| With the gradually exhaustion of fossil fuels and the ever-increasing seriousness of global warming issues,the exploitation of renewable energy such as wind and solar energy has aroused widespread concern.Thanks to the continues development of high voltage direct current(HVDC)transmission,the DC Grid has been considered as a preferrable option for the integration and long distance transmission of large-scale renewable energy.Nevertheless,the realization of DC Grid still faces the challenge of key equipment such as DC-DC converter.DC-DC converter plays an indispensable role in interconnecting DC systems with different voltages.Therefore,the research of DC-DC converter has always been a hot issue all over the world.This paper mainly focuses on the application of DC-DC converter in the DC collection and transmission of large-scale renewable energy.To begin with,the paper analyses the technical requirements of DC-DC converters in various scenarios.Then,the author proposes several applicable DC-DC converter topologies according to the technical requirements.This paper is mainly organized as follows.(1)To begin with,this paper gives a brief introduction to the development of renewable energy and HVDC transmission.After comparing with other several feasible alternatives,the DC Grid is regarded as a preferrable choice in the collection and transmission of large-scale renewable energy.This paper focuses on the research of DC-DC converter,which is one of the critical equipment in forming DC Grid.To begin with,the paper summarizes several promising DC-DC converter topologies,and then briefly analyses the merits and shortcomings of each topology.Based on this,the paper classifies the application scenario of DC-DC converters into two categories,namely the dc collection of renewable energy and the interconnection of HVDC systems.Besides that,the technical requirements of DC-DC converters in different scenarios are given.At last,the paper proposes a feasible topology of DC collection and transmission for large-scale renewable energy,and studies on the DC-DC converters with different purposes in the proposed topology.(2)According to the technical requirement of DC-DC converter applicated in integrating wind power to the DC collector summarized in(1),this paper proposes a novel DC-DC converter topology,termed as unidirectional LC DC/DC converter.The proposed converter has advantages of high stepping ratio,high efficiency,low volume,and fault isolation capability.Then the paper studies the parameter design,power control,and the fault analysis of the unidirectional LC converter.At last,a test system is built on the PSCAD/EMTDC platform to verify the technical feasibility of the proposed converter.(3)According to the technical requirements of integrating photovoltaic to the DC collector summarized in(1),this paper proposes another novel DC-DC converter topology based on the converter in(2).The proposed converter is named cascaded LCT DC-DC converter,which not only retains most of the merits in LC DC-DC converter,but also further increases the stepping ratio.Besides that,the LCT DC-DC converter has electrical isolation capability.Then,the author studies the parameter design,control strategy,semiconductor device selection,and the loss analysis of the proposed converter.Finally,a test model is built on PSCAD/EMTDC platform to verify the technical feasibility of the converter and the effectiveness of the control strategy.(4)As to the long distance transmission issue for the large-scale renewable energy,this paper proposes a feasible HVDC transmission topology.Besides that,this paper also proposes a novel DC/DC converter,termed as hybrid DC AUTO,dedicated to the interconnection of two independent HVDC point-to-point systems.The proposed hybrid DC AUTO has advantages of low capital lost,low transmission loss,and fault riding through capability.At last,a test system is built on PSCAD/EMTDC platform to validate the technical feasibility of the proposed converter,which lays solid foundation for the follow-up analysis towards the interconnected topology.(5)This paper also studies the control strategy and the fault analysis towards the interconnected topology.The merits of the interconnected topology in comparison with point-to-point system are summarized firstly.Then,the specific goals of the system during faulted time are analysed.A feasible control strategy suitable for the interconnected system is proposed according to the fault riding goals.Finally,a test system is built on PSCAD/EMTDC platform to validate the effectiveness of the proposed control strategy.It is expected that the research results of this paper can provide technical support for the application of DC-DC converters in the renewable energy DC collection and DC transmission system. |
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