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Multi-Port High Voltage Gain Modular Power Converter for Offshore Wind Farms

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Multi-Port High Voltage Gain Modular Power Converter for Offshore Wind Farms sustainability Article Multi-Port High Voltage Gain Modular Power Converter for Offshore Wind Farms Sen Song 1, Yihua Hu 1 ID , Kai Ni 1,* ID , Joseph Yan 1, Guipeng Chen 2, Huiqing Wen 3 and Xianming Ye 4 1 Department of Electrical Engineering and Electronics, The University of Liverpool, Liverpool L69 3BX, UK; [email protected] (S.S.); [email protected] (Y.H.); [email protected] (J.Y.) 2 College of Electrical Engineering, Zhejiang University, Hangzhou 310027, China; [email protected] 3 Department of Electrical Engineering and Electronics, Xi’an Jiaotong-Liverpool University, Suzhou 215123, China; [email protected] 4 Department of Electrical, Electronic and Computer Engineering, University of Pretoria, Pretoria 0084, South Africa; [email protected] * Correspondence: [email protected]; Tel.: +44-751-920-3720 Received: 30 May 2018; Accepted: 25 June 2018; Published: 26 June 2018 Abstract: In high voltage direct current (HVDC) power transmission of offshore wind power systems, DC/DC converters are applied to transfer power from wind generators to HVDC terminals, and they play a crucial role in providing a high voltage gain, high efficiency, and high fault tolerance. This paper introduces an innovative multi-port DC/DC converter with multiple modules connected in a scalable matrix configuration, presenting an ultra-high voltage step-up ratio and low voltage/current rating of components simultaneously. Additionally, thanks to the adoption of active clamping current-fed push–pull (CFPP) converters as sub-modules (SMs), soft-switching is obtained for all power switches, and the currents of series-connected CFPP converters are auto-balanced, which significantly reduce switching losses and control complexity. Furthermore, owing to the expandable matrix structure, the output voltage and power of a modular converter can be controlled by those of a single SM, or by adjusting the column and row numbers of the matrix. High control flexibility improves fault tolerance. Moreover, due to the flexible control, the proposed converter can transfer power directly from multiple ports to HVDC terminals without bus cable. In this paper, the design of the proposed converter is introduced, and its functions are illustrated by simulation results. Keywords: high voltage direct current (HVDC); power transmission; DC/DC converter; high voltage gain; modular; multi-port 1. Introduction The global number of offshore wind farms has increased in recent years [1,2]. In Europe, 560 new offshore wind turbines were built in 17 wind farms in 2017 with a total generation capacity of 3148 MW, which is about 20% of the total offshore generation capacity [3]. The power generated offshore is typically transmitted over an average distance of 41 km (in 2017) through submarine cables before reaching a connection point with the existing onshore grid [3]. For example, Hornsea wind farm is located around 40 km from the onshore station. Compared with high voltage alternating current (HVAC) transmission, high voltage direct current (HVDC) transmission is the preferred method to transfer power over a long distance (>40 km) in terms of the factors of economy and power efficiency [4,5]. When delivering the same amount of power, the purchase price for the bipolar HVDC cable is lower than that of two parallel 3-core HVAC ones [6]. Additionally, HVDC has a higher transmission efficiency than HVAC since no inductance-reactive power exists within DC transmission cables. Sustainability 2018, 10, 2176; doi:10.3390/su10072176 www.mdpi.com/journal/sustainability Sustainability 2018, 10, x FOR PEER REVIEW 2 of 15 Sustainabilitytransmission2018 ,efficiency10, 2176 than HVAC since no inductance-reactive power exists within2 ofDC 15 transmission cables. Figure 1 presents three HVDC configurations. The hybrid HVDC system illustrated in Figure 1a uses Figurea medium1 presents voltage three (MV) HVDC—high configurations. voltage (HV) AC/DC The hybrid converter HVDC to systemobtain high illustrated voltage in DC Figure power.1a usesHowever, a medium line-frequency voltage (MV)—high (50/60 Hz) voltage AC/AC (HV) transformers AC/DC converterfor low-voltage to obtain (LV) high—MV voltage conversion DC power. still However,occupy a significant line-frequency portion (50/60 of the Hz) substation AC/AC transformers space. In this for topology, low-voltage the transformer (LV)—MV conversion is replaced still by occupya converter a significant as shown portion in Figure of the 1b,substation which significantly space. In thisreduces topology, the system the transformer size and weight is replaced [7]. These by a convertertwo configurati as shownons in use Figure two1-b,stage which conversion significantly to reducesmeet the the voltage system sizelevel and of weightHVDC [ transmission.7]. These two configurationsHowever, Reference use two-stage [8] points conversion out that the to meetconfiguration the voltage of leveltwo DC/DC of HVDC conversion transmission. stages However, has the Referencehighest power [8] points loss, which out that is thearound configuration four times of that two of DC/DC a one conversion conversion stage stages confi hasguration the highest presented power loss,in Figure which 1c is taking around into four consider times thatation of the a one winding conversion and stagecore losses configuration of transformers presented and in the Figure given1c takingdatasheets into of consideration the semiconductors. the winding The andconverter core losses applied of transformersin Figure 1c not and only the provides given datasheets a high voltage of the semiconductors.gain but also transfers The converterpower directly applied from in multiple Figure1c generators not only provides to HVDC a terminals high voltage without gain bus but cable. also transfersThe converter power design directly is from a technical multiple challenge generators for to HVDCboosting terminals LV directly without to busHV cable.due to The the converter conflict designbetween is athe technical required challenge high voltage for boosting level,LV e.g. directly, ±800 tokV HV [9] due and to the the conflictrestricted between voltage the ratings required of highsemiconductor voltage level, components, e.g., ±800 kVe.g. [9, ]22 and kV the for restricted SiC thyristors, voltage ratingsand 15 of kV semiconductor for SiC transistors components, [10]. e.g.,Fortunately, 22 kV for with SiC thyristors,the development and 15 kVof semiconductors for SiC transistors and [10 converter]. Fortunately, topologies, with the possible development solutions of semiconductorsare provided [11 and–13] converter. topologies, possible solutions are provided [11–13]. LV AC LV-MV MV-HV LV-MV MV DC MV-HV Wind Turbine Generators Wind Turbine Generators Bus AC/AC AC/DC DC/DC Bus DC/DC GB AC/DC = GB AC/DC/AC = . = . = HVDC . = HVDC . transimission transmission GB AC/DC/AC GB AC/DC = = (a) (b) LV-HV Wind Turbine Generators DC/DC GB AC/DC . = . HVDC = transimission GB AC/DC (c) Figure 1. High voltage direct current (HVDC) configurations for wind power transmission: (a) DC- Figure 1. High voltage direct current (HVDC) configurations for wind power transmission: (baseda) DC-based connection connection with two with-stage two-stage hybrid hybridconversion; conversion; (b) DC- (basedb) DC-based connection connection with two with-stage two-stage DC/DC DC/DCconversion; conversion; (c) DC-based (c) DC-based connection connection with the withproposed the proposed modular modular converter. converter. To address the challenges, DC converters with high voltage gains [14–17], modular multilevel To address the challenges, DC converters with high voltage gains [14–17], modular multilevel converters (MMCs) [18–22] and multi-module converters [23–25] are studied extensively. Although converters (MMCs) [18–22] and multi-module converters [23–25] are studied extensively. dual-active bridge (DAB) converters [14,15] and resonant converters [16,17] can obtain high voltage Although dual-active bridge (DAB) converters [14,15] and resonant converters [16,17] can obtain step-up ratios, the voltage stress on their semiconductor components is high, which can be reduced high voltage step-up ratios, the voltage stress on their semiconductor components is high, which can by applying MMCs. By adding sub-modules (SMs), a high output voltage is achieved without be reduced by applying MMCs. By adding sub-modules (SMs), a high output voltage is achieved increasing the voltage stress. However, the MMC topologies based on half-bridge (HB) or full-bridge without increasing the voltage stress. However, the MMC topologies based on half-bridge (HB) or (FB) [18,19] and resonant MMC [20] cannot provide electrical isolation. The isolated MMCs in full-bridge (FB) [18,19] and resonant MMC [20] cannot provide electrical isolation. The isolated MMCs References [21,22] are presented with DC/AC/DC configuration, where medium-frequency high in References [21,22] are presented with DC/AC/DC configuration, where medium-frequency high turns-ratio transformers are employed, resulting in a vast volume. Although resonant MMCs [26] Sustainability 2018, 10, x FOR PEER REVIEW 3 of 15 turns-ratio transformers are employed, resulting in a vast volume. Although resonant MMCs [26] achieveSustainability galvanic2018 isolation, 10, 2176 and a small volume of transformers at the same time, its conversion3 of 15 ratio only satisfies MV applications. Alternatively, transformers can be decentralized into multi-module converters, enabling the installation of high-frequency
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