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Report RSE 17001192

Analysis of the electric systems behaviour in presence of multi terminal HVDC VSClinks



Request Document (17.08 MB, .pdf)

A. Iaria (RSE SpA), D. Cirio (RSE SpA) , M. Rapizza (RSE SpA), A. Pitto (RSE SpA), A. Gatti (RSE SpA)

T&D-C.C. 2016 - Transmission and distribution in direct current

Implementation of control functions for HVDCVSC, aimed to ancillary services supply for the stable operation of the grid and for the restoration's support following a blackout, also in compliance with the European Grid codes. The functions have been verfied on AC grid models with high penetration of variable renewable energy sources, in particular in scenarios of Sardinia and Denmark.

Multi-terminal HVDC links, based on Voltage Source Converter (VSC) technology, may be suitable solutions to transmit the power produced by large amounts of renewable energy sources (RES).

The present activity, carried out in synergy with the European research project “BEST PATHS”, analyses the possible refurbishment of the three-terminal HVDC SA.CO.I.2 between Sardinia, Corsica and the continental grid by means of the adoption of the VSC technology,: the new link SA.CO.I.3 is assumed with bipolar configuration and doubled rated power (2×300 MW).

Some control functions, aimed to provide ancillary services for the stable operation of the Sardinian power grid and for the restoration of the system following a major blackout, have been implemented in the DigSILENT PowerFactory® environment, on a realistic model of the Sardinian/Corsican power system (year 2030) including the two HVDC links SA.CO.I.3 and SA.PE.I. (2×500 MW). These functions are: frequency regulation; small signal oscillation damping (Power System Stabilizer, PSS, mode); black start functions, static compensation of reactive power (STATCOM mode).

Dynamic simulations of generation loss in Sardinia highlighted that in case of low power export toward mainland, fast and continuous power reversal of the SA.CO.I.3-VSC can be crucial in order to contain the Sardinian under-frequency within acceptable operative bounds.

Unlike the LCC technology, in case of short circuits in the Sardinian grid, the VSC is able to preserve a minimum power transfer also in the fault-on period, with consequent advantages on the transient stability. On the other side, a suitable tuning of DC voltage control scheme is needed in order to avoid disturbances on the continental system.

Simulations also proved that SA.CO.I.3-VSC can be useful in the restoration of the Sardinian system following a (possibly complete) blackout. In fact, it can behave as a black start unit, and, thanks to its fast frequency and voltage controls, it ensures higher flexibility, speed and controllability of the restoration process with respect to conventional black start plants. Moreover, in the context of a conventional restoration scheme, SA.CO.I.3-VSC can act as a static compensator in order to support the voltage control along the restoration path.

Finally, the report describes the control schemes for multi-terminal HVDC grids with a special focus on the requirements for their compliance with the ENTSO-E and National Grid (UK) grid codes. It presents some control functions aimed to the provision of grid ancillary services by HVDC converters of future multi-terminal HVDC grids for the integration of offshore wind farms to the mainland AC grid. The control schemes have been implemented and verified in DigSILENT PowerFactory® on a realistic AC/DC grid model.

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