A self restoring system on low voltage level

Abstract: Fortums electric grid in Norra Djurgårdsstaden is a test grid for smart equipment and they are investigating new techniques and ways to improve the quality of the grid. With the quality improvements that are researched, a "self-restoring system" is a part of the research with the intention to lower the amount of outages and shorten the time it takes to restore faults. This thesis can be seen as a part of the optimization process of the grid in Norra Djurgårdsstaden where the benefits with a basic self-restoring system have been investigated on low voltage level. In the thesis the self-restoring system has been classified into a "basic" and an "advanced" category. The basic self-restoring system cross-connect several feeding paths by cross-connecting different low voltage grids and use mechanical equipment to change between cables when a fault in a cable occurs. The advanced self-restoring system uses several feeders and smart grid technology with equipment and softwares which communicate and visualize the grid. The difference between the systems is that the advanced system can visualize the grid and is able to tell when and where faults have occurred to a more detailed level. The advanced system can also calculate the power available and does not need the same amount of cables for redundancy because it can command users to lower their consumption when an outage has occurred. A decision was made to only investigate the technique on low voltage level because a basic system already exists on medium voltage in Norra Djurgårdsstaden. Results show that investing in a basic self-restoring system in Norra Djurgårdsstaden would cost about 2 million SEK and lower the total amount of outages for the customers in the area from 45 minutes per customer and year down to about 41 minutes. The reason why the decrease is only four minutes per year and customer is because faults occurring on higher voltage level cannot be reduced with the system. It is totally about 10 % of the faults that occur on low voltage level. One conclusion from the thesis is that the reduction in quality costs which are because to the lowered outages will not be enough to pay back the investment. More outage-time per customer and year need to be prevented with the system or the customers need to value reduced outages significantly more.