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”.
...
Jo Oct 18, 2022
It is Korean nation that invented and used metal types, one of the epoch-making events in the history of development of printing technology, in the first half of 12th century for the very first time in the world.
People in Koryo, first unified state of the Korean nation, invented and used metal types by further developing wood-block printing and wood-type printing technologies with a long history, thus making a great contribution to the development of publishing and printing technology of the world.
In 1958, metal types moulded in brass in the first half of 12th century were found at Manwoltae in Kaesong, which was the capital of Koryo. These types made of copper, tin and lead as main constituents and silicon, iron and aluminium as subsidiary constituents are 12mm long, 10mm wide and 8mm thick.
According to “Tonggukrisanggukjip” by Ri Kyu Bo, a famous writer in Koryo, 28 copies of fifty-volume “Sangjongryemun” (“Sangjonggogumrye”) were printed on Kanghwa Island between 1234 and 1241. This is still accepted as the longest record in the world associated with the use of metal types.
Other historical books including “History of Koryo” say that several Buddhist books were printed in metal types, which proves that by the beginning of 13th centry printing in metal types was widely conducted on a fairly high level and that the time of its invention in Koryo was the first half of 12th century at the latest.
The fact that Gutenberg from Germany made metal types in 1450 implies that Koryo was as long as 200 years before Europe in the use of metal types.
“Paekunhwasangchorokbuljojikjisimcheyojol”(“Jikjisimgyong” for short) published in Koryo in 1377 is accepted as the world’s oldest copy printed in metal types of all remaining ones and registered at UNESCO. This book was on display in a book exhibition held in Paris, France, under the sponsorship of UNESCO in 1972, where it was appraised as the oldest of all existing printed materials in metal types, and this was informed to every country in the world.
The world’s first metal types are a great pride of the talented and civilized Korean nation with a time-honoured history and brilliant culture.
Kim Kwang Jo, researcher at the Academy of Social Sciences
...
Jo Oct 16, 2022
Pak Kwang Ok, a lecturer at the Faculty of Distance Education, has invented a new teaching method for improving the students’ cognition in which engineers from factories give explanations on specialized knowledge for production sites on a priority basis in a popular style in accordance with the characteristics of online education for working people.
Firstly, the main lecturer is responsible for giving theoretical knowledge and the important parts which are of value in practical use are explained in detail by the engineer.
The lecturer and the engineer discuss teaching contents and make a script beforehand so that students can get a sound understanding of theoretical and practical knowledge. On the basis of it, the main lecturer is in charge of the theoretical parts, whereas the engineer teaches the technical parts that are applied in the reality.
Secondly, mutual response between the lecturers and students is guaranteed.
She makes teaching scenarios by adequately reflecting the question and answer with virtual students.
That has proved effective in helping students acquire both theoretical and practical knowledge at the same time.
...
Jo Oct 14, 2022
Kayagum (Korean harp) was contrived by Uruk, a talented musician in Kaya in the early 6th century. It was called so because it was from Kaya.
After its birth, the kayagum was inherited without many changes in its shape through the periods of the Three Kingdoms, Palhae and Later Silla and Koryo to the feudal Joson dynasty as a unique national stringed instrument loved by our people. The ways of making and playing the kayagum was fixed in “Akhakgwebom” compiled at the end of 15th century.
According to Akhakgwebom, the kayagum has 12 strings stretched on the rectangular resonance box. They are tuned in pentachord. The resonance box was first made of paulownia. In modern days, the top plate stayed the same, but the under and the side ones were made of chestnut.
The kayagum was widely used as an instrument for solos, ensembles and accompaniment to songs for a long period of time and in the course of it, famous kayagum players and pieces were produced. In addition, a solo piece in the form of a chain like kayagum sanjo (a kind of solo) was born toward the end of the 19th century. The kayagum was handed down as an excellent musical instrument congenial to our people’s sentiments since it produced springy, elegant and soft sounds, but the prolonged feudal fetters and the Japanese obliteration policy of national culture before liberation from the Japanese rule prevented the kayagum from developing into a modern one and thus it had some drawbacks.
After liberation, the kayagum was developed into a modern instrument under the wise guidance of our Party. Compared to the conventional one, the improved kayagum has a clearer and greater resonance. The number of strings has increased to 21 and it has a wider range based on a heptatonic scale, which makes it possible to play any pieces freely on it. The ways of playing have also become more diverse and rich.
At present, the kayagum is used not only for solos but also for several kinds of ensembles such as duets, trios, quartets or accompaniments as a major instrument responsible for melodies and rhythms.
Ri Yong Ho, section head at the Academy of Social Sciences
...
Jo Oct 12, 2022
“Expert System” is a subject that studies different methods of designing and developing Intelligent Systems for simulating the ability of an expert. It is closely related to different subjects including “Artificial Intelligence” and “Knowledge Engineering” and it has a wide range of application and a great diversity in designing.
Such characteristics of this subject caused some problems with raising the students’ abilities of systemization, generalization and application onto a due level.
As it was important to remedy such shortcomings, Kim Ok, a researcher at the Faculty of Information Science and Technology, has applied mind mapping to her subject to enhance students’ self-learning abilities and creativity.
As a result, there has been a radical improvement in the students’ learning methods and in the cognitive abilities and creativity for developing intelligent systems.
...
Jo Oct 11, 2022
Paduk is a game in which two people place paduk pieces on the board in turns to win more positions. At first, paduk was as simple as a koni game (one of the Korean folk games), but it was combined with the outlook on the universe of ancient people to develop into the present-day paduk.
In ancient times, people had naïve opinions ― for example, the sky was round and the earth was rectangular. So the flatness of a paduk board denoted the ground and the rectangle of the board signified four seasons. And 361 crossing points meant one year. Why the point in the very middle of the board was called “chonwonjom” (center mark on the paduk board) was that people thought the earth was the center of the universe. It is said that the roundness of paduk pieces symbolized the round universe and the black and white colours of the two sides of the pieces meant the day and the night.
The term “paduk” is a native Korean word which means placing pieces on the board. It was first pronounced like “podol” and it gradually changed into “podok” → “padok” → “paduk”. Paduk is recorded as “padok” in “Hunmongjahoe” published in 1527 and it has been called so in some south coastal areas until today.
Paduk was widely played in the period of the Three Kingdoms as well as in ancient times and it exerted remarkable influences on foreign countries. According to “History of Koryo”, some Koryo people including Kwak Hui Bun and Jo Jong Thong made a playing tour of paduk to Yuan.
During the feudal Joson dynasty, paduk was an intelligent game that attracted interest of the feudal government as well as ordinary people.
Due to the Japanese wicked maneuvres to obliterate national culture, paduk was eclipsed for years, but under the people-centred socialist system of our own style, it was able to develop into a national sporting event, a popular intelligent game. Excellent paduk players display their paduk skills in the paduk events, which are held every year. Furthermore, our players have won medals in international amateur paduk games, exalting the honour of their motherland. In particular, paduk is even more popular with children of preschool age, schoolchildren and youth. They are constantly improving their knowledge and skills in paduk, winning medals in international competitions.
At present, our people enjoy themselves playing paduk in the cultural recreation centres that are found everywhere they go.
Im Sung Bin, section head at the Academy of Social Sciences
...