Influence of transformer tap-changer control mode upon HVDC valve power loss

The Gui-Guang HVDC transmission link in China is taken as an example to study the operation philosophy in two different control modes of converter transformer tap changer, i.e. voltage control mode and angle control one. The HVDC valve power loss is derived here in these two modes. The study results show that the voltage control mode makes the HVDC loss get larger than the angle control one does in the same transmission capacity. The analysis also confirmed that the voltage control mode of converter transformer tap changer is with high stability in relative large bus voltage fluctuation condition. The intuitive relation formula between HVDC operation parameters and valve power loss is also presented here in different tap-changer control modes. This paper present a powerful theory support for long-term stable operation of both HVDC valve and its cooling system with large bus voltage fluctuation on primary side of HVDC converter transformer.