voltage waveform across an arc gap.

However, as the gap opens and the arc begins in a sustained manner, a 20 V (approximately) drop across the gap can be seen. This voltage remains and as the gap increases, the voltage across it increases. At a certain point in time, the arc will cease to continue and the voltage across the gap will return to its setup value.

Further analysis of the voltage waveform under ac performance shows more information. When the gap is closed with no arc present, a transient occurs on the voltage waveform as can be seen in the areas circled red in Figure 6.

Figure 6. Ac analysis of voltage across arc gap.

Another transient occurs when the arc ignites and sustains itself. As the gap is opened further, initially the high frequency components are at a seemingly low magnitude, but as the gap widens their magnitude increases until the gap is so wide (14 mm for 100 V/14 A) the arc cannot sustain itself and ceases. A high transient is also present as the arc ceases.

Analysis of Current Signals

Looking at what’s happening with respect to the current through the system, the waveform below is a preview of the current flowing through the system. First, when the gap is closed, then as the gap opens, and finally when the gap is too big for the current to flow and the arc ceases completely.

Figure 7. DC and ac component of the ARC from a current analysis.

Further analysis on the current flowing through the system shows high frequency components present in the system when arcing is present (Figure 8) and the absence of these signals with the absence of arcing (Figure 9).

Figure 8. No arcing—no high frequency components.

Figure 9. Arcing—high frequency components present.

Frequency Spectrum Analysis

A review of the spectrum of the arc is also of interest here. Figure 11 shows the spectrum with the arc present in the system. It is visible above the base level of the system. At lower frequencies, the level is higher and easier to detect, but at this lower level, systems switching components are present and need to be filtered out to detect the arc signature. A higher resolution ADC may be needed in the lower region of the frequency range.

Figure 10. Arc current spectrum.

Figure 11. No arc spectrum.

At higher frequencies, although the arc is present at a lower magnitude, the switching components of the system are also present at lower magnitudes, therefore the arc can be easier to detect. A lower resolution ADC may suffice in the higher frequency region.

One further piece of valuable information is that the spectrum in Figure 11 changes little under the same conditions irrespective of the current/voltage generating the arc. This indicates that the arc is consistent and therefore detectable in the system.

Conclusions

A solution to dc arcing must be carried out under the following headings:

  • Where in the system the arc can occur and where in the circuit the arc detection is required. This ensures that all arcs are detected.
  • The strength or amplitude of the arcing should then be measured. This is required to make a decisive decision that an arc has occurred. This also eliminates false triggering of arcs due to outside emissions onto the system in question. Therefore, a filter mechanism is required to remove false detection of arcs.
  • Ensure that both series and parallel arcs are accounted for as separate circuits may, or may not, be required for complete detection.
  • Ensure that the electronic circuit can also automatically or manually disable both the PV array and connection to grid, to halt any spread of pending fires.
  • A number of items have been discussed in this document and a summary of these are as follows:

    • Arc detection in PV inverters is a requirement for new developments in solar PV inverters.
    • The analysis of arcing or arc detection is predominantly carried out in the current domain.
    • Tests are all carried out in the dc domain using a test jig aligned with UL1699B directive with two solid electrodes, where high (7 A to 14 A) current is passed through them. These are then separated until arcs occur and continually separated until they are far enough apart that the arcs stop.
    • Maximum power point tracking (MPPT) may play an important role in arc detection and should be considered in developing a solution.
    • Arc detection may potentially be analyzed in the lower frequency spectrum (100 kHz region). A possible solution to arcing is a bandpass filter in a 100 kHz spectrum using the ADSP-CM40s internal ADC.
    • AFCI are available on the market today, which are specifically designed to detect an arc signature in ac circuits.

Arc detection in PV inverters must include a method for predicting the occurrence of arcing, either just before the occurrence of a sustained arc or very early in the in the lifetime of the sustained arc, where the source of the arc can be shut off. Then the PV Inverter can be shut down gracefully, preventing fires and if possible, damage to the inverters.

More Investigation and analysis needs to be done around arc prediction.

References

Haeberlin, Heinrich. Arcing in Photovoltaic DC Arrays—Potential Dangers and Possible Solutions. International Conference Switzerland, 2007.

Norum, Lars E; Schimpf, Fritz. Possibilities for Prevention of Electrical Arcing in PV Systems Norwegian University of Science and Technology (NTNU), 2009.

Norum, Lars E; Schimpf, Fritz. Recognition of Electrical Arcing In DC-Wiring Photovoltaic Systems. Norwegian University Of Science and Technology (NTNU), 2009.

Sclocchi, Michele. Detecting hazardous arc faults. National Semiconductor,
2011.

adsp-cm40x hardware reference. Analog Devices, 2015.

Renewable Energy Generation Web Page. Analog Devices, 2016.

Author

Martin Murnane

Martin Murnane

Martin Murnane is a solar PV systems engineer in the Industrial and Instrumentation segment, focusing on energy/ solar PV applications. Prior to joining Analog Devices, he held several roles in power electronics in energy recycling systems (Schaffner Systems), Windows based application software/database development (Dell Computers), and HW/FW product development using strain gauge technology (BMS). Martin has a bachelor’s degree in electronic engineering from the University of Limerick.