Publication: Centralized control strategies for HVDC connection of offshore wind farms based on diode rectifier units.
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Publication date
2020-07
Defense date
2020-10-23
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Abstract
The possibility of using diode rectifier in the offshore substation for high voltage direct
current (HVDC) connection of offshore wind farms (OWFs) has recently received
an increasing interest due to its lower cost and losses together with its higher reliability
compared with the voltage source converter (VSC) based HVDC link. However,
the main drawback of the diode rectifier solution is that the system frequency has to
be controlled to guarantee the diode rectifier commutation, given that the isolated offshore
AC-grid can not generate the required frequency with the wind turbine generator
systems (WTGSs) conventional control systems. This Thesis presents a direct frequency
control for such an application whose principles are derived from the AC-side dynamics
of a diode rectifier (DR) based HVDC system indicating that the frequency control
can be achieved by reactive power balance at the rectifier station without a capacitor
bank placed at the diode rectifier station. The proposed control system is implemented
by a VSC connected to the rectifier station that is always needed for the centralized frequency
control where the WTGSs conventional control systems are not changed. A hybrid
HVDC system consisting of the DC-parallel connection of a low-power VSC with
the rectifier station is also proposed in this Thesis, which is compared with the traditional
VSC-HVDC connected OWFs. Besides the frequency control and diode rectifier
harmonic currents compensation, the OWFs start-up is addressed in the centralized
hybrid topology, overcoming all the DR-HVDC link drawbacks. Finally, a centralized
control strategy of the parallel operation of the DR-HVDC and VSC-HVDC links for
OWFs connection is presented. The VSC-HVDC controls the voltage and frequency
of the offshore AC-grid needed for the OWFs start-up. The proposed centralized grid
forming control is based on controlling the OWFs voltage by controlling the active
power balance at the VSC-HVDC AC-bus. This leads to an onpitimized system where
the VSC-HVDC can be sized just to support the system energization; while once the
OWFs are in operation, the DR-HVDC will be loaded automatically. Stability analysis
and simulation results provided throughout this Thesis verify the performance of the
proposed control systems.
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Keywords
Wind farms, Wind energy, Offshore wind power plant, Wind turbine generator system, High voltage alternating current, HVAC, High voltage direct current, HVDC, OWF interconnection technology, LCC-rectifier-based HVDC, Diode-based HVDC