DC Link Filter Research Articles

Analysis and Design of MMC-Based High-Power DC–DC Converter With Trapezoidal Modulation

The modular multilevel converter (MMC) based dc-dc converters (MMDC) are essential for voltage matching, galvanic isolation, renewable integration and other applications in high voltage direct current (HVdc) systems. The trapezoidal modulation in these converters has been a proven advantage owing to reduced capacitive requirement and high-power density. However, the existing short overlap (SO) trapezoidal modulation technique presents significant dc link current ripples requiring bulky dc link filter. This paper introduces an extended overlap (EO) modulation technique in MMDC to mitigate the need of dc link filters. The change in overlap angle alters the conduction states, component sizing, harmonic spectrum and other converter parameters. Therefore, a detailed mathematical analysis of MMDC with both SO and EO modes of trapezoidal modulation is carried out in this paper. The effect of change in overlap duration on size of sub-module (SM) capacitor, dc link filter is analyzed by simulation and analytical studies. A single stage three phase MMC experimental setup is built to verify the converter operation in both SO and EO modulations.

A design optimisation tool to minimise volume and failure rate of the modular multilevel converter and the thyristor-controlled rectifier

The unfolding of MVDC (Medium Voltage DC) systems has the prospects to enable the incorporation of power electronic converters with higher power density and reliability. A tool with an integrated design approach is required to minimise the overall system volume by identifying optimal components. In this paper, a component-level early-stage design tool has been developed to attain the minimum achievable volume and failure rate for MVDC power converters. The developed tool optimises the choice of semiconductor switching devices, required heatsink, and other passive components (including dc-link filters and inductors) to minimise failure rate and overall converter volume. The optimisation algorithm employs the non-dominated sorting genetic algorithm (NSGA-II) to evaluate designs based on developed fitness functions. The design tool demonstrates the trade-off when evaluating multiple converter topologies and helps make an informed decision. A comparative study between two converter topologies shows the outcomes in terms of targeted metrics (volume and failure rate). This tool is expected to benefit early-stage design to perform trade-off studies among power electronic converter topologies based on key metrics like volume and failure rate.

A half-bridge distributed static compensator with a DC-link filter capacitor of a reduced size LCL filter

Distributed static compensators (DSTATCOMs), composed of power converters and passive filters, have been extensively utilized in distribution systems to improve their power quality. For the selection of passive filters, LCL filters with smaller sizes and lower costs are increasingly being adopted to replace the conventional L filters. However, the filter components of LCL filters are normally designed for the application of grid-connected inverters, and therefore, these passive components can be further optimized when applied to the DSTATCOM, where the grid-injected current is mainly capacitive. In this study, the novel LCL-filtered half-bridge DSTATCOM topology is proposed, where the conventional filter capacitor of the LCL filter is replaced by the DC-link capacitors with relatively larger capacitances. As a consequence, the grid-side inductance can be dramatically reduced from the order of millihenry to several microhenries. Furthermore, the current stress of semiconductor switches can be decreased. In addition, the power flow analysis and filter design procedure are presented in this study. It is revealed that a constant DC-link voltage can always be maintained, and the voltage variation across each capacitor can be well regulated with properly designed filters and controllers. Experiments were carried out on a down-scaled laboratory prototype, and simulation and experimental results are presented to verify the effectiveness of the proposed DSTATCOM topology.

Current Harmonics Generated by Multiple Adjustable-Speed Drives in Distribution Networks in the Frequency Range of 2–9 kHz

Adjustable-speed drives (ASDs) generate current harmonics due to their nonlinear characteristic and switching function. These current harmonics could pass through a network and deteriorate the power quality, create resonances, and increase network power loss. Higher order 2–9 kHz harmonics, among them, have become a new challenge for drive manufacturers, as these harmonics will be covered in the forthcoming IEC standards (standardization committee SC77A). <styled-content style="color:black" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> To address the existing gaps, this article explores 2–9 kHz current harmonics for a common ASD type that has a diode-bridge rectifier (DR) and a dc-link filter. Through the analytical study and experimental results presented in this article, it is shown that the main source of 2–9 kHz current harmonics at the grid side of this type of ASD is its front-end DR.</styled-content> Moreover, it is found out that the amplitude of these current harmonics can be significantly affected by the operation power of the other ASDs in the network. Furthermore, it is analyzed that these interactions increase the 2–9 kHz current harmonics at an ASD when the other ASDs operate at different load values.

A Novel Inductor for Stabilizing the DC Link of Adjustable Speed Drive

The DC-link filter which includes a magnetic inductor and a storage capacitor is one of the key parts of adjustable speed drives in the market. It significantly affects the stability, reliability, and power density of the motor-drive system. This paper proposes a novel, variable active inductor to improve the performance of DC links in terms of stability, reliability, size, and cost. In contrast to conventional DC-link magnetic inductors, the variable active inductor is made of power electronic circuits, including active switches, passive filters, and smart controllers, which no longer rely on magnetic material. The demonstration shows that the inductor can emulate the electrical characteristics of the magnetic inductor for filtering harmonics and stabilizing the DC link, meanwhile representing a smaller size, lighter weight, and lower cost compared with a conventional one. Furthermore, this paper proposes a variable inductance control method which is able to adaptively tune the inductance value with the operating conditions of the drive system. The DC link can be stabilized, and high performance can be maintained in both balanced and unbalanced grid voltage conditions. A case study of the proposed variable inductor in a motor drive with a three-phase diode-bridge rectifier as the front end is discussed. Experimental results are given to verify the functionality and effectiveness of the proposed variable inductor.

Modeling and analysis of single phase quasi Z source matrix converter fed induction motor drives

Single-phase quasi z-source matrix converter which is used for AC drives and induction heating applications is designed here. The foremost gain of matrix converter is the elimination of dc link filter. By comparing with established matrix converter, the quasi Z source matrix converter provides high voltage gain, requires less number of switches and LC filter functions which is integrated in the proposed network so as to repudiate additional filter components. The proposed topology describes the direct quasi Z source matrix converter in order to increase voltage gain in load side. The sinusoidal PWM technique has been employed and proposed circuit is simulated in MATLAB/SIMULINK. The experimental set up has been implemented and the results shows that the Quasi Z source matrix converter provides high efficiency. Compared with the conventional matrix converter it dispense low input current and high power.

Current and Voltage Harmonics in the Powertrain of Electric Vehicles

Switching electronics, such as the drive inverter, generate significant voltage and current harmonics with frequencies as low as a few kilohertz which are propagated throughout the complete electric powertrain and all connected components. With amplitudes up to several volts and dozens of amperes, such harmonics can cause significant problems and lead to performance limitations or even component failures. In order to maintain the quality of the dc-link voltage and guarantee the sound interaction of all components, power electronics must therefore be equipped with filter capacitors damping these harmonics. However, the filter stages account for a large part of the overall component cost and volume. Usually, the required dc-link filters are designed independently for each component on its own without taking into account the configuration of the complete powertrain. This, however, might result in undersizing of filters and possible system failures or contrary to that to oversized and expensive components. In this study, an analytic approach for the computation of the electric harmonics generated by the drive unit of a hybrid fuel cell vehicle will be derived and validated using measurement data. Using this model, the significance of the electric harmonics for the design of the powertrain as well as the sizing process of the filter capacitors will be analyzed.

Lifetime Prediction of DC-Link Capacitors in Multiple Drives System Based on Simplified Analytical Modeling

Lifetime prediction of dc-link capacitors in a single drive has been discussed before, which indicates that the capacitor in a standard drive meets serious reliability challenges and in a slim drive does not. However, in most of the applications, drives are connected in parallel with the power grid. The large amount of harmonic distortion produced by nonlinearity drives may transmit and couple between grid and drives, which changes the stresses of devices as well as the dc-link filters. Therefore, the estimated results in a single drive cannot be extended to multiple drives any more. This article investigates the lifetime of dc-link capacitors in multiple drives system. First, by decoupling the interactions among grid-connected drives, a simplified equivalent circuit model and its analytical model to obtain the dc-link continuous current in multiple drives is proposed, which releases the designers from configuring the large simulation for multiple drives. Then, applying the lifetime prediction method, the lifetime of dc-link capacitors in multiple drives is investigated, in terms of types of drives, numbers of drives, and grid conditions. The results show that the lifetime of the standard drives extends in the multidrive systems and the lifetime of the slim drives decreases in the multidrive systems, which break the previous mind. Finally, based on the proposed analytical model and lifetime estimation method, the capacitor sizing from reliability aspect for multiple slim drives is given. The outcomes of the lifetime investigation could be a guideline for the design of the capacitive dc link in multidrive systems.

A Single-Stage Soft-Switched Isolated Three-Phase DC–AC Converter With Three-Phase Unfolder

This article proposes a unidirectional, single-stage, three-phase, and high-frequency-link dc–ac converter. The converter uses three high-frequency transformers to provide galvanic isolation. In the primary, the converter has three half-bridge legs. In the secondary, three diode-bridge rectifiers are used. Each rectifier is followed by a half-bridge (unfolder) leg. The modulation strategy proposed in this article ensures zero-voltage switching of all six primary side switches over the entire line cycle without additional snubber circuit. The secondary half-bridge legs are switched at line frequency, incurring negligible switching loss. The proposed converter does not require any interstage bulky dc-link filter capacitor. Thus, the overall filtering requirement is reduced, and the converter reliability is improved. The high-frequency galvanic isolation improves the converter power density. In this article, the modulation strategy and converter operation are described in detail. The operation is validated in a 3.7-kW hardware prototype. Key experimental results are presented in this article.

A Two-Terminal Active Inductor With Minimum Apparent Power for the Auxiliary Circuit

This letter proposes a two-terminal active inductor concept with minimum apparent power processed by the auxiliary circuits, which are implemented by power semiconductor switches and passive elements. It has the same level of convenience as a passive inductor with two power terminals only. Compared with a conventional inductor with the same equivalent inductance and current rating, the energy storage requirement of the inductor can be reduced significantly, as well as the weight and volume. The auxiliary circuit in the active inductor processes both partial voltage and partial current, with the lowest apparent power rating compared to existing solutions. A case study for the dc-link filter inductor in a three-phase diode-bridge rectifier is discussed. Proof-of-concept experimental results are given to verify the functionality and effectiveness of the proposed active inductor.

Lifetime Estimation of DC-Link Capacitors in Adjustable Speed Drives Under Grid Voltage Unbalances

Electrolytic capacitor with a dc-side inductor is a typical dc-link filtering configuration in grid-connected diode rectified adjustable speed drives (ASDs). The criteria to size the dc-link filter are mainly from the aspects of stability and power quality. Nevertheless, the reliability of the dc-link filter is also an essential performance factor to be considered, which depends on both the component inherent capability and the operational conditions (e.g., electrothermal stresses) in the field operation. Nowadays, unbalanced voltage has the most frequent occurrence in many distribution networks. It brings more electrical-thermal stress to the component, affecting the reliability of the capacitors. In order to study the reliability performance of the LC filter in an ASD system quantitatively, this paper proposes a mission profile based reliability evaluation method for capacitors. Different from the conventional lifetime estimation, a nonlinear accumulated damage model is proposed for the long-term estimation, considering the nonlinear process of equivalent series resistor growth and capacitance reduction during the degradation. Based on the proposed lifetime estimation procedure, four case studies are investigated: first, lifetime benchmarking of capacitors in LC filtering and slim capacitor filtering configurations; second, scalability analysis for the lifetime of capacitors in terms of system power rating and grid-unbalanced levels; third, lifetime estimation of capacitors in the dc-link filter with long-term mission profile; and fourth, the impact of the capacitor sizing on the lifetime of the dc-link capacitor under grid-balanced and grid-unbalanced conditions. The results serve as a guideline for proper selection of dc-link configurations and parameters to fulfill a specification in ASDs.

A Single Phase AC/DC/AC Converter With Unified Ripple Power Decoupling

In single phase ac/dc/ac converters, the low frequency ripple powers exist both at the source and load sides. Usually, large dc-link filter components are used to buffer the ripple powers, which increases volume and weight. To overcome the drawback, this paper presents a single phase ac/dc/ac current source converter with unified ripple power decoupling. The converter only consists of three bridge arms and a decoupling circuit. The three bridge arms play the role of rectification and inversion with sharing a bridge arm. And the decoupling circuit is in series with the dc-link energy storage unit to buffer the ripple powers. The circuit configuration and operation principles are introduced first. Then, a modulation strategy based on Cartesian space is developed to achieve sinusoidal input and output currents. The control idea that the dc-link current is regulated by the decoupling circuit and the averaged decoupling capacitor voltage is maintained by the rectifier is adopted. The ripple power buffer is automatically achieved. Finally, the theoretical analysis is favorably verified by the simulations and experimental results.

A Design Optimization Tool for Maximizing the Power Density of 3-Phase DC–AC Converters Using Silicon Carbide (SiC) Devices

The emergence of wide-bandgap devices, e.g., silicon carbide (SiC), has the potential to enable very high-density power converter design with high-switching frequency operation capability. A comprehensive design tool with a holistic design approach is critical to maximize the overall system power density, e.g, by identifying the optimal switching frequency. This paper presents a system level design tool that optimizes the power density (volume or mass) of a three-phase, two-level dc-ac converter. The design tool optimizes the selection of the devices, heatsink and passive components (including the design of the line, electromagnetic interference (EMI), and dc-link filters) to maximize the power density. The structure of the optimization algorithm has been organized to reduce the number of potential design combinations by over 99%, and thus, produces fast simulation times. The design tool predicts that when SiC devices are used instead of Si ones, the power density is increased by 159.4%. A 5 kW, 600-V dc-link, three-phase, two-level dc-ac converter was experimentally evaluated in order to confirm the accuracy of the design tool.

Three-Phase Single-Stage AC–DC Converter Based on Magnetic-Combination Transformer With Power Factor Correction

To improve the power factor and reliability for medium/high-power supply, a three-phase single-stage ac–dc converter based on magnetic-combination transformer with power factor correction (PFC) function is presented in this paper. According to the three-phase instantaneous power balance principle and the concept of the magnetic-flux superposition, a novel magnetic-combination transformer is proposed, which can make three-phase symmetrically and sinusoidally fluctuant power add to be constant. Therefore, the induced voltage of the secondary winding is constant. To realize sinusoidally fluctuant power in the input stage, dc-link filter capacitance can be reduced to a several-microfarad level. Single-stage PFC function, no electrolytic capacitor, and constant output voltage can be realized in the converter. The principle and the structure of the novel magnetic-combination transformer are analyzed in detail in this paper. Simulation and experimental results are presented to verify the feasibility of the proposed converter.

Impact of thermal stress on the ageing behaviour of aluminum electrolytic capacitors for automotive applications

DC link filter capacitors used in electric drives for automotive applications are most of the time aluminum electrolytic capacitors. However their high sensitivity to temperature results in very weak components. So, a lot of studies have been made in order to estimate their ageing. This paper deals with ageing analysis on the basis of an electrical model taking into account a diffusion phenomenon inside the anode of the capacitor. This accurate model, correlated to the temperature, allows comparing the behaviour of capacitors along their lifespan under constant and cycling thermal stress.

Hybrid Modulation Scheme for a High-Frequency AC-Link Inverter

This paper describes a hybrid modulation scheme for a high-frequency ac-link (HFACL) multistage inverter comprising a front-end dc/ac converter, followed by isolation transformers, an ac/pulsating-dc converter, and a pulsating-dc/ac converter. The hybrid modulation scheme enables 1) removal of the dc-link filter evident in conventional fixed dc-link (FDCL) inverters placed after the ac/pulsating-dc converter stage and before an end stage voltage source inverter and 2) significant reduction in switching loss of the inverter by reducing the high-frequency switching requirement of the pulsating-dc/ac converter by two-third yielding higher efficiency, improved voltage utilization, and reduced current stress. Unlike the FDCL approach, in the HFACL approach, hybrid modulation enables the retention of the sine-wave-modulated switching information at the output of the ac/pulsating-dc converter rather than filtering it to yield a fixed dc thereby reducing the high-frequency switching requirement for the pulsating-dc/ac converter. Overall, the following is outlined: 1) hybrid modulation scheme and its uniqueness, 2) operation of the HFACL inverter using the hybrid modulation scheme, 3) comparison of the efficiency and losses, current stress, and harmonic distortion between the hybrid-modulation-based HFACL inverter and the FDCL inverter, and 4) scaled experimental validation. It is noted that the term hybrid modulation has no similarity with the modulation scheme for a hybrid converter (which are conjugation of two types of converters based on a slow and fast device) reported in the literature. The term hybrid modulation scheme is simply chosen because at any given time only one leg of the inverter output stage (i.e., pulsating-dc/ac converter) switch under high frequency, while the other two legs do not switch. The outlined hybrid modulation scheme is unlike all reported discontinuous modulation schemes where the input is a dc and not a pulsating modulated dc, and at most only one leg stays on or off permanently in a 60° or 120° cycle.

Multi-level integrated optimal design for power systems of more electric aircraft

This paper proposes a multi-level optimal design method for a complex actuation system of more electric aircraft. The multi-level structure consists in sharing the optimization process in several levels, here 2, a “system level” which involves main coupling variables and a “component level” with one optimization loop for each device. The interest of this method is to separate the optimal design of each component, making easier the convergence of loops. This method is applied to a relatively complex power conversion system including a high speed permanent magnet synchronous machine (HSPMSM) supplied by a pulse width modulation (PWM) voltage source inverter (VSI) associated with a DC-link filter. Its interest is shown through a comparison with classical design approaches employed in previous works.

A New Random SPWM Technique for AC-AC Converter-Based WECS

A single-stage AC-AC converter has been designed for a wind energy conversion system (WECS) that eliminates multistage operation and DC-link filter elements, thus resolving size, weight, and reliability issues. A simple switching strategy is used to control the switches that changes the variable-frequency AC output of an electrical generator to a constant-frequency supply to feed into a distributed electrical load/grid. In addition, a modified random sinusoidal pulse width modulation (RSPWM) technique has been developed for the designed converter to make the overall system more efficient by increasing generating power capacity and reducing the effects of inter-harmonics and sub-harmonics generated in the WECS. The technique uses carrier and reference waves of variable switching frequency to calculate the firing angles of the switches of the converter so that the three-phase output voltage of the converter is very close to a sine wave with reduced THD. A comparison of the performance of the proposed RSPWM technique with the conventional SPWM demonstrated that the power generated by a turbine in the proposed approximately increased by 5% to 10% and THD reduces by 40% both in voltage and current with respect to conventional SPWM.

Propulsion System Architecture and Power Conditioning Topologies for Fuel Cell Vehicles

Efficient power conversion technologies are strategic to the advancement of fuel cell vehicles (FCVs). This paper presents an overview of power conditioning system (PCS) architectures for an FCV propulsion system. Two new power conversion topologies to obtain variable ac from the fuel cell system are proposed. The switching of the power devices are based on the proposed hybrid modulation technique that minimizes the switching losses in the inverter. The proposed converters do not use the dc-link filter capacitors. Elimination of the dc-link capacitor not only reduces the size and volume but also helps in retaining the sine wave modulated information at the input of a three-phase inverter. Bidirectional converter topologies are proposed for interfacing low-voltage battery and fuel cell dc bus. Experimental results are presented for the proposed topologies.

An Artificial Intelligence Based System Identification of AC-DC Power System Including a Three-Phase Controlled Rectifier

The artificial intelligence techniques are widely applied to many works of engineering. This paper presents the application of the artificial intelligence techniques to identify the AC-DC power system parameters. The DC-link filters are normally included in such power systems. For some applications, accurate DC-link filter parameters are very important for the system analysis and design. According to the measurement errors and a large variation in a capacitor value, the parameter identification is needed to provide the accurate parameters. Therefore, the paper presents the identification of DC-link filter parameters using the adaptive tabu search technique and the genetic algorithm. The experimental results from the testing rig and the averaging model of the system are used in the searching process. The identification results show that the accurate DC-link filter parameters including line parameters on the AC side can be easily obtained via the proposed method. Moreover, the resulting parameters are very useful for a stability analysis of the power electronic systems due to a constant power load.