DC Bus Capacitor Research Articles

Unified Power Quality Conditioner Performance based on Multi-level Inverter Topologies using Intelligent Controllers

This paper presents unified power quality conditioner (UPQC) system based on three-level neutral point clamped (NPC) using Fuzzy and ANN controllers. The proposed UPQC is able to mitigate source current harmonics and compensate all voltage disturbances. It is designed by the integration of series and shunt active filters (AFs) sharing a common DC bus capacitor. The DC voltage is maintained constant using proportional integral voltage controller. The synchronous reference frame (SRF) theory is used to get the reference signals for the shunt APF and the power reactive theory (P-Q theory) for the series APF.Conventional control schemes require complex calculations, the use of intelligent systems help to reduce the control system size and the operation complexity. The shunt and series active power filter (APF) reference signals derived from the control algorithm are injected in Fuzzy and Ann intelligent controllers to generate switching signals. The performances of the proposed UPQC system are evaluated using Matlab-Simulink software and SimPowerSystem Toolbox under various operation conditions. The simulation results show the performance of the proposed UPQC based on intelligent controllers to improve the power quality in steady and transient conditions operation.

Performance of Reduced-DC-Source-Based Three-Phase High-Resolution Multilevel Inverter With Optimal Asymmetry

Asymmetrical cascaded bridge multilevel converters use voltage sources of different magnitudes to generate a higher number of levels. However, the availability of sources of different ratings feeding the bridges is a limitation. This article overcomes such a limitation by utilizing the concept of capacitor voltage balancing using the level doubling network (LDN) principle in the space-vector plane. With the use of optimal asymmetry, the presented topology can generate 43 levels at the output line voltage by using only 12 switches per phase. Hence, the most notable attribute of the presented topology is that it does not require isolated auxiliary power supplies to develop such a high-resolution output voltage. LDNs in this topology are self-balanced, and the auxiliary full-bridges require a closed-loop controller to balance their dc bus capacitor voltage. To validate the proposed concept, MATLAB simulations and experimental results are presented in this article.

New Configuration of Five-Level NPC Inverter with Three-Level Boost Converter for Photovoltaic Solar Energy Conversion

This paper deals with the modeling and control of a new two-stage photovoltaic conversion cascade composed of a Three-Level Boost (3LB) converter and a three-phase NPC five-level inverter (5LI). The main objective is to ensure the continuity of energy transfer from a renewable continuous source to the electrical grid while regulating the voltages of the DC bus capacitors with a new proposed configuration. The 3LB is controlled in MPPT based on fuzzy logic. A PI regulator is introduced into the 3LB regulation loop in order to maintain the equality of the two output voltages. The overall voltage of the DC bus being regulated by the control of the multilevel inverter interconnected to the grid, using synchronous reference frame control. Therefore, in our case there is the problem of voltages imbalance caused by the 5LI. We propose in this work to introduce two Clamping Bridges (CBs) each one is composed by a transistor in series with a resistor. These CBs are introduced in parallel with two capacitors of the DC bus. This solution avoids us to use the control algorithm by redundant configurations in the regulation loop of the 5LI. The results obtained show the effectiveness of the proposed control in stabilizing and maintaining the four DC bus voltages at equal values. With four balanced capacitor voltages, we get symmetrical inverter output voltage waveforms, which directly affects the quality of the current injected to the grid.

Design of Common DC-Link Capacitor in Multiple-Drive System Based on Reduced DC-Link Current Harmonics Modulation

Many industrial applications are recently utilizing the advantages of a common dc-bus-based multiple-drive (multidrive) system, such as low installation cost, space savings as well as improved reliability, and flexibility. One of the main drawbacks of such systems is the requirement for a bulky dc capacitor due to the presence of large dc-link current ripples from the harmonic interaction between these multidrive and pulsewidth modulation (PWM) switching. This problem not only increases the cost and volume of the dc-link capacitors but also decreases its lifetime, which must be considered in the design from the industrial aspect. This article proposes a new dc-bus capacitor design method for a multidrive system, focused on the capacitor sizing, cost reduction, and lifetime improvements. First, an analytical model to obtain the dc-link capacitor current in a multidrive system is proposed, which would aid the designers to configure the dc-capacitor current in systems having numerous multiple drives. Next, using the developed analytical model, the dc-link current harmonics are reduced through optimal interleaving of the fundamental output currents in addition to the phase shifting of the carrier waves of the parallel connected inverters. Finally, new capacitor sizing/design is proposed considering the lifetime, power loss, cost, and volume of the dc-link capacitor bank. The proposed approach reduces the volume and cost of dc-cap bank by 50%, and increases the lifetime by four years ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 35 000 h), considering the worst-case scenarios. Experimental and analytical results are given for validation of the proposed method. The results could serve as a guideline for designing the dc-link capacitor in multidrive applications in a low-cost manner.

A Nonlinear Controller for Neutral Point Piloted (T-Type) Multilevel Inverter-Based Three-Phase Four-Wire DSTATCOM

In recent times, neutral point piloted (NPP) or half-leg T-type multilevel inverter (MLI) becomes a promising topology for medium-voltage high-power applications. This T-type MLI is the best substitute for diode clamped multilevel inverter (DCMLI) due to its simple structure and absence of clamping diodes. However, the neutral point voltage control or dc bus capacitors' balance of NPP T-type MLI is a challenging task. Therefore, this paper presents a nonlinear sliding mode controller (SMC) for dc-link voltage balance and reference current control in distribution static compensator (DSTATCOM). For this, a three-level T-type MLI-based three-phase four-wire DSTATCOM is considered to eliminate current-related power quality issues such as harmonics, reactive power, load unbalance, and neutral current. The proposed NPP T-type-based DSTATCOM is compared with DCMLI and active DCMLI-based DSTATCOMs in terms of switching losses and cost. Simulation studies are carried out in MATLAB Simulink environment and further corroborated with experimental studies. Furthermore, the ability of the proposed SMC-based controller is compared with the PI controller under different operating conditions.

A V2G Enabled Bidirectional Single/Three-Phase EV Charging Interface Using Modular Multilevel Buck PFC Rectifier

The battery charging power electronics interface of an electric vehicle (EV) must be capable of bidirectional power flow to enable both grid-to-vehicle (G2V) and vehicle-to-grid (V2G) operations. In the presence of a single/three-phase AC supply, the front-end of the EV charger employs a power factor correction (PFC) rectifier, which should have the bidirectional capability to facilitate V2G mode. A conventional active rectifier functions in boost mode while performing PFC and voltage regulation. In most of the currently available EVs, however, the battery nominal voltage is low and, hence, a downstream high step-down DC-DC converter and high voltage DC bus capacitor are required in the charging interface. To overcome these issues, this work proposes a bidirectional AC-to-DC buck rectifier topology that can operate in G2V and V2G modes, both in single- and three-phase versions. The proposed topology utilizes the switched capacitors principle to achieve self-balancing of voltages in the capacitors. In addition, it is highly modular in structure. This paper describes the proposed topology, its working and modulation and its applications. The hardware proto model is used to validate the proposed power converter and the control approach to achieve PFC and voltage regulation. In addition, a comparison with other topologies is presented to demonstrate its competence.

Capacitor Technologies: Characterization, Selection, and Packaging for Next-Generation Power Electronics Applications

DC-bus capacitors take up substantial space in an electric vehicle (EV) traction inverter, limiting the traction drive’s volumetric power density. Film capacitors are typically used, but other capacitor technologies with higher energy densities can help reduce the overall size. In this article, several commercial capacitor technologies are considered for use as dc-bus capacitors for EV traction inverters. They are characterized, evaluated, and compared for optimized design for volume reduction. This article also proposes a novel capacitor packaging technique that utilizes symmetrically distant parallel capacitor branches from termination, which improves electrical and thermal performance compared to a traditional flat-printed circuit board-based design. The proposed design was prototyped for a 100-kW traction inverter, and then, the thermal and electrical characteristics were evaluated under various operating conditions. Results show that the proposed symmetrical design has 40% lower layout inductance and 80% lower temperature difference than a traditional package among the parallel capacitor branches.

Optimized Controller Gains Using Grey Wolf Algorithm for Grid Tied Solar Power Generation with Improved Dynamics and Power Quality

This study proposes a control algorithm based on synchronous reference frame theory with unit templates instead of a phase locked loop for grid-connected photovoltaic (PV) solar system, comprising solar PV panels, DC-DC converter, controller for maximum power point tracking, resistance capacitance ripple filter, insulated-gate bipolar transistor based controller, interfacing inductor, linear and nonlinear loads. The dynamic performance of the grid connected solar system depends on the effect operation of the control algorithm, comprising two proportional-integral controllers. These controllers estimate the reference solar-grid currents, which in turn generate pulses for the three-leg voltage source converter. The grey wolf optimization algorithm is used to optimize the controller gains of the proportional-integral controllers, resulting in excellent performance compared to that of existing optimization algorithms. The compensation for neutral current is provided by a star-delta transformer (non-isolated), and the proposed solar PV grid system provides zero voltage regulation and eliminates harmonics, in addition to load balancing. Maximum power extraction from the solar panel is achieved using the incremental conductance algorithm for the DC-DC converter supplying solar power to the DC bus capacitor, which in turn supplies this power to the grid with improved dynamics and quality. The solar system along with the control algorithm and controller is modeled using Simulink in Matlab 2019.

Energy Balance Control for Improving Transient Performance of DC Bus Voltage in Power Electronic Transformer for the Voltage-Sensitive Loads

The power electronic transformer (PET), as a main topology for the energy router in the energy internet, consists of the rectifiers, the dual active bridge (DAB), and the inverter, and these three parts are connected by two dc buses. So, the performance of the dc bus voltages is very important because it can totally affect the output waveforms of the dc and ac voltage, especially for the voltage-sensitive loads. Compared with the proportion integration (PI) control scheme, the energy control method utilizes the energy as the control variable, and the control strategy derived from the energy relationship, including the passive elements and all the interfaces, is more direct and explicit. In this paper, considering the energy between the dc bus capacitors and the input inductor and the load and the source in the PET topology, the energy balance control (EBC) strategy is proposed. For the two dc bus voltages, the energy balance relationship of the different time scales is used to decouple the interaction in the control scheme. The EBC strategy can obviously reduce the fluctuation and the transient time of the two dc bus voltages when the load power or voltage reference is changed. Thus, under the limited voltage fluctuation, the EBC strategy can reduce the dc bus capacitance in order to reduce the volume and weight of the converter and enhance the reliability. The simulation and experimental results verify the effectiveness of the proposed control strategy.

Partial Power Processing for Power Decoupling Network in Three-Phase Three-Leg Four-Wire Three-Level T-Type Inverter With Reduced Split DC-Bus Capacitance

The three-phase three-leg four-wire (3P3L4W) three-level (3L) inverter demands large split dc-bus capacitors for limiting the voltage ripple resulting from the neutral current, which mainly pulsates at the fundamental output frequency, and the midpoint current, which mainly pulsates at triple the output frequency. This article embeds a power decoupling network (PDN) in the basic 3P3L4W 3L inverter for reducing the split dc-bus capacitance. To curb excessive power losses, partial power processing is proposed to make the PDN partially bypass the neutral current and the midpoint current, such that the split dc-bus capacitance could be reduced at the expense of affordable power losses. Finally, a 10-kVA PDN embedded 3P3L4W 3L T-type inverter is built and tested in the laboratory, and experimental results are presented to verify the feasibility of the proposed work.

Analysis and Design of a High-Efficiency SiC MOSFET 6-Phase Boost Rectifier

In this paper, the analysis and the design of a high-efficiency power electronic conversion system for offshore wind applications are presented. This system is composed of a 6-phase AC–DC converter based on the SiC power semiconductors, to be used to control the achievable power from the wind turbine electrical generator. Thanks to the phase redundancy, the proposed boost rectifier is suitable for applications where reliability and fault tolerance capability are the main targets. To select the appropriate power semiconductor devices, voltage and current ratings have to be determined. After that, the power loss equations are derived in order to evaluate the conversion efficiency. To design the appropriate DC-bus capacitor configuration, an analytical investigation is carried out by estimating the DC-bus RMS current and the voltage ripple. Finally, the thermal sizing of the system is calculated to identify a suitable heatsink. To validate the proposed analysis, the analytical results are compared to simulation ones using the Plexim/PLECS tool in the MATLAB/Simulink environment. For further validation, a prototype of the converter is built and the experimental results are carried out. The results demonstrate that the peak efficiency of the 6-phase boost rectifier can reach 98% at 100 kHz switching frequency.

A Cascaded Controller for a Grid-Tied Photovoltaic System With Three-Phase Half-Bridge Interleaved Buck Shunt Active Power Filter: Hybrid Control Strategy and Fuzzy Logic Approach

This paper focuses on the control development of a grid-tied photovoltaic system coupled with a shunt active power filter. The considered power system includes a PV panel, a single-capacitor three half-bridge interleaved buck converter connected to a three-phase power grid and a nonlinear load. This study aims to accomplish simultaneously three objectives: i) to ensure a high compensation level of current harmonics and reactive power consumed by the nonlinear load in order to perform power factor correction at the grid side; ii) to extract the maximum power from the PV panel regardless the climatic conditions; iii) to regulate the dc bus capacitor voltage to its reference provided by the MPPT controller. To this end, a controller using a two-loop cascade strategy is designed. First, the inner loop is developed with the hybrid automaton approach to formulate the switching control signals with the aim of guaranteeing a unitary power factor. Second, the outer loop is designed using fuzzy logic control to ensure the regulation of the photovoltaic voltage, while the perturb and observe algorithm is employed to perform maximum power point tracking. Finally, the system with the developed control scheme is simulated using MATLAB/SimPowerSystems environment. The obtained results confirm that the proposed controller is able to achieve the previous objectives under varying environmental conditions.

Second Harmonic Current Reduction Schemes for DC–DC Converter in Two-Stage PFC Converters

For the two-stage single-phase power factor correction (PFC) converter, its instantaneous input power pulsates at twice the line frequency, generating the second harmonic current (SHC) at the dc-bus port. The dc–dc stage is used to regulate the output voltage or the dc bus voltage in different applications, and the SHC reduction schemes for the dc–dc stage with different control objectives are discussed in this article. When the dc–dc stage regulates the output voltage, it is pointed out that the control bandwidth of the dc–dc stage is required to be high enough and the dc bus capacitor should be large enough to suppress the SHC, and the design of the dc bus capacitor is given. When the dc–dc stage regulates the dc bus voltage, a virtual impedance is added in series with the input/output of the dc–dc stage to suppress the SHC, and a virtual impedance is added in parallel with the dc bus to improve the dynamic performance. The selection and design of the virtual impedances are also given. Based on that, three specific control schemes are proposed for the dc–dc stage to suppress the SHC, and the parameters design method is also presented. Finally, a 3.3-kW two-stage single-phase PFC converter is fabricated and tested in the lab to verify the effectiveness of the proposed SHC reduction schemes.

Multiphase Traction Inverters: State-of-the-Art Review and Future Trends

Multiphase inverters (MPIs) continue to increase in popularity owing to their compelling features that include enhanced fault-tolerance capability, improved per-phase power handling, and reduced dc-bus capacitor sizing. This article presents a comprehensive review on MPIs and their application in transportation electrification. More specifically, voltage source inverter (VSI) and nine-switch inverter (NSI) are the two MPI topologies reviewed herein, due to their popularity and potential for employment as traction inverters. The state-of-the-art review covers modeling and control techniques, dc-capacitor sizing, modulation strategies, inverter losses, and cost. Promising future trends of MPIs in terms of topologies, switching devices and integrated design are also investigated.

GaN-Based High-Power-Density AC–DC–AC Converter for Single-Phase Transformerless Online Uninterruptible Power Supply

This article introduces a new transformerless ac-dc-ac converter topology suitable for a high-power-density high-frequency single-phase online uninterruptible power supply (UPS) with a common neutral between the input and output ac ports. The proposed converter comprises an input power factor correction rectification (ac-dc) stage which is followed by an inversion (dc-ac) stage. The rectification stage operates in boost mode during the input positive half line cycle and buck-boost mode during the input negative half line cycle, while the inversion stage operates in buck and buck-boost modes in the output positive and negative half line cycles, respectively. The rectification stage utilizes boundary conduction mode control enabling soft-switching and allowing high-frequency operation. The inversion stage is operated in continuous conduction mode, wherein a digital controller regulates the output voltage of the converter across both resistive and reactive loads. The proposed online UPS utilizes a single (nonsplit) dc-bus between the rectification and inversion stages, resulting in a 50% reduction in dc-bus capacitance requirement compared to conventional split-bus online UPS topologies. Additionally, two battery interface solutions are also investigated which provide a trade-off between passive volume and additional switch-count. To verify the performance and control of the proposed online UPS, a GaN-based electrolytic-free 1-kVA prototype online UPS is designed, built, and tested. The prototype ac-dc-ac converter achieves a peak efficiency of 95.2% and maintains a high efficiency of above 92.3% across the full output power range. The electrolytic-free prototype ac-dc-ac converter for the online UPS achieves a power density of 26.4 W/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> .

A novel adaptive voltage and current remittance strategy by series and shunt multilevel converters

A new, effective pi-based control method for a neutral 3-level inverter with a unified series and shunt converter is provided in this work. The proposed series of shunting and 3-levels can restrict harmonic current source and combat interruptions of voltage such as voltage drops, swells, imbalances and harmonics. With serial integration and shared DC bus capacitors, it is constructed with Active filters (AFs). With the proportional integrated voltage controller, the DC voltage is continually maintained. The Synchronous SRF theory is utilized to gather APF reference signals and power response theory (p-q theory). For the switching signal the shunt and series APF signal generated by the control algorithm and the observed signal was supplied into two controllers. In the management of the APF Series, PI approaches are employed to enhance capacities and shift 3-tier conversion devices. The power, power and voltage harmonics and any correction for any other voltage disturbance are assessed in the proposed 3 level range and shunt converter with the Matlab-Simulink Software and SimPower System Tool Box. The results of the simulation show the performance of the three-story series & shunt converter's non-linear load for improving energy quality at the sharing point.

A Complete Impedance Model of a PMSG-Based Wind Energy Conversion System and Its Effect On the Stability Analysis of MMC-HVDC Connected Offshore Wind Farms

Impedance modeling of a permanent magnet synchronous generator (PMSG)-based wind energy conversion system (WECS) usually ignores the machine-side dynamics, instead, the machine-side system is simplified as a constant power source, while merely keeping the detailed grid-side converter model. This simplification is mainly based upon the intuition that the dc-bus capacitor is large enough so that the machine- and grid-side dynamics can be decoupled. However, the simplification probably does not hold in practice since there is always a tendency to minimize the capacitor size to achieve a smaller package for installation. Therefore, the machine-side system should not be ignored, otherwise maybe leading to error for stability analysis. To address this issue, this paper developed a complete ac-side impedance model of the PMSG-based WECS. Then, impact factors about the coupling characteristics between machine- and grid-side systems of the PMSG-based WECS are discussed. Finally, a case study of sub/super synchronous oscillation is carried out on a practical offshore wind farm integrated with a modular multilevel converter based high-voltage dc (MMC-HVDC) transmission system in China. The results show that ignoring the machine-side system would lead to an optimistic stability judgement, which confirms the necessity of the complete model of PMSG-based WECS. Furthermore, the system can be stabilized by either changing the couplings between machine- and grid-side systems of PMSG-based WECS or implementing active damping in the wind farm side MMC.

Advanced Switching Sequences Based Model-Predictive Control for Single-Phase NPC Converters

This article presents a continuous control set model-predictive control (MPC) technique based on advanced switching sequences (SSs) for a grid-connected single-phase three-level neutral-point-clamped converter. The SSs are developed considering the tracking performance of the current feed to the utility grid, the dc-bus capacitor voltage balancing, and the switching loss of the semiconductor devices. In the proposed strategy, with the same sampling frequency, the number of pole state changes (SCs) per control cycle is only two, while in the optimal SS and the modulated MPC (pulse pattern similar to the conventional space vector modulation technique) schemes, the number of pole SCs is four. The reduction in the number switching SCs per control cycle considerably reduces the switching loss of semiconductor devices. Moreover, to avoid the challenges of utilizing the weighting factor, a single-objective cost function is adopted to track the reference current, and the dc-bus capacitor voltages are balanced using the redundancy nature of the voltage vectors. Furthermore, the proposed scheme can ensure a fixed-switching-frequency operation, which is advantageous for selecting filter parameters during real-life implementation. The effectiveness of the proposed strategy is validated by the results obtained from both the simulation in MATLAB/Simulink and the experiment carried out using a downscaled laboratory test rig.

Evaluation and Analysis of Soft Computing Techniques for Grid Connected Photo Voltaic System to Enhance Power Quality Issues

The major problem in grid connected transmission and distribution networks are the power quality issues due to the unbalance loads. The power quality issues relate to the current and voltage based issues generated at the point of common coupling. Some of the current based power quality issues relate to reactive power extraction, poor zero voltage regulation, harmonics in the load currents undesired neutral current and power factor and. Several DSTATCOM compensation techniques has been introduced for compensation of reactive power and power quality issues in the three phase, four wire system. The proposed system focus on DSTATCOM soft computing techniques such as: Widrow–Hoff least mean square, vectorial filter, discrete adaptive filter and quasi Newton control algorithms. The proposed algorithms controlled the switches pulses to drive the voltage source converter section in the DSTATCOM network thereby mitigating power quality issues and reactive power generation in the grid connected photovoltaic system. The soft computing techniques has maintained constant DC bus capacitor voltage and common coupling voltage along with unity power factor, zero voltage regulation under linear and non-linear load conditions, eliminating the harmonics in the load currents. The proportional integral controller is used for error correction between the reference and capacitor voltage. The novel drift-free maximum power point technique has shown better results under varying temperature and irradiances conditions. The DSTATCOM soft computing techniques has been implemented and simulated in MATLAB/SIMULINK.

A flexible virtual inertial control algorithm for ship with propulsion load and pulse load

The pulse load of an integrated power system (IPS) ship can produce very high power intermittently in a short period of time. Owing to the limited DC bus capacitance, the periodic pulse load charging and discharging process can make the IPS system state variables have a periodic alternating process during electromagnetic transients, especially at the bus voltage. In order to alleviate this problem, it is necessary to find a suitable control strategy. At first, this article establishes a mathematical model of DC bus voltage dynamics and propulsion motor load. Then, a flexible energy scheduling algorithm is proposed to coordinate the propulsion motor load and the pulse load based on the flexibility of IPS energy scheduling and the concept of virtual inertia. This algorithm can adaptively change the virtual inertia of the DC bus according to the pulse load power level and the propulsion motor load level to mitigate the impact of the pulse load on the IPS system. At the same time, the algorithm is also distributed and has little impact on the propulsion system. Finally, the effectiveness of the algorithm is verified through the mixed simulation of PSCAD and MATLAB and the hardware in the loop test.