Low-frequency Voltage Oscillations Research Articles

A pulse train controlled Buck converter based on a Single Memristive multi-Vibrator

ABSTRACT Memristors (MRs) have been used in integrated circuit design due to their nanoscale size and low power consumption. In this study, a flux-controlled binary MR emulator is designed by using off-the-shelf circuit components. Then, two MR-based multi-vibrators are constructed and applied to pulse train (PT) controlled Buck converters. Compared with traditional PT controlled Buck converters requiring two pulse train generators, only one single memristive pulse generator with controllable duty cycle is used to regulate the output voltage. To suppress the low-frequency oscillations of the PT-controlled Buck converter in continuous conduction mode (CCM), an inductor current based PT (IC-PT) control method is devised with the proposed MR-based pulse generator. Simulation and experimental results both show that the IC-PT-controlled Buck converter with the MR-based pulse generator can achieve fast transient response and minimum low-frequency voltage oscillations in the CCM.

Active hybrid energy storage management in a wind-dominated standalone system with robust fractional-order controller optimized by Gases Brownian Motion Optimization Algorithm

Accessibility to sustainable and reliable electrical energy for remote area power supply (RAPS) systems under high penetration levels of wind energy requires adopting appropriate technologies and controllers. In recent years, energy storage has gained significant attention for smoothing inherent voltage and power fluctuations of standalone systems. This paper presents a filtration-based fractional-order PI (FOPI) controller optimized by Gases Brownian Motion Optimization (GBMO) algorithm to manage an active battery-supercapacitor hybrid energy storage system (HESS) in a wind-dominated RAPS system. The GBMO algorithm has a high accuracy and convergence rate among optimization algorithms introduced in recent years. Moreover, a fuzzy logic controller (FLC) guarantees the wind turbine generator's maximum power point tracking (MPPT) at any wind condition while mitigating its fluctuating torque. The proposed RAPS system uses the advantages of the FOPI controller, the GBMO optimization algorithm, the FLC-based MPPT algorithm, and a high-pass filter to manage precise coordination between different components of the RAPS system. The high-pass filter decouples high and low-frequency voltage oscillations to modify the HESS performance and relieve battery stress, thereby extending its lifespan. The proposed controller's performance and robustness are verified in comparison with an optimal PI controller based on GBMO under turbulent wind speed and load step changes in a detailed model of the system. The results demonstrate the superiority of the optimal FOPI controller over the optimal PI controller.

A Double-Side Feedback Pulse Train Control for the Output Voltage Regulation of Two-Stage Wireless Power Transfer System

Pulse Train (PT) control is a nonlinear voltage regulation method with favorable characteristics of quick response time and simple structure. In this paper, a PT-based feedback control strategy is proposed for stabilizing the output voltage of two-stage wireless power transfer (WPT) systems. Low-frequency voltage oscillations can be observed in PT controlled front-stage power converters, which significantly degrades the output power quality of the WPT system. To solve this problem, by using feedback variables sampled from both output sides of a power converter and WPT, a capacitor current and output voltage feedback PT (CC&OV-PT) controlled two-stage WPT system is further devised in this paper. A discrete time model of the two-stage WPT system is established, and the performance of low-frequency voltage oscillations suppression for the double-side feedback CC&OV-PT controlled WPT system is analyzed. Simulation and experimental verification have been conducted, both of which show that the CC&OV-PT controlled WPT system achieves fast response with effective low-frequency oscillation suppression.

Baseline for Split DC Link Design in Three-Phase Three-Level Converters Operating with Unity Power Factor Based on Low-Frequency Partial Voltage Oscillations

The study sets a baseline for split DC link capacitance values and voltage set points in three-phase three-level AC/DC (or DC/AC) converters operating with unity power factor. In order to equalize the average values of partial DC link voltages, the controller generates a zero-sequence containing DC components only while employing neither dedicated DC link capacitance balancing hardware nor high-order zero-sequence component injection. Such a baseline is required in order to evaluate the effectiveness of different DC link capacitance reduction methods proposed in the literature. Unlike most previous works, utilizing neutral point current based on cumbersome analytical expressions to determine neutral point potential oscillations, the instantaneous power balance-based approach is employed in this paper, resulting in greatly simplified and more intuitive expressions. It is demonstrated that while the total DC link voltage is low-frequency ripple-free under unity power factor balanced AC-side operation, split DC link capacitors absorb triple-fundamental frequency power components with one-sixth load power magnitude. This yields significant opposite phase partial voltage ripples. In such a case, selection of DC link capacitances and voltage set points must take into account the expected values of AC-side phase voltage magnitude and split DC link capacitor voltage and current ratings. Simulation and experimental results validate the proposed methodology by application to a 10 kVA T-type converter prototype.

Voltage Balancing of the DC-Link Capacitors in Three-Level T-Type Multiphase Inverters

This article illustrates an algorithm to balance the voltages across the dc-link capacitors of a three-level multiphase inverter feeding a star-connected load with an odd number of phases. The proposed strategy keeps the dc-link capacitors balanced in any operating condition and minimizes the low-frequency voltage oscillations, even during open-phase faults and with non-sinusoidal output currents. Finally, the article identifies the operating region where the voltage oscillations of the dc-link capacitors can be canceled. The developed algorithm can maintain constant voltages even if the capacitors are unbalanced, and its performance is compared to that of other carrier-based algorithms. Experimental tests are carried out on a five-phase induction motor in healthy and faulty conditions.

Three-Level Unidirectional Rectifiers under Non-Unity Power Factor Operation and Unbalanced Split DC-Link Loading: Analytical and Experimental Assessment

Three-phase three-level unidirectional rectifiers are among the most adopted topologies for general active rectification, achieving an excellent compromise between cost, complexity and overall performance. The unidirectional nature of these rectifiers negatively affects their operation, e.g., distorting the input currents around the zero-crossings, limiting the maximum converter-side displacement power factor, reducing the split DC-link mid-point current capability and limiting the converter ability to compensate the low-frequency DC-link mid-point voltage oscillation. In particular, the rectifier operation under non-unity power factor and/or under constant zero-sequence voltage injection (i.e., when unbalanced split DC-link loading occurs) typically yields large and uncontrolled input current distortion, effectively limiting the acceptable operating region of the converter. Although high bandwidth current control loops and enhanced phase current sampling strategies may improve the rectifier input current distortion, especially at light load, these approaches lose effectiveness when significant phase-shift between voltage and current is required and/or a constant zero-sequence voltage must be injected. Therefore, this paper proposes a complete analysis and performance assessment of three-level unidirectional rectifiers under non-unity power factor operation and unbalanced split DC-link loading. First, the theoretical operating limits of the converter in terms of zero-sequence voltage, modulation index, power factor angle, maximum DC-link mid-point current and minimum DC-link mid-point charge ripple are derived. Leveraging the derived zero-sequence voltage limits, a unified carrier-based pulse-width modulation (PWM) approach enabling the undistorted operation of the rectifier in all feasible operating conditions is thus proposed. Moreover, novel analytical expressions defining the maximum rectifier mid-point current capability and the minimum peak-to-peak DC-link mid-point charge ripple as functions of both modulation index and power factor angle are derived, the latter enabling a straightforward sizing of the split DC-link capacitors. The theoretical analysis is verified on a 30 kW, 20 kHz T-type rectifier prototype, designed for electric vehicle ultra-fast battery charging. The input phase current distortion, the maximum mid-point current capability and the minimum mid-point charge ripple are experimentally assessed across all rectifier operating points, showing excellent performance and accurate agreement with the analytical predictions.

Uniform carrier‐based PWM method for three‐phase three‐level three‐wire and four‐wire converter system with neutral‐point balancing

In this study, a simple carrier-based pulse-width modulation (CBPWM) method is proposed for a three-phase three-level neutral-point-clamped converter with neutral-point (NP) balancing, which can be applied to three-phase three-wire (TPTW) and three-phase four-wire (TPFW) converter systems simultaneously. First, the effect of each switching state for the NP potential in TPTW and TPFW systems is analysed in detail, and a newly NP control strategy by reconstructing the switching state and adjusting the corresponding duration time is deduced for all operation conditions, it only needs to add the offset voltage to the modulation reference voltage to guarantee the NP balance and is easy to implement. Then, under all operating conditions, the mathematical model of NP current is derived and analysed to evaluate the low-frequency voltage oscillation in the NP. Finally, the effectiveness and feasibility of the proposed CBPWM strategy are verified by the experimental results based on a 12 kVA laboratory prototype.

Theoretical Harmonic Spectra of PWM Waveforms Including DC Bus Voltage Ripple—Application to a Low-Capacitance Modular Multilevel Converter

This article develops a new closed-form analytical solution to the harmonic spectrum of the pulsewidth modulated (PWM) output voltage of the single-phase inverter connected to the dc bus with considerable voltage ripple. The solution is based on a double Fourier series expansion in two variables. As the single-phase inverter forms the basic building block of most converters, the developed expressions can be applied to various topologies. This article first identifies the interactions of a single-frequency sinusoidal modulation signal and the carrier signal with the dc bus voltage harmonics. The dc bus voltage harmonics are constrained in this article to multiples of the inverter output voltage fundamental frequency. The analysis is then extended to include a multifrequency modulation signal. Starting from the general solution, new expressions for the output voltage harmonics of the modular multilevel converter (MMC) are developed. The low-capacitance MMC is chosen in this analysis due to inherent low-frequency voltage oscillations in the converter internal capacitors. The MMC analytical harmonic spectrum can incorporate the effects of the circulating current control and third harmonic injection PWM, by including the second- and third-order harmonic components in the modulation signal, respectively. The MMC analytical harmonic spectrum is benchmarked against the experimental results.

Optimal Neutral-Point Voltage Balancing Algorithm for Three-Phase Three-Level Converters With Hybrid Zero-Sequence Signal Injection and Virtual Zero-Level Modulation

This article presents an optimal carrier-based voltage balancing scheme for three-phase three-level converters. The proposed scheme utilizes two available degrees of freedom, i.e., zero-sequence signal injection and virtual zero-level modulation (VZM), to eliminate the low-frequency neutral-point voltage oscillation. It is universally effective over the full power factor and modulation index range and easy to implement in digital controllers. The hybrid algorithm combines the merits of both approaches, which offers the optimal performance regarding controllability, switching device power losses, and output harmonics. The main drawbacks of VZM, i.e., the increased switching loss and high-frequency harmonics due to additional switching transitions, have been minimized in the proposed scheme. The performance of the proposed scheme is evaluated through simulation and experiment.

Design and Hardware-in-the-Loop Implementation of Fuzzy-Based Proportional-Integral Control for the Traction Line-Side Converter of a High-Speed Train

Power quality is one of many issues affecting the traction power supply system. Prominent among the causes of poor power quality is voltage low-frequency oscillation (VLFO). In this paper, a fuzzy-based PI (FPI) controller to optimize the performance of the traction line-side converter (TLSC) and suppress the effect of VLFO is proposed. Firstly, the mathematical model of China’s railway high-speed five single-phase TLSC is developed, and then the FPI control unit is designed based on specific requirements. The fuzzy antecedent and consequence rules were generated based on the expert and previous knowledge of TLSC operation. An offline simulation of the proposed control scheme under different loads and parameters is conducted to verify the designed. To validate the model, the traction power supply system (TPS) is built on the field-programmable gate array (FPGA) real-time digital simulator (FPGA-RTDS), while the FPI control algorithm is load on modeling tech rapid control prototyping (RCP) real-time digital controller (RTDC). Hardware-in-the-loop (HIL), and offline simulation studies between current decoupling (PI) control, sliding mode control (SMC), and the proposed control method confirms in addition to excellent dynamic performance; the proposed method can successfully suppress the effect of VLFO.

A Passivity-Based Control of Euler–Lagrange Model for Suppressing Voltage Low-Frequency Oscillation in High-Speed Railway

The traction network voltage low-frequency oscillation (LFO) in high-speed railways easily leads to the traction blockade of electric multiple units (EMUs), which seriously affects the normal operation of high-speed railways. A passivity-based control (PBC) strategy for single-phase EMUs rectifier is proposed in this paper. First, for the single-phase EMU rectifier, the Euler–Lagrange (EL) mathematical model in dq frame is built, which can realize the decoupling of active power and reactive power. Second, according to the unique characteristics of the rectifier, it is proven that the rectifier is strictly passive, which is the premise of PBC controller design. Third, using the deduced EL mathematical model and the passivity of rectifier, the PBC controller for the single-phase rectifier of EMUs is designed using the damping injecting method. Next, compared with dq current control and interconnection and damping assignment PBC (IDA-PBC), it can be verified that the PBC controller has better dynamic and static characteristics, and can significantly suppress the voltage LFO of traction network. In these strategies listed in this paper, using the PBC, the single-phase rectifier input current has minimal total harmonic distortion, and the dc-link voltage has minimal oscillation. Finally, the dSPACE semiphysical experiment platform of the cascade system of EMUs and traction network is built. Simulation results are validated by the dSPACE semiphysical experiments, which indicate that the PBC has the better inhibitory effect for LFO as compared with that of the IDA-PBC and dq current control in single-phase EMUs rectifier.

Parallel‐connected buck converters controlled by capacitor current feedback based PT with current‐sharing ability

Parallel-connected power converters have been widely used in the application scenarios required high power to deal with the high load current. In this study, the pulse train (PT) controller with the advantages of fast response speed and simple structure is newly applied to the parallel-connected buck converters with current-sharing ability. However, as the converters operated in continuous conduction mode, undesirable low-frequency voltage oscillation will be brought about by the PT control method. To resolve this problem, a novel capacitor-current-feedback-based PT (CCF-PT) control method is proposed to eliminate the low-frequency oscillation of the parallel-connected buck converters. The approximate discrete time model of the CCF-PT-controlled buck converters is established, and the bifurcation diagrams of output voltage and inductor current changing along with the current scale factor, the capacitor-current-feedback coefficient and the load resistance are drawn to show the operation stability of the converters. Simulation and experimental results are obtained, which indicate that the proposed parallel-connected buck converters with CCF-PT control have excellent performance such as fast response, wide load range and effective suppression of low-frequency oscillation.

An Approach to Suppress Low Frequency Oscillation in the Traction Network of High-Speed Railway Using Passivity-Based Control

The traction blockade in depots of multiple electric multiple units (EMUs) is generally considered to be caused by the voltage low frequency oscillation (LFO) of the high-speed railway traction network. As a possible solution, a strategy using single-phase EMUs rectifiers with passivity-based control (PBC) is proposed in this paper. First, the mathematical model of the four-quadrant pulse width modulation rectifier is constructed based on the port controlled Hamiltonian with dissipation. Next, with the insertion of damping, the new energy function can be minimized at the equilibrium point of the system, which enables the derivation of the rectifier control law based on the interconnection and damping assignments PBC. Comparison of simulation results with those of the traditional proportional integral (PI) scheme, autodisturbance rejection control and multivariable control verifies that the proposed PBC controller has better dynamic and static performance. Among all strategies considered, the resulting line current has the least total harmonic distortion, and the dc-link voltage of single-phase EMUs rectifier has the least oscillation and the shortest adjustment time, when using the PBC. Meanwhile, the simulations of multi-EMUs accessed a traction network show that the proposed method significantly suppresses the voltage LFO of the traction network. Finally, the LFO signal modal analysis is performed using the fast Fourier transform and the estimating signal parameters via the rotational invariance techniques. It indicates that PBC can suppress the symmetrical frequency components and effectively reduce the third, fifth, and other harmonic components compared with the PI control in EMUs.

A Double Modulation Wave CBPWM Strategy Providing Neutral-Point Voltage Oscillation Elimination and CMV Reduction for Three-Level NPC Inverters

This paper proposes a double modulation wave carrier-based pulse width modulation (CBPWM) strategy for the three-level neutral-point-clamped (NPC) inverter. This modulation strategy not only overcomes one of the main problems of the NPC inverter, which is the low-frequency voltage oscillation on the neutral point, but also reduces the common-mode voltage (CMV) generated by the inverter. Through the rigorous theoretical derivation, the unified duty cycles of each phase are derived, and the double modulation waves can be easily obtained according to different optimization objectives. However, in the previous double wave CBPWM work, the modulation strategy could not reduce the CMV, whose amplitude is still the same as that of the conventional CBPWM strategy. By rearranging the switching sequence, the vectors corresponding to the large CMV amplitude have been eliminated, and the amplitude of CMV can be reduced by half. Considering the occurrence of neutral-point voltage perturbations, a voltage controller is adopted to recover from the imbalance. Finally, the performance of the proposed modulation technique is verified by the experimental results.

Hybrid modulation strategy for eliminating low‐frequency NP voltage oscillations in NPC using redistribution of NTV duty ratios

Neutral point (NP) clamped (NPC) inverters produce low-frequency NP voltage oscillations when modulated with nearest three-vector (NTV) methods for certain loading conditions. Non-NTV modulations can address these oscillations at the expense of increased switching frequency, output voltage distortion and complexity. This study presents a new generalised algorithm to implement hybrid modulation strategies which can also eliminate NP voltage oscillations using a combination of the aforementioned methods. The proposed algorithm avoids abrupt changeover between the two modulations and ensures minimum contribution of non-NTV operation and hence its drawbacks. Moreover, the algorithm is computationally efficient compared with its counterparts, since it only uses redistributed NTV duty ratios to generate NTV and non-NTV modulations instead of using two separate complex algorithms which result in increased computational burden. A non-NTV method called selected three-vector (STV) modulation is chosen to perform the detailed analysis of above proposed algorithm for hybrid NTV–STV modulation. Additionally, a simplified STV (SSTV) is derived to develop a new hybrid NTV–SSTV modulation strategy which further reduces losses and complexity compared with the hybrid NTV–STV modulation, without affecting the performance of NPC. The proposed algorithm and the derived hybrid modulation strategy are validated using simulation and experimental results.

Investigation of a Fault-Tolerant Three-Level T-Type Inverter System

In recent years, a three-level T-type inverter has attracted considerable attention due to its advantages, such as simple structure and higher efficiency. However, the reliability of a three-level T-type inverter is particularly important as increased power switches are used. Therefore, a fault-tolerant three-level T-type inverter is proposed in this paper. Fault diagnosis and fault-tolerant control strategies for power switches both in half-bridge and neutral-point bridge are investigated. Under fault-tolerant operation, redundant fourth-leg is used to balance neutral-point voltage and the low-frequency voltage oscillation is eliminated completely. The fault ride-through capability and high-quality output waveforms can be obtained. The effectiveness of the proposed inverter topology and control methods is validated by the simulation and experimental results.

Vehicle-Grid System Modeling and Stability Analysis With Forbidden Region-Based Criterion

The voltage low frequency oscillation (LFO), between traction network and electric locomotives or electric multiple units in high speed railway, is a typical phenomenon happening in cascade systems with power electronic converters accessed, which is regarded as the stability problem of a vehicle-grid (V2G) system in general. To address the problem, the mathematical model of the V2G system is deduced in detail first, and a generalized dq -decoupled small-signal model of a single-phase pulse width modulation rectifier is established in this process. Then, a novel stability criterion based on forbidden region for dq-decoupled multi-input multi-output cascade systems is proposed to analyze the stability of the V2G system. The proposed criterion has less conservatism than singular-value criteria and norm criteria, and can be applied for systems with either high or low power factor. The comparison with other criteria and simulation results verifies that the proposed forbidden region-based criterion performs well in the analysis of the critical condition of LFO. Moreover, the impact factors of LFO, both grid parameters and vehicle parameters, are researched based on the proposed stability analysis criterion and the simulation verification, which provides design guidance for the V2G system to restrain LFO.

Single pass full penetration joining for heavy plate steel using high current GMA process

As an arc voltage decreases, an arc length shortens and a surface of molten pool right under the arc becomes dented at a high current range in gas metal arc welding (GMAW) using 100% CO2 as a shielding gas and a carbon steel solid wire. And the arc is finally generated at a lower position under the surface of the molten pool and in a space formed by a wall of molten pool. A spatter generation level is extremely low even at a high current range in this metal transfer mode. This arc phenomenon is referred to as “buried arc,” and the arc stabilization technique at the high current buried arc more than 450A is described in this report. The arc phenomenon taken by an X-ray observation system and a high-speed video camera were observed using a low-carbon steel as a base metal, a carbon steel solid wire of 1.2 mm, 100% CO2 as the shielding gas, and travel speed of 30 cm/min. The high current buried arc stabilized by an external characteristic control of welding power source (maximum current 1000 A, maximum wire feed rate 100 m/min) showed three kinds of metal transfer mode including rotating transfer mode at the current range more than 450 A. Consequently, a new current waveform control combined an adequate gradient of the external characteristic and a low-frequency voltage oscillation, which realizes more stable high current buried arc, has been developed in order to improve a weld bead formation using these particular metal transfer modes.

Method to Minimize the Low-Frequency Neutral-Point Voltage Oscillations With Time-Offset Injection for Neutral-Point-Clamped Inverters

This paper proposes a method to reduce the low-frequency neutral-point voltage oscillations. The neutral-point voltage oscillations are considerably reduced by adding a time offset to the three-phase turn-on times. The proper time offset is simply calculated considering the phase currents and dwell time of small- and medium-voltage vectors. However, if the power factor is lower, there is a limitation to eliminate neutral-point oscillations. In this case, the proposed method can be improved by changing the switching sequence properly. Additionally, a method for neutral-point voltage balancing is also proposed. The change in switching frequency by the proposed method and its effects on the outputs are negligible. Simulation and experimental results verify the validity and feasibility of the proposed strategy.

Novel Carrier-Based PWM Strategy of a Three-Level NPC Voltage Source Converter without Low-Frequency Voltage Oscillation in the Neutral Point

A novel carrier-based PWM (CBPWM) strategy of a three-level NPC converter is proposed in this paper. The novel strategy can eliminate the low-frequency neutral point (NP) voltage oscillation under the entire modulation index and full power factor. The basic principle of the novel strategy is introduced. The internal modulation wave relationship between the novel CBPWM strategy and traditional SPWM strategy is also studied. All 64 modulation wave solutions of the CBPWM strategy are derived. Furthermore, the proposed CBPWM strategy is compared with traditional SPWM strategy regarding the output phase voltage THD characteristics, DC voltage utilization ratio, and device switching losses. Comparison results show that the proposed strategy does not cause NP voltage oscillation. As a result, no low-frequency harmonics occur on output line-to-line voltage and phase current. The novel strategy also has higher DC voltage utilization ratio (15.47% higher than that of SPWM strategy), whereas it causes larger device switching losses (4/3 times of SPWM strategy). The effectiveness of the proposed modulation strategy is verified by simulation and experiment results.