A 25-MW soft-switching HVDC tap for ±500-kV transmission lines

This paper presents an optimization of a previously proposed high-voltage dc current transmission system (HVDC) tap system where the power rating of the soft-switching dc-dc converter has been increased to supply medium loads (up to 25 MW) from ±500-kV HVDC transmission lines. This dc-dc converter is connected in series with a pole of the HVDC transmission line. It drains energy to a dc capacitor through an air-core transformer and a single-phase diode rectifier connected at the secondary. A pulsewidth-modulation (PWM) voltage-source converter is directly connected to this dc capacitor to supply ac loads. The HVDC tap comprises all those components and can supply high-quality power to isolated or interconnected load areas. The controller of the HVDC tap is independent from the control systems of the main HVDC converter stations. It does not need any communication channel to the control of the main HVDC converter stations. A complete bipolar ±500-kV HVDC system, together with a refined model of dc transmission line were implemented to simulate the performance of the proposed HVDC tap. It has revealed to be very robust and to be an attractive solution for feeding small loads from long dc transmission lines.