Direct Current Converter Research Articles

Proposing Wavelet-Based Low-Pass Filter and Input Filter to Improve Transient Response of Grid-Connected Photovoltaic Systems

Available photovoltaic (PV) systems show a prolonged transient response, when integrated into the power grid via active filters. On one hand, the conventional low-pass filter, employed within the integrated PV system, works with a large delay, particularly in the presence of system’s low-order harmonics. On the other hand, the switching of the DC (direct current)–DC converters within PV units also prolongs the transient response of an integrated system, injecting harmonics and distortion through the PV-end current. This paper initially develops a wavelet-based low-pass filter to improve the transient response of the interconnected PV systems to grid lines. Further, a damped input filter is proposed within the PV system to address the raised converter’s switching issue. Finally, Matlab/Simulink simulations validate the effectiveness of the proposed wavelet-based low-pass filter and damped input filter within an integrated PV system.

A new topology for bidirectional multi-input multi-output buck direct current-direct current converter

In this paper, a new topology for bidirectional multi-input multi-output buck direct current–direct current converter is proposed. For the proposed converter, the number of inputs and outputs is arbitrary and independent from each other. Moreover, the number of components of the proposed structure is less than the conventional type of the bidirectional multi-input multi-output buck direct current–direct current converter. The proposed converter can be used in applications that require separate voltage levels with bidirectional current path. The possibility of using different energy sources with different voltage-current characteristics, lower cost, continuous input current, and transformerless output are the major advantages of the converter. In this paper, the different operating modes of the proposed converter are explained. The performance and correctness operation of the proposed converter are validated by the simulation and experimental results.

Wireless power transmission using a resonant coil consisting of conductive yarn for wearable devices

This paper presents a wireless power transmission method that uses magnetic resonant coupling with resonant coils made of conductive yarn that can be integrated into clothing. The conductive yarn consisted of silver-plated copper and polyester filaments. The transmission characteristics of the yarn and copper resonant coils were compared using simulation and measurement tools. It was determined that the signal loss of the conductive yarn resonant coils was higher than that of the copper coils. However, the increase in the signal loss variation was less than 1 dB when the transmission distance was less than 6.5 cm. The conductive yarn resonant coils were placed in the shoes and trouser cuffs of a human subject, and the transmission efficiency was measured after the received alternating current power was converted to direct current power using an alternating current–direct current converter. The measurement results showed a maximum transmission efficiency of 50%; the average transmission efficiency was 45%, despite the change in the coil shape and alignment conditions due to body movement. In addition, the results demonstrated an excellent transmission efficiency of over 40% when the subject was moving at a speed of 6 km/h. It was also shown that the textile coils can be an integrative solution, overcoming clothing comfort and continuous power supply for a new generation of wearable devices.

Sizing standalone battery charging systems based on Photovoltaic (PVTCP) Temperature Crossing Points using Voltage Source Photovoltaic Model (VSPVM)

Rural and semi-rural communities in third world countries harness solar energy mostly by using standalone Photovoltaic (PV) battery charging systems. Basic electronics circuits that do not include direct current to direct current (DC-DC) voltage converters are employed. These provide raw voltage levels from the solar PV modules that sometimes charge batteries insufficiently, leading to shorter battery lives. By modelling the solar PV module using a voltage source circuit representation, the effects of temperature on the PV module voltage could easily be illustrated to these rudimentary trained communities that deal mostly with voltage sources and not current sources. A Voltage Source PV Model (VSPVM) was developed from the well understood PV cell mathematical model. Microsoft Excel (MSE) was used as the data fitting environment and the PSpice environment was used to capture the electronic circuit topology proposed for the VSPVM. Validating the model against experimental data fitted maximum power points within 5% of the experimental data. Observations made on I-V characteristics plotted on the same graph showed interesting patterns of crossing points referred to here as Photovoltaic Temperature Crossing Points (PVTCP). A low temperature cluster and a high temperature cluster which were indicative of thresholds of some sort were observed. For hot climate regions, the power point voltage which exists between the two clusters could be considered as a guide to the possible range within which a PV battery charging system should operation.

Calculation method of winding eddy‐current losses for high‐voltage direct current converter transformers

In this paper, a correction method is introduced to make a more accurate calculation of winding eddy-current losses of high-voltage direct current (HVDC) converter transformer. The assumption that winding eddy-current losses of HVDC converter transformers follow the frequency with the exponent 2 proposed by IEC 61378-2 Std. is thought to be inappropriate and the skin effect should be taken into consideration. The frequency characteristic of winding eddy-current losses is proposed by analysing the distribution of magnetic flux in conductors, in which skin effect is taken into consideration. The correction method with a correction factor is then introduced to correct the assumption. To verify the validity of the correction method, a two-dimension axisymmetric finite element analysis of a HVDC converter transformer is conducted. The geometrical complexity of windings and electrical connection are fully taken into account, it is shown that the correction method shows a significant advantage. Influence of different methods is revealed by applying the actual non-sinusoidal load current and it is shown that the error of correction method is the least. At last, harmonic losses measurements are conducted on three coils to investigate the limits of the correction method.

Failure analysis and maintenance of a surge capacitor on the neutral bus in a ±500 kV HVDC converter station

This study analyses a surge capacitor failure on the neutral bus in a ±500 kV high voltage direct current (HVDC) converter station. Both overvoltage and insulation of the surge capacitor are investigated through simulation, practical record data analysis, and experimentation. A highly accurate HVDC model is established on the basis of the topological structure and parameters of the HVDC project. Using this model, the failure process is simulated by combining neutral bus overvoltage record analysis and voltage divider performance test. Results indicate that the 246 kV uncharacteristically high overvoltage on the neutral bus is a measurement error caused by voltage divider fault. From the field test, difference equation of the voltage divider is derived to reconstruct the actual overvoltage, the amplitude of which is 73 kV. The amplitude is significantly lower than that of the switching impulse withstand level of the surge capacitor, which should not cause capacitor failure. Therefore, the primary cause of capacitor failure is insulation damage rather than overvoltage. Finally, maintenance measures are proposed and applied in the station.

Influences of temperature on partial discharge behavior in oil-paper bounded gas cavity under pulsating DC voltage

The oil-paper insulation of valve-side windings in a high voltage direct current (HVDC) converter transformer is subjected to both complicated voltage and severe thermal stresses. Partial discharge (PD) occurring in oil-paper bounded cavities is influenced by voltage stress type as well as temperature. This paper presents PD behaviors in an oil-paper bounded gas cavity under pulsating DC voltages. The voltage across the gas cavity and PD inception voltages at different test conditions are analyzed. Influences of temperature on statistical PD distributions and parameters are also presented. A numerical model is used to interpret PD behaviors under pulsating DC voltages and various temperatures. Experimental results and theoretical statements show that the temperature exhibits a great influence on the behaviors of cavity discharge under pulsating DC voltages. Volume resistivity, gas pressure, charge decay time constant, and electron generation rate are the primary parameters influencing PD behaviors at different temperatures.

Development of a Novel Bidirectional DC/DC Converter Topology with High Voltage Conversion Ratio for Electric Vehicles and DC-Microgrids

The main objective of this paper was to study a bidirectional direct current to direct current converter (BDC) topology with a high voltage conversion ratio for electric vehicle (EV) batteries connected to a dc-microgrid system. In this study, an unregulated level converter (ULC) cascaded with a two-phase interleaved buck-boost charge-pump converter (IBCPC) is introduced to achieve a high conversion ratio with a simpler control circuit. In discharge state, the topology acts as a two-stage voltage-doubler boost converter to achieve high step-up conversion ratio (48 V to 385 V). In charge state, the converter acts as two cascaded voltage-divider buck converters to achieve high voltage step-down conversion ratio (385 V to 48 V). The features, operation principles, steady-state analysis, simulation and experimental results are made to verify the performance of the studied novel BDC. Finally, a 500 W rating prototype system is constructed for verifying the validity of the operation principle. Experimental results show that highest efficiencies of 96% and 95% can be achieved, respectively, in charge and discharge states.

Predictive direct power control of multilevel direct current link converter for grid connected battery energy storage systems

This paper proposes the adoption of a multilevel dc link converter for a grid connected battery energy storage system in low voltage applications. The multilevel dc link converter is the simple upgrade of a typical two level voltage source converter with the incorporation of some cascaded modular chopper cells which constitute a switched-battery circuit. With the additional chopper cells, more candidates of voltage vectors are available for switching vector selection in power control. Compared to two level voltage source converter using a look-up-table direct power control or finite control set model predictive direct power control, the multilevel dc link converter which employed the latter power control tracks the reference power with much lower power ripple, as well as results in much lower total harmonic distortion in grid current. Therefore, the power quality can be improved significantly. Simulation results are presented to verify that a 3 level dc link converter is more effective than a two level voltage source converter for the grid-connected battery energy storage systems in both generating mode and charging mode.

Rationalisation and validation of dc power transfer limits for voltage sourced converter based high voltage DC transmission

Maximum available power (MAP) is an index for analyzing power-voltage stability in high-voltage direct current (HVdc) converters. Earlier work on calculating MAP for voltage sourced converters (VSC) was not consistent with the methods used for line commutated converters (LCC). The Thevenin voltage source used in VSC-HVdc analysis was set to 1.0 pu, but for LCC-HVdc, it was calculated so as to provide rated conditions at the converter busbar. This study attempts to rationalize the approaches used for VSC-HVdc and LCC-HVdc calculations by making them mutually consistent. However, as the VSC can operate at much lower short-circuit ratio (SCR) than can the LCC, the Thevenin equivalent voltage at low SCRs can be significantly larger than 1.0 pu. This would require an unreasonably large tap changer setting. As an alternative, the paper recommends that for calculating MAP for a VSC, an ac network equivalent be used where some reactive power is locally supplied by shunt capacitors. This renders a Thevenin voltage close to 1.0 pu. This study also considers the impact of the VSC’s internal voltage limits on the MAP. The key results are validated using electromagnetic transients simulation.

A single‐stage LED streetlight driver with PFC and digital PWM dimming capability

SummaryThis paper proposes a single‐stage light‐emitting diode (LED) driver that offers power‐factor correction and digital pulse–width modulation (PWM) dimming capability for streetlight applications. The presented LED streetlight driver integrates an alternating current–direct current (AC–DC) converter with coupled inductors and a half‐bridge‐type LLC DC–DC resonant converter into a single‐stage circuit topology. The sub‐circuit of the AC–DC converter with coupled inductors is designed to be operated in discontinuous‐conduction mode for achieving input‐current shaping. Zero‐voltage switching of two active power switches and zero‐current switching of two output‐rectifier diodes in the presented LED driver decrease the switching losses; thus, the circuit efficiency is increased. A prototype driver for powering a 144‐W‐rated LED streetlight module with input utility‐line voltages ranging from 100 to 120 V is implemented and tested. The proposed streetlight driver features cost‐effectiveness, high circuit efficiency, high power factor, low levels of input‐current harmonics, and a digital PWM dimming capability ranging from 20% to 100% output rated LED power, which is fulfilled by a micro‐controller. Satisfying experimental results, including dimming tests, verify the feasibility of the proposed LED streetlight driver. Copyright © 2016 John Wiley & Sons, Ltd.

Valve saturable reactor iron losses of ultrahigh‐voltage direct current converter

Thyristor-based line-commutated converters (LCCs) are widely used to achieve AC/DC and DC/AC power conversions in UHVDCs (ultrahigh-voltage direct currents) transmission. Valve saturable reactors are used to protect power thyristors. To improve the efficiency of bulk electricity transmission over long distance, the operating voltage and current of UHVDCs and therefore the thermal stresses applied to the valve saturable reactors have steadily increased over the last decades. Valve saturable reactor iron losses were studied by accounting for the reverse recovery characteristics of the valve thyristors and the non-linear characteristics of the saturable reactors. The impact of UHVDC current on valve saturable reactor iron losses was quantified and verified. The valve saturable reactor iron losses of an LCC are generated not only at the turn-on and turn-off periods of the valve, but also at the off-state of the valve. The amount of the losses are affected by the AC busbar stray capacitances of the converter, the valve firing and extinction voltages, the rates of change of valve commutation current, and the reverse recovery current of the valve thyristors. The conclusion obtained will help to understand better the forming mechanism of valve saturable reactor iron losses and optimise the valve saturable reactor design.

One Cycle Control Based Maximum Power Point Tracker Applied in Photovoltaic Systems

The utilization of solar energy has grown in the last decades, and due to the scarcity of fossil fuels and the awareness of the increasing greenhouse gases in the atmosphere, this energy tends to become predominant. For generating photovoltaic energy, photovoltaic modules are connected to converters to define the amount of energy provided to a load. A maximum power point tracking method is used to maximize the photovoltaic module output power considering the solar radiation and panel temperature. This work proposes the implementation of one-cycle control (OCC) to track the maximum power point of a photovoltaic module in direct current converters. This paper contains an analysis of response speed, reference tracking and transient, showing the applicability of the OCC technique.

Analysis of Non-Isolated Bidirectional Active Clamped DC-DC Converter for PV and Battery Integrated Systems

Background/Objective: A novel soft switching technique is implemented in a Bidirectional DC-DC converter by using an Active clamped Auxiliary switch and in turn it achieves continuous current of inductor and operates at constant frequency is used to turn ON and turn OFF the switches. Methods/Statistical Analysis: The Power electronics converter systems have DC bus voltages they are connected to batteries or supercapacitors. Bidirectional converters which are connected to battery, supercapacitor allows them to charge or discharge. A new soft switching Bi-directional direct current to direct current converter is presented in this paper. This converter is operated in zero voltage and zero current switching, continuous inductor current and constant frequency that frequency is used to turn ON and turn OFF the switches. The switching stresses are reduced in this proposed method by auxiliary switches compared to traditional methods. The simulation results are obtained by using MATLAB software in this paper. Findings: The proposed converter is implemented by using an Auxiliary switch with soft switching technique. The output is ripple free DC voltage of 63.5V. Improvements: The Bidirectional Converter will operate at continuous inductor current also with less ripples in output voltage.

A Novel Transformer-less Interleaved Four-Phase Step-Down DC Converter With Low Switch Voltage Stress and Automatic Uniform Current-Sharing Characteristics

In this paper, we propose a novel transformer-less direct current (dc) converter that features low switch voltage stress and automatic uniform current sharing. An interleaved four-phase voltage divider operating from a 400 V dc bus is used to achieve a high step-down conversion ratio with a moderate duty ratio. Based on the capacitive voltage division, the proposed converter achieves two major objectives, i.e., increased voltage conversion ratio, due to energy storage in the blocking capacitors, and reduced voltage stress of active switches and diodes. As a result, the proposed converter permits the use of lower voltage rating MOSFETs to reduce both switching and conduction losses, thereby improving the overall efficiency. In addition, due to the charge balance of the capacitors, the proposed converter enables automatic uniform current sharing of the interleaved phases without adding extra circuitry or complex control methods. The operation principles and performance analyses of the proposed converter are presented, and its effectiveness is verified by a 500 W output power prototype circuit that converts 400 V input voltage into 24 V output voltage.

Fault Analysis of Grid Connected Photovoltaic System

A new method of current control strategy for grid connected photovoltaic (PV) system is presented in this paper. The connection of photovoltaic system with the grid is a difficult task as the solar irradiation is a nonlinear quantity. The objective of this work is to develop a model of the photovoltaic system with maximum power point tracking (MPPT) system connected to 11 KV grid by implementing new control technique so that maximum active power transfer from PV to grid can be taken place without injection of harmonics. The considered system consists of a PV system, MPPT controller, boost converter, voltage source inverter (VSI), 3-Φ filter, a control system, a distribution network, load and grid. In the beginning, a model of a photovoltaic array was developed and then a MPPT controller and a direct current to direct current (DC-DC) converter are designed. To connect PV system to grid, a power electronics converter is needed which can convert DC voltage into three- phase AC voltage. Three-phase VSI using insulated gate bipolar transistors (IGBTs) is used. By means of a step-up transformer and filter, this three-phase VSI is connected to the distribution network. The proposed control of the three-phase grid-connected solar PV system is designed in the synchronously-rotating d-q reference frame. Here, Vdc is measured, then compared with Vdcref and accordingly the error is fed to proportional-integral (PI) controller from which Idref is generated. As PV system should inject only active power, so reactive power injected to grid is made zero by making Iqref zero. There after the final model is simulated by using MATLAB/SIMULINK and different output waveforms are analyzed for different conditions. Finally the fault analysis is carried out to observe the behavior of the system.

FPGA-Based High-Frequency Digital Pulse Width Modulator Architecture for DC-DC Converters

Digital pulse width modulator is an integral part in digitally controlled Direct Current to Direct Current (DC-DC) converter utilized in modern portable devices. This paper presents a new Digital Pulse Width Modulator (DPWM) architecture for DC-DC converter using mealy finite state machine with gray code encoding scheme and one hot encoding method to derive the variable duty cycle Pulse Width Modulation (PWM) signal without varying the clock frequency. To verify the proposed DPWM technique, the architecture with control input of six, five and four bits are implemented and the maximum operating frequency along with power consumption results is obtained for different Field Programmable Gate Array (FPGA) devices. The post layout timing results are presented showing that architecture can work with maximum frequency of 326 MHz and derive PWM signal of 3.59 MHz. Experimental results show the implementation of the proposed architecture in low-cost FPGA (Spartan 3A) with on-board oscillator clock frequency of 12 MHz which is multiplied internally by two with Digital Clock Manager (DCM) and derive the PWM signal of 1.5 MHz with a time resolution of 1 ps.

Power Electronics Basics: Operating Principles, Design, Formulas, and Applications [Book News

This book contains the fundamental subjects of the interdisciplinary field of power electronics, which draws knowledge from circuit and control theory, signal processing, electronics, power semiconductor devices, magnetics, power network analysis, renewable energy, and power quality. The book begins with basic concepts, semiconductor power devices, structures, operation and characteristics, and silicon carbide devices and continues with electromagnetic components. It includes the main switching converters: line-commutated converters, direct-current to directcurrent converters, voltage source and current source inverters, resonant converters, and the new multilevel, modular, multicell converters. A preface, two appendixes and indexes are also included. The book is oriented to modern students and is concise while suggesting significant team work. It is valuable to every undergraduate or graduate student, practicing engineer or technician, industry professionals, researchers, and academics willing to study the broad field of power electronics. It offers a solid background on the underlying theory, while making an overview about the latest trends in converters and control methods of the power electronics industry. The book is also fundamental for power electronics professors and instructors, serving as a textbook for undergraduate classes and as a model for more advanced courses.

Wave Process in Scale-Down Model of UHVDC Converter Transformer Under the Lightning Impulse Voltage

An experimental scale-down model of ultra high voltage direct current converter transformer is designed and manufactured to study the wave process in the winding under impulse voltage. Nine outgoing lines are configured on different winding coils of the scale-down model. The voltage waveforms of different outgoing lines are obtained by the lightning impulse voltage. The simulation model of scale-down transformer is established to calculate the distributed parameter matrices through the finite-element method. The equivalent circuit of the transformer winding is proposed to calculate the voltage waveforms and the potential gradient distribution. The comparative analysis of test and simulation results shows that the oscillating voltage occurs in the winding, and the simulation results can reflect the wave process characteristics.

Hybrid DC–DC converter with high efficiency, wide ZVS range, and less output inductance

SummaryIn this paper, a new soft switching direct current (DC)–DC converter with low circulating current, wide zero voltage switching range, and reduced output inductor is presented for electric vehicle or plug‐in hybrid electric vehicle battery charger application. The proposed high‐frequency link DC–DC converter includes two resonant circuits and one full‐bridge phase‐shift pulse‐width modulation circuit with shared power switches in leading and lagging legs. Series resonant converters are operated at fixed switching frequency to extend the zero voltage switching range of power switches. Passive snubber circuit using one clamp capacitor and two rectifier diodes at the secondary side is adopted to reduce the primary current of full‐bridge converter to zero during the freewheeling interval. Hence, the circulating current on the primary side is eliminated in the proposed converter. In the same time, the voltage across the output inductor is also decreased so that the output inductance can be reduced compared with the output inductance in conventional full‐bridge converter. Finally, experiments are presented for a 1.33‐kW prototype circuit converting 380 V input to an output voltage of 300–420 V/3.5 A for battery charger applications. Copyright © 2015 John Wiley & Sons, Ltd.