Jo Apr 23, 2022
A research team led by Hong Yong Jun, a researcher at the Institute of Electric Engineering, has developed and introduced a single-phase grid connected VSI(Voltage Sourced Inverter). It can not only transfer active power to the national power grid by converting the DC power generated by PV(Photovoltaic) modules into AC active power and synchronizing it with the voltage and frequency of the grid, but also control reactive power independently.
They realized the control of reactive power in the grid-connected VSI by applying the IRP (instantaneous reactive power) theory, the base of the FACTS(flexible AC transmission system). For this control, they had to ensure the exact tracking of the frequency of the grid voltage by introducing the fictitious phase by the SOGI(second-order generalized integrator) and FLL(frequency locked loop).
Then, they took control of grid injected current by a PR(proportional resonant) regulator with proper parameters, and added anti-islanding detection by active frequency drift.
The THD(total harmonic distortion) of grid-injected current is 3.6% and the time required for anti-islanding detection is less than 0.1s.
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Jo Apr 21, 2022
A research group led by Jang Ki Ho, a section head at the Faculty of Heat Engineering, has developed a 10kV electric boiler designed to use the electricity from a power system directly, and they are now working hard to make further achievements in their research.
Boilers are largely divided into two types, i.e. a low voltage electric boiler (LVEB) and a high voltage electric boiler (HVEB).
In a LVEB, heat is generated by Joule heating effect in an electrical resistor as an electrical heating element below 380V and is transferred to fluids. Usually, the electrical heating element used in a LVEB produces several kilowatts of thermal power.
However, hundreds or thousands of kilowatts are needed to heat various kinds of liquid for swimming pools, ponds used for aquaculture, sensitive products used in chemical and agro alimentary processes and buildings. The heat power output of boilers used for controlling the load in a wind power station or a nuclear power station is the same. Even if several heating elements below 380V can be used as one, it remains uneconomic, because it needs a transformer that maintains the supply system voltage at 380V, and it also causes a considerable energy loss.
On the contrary, an HVEB(10kV) offers several advantages: it produces thousands of kilowatts of high electro-thermal power, it needs comparatively low investment, it is easy to make, and it can be operated without a transformer from the direct supply from an electric power system.
The problem is the safety of a high-voltage electric boiler. It is difficult to predict the amount of thermal power in the step of its design because it depends on the temperature, velocity and specific resistance of the fluid that are also changed in relation to each other. To solve these problems, they proposed a 10kV electric boiler equipped with a ceramic insulation tube inside and determined its geometrical sizes and operation parameters through a numerical analysis of coupled thermal, hydro mechanical and electrical fields in the water heating process. On the basis of this, they completed it through tests and practices.
The investment of an 10㎸ electric boiler is under ½, and the installation area under ¼, compared to a LVEB of the same capacity. And there is no consumed power due to the operation of a transformer.
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Jo Apr 19, 2022
See-saw is one of the folk games enjoyed by the Korean people.
There exist several arguments for the origin of see-saw. Some historians claim that it originated from a woman who went on the see-saw outside the wall of the prison to see her imprisoned husband in the period of the Koryo Dynasty while some others believe that it started from the women’s practice of looking over the fence on a piece of board in the yard as a sign of resistance to feudal restraints. It is also said to be derived from the principle of a treadmill used for pounding cereal in the past.
See-saw used to be done mostly on the Lunar New Year’s Day and Jongwoldaeborum (January 15 by the lunar calendar). That was because it was appropriate time for people to enjoy themselves breathing in as much fresh air as they wanted in the currents of warm spring air after the long period of the severe cold striking from morning to evening.
It was so good for women’s health and life that there went an old saying, “A see-saw in January keeps pricks away from soles that year.”
As it is easy and can be played everywhere, middle-aged women as well as young girls can train themselves jumping up and down merrily. In addition, it is in accordance with active and optimistic Korean women’s sentiments of life.
Thanks to the policy of the national heritage conservation of our Party and state, see-saw, the Korean folk game, is still encouraged as the one enjoyed by our women and as one of the national sporting events.
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Jo Apr 18, 2022
There are a great number of world-famous specialities in the DPRK, the land of beautiful mountains and rivers, and among them is Koryo Insam.
Koryo Insam, a special medicinal plant in the DPRK with a time-honoured tradition, has such a mysterious and unique medicinal effect that it is well-known all over the world as a cure-all and a tonic bringing eternal youth.
Even though a lot of kinds of plants in the insam(jinseng) family are growing and are being cultivated in many countries and regions in the world, none of them is more effective, greater in demand and more time-honoured than it.
The word insam originated from its root which resembles a human body. We call those growing naturally in the mountains wild insam while those cultivated artificially is called insam.
Well-known insam plants that have been used by people all around the world so far include Koryo(Korean) Insam, Chinese Insam(Samchil), Japanese Insam(Jukjol insam), Siberian Insam(eleuterococcus senticosus), Western Insam(Hwagisam), etc.
The most famous of all these is none other than Koryo Insam.
The custom of cultivating and using Koryo Insam was nominated as part of the national intangible cultural heritage and the method of cultivating and processing it has still been inherited.
It is classified into two according to the processing method: hongsam(red) and paeksam(white).
Hongsam is insam steamed and dried at a certain temperature and paeksam refers to insam which is cleaned and dried. Insam in its original state is called susam.
As insam is rich in essential amino acid, various vitamins and minerals, etc., it is used as a basic ingredient for a variety of Koryo medicine and tonics.
It, in addition, renews vigor, relieves fatigue, improves mental and physical abilities and promotes health.
Nowadays a large number of insam products such as insam tea, insam extract, insam liquor, insam sweet medicine, etc. are making an active contribution to the improvement of our people’s health and cosmetics produced with insam as major ingredients are also in great demand.
A foreign specialist in crude drug wrote that the value of insam is represented clearly by the international market price and Koryo Insam cultivated in the DPRK is twice to four times more expensive than those from China or Japan although they are of the same kind, and that it is mainly attributable to the cultivation techniques including choice of superior kind, fertile soil, scientific manuring and prevention from blights and harmful insects.
Under the wise leadership and warm care of our Party, farms, factories and institutes for professional cultivation, processing and research are well established in Kaesong, the original home of insam, and they are working hard to conserve and preserve the peculiar features of Kaesong Koryo Insam, a speciality of our country, and to boost its yield.
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Jo Apr 14, 2022
A research group led by Ri Ju Hyok, a section head at the Faculty of Heat Engineering, has invented a vaporizer that vaporizes cryogenic liquid oxygen to supply to consumers.
It is an atmospheric source cryogenic vaporizer that turns cryogenic liquid oxygen into gas by cryogenic distillation method.
In practice, the demand for oxygen is unpredictable. To meet this flexible demand, liquid oxygen is usually stored in the cryogenic tank and is vaporized whenever needed. The vaporization of liquid oxygen is conducted by means of a cryogenic vaporizer.
The vaporizer is a vertical heat exchanger, its cross section is of an 8-pointed star type and it vaporizes cryogenic liquid by atmospheric heat.
To determine the optimal parameters of a vaporizer, they studied 4 items: growth process of frost on the surface of the heat exchanger when liquid oxygen is vaporized, the property of frost, the pulse phenomenon in the heat transfer pipe and the destruction of frost on the surface, and the characteristics of air flow in the surrounding of a heat exchanger.
The cryogenic oxygen liquid vaporizer is extruded of aluminum alloy of good conductivity.
As its heat source is based on only the air in the atmosphere, environmental problems such as pollutant emission and noises are not arisen.
It is also applicable to the vaporization of other cryogenic liquids like nitrogen, argon, etc. as well as oxygen.
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Jo Apr 13, 2022
Choe Yong Son, a researcher at the Faculty of Physics, realized the computerization of an experiment device to measure the characteristics of glow discharge plasma.
Compared to the previous devices with analog meters, the computerized device is equipped with an embedded system and a touch-panel interface, which makes it possible to supervise and analyze experimental processes, and to display the results. It also ensures high accuracy in measurement and short time of measurement along with intuitiveness and effectiveness of experiments.
The device consists of a power supply, a probe voltage stabilizer, a triangular signal generator, a discharge current sustainer, a current-to-voltage converter, a voltage signal detector, a matching unit, and a computer. The main characteristics to measure are electron density and electron temperature. A linearly increasing voltage is applied to the electrostatic probe, which is placed in the discharge region, and the current to the probe is captured at a regular interval, in order to gain voltage-current measurement points of the probe.
These measured data are treated by 3-order spline interpolation and then by weighed polynomial method to obtain a voltage-current curve of the probe. Necessary features are to be derived from the characteristic curve, and these features are utilized to determine electron temperature and electron density. All these operations are automatically performed through its embedded system and touch-panel interface.
This device can increase the measurement accuracy of probe voltage and current by more than 20 times and, as a result, as many measurement points as possible are available. Therefore, we can eliminate scale errors, which were indispensable in the past, and decrease random errors down to below 0.5% (in analog meter: 4.2%). In addition, offline experiments may become possible by the combination of serial communication and network communication.
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