Switching Harmonics Research Articles

Modeling, Analysis, and Design of Multifunction Grid-Interfaced Inverters With Output LCL Filter

The purpose of this paper is to investigate the model, control, and implementation of a multifunction grid-interfaced inverter with output LCL filter, which can provide high performance active power current and compensate the existent harmonics simultaneously in a distributed network. Equipped with LCL filter, the proposed multifunction inverter offers reduced switching harmonics and superior output current shapes. However, the multifunction inverter with output LCL filter brings some challenges, including LCL resonance, phase lag, and complexity in system design. To address these issues, this paper analyzes and develops general Thevenin/Norton models for the multifunction grid inverters with LCL filter. Based on the general models of system, this paper presents guidelines of the control and the procedure of filter design for this application. In particular, a proportional-resonant (PR) plus odd-harmonic repetitive control (OHRC) scheme is designed for the outer current loop. The phase compensation method and detailed design criteria for the proposed control scheme are presented. Furthermore, the proposed OHRC scheme is compared with the previous multiple resonant control (MRC) based on their internal relationship. Simulation and experimental results are provided to validate the effectiveness and advantages of the proposed control strategy.

Two Degrees of Freedom Active Damping Technique for $LCL$ Filter-Based Grid Connected PV Systems

In grid connected photovoltaic (PV) systems, lowpass filters are utilized to reduce injected current harmonics. LCL filters have recently drawn attention for PV system grid interfaces due to their small size and they have shown better attenuation to switching harmonics than simple L filters. However, the LCL filter causes resonance resulting in oscillation and instability issues. This paper proposes an effective active damping technique by introducing a two-degree-of-freedom (2DOF) PID control structure. The 2DOF control structure allows the independent action of PI and D terms giving two degrees of freedom. The design is based on a typical three-phase grid-tied PV system. The active damping control loop is formed by using the existing grid side inductor currents and thus eliminating the need of additional sensors. The relative stability is illustrated in frequency domain by using bode plots. A real-time hardware-in-loop study is performed to validate the performance of the proposed 2DOF technique to damp out the LCL filter resonance.

Asymmetric SVM Technique for Minimizing Switching Loss of Inverter

In power electronics, the pulse width modulation (PWM) inverter is one of the families of electrical circuits that converts the electrical supply from one level to another using semiconductor-based electronic switches. The important characteristic of these types of circuits is that the switches are operated only in one of two states: ON or OFF. The continuous conduction operations of these transitions cause switching power losses, which lead to power loss in PWM inverter. The purpose of the switch is to assist the process of transferring power from source to load. So, these power losses affect the performance and efficiency of the load. To overcome these problems, in this paper, asymmetric space vector pulse width modulation (ASVPWM) technique is proposed. In the proposed ASVM, the additional voltage sectors’ switching states are determined by state transition matrix. Subsequently, using the proposed technique, the speed of the induction motor (IM) is controlled. The error speed and change of error speed of an IM are calculated using a fuzzy logic controller (FLC). The output of FLC is applied to the ASVPWM inverter and the stability of the IM is improved. Then, the switching loss and harmonics effect of the proposed ASVPWM inverter are analyzed.

Calculation of Magnet Losses in Concentrated-Winding Permanent-Magnet Synchronous Machines Using a Computationally Efficient Finite-Element Method

The proposed hybrid method combines computationally efficient finite-element analysis (CE-FEA) with a new analytical formulation for eddy-current losses in the permanent magnets (PMs) of sine-wave current-regulated brushless synchronous motors. The CE-FEA only employs a reduced set of magnetostatic solutions yielding substantial reductions in the computational time, as compared with the conventional FEA. The 3-D end effects and the effect of pulsewidth-modulation switching harmonics are incorporated in the analytical calculations. The algorithms are applied to two fractional-slot concentrated-winding interior PM motors with different circumferential and axial PM block segmentation arrangements. The method is validated against 2-D and 3-D time-stepping FEA.

Compact Cascaded H-Bridge Multilevel Inverter with Single DC Source for Pure Electric Vehicle

The electric vehicle with compact inverter size using modified H-bridge inverter with modified topology has proposed. The modified topology use single DC source and with proposed SSVPWM. The SSVPWM was generated using sampled reference frame algorithm this algorithm used only sampled amplitude of reference phase voltages. This reduces computation time in DSP controllers where conventional algorithm requires more machine cycle to execute. The inverter output is fed to induction motor which propels the rear wheel. The sudden acceleration and de-acceleration will increase the switching stress and harmonics in the output. The neuro fuzzy optimization is used to mitigate the harmonics and bring down to the EN 50160 gride code. This model has been developed successfully using dsPIC with 5-level H-bridge inverter and has been tested in 2.2 kW induction motor.

Extension of Pulsewidth Modulation From Carrier-Based to Dither-Based

The carrier-based pulsewidth modulation (PWM) is extended to a dither-based PWM by replacing a conventional triangular carrier with a binary dither signal. Such a simple extension is generally impractical due to serious performance degradation. However, this paper presents a novel binary dither generator that helps enhance the performance of the dither-based PWM and makes the idea possible. The binary dither is generated as determined by the sign of the filtered modulation noise. The dither generated in this way automatically eliminates low-frequency modulation noise and randomizes the switching frequency. As indicated by experiments, the resulting dither-based PWM has relatively lower THD+N and less pronounced high-frequency switching harmonics, as compared with the conventional fixed-frequency triangular carrier PWM.

Asymptotic Time Domain Evaluation of a Multilevel Multiphase PWM Converter Voltage Quality

Voltage total harmonic distortion and quadratic voltage ripple criteria are accepted figures of merit of a multilevel pulse width modulation (PWM) converter voltage quality. The dominating frequency domain approach does not deliver simple closed-form expressions and practically requires accounting up to 40-50 switching harmonics. Assuming infinitely high switching frequency, suggested time domain approach to a multilevel multiphase PWM voltage quality evaluation provides closed-form piece-wise analytical solutions. These simple formulas clearly reveal the dependence on modulation index for arbitrary voltage level and odd phase numbers. The results are applicable to any type of multilevel converters (including interleaved, open winding and other) operated with nearest level/nearest space vector modulation and relatively high switching frequency.

A Current Controller Design for Current Source Inverter-Fed AC Machine Drive System

A current source inverter (CSI) requires a capacitor filter for the commutation of switching device as well as for attenuating switching harmonics. Hence, the CSI-fed ac machine has a second-order system in the continuous time domain. This paper presents a design methodology for the closed-loop current controller of the CSI-fed ac machine drive system. A multiloop current controller design using a pole/zero cancellation method is employed with a transfer function matrix. To decouple the cross-coupling terms which cause mutual interferences between the d- and q-axes in the synchronous reference frame, two types of controller are proposed and implemented using different decoupling method. Additionally, active damping methods are incorporated to enhance the stability of the system. A stability analysis in discrete-time domain is investigated to verify the feasibility of the proposed closed-loop current controller. To evaluate the effectiveness of the proposed current controller, computer simulations and experimental tests were performed and the results are discussed.

Evaluations of current control in weak grid case for grid‐connected LCL‐filtered inverter

For grid-connected inverters, switching harmonics can be effectively attenuated through an LCL-type filter. In order to suppress resonance and guarantee good performance, many strategies (e.g. active damping (AD), harmonic resonant control, repetitive control and grid feedforward) have been proposed. However, the wide variation of grid impedance value challenges system stability in practical applications. The aforementioned methods need to be investigated. This study evaluates the applicability of each part of the overall control in a weak grid case with the use of a stability criterion. It has been demonstrated that the feedback-based AD control can work well in a wide range of grid conditions. However, the resonant and repetitive control methods meet constraints. The grid feedforward method brings in an extra positive feedback path, and consequently results in high harmonics or even instability. Finally, a recommendation for system design has been presented. Simulations and experiments have been provided to verify the analysis.

Iron Loss and Inductance Analysis Considering Magnetic Nonlinearity in Multi-Segmented Plate Permanent Magnet Linear Motor

In multi-segmented linear motor magnetic field includes coupling and uncoupling parts, so electromagnetic parameter characteristics considering magnetic nonlinearity are different from that of rotary or short-stator counterparts, especially about parameters variation at transition. In this paper magnetic field characteristics of multi-segmented permanent magnet linear motor are analyzed by combined simulation model, and then we evaluate iron loss and inductance in equivalent circuit taking switching harmonics into account. Meanwhile, due to the action of PM magnetic field at transition, parameters involved with the length ratio of mover to one segment show distinct changes. We present a comparative analysis for multi-segmented motor equipped with distributed and concentrated windings. Based on two types of fabricated prototypes, experimental results validate the analysis results.

Power Compensation and Power Quality Improvement Based on Multiple-Channel Current Source Converter Fed HT SMES

The purpose of this paper is to propose the high-performance modulation strategy, power control and current regulation schemes for a multiple-channel current source converter (CSC) fed high temperature (HT) superconducting magnetic energy storage (SMES), which can in turn function as the frequency stabilizer and active power filter for the distributed generation system. The key is to design and implement the interleaved space vector modulation technique to suppress the switching harmonics, the dynamic capacitor voltage control to damp LC resonance as well as regulate grid currents accurately, and the DC current balancing strategy to avoid the circulating currents. The multiple-channel CSC fed HT SMES with the proposed control scheme is verified to provide good performance in power compensation and power quality improvement.

Position Estimation in Switched Reluctance Motor Drives Using the First Switching Harmonics Through Fourier Series

Position estimation using only active phase voltage and current is presented to perform high-accuracy position sensorless control of a switched reluctance motor (SRM) drive. By extracting the amplitude of the first switching harmonic terms of phase voltage and current for a pulsewidth modulation period through the Fourier series, flux linkage and position are estimated without external hardware circuitry, such as a modulator and a demodulator, resulting in increasing cost, as well as large position estimation error produced when the motional back electromotive force is ignored near zero speed. Hence, the proposed position estimation scheme covers the entire speed range, including the standstill under various loads, and it has high-resolution information depending on switching frequency. A two-phase SRM drive system, consisting of an asymmetrical converter and a conventional closed-loop proportional-integral current controller, is utilized to validate the performance of the proposed position estimation scheme in comprehensive operating conditions. The estimated values very closely track the actual values in dynamic simulations and experiments. It is shown that the proposed position estimation scheme using the Fourier series is sufficiently accurate and works satisfactorily at various operating points.

A Noise-Shaped Buck DC–DC Converter With Improved Light-Load Efficiency and Fast Transient Response

A noise-shaped synchronous buck dc-dc converter based on a third-order active-passive continuous-time sigma-delta modulator is presented. Detailed system modeling and loop design methodology are discussed. A dual-mode compensator is proposed to achieve the loop stability over a wide load range. The light-load efficiency is improved by the proposed dynamic width control and sigma-delta pulse-skip mode techniques. In order to get a fast transient response, the linear-nonlinear control is adopted. Moreover, an on-chip soft-start circuit is proposed to save the system cost. The proposed converter has been fabricated using Chartered 0.35- μm 2P4M dual-gate mixed-signal CMOS process. Experimental results show that the converter effectively suppresses the switching harmonics at both continuous conduction mode and discontinuous conduction mode. Compared with the conventional pulsewidth modulation operation, the first harmonic peak has been reduced by -35 dB. The converter achieves a peak efficiency of 92% at 200 mA, and a light-load efficiency of 78% at 5 mA. With the load current skipping between 50 and 450 mA, the maximum output overshoot voltage is 87 mV, and the recovery time is below 12 μs. The start-up time for the output voltage is 160 μs. By configuring the soft-start circuit, this time can be prolonged to 680 μs. The area of the whole chip is 1.4 mm2.

Stability Improvement of Distributed Power Generation Systems with an LCL-Filter Using Gain Scheduling Based on Grid Impedance Estimations

This paper proposes a gain scheduling method that improves the stability of grid-connected systems employing an LCL-filter. The method adjusts the current controller gain through an estimation of the grid impedance in order to reduce the resonance that occurs when using an LCL-filter to reduce switching harmonics. An LCL-filter typically has a frequency spectrum with a resonance peak. A change of the grid-impedance results in a change to the resonant frequency. Therefore an LCL-filter needs a damping method that is applicable when changing the grid impedance for stable system control. The proposed method instantaneously estimates the grid impedance and observes the resonant frequency at the same time. Consequently, the proposed method adjusts the current controller gain using a gain scheduling method in order to guarantee current controller stability when a change in the resonant frequency occurs. The effectiveness of the proposed method has been verified by simulations and experimental results.

Fuzzy dispatching of solar energy in distribution system

This paper presents a novel approach for solar energy using in distribution system as distributed generation (DG) unit. A nonlinear fuzzy controller tunes the modulation index of PWM inverter to feed the load in the grid via photovoltaic arrays. The controller also dispatches two dc sources to control input of inverter. The proposed system controls the voltage even during changing sunlight voltage condition or unbalanced load. A low pass LC filter is linked to the output of voltage source converter to bypass switching harmonics. The evolutionary method based on fuzzy theory is used to determine the value of modulation index and disperse the sources from a fuzzy rule-based defined on load voltage error of the point of common coupling. This system gives a full flexibility to the grid to obtain power from the solar photovoltaic units depending on its cost and load requirement at any given time. Simulation results illustrate the effectiveness of performance of proposed method.

Compensation of Nonlinearities in Diode-Clamped Multilevel Converters

The application of multilevel converters for exciting permanent-magnet machines with low-phase inductance to dc-link voltage ratios facilitates a reduction in high-frequency switching harmonics. However, converter nonlinearities and, in-particular, on-state device voltage drops, create additional low-frequency harmonics. This paper therefore proposes a generic compensation scheme to accommodate the effects of such converter nonlinearities and, in doing so, improve the harmonic quality of the machine phase currents. Experimental results gathered from a prototype five-level diode-clamped converter validate the benefits of the proposed scheme by showing quantitative reductions in low-frequency harmonics.

Natural Harmonic Elimination of Square-Wave Inverter for Medium-Voltage Application

In this paper, a low-frequency square-wave inverter with a series-connected pulsewidth modulation (PWM) inverter is investigated for high-power applications. The series compensators produce only the desired harmonic voltages to make the net output voltage sinusoidal with small PWM switching harmonics only. An open-loop control strategy for the series compensator is proposed in this paper. This strategy indirectly sets the compensator DC bus voltage to the desired level. No external DC source or active power at fundamental frequency is required to control this dc bus voltage. Different variations of this basic strategy are presented in this paper for medium-voltage applications. Theoretical analysis of this strategy is presented in this paper with simulation and experimental results.

Active power flow control in a distribution system using discontinuous voltage controller

This paper proposes a hybrid discontinuous control methodology for a voltage source converter (VSC), which is used in an uninterrupted power supply (UPS) application. The UPS controls the voltage at the point of common coupling (PCC). An LC filter is connected at the output of the VSC to bypass switching harmonics. With the help of both filter inductor current and filter capacitor voltage control, the voltage across the filter capacitor is controlled. Based on the voltage error, the control is switched between current and voltage control modes. In this scheme, an extra diode state is used that makes the VSC output current discontinuous. This diode state reduces the switching losses. The UPS controls the active power it supplies to a three-phase, four-wire distribution system. This gives a full flexibility to the grid to buy power from the UPS system depending on its cost and load requirement at any given time. The scheme is validated through simulation using PSCAD.

A Simple Voltage Sensorless Active Damping Scheme for Three-Phase PWM Converters With an $LCL$ Filter

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Active three-phase pulsewidth-modulated rectifiers are connected to the grid by an inductance or an <formula formulatype="inline"><tex>$LCL$</tex> </formula> filter. Although an <formula formulatype="inline"><tex> $LCL$</tex> </formula> filter is more effective since it more efficiently attenuates the switching harmonics, it causes stability problems. To enable a stable operation, either a damping resistor or an active damping (AD) algorithm has to be used. However, while a damping resistor causes additional losses, an AD algorithm usually requires extra sensors. To avoid these disadvantages, this paper shows simulation and experimental results of a simple ac voltage sensorless AD solution that is suitable for industrial applications. </para>

Analysis and Reduction of Time Harmonic Rotor Loss in Solid-Rotor Synchronous Reluctance Drive

Synchronous reluctance machines with solid rotor construction have advantages in certain high-speed applications such as flywheel energy storage systems. However, the solid rotor allows the flow of eddy currents, resulting in rotor loss and heat generation. The switching harmonics in the stator voltage supplied by a pulsewidth modulation (PWM) inverter are one of the sources of the rotor loss. This paper performs an analysis for the time harmonic loss in a solid-rotor synchronous reluctance machine, and investigates design and control issues associated with the inclusion of a three-phase LC filter for reduction of the rotor loss in solid rotor. A two-phase dynamic model of the machine which incorporates the LC filter dynamics is presented. This model is used to predict rotor losses due to switching harmonics generated by the three-phase PWM inverter. A model-based current regulator is utilized, which is modified to include the effects of the LC filter. Experimental results validate the proposed approach.