Experience from a bipolar HVDC system with a Voltage Source Converter and a Line Commutating Converter

摘要

The Skagerrak HVDC interconnectors connect Kristiansand substation in Norway and Tjele substation in Denmark. Skagerrak 1, 2 and 3 were commissioned in 1976, -77 and -93 respectively, and have a total power capacity of 1050 MW. The new HVDC interconnector between Norway and Denmark, Skagerrak 4, was commissioned in 2014. The main drivers for the new interconnector are security of supply, climate policy and economy. Skagerrak 4 has a power capacity of 715 MW and is based on multilevel Voltage Source Converter (VSC) technology. This is the first 500 kV VSC-based HVDC link. Skagerrak 1, 2 and 3 are based on Line Commutating Converter (LCC) technology. Skagerrak 1 and 2 operate as a balanced bipolar HVDC scheme. In order to minimize the electrode current through the sea, Skagerrak 4 was installed in a bipole configuration together with Skagerrak 3. This is the first bipole in the world that combines a VSC and a LCC. It has required special considerations in both the control system and in the main circuit design. However, the combination has made it possible to get the advantages of a VSC converter while using the Skagerrak 3 LCC converter to balance the Skagerrak 4 neutral current. VSC and LCC converters are fundamentally different regarding change of the power flow direction. A VSC changes power direction by changing the d.c. current direction, whereas a LCC changes the d.c. side polarity. Hence, Skagerrak 4 has been equipped with d.c. side switches, making it possible to change its d.c. side voltage polarity during power reversals. The paper discusses several aspects of this bipole configuration, including control system challenges, pros & cons as well as first year operating experiences. The substation at the Norwegian side of the interconnectors across Skagerrak sea includes a complex combination of both stationary and dynamic reactive power compensation devices. The new bipole controller handles the overall reactive power control (RPC) of the substation. The concept of the RPC and the experiences from the voltage control in the substation after the installation of Skagerrak 4 is presented in this paper.