<p>The background to this work is the introduction of the RCC Ground Fault Neutralizer in countries that earlier have used solidly earthed medium voltage networks. The RCC Ground Fault Neutralizer is a protection system that is capable of handling earth faults without disconnecting the feeder. Since some equipment, that cannot be immediately replaced, only can withstand the full voltage for a limited time, the operating time of the system needs to be limited. This is also true during the commissioning when calibrating the system means that the voltage against earth needs to be increased.</p><p>The primary purpose with this work is to create a possibility to automatically calibrate the settings for the inverter that are used when reacting to an earth fault. A goal with the new automatic calibration is that it should be faster than a preset time per phase. A second goal is to create a more user friendly control of the neutral voltage using the inverter. The purpose of this is to prepare the system for the possibility that PD measurement in combination with the control of the neutral voltage offers. This way isolation defects could be found and possibly also delayed.</p><p>The solution that was chosen was as simple as possible and is a combination of setting the inverter to a calculated value and proportional control. This was implemented in the service software which is normally used for commissioning, surveillance and manual control.</p><p>The solution was proven to be good enough to clear the goal that was set up for this work. The first tests were performed against a net model. After the solution had proven to be successful in the net model more test where performed on a system installed in a real substation. Also the PD control software was tested but only in the net model and because higher voltages is needed to test PD activity no real test with PD was performed. Hopefully this new software can be used to further develop this idea in the future.</p>

The background to this work is the introduction of the RCC Ground Fault Neutralizer in countries that earlier have used solidly earthed medium voltage networks. The RCC Ground Fault Neutralizer is a protection system that is capable of handling earth faults without disconnecting the feeder. Since some equipment, that cannot be immediately replaced, only can withstand the full voltage for a limited time, the operating time of the system needs to be limited. This is also true during the commissioning when calibrating the system means that the voltage against earth needs to be increased. The primary purpose with this work is to create a possibility to automatically calibrate the settings for the inverter that are used when reacting to an earth fault. A goal with the new automatic calibration is that it should be faster than a preset time per phase. A second goal is to create a more user friendly control of the neutral voltage using the inverter. The purpose of this is to prepare the system for the possibility that PD measurement in combination with the control of the neutral voltage offers. This way isolation defects could be found and possibly also delayed. The solution that was chosen was as simple as possible and is a combination of setting the inverter to a calculated value and proportional control. This was implemented in the service software which is normally used for commissioning, surveillance and manual control. The solution was proven to be good enough to clear the goal that was set up for this work. The first tests were performed against a net model. After the solution had proven to be successful in the net model more test where performed on a system installed in a real substation. Also the PD control software was tested but only in the net model and because higher voltages is needed to test PD activity no real test with PD was performed. Hopefully this new software can be used to further develop this idea in the future.

Denna rapport analyserar följderna av en spänningshöjning i ett mellanspänningsnät från 6 kV till12 kV alternativt till 20 kV. Rapporten undersöker ett befintligt elnät i södra Sverige som ägs avMellersta Skånes Kraft ek. För. (MSK). Det består av 177 stycken transformatorer, har ca 2500kunder och överförde totalt runt 32 GWh år 2016.Rapporten är uppdelad i tre delar, den första delen behandlar de tekniska aspekterna av en förhöjdspänningsnivå, den andra delen behandlar de ekonomiska följderna detta kan medföra och dentredje delen presenterar slutsats och diskussion.Elnätet simulerades i programvaran Neplan och elektriska beräkningar utfördes på olika driftfallunder årets månader. Lasterna i nätet fastställdes bland annat genom att använda Velandersmetod. Information och indata av nätets komponenter inhämtades genom produktblad, kontaktmed tillverkare och MSK samt genom diverse litteratur.Rapporten visar att en ökad spänningsnivå i elnätet minskade transmissionsförlusterna ochmedförde lägre kostnader för elnätföretaget. Det medförde också andra tekniska fördelar,exempelvis att spänningsfallet minskade, belastningsgraden av nätets komponenter minskade samtatt kortslutningsströmmen blev lägre. En högre spänningsnivå ökade nätets generering av reaktiveffekt.Elnätföretagets intäktsram förändras vid en förhöjd spänningsnivå, dels till följd av det tillägg somerhålls vid effektivt utnyttjande av elnät, dels då vissa anläggningsdelar i nätet ersätts. Lönsamheteni att investera inför en ökad spänningsnivå berodde på flera faktorer, bland annat vilka typer avanläggningsdelar som behövde ersättas, kapitalstrukturen på investeringen och elnätföretagetskalkylränta. I lönsamhetsanalysen använder rapporten internräntemetoden.I fallet för MSK antogs samtliga av elnätets transformatorer behöva ersättas vid en ökadspänningsnivå, vilket således ökade intäktsramen inför nästa tillsynsperiod med totalt ca 3,4 Mkr.Internräntan för investeringen bestämdes till 4,62%. En viss osäkerhet råder dock kring ett antalav elnätets olika anläggningsdelar (kablar, ledningar och transformatorer) vilket kan påverkainvesterings omfattning, utfall och om nätet kan hantera en förhöjd spänningsnivå i sitt nuvarandeskick. / This report analyses the consequences of an increased voltage level in a medium voltage gridfrom 6 kV to 12 kV alternatively to 20 kV. The report examines an existing electricity grid insouthern Sweden, owned and operated by the power company Mellersta Skånes Kraft ek. För.(MSK). It consists of 177 transformers, has approximately 2500 customers and transferred 32GWh (incl. losses) in 2016.The report is divided into three parts, the first part addresses the technical aspects of an increasedvoltage level, the second part addresses the economic consequences that this implies and thethird part presents the conclusion and discussion.The grid was simulated in Nepal software and electrical calculations were performed on 12different operating modes, one for each month of the year. The loads in the network weredetermined by using Velander's method. Information and input-data of network componentswere obtained through product sheets, direct contact with manufacturers and MSK and as wellas through various literature.The report shows that an increased voltage level in the grid reduced transmission losses andresulted in lower costs for the grid owner. It also brought other technical benefits, such as lowervoltage drops, lower load intensity of network components, and lower short circuit current. Ahigher voltage level increased the network's generation of reactive power.The grid company’s revenue frame changes with an increased voltage level, partly because of thebonus that is obtained through efficient utilization of the grid, partly by replacing certaincomponents in the grid. The profitability of investments imposed by an increased voltage levelquickly became complicated due to several factors. Including what type of components thatneeded replacement, the capital structure of the investment and the electricity company's rate ofinterest. In the profitability analysis, the report uses the internal rate of return method.In the case of MSK it was assumed that all the power grid transformers needed replacement withan increased voltage level, thus increasing the revenue frame for the next supervisory period byapproximately 3.4 million SEK. The internal rate of return for the investment was found at 4.62%.However, some uncertainty exists about several components (cables, wires and transformers) inthe grid, which may affect the profitability of the investment and the grid’s capability to handle anincreased voltage level in its present state.

The grid in the northern part of Sweden is characterized by long radial lines with just a few customers. To secure electricity supply it is necessary to have a redundant feeding alternative. A loop structured grid which is used in the more populated urban area is a poor solution in the rural areas both regarding economy and technology due to the long distances. To feed the northern grid in Sweden a battery energy storage system (BESS) is therefore a solution that could secure the power delivery. An important aspect to investigate is how the electric power quality changes with the new feeder, but more importantly to secure disconnection of supply at fault occurrence.   This thesis investigates how a battery storage system installed at the end of a mid-voltage line affects the electric power quality and the protection system compared to a reference case which represents the line today. The mid-voltage line is an existing line in the northern grid and is a good representation of the general northern grid.     The results obtained from this study show that the loop impedance does not necessarily have to deteriorate with the new feeder, which means that the voltage quality in this aspect does not change. This is only true for this particular grid when the impedance contribution from the mid-voltage grid is small and the contribution from the low-voltage grid is large because of long and weak distribution low-voltage lines.     The inverter is the limiting factor for the short circuit currents, and the short circuit power and current decrease with the new feeder. At fault occurrence in the low voltage grid the short circuit current does not affect the fuse blow. However the inverter will disconnect for all fault occurrences in the mid-voltage grid, and for faults with high short circuit power in the low voltage grid. This means that the selectivity will decrease, but the system will have a high security level.   For this particular grid the results show that the transformer can be isolated from the ground. A residual overvoltage relay is enough for disconnection of grounding faults. Directional ground fault protection is not necessary for this line with this particular characteristics. If the sensitivity for the disconnection of grounding faults with high transition impedances is adjusted according to ground fault currents in island operation, the sensitivity then decreases at the event of a fault during normal operations when the battery storage system is charging. The sensitivity might also be tuned according to normal operation, which then results in a higher sensitivity than necessary during island operation.