Switching Frequency Harmonics Research Articles

A Novel Discrete Hybrid Dual Random SVPWM Scheme for Reducing PMSM Harmonic Intensity

Aiming at suppressing the switching frequen-cy harmonics, a novel discrete hybrid dual random control (DHDRC) technique is proposed in this article. Based on the space vector pulsewidth modulation (SVPWM) strategy, the DHDRC-SVPWM technique is applied to randomize the switching period and pulse position by selecting dual discrete random factors from predefined discrete sequences. Accordingly, the original switching narrowband harmonic energy of SVPWM is extended to a wide range of frequencies. To analyze the harmonic dispersion degree quantitatively, the power spectrum is deduced to assess the performance of the discrete random SVPWM. Then, the connection between the discrete sequence and the harmonic energy distribution is obtained under random variables following a discrete uniform distribution. The obtained results demonstrate that the proposed technique significantly expands harmonic clusters in the frequency domain, thereby reducing the harmonic intensity. The present study provides a reference to investigate random SVPWM and improve electromagnetic interference and noise in power converters.

Redesign of common dc-link capacitors for dual-three phase machine drive system based on PWM sequence reorder

The dual three phase machine drive system with the common dc-link capacitors has been widely used in many industrial applications due to the advantages such as small stallion volume, reduced cost and flexibility. The large ripple current caused by the pulse width modulation (PWM) switching has to be reduced by choosing the large capacitor, while the bulky dc-link capacitor is still the main drawback for the improvement of power density. Therefore, this paper proposes a common dc-link capacitors reduced design method for dual three phase machine drives system with the proposed PWM sequence reorder aiming at the cost, volume and lifetime. First, the derived dc-link current expression are given according to the phase current and average current. Secondly, the PWM sequence reorder method on two inverters is used to reduce the dc-link capacitors switching frequency harmonics. Finally, the design principle of the new capacitors is proposed considering the cost, volume, lifetime. The results show that the lifetime of the capacitors with the proposed reorder PWM method can be extended significantly. The experiment results are given to verify the effectively of the proposed method. The results could provide a good guideline for the dual-three phase drive system design with low cost and high reliability.

Theoretical Analysis of a Fractional-Order LLCL Filter for Grid-Tied Inverters

The LLCL-filter-based grid-tied inverter performs better than the LCL-type grid-tied inverter due to its outstanding switching-frequency current harmonic elimination capability, but the positive resonance peak must be suppressed by passive or active damping methods. This paper proposes a class of fractional-order LLCL (FOLLCL) filters, which provides rich features by adjusting the orders of three inductors and one capacitor of the filter. Detailed analyses are performed to reveal the frequency characteristics of the FOLLCL filter; the orders must be selected reasonably to damp the positive resonance peak while reserving the negative resonance peak to attenuate the switching-frequency harmonics. Furthermore, the control system of the grid-tied inverter based on the FOLLCL filter is studied. When the positive resonance is suppressed by the intrinsic damping effect of the FOLLCL filter, the passive or active damper can be avoided; the grid current single close-loop is adequate to control the grid-tied inverter. For low-frequency applications, proportional-resonant (PR) controller is more suitable for the FOLLCL-type grid-tied inverter compared with the proportional-integral (PI) and fractional-order PI controllers due to its overall performance. Simulation results are consistent with theoretical expectations.

Harmonic-Invariant Scaling Method for Power Electronic Converters in Power Hardware-in-the-Loop Test Beds

Power hardware-in-the-loop (PHIL) is an experimental technique that uses power amplifiers and real-time simulators for studying the dynamics of power electronic converters and electrical grids. PHIL tests provide the means for functional validation of advanced control algorithms without the burden of building high-power prototypes during early TRL. However, replicating the behavior of high-power systems with laboratory SDC can be complex. Inaccurate scaling of the SDC coupled with an exclusive focus on instantaneous voltages and currents at the fundamental frequency can lead to PHIL results that are only partially relatable to the high-power systems under study. Test beds that fail to represent switching frequency harmonics cannot be used for studying harmonic penetration or loss characterization of large-scale converters. To tackle this issue, this paper proposes a harmonic-invariant scaling method (HISM) that exploits the VA rating of preexisting laboratory SDCs for more accurately replicating harmonic phenomena in a PHIL test bench. Firstly,, a theoretical analysis of the proposed method is presented and, subsequently, the method is validated with MATLAB simulations and experimental tests.

Mathematical Modeling and Simulation of AC-AC Three Phase Matrix Converter with LC Filter driving Static Resistive Load

During the last two decades, it has been seen that power conversion from one form to another form of energy was a serious issue in power electronics. After the production of power converters such as rectifiers that convert AC-DC, inverters that convert DC-AC, voltage regulators, choppers, and cycloconverters make possible power conversion. In 1980, Venturini presented the concept of the Matrix Converter in power electronics; it consists of bidirectional IGBT switches and can conduct current and block reverse and forward voltage. Nowadays matrix converter is frequently utilized for power conversion because it can convert AC-AC, AC-DC, DC-DC, and DC- AC directly without an energy storage device which results in less complexity and cost. In matrix converters, high switching frequency harmonics are produced due to the switching of converters and need to be minimized. In this paper, an LC filter is employed to mitigate the problem of harmonics at the output of the three-phase matrix converter. A three-phase matrix converter with an LC filter is presented in this paper and the SVM technique is used for the generation of pulses. Simulation results are carried out through MATLAB software.

A Current-Control Strategy for Grid-Connected Converter Based on Inductance Non-Linear Characteristic Compensation

Inductance is a necessary device for a grid-connected converter (GcC) to attenuate the switching-frequency harmonics in injected grid currents. However, in practice, the inductance decreases with an increase in output current. Especially when the amplitude of sinusoidal currents is higher, the inductance will vary over a wide range as the current value changes in a period. This variation may lead to system instability and cause output current fluctuation. To solve this issue, the model of the GcC with a proportional resonant regulator is firstly built, and the system stability with different current values is analyzed using the Nyquist criterion. The results show that the system stability decreases with an increase in current absolute value. Further, a loop gain compensation unit is embedded into the current regulator to maintain the loop gain constant and ensure the stability of the system under a wide variation range of current values. With this scheme, the compensation unit is only determined by the rated value and the non-linear characteristic of the filter inductance. Therefore, the loop gain compensation unit is independent of the original control system, and the traditional controller parameter design method can also be inherited. Finally, the simulated and experimental results from a 50 A static var generator (SVG) with wide filter inductance variation (using Mega-Flux core) have verified the correctness of the analyses and the effectiveness of the proposed method in this paper.

Characterization and Selection of Probability Density Function in a Discrete Random Switching Period SVPWM Strategy

Aiming at suppressing the switching frequency harmonics in the integrated permanent magnet synchronous motor (PMSM) systems, a discrete random switching period (DRSP) space vector pulse width modulation (SVPWM) technique is proposed in the present study. In the proposed DRSP-SVPWM technique, the switching period is randomized according to a predefined frequency sequence. Accordingly, the narrow-band harmonics of the switching frequency are extended to a wider range. Then, a mathematical correlation between the probability density function and the harmonic distribution is established. Moreover, the effects of discrete uniform and beta distributions on the harmonic spreading are studied. Based on the proposed DRSP-SVPWM technique, the mathematical expression of the harmonic power spectrum is derived, demonstrating the intrinsic dependency between the discrete random factor and the harmonic energy distribution. Finally, a series of simulations and experimental results are compared to evaluate the performance of the proposed technique. The present study is expected to provide a theoretical basis for the design and selection of discrete frequency sequences. Meanwhile, it provides a reference to investigate the random SVPWM and improve noise and electromagnetic interference in power converters.

Adaptive Neuro-Fuzzy Voltage Control for LCL-Filter Grid-Connected Converter

Inductance – Capacitance – Inductance (LCL) filter is a very attractive candidate for renewable energy system applications due to its high efficiency. High attenuation of the switching frequency harmonics, small size, low fee, and improving the overall harmonic distortion (THD). This paper presents how voltage is affected by increased loads or voltage sag. Therefore it is necessary to control it with certain controllers. The Adaptive Neuro-Fuzzy Inference System (ANFIS) is used as an intelligent controller, the voltage constraint as training data for ANFIS obtained from PI. The filter works in a good connection between the inverter and the grid and rewords unwanted harmonics from using the inverter. The mathematical models for the LCL filter are investigated. The proposed approach shows more effective results than previous performance for voltage controlling and harmonic reduction. It gives overshoot (0.5%), steady state error (0.005), settling time (0.03 sec), rise time (0.005 sec), and improving THD 8.67% to 2.33% by comparing these results of ANFIS respectively with the results of PI which gave(3%),(0.01),(0.2sec)and( 0.02sec).

Low Switching Frequency SPWM Strategies for Open-Winding Machine With Low Current Harmonics

In this article, two novel sinusoidal pulsewidth modulation (SPWM) strategies with simple implementation are proposed for an open-winding machine fed by isolated dc-bus dual two-level three-phase inverters to reduce switching frequency and current harmonics. The proposed strategies utilize zero sequence voltage injection and unbalanced reference voltage distribution to generate discontinuous PWM with low current harmonics and less switching action. Two different zero sequence voltages are introduced in this article corresponding to two proposed PWM strategies with different switching actions in a PWM cycle. The novelty of this article is low current harmonics and less switching actions in a PWM cycle compared with the conventional SPWM strategy. Moreover, compared with the subhexagonal center PWM strategy, the proposed SPWM strategies have advantages of easy implementation and half switching commutations with the same current harmonic characteristics. The superiority of the proposed SPWM strategies is validated by simulation and experiment results.

A Transformerless AC-AC Converter with Improved Power Quality Employed to Step-Down Power Frequency at Output

Variable voltage and frequency are required to govern the torque-speed characteristics of many industrial drive systems. Traditionally, this is achieved with a power converting system implemented with multistage converters. This technology is based on rectifying AC power into DC and then DC into AC with an inverter circuit. The power quality concerns of both conversion stages are tackled by selecting high switching frequency PWM control and harmonics mitigation filters. Also, using a bulky DC-link capacitor is one of the big sources of low system reliability, so this approach increases the conversion losses, circuit, and control complications. The frequency step-down conversion is very attractive with direct AC-AC converters as it has a simple control and circuit structure, but these converters face poor power quality challenges once the output frequency is decreased with respect to an input. In these converters, the total harmonic distortion (THD) of the output voltage becomes very poor once the output frequency is reduced. The problem of high THD of the output is addressed in the power converting circuits implemented with line frequency multi-winding transformers. The required number of output winding and switching devices (diodes and thyristors) increases once the value of the output frequency is decreased. This will increase the overall volume, cost, and losses. The use of a bulky and costly line frequency transformer may be eliminated if AC voltage controllers have non-inverted and inverted voltage buck capabilities, such existing topologies either have complex control schemes or require a large number of operating devices. Therefore, in this research article, a new transformerless frequency step-down converter employing fewer devices is proposed. This approach is realized with a high-frequency controlled rectifier for the required voltage stabilization and a low-frequency inverter bridge for frequency control. Its validation is supported by the results attained from Simulink and practical-based prototypes.

An Improved FCS-MPC Based on Virtual Vector Expansion and Sector Optimization for 2L-VSCs

Conventional finite control set model predictive control (FCS-MPC) outputs one voltage vector per period by vector traversal algorithm, which leads to low control accuracy, unnecessary high computational burden and unfixed switching frequency. In this paper, an improved FCS-MPC based on virtual vector expansion and sector optimization is proposed to improve system performance and reduce computational burden for 2L-VSCs. Three aspects are improved as follows. First, a sector optimization method based on effective vector radiation range is proposed to directly determine the optimal vector without vector traversal. Second, a virtual vector expansion method based on vector synthesis algorithm with fixed 1/4 & 1/2 & 3/4 duty cycle is proposed to expand optional vectors from 8 to 44, and improve vector accuracy and system performance. Third, a modulated vector output mode is adopted per period to fix switching frequency and decrease current harmonics. Finally, the advantages of proposed MPC are verified by simulation and experiment.

Effect of Semiconductor Parasitic Capacitances on Ground Leakage Current in Three-Phase Current Source Inverters

This paper investigates the influence of power semiconductor parasitic components on the ground leakage current in the three-phase Current Source Inverter topology, in the literature called H7 or CSI7. This topology allows reducing converter conduction losses with respect to the classic CSI, but at the same time makes the topology more susceptible to the parasitic capacitances of the semiconductors devices. In the present work, a grid-connected converter for photovoltaic power systems is considered as a case study, to investigate the equivalent circuit for ground leakage current. The same analysis can be extended to applications regarding electric drives, since the HF model of electric machines is characterized by stray capacitance between windings and the stator slots/motor frame. Simulation results proved the correctness of the proposed simplified common-mode circuit and highlighted the need of an additional common-mode inductor filter in case of resonance frequencies of the common-mode circuit close to harmonics of the power converter switching frequency. Experimental results are in agreement with the theoretical analysis.

Sensorless Field-Oriented Control for Open-End Winding Five-Phase Induction Motor With Parameters Estimation

This paper proposes a sensorless field-oriented control (FOC) of an open-end stator winding five-phase induction motor (OESW-FPIM). The FOC technique used is associated with dual Space Vector Modulation (SVM) to provide a constant switching frequency and lower harmonics distortion. Furthermore, a simple hybrid observer is proposed which combines a model reference adaptive system (MRAS) and a sliding mode (SM) observer. The examined observer is designed for the estimation of the rotor flux and rotational speed as well as for the estimation of the load torque disturbances. Lyapunov theorem is used in this paper to prove the observer's stability. The work presented in this paper aims to enhance the researched motor's sensorless control and its robustness against external load disturbances and parameters variation. In the proposed MRAS-SM observer, the reference model is replaced by a SM model which uses a sigmoid function as a switching function to overcome the chattering problem. This combination is intended to make use of the advantages of both strategies. At the same time, to preserve the high-level performance of the sensorless FOC technique and to reduce system uncertainties, an estimation algorithm is developed to identify the rotor resistance and the stator resistance simultaneously during motor operation. The parameter estimation algorithm is combined with the proposed control to improve the speed estimation and control accuracy, particularly at low-speed operation. Finally, the effectiveness of the proposed control is validated in real-time by utilizing a hardware-in-the-loop (HIL) platform.

DC Capacitor-Less Solid-State Variable Capacitor

H -bridge-based single-phase applications have second-order harmonics on the dc-link voltage. Conventionally, the second-order harmonics are filtered by bulky dc capacitors to maintain the constant dc-link voltage, which results in poor power density and low reliability. However, in applications, such as the solid-state variable capacitor (SSVC), the constant dc-link voltage is unnecessary since the dc link is disconnected from the load or source. This article proposes a dc capacitor-less SSVC, which removes the dc capacitors from the topology. The dc voltage is supported by the ac-side voltage directly. Only a 0.03 per unit (p.u.) capacitor is needed to filter the switching-frequency harmonics. This enables the proposed SSVC to use the snubber capacitors to achieve the function of the 0.03-p.u. filtering capacitor. Other capacitors can be removed from the dc-link busbar. The modulation, modeling, and control strategy of the proposed dc capacitor-less SSVC are provided in this article. The proposed dc capacitor-less SSVC is validated by the experiment.

Current Control and Active Damping for Single Phase LCL-Filtered Grid Connected Inverter

LCL filter has been widely used in the grid connected inverter, since it is effective in attenuation of the switching frequency harmonics in the inverter. However, the resonance in this filter causes stability problems and must be damped effectively to achieve stability. There are some methods to damp the resonance; one method is passive damping of resonance by adding a series resistor with the filter capacitor, but passive element reduces the inverter efficiency. Other method uses active damping (AD) by adding a proportional control loop of filter capacitor current, but this method needs additional sensor to measure filter capacitor current; moreover, when the control loops are digitally implemented, the computation delay in AD control loop will lead to some difficulties in choosing control parameters and maintaining system stability. This paper presents current control scheme for the grid connected inverter with the LCL filter. The proposed scheme ensures the control of injected current into grid with AD of the resonance in the LCL filter while keeping system stability and eliminating the effect of computation delay of the AD loop. An estimation of filter capacitor current with one step ahead is performed using the discrete time observer based on measuring the injected current. This reduces the cost and increases the robustness of the system. Proportional Resonant (PR) controller is used to control the injected current. Design of control system and choosing its parameters are studied and justified in details to ensure suitable performance with adequate stability margins. Simulation and experimental results show the effectiveness and the robustness of the proposed control scheme.

Design, control and comparative analysis of an LCLC coupling hybrid active power filter

This study provides the design, control and comparative analysis of the proposed LCLC coupling hybrid active power filter (LCLC‐HAPF). Compared with the conventional L filter coupling active power filters (APFs), LC filter and LCL filter coupling HAPFs (LC‐HAPFs and LCL‐HAPFs), the proposed LCLC‐HAPF provides better switching frequency harmonics elimination, lower dc‐link voltage rating and switching loss. Although the LCLC‐HAPF has some advantages, two main challenges will still be confronted in practical implementations. Those are stability problems caused by resonance of LCLC filter, and parameter design for the LCLC filter. In this study, a new active damping method is proposed to overcome the high order resonance problem existing in the proposed LCLC‐HAPF. After that, the stability analysis and parameter design are given for the proposed LCLC‐HAPF. Finally, the effectiveness of the proposed LCLC‐HAPF topology is validated through both simulation and experimental studies in comparison with the conventional APFs and HAPFs.

Modular Multilevel Converter Switching Frequency Harmonics Analysis and Suppression Through Cell Voltage Control

This paper investigates the impact of reference's bias on the high frequency voltage spectrum for the modular multilevel converters (MMC). Analytical solution of the voltage spectrum under carrier Phase Shifted Pulse Width Modulation (PSPWM) is derived using double Fourier integral analysis. The distribution of the switching frequency voltage harmonics is found to be directly related to the ratio between the dc-link voltage and the cell voltage. A control method utilizing this phenomenon is thereby proposed to suppress switching frequency harmonics in the Differential Mode Voltage (DMV) and Common Mode Voltage (CMV). Cell voltage control trade-offs considering different cell numbers, different ac voltages, 3rd harmonics injection, and cell voltage stress are also investigated. The analysis and control method are validated through experiments on a 1.25 megawatt MMC testbed.

ADRC-Based Current Control for Grid-Tied Inverters: Design, Analysis, and Verification

The conversion and utilization of renewable energy generations often require grid-tied inverters. When an LCL filter is applied to attenuate the high switching frequency harmonics, it is complex to design a controller with proper parameters due to the characteristics of the LCL filter and system uncertainties. In this article, with LCCL filter, the order of the inverter system is degraded from third order to first order, and an active disturbance rejection control-based current control strategy for LCCL -type grid-tied inverters is proposed. The proposed strategy is able to treat the unknown dynamics and the external disturbance of the inverter system as overall disturbance through a single structure, and the closed-loop system is regulated by an improved control law with reference differential feedforward. Moreover, with parameter uncertainties considered, the robustness of the proposed strategy is studied through an internal model control structure. A 2-kW experimental prototype has been tested to verify the effectiveness of the proposed scheme.

A Modular Multilevel Converter With Filter Capacitor for Long-Cable-Fed Drive Application

This paper proposes a modular multilevel converter (MMC) with a filter capacitor for medium-voltage long-cable-fed drive applications. Arm inductors that are inherent components in each phase of MMC are tuned with a filter capacitor to behave like a low-pass LC filter at the MMC output terminal. As a result, the output voltage of the converter is highly sinusoidal with negligible switching frequency harmonics. The overvoltage phenomenon that occurs at motor terminal due to cable resonance and voltage reflection is almost eliminated, and hence, with the proposed concept, the motor can be installed at very long distances from the converter. The voltage amplification at the motor terminal is negligible for different cable lengths with the proposed filter. Additionally, the output voltage quality of the converter is not degraded due to internal fault and subsequent bypass in power cells. The validity of the proposed concept is supported by simulation and experimental results.

Developing a grid-connected DFIG strategy for the integration of wind power with harmonic current mitigation

<p>The aim of this paper is to present a study of the efficiency of the electrical part of a wind generation system. Two back-to-back PWM voltage-fed inverters connected between the stator and the rotor are used to allow bidirectional power flow. The second inverter grid side, has a role of a power active filter, to eliminate the harmonic generated by the non linear load, in the same time gives an active and reactive power needed by the rotor of DFIG. The harmonics of switching frequency in the current stator, pose a major problem in the moment where commutations in the diode bridge, to solve this problem, we introduce a small-sized passive LC filter for the purpose of eliminating high-frequency shaft voltage and grid current from a DFIG driven by a voltage-source pulse width-modulation rotor inverter controlled with SVM. The control theory is discussed, and the controller implementation is described. Design criteria are also given. The results of simulation tests show excellent static and dynamic performances.</p>