Abstract
This paper proposes a communication-free control strategy at the offshore wind farm (OWF) level to enhance onshore fault ride-through (FRT) grid code compliance of the voltage source converter (VSC)-based multi-terminal high voltage direct current (MT-HVDC) grid. In this proposal, the emerging virtual synchronous generator (VSG) concept is employed to equip the Type 4 wind turbine generator (WTG)s with inherent grid forming ability. Accordingly, it is proposed to switch the offshore HVDC converters control mode from grid forming to grid feeding during onshore FRT period to realize direct wind power in-feed reduction as a function of the severity of MT-HVDC grid’s overvoltage. The related dynamics are mainly characterized by the high-speed current control loop, so improved OWF response is achieved during onshore FRT period as conventional voltage/frequency modulation strategies are not employed. New analysis/amendments are also proposed to study and improve the transient active power reduction sharing between the WTGs in first few power cycles under wind wake effect. Finally, with the objective of a smooth transfer of HVDC converters and WTGs in several proposed operation states, a set of state machines are proposed considering whole WTG’s dynamics. Comprehensive time-domain simulations are performed with averaged electromagnetic transient models to demonstrate the improved onshore FRT behavior in terms of minimizing the electrical stress at both MT-HVDC grid and OWF levels.
Highlights
Significant power imbalance occurs between the reduced active power exported by ON-HVDCCs and the wind power in-feed by OF-HVDCCs which will be stored as an electrical field in the respective capacitors of voltage source converter (VSC) and HVDC lines
Wind power in-feed reduction can be done by emulating the fault conditions inside the offshore wind farm (OWF) via the appropriate exercise of OF-HVDCC with careful tuning [14] that relies on local measurements and controllers
The described control strategies are done indirectly as the OF-HVDCC requests the individual wind turbine generator (WTG) to reduce their injected wind power rather than reducing the active power directly by itself, Fig. 1. It is since the voltage of the OWF's ACC network is formed by the OF-HVDCC in voltage/frequency control mode and the WTGs GSC are operated as grid feeding converters in active/reactive power control mode, i.e., conventional PLL units are utilized to achieve grid synchronism that has no grid forming capability
Summary
The second economic and elaborate solution provides control frameworks at the OWF level to reduce the wind power in-feed effectively [6]– [11] This can be done by sending active power reduction orders to individual WTGs through a dedicated fast communication channel [10], [12], [13] with reliability concerns. The described control strategies are done indirectly as the OF-HVDCC requests the individual WTGs to reduce their injected wind power rather than reducing the active power directly by itself, Fig. 1 It is since the voltage of the OWF's ACC network is formed by the OF-HVDCC in voltage/frequency control mode and the WTGs GSC are operated as grid feeding converters in active/reactive power control mode, i.e., conventional PLL units are utilized to achieve grid synchronism that has no grid forming capability. Fast OWF volt age amplitude reduction/fast OWF frequency rise/dispatching communi cation signals
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