Potentialstyrning i mellanspänningsnät : kalibrering av restströmskompensering och isolationsprovning

Abstract: 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.