Variable DC Link Voltage Research Articles

Novel Electric Vehicle Traction Architecture With 48 V Battery and Multi-Input, High Conversion Ratio Converter for High and Variable DC-Link Voltage

Novel Electric Vehicle Traction Architecture With 48 V Battery and Multi-Input, High Conversion Ratio Converter for High and Variable DC-Link Voltage

Hybrid Modular Multilevel Converter Design and Control for Variable Speed Pumped Hydro Storage Plants

Hybrid modular multilevel converter control and design for pumped hydro storage plants is presented, addressing application-specific shortcoming of conventional back-to-back half-bridge modular multilevel converter: high common-mode-voltage machine stress. Common-mode-voltage-free operation in the entire or partial frequency range is enabled by variable DC link voltage control, through introduction of minimal full-bridge submodule share in hybrid active front-end converter stage. The developed generalized converter design approach for arbitrary DC link voltage range operation and additional internal energy balancing control layers enable down-to-zero DC voltage control. The results are verified through high-fidelity switched-model simulations of 6 kV converter, with 10/6 ratio of full-bridge to half-bridge submodules in active front-end. The analyzed hybrid active front-end stage benefits from lower converter losses for equal machine operation flexibility compared to full-bridge design, while trade-off between grid-side power factor range and full-bridge submodule share is offered within the design stage. Compared to the state-of-the-art, zero-to-rated DC link voltage operation is possible at lower full-bridge submodule share (62% against 75 %), at the penalty of reduced grid-side power factor. Alternatively, operation at higher full-bridge submodule share (62% against 50% existing solution) enables grid-side reactive power support over wide speed range, without branch current overload.

Coupled Current Tracking Capability and Stability Analyses of 3Φ3W LCL Converter With Decoupled Control and Variable Filter Inductances

In a three-phase three-wire ( 3Φ3W) converter, the average common-mode voltage between the neutral point of its ac source and the dc-source positive or negative rail over one switching cycle can be proved to be an arbitrary value. Therefore, 3Φ3W LCL (a filter that is composed of inductors, capacitors, and inductors) converters can be decoupled and controlled phase by phase in a natural frame, which results in equivalent three single-phase controls. With the single-phase control, inductance, dc-link voltage, and grid-voltage variations can be taken into account readily, yielding low-total harmonic distortion (THD) currents under distorted and unbalanced grid voltages. However, the 3Φ operations are still coupled each other and, moreover, the 3Φ filter inductances are varying with their currents simultaneously. This article presents system modeling of the decoupled digital controlled LCL converter from which coupled current tracking capability and stability of the system are investigated. Comparison between coupled and decoupled current tracking capability and stability has been verified with pole-zero plot and step response. The tracking performances of a 10 kVA with two types of feedback signals are compared and discussed. Moreover, simulated and experimental results of a 20-kVA 3Φ3W LCL grid-connected converter with wide filter inductance variation and distorted grid voltages have verified the analyses and discussions.

Meeting IEC and ER G4/5 harmonic standards in nine‐level cascaded H‐bridge inverters using an improved SHM‐PAM scheme

This study introduces a new switching scheme for nine-level cascaded H-bridge (CHB) inverter to comply with the harmonic standards (IEC 61000-2-12, IEC 61000-3-6, and ER G4/5) using an improved selective harmonic minimisation pulse amplitude modulation (SHM-PAM) scheme. In this scheme, the optimised switching instances and variable DC-link voltages are determined by solving some new constraints based cost functions using particle swarm optimisation (PSO) technique. The theoretical analysis and optimisation results of the proposed modulation scheme are validated through MATLAB simulations and experimentally on a laboratory-scale prototype of CHB inverter. The proposed modulation scheme utilises a minimal number of switching instances in a fundamental period and optimisation variables in the problem formulation in comparison to conventional SHE-PWM and SHE-PAM schemes. The key performance of the proposed scheme in terms of harmonic and loss analysis is evaluated over the wide range of the power factors and modulation index. Finally, the suitability of the proposed scheme is tested for the closed-loop speed control of a 5 hp, 415 V three-phase induction motor.

Computation-Efficient Online Optimal Tracking Method for Permanent Magnet Synchronous Machine Drives for MTPA and Flux-Weakening Operations

In this article, a novel online optimal tracking method is proposed for permanent magnet synchronous machine drives for maximum torque per ampere (MTPA) and flux-weakening (FW) operations. The conventional model-based optimal tracking methods impose heavy computational burden, because the optimality criteria are usually used along with related boundaries to develop different optimization problems for different operation regions. Compared with the conventional methods, the proposed method reduces the mathematical complexity of the optimization problem and improves the computational efficiency in real-time implementation. Only one optimization problem is solved in both MTPA and FW regions, while the enhanced projection operations are developed to ensure the current references are within the voltage and current constraints. Magnetic saturation, phase winding resistance, nonlinearity of the inverter, and dc-link voltage variation are all taken into consideration. The feasibility of the proposed online optimal tracking method has been validated on an interior permanent magnet synchronous machine (IPMSM) test bench with an off-the-shelf motor control unit. The execution time and convergence rate of the proposed method have also been evaluated with benchmark conventional methods and presented in this article.

A Novel DC-DC Converter Based Closed Loop Control of BLDC Motor for SPV fed Water Pumping System

This Paper Presents Novel DC-DC Converter Based Closed Loop Control of BLDC Motor for SPV fed Water Pumping System Solar Photovoltaic (SPV) Array fed Water pumping System Utilizing Buck-boost DC-DC Converter in order to extract the maximum available power from Solar system. Solar energy has the greatest availability compared to other energy sources. For such solar PV systems, maximum power point tracking control is preferred for efficient operation. This concept is dealing with INC method which is one of the MPPT methods. This study deals with a buck–boost converter controlled solar photovoltaic (SPV) array fed water pumping in order to achieve the maximum efficiency of an SPV array and the soft starting of a permanent magnet brushless DC (BLDC) motor. The current sensors normally used for speed control of BLDC motor are completely eliminated. The speed of BLDC motor is controlled through the variable DC-link voltage of a voltage-source inverter (VSI). The VSI is operated by fundamental frequency switching, avoiding the losses due to high-frequency switching, in order to enhance the efficiency of the proposed system.

Mains Current Distortion Suppression for Third-Harmonic Injection Two-Stage Matrix Converter

The third-harmonic injection two-stage matrix converter (3TSMC) not only possesses the advantages of a TSMC, but also enhances the input reactive power control capability. Due to the input current deviation caused by the voltage ripple across the input filter capacitors, the mains currents of the 3TSMC are distorted at sector boundaries. To improve mains current quality, this article proposes a mains current distortion suppression method for a 3TSMC, which adds a prefilter to the output side of the input voltage selector to reduce the input current harmonics. As a result, the input filter capacitor voltage ripple and the input current deviation are reduced. The details of design of the prefilter are explained mathematically. To compensate for the dc-link voltage variation after topology improvement, an algorithm based on calculation of the voltage drops across the prefilter inductors is proposed, which can further improve the mains current performance. The feasibility of the proposed method is verified by simulation and experimental results.

Enhanced Frequency Control Method for Microgrid-Connected Flywheel Energy Storage System

Flywheel energy storage system (FESS) can be used for frequency regulation in microgrids. In this article, an enhanced frequency control system is presented for FESS to reduce the frequency variations of microgrid. A three-layer control system is proposed for machine-side converter of the FESS including dc-link voltage controller, speed controller, and field-oriented control system. Unlike the conventional control methods, in the proposed method, the frequency controller is implemented in the grid-side converter to increase the ramp rate of the FESS and reduce the frequency deviation against load changes. Also, the proposed three-layer control system of the machine-side converter reduces the dc-link voltage variations. Furthermore, a mode coordination system is proposed to reduce the dc-link voltage variations when the operation mode of FESS changes from flywheel speed control mode to the microgrid frequency control mode. To demonstrate the advantageous of the proposed method in comparison to the conventional method, at first, the linearized model of the FESS in microgrid is extracted, and then the small-signal analysis is carried out for optimal design of proportional integral (PI) controllers. Two control strategies are theoretically presented and experimentally compared in a prototype microgrid including a diesel generator emulator and a load emulator. The frequency control system is studied in different loading conditions. Experimental results verify the theoretical concepts proposed in this article, and show the superiority of the proposed method over the conventional method.

Improved SHM‐PAM‐based five‐level CHB inverter to fulfil NRS 048‐2:2003 grid code and to apply as shunt active power filter with tuned proportional‐resonant controller for improving power quality

This study proposes a selective harmonic minimisation-pulse amplitude modulation (SHM-PAM) technique utilising least number of switching based on optimised waveform pattern for five-level cascaded H-bridge (CHB) inverter to satisfy the NRS 048-2:2003 grid code standard. Here, particle swarm optimisation method is used to evaluate the solution of the optimised switching angles and variable DC-link voltages. A comparison between the proposed SHM-PAM and conventional selective harmonic elimination-pulse width modulation (SHE-PWM) techniques has been carried out by harmonic performance analysis (total harmonic distortion (THD), weighted current THD and harmonic loss factor) and loss analysis (conduction, switching and their cumulative losses) of the switches of five-level CHB inverter under different load power factor angles for medium-voltage application. The performances of the conventional SHE-PWM and proposed SHM-PAM techniques are examined through experimentation using a three-phase five-level CHB inverter. Finally, the proposed technique based 5-level CHB inverter is applied to shunt active power filter using tuned proportional plus parallel resonant current controller for improvement in power quality under non-ideal grid conditions.

Low Cost Simple Look-Up Table-Based PMSM Drive Considering DC-Link Voltage Variation

This paper proposes a permanent magnet synchronous machine (PMSM) table-based torque control method considering a variable DC-link voltage that can be used in a low cost DSP. The current reference generation based on two-dimensional look-up table (2D-LUT) is widely used for PMSM drives used in industrial applications because of its torque control performance and operation stability. In general, a 2D-LUT is established based on the flux and torque to overcome the variation in DC-link voltage. However, this method requires a flux estimator for estimating the instantaneous flux, which is defined as a division of the operating speed used to obtain the flux data. Therefore, to obtain the operating flux, a variable division calculation or complex controller is used, which can be difficult to process through a low cost digital signal processor. In this paper, a novel look-up table-based control method that uses the newly established speed-torque 2D-LUT is proposed. This 2D-LUT inherently implements data on the d-/q-axis currents throughout the operating regions, not only the speed and torque, but also the DC-link voltage variation. The proposed method was verified through an experiment on the torque control a variation in the DC-link voltage.

Variable Structure Back-Stepping Control of Two-stage Three Phase Grid Connected PV Inverter

This work presents a detailed analysis of a three phase grid tied photovoltaic inverter with variable structure back-stepping control approach. A nonlinear model of the system is derived and presented in rotational frame using the direct quadrature zero transformation (DQ). For the derivation of active and reactive power loops of the inverter, nonlinear back stepping approach is used. Moreover, sliding mode control method is used to derive the inner current loops while for the outer loop a virtual controller is derived using the Lyapunov function. The control loops are implemented in MATLAB/Simulink environment. To test the controller performance, active power variation, DC link voltage variation and reactive power variations are inflicted. The obtained results under the proposed control scheme are compared with boundary layer design based sliding mode controller. From the comparative analysis it is concluded that the proposed controller exhibit superior and robust performance under all test conditions.

Capacitor-Energy-Based Control of Doubly Salient Brushless DC Generator for Dynamic Performance Optimization

A DC-link voltage regulation method is proposed based on capacitor energy control (CEC) for the doubly salient electro-magnetic generator (DSEG) system. Instead of tracking the DC-link voltage, the stored energy in the DC-link capacitor is set as the control target in the outer loop. A proportional component that converts the command output current into the command field current is developed between the capacitor energy loop and field current loop. The coefficient in the proportional component is investigated based on the mathematical model of the 12-8 pole DSEG, considering the load and speed variation. The dynamic response of the DSEG system is accelerated and the DC-link voltage variation is reduced with the proposed method. Furthermore, the adaptability to speed variation of the controlled is optimized. Since the DC-link capacitance is an additional parameter, the influences of capacitor degradation are investigated. The conventional voltage control and the CEC are implemented and compared on a DSEG platform. The optimized performance of the proposed method is verified by both finite element analysis (FEA) and experiments. The proposed method provides optimized dynamic performance over the entire speed range, enabling the DSEG system as a competitive option in the aircraft application.

Modified rotor-side converter control design for improving the LVRT capability of a DFIG-based WECS

The rapid integration of wind-power generation with existing power grids has caused reliability and stability concerns owing to the negative impact on the dynamic behaviors of power systems. During a fault, large electromotive force is induced in the rotor circuit of a doubly fed induction generator (DFIG) as the circuit is highly vulnerable to it. Such circumstances increase the importance of the low-voltage ride-through (LVRT) capability of a DFIG to ensure stability of the electric grid during transient conditions. Considering these factors, this study focuses on the mitigation of rotor overcurrents and DC-link voltage variations by modifying the control structure of the DFIG converter, thereby enhancing its LVRT capability. Additional voltage terms are injected into the rotor-voltage references to improve the transient behavior of the DFIG control system. In the proposed design, transient rotor currents and DC-link voltage variations are effectively suppressed. Because the voltage terms are introduced outside the current loops, there is no impact on their stability. Furthermore, electromagnetic torque oscillations during the faults are considerably suppressed. Finally, the validity of the proposed design during abnormal grid conditions is demonstrated via MATLAB/Simulink. The results confirm the feasibility and effectiveness of the improved converter control design to enhance the LVRT capability of a DFIG.

Development of PCS to utilize differential pressure energy in district heating systems with reduced DC-link voltage variation

This paper proposes a power conversion system (PCS) to utilize the differential pressure (DP) energy in district heating systems with reduced DC-link voltage variation. A differential pressure control valve (DPCV) is usually used to regulate pressure for heat transmission to remote areas in district heating systems. However, there is a high probability of malfunctions and failures of the DPCV, due to cavitation, since high-pressure fluids are used in district heating systems. Therefore, a PCS for utilizing DP energy in the district heating system replacing the DPCV is proposed in this paper. In addition, the DC-link voltage of the PCS greatly fluctuates when a sudden change occurs in a hydraulic turbine. It is effectively relieved by using the feed-forward control method. The effectiveness of the PCS for utilizing the DP energy in district heating systems with reduced DC-link voltage variation is verified by simulation and experimental results.

Design of a robust controller for DC/DC converter–electrolyzer systems supplied by [formula omitted]WECSs subject to highly fluctuating wind speed

A buck-based, isolated, high-voltage-ratio DC/DC converter that allows supplying a proton exchange membrane (PEM) electrolyzer from a micro-wind energy conversion system (μWECS) has been recently presented. It exhibits low ripple at the switching frequency on the output voltage and current and represents an attractive solution for low-cost hydrogen production. In this paper, a more accurate mathematical model of such a converter is derived and discussed. Then, a model-based robust controller is designed in the frequency domain using the Internal Model Control structure and in the context of H2∕H∞ optimal control. The controller satisfies the condition of robust stability and behavior, i.e., it guarantees stability and the desired behavior in the presence of parametric variations and unmodeled dynamics. In particular, the robustness in the presence of variations of DC-link voltage and buck input inductance is verified from the theoretical point of view. The validation of the controller is performed by integrating it into a detailed switching model of the DC/DC converter, which is implemented on a widely used circuit-oriented simulator. Good results are obtained in terms of dynamic and steady-state behavior, even in the presence of the above variations. A comparison is also carried out with the results obtained using an integral controller designed on the basis of the above mathematical model. Such a comparison shows the superiority of the robust controller over the integral controller in all the operating conditions, especially when the DC-link voltage is subject to significant variations and is affected by a non-negligible low-frequency ripple due to the presence of the diode rectifier at the output of the μWECS.

Analysis and design of single‐stage, two‐mode AC/DC converters for on‐board battery charging applications

On-board battery chargers (OBCs) offer the feasibility of charging electric vehicles or plug-in hybrid electric vehicles with a standard household power supply. In this study, an OBC using two-mode topology (step-up or step-down) is presented. One of the main concern with two-mode topologies is the hard transitions of the converter operating from boost mode to buck mode or vice versa, which can deteriorate the performance of converter. This study provides smooth transitions between the modes and can have an output that could be more or less than the peak of input voltage. The detailed analysis of operating modes involved in the converter are analysed in the study. Moreover, the stress analysis on the components is also detailed in this study and the results are compared with existing two-mode converters. A simple dual-loop control structure with the proposed modulation technique eliminates the sub-harmonic oscillations and stabilizes the output voltage is also presented in the study. Experimental validation is performed for a 1.0 kW laboratory prototype and the results obtained are detailed in the study. This proposed converter topology can be applied as a single-stage OBC or as a power factor correction converter with variable DC link voltages in isolated two-stage OBC.

Improved Online Optimization-Based Optimal Tracking Control Method for Induction Motor Drives

An online optimization-based optimal tracking control method to control IM drives in the entire speed and torque ranges is proposed in this article. The main feature of the proposed method is considering the nonlinear core saturation effects in generating the optimal reference currents in MTPA, field-weakening, and maximum torque per voltage (MTPV) regions. Constrained optimization problems are formulated and solved using Lagrange multipliers method to derive improved nonlinear MTPA and MTPV optimality conditions. Unlike most of the existing loss minimization methods in the literature, using optimal reference point look-up tables is avoided in the proposed method. Moreover, the effects of stator resistive drops, dc-link voltage variations, and inverter nonlinear voltage drop, which are usually neglected in the literature, are considered in generating optimal reference currents. A new straightforward and well-structured optimal reference point calculation scheme is proposed by dividing the operating plane of IM into four regions. The optimal reference currents in each region are then generated by solving the corresponding optimization problem. The smooth operation and transition of the proposed control method between MTPA and field-weakening regions are verified using simulations. The effectiveness of the proposed method is experimentally validated on a 4.7-kW, 4-pole, three-phase induction motor drive.

Online Optimal Tracking Method for Interior Permanent Magnet Machines With Improved MTPA and MTPV in Whole Speed and Torque Ranges

This article proposes a new online optimal tracking method with improved maximum torque per ampere (MTPA) and maximum torque per volt (MTPV) considering magnetic core saturation. The conventional optimality criteria for MTPA and MTPV do not consider magnetic core saturation, and therefore results in deviation from the optimal trajectory especially at higher load currents. This article establishes a mathematical formulation to consider magnetic core saturation in both MTPA and MTPV criteria and proposes corresponding new optimal conditions. The appropriate optimality conditions along with associated constraints form an optimization problem corresponding to an operating region. A simple and straightforward logic sequence proposed in this article identifies the operating regions and initiates corresponding online optimization to find the optimal reference currents at every sampling interval. Moreover, the proposed method ensures that the reference currents are always attainable by considering the inverter nonlinearity, winding resistance, and variation in dc-link voltage into the optimization problem. The proposed online optimal tracking method is successfully implemented in an off-the-shelf motor control unit, and the performances are experimentally validated for wide operating regions.

Design and Development of Standalone Solar Photovoltaic Battery System with Adaptive Sliding Mode Controller

In this paper, a solar photovoltaic-battery located standalone system is proposed. The configuration of bidirectional buck-boost converter has been proposed for charging (buck) and discharging (boost) the battery storage system. Due to solar energy sources irregular nature and variations in load demand, DC micro grids mostly depend on control methods to overcome with DC link voltage variations and power stability. Adaptive sliding mode controller with washout filter is applied due to its good strength for unexpected change of energy resources and loads. This system includes a solar photovoltaic array basis on maximum power tracking from PV, boost converter to maximize power from the PV. A bidirectional DC-DC buck-boost converter is proposed being charging battery in buck mode and discharging battery in boost mode. Incremental conductance implemented to obtain maximum power from photovoltaic array. In this paper, study of comparison between proportional-plus integral method and sliding mode control method with washout filter has been also simulated in the MATLAB using Simulink and the Sim Power System toolbox.

Analysis and Control of Current Ripples of Z-Source Inverters

Load current ripple is an important issue in the design and control of inverters since it influences the torque ripple and noise amplitude of a motor drive. The current ripple of Z-source inverter (ZSI) is analyzed in this paper. The expression of current ripple is derived and the current ripples of ZSI and voltage-source inverter (VSI) are compared. A variable DC-link voltage and switching frequency method was adopted to reduce conduction and switching losses without increasing the predicted peak current ripple. These theoretical findings were further verified by simulations and experiments.