Journal of information and
communication convergence engineering
(J. Inf. Commun. Converg. Eng.)

Because of the continual increase in energy consumption around the world, efforts are being made to produce pollution- free energy to replace the limited energy provided by existing fossil fuels. However, an examination of power systems shows that very large losses are inevitable in the systems themselves, apart from the energy wasted by users. Although the conversion, transmission, and use losses of various facilities have been reduced, little attention has been paid to the magnetic field generated in electric power systems. Although the magnetic field generated when current flows cannot be regarded as wasted energy, it is clear that it disappears without any recycling. If it could be converted into energy and used, the amount of energy produced by power conversion in one switchboard might be insignificant [1-4].

Currently, although much research on energy harvesting is being conducted at home and abroad, most energy harvesting technologies focus on the conversion of electric energy. The importance of electric energy is so great that electric energy reproduction is an inevitably necessary technology for mankind. In particular, the method of obtaining electric energy through a magnetic field can produce relatively more energy than other methods. However, it is difficult to easily access because of AC power generation and multi-phase systems, and it is necessary to expand the market through continuous research and development [5-8].

In this study, we developed a conversion core that produces electric energy by utilizing the unused magnetic field in a switchboard. This conversion core makes it possible to utilize power that is normally wasted. The core is composed of a core, filter, and battery. A prototype was installed in a switchboard to conduct tests on the output, battery storage, and output boosting of multiple batteries.

A. Core

The core part is composed of a ferrite core, coil, and bobbin. The shape, size, and location of the ferrite core were designed to concentrate the magnetic field generated by a busbar. After designing it as a 3D object based on the theoretical characteristics, the density of the magnetic flux was verified, corrected, and supplemented using a simulation tool of the host organization to achieve the optimal performance. Each switchboard contains several switches. Thus, the mutual influence was considered in the design of a single configuration consisting of a ferrite core and coil. The output from the single core part had the greatest impact on the final output goal of the “production of 1 mW per 1 A.” Thus, it was a core technology.

A simulation was conducted in a virtual state by analyzing the magnetic field of the conductor and core using a 3D simulation tool. The distribution of the magnetic field was analyzed by designing the core with a shape suitable for a multiphase busbar system. After selecting a core with a capacity that would not saturate the primary magnetic field and concentrating it on the core as much as possible, an inductance (number of turns) that was capable of the maximum energy conversion in the concentrated magnetic field lines was designed.

B. Filter

The filter part was made by mounting the MCU and passive elements on a PCB. Even if multiple units (dozens if necessary) are required for a switchboard, the minimum power is still based on the power produced by a single core unit. In general, 3-phase, 4-wire-type multi-phase power is used in a switchboard. Thus, there are phase differences of 120°, and mutual interference occurs as magnetic fields identical to the AC waveforms of the currents are formed. Because the interference phenomenon that occurs at this time changes rapidly with the frequent changes in the amount of current, the summed current waveform (magnetic field waveform) is deformed to a level where a 60 Hz sine wave cannot be recognized. In addition, because the inductance value was obtained after winding a coil around a ferrite core, the capacitance had to be adjusted accordingly. A separate verification of the waveform after filtering was not performed, but the effect and efficiency of the filtering were indirectly verified by testing the same “1 mW production per 1 A,” as discussed above.

The magnetic field collected by the coil part was converted into electric energy. Although the amount of energy was larger than that of a conventional harvesting system, filtering was needed to reduce losses as much as possible because it was small and not suitable for the circuit configuration of a general power storage system. The implementation of a circuit was required. This circuit organically adjusted the circuit configuration according to the state of the input voltage and boosted undervoltage. It was configured as shown in Fig. 3.

C. Battery

The battery part consisted of a battery and BMS. The BMS consisted of a PCB, an MCU, and passive elements. The design of the battery unit has already been commercialized under the condition of a single input (sufficient for charging power), but it is difficult to apply the existing technology to the power generated by the coil unit of this research and development project. After passing through the filtering unit, the power generated in the coil unit and delivered to the battery unit is extremely small. Multiple coil units could be used in a switchboard, all feeding a single battery unit, resulting in a multi-input situation with extremely small amounts of power in the battery unit. Battery management system (BMS) control technology had to be developed to optimize these inputs. When charging a battery, the desired voltage must be kept constant to improve the lifespan or charging quality of the battery. Thus, the efficiency changes with the BMS design.

A circuit was required to use the filtered power to charge the battery. We developed a BMS that could protect the battery and circuit in the event of overheating, overvoltage, or overcurrent. The output from the battery was at a voltage level that could not be utilized by normal users. In order to secure a sufficient amount of power through multiple batteries, a system was developed that boosted the outputs of multiple batteries and minimized losses due to voltage level differences between batteries connected in parallel.

D. Test Results

A prototype was installed in a switchboard to conduct tests on the output, battery storage, and output boosting of multiple batteries. The voltage, current, and power were measured using an oscilloscope and a digital multi-meter.

The tests were conducted using a coil with 13,000 turns, bus current of 600 A, reference voltage of 5 VDC, current of 129.3-132.2 mA, and power of 646.5-661.9 mW. The power conversion ratio of the core used to generate energy by harvesting the magnetic field generated by the switchboard busbar was converted into a value per 1 A of bus current, and an output of 1.08-1.01 mW was measured.

In this study, we developed a conversion core that produces electric energy by utilizing the magnetic field that is discarded without being used in a switchboard. For efficient power reproduction, a ferrite core was designed to collect the magnetic field, and the most suitable type of wire and number of turns for the core were determined. The magnetic field was analyzed using a 3D simulation, and numerous lowpower multi-input filtering circuits were designed. The power conversion ratio of the core that was used to generate energy by harvesting the magnetic field generated by a busbar in a switchboard was converted into a value per 1 A of bus current, and an output of 1.08-1.01 mW was measured. Supplying this technology to the market through commercialization is expected to greatly assist in disseminating energy harvesting, which has not yet spread widely to the general public. The micro-power multi-input processing part will be more useful when the energy harvesting market spreads in the future.