The prevention and cure of electric fire which is the most harmful to fire is very important, and more attention is paid to preventing fire from burning. Arc Fault Circuit Breaker (AFCI), an arc protection switchgear that initially prevents the occurrence of electrical fires, emerges as required.
Mission "WHY"
Electrical fire mainly comes from: (1) Series arc or parallel arc short circuit caused by poor contact or insulation damage of electrical lines (mostly carbonization due to line aging); (2) Grounding (arc) fault caused by damaged insulation of electrical lines to the ground; (3) abnormal temperature rise caused by poor contact of electrical lines, improper arrangement of electrical equipment and overload. Among them, arc short circuit concealment is not easy to find and becomes the primary electrical fire prevention. When the temperature of each arc exceeds 5500 C, the thermal particles emitted by the high-strength thermal arc accumulate over time to ignite the insulation material around the line, causing electrical fire.
The current value of the series fault arc is less than the rated current of the circuit, and the current value of the parallel fault arc may be larger than the rated current, while the starting inrush current of the household appliances is larger than the rated current. To protect two kinds of arc faults, it is necessary to bypass the equipment to start inrush current. Traditional thermal magnetic system (such as terminal overcurrent protection or leakage protection) can not detect arc current and identify arc fault, which can not meet the requirements of preventing electric fire. According to statistics, fire caused by arc fault accounted for 51% of the total electric fire in China from 2005 to 2009, and direct economic losses accounted for 36% of the total fire.
With the development and application of intelligent distribution technology, AFCI, which can realize arc detection and fault signal recognition, can meet the requirements of arc fault protection by relying on protection algorithm and avoid electric fire. The National Electrical Code (NEC) of the United States defines it as a device that provides arc fault protection by identifying arc characteristics and cutting off circuits when an arc is detected.
Mission "HOW"
AFCI includes general structures such as operation mechanism, contact system, release mechanism, test button, connection terminal, shell and so on. Its characteristic structure also includes arc detection circuit and arc fault electronic identification circuit (including microprocessor). It realizes intelligent arc detection and fault arc recognition based on PCB hardware and preset protection algorithm.
The process of AFCI protection: (1) arc detection. The arc in the circuit is monitored by advanced electronic technology. (2) Identification of arc characteristics. Aiming at the detected arc, its characteristics are analyzed to identify whether it is a fault arc or not. In AFCI manufacturing, hundreds of possible operating states need to be tested and programmed into arc characteristic filters to identify "normal" and "dangerous" arcs. (3) Matching analysis of protection characteristics. The protection characteristic meets UL1699 standard: When AFCI detects 8 half-cycle arc fault within 0.5s on AC power supply line, AFCI performs trip-off circuit, and trip-off time should be less than 0.2s. (4) Cut off the circuit to realize fault protection. When the arc fault protection characteristic is satisfied, the tripping signal is sent out and the circuit is cut off.
AFCI detects faulty arc and triggers tripping when the protection characteristics are satisfied by the analysis of protection algorithm. Typical way: detect the load current, amplify the current signal and transmit it to the arc characteristic filter to determine whether the current signal frequency is greater than the power supply frequency and less than the power line communication frequency. The output signal of the filter is compared with the set threshold of arc current. When the threshold of arc current is larger than that, the accumulator is added. AFCI periodically checks the output of the accumulator and triggers the tripping when it exceeds the threshold.
The combination of AFCI and GFCI (or RCCB) can provide both arc and leakage fault protection for home electrical fault protection, usually in the form of AFCI-GFCI socket. EATON Corporation has developed a terminal product which integrates grounding fault (leakage) protection function in AFCI shell.
Mission "WHAT"
Technology status
AFCI uses electronic technology to identify arc state, and fault arc detection is the key link of arc fault protection. Research on arc and fault arc detection began in the late 1980s and early 1990s in the United States. Major arc detection and fault identification methods are based on the optical, thermal, acoustic and electromagnetic characteristics of arc discharge.
(1) According to the characteristics of arc waveform, arc fault can be identified by judging whether the current waveform derivative and the accumulated arc cycle exceed the set threshold.
(2) According to the abrupt change of arc high frequency energy, the arc can be identified by detecting the abrupt change of energy in the high frequency part of current signal, and the arc fault can be identified by detecting the number of arcs.
(3) Using high-frequency wavelet transform: high-frequency sampling of load current, calculating non-zero-crossing discrete wavelet coefficients, together with low-frequency current zero-crossing signal to determine whether the threshold is met.
(4) Fourier transform: Short-time Fourier transform is used to analyze the changes of fundamental, odd and even harmonic components of sampled signals, and to extract and judge the characteristics of series arc fault.
(5) Time-frequency analysis: Based on the difference of delay attenuation time between high and low pulses reflecting current abrupt change, the time domain above and below the threshold is taken as the judgment basis.
(6) Using high frequency signal contrast: by judging whether the high frequency current generated periodically is different from the normal switching arc, and detecting whether the spectrum range is different from the ordinary high frequency harmonics generated by power electronic devices and other applications.
(7) Arc wavelength switching: the ultraviolet light in the received arc light is converted into visible light, and the photoelectric converter is converted into trigger signal.
Industry competition
AFCI, as an effective terminal protection device to prevent electric fire caused by arc faults, has been rapidly popularized and promoted in North America. In the field of building electrical fire protection, arc fault protection technology will have broad market prospects and development potential.
Mission "WHERE"
Based on the physical and chemical characteristics of arc discharge and the characteristics of arc waveform, the application of various new detection techniques to arc detection and arc fault identification will further enhance the level of arc protection. Improving arc dynamic model, simulating arc characteristics and adopting new algorithm to improve fault information processing speed are important research topics in the field of arc fault protection.
(1) Arc detection and fault identification
Applying mature intelligent control theory and pattern recognition technology to arc detection and fault identification will greatly improve the speed and accuracy of arc fault identification.
(2) Multi-protection integration cooperation
It solves the coordination of switches at each node in the distribution system where AFCI is located, and provides comprehensive fault protection for the circuit. It integrates overload, short circuit, grounding fault, arc fault and other protections into a composite China circuit breaker, and optimizes the coordination of various protection functions.
(3) Arc fault protection of photovoltaic system
Give full play to the role of clean photovoltaic energy, and develop a special AFCI for photovoltaic DC system, which involves series DC arc fault protection of photovoltaic inverters, shunt boxes and photovoltaic cell modules.
(4) To meet the new requirements of smart grid for switchgear
The technologies of communication, networking, intellectualization, bus technology and networking will play a more effective role in AFCI.
(5) Serialization and standardization of AFCI products
The serialization, standardization and modularization of AFCI will greatly improve its application in terminal distribution.
Epilogue
As an important terminal fire protection equipment for low voltage distribution system, especially for building power supply and distribution, there is still a big gap between China and foreign AFCI technology development and product application. Foreign-funded enterprises have begun the strategic layout of AFCI in China, and the number of patent applications in China is increasing year by year. Chinese universities should focus on promoting the transfer of AFCI patented technology and seeking the profit of proprietary technology; local enterprises should strengthen the research and development of AFCI technology and patent, reserve strategic assets for long-term development, and learn from the market strategy of foreign enterprises, and strive to cultivate core competitiveness. With the increasing demand for home fire safety protection products and the implementation of domestic AFCI technical standards, the research and application of AFCI technology will deepen and bring huge social and economic benefits.
