Transient Analysis in DC Distribution Grids

Abstract

The latest developments in power electronics along with the increased generation from Renewable Energy Sources (RES), the new prosumers who both generate and consume electricity and the rapid adoption of DC loads in the household are forcing us to rethink the manner with which power is distributed and consumed. The proposal to make DC technology an integral part of the distribution network seems to have more merit. This change would lead to energy savings in power conversion and enable the creation of meshed DC grids, highly controllable and flexible networks which will be able to facilitate the more sustainable energy future towards which we strive. To build such a system, however, a different approach than the one used in AC systems is needed, due to the peculiarities of DC technology. In our research for the best possible way to protect these systems, it is imperative to constantly test the protection circuit not only as a standalone, but also incorporate it in the systems that will be built and test them together. In other words, transient analysis of the systems with the proposed protection included is an integral part in gauging the impact of the latter on the former during operation, and therefore in assessing and improving the protection for this specific application. This thesis presents such a transient analysis of various configurations of DC distribution systems. A combination of theoretical and practical methods were used to investigate the behavior of the systems' variables after the occurrence of faults and current interruption. A number of networks of varying complexity were built with detailed cable models using the EMTP/ATP software package in order to simulate a multitude of faults and assess each network's response at various positions and most crucially, the effectiveness of the protection scheme in reducing the impact of the transients after the fault. The travel speed of the transients was also investigated. At the same time, a set of measurements was conducted on a DC street lighting network to study the transient behavior of a real system. A separate model was built to resemble the measured system, in order to compare between the simulations and the measurements. The evaluation of the measurement and simulation results led to conclusions that will contribute to the creation of better, safer and more versatile DC distribution grids. The comparison showed that the models used can approach the behavior of the real system to a reasonable degree. The other simulations indicated that the transient impact is significantly smaller in locations far from the fault, but continued operation of the healthy pole cannot be guaranteed, especially after earth faults.