HVDC transmission system using multilevel power converters based on dual three-phase two-level inverters

Multilevel converters are now an attractive solution for high-voltage direct-current (HVDC) electrical energy transmission systems. Unlike the well-known two-level voltage source converters, multilevel converters use 3 or more voltage levels or steps per leg to modulate the ac voltages, decreasing voltage distortion and reducing electromagnetic interference. This work presents a HVDC transmission system based on a new multilevel structure using a dual two-level converter topology. This structure attains multilevel operation and advantages using two well known three-phase voltage source two-level inverters connected to one three-phase open windings transformer. The proposed dual converter structure has two independent dc links allowing each inverter to process half of the total power. This arrangement is fitted with a control system designed to control the active and reactive power towards their specific set point values, while balancing the voltages of the two dc link capacitors in real time. Obtained results show the effectiveness of the proposed HVDC transmission system.     

Sensitive load voltage compensation against voltage sags/swells and harmonics in the grid voltage and limit downstream fault currents using DVR

In the past decades, the power quality (PQ) has become a critical issue in highly automated industries and sensitive load centers. The voltage quality is the most important part of the PQ for customers. The voltage disturbances in the form of voltage sags, swells and harmonics can cause huge financial losses. In the past few years, the power electronic solutions have been proposed to avoid these problems. The dynamic voltage restorer (DVR) is one of the solutions. This paper proposes a new topology for single-phase DVR which is able to compensate multiple voltage disturbances and limit the downstream fault currents with a simple topology. The proposed DVR is based on the single-phase direct ac/ac converter which is energized from the main grid. As a result, there is no need for energy storage elements and dc link capacitors. The proposed topology can deal with high voltage with low voltage rating switches. Therefore, the switches can be IGBTs without needing an auxiliary forced commutation circuit as in thyristors which are high voltage switches. The performance of the proposed topology is validated by digital simulations and a laboratory scale prototype implementation.     

Mathematical model of five-level ac/dc converter in abc reference frame

This paper proposes a mathematical model for three-phase multilevel diode clamped ac/dc converter (DCC) in abc-reference frame, in order to study dynamic responses and stability of the system, where multilevel converters are employed. An attempt is made to model the converter adopting 5-level DCC and can be easily extended to DCC with any number of levels. The converter output and dc capacitor voltages are expressed in terms of input source currents and switching functions; which are basic inputs provided to a converter. Connection and capacitor currents of the converter have been analyzed. The effectiveness of the proposed mathematical model is verified by the computer simulations and experimental results. The obtained mathematical model has been validated with the physical converter model in MATLAB/Simulink environment and the prototype model using dSPACE DS under closed loop control system. The developed mathematical model equations are able to represent the physical converter with similar characteristics.     

An energy optimized control scheme for a transformerless DVR

The dynamic voltage restorer (DVR) is considered as the most effective and economic solution for voltage disturbances. This paper presents an energy optimized control scheme for a transformerless DVR. The DVR structure is based on a cascaded H-bridge multilevel inverter topology to eliminate the need of insertion transformers. The proposed control algorithm maintains a balanced load-side voltage even during the compensation of unbalanced disturbances with a minimum active power injection. Moreover, the proposed scheme maximizes the ride-though capability of the DVR during voltage sags. This feature is verified by using capacitors instead of dc sources as energy storage elements for the DVR. Furthermore, the proposed minimum energy scheme prevents the rise in the dc-side voltage of the inverter when compensating voltage swells. The performance of the proposed DVR system is evaluated for compensating different types of voltage disturbances. The results validate the robustness and the accuracy of the proposed system.     

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.     

一种具备故障阻断能力的低导通损耗模块化多电平换流器

论文面向具备直流侧故障阻断能力的MMC子模块拓扑开展研究,通过详细分析半桥型和全桥型MMC子模块在直流侧故障时的工作特性,提出一种基于逆阻型元件的新型子模块:该拓扑无需向子模块正常电流通路中插入额外功率器件,即可改变故障电流路径。与现存具备故障阻断能力的子模块拓扑相比,提出的拓扑在正常工况下具有更低的导通损耗。为了验证提出拓扑的有效性,在Matlab/Simulink环境下搭新型MMC换流器仿真平台,并搭建相应样机,全面验证所提的子模块拓扑具备直流侧故障阻断能力。     

一种改进型级联多电平拓扑及其频谱分析

近些年,多电平拓扑结构特别是级联和二极管钳位这两种拓扑在中高压大功率领域中得到广泛的关注和应用.本文对这两种多电平拓扑结构的优缺点进行分析,根据目前电力电子器件的发展特点,将具有相同母线电压的一个二极管钳位H桥拓扑与若干个H桥级联,形成一种具有实用价值的改进型逆变器拓扑结构,并对其进行研究.针对这种拓扑结构,分析了一种...     

一种具有自修复功能的多电平变换器拓朴

从提高复杂多电平变换器的可靠性出发，提出了一种具有类似生物系统自修复功能的多电平变换器拓扑。电路拓扑工作原理的分析结果表明，该拓扑在正常工作情况下所有开关管均承受单位电平的电压应力，并且以较少的飞跨电容实现每个电压电平的自动平衡。该拓扑不需要任何复杂的中点电位平衡电路或控制措施。文中详细分析了电路中开关管故障对多电平变换器系统运行和器件电压应力的影响。分析结果表明，当某些开关管出现短路或断路故障时，通过简单的软件重构改变驱动信号，利用冗余开关状态组合，电路仍能得到所有电平的正常输出和直流侧电容的电压自平衡，真正实现了原有功能的自修复。整个多电平变换器系统的可靠性由此提高。文中以五电平变换器为例进行了分析研究，并通过仿真实验进行验证。     

A Hybrid DC Circuit Breaker Combining a Multilevel Converter and Mechanical Contactors: Verification of the Principles of Operation by Experiment and Simulation

This paper presents a new hybrid dc circuit breaker producing no conducting power loss under normal operating conditions and having the capability to interrupt a large amount of short‐circuiting current within several milliseconds. It consists of two reactors for current limiting and current control, two mechanical contactors, and a multilevel converter based on the cascade connection of multiple full‐bridge dc/dc converter cells. It is characterized by achieving current commutation from the main mechanical contactor to the multilevel converter with fast current control. This results in eliminating auxiliary power semiconductor switches that produce a nonnegligible conducting power loss under normal operating conditions. This paper discusses both principles of operation and control method of the new hybrid dc circuit breaker. Experimental verification is performed by a downscaled system rated at 300 V and 150 A, along with verification by computer simulation.     

基于半压钳位子模块的MMC直流短路故障穿越研究

基于模块化多电平换流器MMC(modular multilevel converter)的柔性直流输电技术,在高压大容量输电领域有广阔的应用前景。直流故障穿越是MMC应用必须解决的关键问题。目前,具有直流故障穿越能力的MMC改进拓扑在功率器件成本和换流器损耗方面依然偏高,并且缺乏物理实验验证。首先,提出一种基于半压钳位子模块的MMC改进拓扑;然后利用子模块电容电压来主动抑制二极管续流效应,迅速清除故障电流和实现自动重启,并且额外成本很低;最后,相应地搭建了1 k V/20 k W物理样机,通过物理实验详细地研究了所提拓扑的直流故障清除和恢复过程,并验证了该拓扑的直流故障穿越能力。     

稳态运行和直流故障下桥臂交替导通多电平换流器的控制策略

寻找一种可有效处理直流侧故障的拓扑结构是目前电压源换流器型高压直流输电(VSC-HVDC)工程应用中的关键性问题。桥臂交替导通多电平换流器(AAMC)具有能够处理直流故障、模块电容数量少、占地空间小等优点,具有广阔的发展前景。针对目前AAMC有关主回路参数确定运维控制等内容有关文献研究不多的情况,首先对AAMC拓扑结构和基本原理进行研究和分析,推导了直流电压和交流阀侧电压的最优关系并分析了子模块数量选取原则;其次研究了稳态运行时正弦波形成电路和导通开关协调配合,并提出了基于电压极性选择的子模块均衡策略;接着分析了直流故障下的控制策略,指出故障期间闭锁导通开关中的IGBT,并通过桥臂模块产生负电平利用串联二极管的反向阻断能力能够完成直流熄弧过程。最后在PSCAD/EMTDC搭建15模块AAMC系统模型,仿真结果验证了设计原则和控制策略的有效性。     

飞轮储能动态电压恢复器的拓扑结构和充电策略

基于飞轮储能的动态电压恢复器的传统拓扑结构采用辅助整流器给飞轮充电,提高了系统的复杂程度,也增加了成本.为此,提出了两种新型拓扑结构:并联结构和串联结构.并联结构通过引入转换开关,使动态电压恢复器的电压源型变换器工作于整流状态给飞轮充电;串联结构使动态电压恢复器的电压源型变换器工作于逆变状态给飞轮充电,进一步简化了结构.并针对串联结构提出了自充电策略.最后将3种拓扑结构在经济和系统性能等方面进行了比较.实验结果验证了拓扑结构和充电策略的正确性.     

一种新型的无变压器级联型多电平变换器拓扑

H桥级联多电平逆变器在高压大容量变频调速领域得到了广泛的应用,其最大的不足之处是必须使用庞大、复杂而昂贵的多绕组移相隔离变压器。为了省去传统H桥级联多电平变换器中的多绕组移相变压器,提出了一种新型的无变压器级联型多电平变换器拓扑结构。该拓扑由通用拓扑结构派生而来,全部由基本单元级联而成,不需要大量独立直流电源,省去了多绕组移相隔离变压器,具有模块性强、结构简单、易于扩展等优点。和现有的主要多电平拓扑结构相比,随着电平数的增多,使用元件较少,更适合于五电平及以上多电平使用。以五电平拓扑为例进行了研究,分析了其工作原理和悬浮电容电压平衡控制方法,仿真和实验结果验证了其可行性。     

Design and Experimentation of a Dynamic Voltage Restorer Capable of Significantly Reducing an Energy-Storage Element

This paper deals with a dynamic voltage restorer (DVR), or a voltage-sag compensator, which consists of a set of series and shunt converters connected back-to-back, three series transformers, and a dc capacitor installed on the common dc link. The DVR is characterized by installing the series converter on the source side and the shunt converter on the load side. This system configuration allows the use of an extremely small dc capacitor intended for smoothing the common dc-link voltage. This paper provides a design procedure of the dc capacitor under a voltage-sag condition and proposes a control method for the series converter, which is capable of reducing the voltage ratings of both the series converter and the series transformers. Experimental results obtained from a 200-V 5-kW laboratory system are shown to confirm the validity of the design procedure and the effectiveness of the control method.     

HVDC transmission systems: Bipolar back-to-back diode clamped multilevel converter with fast optimum-predictive control and capacitor balancing strategy

Voltage source multilevel power converter structures are being considered for high power high voltage applications where they have well known advantages. Recently, full back-to-back connected multilevel neutral diode clamped converters (NPC) have been used in high voltage direct current (HVDC) transmission systems. Bipolar back-to-back connection of NPCs have advantages in long distance HVDC transmission systems, but highly increased difficulties to balance the dc capacitor voltage dividers on both sending and receiving end NPCs.This paper proposes a fast optimum-predictive controller to balance the dc capacitor voltages and to control the power flow in a long distance HVDC system using bipolar back-to-back connected NPCs. For both converter sides, the control strategy considers active and reactive power to establish ac grid currents on sending and receiving ends, while guaranteeing the balancing of both NPC dc bus capacitor voltages. Furthermore, the fast predictive controller minimizes the semiconductor switching frequency to reduce global switching losses.The performance and robustness of the new fast predictive control strategy and the associated dc capacitors voltage balancing are evaluated.     

基于双极性直接式AC/AC变换的单相动态电压恢复器

动态电压恢复器(DVR)是串接于电源与负荷之间用以实现快速补偿系统电压波动的电力电子装置。针对传统基于电压源型逆变器的DVR存在的储能设备所带来的问题以及基于脉宽调制(PWM)型直接式AC/AC变换的DVR所存在的换流、飞跨电容电压平衡等问题,文中提出了一种基于双极性直接式AC/AC变换的单相DVR。该DVR所使用的PWM型AC/AC变换拓扑具有输入、输出共地特点,同时,该DVR能够在较为简单的控制策略下实现系统电压的双极性调控且其在运行过程中有效解决了换流问题。为了验证所提出的DVR的工程价值,在理论分析的基础上,搭建了1 kW的实验平台对其合理性与有效性进行了验证。     

Compensation of voltage disturbances in distribution systems using single-phase dynamic voltage restorer

In this paper, a new topology is proposed for a single-phase dynamic voltage restorer (DVR) using direct ac/ac converter. This topology does not require dc-link energy storage elements. The proposed topology has a simple structure and can compensate several types of voltage disturbances such as voltage sags, swells, harmonics and flickers. This topology will not face any problem in long time compensation due to the fact that it provides the required energy directly through grid. The proposed topology can be easily extended to n-phase systems such as three-phase based on the same principle of the operation. In n-phase systems, the voltage sags and swells can be properly compensated regardless of the balanced or unbalanced operation. A new control method is also proposed for direct ac/ac converter in the proposed topology. The simulation and experimental results verify the effectiveness of the proposed topology and its control method in voltage restoration.     

一种新型自均压二极管钳位级联有源滤波器

提出一种新型二极管钳位级联多电平拓扑,在半桥单元拓扑上增加钳位二极管,为直流电容提供单向钳位通路。在该拓扑中,只需对阀体两端单元的直流电压进行控制就可实现均压控制,且每相阀体只需2个直流电压传感器,结构简单。将拓扑拓展到三相结构,提出了二极管钳位多电平有源滤波器(APF)拓扑及其控制方法。搭建了7电平二极管钳位多电平APF仿真模型和实验样机。实验结果表明,该拓扑自均压控制和谐波补偿效果良好,控制算法简易、可行。     

Application of IGCT in High-Power Rectifiers

This paper investigates the possibility of applying an integrated gate commutated thyristor (IGCT) into high-current high-power rectifier systems. The optimal power converter topology that fully utilizes the functionality of the IGCT is proposed. The proposed power converter consists of a front-end diode rectifier and three-level step-down dc/dc converter. As compared to two-level operation, half the input dc voltage and twice the effective switching frequency of three-level operation can provide a wider operation range of continuous conduction mode resulting in a higher average output current of less ripple size than two-level case. A laboratory prototype of a 5-kA 3.1-MW rectifier has been simulated and tested to confirm the feasibility of the IGCT in the proposed power converter topology. The proposed solution generates a relatively smooth output dc current with less contents of high-frequency harmonics, thereby avoiding the high-frequency harmonic-related problems found in insulated-gate-bipolar-transistors-based high-current rectifiers. The IGCT would play a significant role to improve the cost-effective performance and reliability in low- and medium-ranged high-current high-power rectifier systems.     

CPS-SPWM技术在级联H桥型变流器上的具体实现方法

CPS-SPWM技术是多重化技术和SPWM技术的有机结合。该技术能够在较低的器件开关频率下实现较高等效开关频率的效果,通过低次谐波的相互抵消提高等效开关频率,而不是简单地将谐波向高次推移,因而具有良好的谐波特性。具有独立电源的级联H桥多电平变流器具有每个H桥单元结构相同、所用元器件数少、易于实现电路的模块化设计和封装等一系列优点,因而在无功补偿和有源电力滤波器等领域有广泛的应用前景。重点提出了基于CPS-SPWM技术的级联H桥型多电平变流器,是CPS-SPWM技术与级联H型变流器拓扑结构的结合,同时具备二者的优点,在大功率变流器领域具有很好的应用前景。