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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Jo Sep 25, 2022
A research team led by Kim Pyong Hun, a section head at the General Assay Office, has developed an electrolytes analyzer whereby a rapid analysis of the concentration of ionic electrolytes in human serum is made by using selective ionic electrodes.
The electrolytes analyzer is a medical analytical instrument used to diagnose several diseases by rapidly determining the concentrations of ionic electrolytes such as potassium, sodium, chloride and calcium and pH in serum or urine.
It consists of power supply unit, sample injection unit, detection unit, waste disposal unit, etc.
The power supply unit supplies power to the signal amplifier and the controller. The sample injection unit has a sample injection needle which injects samples, and a switching valve which selects necessary liquor such as calibration solutions, elution solutions, activation solutions for electrodes, etc. The detection unit has six electrodes, that is, a potassium ionic selective electrode, a sodium ionic selective electrode, a chloride ionic selective electrode, a calcium ionic selective electrode, a pH electrode and a reference electrode. The waste disposal unit has a peristaltic pump for drainage and a box for wastes.
The application range of the electrolytes analyzer is 0.5~15.0mmol/L for potassium, 30.0~200.0 mmol/L for sodium, 20.0~200.0 mmol/L for chloride, 0.10~6.00 mmol/L for calcium and pH 4.00~9.00.
The CV of the electrolytes analyzer is ≤1.5% for potassium, sodium and chloride and ≤2.0% for calcium and pH.
The operation conditions are 220V, 170 W, 5~35℃ temperature and ≤65% humidity.
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Jo Sep 23, 2022
Ro Won Myong, a lecturer at the Faculty of Applied Mathematics, has investigated necessary optimality conditions and a numerical method for solving variable-order fractional optimal control problems where the final time and state are simultaneously free and the dynamic system is described as a binomial variable-order fractional differential equation containing variable-order fractional derivatives in the sense of Caputo. These problems have been the focus of fractional order differential equations and their applications.
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Jo Sep 21, 2022
Digital watermarking is being widely applied in various fields such as copyright protection of paintings, music, movies and documents together with broadcast monitoring, transaction tracking, copy protection and ownership authentication nowadays when digital multimedia is rapidly spread through the Internet.
Recently, the most actively studied digital watermarking covers image watermarking, document watermarking and video watermarking. In particular, image watermarking holds a high proportion.
Image watermarking can be divided into several kinds according to representation, embedding domain and objectives of applications ― visible watermarking and invisible watermarking from the aspect of representation; spatial domain watermarking and transformed domain watermarking according to embedding domain; robust watermarking and fragile watermarking for objectives of applications.
There are several indicators of the performance of watermarking including imperceptibility, robustness, capacity, security and computational complexity. In particular, imperceptibility and robustness are the two major features that are trade off, so a number of studies for an effective solution to it have been conducted.
Among them are approaches to raise imperceptibility by embedding watermarks into regions with the lowest human visual characteristics, approaches to get the highest robustness under given imperceptibility levels and approaches to solve single object optimization problems of empirical weighted imperceptibility and robustness by advanced evolutionary algorithms. However, all these have several drawbacks since they cannot correspond properly to arbitrary images and every attack.
Son Chol Min, a researcher at the Faculty of Applied Mathematics, selected blocks with lower human visual characteristics and embedded watermarks into their DC-transformed domains so as to raise imperceptibility. On the basis of it, he has presented a design method of an optimal blind watermarking algorithm which guarantees the highest imperceptibility and robustness for arbitrary images and against any types of attacks by solving multi-objective optimization problems between imperceptibility and robustness based on MOEA/D.
The simulation results showed that the imperceptibility of images watermarked by the proposed method was very high and this method outperforms previous ones in terms of robustness against various attacks.
You can find detailed information about this in his paper “Design of Optimal Blind Watermarking Technique based on MOEA/D” presented to the SCI Journal “IET Image Processing”.
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