Automation i Distributionsnät
As a part of Göteborg Energi?s work towards a smarter electric grid, the company has chosen to investigate the possibilities of automation in their middle voltage grid. In Europe, there are example cases where these technologies already are installed which many articles on the CIRED conference 2011 proved. Netcontrol Oy in Finland has, together with Tekla, created an automated SCADA system that Vattenfall in Finland today uses. The grid mainly consists of 10 kV open loop fed city grid and this automation is mainly designed for this type of grid. The intention of the automation is to when a fault occurs, the automation disconnects the faulted cable in the loop and then reconnects, this in as little time as possible in order to not make customers suffer.There are several different theories about how automation can be implemented, the two main principles are local automation and centralized automation. The local automation is implemented at station level with fault detectors that communicate with each other and thus disconnects a fault in the circuit using logic steps. This is the fastest option and Göteborg Energi has already installed several devices in the grid, which could make this option possible to realize. The other alternative is to let the SCADA do the sectionalizing using information from the indicators in the loop. This poses a problem though when the system becomes more complex. The operational staff might have problems with letting the SCADA make its own decisions and thereby lose control over the events in the grid. What the system could do instead is to make a suggestion on how the operational staff should handle the situation. However, the fault durations in this case will end at its best around 2-5 minutes and this makes the customer suffer for a longer time than the earlier principle.Hence, local automation is recommended, partly because it is today implemented in the grid and also because the fault duration is much shorter. This option can, with today?s technology disconnect a single-phase to ground fault without a power failure, and thus does no customer suffer during the fault. This should be supplemented with some sort of residual current compensation of the earth fault in order to remove the active fault current, thus minimizing the probability of a double earth fault or short circuit. Göteborg Energi should contact the suppliers of these devices and find out what they consider appropriate, and implement the technology in a loop as a pilot project to investigate its function.