Mitigation of harmonic instability in offshore wind farms using supplementary signals in the HVDC control

This paper deals with the harmonic instability problem in offshore wind farms that can be caused by the interaction of HVDC controller with the resonance frequencies of the grid. The circuit comprising of HVDC filters, transformers and long submarine cables linking offshore wind farms to the main grid onshore have natural resonances, which are close to the operating frequency of the HVDC converter. The converter control may amplify the oscillation to such an extent that it can surmount the inherent damping in the circuit and lead to harmonic instability. Therefore, the converter control has to be designed taking into account all possible operating conditions and grid topologies. In this paper a new damping control approach is presented that adds supplementary signals to the dq-components of the converter output voltage. The parameters of the damping controller are estimated using the heuristic optimization algorithm called MVMO. The eigenvalue analysis of the model validates the proposed damping control approach. Simulation results in the time domain demonstrate the effectiveness of the proposed method in damping the oscillations.