Six-pulse Rectifier Research Articles

Effect of Supply Unbalance on Harmonic Emissions of Uncontrolled Six-Pulse Rectifiers

Standards dealing with power quality disturbances and emission limits for equipment and installations are constantly evolving as additional knowledge is gained. Often, each type of disturbance, such as harmonics, is considered individually and without regard for the possible effects of other types of disturbances, such as unbalance. The interdependencies between the disturbance phenomena are often disregarded. The work reported in this paper is focused on quantifying and correlating the effect of supply system unbalance on the harmonic emissions of the most basic harmonic-producing converter, the uncontrolled six-pulse rectifier. Six-pulse rectifiers with different loading conditions are supplied by sources with varying levels of unbalance to determine the effect of the unbalance on harmonic emissions via simulation. Different unbalanced supply conditions, all within credible levels, are considered and the effects on the harmonic emissions of the six-pulse rectifier are evaluated. The results of these simulations show that as source unbalance increases, THD and triplen harmonics also increase in a nearly-linear pattern. This type of relationship may be appropriate for consideration in the ongoing efforts in standardization related to both harmonics and unbalance so that more coordinated decisions can be made.

Electric vehicles charging station configuration with closed loop control

<p><span lang="EN-US">Recently, the demand for electric vehicles (EV) has been on the rise in global markets due to the orientation of national policies to reduce emissions and global warming through the electrification of the transportation sector and the use of clean energy sources. Electric vehicles function on batteries, which must be recharged, either by slow charging at home or by fast charging with a direct current. In the fast-charging process, the batteries can be charged in less than 15 minutes. In this paper, an off-board charger with a three-phase, six-pulse voltage rectifier was designed using the MATLAB/Simulink program. The closed control circuit was simulated, where the simulation results were influenced by changes to the input voltage. When the input voltage was increased or decreased by 5%, this control maintained the value of the current and voltage at the output to be equal to the reference values required to achieve fast charging. The simulation results showed that in the first case where no filter was used, the output voltage and current had a high amount of ripple that exceeded the permissible value. Therefore, a low-pass filter was designed to reduce the ripple factor to a value that was within the permissible limit.</span></p>

Impact of power supply fluctuation and part load operation on the efficiency of alkaline water electrolysis

Contrary to traditional electrolysers which operate continuously at their nominal load, future alkaline electrolysers need to be able to operate over a wide load range due to the variability of renewable electricity supply. We have investigated how the residual ripples from thyristor-based power supplies are influenced by the operating load of the system, and how these ripples affect the efficiency of alkaline electrolysers. For this, a simulation tool was developed which combines a six-pulse bridge thyristor rectifier model with closed-loop current control and semi-empirical electrolysis models. The electrolysis models can simulate the potential response to both direct and high amplitude alternating currents for lab-scale and industrial electrolysers. The electrolysis model of the lab-scale electrolyser was validated with experiments with a square wave current input. The models show that without filters the ripples result in a total system efficiency loss of 1.2–2.5% at full load and of 5.6–10.6% at a part load of 20% depending on the type of electrolyser. The implementation of an optimized L-filter suppresses residual ripples and reduces the efficiency losses to 0.5%–0.8% at full load and to 0.8–1.2% at the minimum load.

Doubly Fed Induction Machine-Based DC Voltage Generator with Reduced Oscillations of Torque and Output Voltage

The doubly fed induction machine (DFIM)-based DC voltage generator is equipped with a stator-connected diode rectifier. The six-pulse diode rectifier as a nonlinear circuit introduces harmonics in the stator and rotor current and distorts the machine stator voltage, as well as the stator flux. This causes electromagnetic torque oscillations and instantaneous power components oscillations. The torque oscillations adversely impact the mechanical parts of the drive-train and oscillations of the p component of instantaneous power influence DC-bus voltage oscillations. The oscillations can be somewhat cancelled by control methods. However, cancellation of electromagnetic torque is not strictly coupled with cancellation of oscillations of the p component of instantaneous power. The paper presents an analysis of influence of the control methods aimed at a reduction of torque oscillations on the output voltage oscillations level in the stand-alone DFIM-based DC voltage generator. Field-oriented control FOC with current controllers and space vector modulation-based direct torque control DTC-SVM with flux module regulation have been compared with control in which electromagnetic torque is one of the commanded variables, whereas the second variable is the dot product of stator flux and rotor current space vectors. The contributions of this paper are the introduction of a new variable in the second control path in the DTC-SVM method instead of flux vector length and the proof that it can reduce torque and DC-bus voltage oscillations in the DFIG-DC system. Additionally, this paper reveals that for proper stator-to-rotor-turns ratio of a doubly fed machine necessary for reduction of the rotor converter power, lower DC-bus voltage can be obtained than is required for full realization rotor side voltage requested by rotor current controllers. This is the reason why, regardless of the control method, torque oscillations cannot be always fully cancelled, and a comparative study of the methods at these conditions has been conducted in simulation and in laboratory tests.

Control of a Waveshaper-MMC With Thyristor-Based Front-End Converter for Open-End Winding Variable Speed Medium-Voltage Induction Motor Drive

For variable frequency induction motor drive application, conventional modular multilevel converter (MMC) faces some issues, like high capacitor voltage ripple at low frequency, high circulating current, and high component count. This article proposes a novel configuration to address these issues, where a six-pulse thyristor-based controlled rectifier is combined with a waveshaper-based MMC (WS–MMC). Each phase of the converter consists of a WS and director valve, and then the phases are connected in series to create a common dc-link. The use of a thyristor-based controlled rectifier as a front-end converter increases reliability and reduces component count to a large extent. Unlike conventional MMC, circulating current does not exist in this converter, and the capacitor voltage ripple does not depend on the operating frequency. As a consequence, the current rating of the switching devices is reduced for the same power output, and capacitor voltage ripple at low frequency stays within the specified limit. Besides this, the component count of the proposed configuration is far less than the MMC-based topologies available so far. The operation principle, relevant control techniques, and analysis of the converter are discussed in detail in this article. The complete system is simulated in MATLAB/SIMULINK, and the performance of the control methods has been validated at different operating frequencies with various load torques. The results show satisfactory performance.

An Improved Algorithm for AC Impedance Extraction

The overall system stability of interconnected electrical and electronic systems is associated with the input and output impedances of each individual system, therefore, accurate AC impedance measurement for a linear and nonlinear systems are imperative. These impedances are normally measured using small-signal frequency injection techniques. However, the harmonic transfers from the output side to the input side and vice versa deteriorates the measured results, making them ambiguous. These harmonic transfers cause the problems of frequency aliasing as well as spectral leakages leading to inaccurate measurements of results. In order to achieve the precise and accurate results, there is a need to adopt such an impedance measuring technique which is the best among the available techniques. A new algorithm is proposed to modify the range of frequencies of interest and inject such small-signal frequencies which do not interfere with the system harmonics. The measurement of the AC impedance of the three-phase RL circuit utilizing three different small-signal injection techniques is performed. From the results displayed, it is clear that all the techniques perform well, therefore, the most simple, single phase line-current injection was selected for the AC impedance measurement of the nonlinear switching circuit such as six-pulse rectifier. The issues discussed above are more serious in nonlinear switching circuit because of the presence of higher order switching harmonics in the circuit. The proposed algorithm is applied to measure the output-impedance of the three-phase AC power supply as well as the AC input-impedance of the six-pulse rectifier. These results of the switch vs average model and that of average model vs. experimental prototype are displayed for comparison, and it is clear that the proposed algorithm offers much-improved results.

Novel Control Approach for a Hybrid Grid-Forming HVDC Offshore Transmission System

This article describes a hybrid topology of high-voltage direct current (HVDC) for offshore wind farms using a series connection of a voltage source converter (VSC) and six-pulse diode rectifier (6P-DR). In this topology, the offshore side VSC (OF-VSC) acts as a grid-forming converter to maintain the PCC (point of common coupling) voltage of offshore wind farms (WF) and frequency. In addition, the OF-VSC functions as an active power filter to suppress the 5th, 7th, 11th, and 13th order harmonic current components produced by the 6P-DR, making it almost sinusoidal. Due to the 6P-DR being used in the hybrid converter, this new configuration reduces the total cost of the converters and losses, while preserving the power flow to the onshore gird. Compared to the fully-rated converter and hybrid converter based on a 12-pulse diode rectifier, the power loss and cost are reduced, and in addition, the proposed hybrid converter does not require a phase shift transformer nor a high number of diodes. A 200 MW in an HVDC transmission system using the hybrid configuration was simulated in PSCAD. The results show that the system operated correctly and the harmonic components were filtered.

Application of an improved harmonic detection algorithm in harmonic control of charging station

With the rapid prosperity of the electric vehicle industry, the demand for charging stations providing charging services has become increasingly strong, which followed by serious power quality problems. Because the current level of the charging station changes gradually with time, the harmonic current generated by the charging station will also change gradually with time. The influence of the low-pass filter leads to the slow start-up and tracking speed of harmonic current detection method based on ip-iq algorithm, which cannot meet the need of harmonic control in charging station. In this paper, the harmonic characteristics and basic structure of six-pulse rectifier charging pile are analyzed in detail, and the charging station model based on six-pulse rectifier charging pile is established. At the same time, in order to meet the need of harmonic suppression, a harmonic current detection algorithm based on variable step size LMS/LMF algorithm is proposed and analyzed comprehensively. The simulation and experimental results show that compared with the traditional harmonic detection algorithm, the improved harmonic detection algorithm has obvious advantages in many aspects, such as start-up and tracking speed, and has good engineering application prospects.

A Novel Dual Integrated LLC Resonant Converter Using Various Switching Patterns for a Wide Output Voltage Range Battery Charger

This paper proposes a novel dual integrated LLC resonant converter (DI-LRC) with a wide output voltage range using various switching patterns. The primary side of the proposed DI-LLC converter consists of two resonant tanks and six switches, while the secondary side consists of a six-pulse diode rectifier. Depending on the switching pattern of the primary switch, the DI-LRC converter is performed by single full-bridge operation with a voltage gain of 1, series-connected full-bridge operation with a voltage gain of 0.5, series-connected half bridge operation with a voltage gain of 0.25, and parallel-connected full-bridge operation with a voltage gain of 2. Accordingly, the proposed DI-LRC converter has four voltage gain curves with different variations and achieves a wider output voltage range than the conventional single voltage gain curve in a given operating frequency range. In this paper, the equivalent circuits derived for each switching pattern are proposed to analyze the operating characteristics of the proposed converter according to each switching pattern, and each Q factor and voltage gain are calculated based on the analyzed equivalent circuit. The performance of the proposed converter and switching pattern is verified using the simulation and experimental results of the prototype battery charger, which is designed to be 4-kW class.

Field Validated Generic EMT-Type Model of a Full Converter Wind Turbine Based on a Gearless Externally Excited Synchronous Generator

The integration of wind power plants introduces new dynamics into power systems, forcing reconsiderations of how they are studied, planned, and operated. High quality models are essential to these studies. Manufacturer-specific electromagnetic transient (EMT) wind turbine models are usually available only as black-boxes, which hinders analysis and research. To overcome this issue, this paper proposes a generic EMT-type model for a specific type-IV wind turbine system, which is validated against field measurements from a wind turbine of the same type. More precisely, it proposes a wind turbine model based on an externally excited synchronous generator system connected to a full converter composed of a six-pulse diode rectifier, a dc–dc boost stage and a two-level voltage source converter. The required control features and internal protection schemes are considered and described. Two different fault ride-through control strategies, in line with existing grid codes, are implemented. A corresponding EMT-type hybrid model representation is also developed based on newly proposed switched equivalent circuits and average models for the considered hardware, control, and power electronics stages. It allows for the use of larger simulation time steps, hence considerably improving computation times.

A New Design Method of an <italic>LCL</italic> Filter Applied in Active DC-Traction Substations

This paper concentrates on the LCL filter with damping resistance intended to connect the shunt active power filter of an active dc-traction substation to the point of common coupling with the transmission grid. In order to find design conditions and conceive a design algorithm, attention is directed to the transfer functions related to currents and the associated frequency response. The mathematical foundation of the design method is based on meeting the requirements related to the significant attenuation of the high-frequency switching current, concurrently with the unalterated flow of the current that needs to be compensated by active filtering. It is pointed out that there are practical limitations and a compromise must be made between the two requirements. To quantify the extent to which the harmonics to be compensated are influenced by imposing the magnitude response at both highest harmonic frequency to be compensated and switching frequency, a performance indicator is defined. As an additional design criterion, the damping power losses are taken into consideration. The validity and effectiveness of the proposed method are proved by simulation results and experimental tests on a laboratory test bench at small scale reproducing the specific conditions of a dc-traction substation with six-pulse diode rectifier.

An Improved Topology for Multipulse AC/DC Converters Within HVDC and VFD Systems: Operation in Degraded Modes

An improved topology for a multipulse ac/dc converter based on six-pulse rectifier modules operating in a degraded mode is described. Its main application is HVDC and variable-frequency-drive systems, where reliability, availability, and maintainability are the most critical requirements. A new concept, called “three-phase electronic Z/z transformers,” which provides separated voltages with reconfigurable phase-shifting angles and voltage ratios, is discussed. The number of acceptable faulty rectifier modules is defined at the design stage. Such features enable ac/dc rectifier systems to be modular and reconfigurable. In contrast with conventional topologies, the proposed structure can operate optimally as an 18-pulse, 12-pulse, or six-pulse rectifier in faulty conditions of local transformers or rectifier modules. An implementation method for a degraded 24-pulse rectifier with its control architecture including the Z/z transformers’ command unit is discussed. A 1.5-kV, 18-kVA, 18-pulse ac/dc converter associated with a real-time system control is designed and tested. Promising results validate the new concept.

MVDC Supply Technologies for Marine Electrical Distribution Systems

The increase in the popularity of the medium voltage dc (MVDC) electrical distribution, as a possible evolution of the medium voltage ac (MVAC) electrical distribution for the ship onboard power systems, arises a need for a comparative evaluation and demonstration of feasible technologies for the MVDC supplies. For designing the MVDC supplies in the range from 5-35 kV, different available technologies can be considered for the designs and have a direct influence on the overall system performance. In this paper, different technologies for prime movers, electrical generators and rectifiers are discussed in terms of feasibility for the MVDC supplies. Different supply configurations can be envisioned from these based on the commercial availability, quality of supply, efficiency, dynamic performance and volume. Multi-phase multipulse supply configurations are identified and proposed for the marine MVDC systems. Combination of multi-phase generators and multi-pulse rectifiers offer reliable, simple and fault tolerant solution with acceptable dynamics. To explore and highlight these benefits, a six-pulse rectifier sub-module is designed and analysed in two arrangements for multi-pulse configurations, namely parallel and series. It has been shown that with appropriate selection of semiconductor devices, coupled with properly selected fast fuses, excellent fault current (thermal) withstand capabilities can be achieved.

Reduced Active Switch Front-End Multipulse Rectifier With Medium-Frequency Transformer Isolation

This paper presents a reduced switch count multipulse rectifier with medium-frequency (MF) transformer isolation. The proposed topology consists of a three-phase push–pull based ac to dc rectifier with a MF ac link employing two active switches. A three-phase, five-limb, multiwinding MF transformer is employed for isolation. The secondary side of the transformer is connected in a zig-zag configuration and is fed to two six-pulse diode rectifiers, achieving 12-pulse rectifier operation. The primary advantage of the proposed system is reduction in size/weight/volume compared to the conventional 60 Hz magnetic transformer isolation rectifier system. Operating the transformer at 600 Hz is shown to result in three times reduction in size. Furthermore, the proposed system employs only two active semiconductor switching devices operating under a simple pulse width modulation scheme. Also, the zig-zag transformer connection helps to balance leakage inductance on the secondary side. Detailed analysis, simulation, and experimental results on a 208Vl-l, 3.15 kW laboratory prototype are presented to validate the performance of the proposed approach.

Generator with modulated magnetic flux for wind turbines

AbstractIn the paper, the concept of a permanent magnet synchronous generator (PMSG) with uniquely designed stator windings for wind turbines is presented. Two 3-phase windings in the stator are used, one of which is connected in the star, while the other in the delta configuration. Six-pulse rectifiers, mutually coupled by the pulse transformer, whose primary winding is supplied by the so-called “current modulator”, are placed at the outputs of both windings. The modulator output current should meet all the necessary and strict requirements. Both rectifiers operate on a common DC circuit. These solutions provide the sinusoidal magnetomotive force in the stator of the PMSG and the quasi-sinusoidal (taking into account the non-linearity of the magnetic circuit) magnetic flux. In light of the generator principle, it has been called the “PMSG with modulated the magnetic flux”. The slightly higher complexity in the structure of the generator, as compared to the normal three-phase construction, is compensated by the exceptional simplicity of the power electronics section of the system, which allows high efficiency to be reached. The current modulator (as well as the pulse transformer) is a power electronics converter with a relatively low output power as compared to the overall output power of the system. In comparison to other known solutions, the expected cost of the system should be lower. It is also expected that a high degree of reliability in terms of its operation will be achieved, and consequently, that the the ongoing costs of its maintenance will be reduced. In the paper, concept, theoretical basis of operation, and results of the studies of the simulation models of the generator, including the basic power electronics section, are presented.

Simulations of Wind Energy Conversions Systems with Asymmetrically Rnsic

The electric energy losses as well as electromagnetic pollution of the environment by the modern electric or electronic apparatus running in a commutating regime may be reduced to minimum with the help of power electronics, field which will be well developed in the next century in order to modernize industry, transport, telecommunications, etc. The paper present a wind generator system based on a new converter configuration with a asymmetrical rectifier with near sinusoidal input currents (Asim-RNSIC), we propose a method to reduce by 15%-25% of the reactive elements (capacitors and inductances) of a asymmetrical rectifiers with near sinusoidal input currents (Asim-RNSIC). This method consists of the parallel connection of two RNSIC converters of the same type, dimensioned for half of the converted nominal power and whose entry currents are phase-shifted with an angle of 30°-40° by the correct choosing of the inductances on the AC part. Thus, it results a sufficient compensation of the type 5 current harmonics generated in the power grid when the capacitors and inductances of the two rectifiers are lowered accordingly. The converter, named Asim-RNSIC, is economically and technically more competitive compared to the three-phase six-pulse full-bridge diode rectifier with passive filters.

Virtual Flux Sensorless Control for Shunt Active Power Filters with Quasi-resonant Compensators

This paper presents a sensorless control strategy for pulsewidth modulated active filters. The complete solution uses only two current sensors, which are located in the grid interface. The compensation is performed by the imposition of sinusoidal current references in the grid, and does not require algorithms for disturbance estimation. The virtual flux concept for grid synchronization and multiple quasi-resonant compensators in the current scheme are employed for selective harmonic mitigation. A discretization analysis is presented in order to obtain a precise digital realization of the current controllers. It is proposed as a regulator for the direct-current bus voltage in the active filter, which employs only information of the modulated voltage and is compatible with the previous sensorless synchronization solution. Experimental results from a 1-kVA prototype converter are presented, covering the solution with and without the voltage sensor in the direct current bus. Analyses of stationary and transient responses are performed by using a six-pulse diode rectifier as nonlinear load.

Testing and Commissioning of a 135 MW Pulsed Power Supply at the Wuhan National High Magnetic Field Center

At the Wuhan National High Magnetic Field Center, two 67.5-MW pulsed power supply units have been installed to energize long-pulse magnets. Each unit based on a 12-pulse converter consists of two 6.9 kV/1.2 kV (1.0 kV) cast coil transformers and two six-pulse rectifiers connected in series. The units are connected to a 100 MVA/185 MJ inertial energy storage motor-generator set and can operate independently as well as in parallel or in series. Each unit is designed to provide no-load voltages of 3.2 kV (2.7 kV) and full-load voltages of 2.7 kV (2.25 kV) at the rated current of 25 kA for a 3-s pulse once every half hour. After completion of various performance tests, the power supply commissioning started in October 2012. At present, the power supply has energized a long-pulse magnet more than 100 times to produce flat-top pulses at various field levels, including a 50-T field pulse with 100-ms flat-top duration. In this paper, the final design of the power supply components is described in detail. Testing and commissioning results of the power supply taken with a dummy load and a long-pulse magnet are also presented.

A novel feeding topology for DC electric arc furnaces utilizing PWM-AC choppers

SUMMARY The main contribution of this paper is the study of a new feeding system for direct current (DC) electric arc furnaces. This system includes pulse-width modulation-alternating current (PWM-AC) choppers along with six-pulse diode rectifier. In contrast with traditional power supplies such as 12-pulse thyristor-based rectifier and 12-pulse diode rectifier–DC chopper-based power supplies, the proposed configuration stands out because of the reduced number of rectifier units and output reactors. Furthermore, the proposed power supply system is free of any current/power-sharing problems, and only one proportional-integral controller is sufficient to control the DC arc current or its power. It is shown through analysis and simulation that in comparison with traditional feeding systems, the proposed topology can lead to smaller input filter size, total unbalance and harmonic distortion reduction, and power factor improvement. Copyright © 2012 John Wiley & Sons, Ltd.

Low-Modulation-Index Operation of a Five-Level Diode-Clamped PWM Inverter With a DC-Voltage-Balancing Circuit for a Motor Drive

This paper presents a five-level diode-clamped pulse width-modulated (PWM) inverter intended for a medium-voltage motor drive with a constant-torque load but no regenerative braking. It is applicable to drilling rigs, extruders, and rubber mixers. The power conversion system consists of a three-phase six-pulse diode rectifier, a five-level diode-clamped PWM inverter, and a dc- voltage-balancing circuit including a single coupled inductor. The five-level inverter is characterized by injecting a common ninth-harmonic zero-sequence voltage on each of the three-phase reference voltages in a low-modulation-index region. This injection contributes not only to mitigating magnetic-flux fluctuation of the coupled inductor but also to making uniform the power losses produced by eight insulated-gate bipolar transistor modules per leg. In addition, the dc-voltage-balancing circuit introduces a control method capable of reducing an amplitude of magnetic flux of the coupled inductor. The combination of this control method and the ninth-harmonic zero-sequence voltage injection keeps the peak value of the magnetic flux within its designed value.