A modular power electronic transformer based on a cascaded H-bridge multilevel converter

In this paper a modular power electronic transformer (PET) for feeding critical loads is presented. The PE-based transformer is a multi-cellular step-down converter that can directly connect to medium voltage levels on the primary side and provide a low voltage, highly stable interface for consumer applications. The presented structure consists of three stages: a cascaded H-bridge (CHB) rectifier, an isolation stage, and an output stage. The CHB rectifier serves as an active rectifier to ensure that the input current is sinusoidal, and it converts the high AC input voltage to low DC voltages. The isolated DC/DC converters are then connected to the DC links and provide galvanic isolation between the HV and LV sides. Finally, a three-phase inverter generates the AC output with the desired amplitude and frequency. This paper introduces a new control strategy to maintain DC voltage balance among the CHB converter cells, even if the attached loads are different. The effects of voltage offsets and device mismatches on the equal load-current sharing are investigated, and an active load-current sharing method is presented to balance the load power among the parallel-output cells. The validity of the proposed controllers and the PET performance are verified by simulation and experimental results.     

一种三相级联型电力电子变压器及其控制策略研究

针对直流微网的并网问题,设计一种三相级联型电力电子变压器,高压交流输入级采用星形级联型H桥拓扑,低压直流输出级使用输出并联的隔离式双向主动全桥结构。在直接接入交流配电网的同时,为分布式能源提供了一个稳定的直流接口。文中针对两级分别设计了闭环的控制方法,特别是对于高压级的星形级联型拓扑提出了一种简单的相间和相内直流侧平衡控制方法。通过仿真和实验验证了上述拓扑结构和控制策略的可行性和有效性。     

A novel low voltage ride through strategyfor cascaded power electronic transformer

In view of the operating characteristics for voltage sags of AC side of the power electronic transformer(PET), a lowvoltage ride through(LVRT) strategy adapted to bidirectional power exchange of PET is proposed for the purposes of maintaining the system stability, assisting the system voltage recovery and protecting PET safety. During the asymmetric voltage sag, the negative sequence current of PET is eliminated to ensure the symmetry of the injected current. According to the degree of positive sequence voltage sag, the reactive current injection is provided to assist in voltage recovery. According to the PET active power condition before the voltage sag, the level and direction of which are maintained as far as possible without exceeding the limit, for which the disturbance to the AC and DC grids is reduced. Finally, the effectiveness of the proposed LVRT strategy is verified by simulation model.     

一种新型结构的电力电子变压器

电力电子变压器是一种新型智能电力变压器,在配电网中应用前景广阔。本文提出了一种新的模块化电力电子变压器拓扑结构,将双向隔离DC/DC变换器与模块化多电平变换器子模块连接在一起,直接得到低压直流输出,并利用四桥臂变换器产生低压交流输出,这种结构方式更利于模块化设计,减小装置体积。文中给出了模块化多电平变换器,双向隔离DC/DC变换器以及四桥臂变换器的控制方法,最后通过仿真和实验验证了该拓扑的性能。     

A new single‐phase multilevel converter topology with reduced power electronic devices,voltage rating on switches,and power losses

In this paper, a new extended single‐phase structure for multilevel converter is presented which consists of several bidirectional and unidirectional switches along with dc voltage sources. To generate all possible levels at output voltage waveform, 2 methods are presented for determination of the amplitudes of dc voltage sources. The proposed structure is compared with traditional cascade H‐bridge multilevel converter and other recently proposed structures in terms of the number of power electronic components, voltage rating on switches, and power losses. Based on the comparison results, it is demonstrated that the proposed structure needs minimum number of IGBTs, gate drivers, and anti‐parallel diodes. Moreover, the voltage rating on bidirectional and unidirectional switches in the proposed structure is less than other similar topologies. Also, power losses analysis on the proposed topology is investigated. It is shown that the power loss of proposed topology is less than H‐bridge multilevel converter topology. The number of on‐state switches in the current path of proposed topology is much lower than other topologies which lead to the reduction of voltage drop on the switches and power losses. Both experimental and simulation works are provided to verify the performance of the presented structure.     

几种模块化多输出电力电子变压器拓扑的不平衡负载补偿能力比较

电力电子变压器除了能实现传统变压器的变压/隔离功能之外,还需要根据用户需求提供一些额外功能,其中之一是多输出功能,即给多个独立的交/直流负载供电。因为负载不平衡工况会对电力电子变压器的运行提出挑战,因此当设计一台具有多输出功能的电力电子变压器时,需要确定合适的拓扑与控制策略。本文对三种电力电子变压器拓扑的补偿不平衡负载的能力进行了比较,包括:独立相连接拓扑,交叉相连接拓扑与自平衡拓扑。独立相连接拓扑通过在输入级中注入零序分量来实现不平衡负载的补偿;交叉相连接拓扑通过输入级调制度以及无功分量注入实现不平衡负载的补偿;自平衡拓扑通过拓扑自身的连接方法以及多端口高频变压器实现不平衡负载的补偿,拓扑较为复杂。本文分析了上述三种电力电子变压器拓扑的原理以及控制策略,并在SIMULINK中验证了这几种拓扑补偿不平衡负载的功能。     

风电用变压器型电压跌落器及保护系统的研究与设计

风电机组并网运行前,需要进行低电压穿越能力的测试,专门的电压跌落模拟装置必不可少。基于此,研究了一种风力发电用变压器型电压跌落装置,可以模拟不同类型的故障,实现不同深度的电压跌落,电压跌落持续时间可调。为保证电压跌落测试系统能安全可靠工作,设计了完备的故障保护系统。通过建模仿真与现场测试实验,该装置可以满足实际工程需求,为风电机组低电压穿越测试中电压跌落装置实现方案提供选择与参考。     

MMC柔性直流输电系统网侧故障时紧急功率支援控制

为了解决MMC-HVDC(Modular Multilevel Converter Based on HVDC,MMC-HVDC)交流侧系统故障时的过流问题,以及增强MMC换流器的低压穿越能力,通过对换流器功率数学模型及控制方式进行分析,发现了换流器有功和无功功率解耦的PI控制方式。提出了当交流侧发生对称和不对称故障时,通过控制PI值限制功率输出,同时由交流电压偏差有效值生成正负序补偿电流的紧急功率支援控制策略。将这种控制策略添加到电磁暂态仿真系统当中,当系统网侧发生对称或不对称故障时,利用数值仿真技术分析了换流器阀侧的电能质量。仿真结果验证了所提出的控制方法对故障时过流抑制的有效性,同时增强了换流器的低压穿越能力。     

电力电子变压器在输电系统中的控制策略研究

电力电子变压器(PET—PowerElectronicTransformer)是一种新型的电能转换设备,它通过电力电子变换技术实现电力系统中的电压变换和能量传递。研究了PET在电力系统中的控制问题,提出了在单机无穷大系统中与发电机相连的PET的控制策略,并在MATLAB环境中进行了时域仿真研究。仿真结果表明,在系统中出现扰动的条件下,PET能有效的提高系统阻尼,改善系统电压特性,提高了电力系统的稳定性。     

谐振变换器在电子模拟功率负载中的应用

分析了电路的基本工作原理,阐述了谐振变换器应用于电子模拟功率负载中各主要参数的设计方法,采用UC3879为控制核心,实现了系统的恒流输入控制,试验结果证明系统性能优良,给出了在软开关条件下的实验波形。     

基于电力电子变压器的三端协调控制

为研究电力电子变压器对主网、交流微网、直流微网的三端协调控制,根据微网内分布式能源出力与功率缺额,提出了一种确定储能荷电状态预期值的方法。并基于此荷电状态值,综合考虑微网内源、荷、储三者关系,将微网运行状态划分为电源型微网和负载型微网。电力电子变压器根据微网类型,控制交直流微网功率流向与大小,最大化消纳分布式能源,以减少微网对配网的功率回馈次数及回馈时产生的功率损耗。仿真结果和实验数据验证了所提划分方法和控制策略的有效性。     

A new control method of a resonant switched‐capacitor converter and the application for balancing of the split DC voltages in a multilevel inverter

This paper proposes a new voltage‐balancing circuit for the split DC voltages in a diode‐clamped five‐level inverter. The proposed circuit is based on a resonant switched‐capacitor converter (RSCC), which consists of two half‐bridge inverters, a resonant inductor, and a resonant capacitor. A new phase‐shift control of the RSCC is proposed to improve voltage balancing performance. Theoretical analysis reveals the rating of the RSCC and stored energy in the resonant inductor. Experimental results confirm the reduction of the inductor to one‐tenth in volume compared to a conventional voltage‐balancing circuit based on buck‐boost topology. Moreover, the proposed phase‐shift control has demonstrated that it is possible to eliminate the voltage deviation between the DC capacitors. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 168(2): 69–79, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20719     

中高压电力电子变压器拓扑与控制应用综述

电力电子变压器(PET)作为一种新型的电能路由设备,在智能电网与能源互联网领域有着极大的应用价值。本文针对中高压的应用背景,对典型的PET拓扑及相关的控制策略进行了分析,总结了各类拓扑在系统效率、可靠型以及控制复杂度等应用方面的特点,其中带有多模块级联高压侧变换器的三级式PET拓扑电平数易于扩展,且控制上较为简单,但在系统体积与效率方面仍有优化的空间。本文还对于隔离级中两种不同类型的变换器进行了具体控制方法上的归纳与对比,对两种不同变换器在中高压应用场合的适应性进行了分析。     

Multi‐function combined operation and control strategy of electronic power transformer for power quality improvement

High penetration of power electronic devices and nonlinear loads can negatively affect the power quality of the distribution system. The conventional method of improving the power quality is to employ an extra distribution static synchronous compensator (D‐STATCOM ) or an active power filter (APF ) for compensating reactive power and mitigating current harmonics. This paper proposes an electronic power transformer (EPT ) without the aforementioned additional equipment, which can realize multi‐function coordinated operation, including active power transmission, voltage conversion, reactive power compensation, and current harmonics suppression, based on the distribution system need. The multifunctional EPT consists of three stages, namely cascaded multilevel AC /DC rectifiers, dual‐active bridge (DAB ) converters with medium‐frequency transformers, and DC /AC inverters. The mathematical model of each stage is established and analyzed. Meanwhile, the control strategies of three stages are presented. Finally, a simulation model is built by MATLAB /Simulink to demonstrate the effectiveness of the strategy and operating principle of the proposed EPT . © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Optimal switching Lyapunov‐based control of power electronic converters

This paper presents new ideas and insights towards a novel optimal control approach for power electronic converters. The so‐called stabilizing or Lyapunov‐based control paradigm is adopted, which is well known in the area of energy‐based control of power electronic converters, in which the control law takes a nonlinear state‐feedback form parameterized by a positive scalar λ . The first contribution is the extension to an optimal Lyapunov‐based control paradigm involving the specification of the optimal value for the parameter λ in a typical optimal control setting. The second contribution is the extension to more flexible optimal switching‐gain control laws, where the optimal switching surfaces are parameterized by a number of positive scalars λ _j . Systematic derivation of gradient information to apply gradient‐descent algorithms is provided. The proposed techniques are numerically evaluated using the exact switched model of a DC–DC boost converter. Copyright © 2016 John Wiley & Sons, Ltd.     

电力电子变压器研究综述

电力电子变压器(PET)除了具备传统变压器的功能之外,还具有潮流控制、电能质量调节和维持系统稳定等优点,符合近年来智能电网的发展需求,受到了极大关注。就目前PET的几个主要关键技术发展进行了综述,首先介绍了对PET研究的必要性及其工作原理,其次对现阶段国内外提出的PET拓扑结构作了仔细分析,再次,通过对其典型控制策略进行了总结,得出其控制的实质,由此建立其数学模型及相应的控制方法,然后分析了PET功能拓展的几种方案和其应用领域,最后总结了国内外对PET总体的发展概况,并对其今后发展方向进行了展望。本文旨在对电力电子变压器的研究进行阶段性总结,希望对其领域的研究提供参考价值。     

基于能量回馈的电力电子变压器软启动策略

电力电子变压器(PET)中存在大量初始能量为0的电容器,若直接启动PET通过常规闭环控制给这些电容充电以建立直流电压,则将导致PET内部各个变换器的电流过流,可能会导致器件的损坏。为解决此问题,提出了一种基于能量回馈的PET软启动策略。基于PET各部分变换器的电流和功率模型,该策略能够在软启动全过程中实时计算和控制峰值电流,达到了限制启动冲击电流的目的。该策略始终令双有源桥(DAB)变换器工作于峰值电流控制模式下,在保证安全的前提下最大化利用DAB变换器的功率传输能力以加快软启动速度。该策略将低压直流母线电容适度过充,并适时将其中部分能量回馈到高压侧,使得在级联H桥变换器即将进入闭环控制前其高压直流母线电压就已达到参考值,从而彻底消除交流侧的冲击电流。所提策略避免了复杂的参数设计过程,完全依靠理论模型而不是经验来指导软启动全过程的设计。仿真与实验结果验证了所提策略的正确性和有效性。     

级联H桥型电力电子变压器平均值模型

针对电力电子变压器仿真效率较低和死区效应越来越显著的问题,基于级联H桥型双有源桥结构电力电子变压器,提出一种考虑死区效应的平均值模型。首先,计及寄生参数将双有源桥单元简化为一阶RL电路,分析各个死区工况下电压电流特性,求解电流的平均化解析表达式,通过等效电流源代替全桥及高频变压器实现对双有源桥单元的平均值建模。其次,基于状态空间平均法,通过占空比将H桥整流单元各工况整合为一个统一的模型,进而使用受控源代替H桥整流单元与交流电网、双有源桥的接口电路。最后,建立输入侧串联输出侧并联型的级联H桥型电力电子变压器平均值模型并给出计算流程。在PSCAD/EMTDC仿真软件上分别搭建电力电子变压器详细模型与平均值模型,仿真结果表明所提出的模型具有较高的精度和运行效率。     

Low‐Voltage‐Ride‐Through Performance of an HVDC Transmission System Using Two Modular Multilevel Double‐Star Chopper‐Cells Converters

This paper presents an intensive discussion on a long‐distance high‐voltage direct‐current (HVDC) transmission system that combines two modular multilevel cascade converters based on double‐star chopper cells (MMCC‐DSCC) with DC power cables. Hereinafter, a single MMCC‐DSCC is referred to as a DSCC converter or just as a DSCC for the sake of simplicity. The HVDC transmission system is required to provide low‐voltage‐ride‐through (LVRT) capability to enhance transmission system availability. This paper proposes a new LVRT method without any direct information exchange between the two DSCC converters. The validity of the method is verified, using simulated waveforms from the software package of “PSCAD/EMTDC” and experimental waveforms from a three‐phase 200‐V, 400‐Vdc, 10‐kW, 50‐Hz downscaled HVDC system with a set of 300‐meter‐long DC power cables.