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This paper introduces a new iterative design procedure of L and LCL filters for low-voltage active-front-end PWM two-level voltage source converters. The analytical expression of the converter harmonic voltages by Bessel functions is applied to design the filter parameters for defined maximum grid current harmonics. Different filter designs are derived for various resonance frequencies and inductance split factors of LCL filter. The minimum amount of stored energy of passive components is used to select a final filter design. A voltage-oriented control scheme, including active damping, is applied in the (400 V and 50 kVA) experimental setup. Both simulation and experimental investigations are presented to verify the accuracy of the filter design procedure. Finally, the steady state and transient performance of the active front end with different LCL filters are depicted. Index Terms--Active damping, active front end, LCL filter.

The application of a particular medium-voltage converter in a certain application depends on numerous criteria. However, in completely new installations, the choice of the system voltage is very often uninhibited of external constraints. Then, the voltage level shall be selected to enable the most efficient power conversion at the lowest cost. This paper is dedicated to help in finding the best voltage level for three-level neutral-point-clamped voltage source converters (3L-NPC VSCs) with respect to the power semiconductor devices. Three insulated-gate-bipolar-transistor-based 3L-NPC VSCs of different voltage levels (2.3, 3.3, and 4.16 kV) are investigated and compared regarding their maximum output power, semiconductor efficiency, and semiconductor cost per MVA output power. The effects of thermal cycling, the loss distribution within the converter, and switching frequencies from 300 to 1050 Hz are considered in the evaluation. Index Terms--Insulated gate bipolar transistor (IGBT) converters, medium-voltage (MV) converters, three-level converters.

This paper, along with an earlier published paper as Part 1, provides a comprehensive review of the state of the art of high-power converters (above 1 MW) for adjustable-speed ac drives. In this highly active area, different converter topologies have been developed for various drive applications in the industry. Due to its extensive coverage, the subject is divided into two parts: multilevel voltage source and current source converter topologies. This paper is focused on the second part and covers the current source converter technologies, including pulsewidth-modulated current-source inverters (CSIs) and load-commutated inverters. In addition, this paper also addresses the present status of the direct converter, which is also known as cycloconverter (CCV). This paper focuses on the latest CSI and CCV technologies and an overview of the commonly used modulation schemes. It also provides the latest technological advances and future trends in CSI- and CCV-fed large drives. This paper serves as a useful reference for academic researchers and practicing engineers in the field of power converters and adjustable-speed drives. Index Terms--Current-source inverter (CSI), cycloconverter (CCV), high-power drives, load-commutated inverter (LCI).

This paper investigates the effects of grid voltage distortions caused by thyristor converters on the steady-state performance of a pulsewidth modulation (PWM) rectifier with dc voltage link for low-voltage applications. The disturbance transfer function of the current control loop is used to analyze the effects of grid distortions. Influences of the current controller, the phase-locked loop parameters, and the filter inductance on the generated current harmonics are presented. For existing grid voltage harmonics, the amplitude of the corresponding current harmonics are determined and compared to measured current harmonics using a 400-V 40-kW test bench consisting of a PWM rectifier with voltage dc link parallel to a 40-kW thyristor converter. Index Terms--Harmonic distortion, pulsewidth modulation (PWM) active front ends, thyristor converters.

This paper presents a technology review of voltage-source-converter topologies for industrial medium-voltage drives. In this highly active area, different converter topologies and circuits have found their application in the market. This paper covers the high-power voltage-source inverter and the most used multilevel-inverter topologies, including the neutral-pointclamped, cascaded H-bridge, and flying-capacitor converters. This paper presents the operating principle of each topology and a review of the most relevant modulation methods, focused mainly on those used by industry. In addition, the latest advances and future trends of the technology are discussed. It is concluded that the topology and modulation-method selection are closely related to each particular application, leaving a space on the market for all the different solutions, depending on their unique features and limitations like power or voltage level, dynamic performance, reliability, costs, and other technical specifications. Index Terms--High-power applications, multilevel converters, voltage-source inverters (VSIs).

This paper compares the expense of power semiconductors and passive components of a (2.3 kV, 2.4 MVA) two-level, three-level neutral-point-clamped, three-level flying-capacitor, four-level flying-capacitor, and five-level series-connected H-bridge voltage source converter on the basis of the state-of-the-art 6.5-, 3.3-, 2.5-, and 1.7-kV insulated gate bipolar transistors for industrial medium-voltage drives. The power semiconductor losses, the loss distribution, the installed switch power, the design of flying capacitors, and the components of an LC sine filter for retrofit applications are considered. Index Terms--Filter design, medium voltage, multilevel converters, power electronics.

The three-level neutral-point-clamped voltage-source converter (NPC VSC) is widely used in high-power medium-voltage (MV) applications. To overcome its major drawback--the unequal loss distribution among the semiconductors-the use of active NPC (ANPC) switches was previously proposed. In this paper, a simple feedforward loss-control scheme for the ANPC converter is presented. The principle of operation of the ANPC VSC is experimentally verified for the first time. Electrical measurements on a low-voltage test bench prove the proper function of the ANPC converter. Infrared junction temperature measurements confirm the improved loss distribution using the new loss-control scheme. Finally, the authors demonstrate the superior performance of the loss-controlled ANPC converter in various MV applications. Index Terms--Medium-voltage (MV) converters, multilevel converters.

This paper describes the design of a (4-kV, 4.16-MVA) three-level neutral-point-clamped-, three-level flying-capacitor-, four-level flying-capacitor-, and nine-level series-connected H-bridge voltage-source converter on the basis of state-of-the-art 6.5-, 4.5-, 3.3- and 1.7-kV insulated gate bipolar transistors. The semiconductor loss distribution and the design of semiconductors and passive components are compared for a medium switching frequency assuming a constant converter efficiency of about 99%. To evaluate the converter characteristics in high switching frequency applications, a second comparison is realized for the maximum switching frequencies assuming a constant expense of semiconductors in all converters. Index Terms--Medium-voltage drives (MVDs), multilevel converters, power electronics.

The design of a power converter must guarantee that the operating junction temperatures [V.sub.j] of all devices do not exceed their limits under all specified operating conditions. Usually, this is ensured by a simulative or analytical junction temperature estimation based on simple electrical and thermal models and semiconductor datasheet values. This paper discusses the difficulties and quantifies the limitations of this approach on the example of a three-level neutral point clamped voltage source converter (NPC VSC) with insulated gate bipolar transistors. The calculations are compared to the results of direct junction temperature measurements with an infrared camera. The paper also provides the experimental proof for the unequal loss and junction temperature distribution in the three-level NPC VSC. Index Terms--Insulated gate bipolar transistors (IGBT), Junction temperature measurement, neutral point clamped voltage source converter (NPC VSC), thermal modeling.

The low-frequency line transformer in todays ac rail vehicles suffers from poor efficiency and a substantial weight. Future traction drives may operate directly from the mains without this transformer. A feasible concept for a transformerless drive system consists of series connected medium voltage converters applying modern high-voltage insulated gate bipolar transistors (HV-IGBTs). In a first design step, the switching characteristics and losses of 6.5-kV IGBTs are compared to 3.3-kV and 4.5-kV IGBTs which are already commercially used in traction applications. Based on the considered HV-IGBTs, the properties of multilevel converters are analyzed and their applicability to the transformerless system is evaluated. The paper focusses on a loss analysis of the converters. Reliability aspects and harmonic spectra are briefly discussed. Taking these design aspects into account, the three-level neutral point clamped converter turns out to be a reasonable solution to realize line and motor converter modules in a transformerless traction system. Index Terms--Flying capacitor (FC) converter, gate turn-off thyristors (GTOs), high-voltage insulated gate bipolar transistors (HV-IGBTs), neutral point clamped (NPC) converter.

The three-level neutral-point-clamped voltage-source converter (NPC VSC) is widely used in high-power medium-voltage applications. The unequal loss distribution among the semiconductors is one major disadvantage of this popular topology. This paper studies the loss distribution problem of the NPC VSC and proposes the active NPC VSC to overcome this drawback. The switch states and commutations of the converter are analyzed. A loss-balancing scheme is introduced, enabling a substantially increased output power and an improved performance at zero speed, compared to the conventional NPC VSC. Index Terms--Medium voltage, power losses, three-level converter, voltage-source converter (VSC).

This paper evaluates three-level topologies as alternatives to two-level topologies in converters for low-voltage applications. Topologies, semiconductor losses, filter aspects, part count, initial cost, and life-cycle cost are compared for a grid interface, a conventional drive application, and a high-speed drive application. Index Terms--Life-cycle cost, multilevel converters, semiconductor losses.

This paper evaluates the benefits of three-level topologies as alternatives to two-level topologies in low-voltage converters primarily operated in rectifier mode. The main evaluation aspects are input filter size, semiconductor losses, maximum switching frequency, part count, initial cost, and life cycle cost. Semiconductor loss characteristics of various three-level topologies are discussed. A detailed converter comparison is based on a 100-kW 400-[V.sub.rms] rectifier using commercially available Si insulated gate bipolar transistor modules. Index Terms--Filter, losses, multilevel converter, rectifier.

In this article, a detailed hard switching behavior comparison between a 1700-V silicon carbide metal-oxide-semiconductor field-effect-transistor and a 1700-V silicon insulated-gate bipolar-transistor module was performed in an identical low inductance commutation circuit for comparable driving conditions. To accomplish this, devices using same module formats were used and carefully selected to match thermal characteristics in the exact same commutation circuit. Furthermore, a simulation of the modules in inverter operation based on the experimental results and complimented by datasheet based on-state values and Foster thermal models has been performed to study losses, junction temperatures, and current utilization. These results are further discussed highlighting advantages and limitations regarding these state-of-the-art SiC devices versus relative cost when contrasted with a comparable Si-based device. [ABSTRACT FROM AUTHOR]

The need for lighter and smaller railway traction drives and recent advances in power semiconductor technology have resulted in a new energy conversion paradigm, one without the classical heavy and bulky line-frequency transformer. Several multilevel topologies have been introduced over the past years, all having in common the use of medium-frequency (MF) energy conversion to reduce the weight and volume of the magnetic components. This paper is concerned with the design of the input filter for an MF traction converter, focusing on a solution that minimizes the filter's overall mass. Two filter topologies are investigated, namely a single-pole filter (inductor) and a passively damped $LCL$ filter. The paper first describes the requirements to be fulfilled, establishes the spectrum to consider for the design, and proposes a method to determine the initial parameters of the filter's components. The results show that an $LCL$ -type filter achieves considerable weight savings in comparison to the single-pole solution. [ABSTRACT FROM AUTHOR]

In this study, a model predictive control (MPC) strategy that combines finite-control-set MPC with selective harmonic elimination (SHE) modulation pattern in its formulation is proposed to govern multilevel power converters. Based on a desired operating point for the system state (converter current reference), an associated predefined SHE voltage pattern is obtained as a required steady-state control input reference. Then, the cost function is formulated with the inclusion of both system state and control input references. According with the proposed reference and cost function formulation, the predictive controller prefers to track the converter output current reference in transients, while preserving the SHE voltage pattern in steady state. Hence, as evidenced by experimental results, a fast dynamic response is obtained throughout transients while a predefined voltage and current spectrum with low switching frequency is achieved in steady state. [ABSTRACT FROM PUBLISHER]

Modular multilevel converters have several attractive features such as a modular structure, high voltage capability and scalability, excellent quality of the output voltage waveforms, as well as, low expense for redundancy. However, the main drawback of this converter is the complexity of the control system due to the multiple control objectives which include the input, output, circulating current and the capacitor voltages. Several control schemes for the capacitor voltages have been proposed, being usually divided into the control of the average voltage and the balancing of the capacitor voltages among the submodules and among the arms. These control schemes can effectively control the capacitor voltage when the reference is the same for all arms, but they cannot manage arbitrary voltage references when each arm capacitor voltage must be controlled independently as required, for example, to achieve maximum power point tracking in photovoltaic applications. This paper proposes a control strategy for a modular multilevel converter capable to manage the average capacitor voltage of each arm independently. The proposed scheme is based on a decomposition of the energy of the arms in different components, according to theirs symmetries. Results validating the performance of the proposed control scheme are shown and the impact on the output current is also discussed. [ABSTRACT FROM PUBLISHER]

Modular multilevel converters have several attractive features such as a modular structure, the capability of transformer-less operation, easy scalability in terms of voltage and current, low expense for redundancy and fault tolerant operation, high availability, utilization of standard components, and excellent quality of the output waveforms. These features have increased the interest of industry and research in this topology, resulting in the development of new circuit configurations, converter models, control schemes, and modulation strategies. This paper presents a review of the latest achievements of modular multilevel converters regarding the mentioned research topics, new applications, and future trends. [ABSTRACT FROM AUTHOR]

A new modular drivetrain for large wind turbines currently under development is presented. The electromechanicaldrivetrain has a direct-drive configuration with a modular flux-switching permanent-magnet generator. The full-powerconverter consists of 12 parallel back-to-back two-level voltage source converter modules. A novel method of modulationis used to improve harmonic quality while maintaining a low switching frequency. Benefits of the new design includesimplified manufacturing and transport, increased system availability due to fault tolerance and redundance, andavoidance of an LCL filter. Measurement results are shown from a 500-kW test rig. [ABSTRACT FROM AUTHOR]

In order to fulfill the grid codes and meet power quality requirements in grid-connected converters without increasing the switching frequency or system size, higher order line filters are often used. This paper considers the active damping of $LC\!L$-filters in grid-connected converters. Analysis in the discrete time domain shows that the damping can be increased when proportional plus derivative control of the capacitor current is applied, and it is found that this scheme has a simple implementation in a discrete time system as it is equivalent to proportional feedback of the last two samples. To find optimal damping controller gains, an analytical solution is derived for the open-loop system, and for the closed-loop system, a numerical optimization algorithm is developed. The performance of the proposed methods is compared using analysis in the $z$-domain, simulations, and experimental investigation. The analytical solution has good damping, an easy implementation, and is seen to have robustness against parameter variation, whereas the numerical solution is computation intensive but has even higher damping. [ABSTRACT FROM AUTHOR]

This study is focused on evaluating the advantages of 6.5-kV SiC p-i-n diodes in a 4.16-kV, 8-MVA neutral point clamped (NPC) voltage source converter. Electrothermal models of the power semiconductors are elaborated based on measurement data of a SiC diode module prototype, as well as of commercial Si devices. These models are integrated into a converter simulation, where the junction temperatures of each device are used to determine the maximum power output under different device configurations. For the analysis, a parallel connection of two 6.5-kV insulated-gate bipolar transistors (IGBTs) as well as a series connection of two 3.3-kV IGBTs are considered as switches. The influence of the current change rate during the IGBT turn-on is also studied. SiC p-i-n diodes are used as a replacement for the NPC diodes of the converter and as antiparallel diodes in IGBT modules. The results obtained indicate that an increase in the output power of 17% can be achieved using SiC NPC diodes for the studied current change rates. Alternatively, for a constant output power, the switching frequency of the converter can be increased by 69%, reducing the converter size and system costs by the use of smaller filters. [ABSTRACT FROM AUTHOR]

The parallel connection of IGBTs is being applied in various low- and medium-voltage converters. The selection of the devices, the manual parametrization of gate units, and the substantial device derating are substantial disadvantages of state-of-the-art converters with parallel-connected insulated-gate bipolar transistors (IGBTs). A new, low-expensive, and automated delay-time compensation method without additional current measurements was introduced in the 2011 IEEE Energy Conversion Congress and Exposition. This paper briefly reviews the structure and function of this new scheme, shows the extension of the scheme for three parallel-connected IGBTs, and investigates the performance of the new delay-time compensation principle for sinusoidal current operation at different $\cos\phi$ and m_{a} values, as in a converter for drives. [ABSTRACT FROM AUTHOR]