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教育研究プラットフォームIDER研究成果/ポストドクター・GCOE研究員Low Frequency Transmission System (LFAC) using Underground Cable in City Area Achara Pichetjamroen D ivision of Electrical, Electronic and Information Engineering , Graduate School of Engineering , Osaka University Ise Laboratory Abstract Power transmission with underground cables is more suitable for city area, instead of an over head transmission line because of limited space and pleasant atmosphere. In general, the ac transmission system is operating at frequency 50 or 60 Hz . At this range of frequency, t he problem of large charging current of cable capacitance that causes large losses. HVDC, high voltage dc transmission system, is one of alternatives because charging current of cable capacitance is proportional to frequency, so that there is no capacitanc e charging current. However, it still has the difficulties of control for multi - terminal system which has more than two terminals with converters. For these reasons, we propose a low frequency ac transmission system that can reduce capacitance charging cur rent due to lower frequency than conventional ac transmission system, and also propose a new control scheme for this multi - terminal system. The objective of this research are : (1) To investigate the low frequency ac transmission system including convert er control that can reduce charging current of cable capacitance by using a 12 - pulse ac - ac converter to convert frequency from 60 Hz to 1 - 5 Hz for transferring power via underground cable and reconvert frequency from 1 - 5 Hz to 60 Hz. (2) To design control scheme for low frequency ac transmission system to apply to multi - terminal system. In the first step on this research was numerical simulation of cycloconverter; the cycloconverter can convert an ac waveform to another one of lower frequency without dire ct - current link. Then, t he 6 - pulse ac - ac converter consists of two sets of cycloconverters which contain thyristor devices was simulated. The result shows that with this control scheme the power flow direction can be controlled correctly. Figure 1 shows the configuration of low frequency ac transmission system (LFAC). In the system, 2 sets of 12 - pulse ac - ac converter connected to cable are required. The operation of this system is changing frequency from ac input voltage 60 Hz to ac voltage 1 - 5 Hz to be l ow frequency during power transmission in cable, and then changing ac voltage 1 - 5 Hz to ac voltage 60 Hz. From this configuration, the control scheme needs to be designed with appropriate parameters to control power flow direction by using some control met hod such as phase d ifference control . To verify the control parameter in this system, PSCAD/EMT DC simulation program was introduced to this study . In the near future, the next generation power devices such as GaN would be applied to this high power ca pacit y system for testing the high power application of the new devices. We believe that these devices could be made power system higher efficiency , high er power capacity, more robust, smaller size, lower price etc. Fig.1 Low frequency ac transmission l ine system configuration 108

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