EN 
PT_BR 
The safety of direct current circuits in photovoltaic systems has received great attention in many countries.
Direct current circuits are where the highest voltage levels are found, which can be dangerous especially during accident situations (such as fires).
In direct current circuits there is also the risk of generating electric arcs, which are fire starters.
Although photovoltaic systems are not statistically major causes of fires, the presence of electrical arcs (normally caused by poor contacts) is potentially dangerous and must be avoided at any cost in photovoltaic systems.
MLPE and the safety of DC circuits
MLPE (module level power electronics) is the name we give to the family of microinverters and power optimizers. MLPE-type systems naturally have some advantages in relation to the safety of direct current circuits.
Some safety features of DC circuits can also be achieved in conventional string inverters with the use of accessories, but in systems with MLPE these features are already native.
Photovoltaic systems with microinverters are naturally safe and meet the requirements for quick disconnection and safety voltage level required in some countries, because in the event of disconnection of the connection to the electrical grid (i.e., turning off the general circuit breaker of the photovoltaic system) no voltage will be released. presents at the output of the inverters.
Furthermore, photovoltaic modules are used individually and never connected in series, which always keeps open circuit voltages at safe levels.
The use of power optimizers linked to photovoltaic modules has become very common in the world market, including in Brazil.
In this type of system, strings are constructed indirectly, as shown in Figure 2. Strings are formed by optimizers in series and not by modules in series.
Optimizer technology presents some advantages from the point of view of efficiency during normal operation. In addition to the operational advantages, the presence of optimizers naturally offers greater security to photovoltaic systems with regard to rapid shutdown and the safety of direct current circuits – by reducing the voltage levels present in the circuit.
Rapid shutdown – rapid shutdown of the photovoltaic system
The rapid shutdown system of the photovoltaic system, known as rapid shutdown, is a resource used and required by law in some countries in which the rules and restrictions for the use and installation of photovoltaic systems are stricter.
Article 690.12 of the National Electric Code (USA) 2014, for example, requires a shutdown mechanism on all hidden conductors that are not close to modules and inverters. D
and according to the code, all conductors that are more than 3 meters away from the inverter must be de-energized in less than 30 seconds, in case of emergency, and cannot present a voltage above 30V at their terminals.
There must be a visible and identified mechanism for emergency shutdown of the photovoltaic system. This measure was adopted mainly to ensure the safety of firefighters during fire situations.
Although photovoltaic systems are not associated with the main causes of fires in countries where these statistics have already been collected, the rapid shutdown offers the maximum security necessary in case of emergency situations.
In other words, the rapid shutdown system is not the photovoltaic industry's response to preventing photovoltaic systems from causing fires.
It is just a feature that increases the safety of photovoltaic systems in any type of situation (including fires caused by any cause), avoiding risks for firefighters or technicians who may be maintaining the systems.
In Brazil the rapid shutdown It is not yet a reality, that is, there is no obligation to use this type of resource, but the technology is already available in the country.
We have already seen some reports of early fires with photovoltaic systems in Brazil.
A very well-known case, with a video that circulated widely on the internet a few months ago, was that of a car dealership in which several mistakes were made: inverters installed high, absence of a visible disconnector switch, stringbox installed very close to the inverter and also the lack of preparation and training of photovoltaic system users.
If the system were installed in accordance with the North American NEC standard, it should have an isolating device and a switch button. rapid shutdown visible, preventing attempts to extinguish the fire, by those present or the fire department, from causing more serious accidents.
Not only the system rapid shutdown It would have made it possible to extinguish the beginning of a fire (probably caused by an electric arc), as well as guarantee the safety of firefighters in the event of a larger fire.
According to the documents Safety Risks and Solutions in PV Systems It is Application Note – Rapid Shutdown in SolarEdge Systems, from inverter manufacturer SolarEdge, fast shutdown (rapid shutdown) of the photovoltaic system can be triggered by one of the following events:
- The inverter's connection to the electrical grid (on the AC side) is lost due to the tripping of a circuit breaker, a grid failure or any other reason that prevents the injection of energy into the electrical grid;
- The inverter is intentionally turned off by the user, and its operation is terminated;
- The inverter's DC disconnector switch (string input) is turned off;
- An anomaly is detected by the electric arc detection system.
Arc flash detection
Electric arcs, as mentioned above, are major fire starters in photovoltaic systems. Arcs are caused by the passage of electric current through the air when there is poor contact in some part of the installation, normally in the connections of the strings in the electrical panels or in the inverter.
Poor quality connectors (which are not rare in installations) can also cause poor contacts and produce electrical arcs.
Sections 690.11 and 690.12 of the North American NEC 2014/2017 standard require photovoltaic systems to be able to detect and eliminate electrical arcs through automatic inverter shutdown.
The UL1699B standard requires the presence of an electric arc detection system in DC circuits with voltages above 80 V.
With the electric arc detection feature, the photovoltaic system has the ability to detect and interrupt electric arc faults caused by poor contacts in conductors, connections, modules or any other component of the system.
This is a subject that is still not talked about much in Brazil. Electric arc detection is still far from being a concern here in the country.
In addition to there being no regulatory guidelines for this purpose, most of the public is unaware of the danger of electric arcs. This is even more serious when we see the existence of many precarious photovoltaic installations in the country, “designed” and carried out by untrained professionals.
It is not uncommon to use generic components (connectors, disconnect switches, surge protectors), without international certifications, in addition to gross errors in the execution of installations and electrical connections, which are the majority of causes of electric arc creation.
SafeDC by SolarEdge
Power optimizer technology was introduced in Brazil by SolarEdge, an Israeli manufacturer with a global presence.
Among the resources available in this brand's optimizers is the technology commercially registered as SafeDC, which consists of a system for interrupting the current and automatically reducing the optimizer's output voltage in the event of the inverter turning off.
In the SafeDC strategy developed by SolarEdge, power optimizers remain in operation while receiving an operating signal from the inverter.
This signal is constantly renewed, except if the inverter is turned off or if a fault is detected. If no signal is received from the inverter, the optimizers disconnect its terminals, interrupting the supply of electrical current and bringing the output voltage to the level of 1 V.
Shutdown of the optimizers (i.e., disconnection of the DC circuit at module level) occurs if any inverter shutdown condition is reached and also when the internal temperature of the optimizers exceeds 85 OW.
Conclusion
The direct current circuits of photovoltaic systems can become safer with the adoption of new features, many of which are not yet mandatory in Brazil.
In this article, we exemplify three important safety features of photovoltaic systems: rapid shutdown, electric arc detection and reduction of voltage levels in photovoltaic strings (SafeDC).
O rapid shutdown and electric arc detection are regulatory requirements that are already beginning to be adopted in some countries, but do not yet exist in Brazil.
SolarEdge's SafeDC proposal, naturally, already makes it possible to meet the requirements for quick and safe shutdown of direct current circuits in photovoltaic systems, provided for in the North American standard NEC 2014/2017.
We can highlight the following characteristics and the following features made possible by the SafeDC feature set, rapid shutdown and arc detection:
- Full rapid shutdown of the photovoltaic system, including direct current circuits, in case of emergency;
- Electric arc detection in accordance with standards UL1699B and NEC 2014/2017 (sections 690.11 and 690.12) – not yet adopted in Brazil;
- In the event of an electric arc, the quick shutdown system (rapid shutdown) is activated and the photovoltaic system remains off until it is manually reset by an operator;
- The output voltage of the optimizers is reduced to 1 V when a rapid shutdown occurs – it is a native feature of the optimizers that SolarEdge calls SafeDC;
- The voltage present in the direct current (DC) circuits becomes less than 30 V after the quick shutdown command (considering a limit of 30 optimizers in the strings).
