O investor is an essential device for AC/DC conversion and monitoring of the electrical grid in photovoltaic power generation systems. However, possible direct current (DC) electrical arcs can cause damage to components and pose a fire risk.
This article features a capabilities de based protection em tests de laboratory that monitor and analyze DC electrical arcs, in order to protect the photovoltaic system and guarantee human safety.
Test results show that this capabilities can effectively improve a reliability of the inverter and the Safety of operators, in order to avoid damage to system equipment, local property and, mainly, prevent accidents caused by DC electric arcs.
Context
According to the most current data, globally installed photovoltaic capacity was approximately 240 GW in 2022, with a cumulative installed capacity of 1,2 TW worldwide. As one of the central devices in the photovoltaic system, inverters are undergoing constant technological updates and innovations with the continuous maturation and development of the photovoltaic market. Increasing inverter reliability is an important trend in the development of conversion technology and has a crucial impact on the power generation and safety of photovoltaic systems.
During photovoltaic system operation, DC electric arc represents a serious risk to plant safety and has received increasing attention in practical applications. Statistics show that most fire incidents in photovoltaic plants are caused by DC arc flashes. This further demonstrates the importance of strengthening safety monitoring and DC arc protection in the photovoltaic system.
Currently, there is no unified solution or international regulatory policy for the regulation and formulation of standards for DC arc fault testing in photovoltaic systems. The United States was the first country to begin researching and developing standards for fire problems in photovoltaic systems. UL (Underwriters Laboratories) first issued the UL Outline standard in 2011 and has updated it several times.
The latest version is UL 1699B-2018. This standard specifies test requirements and performance indicators for arc fault protection devices (AFCI) in photovoltaic systems. The 2017 edition of the National Electrical Code (NEC) introduced requirements for AFCIs for the first time, stipulating that AFCI devices complying with the UL 1699B standard must be installed on DC circuits of photovoltaic systems.
Furthermore, the installation specifications of photovoltaic systems in several European countries have gradually increased the requirements for the introduction of AFCIs based on the IEC 63027 standard. With the increasing emphasis on the operational safety of photovoltaic plants, the testing and failure protection technology DC arc in photovoltaic systems has become a requirement to be followed by inverter manufacturers.
Therefore, developing an effective DC arc fault test solution to evaluate and develop inverters, aiming to improve the reliability and safety of photovoltaic systems, has become one of the most relevant fields of research currently in the sector.
Theoretical analysis
Formation of electric arcs
The electric arc refers to a luminescent phenomenon produced by the discharge of ionized air when breaking the dielectric strength between two conductors with a potential difference, when the distance between them is relatively small. When the current supplied by the photovoltaic system is interrupted, as for example in a switching system, the high voltage between the conductors will produce an electric arc capable of emitting intense heat and reaching high temperatures.
Although the conductors are disconnected at this point, current will continue to flow through the electrical arc between them. Due to the constant current characteristic on the DC side, it is difficult to extinguish the electric arc after it is formed. The electric arc will only disappear when the distance between the conductors is sufficiently long or when the flow of current through the circuit is interrupted. An effective way to extinguish DC arcs in photovoltaic systems is precisely to interrupt the flow of current through the circuit.
Classification of electric arcs
Electric arcs in photovoltaic systems can be divided into three types: series arc, parallel arc and earth arc.
Series Arc
It occurs in DC cables of the same circuit with current flowing through them, generally due to small spacings between loose or poorly crimped connectors.
Parallel Arc
It occurs between the positive and negative poles of an array or device, usually due to damaged cables or loose connections within string boxes or other points in the circuit.
Bow to earth
It occurs between positive or negative cables and components directly connected to ground. Most often, this happens due to faulty or damaged conductor insulation.
Photovoltaic systems have some inherent characteristics that make DC series arcs more likely to occur, such as high DC voltage, outdoor installation environments, etc. Due to the high direct current electrical voltage, photovoltaic systems are highly prone to suffering from series electrical arcs.
DC arcs in PV systems have the following characteristics:
Electric arc is a high-power discharge phenomenon. Accompanied by the arc, a large amount of electrical energy is converted into thermal energy, resulting in extremely high temperatures in its proximity.
The electric arc is a self-sustained discharge phenomenon, it can maintain its combustion stable for a long period without extinguishing and not necessarily only under high current or high voltage conditions.
The arc is a very light plasma. The gas flow in the arc region, including natural convection and the magnetic field generated by the external environment and even the arc current itself, can cause the physical appearance of the arc to be remodeled frequently.
Arc intensity increases with increasing voltage, current and spacing, while stability decreases with increasing spacing. The arc characteristic curve is shown in the figure below.
Most current photovoltaic systems are designed based on voltages from 600Vdc to 1500Vdc, with high current modules of 182/210mm generally being used, all operating at currents of 14A or higher. Therefore, once the electric arc is generated, there is a high probability of open flames emerging and causing a fire.
Related standards
Currently, the main international standards for AFCI in photovoltaic systems include:
UL Outline Standard: The UL Outline standard was first issued by UL in 2011 and subsequently updated UL 1699B: several times. The latest version is UL 1699B-2018. This standard specifies the testing requirements and performance indicators for arc fault protection devices (AFCI) in photovoltaic systems.
IEC 63027 standard: the international standard developed by the International Electrotechnical Commission (IEC) in 2017, specifying the final performance requirements for AFCI in photovoltaic power generation systems.
AS/NZS O 5033 Standard: standard issued by Australia and New Zealand for the first time in 2019, under code AS/NZS 5033:2019, specifies the functional requirements and test methods for AFCI in photovoltaic systems.
NE C 2017 Section 690.11: The 2017 edition of the National Electrical Code (NEC) introduced requirements for AFCI for the first time, stipulating that AFCI devices conforming to the UL 1699B standard must be installed on DC circuits of photovoltaic systems.
CSA Standard C22.2 No. 293: safety standard for photovoltaic systems issued by the CSA Group of Canada. Since the 2019 edition, it has incorporated the functional requirements and testing provisions of AFCI devices, requiring reference to the UL 1699B standard.
Furthermore, specifications for the installation of photovoltaic systems in several European countries have gradually increased the introduction of AFCIs into the requirements, with reference to the IEC 63027 standard.
In summary, UL 1699B and IEC 63027 are currently the two main international standards related to AFCI detection and protection. They have high consistency in defining AFCI functions, technical requirements, test methods, among others. Standards in other countries and regions often refer to or reference these two standards. This helps promote interoperability and internationalization of AFCI technology and products.
Specific requirements for key parameters vary between different AFCI standards. In comparison, the UL 1699 standard has the most stringent and detailed requirements for AFCI performance and technology. IEC 63027 essentially adopted the rule from UL 1699, but leaves open some specific numerical requirements. AS/NZS 5033 primarily refers to the technical requirements of UL 1699, but does not require high technology product levels and focuses more on basic AFCI products. Some of the main parameters of the more stringent UL1699B standard are the following:
In general, all of these requirements propose minimum characteristics of AFCI devices, such as response speed, detection sensitivity and protection range. This encourages the improvement of AFCI technology and optimizes the performance of safe products in the sector.
Solution
When there is damage (to the module frame, cable insulation, DC connectors, etc.) that leads to the occurrence of an electric arc in any position on the DC side, according to Joule's Law, the thermal effect at the short circuit point is directly proportional to the square of the current, so the greater the current, the greater the risk of fire.
Therefore, an effective way to reduce this risk is to disconnect the inverter from the AC grid, stopping the DC to AC conversion and consequently stopping the current flow in the photovoltaic arrays and the electric arc. In inverters with AFCI devices, disconnection from the AC network is done automatically by the inverter when it enters fault mode, ceasing to perform the conversion.
How to identify arc faults?
Consequently, the key to the DC arc flash protection function in photovoltaic systems is to identify the arc flash current characteristic and promptly stop the flow of DC current in the photovoltaic arrays.
Essentially, an electric arc occurs when the dielectric strength of a gas breaks due to a strong electric field between conductors, forming a continuous plasma that produces intense ultraviolet radiation and intense heat. The mechanisms and locations of possible electrical arc occurrences in photovoltaic systems are diverse. Therefore, the current characteristic of an electric arc is usually identified through measurement and use of spectral analysis methods.
Due to the ionization of air during the electric arc, the plasma will be in a disordered state and the current flowing through the arc will have strong fluctuations. This highly volatile current produces a very wide band of noise in the spectrogram, known as “white noise” in spectral analysis, whereas normal direct current without interference shows a relatively stable state.
As shown in the figure below, the normal direct current spectrum only shows the switching frequency of the inverter when there is no electric arc. However, a certain “noise frequency” will appear from the moment the electric arc appears, and an even more disorderly “noise frequency” will be generated during the continuous electric arc.
When the inverter is operating with the DC arc flash protection function activated, it tests the input current of each string in real time. When identifying a characteristic curve of an electric arc (as shown above), the inverter automatically goes into fault mode, disconnecting from the AC network and reporting an error in the monitoring system. Consequently, the current flow on the DC side is stopped and the electric arc is extinguished in time and safely.
Technical characteristics
Since its inception, GoodWe has focused on safety issues for solar photovoltaic power plants and seeks to continually improve AFCI technology, including algorithms and accuracy testing. After several technological changes, there has been a significant advancement in the DC arc flash identification functions embedded in GoodWe inverters, and a new generation of AFCI technology, called AFCI 3.0, was recently launched.
Artificial intelligence integrated with deep learning
Unlike the traditional solution, where arc identification algorithms and threshold settings are based on theoretical data, GoodWe's AFCI calculates and analyzes data in bulk, learning from characteristics of real arcs occurring in the field and forming a library of behavior of arches. This aims to eliminate false alerts caused by environmental noise. Built-in intelligence includes deep learning, allowing the system to automatically learn and continuously improve spectra encountered in practice, adapting to different scenarios.
Powerful data acquisition capabilities
GoodWe's AFCI has a specific sensor for DC arcs, which provides powerful precision in acquiring data on the characteristics of arcs encountered in practice. Once the arc is identified, it is promptly reported to the chip, allowing for a quick and efficient response.
Adaptable and flexible
Conforming to different cable length and operating current requirements of high-power photovoltaic modules used in large power plants, GoodWe's AFCI has wide-range detection and is capable of stopping current flow within a few milliseconds, significantly suring the minimum performance required by UL 1699B.
Performance check
In order to accurately assess the performance level of GoodWe's AFCI 3.0 technology, the certifier TÜV Rheinland established a verification team to carry out a thorough validation of the technologies applied by GoodWe in its equipment. The evaluation process used is presented below.
Analysis of AFCI application demands
Evaluated contents: evaluate whether the application scenarios are comprehensive and whether the applications in each scenario are appropriate and meet the real practical needs of the photovoltaic sector.
Assessment method: documentary assessment.
Assessment of the AFCI mechanism
Contents evaluated: evaluate whether the technical solution meets the requirements foreseen for the scenario and whether the performance indicators demonstrate technological advancement.
Assessment method: documentary assessment + testing and verification carried out in the laboratory.
AFCI Performance Check
Contents evaluated: arc detection distance, maximum input current, detected energy, shutdown time and maximum operating current.
Verification method: in laboratory and on-site witnessing, testing and verification.
Thorough evaluation of results
Contents evaluated: arc detection distance, maximum input current, detected energy, shutdown time and maximum operating current.
Verification method: in laboratory and on-site witnessing, testing and verification.
Assessment of application demands and technical solutions
GoodWe mainly applies AFCI technology in products for residential, commercial and industrial (C&I) photovoltaic systems, with clear usage scenarios and in compliance with local policies and standards.
During testing, GoodWe demonstrated suitable technical indicators and efficient solutions.
Through the review, the verification team came to the conclusion that the technical solutions proposed by GoodWe exceed regulatory and market technical standards to promptly identify and extinguish DC arc faults. The AFCI 3.0 solution developed and presented offers robust and accurate hardware and software for both the simulation and testing of AFCI devices, and for the development and construction of devices within its quality standards.
Performance check
Based on the technical indicators developed by GoodWe and the previously mentioned safety classification evaluation requirements, the verification team approved the test solution presented in the laboratory and on site, which complies with international reference standards.
AFCI Testing and Validation
According to the above plan and through repeated tests, the verification results are as follows:
AFCI Performance Testing and Verification Result:
Final evaluation
Based on the results of the technical evaluation and testing, we can reach the following conclusion:
- GoodWe's AFCI 3.0 technology has technological advancement, adequate indicators and reliable and stable performance in the tests carried out;
- AFCI meets the performance requirements of the most relevant current standards, such as US NEC 2020 and UL1699B, with some indicators exceeding standard requirements;
- GoodWe's string inverters, integrated with AFCI function, can effectively eliminate and prevent the risks of electric arcs and fires in photovoltaic systems, in addition to reducing material and human losses;
- GoodWe's string inverters, integrated with AFCI function, can effectively identify and eliminate electrical arcs and, thus, prevent fires in photovoltaic systems, eliminating material losses and ensuring human safety.
Conclusion
Based on the above analysis, it is evident that photovoltaic inverters are becoming smarter, more digital and safer. Through the study of the European standard (IEC 63027) and relevant international standards, the technology for testing and identifying direct current electric arcs proves to be the key to ensuring the safety of photovoltaic systems. Manufacturers, installers and regulatory bodies in regions beyond the United States and Europe must also work closely to develop and comply with relevant standards and specifications in order to ensure that PV systems meet the highest safety standards during design, installation and operation.
With the development and expansion of photovoltaic technology, protection technology against electric arcs in photovoltaic systems is constantly evolving and advancing. We hope that this article will play a positive role in boosting the development of safe photovoltaic systems and promoting industrial cooperation and knowledge sharing. Through collaborative efforts, we are confident in establishing a more reliable, safe and sustainable photovoltaic system, thus contributing to the promising future of clean energy.
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