Resonances due to long HVAC offshore cable connections: studies to verify the immunity of dutch transmission network

Studies carried out by the Dutch transmission system operator TenneT have shown that the harmonic impedance of the 380 kV transmission grid, as observed from the point of connection for planned offshore wind power plants, could lie on a low resonance frequency of about 100 Hz. There is a concern that the low resonance frequencies could result in possible elevated voltages (Temporary Overvoltages, TOVs). The resonance phenomena might be triggered by switching events, most significantly those that involve power transformers. Based on the harmonic impedance analysis, a full transient study is performed by DNV GL in order to better understand these phenomena and the potential impact of the long (100 km) HVAC offshore cable connections on the transmission grid. This paper describes the considered topics, starting from the harmonic frequency scans to the discussion of the resonance phenomena. Various time domain transient case scenarios are studied as well to verify the immunity of the Dutch transmission network to a long HVAC offshore cable connection and conclusions and recommendations are drawn.