Abstract
In most DC power systems, power electronic devices can introduce ripple content into the DC grid, where large input ripple currents on the DC link can have a negative influence. Under these circumstances, DC side filters play an important role in the reduction of ripple content. In this paper, based on a full detailed closed loop model of the entire offshore wind farm multi-terminal DC network, the effect of filter capacitors connected at different sections of the system on the limitation of the AC ripple content, particularly in the DC cables, is studied. In contrast to other work on HVDC for offshore wind, where simplified or equivalent circuits are mainly used while concentrating on the power and control system, this study can be regarded as the specific study of filter capacitors operating within a detailed system model. Utilising the advantages of PLECS, which is a highly effective tool in the simulation of power electronic circuits, no small-signal or simplified equivalent models are used in the system, and the entire study is based on detailed and accurate models of the semiconductor elements and transformers, which help to provide more realistic simulation results and a better understanding of the system. Another novel point in this paper is a new concept, relative losses, which is proposed to simplify extensively the calculation of losses in this research. Finally, the size of the filter capacitors at different sections of the system under different situations is suggested.
Highlights
The proposed use of multiple and different DC-DC converters to step up the voltage in a multi-connected offshore DC grid is a new solution to collect power from a large offshore wind farm; Investigation of requirements for filters at different sections of the multi-connected system to limit the DC voltage ripple content and AC losses on DC cables; The proposal and definition of the concept of relative AC losses and presentation of an AC efficiency study on multi-connected DC cables; The development of a simplified and downscaled hardware model of the system to verify the effect of filter capacitors at different sections of the system
Different values of the output capacitors of the 5 MW DC-DC converters (Co in Figure 3) and of the input capacitors of the IPOS converters in Figure 4 are simulated to monitor their effects on the ripple content of the voltages and AC losses caused by the ripple content of currents in the multi-connected section of the system
This study aims at giving a general understanding of the operation of the system, as well as selecting appropriate capacitor filters based on the voltage and current ripple content on the cables under different IPOS converter structures
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. This paper uses a detailed offshore wind farm based multi-terminal DC system which includes a full closed-loop control circuit at the first stage; the research focuses on the variation in cable ripple content due to the value of filters at different sections of the system, under the same wind speed. Another point is this study is that it focuses on the AC ripple content, in the DC cables at different sections of the system, which, important, is less common in existing studies. The proposed use of multiple and different DC-DC converters to step up the voltage in a multi-connected offshore DC grid is a new solution to collect power from a large offshore wind farm; Investigation of requirements for filters at different sections of the multi-connected system to limit the DC voltage ripple content and AC losses on DC cables; The proposal and definition of the concept of relative AC losses and presentation of an AC efficiency study on multi-connected DC cables; The development of a simplified and downscaled hardware model of the system to verify the effect of filter capacitors at different sections of the system
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