VFT insulation coordination study of a 400 kV GIS

In November 2008 the Danish government decided that all overhead lines below 400 kV should be replaced by underground cables. This is due to a demand of reducing the overall visibility of the transmission system, sometimes referred to as the beautification of the transmission system. The agreement furthermore included to reinforce some of the existing 400 kV transmission lines. This is due to both increasing wind penetration and power flow between Scandinavia, Germany and possible future connections. As a part of reinforcing the 400 kV transmission system in Jutland, Denmark, the Danish TSO (Energinet.dk) is in the process of constructing a new gas insulated substation (GIS) in Revsing. As a part of this process, new Eagle pylons will replace some of the existing Donau pylons. The new Eagle type pylon is meant to reduce the visual impact of transmission lines.
The reliability of the substation in Revsing is of great importance as it is part of the 400 kV systems backbone between Sweden, Norway, Germany and the offshore windfarms in Horns Rev. The design of the insulation coordination for GIS must therefore be studied carefully.
During a disconnector operation in GIS, very fast transient (VFT) may generate overvoltages (VFTO) inside the enclosure. Because the gas insulated system must be viewed as non-self-restoring, it is important to ensure that the voltage inside the GIS does not exceed the insulation strength. This must therefore be accounted for, when conducting an insulation coordination study of a GIS.
This article describes how the VFT phenomenon occurs inside the GIS and how it may generate overvoltages. This includes an explanation of how it is generated, what causes it and why it is so fast.
A schematic consisting of the surge impedances from the manufacturer of the GIS is simulated and compared to the same model with a number of capacitances added (representing the corresponding component). These added capacitances were not modelled by the manufacturer, but were added in order to further increase the level of detail. This is important as VFT may also be generated by circuit breaker (BRK) operations, a ground switch or due to a fault [1]. A detailed model is more likely to detect a VFT generated by e.g. a fault than a simplified model.
It is shown via simulations in a EMTP software, how the level of modelling detail affects the results. As may be seen from the simulation results, there is a significant difference between the voltage characteristics when simulating the GIS with and without the added components. The difference is approximately 1.5 p.u., and is apparent 35 ns from when the disconnector operation is performed. It is of interest to investigate what the primary cause of this difference is. A further analysis of this difference, lead to a closer look at how the breaker is represented in the model from the manufacturer and the model with the added components.
A comparison of the two models revealed the importance of detailed modelling, especially all of the capacitances which are present in the GIS for e.g. circuit breaker, disconnector (DS) and spacers.