Three-phase Output Currents Research Articles

Inverter fault diagnosis based on Fourier transform and evolutionary neural network

The fault diagnosis of the inverter is fundamental to energy intelligence. Due to the complex characteristics of the inverter (e.g., high-dimensional decision and poor stability), it is challenging to solve the problem using traditional fault diagnosis methods. Recently, artificial intelligence (AI)-based approaches have emerged as the most promising methods. However, they often require to set hyperparameters manually, which hinders further AI-based applications in fault diagnosis of inverters. To fill the gap, we propose an inverter fault diagnosis method using fast Fourier Transform (FFT) and evolutionary neural network. This method combines the amplitude of low-frequency harmonic component of the three-phase inverter output current which is obtained by FFT and the average value in a period of three-phase inverter output current into the fault eigenvector. This method uses an evolutionary neural network trained by combining genetic algorithm (GA), ant colony optimization (ACO) algorithm and Back-propagation (BP) algorithm to realize fault diagnosis. This method can effectively resist noise interference and reduce the number of independent variables in the part of feature extraction, so that it can simplify the network model. In addition, this method can avoid the network training from trapping in local optima in the part of fault classification, with high accuracy and fast response speed. The experimental results show that the proposed algorithm and method of fault feature extraction can effectively detect and locate the insulated-gate bipolar transistor (IGBT) with open circuit (OC) fault in three-phase inverter, and can be applied to online monitoring.

Online Diagnosis and Ride-Through Operation for Cascaded H-Bridge Converter Based STATCOM With a Single Open-Circuit IGBT

To address the most commonly seen single insulated gate bipolar transistor (IGBT) open-circuit fault problems in cascaded H-bridge (CHB) converter based static synchronous compensator, a simple online fault diagnosis method and the corresponding ride-through operation approach are proposed. The method can quickly determine the exact location of the open-circuit IGBT through three steps, via analyzing the relationships between the dc bias in the three-phase output current of CHB converter, the transient dc rail voltage variations with respect to the averaged dc rail voltage, and the response of dc rail voltage and output current when the modulation approach of the faulty power module is adjusted. Afterwards, a fault ride-through approach is adopted to seamlessly transfer the modulation approach of faulty H-bridge module and it behaves as a half-bridge to provide a half of the rated output ac voltage. Meanwhile, the corresponding dc offset voltage from this faulty module is compensated by producing the same dc voltage in the healthy phase legs to avoid the dc bias in the three-phase output current. It has been demonstrated that the proposed fault diagnosis approach and the fault ride-through approach are effective in various adverse situations including grid voltage sudden disturbances and the load power sudden changes. Comprehensive simulated and experimental results have been provided to validate the effectiveness of the proposed approach.

Design and Implementation of Input AC Filters and Predictive Control for Matrix-Converter Based PMSM Drive Systems

Matrix converters have many advantages, including high-efficiency, single-stage AC/AC energy conversion, bidirectional power flow, a near-unity input power factor, sinusoidal three-phase input currents, and sinusoidal three-phase output currents. However, matrix converters have 360 Hz voltage pulsations at the virtual DC-bus, which produce input harmonic currents and output harmonic currents, which cause unsatisfactory responses. To solve the problem of the input harmonic currents, a systematic design of an input three-phase current modulation method and an input three-phase AC filter that uses two different design methods are proposed. In addition, to improve dynamic responses, two predictive speed controllers are investigated and compared, and a predictive current controller is studied to reduce the output harmonic currents. A digital signal processor and an FPGA are used to execute the control algorithms. Several experimental results validate the theoretical analysis and show that the proposed methods effectively improve the power quality of the PMSM drive system and its input power-source quality.

Compound Fault Diagnosis Method of Modular Multilevel Converter Based on Improved Capsule Network

When submodule (SM) compound fault occurs in the Modular multilevel converter (MMC), the time-domain waveform characteristics of output current and internal circuiting current of MMC are not obvious, especially on the condition of high-level MMC. To address this issue, A compound fault diagnosis method based on an improved capsule network (CapsNet) is proposed in this paper. Firstly, The front end of the network adopts the feature extraction structure of One-dimensional Convolutional Neural Network (1DCNN) combined with Long and short-term memory network (LSTM). Then, the original data of MMC three-phase output currents and three-phase circulating currents are used as fault detection signal. The back-end employs a main capsule layer and a digital capsule layer structure, and realizes the transfer of feature vectors through a dynamic routing algorithm. This feature extraction structure combines the light weight of 1DCNN and the sequential sensitivity of LSTM. While ensuring that the information is fully extracted, the computational cost of the model is greatly reduced. Furthermore, the overlapping sampling method is used to construct and expand the sample sets. Compared with the other three common deep learning methods, their fault diagnosis performance under the condition of different levels and levels changing are analyzed respectively. The experimental results demonstrate that the proposed method has excellent cross-domain learning ability and high fault recognition accuracy.

Reactive power control of grid-connected photovoltaic micro-inverter based on third-harmonic injection

<span lang="EN-US">This paper presents four interleaved flyback DC-DC converters. Each flyback converter contains a separate maximum power point tracker (MPPT). The MPP tracker is used to collect the maximum power value from photovoltaic panel by using (P&O) algorithm with an output of reference current. In latest years, the active third-harmonic injection circuit have received much interest, this technique contributed to getting a better quality of injection current into the utility grid and control the reactive power. For converting direct current to three-phase sinusoidal currents, a line-commutated current source inverter type (CSI) with filter is used. The developed micro-inverter of (1000 W) offers an expanded range of reactive power control with balanced three-phase output power and good efficiency 95.07%. The effectiveness of the suggested system is clarified through the MATLAB program simulation. The system proposed in this paper has proven its effectiveness in obtaining reactive power control, nearly sinusoidal three-phase output currents and it is compared with the traditional PV micro-inverter system. The comparison shows that the PV micro-inverter is best in more specification than the traditional PV system such as efficiency and total harmonic distortion (THD) and the system losses.</span>

Cascaded Multilevel Rectifier with Common High Voltage Direct Current (HVDC) Bus and Its Control Method

In order to realize the voltage equalization and minimize the total harmonic distortion rate of current of cascaded multilevel rectifier with common high voltage Direct Current (HVDC) bus, the cascaded multilevel rectifier with common high voltage DC bus and its control method are studied. The output side DC voltage of the cascade multilevel rectifier with common high voltage DC bus is sampled, and the output DC current signal amplitude is obtained according to the calculated output side DC voltage signal. On this basis, the AC side voltage corresponding to the applied switching state and the switching state corresponding to the rectifier in the sampling period are solved. The driving signal of the switch device is generated and sent to the corresponding switch device to realize the active power factor correction and sinusoidal input current, so as to realize the balanced control of output DC capacitor voltage of cascaded multilevel rectifier with common High-Voltage Direct Current (HVDC) bus. The test results show that this method can effectively control the three-phase output current of the rectifier and ensure the maximum output of the current. The lowest total harmonic distortion rate of the output phase voltage is about 40%, which can effectively ensure the stability of the output phase voltage after control.

Performance Comparison of Reactive Power Control Methods of Photovoltaic Micro-inverter

Photovoltaic power generation becomes an important technology in recent year, because of its advantages such as clean energy, pollution reduction, no gas emissions, maintenance and operation requirements are low. So it is a new path for generating electric power. This paper will study the performance comparison of two photovoltaic systems of three-phase grid-connected micro-inverter and study their design methods of reactive power control. The first design is suggested that the photovoltaic PV system based on an interleaved DC-DC boost type converter with its maximum power point tracking control (MPPT) for each boost converter. A voltage source inverter type (VSI) is used as a three-phase micro-inverter. By controlling the direct and quadrature components of inverter output currents, reactive power controlling is achieved at 90.76% efficiency. The second design is suggested that the photovoltaic panel is connected DC-DC converter of an interleaved flyback type. Each sub-converter is controlled by an individual (MPPT). The circuit of active third-harmonic current injection in recent years have received much interest, this technique contributed to get better quality of current injected into the utility grid and to control the reactive power with good efficiency 95.07%. A line-commutate current source inverter type (CSI) with filter is used. The developed micro-inverter of (1000W) offers an expanded range of reactive power control with balanced three-phase output power. Each system design in this study has proven its effectiveness in obtaining control of reactive power and nearly sinusoidal three-phase output currents. The effectiveness of the suggested systems are clarified by using the MATLAB Simulink program and the results of the simulation show the validity of the suggested micro-inverter system.

Common-Mode Voltage Reduction Method for Three-Level Inverter With Unbalanced Neutral-Point Voltage Conditions

The three-level inverter has been widely researched and adopted in industry. This article further proposes a carrier-based modulation method to reduce the common-mode voltage (CMV) and obtain high-quality output currents for three-level inverter with unbalanced neutral-point voltage conditions. In this method, the zero-sequence voltage is calculated by three-phase modulation waves, three-phase output currents, and voltages across the dc-link capacitors. The limitations of the zero-sequence component are carefully considered in order not to generate the basic voltage vector with high CMV magnitudes. In addition, a deadbeat control scheme for controlling the dc-link voltages asymmetrically with the proposed pulsewidth modulation method is presented, and asymmetrical control of dc-link voltages can be realized with fast response. Compared with the conventional space vector modulation (SVM) and the virtual SVM methods, the proposed scheme can reduce the CMV by half. Simulated and experimental results validate the effectiveness of the proposed method.

Analysis of Input Voltage Switching Ripple in Three-Phase Four-Wire Split Capacitor PWM Inverters

Three-phase, four-wire split capacitor inverters are currently employed in many applications, such as photovoltaic systems, battery chargers for electric vehicles, active power filters, and, in general, in all grid-tied applications that deal with possible grid voltage and/or current unbalances. This paper provides a comprehensive evaluation of the capacitor-switching voltage ripple and dc-link switching voltage ripple for the three-phase, four-wire, split capacitor inverters. Specifically, analytical formulations of the peak-to-peak and rms values of the voltage ripples are originally pointed out in this paper and determined in the case of balanced three-phase and unbalanced (two-phase and single-phase) output (ac) currents. The obtained results can help in designing the considered inverter and sizing of the dc-link capacitors. Reference is made to the sinusoidal PWM modulation and sinusoidal three-phase output currents with an almost unity power factor, representing a grid-connected application. Extensive numerical simulations have been carried out to thoroughly verify all the analytical developments presented in this paper. Furthermore, some experimental tests, having balanced output currents on the ac side, have been accomplished, validating numerical simulations and analytical developments.

An On-Line Multiple Open-Circuit Fault Diagnostic Technique for Railway Vehicle Air-Conditioning Inverters

The safety and reliability of inverters play a significant role in stable power electronic system operations. Considering a railway vehicle air-conditioning inverter as a case study, this paper presents a generic open-circuit fault diagnosis method that can be applied to various inverter and control schemes. First, a detailed model of three-phase output currents under both normal and fault status is derived, and then the concept of space trajectory is introduced based on the three-phase output currents. The open-circuit faults of the inverter lead to output phase current distortion. Therefore, different types of faults have different space trajectories, and the corresponding shape of the space trajectory is considered to be a fault feature. Then, the fault is located by the direction vector and center-of-mass coordinate. The effectiveness and reliability of the diagnosis method are verified and analyzed using a hardware-in-the-loop test platform.

Analysis of a Synergetically Controlled Two-Stage Three-Phase DC/AC Buck-Boost Converter

Three-phase DC/AC power electronics converter systems used in battery-powered variable-speed drive systems or employed in three-phase mains-supplied battery charger applications usually feature two power conversion stages. In both cases, typically a DC/DC stage is attached to a three-phase DC/AC stage in order to enable buck-boost functionality and/or a wide input-output voltage operating range. However, a two-stage solution leads to a high number of switched bridge-legs and hence, results in high switching losses, if the degrees of freedom available for controlling the overall system are not utilised. If the DC/DC stage is used to vary the DC link voltage with six times the AC-side frequency, a pulse width modulation (PWM) of always only one phase of the DC/AC stage is sufficient to achieve three-phase sinusoidal output currents. The clamping of two phases (denoted as 1/3 PWM) leads to a drastic reduction of the DC/AC stage switching losses, which is further accentuated by a DC link voltage which is lower than for the conventional modulation schemes. This paper details the operating principle of a three-phase buck-boost converter system using 1/3 PWM and outlines an appropriate control system design. Subsequently, the switching losses and the voltage/current stresses on the converter components are analytically derived. There, a more than 66% reduction of the DC/AC stage switching losses is calculated without any increase of the stress on the remaining converter components. The theoretical considerations are finally verified on a hardware demonstrator, where the proposed modulation strategy is experimentally compared against several conventional modulation techniques and its clear performance advantages are validated.

A Modified Third Harmonic Pulse-Width Modulation for Reduced Switching Loss in Cascaded H-Bridge Multilevel Inverters

Abstract A cascaded H-bridge multilevel inverter (CHMI) is one of the promising candidates for high-voltage direct current (HVDC) distribution system. The huge number of inverter cells and switching devices, which consists CHMI, imply a great switching loss and it requires increased cost and size of the cooling components. To reduce the huge switching loss, a pulse-width modulation (PWM) strategy is proposed in this paper, which is composed of two different modulation methods. In addition, an additional cell rotation method is proposed to evenly distribute power among inverter cells which allow shaping three-phase output currents with reduced switching loss. The simulation results verify the validity of the proposed modulation scheme.

Mitigation of DC and Harmonic Currents Generated by Voltage Measurement Errors and Grid Voltage Distortions in Transformerless Grid-Connected Inverters

The dc offset and scaling errors in the voltage measurements cause the injection of undesired dc and harmonic currents into the three-phase output currents of the grid-connected inverter. This study proposes an enhanced current control scheme to eliminate the dc and harmonic currents caused by voltage distortion and voltage measurement error in three-phase grid-connected inverters. The proposed current controller is designed in the synchronous ( d-q ) reference frame and composed of a proportional resonant (PR) controller and a repetitive controller (RC). The role of the RC is to regulate the grid current follow the reference value as well as compensate all $6{\rm{n}}\omega _{{\rm{s}}}$ and $(6{\rm{n}}\pm 2)\omega_{{\rm{s}}}\,(n= 1,\,2,\,3,\ldots)$ harmonic components caused by distorted grid voltage and scaling errors. Meanwhile, the PR controller helps to compensate the dc current generated by dc offset measurement errors to guarantee that the three-phase grid currents are balanced and sinusoidal with extremely low dc component. Since the RC employed in the proposed current controller reduces to time delay by four times compared to the conventional RC, the proposed control system ensures good steady-state performance of the grid current without deteriorating its fast dynamic response. The effectiveness of the suggested solution is verified by various experimental tests.

Intelligent Wind Power Smoothing Control using Fuzzy Neural Network

An intelligent wind power smoothing control using fuzzy neural network (FNN) is proposed in this study. First, the modeling of wind power generator and the designed battery energy storage system (BESS) are introduced. The BESS is consisted of a bidirectional interleaved DC/DC converter and a 3-arm 3-level inverter. Then, the network structure of the FNN and its online learning algorithms are described in detail. Moreover, actual wind data is adopted as the input to the designed wind power generator model. Furthermore, the three-phase output currents of the wind power generator are converted to dq-axis current components. The resulted q-axis current is the input of the FNN power smoothing control and the output is a gentle wind power curve to achieve the effect of wind power smoothing. The difference of the actual wind power and smoothed power is supplied by the BESS. Comparing to the other smoothing methods, a minimum energy capacity of the BESS with a small fluctuation of the grid power can be achieved by the FNN power smoothing control. In the experimentation, a digital signal processor (DSP) based BESS is built using two TMS320F28335. From the experimental results of various wind variation sceneries, the effectiveness of the proposed intelligent wind power smoothing control is verified.

Current similarity based open-circuit fault diagnosis for induction motor drives with discrete wavelet transform

Three-phase voltage-source inverters fed induction motor with space-vector controlled scheme are widely used in industrial applications. The suffered failures will degrade the system performance with output torque ripple and harmonic currents. In this paper, a novel diagnosis method is proposed to detect and locate the insulated gate bipolar translator open-circuit fault. Discrete wavelet transform is used as a pre-treatment technique for three-phase output currents, the approximate coefficients are applied to obtain energy vectors. Euclidean distance between every two of the energy vectors are calculated for measuring the current similarity to diagnose fault. When IGBTs occur open-circuit fault, the values of Euclidean distance will be smaller than that under normal conditions, then faults can be detected. Faults can also be located according to extracted features. Simulation and experimental results show high efficiency and merits of the proposed method.

Intelligent wind power smoothing control with BESS

An intelligent wind power smoothing control using recurrent fuzzy neural network (RFNN) is proposed in this study. First, the modeling of wind power generator and the designed battery energy storage system (BESS) are introduced. The BESS is consisted of a bidirectional interleaved DC/DC converter and a 3-arm 3-level inverter. Then, the network structure of the RFNN and its online learning algorithms are described in detail. Moreover, actual wind data is adopted as the input to the designed wind power generator model. Furthermore, the three-phase output currents of the wind power generator are converted to dq-axis current components. The resulted q-axis current is the input of the RFNN power smoothing control and the output is a gentle wind power curve to achieve the effect of wind power smoothing. The difference of the actual wind power and smoothed power is supplied by the BESS. The minimum energy capacity of the BESS with a small fluctuation of the grid power can be achieved by the RFNN power smoothing control. A digital signal processor (DSP) based BESS is built using two TMS320F28335. From the experimental results of various wind variation sceneries, the effectiveness of the proposed intelligent wind power smoothing control is verified.

Intelligent PV Power Smoothing Control Using Probabilistic Fuzzy Neural Network with Asymmetric Membership Function

An intelligent PV power smoothing control using probabilistic fuzzy neural network with asymmetric membership function (PFNN-AMF) is proposed in this study. First, a photovoltaic (PV) power plant with a battery energy storage system (BESS) is introduced. The BESS consisted of a bidirectional DC/AC 3-phase inverter and LiFePO4 batteries. Then, the difference of the actual PV power and smoothed power is supplied by the BESS. Moreover, the network structure of the PFNN-AMF and its online learning algorithms are described in detail. Furthermore, the three-phase output currents of the PV power plant are converted to the dq-axis current components. The resulted q-axis current is the input of the PFNN-AMF power smoothing control, and the output is a smoothing PV power curve to achieve the effect of PV power smoothing. Comparing to the other smoothing methods, a minimum energy capacity of the BESS with a small fluctuation of the grid power can be achieved by the PV power smoothing control using PFNN-AMF. In addition, a personal computer- (PC-) based PV power plant emulator and BESS are built for the experimentation. From the experimental results of various irradiance variation conditions, the effectiveness of the proposed intelligent PV power smoothing control can be verified.

A Low-Speed, High-Torque Motor Drive Using a Modular Multilevel Cascade Converter Based on Triple-Star Bridge Cells (MMCC-TSBC)

This paper provides experimental verification of a modular multilevel cascade converter based on triple-star bridge cells (MMCC-TSBC) for a low-speed, high-torque motor drive. The TSBC is one of direct ac-to-ac power converters capable of achieving bidirectional power flow and drawing/feeding three-phase sinusoidal input (supply-side) and output (motor-side) currents with any power factor at both sides. This paper designs, constructs, and tests a downscaled drive system combining a 400-V, 15-kW, phase-shifted-carrier PWM TSBC equipped with four bridge cells per cluster and a 320-V, 38-Hz, 6-pole, 15-kW induction motor loaded at the rated torque. Experimental waveforms obtained from the downscaled drive system verify stable operation across a broad range from a standstill to the rated speed, showing satisfactory start-up performance.

TSBC方式モジュラー・マルチレベル・カスケード変換器（MMCC-TSBC）を用いた低速高トルクモータ駆動

SUMMARY This paper provides an experiment-based discussion on a modular multilevel cascade converter based on triple-star bridge cells (MMCC-TSBC) for a low-speed high-torque motor drive. The TSBC is a direct ac-to-ac power converter capable of achieving bidirectional power flow as well as drawing and feeding three-phase sinusoidal input (supply-side) and output (motor-side) currents with any power factor at both sides. This paper discusses active dc-capacitor-voltage control applied to the low-speed, high-torque motor drive. A specially designed downscaled system combining a 320-V, 38-Hz, 6-pole, 15-kW induction motor with a 400-V, 15-kW TSBC is constructed and tested to confirm the validity of the motor drive. Experimental waveforms obtained from the downscaled system confirm stable operation with the rated load torque across a range from a standstill to the rated speed, including satisfactory start-up performance.

Predictive Control of Switching System for DC/AC Converter

In this paper the predictive control of switching system is introduced to the DC/AC converter. Our aim is to control the output power of converter closer to the reference. By considering the working status of DC/AC converter is actually switched among several definite subsystems, the switching system model of DC/AC converter is constructed. The tracking problem of converter output power can be changed to be that of its three-phase output currents. Based on the switching system model of converter, the three-phase output currents of those subsystems are predicted and the switching command is determined by examining the discrepancies between the three-phase predictive currents corresponding to each subsystem and the reference ones. Simulations demonstrate that our control scheme for DC/AC converter connected with ultra-capacitor has great advantages in output power tracking and sinusoidal output current waveforms.