Offshore HVDC grids are envisaged to render more reliable and economical solutions for transferring more offshore wind energy to onshore power systems. From the reliability point of view, the most promising operational/control strategy of HVDC grids is the dc voltage droop control. This control provides autonomous power-sharing among converters and promotes the frequency support to onshore power systems from other ends of HVDC grids without using fast communication links. The simultaneous use of frequency and dc voltage droops in an HVDC converter can transform frequency deviations into dc voltage changes. Other converters with dc voltage droop can react to the changes by regulating their power flow. Therefore, autonomous frequency support can be established through an HVDC grid. Although this method is reliable, fast, and economical, it gives rise to interactions between different droops, which can result in a reduction in the HVDC power that system operators can expect during frequency events. The present study investigates the effectiveness and challenges of the droop-based (communication less) control for providing frequency support in offshore HVDC grids. The frequency support is described mathematically, and a static transfer function between onshore frequency and HVDC power is derived as a function of the power system and control parameters. The function can be used to compensate for the adverse effect of frequency and dc voltage droop interactions; however, it is challenging in the case where an HVDC grid has more than one onshore ac systems since the transfer function of one ac system will depend on the parameters of other ac systems. A signal-flow model is developed for offshore HVDC grids to quantify the effects of onshore frequency deviations on different electrical parameters of the grid. Using this model the transfer function mentioned before can be obtained more easily. A point-to point and a four-terminal HVDC connections are simulated and communication less control is numerically compared with communication-based control.
|Title of host publication||Proceedings of the CIGRE Symposium 2019|
|Number of pages||10|
|Publisher||CIGRE (International Council on Large Electric Systems)|
|Article number||13 (YM)|
|Publication status||Published - 2019|
|Event||Cigre Aalborg 2019: International Symposium - Aalborg Congress and Culture Centre, Aalborg, Denmark|
Duration: 4 Jun 2019 → 7 Jun 2019
|Conference||Cigre Aalborg 2019: International Symposium|
|Location||Aalborg Congress and Culture Centre|
|Period||04/06/2019 → 07/06/2019|
Bidadfar, A., Saborío-Romano, O., Altin, M., Cutululis, N. A., Prieto-Araujo, E., Gomis-Bellmunt, O., & Sørensen, P. E. (2019). Droop-Based Frequency Support from Offshore HVDC Grids. In Proceedings of the CIGRE Symposium 2019 [13 (YM)] CIGRE (International Council on Large Electric Systems).