Electric arc protection in photovoltaic inverters

Electric arcing is a feared and unwanted phenomenon in photovoltaic systems.

13 minute(s) of reading

The electric arc is a phenomenon caused by the passage of electric current through the air, and is considered a feared and unwanted phenomenon in photovoltaic systems.

During its occurrence, an effect similar to a spark is noticed, with strong light intensity and high temperature.

Due to the high voltage values and the continuous nature of the electric current in photovoltaic strings, electric arcs can form relatively easily in poor electrical connections or during incorrect opening (without an appropriate isolating device) of a photovoltaic circuit under load. .

Electric arcs that arise when we make the wrong disconnection or when there are insulation failures or poor contacts in cables and connectors can cause a rapid increase in the temperature of the location where the arc is located, which can then lead to components melting or catching fire.

Electric arcs can be classified into 3 types:

arcs in parallel;
arcs in series, and;
arcs to the ground.

Figure 1 – The electric arc is caused by the passage of an electric current through the air. It can occur due to bad contacts or incorrect sectioning of circuits in operation

Parallel arcs occur when there is a short circuit between the positive and negative of the array or device, typically caused by damage to the cables or loose cables within the string-box or some part of the circuit.

The series arc occurs when there is an abrupt interruption of an energized circuit, as is the case with the disconnection of MC4 pairs under load or when there is a faulty electrical connection, with poor contact.

The earth fault arc occurs when one of the circuit cables is damaged and comes into contact with the metal casings connected to earth. Annex D of the NBR 16690 standard shows the three types of electric arcs that can be found in the direct current circuits of photovoltaic systems.

Figure 2 – Types of electrical arcs (parallel, series and to earth) according to annex D of standard NBR 16690

The following figure helps to better understand the concepts of series arc and parallel arc within an electrical circuit. In the image on the left (Figure 3) it can be seen that the electric arc is caused by a union between two different poles, in this case caused by a failure in the insulation of the conductors.

The electric arc is exactly like a short circuit sustained by the passage of electric current through the air. In the image on the right (Figure 3) it can be seen that the electrical arc is caused by a contact failure or an interruption caused in the electrical conductor for any reason.

There is no short circuit in this case. The electric current continues to flow normally, without the effect of the arc being noticeable (except for excessive heating and the fire that the arc can cause).

Figure 3 – Parallel arc (on the left) and series arc (on the right). Source: Leviton

Parallel arc and ground arc

When carrying out an electrical analysis of each type of arc, we see that for each situation the inverter sees a type of fault and has an action to be taken. Earth arcs, as they divert part of the string current to earth, are the easiest to detect.

In this situation, the inverter detects the arc as well as a DR (residual device), that is, taking the difference between the input and output current of its DC terminals. If the current is the same measured in the positive and negative poles, there is no leakage detected. If the current is not the same, an insulation fault with earth is detected.

The parallel arc is seen by the inverter as a sudden drop in current and voltage. Depending on the configuration of the inverter's protection systems, it may be able to detect it.

Both the ground arc and the parallel arc cannot be prevented by an inverter action, as disconnecting the array from the inverter through an internal automatic switch, for example, will not change the electrical behavior of these arcs in such a way as to extinguish them. (suppress arc voltage or current).

In other words, there is no action to be taken by the inverter on ground and parallel fault arcs other than the fault notification. One of the main reasons for the special requirements for photovoltaic cables and devices is precisely to reduce the risk of these two types of arcs.

Cables and devices with double insulation, compatible with the system's exposure to time, voltage and current levels and with secure connections are the best methods to avoid arcs in general.

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Series arc

The series arc arises when a conductor is abruptly interrupted or when there is a bad contact in the direct current circuit. For example, an arc may occur when disconnecting an MC4 connector or removing a fuse from a string box while the system is running.

An arc can also occur when the direct current circuit cables are intentionally or accidentally broken or, more commonly, when there are poor contacts in the electrical connections of the string-box or inverter.

The series arc, unlike the others already presented, can be extinguished easily, simply by the inverter interrupting the current flow. To interrupt the arc, the inverter can switch off electronically or open an automatic internal switch, completely and quickly stopping the electrical current.

The problem with this type of arc, although its extinction is relatively simple, is its detection. The arc itself is characterized as the continuity of current flow even in the absence of a section of conductor.

From the point of view of the electrical circuit, the current flows normally even if a series-type electric arc is occurring in some part of the circuit. There are no changes in voltage or current intensity values.

When the series arc occurs, the inverter perceives voltage and current levels compatible with the system's normal operating values. In fact, the same value of the current “entering” and the current “leaving” the inverter is measured, and detection by the inverter’s internal DR device is not possible.

The electric arc is a phenomenon with intense heating, which causes the ionization of the air around it. This ionization changes the equivalent resistance of the air spacing between the conductors and is also capable of melting the conductor and its insulation.

From an electrical point of view, the circuit with the presence of an electric arc presents small oscillations in equivalent resistance and air spacing.

In addition to the effects of air ionization, there is a potential difference between the exposed conductors, creating the equivalent of a capacitor which, when interacting with the inductance represented by the rest of the line, effectively forms an RLC oscillator.

This phenomenon gives rise to a high-frequency oscillatory behavior (in the order of tens to hundreds of kilohertz) that is effectively a high-frequency alternating current (AC) component added to the originally direct current.

This phenomenon of generating high-frequency alternating current was even used in the first radio transmitters, in which an intentionally created electric arc received a current and voltage signal from a telegraph, which was then transformed into a radio frequency signal.

Figure 4 – The telegraph signal modulated the intensity of the electric arc, which in turn emitted radio waves originating in the oscillation described above

The AFCI protection system

The acronym AFCI originates from the English expression “arc fault circuit interrupter”, which means in free translation: “arc fault circuit interrupter”.

AFCI devices are not mandatory and are still practically unknown in Brazil, but they are already gaining ground in some countries. They can be inserted as components in electrical installations or can be embedded in inverters and other types of equipment.

In the figure below, we see an example of an AFCI device that looks like an electrical circuit breaker and can be added to electrical panels and string boxes. There are AFCI devices for direct current and alternating current on the market.

Figura 5: Dispositivo AFCI (interruptor de arco elétrico) que pode ser montado como um disjuntor comum em um quadro elétrico ou string-box. Fonte: Eaton — Figure 5 – AFCI device (arc flash interrupter) that can be mounted as a common circuit breaker in an electrical panel or string-box. Source: Eaton

AFCI devices have been mandatory by the US NEC (National Electrical Code) since 1999, having increased in scope over the years. According to NEC 2020, AFCI is mandatory in practically all areas of a residence.

While AFCI devices are practically unknown in the Brazilian market, in addition to not being required by any national technical standard, some photovoltaic inverters already incorporate the AFCI function, as is the case with the manufacturer's equipment. Solis.

The AFCI system built into the inverters is designed to detect series arcs in the string-box or in direct current cabling. In the event of a series arc, the AFCI system sends an alert to the inverter's CPU (central processing unit) and interrupts the equipment's operation, which immediately stops the flow of current and extinguishes the electric arc.

The series arc is the most common and most likely to occur on the direct current side of photovoltaic systems. The presence of AFCI devices in inverters is very beneficial and desirable, greatly increasing the safety of installations and preventing fires in photovoltaic systems.

Figura 6: Inversor da série 5G da Solis, disponível no Brasil, que possui sistema AFCI integrado para a detecção de arco elétrico, sendo capaz de evitar incêndios. Fonte: Ginlong Solis — Figure 6 – Solis 5G series inverter, available in Brazil, which has an integrated AFCI system for electric arc detection, being able to prevent fires. Source: Ginlong Solis

How inverter AFCI protection works

The inverters' internal current and voltage sensors are not sensitized by high frequency signals typical of a series arc. To detect the arc, the inverter must have an additional AFCI protection device, which analyzes the high-frequency components of the array's voltage and current and interrupts the circuit when high-frequency threshold values are reached.

Figure 6 shows the voltage and current behavior of a photovoltaic inverter in the event of a series arc. The noisy behavior of the two variables (voltage and current) reveals the existence of high frequency components, which can be detected by an AFCI system.

Figure 7 – System behavior during the occurrence of a series arc. Source: adapted from [1]

It is possible to analyze the behavior of the series arc in the frequency domain using a fast Fourier transform (FFT – Fast Fourier Transform), which is a technique used in electrical engineering for the analysis of signals composed of multiple frequencies.

FFT reveals the composition of an electrical signal, revealing its multiple frequencies. The typical electrical current and voltage of a photovoltaic system have only low-frequency components. When an electric arc occurs, high-frequency components are generated that can be revealed by the FFT technique.

Figure 8 shows in detail how high frequency noise levels behave at arc moment. The following figures were extracted from the work “Real Time Series DC Arc Fault Detection Based on Fast Fourier Transform” [1], which was used as a reference in this article.

Figura 8 - Os itens “a” e “c” mostram o comportamento de uma corrente convencional (sem arco elétrico) no tempo e no domínio da frequência. Os itens “b” e “d” mostram como a corrente medida se altera na ocorrência do arco elétrico — Figure 8 – Items “a” and “c” show the behavior of a conventional current (without electric arc) in the time and frequency domain. Items “b” and “d” show how the measured current changes when the electric arc occurs

In Figure 8, comparing figures “a” and “b” (time domain) we see that the main difference is the appearance of high frequency noise. This noise, caused by the existence of an electric arc in the circuit, would go unnoticed in equipment without an AFCI feature.

Conventional inverters are simply unable to perceive the presence of this noise. By analyzing the FFT of the electric current, which is the technique used by AFCI devices, it is possible to perceive the presence of arc noise, as shown in items “c” and “d” in Figure 8. The difference in the arc signature is notable. frequency between items “c” (normal current) and “d” (electric arc current).

What does the Brazilian standard say about arcs in photovoltaic systems

The ABNT NBR 16690 standard does not require the presence of an AFCI series arc detection device in inverters, and this is clearly written in Annex D of the standard. However, even if not mandatory, the standard recommends that there be some type of detection and protection against series arcs in photovoltaic systems, even implying that in future revisions the item may become mandatory:

The best way to protect yourself against arcing is to prevent its causes in the first place. The use of solar-type cables with double insulation, the use of good connectors and the use of good assembly practices significantly reduce the risk of breakage, bad contacts and accidents with electric arcs.

The use of AFCI protection in inverters is recent and is not mandatory, but it can already be found in various equipment available on the market. All protection is welcome and we know that the more reliable and safe the photovoltaic systems, the greater their acceptance by the consumer public.

Reference

[1] MH Riza Alvy Syafi'i, E. Prasetyono, MK Khafidli, DO Anggriawan and A. Tjahjono, “Real Time Series DC Arc Fault Detection Based on Fast Fourier Transform” 2018 International Electronics Symposium on Engineering Technology and Applications (IES- ETA), Bali, 2018, pp. 25-30, doi: 10.1109/ELECSYM.2018.8615525.

Matthew Vinturini

Specialist in photovoltaic systems and electrical engineer graduated from UNICAMP (State University of Campinas). Science and technology enthusiast, with experience in the field of solar energy, both commercially and in the design, dimensioning and installation of photovoltaic systems.