Renewable energy resources such as photovoltaic panels and wind turbines have been increasing rapidly to prevent global warming. The number of energy storage systems and many types of batteries is also growing to keep supply and demand balance in local areas. Interfaces of these facilities are direct current (DC). Most of modern electrical appliances also use DC power inside them. Therefore, DC power technologies with some features are expected to meet requirements for new electrical power systems, for example smartgrids, microgrids, and other electrical applications. In recent years, DC power applications in data centers, commercial buildings, and dwellings have been developed in Japan, the U.S. European countries, and so on. At the same time, international standardization activities had started. This paper describes trends of DC power technologies and their applications.
Our previous research have revealed that a deliverable power through a low-voltage DC distribution system has an upper limitation. This limitation results from voltage-instability phenomena occurring for the load of a voltage factor below 1. As the next stage, the present paper proposes two formulas expressing the upper limitation of the deliverable power, Plim, in terms of DC system specifications. These formulated expressions contribute to understand dependence of Plim on DC system specifications and to evaluate Plim without a numerical calculation.
Local DC distribution systems are designed, and the voltage drop and the distribution loss of which are evaluated. First, the DC system for heavily populated area is designed. Entire DC power is supplied from the conversion station located at the center of the service area through long-length, low-voltage distribution lines. 4 V maximum voltage drop is assumed and all line sizes and routings are determined. With this design, the daily energy loss of the DC system is calculated and it is shown that the total loss is almost the same as that for the traditional AC system. Though the large part of the loss consists of the transformers loss, the line loss for DC system is relatively large, especially in winter and summer. When connected with the photovoltaic generations, the current in the line decreases during the day. Then, both the voltage drop and the distribution loss decrease, especially for the DC system. If the house density is low, the line length becomes long and the voltage drop or distribution loss greatly increases. This will be solved by using large-sized wires but it leads to the increased cost. To avoid this, the reduction of the service area is effective.
It is very suitable to select the polymer materials for the housings of surge arresters (SAs), because the polymer materials are generally soft and light weight. Therefore, many kinds of polymer-housed SAs using various polymer materials have been developed, and expanding into many countries. Considering these backgrounds, the JEC technical report (JEC-TR) 23002-2008; polymer-housed surge arrester(1) has been established based on the existent relevant standards of arresters, such as JEC-2371-2003; Insulator-housed surge arresters(2) and IEC 60099-4 Edition 2.2, Metal-oxide surge arresters (MOSAs) without gaps for a.c. systems(3) in order to introduce the technology and provide a common guide for testing of polymer-housed SAs. According as the JEC-TR, the various new applications of the polymer-housed SAs, which are caused by superior advantages such as compact, light weight, safe failure mode, anti-seismic performance, anti-pollution performance and cost efficiency design, have been realized recently in Japan. Therefore, this paper gives specific consideration on the superior performance of the polymer-housed SAs and the evaluation methods of the polymer-housed SAs, because there are some issues in the existent standards to be solved.
The overhead ground wire of AC 150mm2 is applied for the large-size transmission lines. According to inspections of transmission lines, melted component wires of AC 150mm2 have been found in some parts of 500kV transmission lines and it is obvious that the damages have been caused by lightning. Hence, the authors have developed lightning-resistant overhead ground wires as well as being evaluated lightning-resistant performance of the conventional AC 150mm2. In the paper, firstly, it is reported on the lightning performance of the conventional AC 150mm2 resulting from a DC arc test which is considered the polarity of currents and injected current waveforms for summer and winter lightning. As a result, the least amount of electric charges which can melt the components of the conventional wire was 180C. Then, the paper outlines the improvement of lightning performance of the overhead ground wire. Some kinds of developed ground wires as a trial piece have been proposed and tested, and here presented the characteristics of two typical developed wires among them. One is coated with conductive ceramic on the outer components and the another is enlarged a diameter of outer components. Both types of the wires show the excellent lightning performance from the DC arc test, comparing with the conventional ground wire of AC 150mm2.
Fast-moving irradiance condition is one of problems that need to be solved in the non-stationary conventional maximum power point (MPP) trackers of PV system. Under sudden irradiance changes, the output power is changed drastically that leads to the shifting in MPP voltage. Conventional MPP algorithms may start continuously to search for finding the optimum point. However, suddenly another shadow can occur prior to complete removing of previous shadow. Continuing the tracking process under this condition will cause to lose energy. This paper presents the electric double layer capacitor (EDLC) as the power compensation method for improving the maximum power transfer of PV system under short-term period of shading. Several scenarios are tested in this work by measurement the percentage of power compensation, for instance the effect of capacitor size to the period of shading, the effects of shading period to the level shading intensity and cell temperature. This paper is directly purposed to reduce the power losses for moving objects powered by solar energy, such as solar car and solar boat systems.