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This paper compares the differences in economic feasibility and dynamic characteristics between underground (U/G) cable and overhead (O/H) line for low-voltage direct current (LVDC) distribution. Numerous low loaded long-distance distribution networks served by medium-voltage alternative current (MVAC) distribution lines exist in the Korean distribution network. This is an unavoidable choice to compensate voltage drop, therefore, excessive cost is expended for the amount of electrical power load. The Korean Electric Power Corporation (KEPCO) is consequently seeking a solution to replace the MVAC distribution line with a LVDC distribution line, reducing costs and providing better quality direct current (DC) electricity. A LVDC distribution network can be installed with U/G cables or O/H lines. In this paper, a realistic MVAC distribution network in a mountainous area was selected as the target model to replace with LVDC. A 30 year net present value (NPV) analysis of the economic feasibility was conducted to compare the cost of the two types of distribution line. A simulation study compared the results of the DC line fault with the power system computer aided design/electro-magnetic transient direct current (PSCAD/EMTDC). The economic feasibility evaluation and simulation study results will be used to select the applicable type of LVDC distribution network.

With power systems switching to smart grids, real-time and on-line monitoring technologies for underground distribution power cables have become a priority. Most distribution components have been developed with self-diagnostic sensors to realize self-healing, one of the smart grid functions in a distribution network. Nonetheless, implementing a real-time and on-line monitoring system for underground distribution cables has been difficult because of high cost and low sensitivity. Nowadays, optical fiber composite power cables (OFCPCs) are being considered for communication and power delivery to cope with the increasing communication load in a distribution network. Therefore, the application of distributed temperature sensing (DTS) technology on OFCPCs used as underground distribution lines is studied for the real-time and on-line monitoring of the underground distribution power cables. Faults can be reduced and operating ampacity of the underground distribution system can be increased. This paper presents the development and improvement of an intelligent cable monitoring system for the underground distribution power system, using DTS technology and OFCPCs as the underground distribution lines in the field.

To save on the island area's power supply cost and protect the clean environment, the Isolated MicroGrid is being duly considered. Consisting of the Wind Turbine Generator (WT), photovoltaic generator, battery system, back-up diesel generator, etc., Isolated MicroGrid, which usually uses the inverter to maintain voltage and frequency of the system, is very weak in terms of voltage and frequency stability compared to the large-scale electrical power system. If wind turbine generator is applied to this weak power system, it could experience many problems in terms of maintaining its voltage and frequency. In this paper, the measurement result of voltage and frequency is presented for MicroGrid, which consists of the Wind Turbine Generator adopting the induction generator and the battery system. MicroGrid’s voltage waveform distortion and Wind Turbine Generator’s output oscillation problems are analyzed using PSCAD/EMTDC. Based on the analyzed result, the importance of type and capacity choice has been suggested in case the Wind Turbine Generator is applied to the Isolated MicroGrid.

A distribution system was designed and operated by considering unidirectional power flow from a utility source to end-use loads. The large penetrations of distributed generation (DG) into the existing distribution system causes a variety of technical problems, such as frequent tap changing problems of the on-load tap changer (OLTC) transformer, local voltage rise, protection coordination, exceeding short-circuit capacity, and harmonic distortion. In view of voltage regulation, the intermittent fluctuation of the DG output power results in frequent tap changing operations of the OLTC transformer. Thus, many utilities limit the penetration level of DG and are eager to find the reasonable penetration limits of DG in the distribution system. To overcome this technical problem, utilities have developed a new voltage regulation method in the distribution system with a large DG penetration level. In this paper, the impact of DG on the OLTC operations controlled by the line drop compensation (LDC) method is analyzed. In addition, a generalized determination methodology for the DG penetration limits in a distribution substation transformer is proposed. The proposed DG penetration limits could be adopted for a simplified interconnection process in DG interconnection guidelines.

The Load Tap Changing (LTC) transformer and Shunt Capacitor (SC) bank are major devices for voltage and reactive power control in a distribution substation. Thus, the coordination operation of a LTC transformer and a SC bank is required to achieve better voltage and reactive power compensation at a distribution substation in the same time. This paper proposes coordinate control method of LTC transformer and SC bank to achieve better voltage and reactive power compensation and operation times of these two devices in the same time. The mathematical formulations of the proposed coordinate control method are introduced. Sample case studies are shown to verify the effectiveness of the proposed coordinate control method.

Temperature rise characteristics of medium voltage class transformers and dielectric characteristics of its insulation oil according to the loading rate were investigated using the transformer that has thermocouples embedded in the windings. The hottest-spot of the transformer locates at about 2/3 height of the windings where they adjoin each other among the 3 phase windings. The temperature difference between the hottest-spot and top-oil is about and the value keeps uniformly in short time after changing the loading rate. Capacitance of the oil varies in inverse proportion to temperature and the inclination rate is about -0.0106 that is not dependent on the aging rate. On the other hand the capacitance is increased depending on the aging rate, so the capacitance characteristics of the oil could be applied to the diagnostic technique.

Recent R&D progress on condition monitoring and diagnosis for power apparatus such as substation equipments including GIS and main transformer, rotating machineries including hydro-generator, turbo-generator, and HV motor, and power distribution equipments including power cable, overhead line system and lightning arrester, and so on in Korea are investigated in this paper. The fields experiences on the CMD carried out by KEPCO are also introduced.

Pole transformers of MV class have utilized an insulation system consisting of natural cellulose-based materials and mineral oil for many years. If transformers are subjected to rapidly changing overload conditions, insulation materials can see high temperatures leading to unexpected aging. Therefore, we have been conscious of developing a transformer that is more thermally stable than conventional types. To measure temperature distribution in the winding of conventional and newly designed transformers, temperature sensors were installed for direct reading of the transformer oil and conductor temperatures of transformers. We also conducted accelerated aging tests for these transformers. The measurement data was utilized in the design of the hybrid insulation system. Also, we found the possibility of applying this insulation system in 13.2 kV pole mounted transformers.

Liquid-immersed pole transformers have utilized an insulation system consisting of natural cellulose-based materials and mineral oil for many years. If the transformers are installed with the abruptly changing load over limitation, its insulation materials are apt to unexpected aging with high temperature. Therefore we have been conscious of developing a transformer that is more thermally stable then conventional types. To measure temperature distribution in the winding of conventional and newly designed transformers, temperature sensors were installed for direct reading of oil and conductor temperatures at transformers. And we conducted accelerating aging test for these transformers. The measuring data had made use of the design to hybrid insulation system. Also we could find the possibility of this insulation system to apply in pole mounted transformers.

In this study, experimental investigation has been carried out for the defect detecting capability of two different way of the OW (oscillating wave) voltage test, such as with-cable-charging and without-cable-charging method. For this purpose, two types of OW voltage tests have been performed at the laboratory on the 22.9 kV CN/CV cables in which artificial needle-type and knifecut-type defects were well integrated. Based on the test results, it could be remarked that the with-cable-charging method is more appropriate to the after laying test due to its consistency of the test condition regardless of the cable length under testing.

This paper describes a diagnostic technique of cable splice by detection of ultrasonic signal. For simulating the ultrasonic signal by partial discharge, an ultrasonic generator was inserted in cable splice. Internal defects were purposely made by needle electrode and ring shape electrode in cable splice. It was shown that defects in cable splice can be detected and the position of defects would be found out by ultrasonic signal measurement.

This paper provides the results of accelerated aging test of 18 kV ZnO surge arresters to investigate the leakage current characteristics. It was impossible to establish the replacement criterion for field operating surge arresters because conventional aging test was only focused on the aging of ZnO elements. In general it is possible to replace the aged arresters before the failure in the field using the leakage current measurement because it must be increased by the aging. But we cannot use this method because we don't know the characteristics of the surge arresters installed in the field. Therefore in this paper we applied operating voltage and accelerating cycle which consists of temperature and humidity to the arresters. Then we measured the variation of total and resistive leakage current and found out the characteristics of the aged surge arresters.