Coordinated control of DFIG-based offshore wind power plant connected to a single VSC-HVDC operated at variable frequency
Mikel De-Prada Gil
1, Anca D. Hansen2, José Luis Domínguez-García1, Nicolaos A. Cutululis2, Oriol Gomis-Bellmunt3
1IREC, Sant Adrià de Besòs, Spain, 2DTU, Roskilde, Denmark, 3CITCEA-UPC, Barcelona, Spain
The existence of HVDC transmission systems for remote offshore wind power plants (OWPPs) is posing new challenges to cope with, but also is an opportunity for academia and industry to propose novel OWPP concepts in the attempt to reduce their levelised cost of energy. This paper investigates the dynamic behaviour and the fault ride-through (FRT) capability of an OWPP configuration based on DFIG wind turbines equipped with reduced size power converters (approx. 5% instead of 30% of the rated power) connected to a common VSC-HVDC, allocated in the offshore substation, which operates at a variable frequency within the collection grid. Several researchers have studied the FRT capability of both DFIG wind turbines itself or a combination between a DFIG-based wind farm connected through VSC-HVDC. However, in both cases the collection grid frequency was fixed to 50Hz and individual power converters of each DFIG wind turbine were sized at 30% of its rated power. This paper analyzes the ability of DFIG wind turbines with reduced sizes power converters (5% of its rated power) in combination with the VSC-HVDC converter to protect themselves without disconnection during grid faults. Specifically, this paper focus on determining how much the size of the power converter can be reduced and on developing a coordinated control strategy between the VSC-HVDC common converter and the individual back-to-back reduced power converters of each DFIG wind turbine for both normal and fault operations in order to maximize the energy production and provide control capability at a reduced cost.
This significant reduction of the power converter size is expected to be achieved as a consequence of the variable speed control provided by the common VSC to all the wind turbines. Thus, the VSC-HVDC converter imposes an optimum variable frequency within the collection grid whilst the smaller power converters inside each DFIG wind turbine are in charge of compensating (partially or totally) the wind speed differences among them. The control of the power electronics converter of both DFIG wind turbines and VSC-HVDC, as well as the pitch control of each turbine, must be redesigned to meet the new requirements of variable frequency operation and provide FRT capability. The common VSC-HVDC operates at rated volt/herz mode to keep the magnetizing flux constant at its rated value.
This paper investigates the dynamic behavior of DFIG wind turbines with reduced size power converters which operate under rated volt/hertz. This novel OWPP design allows each DFIG turbine to rotate at different speed within a certain range defined by the size of its partial scale power converter. Thereby, depending on the wind speed variability among the wind turbines and the power converter capacity, it is possible to ensure that all wind turbines operate at their optimum point.
The results show a good performance of the proposed system under both normal conditions and abrupt wind speed changes. The combination of controllers (VSC-HVDC, individual power converters and pitch control) can potentially reduced the power converter size (implying cost savings).
This paper analyzes the dynamic performance of an OWPP configuration arisen thanks to the use of HVDC technology and its ability to electrically decouple the OWPP collection grid from the onshore power system. The fact that collection grid frequency can vary and be optimized allows rethinking the way the OWPPs are currently designed and enables cost savings by reducing the power converter size of each DFIG wind turbine.
The impact of different power converter sizes and wind speed variability within the WPP on the dynamic behavior of the system under both normal and fault conditions is assessed. A coordinated control between the VSC-HVDC converter and the individual converters of each DFIG turbine is implemented and validated by means of dynamic simulations with DIgSILENT Power Factory.
It is expected that the results of the activities will provide better scientific understanding and guidance to continue advancing on the design of more cost-competitive OWPP concepts. Particular emphasis will be on challenges related to develop of a coordinated control strategy for both normal and fault operations between reduced power electronic converters inside DFIG wind turbines and a common large VSC-HVDC located in the offshore substation that dynamically changes the collection grid frequency, in order to maximize the energy production and partially/totally compensate the wind speed differences among them. It is worth mentioning that the control scheme proposed can also be applied to onshore WPPs, but in this paper, its application is focused on offshore WPPs.