Design requirements for a mobile high-voltage substation to (partly) bypass permanent substations to reduce planned outages

Abstract

The energy transition requires massive expansion and reinforcement of the transmission grid which is challenging because of its high utilization, and regular maintenance, replacement, and repair. The difficulty of performing maintenance on high-voltage equipment and especially the substations lies in the fact that a big part of the substation needs to be taken out of service for the maintenance crew to work safely. When a part of a substation is taken out of service this influences the capacity and redundancy and thus the reliability of the grid. A planned outage is required to take a part of a substation out of service, these planned outages are limited in frequency and duration due to a shortage of especially trained and skilled people in combination with the high utilization of the grid.

The objective of this master thesis is to investigate important design aspects for mobile transmission stations to reduce/prevent planned outages during maintenance, replacement, and expansion projects on TenneT's (the Dutch transmission system operator) permanent substations. A mobile substation is a fully equipped substation mounted on one or multiple trailers to be easily transported. The mobile substation provides a bypass during maintenance on permanent substations allowing continued service without requiring planned outage.
A case study in this work shows that using a mobile substation during maintenance, replacement, and expansion projects on substations can possibly save TenneT several hundred million euros per year. Using the mobile substation on a 380 kV or 220 kV substation would make it possible to work on multiple 380 or 220 kV substations at the same time. It would also be easier to get a planned outage permit for the 150 kV and 110 kV substations. The use of a mobile substation would thus increase the efficiency of the projects and would reduce the amount of required critical resources.

Besides the case study, this work investigates the design requirements for mobile substations. The mobile substation should be able to (partly) bypass all TenneT substations and should contain mobile power transformers. The weight of the mobile substation is limited by road regulations. The transformers will be the heaviest components in the substation thus special attention is needed for their design. For the power transformers, a shell-type core design would be preferred with (high temperature) hybrid-insulation, due to its compact design and robustness for transportation. To comply with road regulations without special permits six 83.33 MVA single-phase transformers with an approximate weight of 36 tonne need to be connected in two banks of three single-phase transformers to reach the standardized capacity of 500~MVA.

The insulation distance in the mobile substation will greatly influence the size of the substation when deployed and during transport. Therefore, this insulation distance is an important design aspect which is covered in this work. The standards for insulation distances were analyzed. This analysis gave rise to doubts if these distances would apply to compact substations with large electrodes. An experiment was conducted at the TU Delft Electrical Sustainable Power Lab to determine the dielectric strength of large electrodes with sharp points in compact substations, and to verify if the insulation distances as specified in the standards should be applied to mobile substations.
It was found that the large electrodes with sharp points have similar breakdown voltages as a rod-rod gap setup. A higher breakdown voltage was found for the large spheres with protrusions compared to the rod-conductor gap which is used by the IEC standard. This indicates that the IEC standard insulation distances would be sufficient for use in a compact substation. Whether the substation can be made more compact by reducing the insulation distances should be investigated by executing experiments on an actual skid in a future study.