Jo Oct 28, 2022
Son Won Chol, an institute head at the Faculty of Materials Science and Technology, who successfully developed a new method of hardfacing a worn drill bit by using a build-up electrode with coatings such as nitrided ferrochromium and WC, has been working hard to make further achievements.
In the past, build-up electrodes with Fe-Cr-V-Mo-Ti, Fe-Cr-Mn-V-Ti and Fe-Cr-W-V were used to repair a drill bit for drilling rocks of low strength (f<5).
In general, a hard alloy with high bending strength is used to drill hard rock thanks to its high stability on impact load and that with low bending strength is used for soft rock.
What he used to develop build-up material of the drill bit with high bending strength and toughness was Fe-Cr-Mn-V-Ti-WC system alloy.
He added WC and nitrided high carbon Fe-Cr in coating flux to form a large amount of carbonitride in weld metal for improving the bending strength and toughness.
Based on the effect of the content of ferro-alloys on the hardness and bending strength of weld metal, it was found that the reasonable contents of ferro-alloys to ensure HRC 62 and bending strength of 480MPa were nitride ferrochromium of 37%, ferromanganese of 8%, ferrotitanium of 8%, ferrovanadium of 8% and tungsten carbide of 12%.
Then, considering the effect of weld layers on the microstructure and properties of weld metal and the geometric dimension of the drill bit, he selected two weld layers and heat input 14.20kJ/cm. Finally, he could ensure hardness of weld metal HRC64.
This method can be used to deposit a worn drill bit at coal mines and mines where the strength of rock is medium (f=5~8).
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Jo Oct 26, 2022
In sedimentology, stratigraphic sequences and cycles are ordered by time spans and physical scales such as thickness, and bounded by discontinuities including unconformities or flooding surfaces.
Song Man Hyok, a section head at the Faculty of Earth Science and Technology, has proposed a spectral analysis method based on wavelet transform (WT) maxima and used it as a quantitative tool to identify multiorder stratigraphic boundaries and cycles in well log data.
The proposed spectral analysis is based on quantitative analysis of the center frequencies and resolutions of Gaussian wavelets in time and frequency, and it uses a combination of the WT maxima based on both the first order Gaussian wavelet with a high time resolution and the seventh order Gaussian wavelet with a high frequency resolution.
WT maxima spectra, which can characterize the evolution of WT maxima across scales and periods along WT maxima lines concerned with sequence boundaries, are used to detect dominant spectral peaks corresponding to the time-period domain WT maxima and to determine WT maxima spectral slopes.
The WT maxima spectral slopes are helpful for discriminating sequence boundaries from intrasequence cyclic variations in well log data, and the time-period domain WT maxima are used to relate the detected boundaries to relevant cycles.
He has introduced interval WT maxima spectra and stationary indexes related to the WT maxima spectra as indicators that could be used for hierarchical ordering of the boundaries and cycles.
Application of the proposed method to well log data shows that this method is efficient in identifying multi-order sequences that relate well to the Milankovitch cycles.
The detailed information about this is found in his paper “Spectral Analysis Based on Wavelet Transform Maxima: Identification of Multi-order Stratigraphic Boundaries and Cycles” presented to the SCI Journal “Mathematical Geosciences”.
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Jo Oct 22, 2022
In general, commercialized medium-sized asynchronous wind turbines are fully-automated facilities designed to operate in a parallel connection to the grid; in case of isolated operation, they need to be combined with diesel generators.
Choe Ki Yong, an institute head at the Faculty of Automation Engineering, has studied a method of producing electricity of maximal quality with the wind, by constructing a new stand-alone hybrid (medium-sized asynchronous wind turbines, UPS with battery and photovoltaic array) power system without a diesel generator.
He has proposed a new architecture of a stand-alone hybrid power system which consists of a medium-sized asynchronous wind turbine, a UPS, a current limiter (reactor), a photovoltaic array, and consumer and dump loads; Accordingly, a supervisory control and data acquisition (SCADA) for this system has been suggested along with the operation strategies depending on the output power of the UPS and the wind turbine, consumer load and the battery voltage of UPS.
The case study was confirmed through the simulation results of the operation of a new stand-alone hybrid (two 110 kW asynchronous wind turbines, 250 kVA UPS with battery, reactor, 36 kW photovoltaic array, consumer and dump loads) power system.
The results of the simulation showed that the system frequency change of the new stand-alone hybrid power system was 60±0.5 Hz and that of the wind + diesel stand-alone hybrid system was 60±1 Hz, for the sudden change of consumer load and gust.
More information can be found in his paper “New architecture and SCADA for stand-alone hybrid (medium-sized asynchronous wind turbine + UPS with battery + photovoltaic array) power system without diesel generator” presented to the SCI Journal “Wind Energy”.
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Jo Oct 20, 2022
The microgrid has more flexibility and reliability to fulfill the system stability, power quality requirement, etc. However, the voltage unbalance might occur when the unbalanced loads including single-phase loads between two phases or between one phase and the neutral are connected in the microgrids. The unbalanced voltages occurred in the microgrids may cause problems with power quality such as power system instability, additional power losses, and adverse effects on the motors and power electric converters.
In order to provide highly reliable and good quality power to the distributed loads, the power-converter based DGs can be used as the distributed active filter to compensate the voltage imbalance, for which the imbalance power should be shared among the distributed generations in the islanded microgrid.
In the islanded microgrids, the droop control method has been widely used for power sharing, but the mismatched line impedance was not considered in most literature for accurate power sharing. With the help of the low-bandwidth communication, the centralized control method was proposed to compensate the voltage imbalance and to share the imbalance power. However, the single point of failure feature results in limited flexibility and low reliability.
In recent years, consensus-based distributed control schemes have been studied for distributed voltage imbalance compensation with accurate sharing of the negative-sequence current among the DGs. However, this method may cause differences in negative-sequence voltage among the DGs to incur the circulating negative-sequence currents in the microgrid.
Kim Sung Hyok, a researcher at the Faculty of Electronics, has proposed a method whereby virtual impedance is adopted to regulate the DG equivalent impedance at the fundamental negative-sequence for sharing the imbalance power in proportion to the DG power rating with consideration of mismatched line impedance. In that case, the line equivalent impedance is defined as the serial combination of the line impedance and the virtual impedance controlled by the DG unit at the fundamental negative-sequence.
In order to ensure the accurate imbalance power sharing regardless of the impact of mismatched line impedance, the discrete form of the consensus algorithm is considered to design the communication law for the global average of per unit imbalance power discovery.
The global average of the imbalance power is used to find the virtual negative-sequence impedance correction term by the designed consensus algorithm. Here, the per unit imbalance power mismatch is fed to a proportional-integral (PI) controller to discover the virtual negative-sequence impedance correction term.
The virtual negative-sequence impedance is regulated adaptively to eliminate the imbalance power sharing mismatch. Accordingly, the load imbalance power is accurately shared among the DGs.
The virtual negative-sequence impedance regulation based on the consensus algorithm is developed to accurately regulate the line equivalent impedance regardless of the detection of line impedance.
The effectiveness of the proposed method has been verified by the simulation and experiment results.
You can find more information about this in his paper “Distributed virtual negative-sequence impedance control for accurate imbalance power sharing in islanded microgrids” presented to the SCI Journal “Sustainable Energy, Grids and Networks”.
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Jo Sep 29, 2022
Cadmium sulfide (CdS) is an important II–VI compound semiconductor material with applications in several heterojunction photovoltaic systems including CdTe and CIGS solar cells. There are several physical and chemical deposition techniques used for deposition of CdS thin films including RF sputtering , evaporation, close space sublimation (CSS), closed space vapor transport (CSVT), metal organic chemical vapor deposition (MOCVD), chemical bath deposition (CBD), spray pyrolysis, successive ionic layer adsorption and reaction (SILAR), etc.
Among these, CBD is one of the simple stand inexpensive large-area deposition techniques. CBD is most widely used since it is not only available at low temperatures and easy to control but also it helps produce uniform, compact and adherent films.
Kim Hyon Chol, a researcher at the Faculty of Electronics, has investigated the influence of deposition parameters such as reactant concentration, bath temperature and deposition time on the CdS thin film properties when a CdS thin film, a window layer of a CdTe solar cell, was deposited by the chemical bath deposition (CBD) method without stirring.
Influence of solution concentration: When the reactant concentration is too high, the reaction rate becomes fast and the color of solution turns yellow, resulting in films with brittle nature and poor uniformity due to the adherence of a great amount of CdS precipitates onto the surface, which is undesirable for solar cell fabrication. On the other hand, low concentration leads to a decrease in deposition rate and non-uniformity of thin films. When the reactant concentration is controlled suitably, the residual solution after the reaction can be colorless to obtain uniform, compact and transparent films.
Influence of bath temperature: The bath temperature affects the surface morphology, microstructure and adhesion of CdS thin films. At lower temperatures, an abundance of CdS particles are directly deposited because of lower solubility products of CdS (Ksp). Meanwhile, higher bath temperature yields thin films of poor quality due to the increase in Ksp and the decrease in the precipitation rate. The increase in the bath temperature decreases the grain size and promotes the formation of CdS thin films.
Influence of deposition time: Deposition time influences the thickness of films with constant reactant concentration. At the initial stage of the formation of a thin film, a large number of voids made by CdS colloid particles form discontinuous films. For CdS thin films with these voids, the performance of solar cell is strongly affected by the decrease in shunt resistance. As the deposition time increases, the surface of a film becomes compact, uniform and continuous.
The measurements of transmittance were carried out for all deposition parameters. The performances of solar cells with different S/Cd ratios, the variation of the crystal phase with NH4Cl concentration and the surface morphology with the bath temperature were studied. Finally, he determined the optimum deposition parameters through the systematic analysis.
You can find more information in his paper “Effect of the reactant concentration, bath temperature and deposition time on the properties of CdS thin film prepared by the chemical bath deposition method” presented to the SCI Journal “optical materials”.
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Jo Sep 27, 2022
A research group led by Choe Kwang Hyok, an institute head at the Faculty of Shipbuilding and Ocean Engineering, has developed a wave power device for providing power of a navigation and marking buoy.
The device is a floating WAB (wave activated body) and it consists of a point absorber, power take-off (PTO) of a wave energy converter, an electric generator, etc.
The point absorber consists of a floating body, a spar and a heave plate.
The floating body is heaved on wave surfaces and the spar is a relatively fixed body by a mooring system. The heave plate is strongly connected with the spar and it can effectively improve the heave response of the device by providing additional damping and added mass.
When the wave power device is on the water surface, the floating buoy and spar oscillate up and down vertically due to the wave. At that time, the link attached to the floating buoy moves up and down by relative amplitude differences occurring in the amplitude and phase of two buoys and the alternating motion of the link changes into rotary motion through the specially designed chain driving device.
The rotary motion of the chain driving device increases up to the rated rotary velocity of the generator and it is transmitted to the generator through the multiplying gear (1:3) and the belt (1:3), which generates power of up to 100w at the wave height 0.2~1m.
The diameters of the floating buoy, the spar and the heave plate are 1.2m, 0.3m, 1m respectively and the whole mass is 980kg.
The wave power device can be utilized to supply power not only to navigation and marking buoys, but also to other measuring and monitoring devices on the sea.
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