Evolution of Harmonic Distortion in a Cablified Grid Island after Separation from the Meshed Transmission Grid - A Case Study from Denmark

Green transition in Denmark with more renewable energy production and electrification of consumption, transport, heating, energy conversion and storage accelerates expansion and reconstruction of the transmission grid. When electricity generation and consumption have common connection cables and substations in the transmission grid, such centres are called prosumers.

The prosumers do not necessarily increase the net energy exchange with the transmission grid but their short-circuit current contribution may significantly increase due to utilization of inverter-based units. To keep the short-circuit current contribution below the required rating, the meshed 150 kV transmission grid will be separated into islands that are interconnected through the 400 kV meshed transmission system. At the same time, 150 kV overhead lines will be replaced with underground cables as part of the ongoing grid reconstruction in Denmark.

This paper presents the simulation results of the 5th harmonic voltage distortion evolution from the present grid stage with the meshed 150 kV transmission grid and, mainly, with overhead lines, through a long-term grid development process with cabling and separation of the meshed grid into 150 kV grid islands. The paper explains foreseen changes of the 5th harmonic voltage distortion within a specific grid island using a measurement validated simulation model for harmonic assessment and benchmarking the simulation results to the harmonic voltage measurements in the present grid stage. The paper also demonstrates identification and usage of early warnings for not-yet-occurred critical increases of the harmonic voltage distortion and proposal of mitigation solutions.