深度｜电力电子变压器技术研究综述

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。

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