AFCI and rapid shutdown: how to improve the safety of PV systems

This article was produced and adapted from technical materials provided by Kehua

In recent years, with the accelerated search for renewable energy sources, photovoltaic energy conversion systems are also developing rapidly.

Driven by the continuous search for minor LCOE (levelized cost of electricity) in photovoltaic systems, components and inverters in systems are developing in the direction of higher current and higher voltage.

This makes the application scenarios for photovoltaic systems increasingly complex – and the hidden danger of fires also increases.

Therefore, more and more attention has been paid to how to improve the safety of photovoltaic plants. The two best-known technologies on the market are AFCI (arc fault circuit interrupter) and rapid shutdown.

Let's talk about the first technology, AFCI. The acronym AFCI (from English) can be translated as “arc fault interruption system”. Its operation provides a reliable guarantee for the safety of photovoltaic systems against electrical arcs, which are the main cause of fire in solar energy systems.

By identifying the characteristic signs of an electrical arc forming in a circuit, the AFCI disconnects the power source (i.e., the solar panels) before the arc fault turns into a fire.

The arc detection function was first proposed in the United States National Electrical Code, which has required since 2014 that photovoltaic systems with a system voltage greater than 80 V be equipped with an appropriate DC arc circuit breaker.

Do you know what an electric arc is?

O electric arc, also popularly known as arc flash, is the phenomenon resulting from the dielectric breakdown of air, in which a plasma discharge is produced, similar to an instantaneous spark, resulting from a flow of current in a medium that is normally insulating.

The arc always occurs in a space filled with gas between two conductive points, which results in a temperature capable of melting, vaporizing, and even causing fires for this reason.

Due to the high temperature they produce, electric arcs are used commercially in welding and plasma cutting machines, in furnaces for the production of steel and aluminum, among other applications.

Electrical arc fires are generally caused by the following reasons:

1. Series arc caused by poor contacts in electrical connections;
2. Parallel arc (short circuit) caused by insulation failure in electrical cables;
3. Earth faults, also caused by broken insulation of electrical cables;
4. Abnormal increase in temperature caused by poor contact in the electrical circuit, inadequate arrangement of electrical equipment and overload.

So we can see that electric arc becomes one of the priorities if we want to prevent fires in installations.

Because the arc temperature can exceed 5,500℃, the hot particles emitted by the high-intensity electric arc accumulate over time and easily ignite the insulating layer material around the cable, causing an electrical fire.

That's why AFCI technology is a tool for fire prevention, as it allows us to detect and interrupt the arc before it causes a fire.

Below we will address four operating principles of the AFCI system:

1. Arc detection: arcs in electrical circuits are monitored by electronic systems;
2. Identification of arc characteristics: to avoid false detections, AFCI algorithms use characteristic filters and analyze patterns to identify “normal” situations and “dangerous” arcs;
3. Correspondence analysis of protection characteristics: the protection characteristics meet the requirements of the UL1699B standard: on the AC power supply bus, when the AFCI detects 8 half-cycle fault arcs within a 0.5s window, the AFCI will trip and cut the circuit, and the trigger time must be less than 0.2 s;
4. Cutting the circuit to perform fault protection: From this, it can be seen that AFCI is an active fire prevention technology in photovoltaic plants, which requires inverter manufacturers to have strong technical strength.

All series inverters KEHUA offer the option of being equipped with the AFCI function. After continuous technical improvements, Kehua's AFCI technology has reached version 3.0. An AI (artificial intelligence) algorithm specially developed by Kehua is used to continuously monitor arc characteristics and improve identification accuracy, which allows you to quickly and accurately detect the occurrence of electric arcs in direct current circuits.

Rapid shutdown

Now let's take a look at another rapid shutdown technology that has received a lot of attention in the market. O rapid shutdown (RSD) – rapid shutdown system – of the photovoltaic system, as the name suggests, is the system that quickly shuts down the photovoltaic modules in the event of a failure.

This concept was also proposed by the National Electrical Code (NEC) in the United States. In 2014, NEC 2014 690.12 published rapid shutdown regulations, which became required in photovoltaic systems in the USA.

In the 2017 version, NEC 690.12 proposed more stringent requirements for rapid shutdown – the distance to the PV array is 3.05 m as a limit. Within 30 s after triggering the quick shutdown device, the system voltage must be reduced to below 30 V outside the limits of the photovoltaic panels and 80 V in the module circuit. This requires “rapid shutdown at module level”, which means that each PV module individually receives a shutdown command.

In the latest 2020 version of the NEC, the term “rapid shutdown” was revised and expanded and “photovoltaic hazard control systems” were proposed. The new standard requires the photovoltaic system to have a “photovoltaic hazard control system”, so that the photovoltaic system can be in a controllable state in a critical situation, that is, the “photovoltaic hazard control system” can be used to perform module-level shutdown.

Within 30 s after the start of disconnection, the voltage within the limit of the PV modules must drop below 80 V. The birth of RSD technology occurred mainly to prevent electrical shock injuries to firefighters carrying out firefighting after the occurrence of a fire in photovoltaic plants.

It can be seen that RSD is a technology to guarantee rescue conditions after a fire in a photovoltaic plant. So how to implement a fast shutdown at the module level? Different types of inverters have different implementation methods.

If it is a microinverter, since its working voltage is generally around 60VDC, it can naturally meet the requirements of fast shutdown. If it is a conventional inverter, just add a quick shutdown box (can be easily purchased on the market), and the implementation method is simple and low cost.

In summary, AFCI and rapid shutdown (RSD) are technologies for two different moments when a fire occurs in a photovoltaic plant. AFCI is used to prevent problems before they happen and RSD is used to ensure rescue conditions and life safety for firefighters after a fire occurs.

Both can improve the safety of photovoltaic plants, but it is more important to prevent fires with the help of technology and improving the conditions of the installations.