Harmonic/Transient Analysis of the RARIK Distribution Network
Author: Andri Viðar Kristmannsson
Year: 2023
Supervisor: Ragnar Kristjánsson
Abstract:
The RARIK distribution grid is the longest power grid in Iceland reaching 9000km. In the last couple of decades has RARIK been systematically phasing out all overhead lines and replacing them with underground cables. Now 70\% of the distribution grid of RARIK are underground cables. RARIK has noticed that the number of damaged electronics being reported to them, due to events in their grid has increased over the years. Thus, it is of great interest for RARIK to study if the increase in underground cables is having negative effects to their grid with regards to transient/harmonic overvoltages. This research starts with reviewing damage reports to locate where it is common for damages to occur. One of the locations found from that analysis was the Borgarnes/Skorradalur area, that area has a reasonably compact grid, thus it was chosen as the case study for this research. The next steps were to create a model of the system, validating it and then preforming simulations. Twelve different cases were created to simulate the response in that area. These cases give a good representation of the general response of the system to different types of switching transients. These transient overvoltages were then evaluated using the ITIC curve to see if the overvoltages might be causing damages. A frequency sweep was also performed to see if the resonant frequency of the system had been changed due to the increased underground cables. These simulations where then compared to identical simulations, performed in an identical grid that utilizes overhead lines instead of underground cables. These comparisons show that the transient response of the current grid is composed of lower frequencies, where the transient overvoltages are also smaller in magnitude. These lower frequency overvoltages can be considered negative with regards to the ITIC curve, as it seems that lower frequencies are generally closer to violating the ITIC curve compared to higher frequencies. These lower frequencies of the transients also increase the likelihood of the transient including frequencies that are close to the resonance of the system, as it could also be seen through the comparisons that the resonant frequency of the grid has shifted towards the lower end of the frequency spectrum.