Steps towards the universal direct current distribution system

Abstract

The traditional ac power system is challenged by emerging distributed renewable energy sources and an increase in installed load capacity, e.g., electric vehicles. Most of these new resources use inherently dc as do more and more appliances. This poses the question, if they should still be connected on ac in the low voltage grid, which was chosen a century ago, because at that time dc could not be easily transformed to higher voltage levels. In this dissertation, steps are set towards a universal dc distribution system that has the capability of replacing current low voltage ac grids. Standardization is very important at this voltage level due to the high number of connected devices. Therefore, an analysis of the future power system requirements is made and a modular architecture is proposed that consists of connected nano- and microgrids. The dc distribution grid could be meshed and these nano- and microgrids could be connected without a converter separating them, which has significant implications on the overall design of the system. Modular bipolar voltage levels can increase the efficiency of the system, but complicate its operation as well. The exact optimal power flow is formulated for bipolar dc distribution grids with asymmetric loading. It can be used to manage congestions that could affect only one pole. Congestions in distribution grids are likely to increase, due to the increase in installed capacity. They are also more severe in dc grids due to the use of power electronic converters that have very hard limits in comparison with ac transformers. A general method to calculate locational marginal prices between any two nodes in the dc grid is formulated. The optimal power flow formulation is extended to multiple periods in order to include storage operation. Protection is one of the main challenges in creating large dc grids, as short-circuit currents can be very high and there is no current zero crossing as in ac. A low short circuit current protection philosophy is formulated to deal with this. It also addresses the challenge of very low fault current contribution in case of islanded operation. Solid-state circuit breakers are proposed as the main protection devices for dc distribution grids in combination with fast fault detection and clearance. The challenges regarding fault discrimination and selectivity are addressed. Additionally a classification of protection zones in dc distribution grids based on risk is proposed. Experimental measurements of a developed prototype using current derivative tripping are shown. Finally, dc ready devices, that can operate on dc as well as ac, are introduced as a means of simplifying the transition towards dc distribution grids. The losses in the rectification components, when operated on dc instead of ac, are analyzed and it is found that the rectification components of wide input range devices do not need to be enhanced.