The rural power grid has traditionally mostly consisted of overhead power lines. In recent years the trend has been to replace the overhead lines with cables instead. The reason is that overhead lines are relatively vulnerable, strong winds and storms can cause trees and branches to fall over the power lines and cause a phase to ground fault. This will then trip the ground fault relays and disconnect the faulty power line. A cable grid is not vulnerable in the same way, and could be considered a solution to make the power grid more reliable. A cable grid does come whit other types of problems instead. It generates about 50 times more phase to ground capacitance compared with the same length of overhead lines. When a phase to ground fault occurs the capacitance in the healthy phases will generate a current to ground and then through the fault. On average a cable grid generates about 2 A per kilometer. Large cable grids can therefore cause very large capacitive currents to flow through the fault.  To counter this, a reactor is placed between the neutral point of the transformer and ground. When a phase to ground fault occurs, the reactor will generate an inductive current which is in the opposite phase compared to the capacitive current. This current will flow through the faulty line and cancel out the capacitive current. In a perfectly tuned power grid the only component left in the fault is a smaller resistive current. Large cable grids will require a large reactor to generate the large inductive current, which might need to flow over a great distance in the grid to reach the fault location. To reduce the inductive current from the central reactor, it is possible to install smaller local reactors in the grid. These will then in the event of a phase to ground fault generate a part of the inductive current, which will reduce the currents from the central reactor. This report will look at the factors related to grounding systems and how these factors affect the ground fault currents. The purpose of the report is to give recommendations to Umeå Energi on where in their grid they should install additional local reactors and also which factors they should consider when doing future expansions and rebuilds of their power grid.

I nuläget används XLPE högspänningskablar vid överföring och distribution av elkraft och har en viktig roll i elsystemet. Kraftöverföring som sker över långa sträckor kopplas vidare från en punkt till en annan och sker med hjälp av kabelavslutningar. Dessa kabelavslutningar har en del sårbarheter ”felfunktion i kabelavslut” som påverkar hela elkraftsystemet. En analys och simulering av fältfördelning i kabelavslut genomfördes genom en teoretisk-respektive praktisk del i rapporten. Den teoretiska delen av rapporten genomfördes med hjälp av FEM programmet Comsol Multiphysics. Resultaten visade att den högsta fältkoncentrationen uppstår vid isolationen av kabeln och orsakade felfunktion och sårbarheter. Vid det linjära fallet för den elektriska delen resulterade en hög permittivitet till att potentialen och det elektriska fältet minskades vilket var densamma med låg konduktivitet. För det olinjära materialet ändrade sig konduktiviteten med elektriska fältet och tiden. Temperaturen vid det linjära fallet visade att vid en hög temperatur blev materialet mer ledande och gav ett högt elektriskt fält i halvledaren. I det olinjära fallet minskades materialets ledande. Detta kunde regleras med olika tröskelvärde (Eb) vilket inte kan i det linjära fallet. Den praktiska delen genomfördes i E.ON:s laboratorium för olika typer av kabelavslutningar som testades med 33 kV. Vissa av de provade kabelavslutningarna användes för  provningssyfte och andra plockades ur drift på grund av felfunktion i kabeln. Ett försök för att kontrollera fältkoncentrationen genomfördes i laboratorium och resulterade till en reducerad fältfördelning i kabelavslutning. Resultatet av den praktiska delen visade hur fältkoncentrationen fördelades i kabelavslutning och att fältfördelning ledde till sammanbrott i kabeln. / Nowadays XLPE high voltage cables are used in transmission and distribution of electrical power and has an important role in the electrical system. Power transmission that occurs over long distances is diverted from one point to another and is done with the help of cable terminations. These cable terminations have some vulnerabilities that affect the entire power system. The theoretical part of the report was carried out with the help of FEM software Comsol Multiphysics. Results showed that the highest field concentration occurs at the insulation of the cable and caused malfunction and vulnerabilities. In the linear case for the electrical part resulted a high permittivity that the potential and the electric field was reduced, which showed the same result for low conductivity. For the nonlinear material the conductivity changes with the electric field and time. The temperature of the linear case showed that at high temperatures the material became more conductive. In the nonlinear case the conductive material was reduced. This could be controlled with different threshold value (Eb) which cannot in the linear case. The practical part was done in E.ON:s laboratory for different type of cable terminations that were tested with 33kV. Some of the tested cable terminations were used for testing purposes and was picked out of operation because of a malfunction in the cable. An attempt to control field concentration was carried out in the laboratory and resulted to a reduced field  istribution in the cable termination. The result of the practical part showed how the field concentration was distributed in the cable termination and that the field distribution led to the collapse of the cable. / <p>QC 20160718</p>