Design of Medium Voltage DC Grids– Impact of Power Flow Control on Grid Structure

In medium voltage direct current (MVDC) grids converters allow power flow control. However, in existing medium voltage alternating current grids such a power flow control is not possible due to an unsuitable grid structure which is mostly radially operated. Therefore, this paper answers the question on how the power flow controllability of converters affects a suitable grid structure of MVDC grids. First, this paper analyzes the possibility and potential benefit of power flow control. After this, the resulting mathematical optimization problem for the power flow control is presented. It represents a quadratically constrained quadratic problem which is cohesively formulated and explicitly includes load flow equations. To consider power flow in the grid structure designing process, this operation optimization problem is integrated in a method for designing MVDC grids. Based on an exemplary supply task, grid structure is designed in four investigations by varying different power flow control relevant parameters. Results show that there is a trade-off between the benefit as well as costs of power flow control and an extensive grid infrastructure. Furthermore, a higher meshed grid structure reduces the need for power flow control and for high converter dimensioning. Separated grids with only one primary substation avoid additional loadings of high voltage level.