Control System interaction in the VSC-HVDC Grid Connected Offshore Wind Power Plant

Conventional offshore wind power plants (OWPPs) are due to the combination of the extensive sub-marine cabling and possible low available short-circuit power at the point of common connection (PCC) susceptible to the harmonic instability phenomena. The instability is caused by the resonances in the electrical system which is generally located within the wind turbine generator (WTG) controller bandwidth. Some of the negative impact of the existence of harmonic instability include (but not limited to) accelerated component ageing due to increased thermal stresses and loss of power production before the source of the instability has been identified and mitigation methods have been designed and implemented. This procedure is not straightforward and can have a long lead time. The harmonic instability can have severe economic consequences for the OWPP owner due to the large investment. Harmonic stability or converter interaction studies have therefore become an important part of the system design studies of a high voltage alternating current (HVAC) grid connected OWPP.
The voltage sourced converter high voltage direct current (VSC-HVDC) has become a preferred choice for grid connection of remotely located OWPPs.
As for the HVAC grid connected OWPPs, there is a need to conduct harmonic stability studies in the design phase of an HVDC grid connected OWPP. As the offshore electrical environment is significantly altered compared to the offshore network in an HVAC connected OWPP, there is a need to define the procedure of the stability study and its application for the HVDC grid connected OWPPs. The purpose of this paper is to investigate the harmonic instability phenomena in HVDC grid connected OWPPs using both frequency and time domain simulations. A good correlation at lower frequencies between the two domains is observed. However, the frequency domain is insufficient at higher frequencies (i.e. in the vicinity of the switching frequency). A combination of the two techniques is thus recommended in the OWPP design phase. Successful application of active filtering to supress possible instabilities is illustrated.