Steady State and transient behavior of underground cables in 380 kV transmission grids

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The extension of the Dutch 380 kV high voltage grid is necessary in order to guarantee security of electricity supply to the consumers. To achieve this extension, there are two new 380 kV connections under construction in the Randstad area, a densely populated area in the western part of the Netherlands. In these 380 kV connections, underground cables are applied. The work described in this thesis forms a part of a monitoring program that is managed by the Dutch transmission grid operator TenneT. In this program, the behavior of the two new underground cable connections in the Dutch 380 kV grid is being investigated and the work described in this thesis contributes to this program. Unlike the common overhead transmission line, which has an inductive behavior, a cable acts as a capacitance when it is in operation. This difference in electrical behavior makes research on the grid behavior necessary. One of the research goals is to evaluate whether the power flows and voltage levels in the 380 kV grid stay within the prescribed limits. Another goal of this work is investigating whether the peak of the transient voltages at the cable terminals after lightning currents will stay below the Basic Insulation Level. The research is divided into two parts: steady state behavior and transient behavior. For the steady state part, PI sections are used to model the underground cable and overhead line to perform load flow studies in order to investigate the power flows and voltage levels in the 380 kV grid. For the transient part, the widely applied Frequency Dependent Phase Model (FDPM), which is a transmission line model that takes into account the frequency dependency of cable and line parameters, is used to model the cable and overhead line sections. For the transient part, the focus is on the modeling work of the complete underground cable system. Field measurements are performed for both the 380 kV cross-bonded cable and the overhead line sections and the results are used for transient cable model verification. The scientific contribution of this work is the determination of the parameters for the 380 kV cross bonding system by using reflection measurements and the validation of the FDPM transmission line model by using field measurements on the actual cross bonded 380 kV cable system. Finally, based on the studies described in this thesis, one can conclude that no remarkable overvoltages occur during the studied steady state and transient situations for the given dimensions of the observed 380 kV system.