A GTO-based scheme for small power tapping from HVDC transmission systems

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Agha Ebrahimi, Mohammad R.
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Ever since the High Voltage Direct Current (HVDC) Transmission Systems were developed and utilized in power systems, power tapping from these transmission systems, be it for supplying isolated communities and/or loads in the vicinity of the transmission system, or to pick up dispersed generation, has presented formidable technical, as well as economical challenges to power system designers. As the size of the intermediate load, or local generation, has decreased, the severity of these challenges has increased. During the years different methods to achieve tapping have been proposed and studied. To this date, however, no practical small tapping station has been commissioned in any HVDC transmission system. The current study, deals with this problem by paying special attention to the new semiconductor technologies available on the market, which have made the production of powerful thyristors with turn off capability, i.e. Gate Turn Off Thyristors, or GTO thyristors, possible. A tapping scheme, using GTO thyristor switches, is proposed, and using the PSCAD/EMTDC digital simulation software package, the operation of the proposed scheme is studied. Simulation results prove the practicality of the proposed scheme and the quality of the power being tapped into a small local load is within the acceptable limits. The performance of the control systems, have been studied and found to be satisfactory. Also, the use of a customized air-core transformer, with a diameter of 5 m, to feed loads of up to 600 kW is shown to be possible. The tapping station is found to be unable to impose any significant negative effect on the performance of the main HVDC system.