3.3 高频变压器的绝缘及冷却
对于高压大容量的高频变压器来说,为了实现较高的功率密度,其绝缘和冷却设计需要重点考虑。从绝缘形式划分,PET中的高频变压器可以分为油(液体)浸式和干式两种。在已研制的PET样机中,两种类型的绝缘技术也均有采
用[19,22-23,30,32,49,64,104,107]。干式变压器的绝缘材料一般采用合成材料绝缘纸、环氧树脂[19,32]等。由于没有液体泄漏、挥发、着火等问题,干式变压器具有环境友好,维护少等优势。但是,干式变压器的散热、绝缘、局部放电等需要特别处理。因此,油浸式高频变压器依然获得了较多应用。油浸式变压器除去绝缘性能好之外,由于铁心和绕组都可以浸泡在绝缘油中,采用导热性能良好的绝缘油时还可以起到很好的散热作用。
另外,由于发热量较大且空间限制较多,高频变压器的冷却一般采用水冷[18,22-23,30,32,96,102,107]、强迫风冷[49,55,64]或者油冷方式[22]。但是,强迫风冷方式噪音大,散热器尺寸较大,散热效率相对较低,而油冷方式存在漏油、易燃、维护多等问题,这两种散热方式在实际应用中相对较少。而水冷方式通常采用去离子水对变压器进行冷却,以提高冷却水的耐压水平和绝缘性能,实现不同电位的元器件冷却水路相互连通,便于水冷系统设计。
4 PET的功率电路紧凑设计技术
PET中含有大量的功率半导体器件、电感、电容、高频变压器等无源器件,以及测量设备、控制保护设备等。为了便于安装维护,PET中的电力电子变流器主电路通常采用模块化设计,机柜式安装。在满足系统局部放电、散热、电气隔离以及机械应力等多种约束的情况下,通过合理的硬件布局与设计,尽可能降低系统的体积,是提高PET功率密度的关键技术。PET功率电路紧凑设计主要包含以下三方面技术。
4.1 变流器主电路紧凑化设计
如前所述,无论是CHB型还是MMC型PET,其主电路中通常均含有大量的子模块(如半桥或全桥模块)[5,23,27,48-49,55,61,63-64]。每个子模块都含有若干功率半导体器件、驱动控制电路、测量设备及直流电容等。在子模块内部,开关器件通常与直流电容采用层叠母排连接,能够极大提高子模块的紧凑程度[27-28,30,64]。此外,PET的功率子模块设计当中通常还需要采用特殊结构(如增加均压环、金属部件钝化处理等)或特殊绝缘材料等方法,实现紧凑空间内的电磁兼容性及绝缘特性,以进一步提高子模块的功率密度。
4.2 绝缘设计
为了保证设备及操作人员的安全,在有限的空间内实现PET良好的绝缘设计对其紧凑化有较大影响。在实际的PET中,为了满足系统绝缘和电气隔离的要求,高电位侧元部件和低电位侧元部件均需要设置良好的绝缘,包括足够的空气间隙、爬电距离和良好的局放处理措施[30,55,64]。但是,在保证绝缘安全的情况下,如何减小元部件之间的布置距离,并实现良好的电磁兼容特性和散热涉及电、磁、热等在内的多物理场耦合优化设计,是一项复杂的工作。例如,为尽可能减少局部放电现象,可以在高电位侧元件外增加用以均衡电场的防护罩[27,30]。但是,不合理的防护罩设计反而可能会增加局放效应并影响系统散热。而为了将处于高电位及低电位侧的不同电气元件布置在有限的空间内,通常做法是增加元件之间的空气间隙,但此种方法会一定程度上增加PET体积,降低功率密度。另外也可考虑采用更好性能的绝缘材料,如环氧树脂等,以提高系统绝缘水平,但这往往会提高系统造价[19]。
4.3 冷却设计
PET中的冷却技术与高频变压器类似,一般也包括自然冷却、强迫风冷、水冷和油冷等方法。但是,PET中不同的元器件发热情况不同,其对散热系统的要求也不尽相同,需要采用不同的冷却设计。例如：
1)功率半导体器件。
相比于PET的其他部分,功率半导体器件的损耗一般最大,也是PET产生损耗的主要来源。对于一些中小功率场合,可以采用强迫风冷的方式实现功率半导体器件的冷却[49,55]。在绝大多数情况下,尤其是对功率密度要求较高的场合,PET中的功率半导体器件一般均采用冷却效率较高的水冷方 式[27-28,30,64],即在固定开关器件的散热片上安装水冷板,通过外部水冷装置的循环实现开关器件冷却。此外,在某些特殊场合,如机车牵引[22],对PET的功率密度要求极高,一般需要采用油浸式变压器,为了尽量减少散热系统的体积,可直接采用油冷的方式实现功率半导体器件的冷却。
2)高频变压器。
高频变压器的损耗,包括铁心损耗、绕组损耗等,是PET损耗的另一个主要来源,其典型的冷却方式在前文已经总结,此处不再赘述。
3)其他部分。
其他部分主要包括直流电容、谐振电容、滤波电抗器等元件。这些元件在实际工作过程中自身发热量一般较小,通常采用自然冷却的方式即能满足设计要求。在发热量较大的场合,这些元件也可以通过增加绝缘后与功率半导体器件或高频变压器共用散热系统。
综上所述,PET功率电路的紧凑设计主要包括子模块紧凑设计技术、绝缘设计和高效率冷却技术等。但是,现有的PET功率密度仍然较低,这一方面是受到商用功率半导体器件发展水平的限制,导致在中高压场合应用的PET均含有大量的子模块和储能电容,极大的增加了PET的体积。另一方面,对于中高压场合,单纯提高高频变压器工作频率并不能显著降低其体积,原副边绕组的隔离电压也是影响高频变压器体积的主要因素[70]。因此,采用耐压等级更高的功率半导体器件和减少高频变压器使用数量是提高PET功率密度的主要途径。
4.4 宽禁带功率半导体在PET中的应用
由前文关于PET电路拓扑的分析可见,为承受高电压,已有的PET拓扑大多采用了级联型的变流器实现电能的交直流变换。在此情况下,显然可以通过采用更高耐压的功率半导体减少级联的功率单元数量,以及功率半导体器件和功率单元中储能电容的数量,从而可以简化PET的电路结构,提高功率密度。但是,目前的电力电子器件水平下,硅基可关断器件,尤其是应用最广泛的IGBT商用产品一般不超过6.5kV[31-32,56-57,87,101]。这就导致中高压的PET采用硅基IGBT时不得不采用大量的功率单元。为解决这一问题,近年来,宽禁带功率半导体,尤其是基于SiC材料的功率半导体器件在PET相关的应用得到较广泛的关注[56-58,77,82-83,87,96,100]。
一般说来,相对于硅基器件,宽禁带半导体,尤其是SiC器件具有如下优点：1)耐压等级高,更适合高压器件;2)开关速度快,适合高频应用;3)热导率高,使得它们非常适用于高温及高功率领域;4)损耗小,可以提高变流器的运行效率。因此,高压SiC器件特别适用于高效率、高功率密度的PET系统。现有的PET中,主要采用的高压SiC器件包括金属-氧化物半导体场效应晶体管(metal-oxide-semiconductor field-effect transistor,MOSFET)和IGBT。
2011年,美国GE公司联合Cree公司、Powerex公司等基于10kV耐压的SiC MOSFET研制了1 MVA的PET(文中称为固态变电站：solid state power substation,SSPS),部分SiC MOSFET的开关频率达到了20kHz[96]。相对于基于传统工频变压器的变电站,该PET比其重量减少了75%,而体积减少了约50%,运行效率达到了约97%。美国北卡罗来纳州立大学分析了基于SiC IGBT的PET在面向无工频变压器的智能变电站(transformerless intelligent power substation,TIPS)中应用优势,采用15 kV SiC MOSFET构建H桥,无需级联即可实现高压侧直接接入,分别开发了Gen-II和Gen-III两代小功率样机,并完成了实验验证[56-57]。在SiC器件的高频化应用方面,2017年,美国GE公司采用1.7kV的SiC MOSFET实现PET中的谐振软开关型DAB,研制的50kW级别的DAB样机在实验中的开关频率达到了175 kHz[77]。
高压、高频、低损耗的SiC器件在PET中的应用可以显著提高PET的功率密度和运行效率。但是相对于电压较低的硅基器件来说,高压SiC器件的应用也给PET的设计带来了一些新的问题。例如,单个子模块的额定工作电压在采用硅基器件时一般从几百V到不超过3kV,而10kV以上耐压的SiC器件使得单个功率模块的工作电压就达到了中压的水平,有限空间内的绝缘处理技术、高压器件的供电及电气隔离技术等比采用硅基器件时的低压场合更加困难。另外,SiC器件工作频率高且开通、关断速度快——dv/dt(电压变化率)可高达50kV/μs,远高于硅器件的情况(硅器件一般小于10kV/μs)[77-78]。这给器件本身及驱动电路、供电电源、控制电路、散热及接地系统等都带来了非常大的电磁干扰,也给高频变压器的寄生参数,尤其是高频下的寄生电容设计及优化带来了挑战。因此,高dv/dt下的PET系统电磁兼容设计比硅器件的情况也更加复杂和困难。
5 PET发展总结及展望
近十年来,PET相关的理论和技术研发在学术界和工业界已经获得了广泛关注,并先后研制了多台实验样机。但是,PET总体上仍然处于关键理论及技术攻关研究阶段,其性能与实际应用还有一定距离。综合分析PET的发展过程及现有技术,可以得到以下结论：
1)由于PET功能远多于传统的工频变压器,将PET仅与工频变压器本身进行效率、造价、功率密度等性能的直接比较不尽合理。实际上,与集成了工频变压器及电能质量治理功能的综合电能管理装置相比,目前的PET功率密度已经达到了相当甚至更高的水平,但运行效率和经济性仍需进一步提高。
2)由于PET电气连接端口形式多样灵活,PET更加适合于交流输入、直流输出的场合应用,而非直接替代现有的工频变压器。尤其是应用于以低压直流为主的配电网时,PET可以取消原有交流低压配电网中各种直流设备前端的并网逆变器,优化整个系统的结构和运行效率。
3)对我国的机车牵引系统来说,现有的车载牵引变压器额定工作频率为50Hz,比欧洲的部分铁路系统的16.7Hz已经高出很多,即车上的牵引变压器本身体积已经减小很多。在此种场合通过PET来替代车载牵引变压器的困难更大。而机车上的空间有限、震动明显、冷却方式受限等问题也给此种替代方案带来了更大的挑战,相关技术也需更加深入的研究。
4)PET的电路拓扑是决定其多方面性能的关键因素。在目前的技术水平下,PET的各种拓扑一般均需要大量的电力电子半导体器件和电容、电感等无源器件,导致其效率、功率密度、可靠性和经济性指标一般较低,这是限制其推广和应用的主要因素。而目前的研究表明,对于中高压应用的PET来说,高频变压器在整个PET系统中的体积和重量比重很小。提高高频变压器的工作频率,例如达到20kHz以上带来的功率密度指标提升十分有限,而综合考虑散热、绝缘等问题后甚至会降低系统功率密度。因此,电能变换环节数量少、运行效率高且结构紧凑的新型电路拓扑是提高PET多方面性能最亟需解决的问题。
5)宽禁带功率半导体,尤其是SiC功率半导体具有耐压等级高、损耗小等突出优势。高压SiC器件对于减少现有PET中功率半导体及无源器件数量、提高系统运行效率和功率密度具有显著的效果。对于中高压的PET来说,10kV以上的高压SiC器件应该会获得越来越广泛的应用。但是,因高压SiC器件应用带来的PET高频电磁场下的绝缘技术、高dv/dt下的电磁兼容设计技术、新型的冷却技术及系统优化技术也需进一步深入研究。
参考文献
[1] She Xu,Huang A Q,Burgos R.Review of solid-state transformer technologies and their application in power distribution systems[J].IEEE Journal of Emerging and Selected Topics in Power Electronics,2013,1(3):186-198.
[2] Feng Jianhua,Chu W Q,Zhang Zhixue,et al.Power electronic transformer-based railway traction systems:challenges and opportunities[J].IEEE Journal of Emerging and Selected Topics in Power Electronics,2017,5(3):1237-1253.
[3] 赵争鸣,冯高辉,袁立强,等.电能路由器的发展及其关键技术[J].中国电机工程学报,2017,37(13):3823-3834. Zhao Zhengming,Feng Gaohui,Yuan Liqiang,et al.The Development and Key Technologies of Electric Energy Router[J].Proceedings of the CSEE,2017,37(13):3823-3834(in Chinese).
[4] Kolar J W,Ortiz G I.Solid-state-transformers:key components of future traction and smart grid systems[C]//Proceedings of the International Power Electronics Conference.Hiroshima,Japan:IPEC,2014.
[5] Huang A Q,Crow M L,Heydt G T,et al.The Future Renewable Electric Energy Delivery and Management (FREEDM) system:the energy internet[J].Proceedings of the IEEE,2011,99(1):133-148.
[6] She Xu,Huang A Q,Lukic S,et al.On integration of solid-state transformer with zonal DC microgrid[J].IEEE Transactions on Smart Grid,2012,3(2):975-985.
[7] McMurray W.Power converter circuits having a high frequency link:US,3517300[P].1970-06-23.
[8] McMurray W.The thyristor electronic transformer:a power converter using a high-frequency link[J].IEEE Transactions on Industry and General Applications,1971,IGA-7(4):451-457.
[9] Harada K,Anan F,Yamasaki K,et al.Intelligent transformer[C]//Proceedings of the 27th Annual IEEE Power Electronics Specialists Conference.Baveno,Italy:IEEE,1996:1337-1341.
[10] Kang M,Enjeti P N,Pitel I J.Analysis and design of electronic transformers for electric power distribution system[C]//Proceedings of the 32th IAS Annual Meeting,Conference Record of the 1997 IEEE Industry Applications Conferenc.New Orleans,LA,USA:IEEE,1997:1689-1694.
[11] Manjrekar M D,Kieferndorf R,Venkataramanan G.Power electronic transformers for utility applications [C]//Proceedings of the Conference Record of the 2000 IEEE Industry Applications Conference.Rome,Italy:IEEE,2000:2496-2502.
[12] Ronan E R,Sudhoff S D,Glover S F,et al.A power electronic-based distribution transformer[J].IEEE Transactions on Power Delivery,2002,17(2):537-543.
[13] 邓卫华,张波,胡宗波.电力电子变压器电路拓扑与控制策略研究[J].电力系统自动化,2003,27(20):40-44,48. Deng Weihua,Zhang Bo,Hu Zongbo.Research on the topology and control scheme of power electronic transformer[J]. Automation of Electric Power Systems,2003,27(20):40-44,48(in Chinese).
[14] 方华亮,黄贻煜,范澍,等.配电系统电力电子变压器的研究[J].电力系统及其自动化学报,2003,15(4):27-31. Fang Hualiang,Huang Yiyu,Fan Shu.Study on power electronic transformer (PET) for distribution power system[J]. Proceedings of the EPSA,2003,15(4):27-31(in Chinese).
[15] 毛承雄,范澍,王丹,等.电力电子变压器的理论及其应用(I)[J].高电压技术,2003,29(12):4-6. MaoChengxiong,Fan Shu,Wang Dan,et al.Theory of power electronic transformer and its application (I)[J]. High Voltage Engineering,2003,29(12):4-6(in Chinese).
[16] 毛承雄,范澍,黄贻煜,等.电力电子变压器的理论及其应用(Ⅱ)[J].高电压技术,2003,29(12):1-3. MaoChengxiong,Fan Shu,Wang Dan,et al.Theory of power electronic transformer and its application (II)[J]. High Voltage Engineering,2003,29(12):1-3(in Chinese).
[17] 赵剑锋. 输出电压恒定的电力电子变压器仿真[J].电力系统自动化,2003,27(18):30-33,46. ZhaoJianfeng.Simulation study of a power electronics transformer with constant output voltage[J].Automation of Electric Power System,2003,27(18):30-33,46(in Chinese).
[18] Heinemann L.An actively cooled high power,high frequency transformer with high insulation capability[C]// Proceedings of the 17th Annual IEEE Applied Power Electronics Conference and Exposition.Dallas,TX,USA:IEEE,2002:352-357.
[19] Kjellqvist T,Norrga S,Ostlund S.Design considerations for a medium frequency transformer in a line side power conversion system[C]//Proceedings of the 35th Annual Power Electronics Specialists Conference.Aachen,Germany:IEEE,2004:704-710.
[20] Glinka M.Prototype of multiphase modular-multilevel- converter with 2 MW power rating and 17-level-output- voltage[C]//Proceedings of the 35th Annual Power Electronics Specialists Conference.Aachen,Germany:IEEE,2004:2572-2576.
[21] Glinka M,Marquardt R.A new AC/AC multilevel converter family[J].IEEE Transactions on Industrial Electronics,2005,52(3):662-669.
[22] Hugo N,Stefanutti P,Pellerin M,et al.Power electronics traction transformer[C]//Proceedings of 2007 European Conference on Power Electronics and Applications.Aalborg,Denmark:IEEE,2007:1-10.
[23] Steiner M,Reinold H.Medium frequency topology in railway applications[C]//Proceedings of 2007 European Conference on Power Electronics and Applications.Aalborg,Denmark:IEEE,2007:1-10.
[24] Martin J,Ladoux P,Chauchat B,et al.Medium frequency transformer for railway traction:soft switching converter with high voltage semi-conductors[C]//Proceedings of 2008 International Symposium on Power Electronics,Electrical Drives,Automation and Motion.Ischia,Italy:IEEE,2008:1180-1185.
[25] Carpita M,Marchesoni M,Pellerin M,et al.Multilevel converter for traction applications:small-scale prototype tests results[J].IEEE Transactions on Industrial Electronics,2008,55(5):2203-2212.
[26] Dujic D,Mester A,Chaudhuri T,et al.Laboratory scale prototype of a power electronic transformer for traction applications[C]//Proceedings of the 14th European Conference on Power Electronics and Applications.Birmingham,UK:IEEE,2011:1-10.
[27] Zhao Chuanhong,Weiss M,Mester A,et al.Power electronic transformer (PET) converter:Design of a 1.2MW demonstrator for traction applications[C]// Proceedings of 2012 International Symposium on Power Electronics Power Electronics,Electrical Drives,Automation and Motion.Sorrento,Italy:IEEE,2012:855-860.
[28] Dujic D,Zhao Chuanhong,Mester A,et al.Power electronic traction transformer-low voltage prototype[J].IEEE Transactions on Power Electronics,2013,28(12):5522-5534.
[29] Besselmann T,Mester A,Dujic D.Power electronic traction transformer:efficiency improvements under light-load conditions[J].IEEE Transactions on Power Electronics,2014,29(8):3971-3981.
[30] Zhao Chuanhong,Dujic D,Mester A,et al.Power electronic traction transformer—medium voltage prototype[J].IEEE Transactions on Industrial Electronics,2014,61(7):3257-3268.
[31] Weigel J,Ag A N S,Hoffmann H.High voltage IGBTs in medium frequency traction power supply[C]//Proceedings of the 13th European Conference on Power Electronics and Applications.Barcelona,Spain:IEEE,2009:1-10.
[32] Hoffmann H,Piepenbreier B.High voltage IGBTs and medium frequency transformer in DC-DC converters for railway applications[C]//Proceedings of the 2010 International Symposium on Power Electronics Electrical Drives Automation and Motion.Pisa,Italy:IEEE,2010:744-749.
[33] Gu Chunyang,Zheng Zedong,Xu Lei,et al.Modeling and control of a multiport power electronic transformer (PET) for electric traction applications[J].IEEE Transactions on Power Electronics,2016,31(2):915-927.
[34] Lai J S,Maitra A,Mansoor A,et al.Multilevel intelligent universal transformer for medium voltage applications [C]//Proceedings of the Fourtieth IAS Annual Meeting.Conference Record of the 2005 Industry Applications Conference.Hong Kong,China:IEEE,2005:1893-1899.
[35] Lai J S,Maitra A,Goodman F.Performance of a distribution intelligent universal transformer under source and load disturbances[C]//Proceedings of the Conference Record of the 2006 IEEE Industry Applications Conference Forty-First IAS Annual Meeting.Tampa,FL,USA:IEEE,2006:719-725.
[36] Jiang Wei,Fahimi B.Multi-port power electric interface for renewable energy sources[C]//Proceedings of the 24th Annual IEEE Applied Power Electronics Conference and Exposition.Washington,DC,USA:IEEE,2009:347-352.
[37] Sabahi M,Goharrizi A Y,Hosseini S H,et al.Flexible power electronic transformer[J].IEEE Transactions on Power Electronics,2010,25(8):2159-2169.
[38] Sabahi M,Hosseini S H,Sharifian M B,et al.Zero-voltage switching bi-directional power electronic transformer[J].IET Power Electronics,2010,3(5):818-828.
[39] Bifaretti S,Zanchetta P,Watson A,et al.Advanced power electronic conversion and control system for universal and flexible power management[J].IEEE Transactions on Smart Grid,2011,2(2):231-243.
[40] She Xu,Huang A Q,Wang Gangyao.3-D space modulation with voltage balancing capability for a cascaded seven-level converter in a solid-state transformer[J].IEEE Transactions on Power Electronics,2011,26(12):3778-3789.
[41] Shi Jianjiang,Gou Wei,Yuan Hao,et al.Research on voltage and power balance control for cascaded modular solid-state transformer[J].IEEE Transactions on Power Electronics,2011,26(4):1154-1166.
[42] Liu Xiaohu,Li Hui,Wang Zhan.A start-up scheme for a three-stage solid-state transformer with minimized transformer current response[J].IEEE Transactions on Power Electronics,2012,27(12):4832-4836.
[43] Zhu Haibin,Li Yaohua,Wang Ping,et al.Design of power electronic transformer based on modular multilevel converter[C]//Proceedings of 2012 Asia-Pacific Power and Energy Engineering Conference.Shanghai,China:IEEE,2012:1-4.
[44] 张明锐,王之馨,黎娜.下垂控制在基于固态变压器的高压微电网中的应用[J].电力系统自动化,2012,36(14):186-192. Zhang Mingrui,Wang Zhixin,Li Na.Application of droop control in high-voltage based on solid state transformer[J].Automation of Electric Power System,2012,36(14):186-192(in Chinese).
[45] Falcones S,Ayyanar R,Mao Xiaolin.A DC-DC multiport-converter-based solid-state transformer integrating distributed generation and storage[J].IEEE Transactions on Power Electronics,2013,28(5):2192-2203.
[46] Hwang S H,Liu Xiaohu,Kim J M,et al.Distributed digital control of modular-based solid-state transformer using DSP+FPGA[J].IEEE Transactions on Industrial Electronics,2013,60(2):670-680.
[47] Qin Hengsi,Kimball J W.Solid-state transformer architecture using AC-AC dual-active-bridge converter[J].IEEE Transactions on Industrial Electronics,2013,60(9):3720-3730.
[48] Zhao Tiefu,Wang Gangyao,Bhattaya S,et al.Voltage and power balance control for a cascaded h-bridge converter-based solid-state transformer[J].IEEE Transactions on Power Electronics,2013,28(4):1523-1532.
[49] 李子欣,王平,楚遵方,等.面向中高压智能配电网的电力电子变压器研究[J].电网技术,2013,37(9):2592-2601. Li Zixin,Wang Ping,Chu Zunfang,et al.Research on medium- and high- voltage smart distribution grid oriented power electronic transformer[J].Power System Technology,2013,37(9):2592-2601(in Chinese).
[50] Contreras J P,Ramirez J M.Multi-fed power electronic transformer for use in modern distribution systems[J].IEEE Transactions on Smart Grid,2014,5(3):1532-1541.
[51] Guillod T,Huber J E,Ortiz G,et al.Characterization of the voltage and electric field stresses in multi-cell solid-state transformers[C]//Proceedings of 2014 IEEE Energy Conversion Congress and Exposition.Pittsburgh,PA,USA:IEEE,2014:4726-4734.
[52] Shah D G,Crow M L.Stability design criteria for distribution systems with solid-state transformers[J].IEEE Transactions on Power Delivery,2014,29(6),doi: 10.1109/TPWRD.2014.2311963.
[53] Yu Xunwei,She Xu,Ni Xijun,et al.System integration and hierarchical power management strategy for a solid- state transformer interfaced microgrid system[J].IEEE Transactions on Power Electronics,2014,29(8):4414-4425.
[54] Yu Xunwei,She Xu,Zhou Xiaohu,et al.Power management for DC microgrid enabled by solid-state transformer[J].IEEE Transactions on Smart Grid,2014,5(2):954-965.
[55] Gao Fanqiang,Li Zixin,Wang Ping,et al.Prototype of smart energy router for distribution DC grid[C]// Proceedings of the 17th European Conference on Power Electronics and Applications.Geneva,Switzerland:IEEE,2015:1-9.
[56] Madhusoodhanan S,Tripathi A,Patel D,et al.Solid-State transformer and MV grid tie applications enabled by 15 kV SiC IGBTs and 10 kV SiC MOSFETs based multilevel converters[J].IEEE Transactions on Industry Applications,2015,51(4):3343-3360.
[57] Mainali K,Tripathi A,Madhusoodhanan S,et al.A transformerless intelligent power substation:a three-phase SST enabled by a 15-kV SiC IGBT[J].IEEE Power Electronics Magazine,2015,2(3):31-43.
[58] Rothmund D,Ortiz G,Guillod T,et al.10kV SiC-based isolated DC-DC converter for medium voltage-connected solid-state transformers[C]//Proceedings of 2015 IEEE Applied Power Electronics Conference and Exposition.Charlotte,NC,USA:IEEE,2015:1096-1103.
[59] Wang Xinyu,Liu Jinjun,Ouyang Shaodi,et al.Research on unbalanced-load correction capability of two power electronic transformer topologies[J].IEEE Transactions on Power Electronics,2015,30(6):3044-3056.
[60] Ahmed H F,Cha H,Khan A A,et al.A highly reliable single-phase high-frequency isolated double step-down AC-AC converter with both noninverting and inverting operations[J].IEEE Transactions on Industry Applications,2016,52(6):4878-4887.
[61] Briz F,Lopez M,Rodriguez A,et al.Modular power electronic transformers:modular multilevel converter versus cascaded h-bridge solutions[J].IEEE Industrial Electronics Magazine,2016,10(4):6-19.
[62] Huang A Q.Medium-voltage solid-state transformer:technology for a smarter and resilient grid[J].IEEE Industrial Electronics Magazine,2016,10(3):29-42.
[63] Shah D,Crow M L.Online volt-var control for distribution systems with solid-state transformers[J].IEEE Transactions on Power Delivery,2016,31(1):343-350.
[64] Wang Dan,Tian Jie,Mao Chengxiong,et al.A 10kV/400V 500kVA electronic power transformer[J].IEEE Transactions on Industrial Electronics,2016,63(11):6653-6663.
[65] Wang Liang,Zhang Donglai,Wang Yi,et al.Power and voltage balance control of a novel three-phase solid-state transformer using multilevel cascaded H-bridge inverters for microgrid applications[J].IEEE Transactions on Power Electronics,2016,31(4):3289-3301.
[66] Wang Xinyu,Liu Jinjun,Ouyang Shaodi,et al.Control and experiment of an h-bridge-based three-phase three- stage modular power electronic transformer[J].IEEE Transactions on Power Electronics,2016,31(3):2002-2011.
[67] 韩继业,李勇,曹一家,等.一种新型的模块化多电平型固态变压器及其内模控制策略[J].中国科学:技术科学,2016,46(5):518-526. Han Jiye,Li Yong,Cao Yijia,et al.A new modular multilevel type solid state transformer with internal model control method[J].Scientia Sinica Technologica,2016,46(5):518-526(in Chinese).
[68] 韩继业,李勇,曹一家,等.基于模块化多电平型固态变压器的新型直流微网架构及其控制策略[J].电网技术,2016,40(3):733-740. Han Jiye,Li Yong,Cao Yijia,et al.A new DC microgrid architecture based on MMC-SST and its control strategy[J].Power System Technology,2016,40(3):733-740(in Chinese).
[69] 季振东,李东野,孙毅超,等.一种三相级联型电力电子变压器及其控制策略研究[J].电机与控制学报,2016,20(8):32-39,47. Ji Zhendong,Li Dongye,Sun Yichao,et al.Research on a three-phase cascaded power electronic transformer and its control strategy[J].Electric Machines and Control[J],2016,20(8):32-39,47(in Chinese).
[70] 李子欣,高范强,徐飞,等.中压配电网用10kVac- 750Vdc/1MVA电力电子变压器功率密度影响因素研究[J].电工电能新技术,2016,35(6):1-6. Li Zixin,Gao Fanqiang,Xu Fei,et al.Power density analysis of 10 kVac–750 Vdc/1 MVA power electronic transformer/solid-state transformer for medium voltage distribution grid[J].Advanced Technology of Electrical Engineering and Energy,2016,35(6):1-6(in Chinese).
[71] 刘维,涂春鸣,兰征,等.具有同步电机特性的电力电子变压器[J].电网技术,2016,40(3):918-924. LiuWei,Tu Chunming,Lan Zheng,et al.Power electronic transformer with synonous generator acteristics[J].Power System Technology,2016,40(3):918-924(in Chinese).
[72] 王婷,王广柱,张勋.基于模块化多电平矩阵变换器的电力电子变压器控制策略[J].电工技术学报,2016,31(18):108-115. Wang Ting,Wang Guangzhu,Zhang Xun.The control strategy of power electronic transformer based on modular multilevel matrix convertors[J].Transaction of China Electrotechnical Society,2016,31(18):108-115(in Chinese).
[73] Afiat Milani A,Khan M T A,Chakrabortty A,et al.Equilibrium point analysis and power sharing methods for distribution systems driven by solid-state transformers[J].IEEE Transactions on Power Systems,2018,33(2):1473-1483.
[74] Appel T,Benner D.Fundamental investigation of very fast transients in power-electronic winding for Solid-State Transformers[C]//Proceedings of the 19th European Conference on Power Electronics and Applications.Warsaw,Poland:IEEE,2017:P.1-P.8.
[75] Costa L F,Buticchi G,Liserre M.Quad-active-bridge DC-DC converter as cross-link for medium-voltage modular inverters[J].IEEE Transactions on Industry Applications,2017,53(2):1243-1253.
[76] Cuartas J M,Da La Cruz A,Briz F,et al.Start-up,functionalities and protection issues for CHB-based solid state transformers[C]//Proceedings of 2017 IEEE International Conference on Environment and Electrical Engineering and 2017 IEEE Industrial and Commercial Power Systems Europe.Milan,Italy :IEEE,2017:1-5.
[77] Dong Dong,Agamy M,Mandrusiak G,et al.Design of high-speed H-bridge converter using discrete SiC MOSFETs for solid-state transformer applications[C]// Proceedings of 2017 IEEE Energy Conversion Congress and Exposition.Cincinnati,OH,USA:IEEE,2017:1379-1386.
[78] Ferreira Costa L,De Carne G,Buticchi G,et al.The smart transformer:a solid-state transformer tailored to provide ancillary services to the distribution grid[J].IEEE Power Electronics Magazine,2017,4(2):56-67.
[79] Gao Rui,She Xu,Husain I,et al.Solid-state- transformer-interfaced permanent magnet wind turbine distributed generation system with power management functions[J].IEEE Transactions on Industry Applications,2017,53(4):3849-3861.
[80] Garcia Rodriguez L A,Jones V,Oliva A R,et al.A new SST topology comprising boost three-level AC/DC converters for applications in electric power distribution systems[J].IEEE Journal of Emerging and Selected Topics in Power Electronics,2017,5(2):735-746.
[81] Guillod T,Krismer F,Kolar J W.Protection of MV converters in the grid:the case of mv/lv solid-state transformers[J].IEEE Journal of Emerging and Selected Topics in Power Electronics,2017,5(1):393-408.
[82] Huang A Q,Zhu Qianlai,Wang Li,et al.15 kV SiC MOSFET:an enabling technology for medium voltage solid state transformers[J].CPSS Transactions on Power Electronics and Applications,2017,2(2):118-130.
[83] Huber J E,Böhler J,Rothmund D,et al.Analysis and cell-level experimental verification of a 25 kW All-SiC isolated front end 6.6 kV/400 V AC-DC solid-state transformer[J].CPSS Transactions on Power Electronics and Applications,2017,2(2):140-148.
[84] Huber J E,Kolar J W.Optimum number of cascaded cells for high-power medium-voltage AC-DC converters[J].IEEE Journal of Emerging and Selected Topics in Power Electronics,2017,5(1):213-232.
[85] Isobe T,Tadano H,He Zijin,et al.Control of solid-state-transformer for minimized energy storage capacitors[C]//Proceedings of 2017 IEEE Energy Conversion Congress and Exposition.Cincinnati,OH,USA:IEEE,2017:3809-3815.
[86] Itoh J I,Aoyagi K,Nakanishi T.Bidirectional single- phase Solid-State Transformer using multi cell for volume reduction of high voltage capacitor[C]//Proceedings of 3rd International Future Energy Electronics Conference and ECCE Asia.Kaohsiung,Taiwan,China:IEEE,2017:332-337.
[87] Madhusoodhanan S,Mainali K,Tripathi A,et al.Harmonic analysis and controller design of 15 kV SiC IGBT-based medium-voltage grid-connected three-phase three-level NPC converter[J].IEEE Transactions on Power Electronics,2017,32(5):3355-3369.
[88] Syed I,Khadkikar V.Replacing the grid interface transformer in wind energy conversion system with solid-state transformer[J].IEEE Transactions on Power Systems,2017,32(3):2152-2160.
[89] 高范强,李子欣,徐飞,等.一种高频链模块化电力电子变压器[J].电工电能新技术,2017,36(5):51-58.Gao Fanqiang,Li Zixin,Xu Fei,et al.Power electronic transformer based on modular convertor with high-frequency link[J].Advanced Technology of Elrctrical Engineering and Energy,2017,36(5):51-58(in Chinese).
[90] 刘闯,齐瑞鹏,刘海军,等.一种减小三相级联型PET各中间直流侧电容的方法[J].电力自动化设备,2017,37(11):46-53. Liu Chuang,Qi Ruipeng,Liu Haijun,et al.Method of decreasing intermediate DC-Link capacitors for three-phase cascaded power electronics transformer[J],Electric Power Automation Equipment,2017,37(11):46-53(in Chinese).
[91] 刘闯,支月媚.混合级联式电力电子变压器拓扑结构及控制策略[J].电网技术,2017,41(2):596-603.Liu Chuang,Zhi Yuemei.Hybrid cascaded power electronics transformer topology and control scheme[J].Power System Technology,2017,41(2):596-603(in Chinese).
[92] 涂春鸣,兰征,肖凡,等.模块化电力电子变压器的设计与实现[J].电工电能新技术,2017,36(5):42-50.Tu Chunming,Lan Zheng,Xiao Fan,et al.Design and implementation of modular power electronic transformer[J].Advanced Technology of Elrctrical Engineering and Energy,2017,36(5):42-50(in Chinese).
[93] 涂春鸣,孟阳,肖凡,等.一种交直流混合微网能量路由器及其运行模态分析[J].电工技术学报,2017,32(22):176-188. Tu Chunming,Meng Yang,Xiao Fan,et al.An AC-DC hybrid microgrid energy router and operational model analysis[J].Transaction of China Electrotechnical Society,2017,32(22):176-188(in Chinese).
[94] 王优,郑泽东,李永东.中高压电力电子变压器拓扑与控制应用综述[J].电工电能新技术,2017,36(5):1-10. Wang You,Zheng Zedong,Li Yongdong.Review of technology and control application of medium and high voltage power electronic transformer[J].Advanced Technology of Elrctrical Engineering and Energy,2017,36(5):1-10(in Chinese).
[95] 王丹,毛承雄,陆继明.自平衡电子电力变压器[J].中国电机工程学报,2007,27(6):77-83. WangDan,Mao Chengxiong,Lu Jiming.Auto-balancing electronic power transformer[J].Proceedings of the CSEE,2007,27(6):77-83(in Chinese).
[96] Das M K,Capell C,Grider D E,et al.10 kV,120 A SiC half H-bridge power MOSFET modules suitable for high frequency,medium voltage applications[C]//Proceedings of 2011 IEEE Energy Conversion Congress and Exposition.Phoenix,AZ,USA:IEEE,2011:2689-2692.
[97] Ortiz G,Leibl M,Huber J,et al.Efficiency/power-density optimization of medium-frequency transformers for solid-state-transformer applications[C]//Proceedings of the 10th IEEE International Conference on Power Electronics and Drive Systems.Kitakyushu,Japan:IEEE,2013.
[98] Basu K,Mohan N.A single-stage power electronic transformer for a three-phase PWM AC/AC drive with source-based commutation of leakage energy and common-mode voltage suppression[J].IEEE Transactions on Industrial Electronics,2014,61(11):5881-5893.
[99] Chen Hao,Divan D.Soft-switching solid-state transformer (S4T)[J].IEEE Transactions on Power Electronics,2018,33(4): 2933-2947.
[100] Aggeler D,Biela J,Kolar J W.Solid-state transformer based on SiC JFETs for future energy distribution systems[C]//Proceedings of the Smart Energy Strategies Conference 2008.Zürich,Switzerland:vdf Hochschulverlag AG an der ETH Zürich,2008.
[101] Dujic D,Steinke G K,Bellini M,et al.Characterization of 6.5 kV IGBTs for high-power medium-frequency soft-switched applications[J].IEEE Transactions on Power Electronics,2014,29(2):906-919.
[102] Ortiz G,Leibl M,Kolar J W,et al.Medium frequency transformers for solid-state-transformer applications- design and experimental verification[C]//Proceedings of the 10th International Conference on Power Electronics and Drive Systems.Kitakyushu,Japan:IEEE,2013:1285-1290.
[103] Villar I,Viscarret U,Etxeberria-Otadui I,et al.Global loss evaluation methods for nonsinusoidally fed medium-frequency power transformers[J].IEEE Transactions on Industrial Electronics,2009,56(10):4132-4140.
[104] Du S B Y,Wang Gangyao,Bhattaya S.Design considerations of high voltage and high frequency transformer for solid state transformer application[C]// Proceedings of the 36th Annual Conference on IEEE Industrial Electronics Society.Glendale,AZ,USA:IEEE,2010:421-426.
[105] Baek S,Bhattaya S.Analytical modeling and implementation of a coaxially wound transformer with integrated filter inductance for isolated soft-switching DC-DC converters[J].IEEE Transactions on Industrial Electronics,2018,65(3): 2245-2255.
[106] Rauls M S,Novotny D W,Divan D M.Design considerations for high-frequency coaxial winding power transformers[J].IEEE Transactions on Industry Applications,1993,29(2):375-381.
[107] Klontz K W,Divan D M,Novotny D W.An actively cooled 120 kW coaxial winding transformer for fast ging electric vehicles[C]//Proceedings of 1994 IEEE Industry Applications Society Annual Meeting.Denver,CO,USA:IEEE,1994:1049-1054.
[108] Rauls M S,Novotny D W,Divan D M,et al.Multiturn high-frequency coaxial winding power transformers[J].IEEE Transactions on Industry Applications,1995,31(1):112-118.
[109] Baek S,Bhattaya S.Analytical modeling of a medium-voltage and high-frequency resonant coaxial- type power transformer for a solid state transformer application[C]//Proceedings of 2011 IEEE Energy Conversion Congress and Exposition.Phoenix,AZ,USA:IEEE,2011:1873-1880.
