Showing papers on "Harmonic published in 2013"

An Islanding Microgrid Power Sharing Approach Using Enhanced Virtual Impedance Control Scheme

Abstract: In order to address the load sharing problem in islanding microgrids, this paper proposes an enhanced distributed generation (DG) unit virtual impedance control approach. The proposed method can realize accurate regulation of DG unit equivalent impedance at both fundamental and selected harmonic frequencies. In contrast to conventional virtual impedance control methods, where only a line current feed-forward term is added to the DG voltage reference, the proposed virtual impedance at fundamental and harmonic frequencies is regulated using DG line current and point of common coupling (PCC) voltage feed-forward terms, respectively. With this modification, the impacts of mismatched physical feeder impedances are compensated. Thus, better reactive and harmonic power sharing can be realized. Additionally, this paper also demonstrates that PCC harmonic voltages can be mitigated by reducing the magnitude of DG unit equivalent harmonic impedance. Finally, in order to alleviate the computing load at DG unit local controller, this paper further exploits the band-pass capability of conventionally resonant controllers. With the implementation of proposed resonant controller, accurate power sharing and PCC harmonic voltage compensation are achieved without using any fundamental and harmonic components extractions. Experimental results from a scaled single-phase microgrid prototype are provided to validate the feasibility of the proposed virtual impedance control approach.

An Improved Repetitive Control Scheme for Grid-Connected Inverter With Frequency-Adaptive Capability

Abstract: The power quality of grid-connected inverters has drawn a lot of attention with the increased application of distributed power generation systems. The repetitive control technique is widely adopted in these systems, due to its excellent tracking performance and low output total harmonic distortion (THD). However, in an actual system, the ratio of the sampling frequency to the grid frequency cannot always maintain an integer, and then, the resonant frequencies of the repetitive control technique will deviate from the real grid fundamental and harmonic frequencies. This will degrade the performance of the system, particularly when the grid frequency varies. Even if the ratio is a fixed integer, the auxiliary function for stabilization in the conventional repetitive control technique will also increase the steady-state tracking error and THD of the system. In this paper, an improved repetitive control scheme with a special designed finite impulse response (FIR) filter is proposed. The FIR filter cascaded with a traditional delay function can approximate the ideal repetitive control function of any ratio. The proposed scheme varies the FIR filter according to varied grid frequency and maintains its resonant frequencies matching the grid fundamental and harmonic ones. Finally, the simulation and experimental results show that the improved repetitive control scheme can effectively reduce the tracking error and compensate harmonics of the inverter systems.

An Advanced Current Control Strategy for Three-Phase Shunt Active Power Filters

Abstract: This paper proposes an advanced control strategy to enhance performance of shunt active power filter (APF). The proposed control scheme requires only two current sensors at the supply side and does not need a harmonic detector. In order to make the supply currents sinusoidal, an effective harmonic compensation method is developed with the aid of a conventional proportional-integral (PI) and vector PI controllers. The absence of the harmonic detector not only simplifies the control scheme but also significantly improves the accuracy of the APF, since the control performance is no longer affected by the performance of the harmonic tracking process. Furthermore, the total cost to implement the proposed APF becomes lower, owing to the minimized current sensors and the use of a four-switch three-phase inverter. Despite the simplified hardware, the performance of the APF is improved significantly compared to the traditional control scheme, thanks to the effectiveness of the proposed compensation scheme. The proposed control scheme is theoretically analyzed, and a 1.5-kVA APF is built in the laboratory to validate the feasibility of the proposed control strategy.

Three-Phase Cascaded Delayed Signal Cancellation PLL for Fast Selective Harmonic Detection

Abstract: Fast and accurate selective harmonic detection has crucial value for many power system harmonic compensation systems. The existing detection methods are known to have a few drawbacks, such as long delay time, sensitivity to grid frequency variation, and difficulty to achieve zero steady-state error. In order to overcome these drawbacks, this paper proposes a selective harmonic detection system based on the three-phase cascaded delayed signal cancellation phase-locked loop. The system can be flexibly configured to detect any individual harmonic from the source with various background harmonics. It also features very short transient and excellent adaptability under small and considerable frequency variations, as verified by comprehensive experimental results. Finally, this paper provides guidance on how to tailor the detection system for different applications and a solution to address the practical implementation issues.

Augmenting Second Harmonic Generation Using Fano Resonances in Plasmonic Systems

Abstract: Significant augmentation of second harmonic generation using Fano resonances in plasmonic heptamers made of silver is theoretically and experimentally demonstrated. The geometry is engineered to simultaneously produce a Fano resonance at the fundamental wavelength, resulting in a strong localization of the fundamental field close to the system, and a higher order scattering peak at the second harmonic wavelength. These results illustrate the versatility of Fano resonant structures to engineer specific optical responses both in the linear and nonlinear regimes thus paving the way for future investigations on the role of dark modes in nonlinear and quantum optics.

Selective Harmonic Mitigation Technique for Cascaded H-Bridge Converters With Nonequal DC Link Voltages

Abstract: Multilevel converters have received increased interest recently as a result of their ability to generate high quality output waveforms with a low switching frequency. This makes them very attractive for high-power applications. A cascaded H-bridge converter (CHB) is a multilevel topology which is formed from the series connection of H-bridge cells. Optimized pulse width modulation techniques such as selective harmonic elimination or selective harmonic mitigation (SHM-PWM) are capable of preprogramming the harmonic profile of the output waveform over a range of modulation indices. Such modulation methods may, however, not perform optimally if the dc links of the CHB are not balanced. This paper presents a new SHM-PWM control strategy which is capable of meeting grid codes even under nonequal dc link voltages. The method is based on the interpolation of different sets of angles obtained for specific situations of imbalance. Both simulation and experimental results are presented to validate the proposed control method.

A High Power Density Single Phase Pwm Rectifier with Active Ripple Energy Storage

Abstract: It is well known that there exist second-order harmonic current and corresponding ripple voltage on dc bus for single phase PWM rectifiers The low frequency harmonic current is normally filtered using a bulk capacitor in the bus which results in low power density? This project proposed an active ripple energy storage method that can effectively reduce the energy storage capacitance The feed- forward control method and design considerations are provided

Surpassing Fundamental Limits of Oscillators Using Nonlinear Resonators

Abstract: In its most basic form an oscillator consists of a resonator driven on resonance, through feedback, to create a periodic signal sustained by a static energy source. The generation of a stable frequency, the basic function of oscillators, is typically achieved by increasing the amplitude of motion of the resonator while remaining within its linear, harmonic regime. Contrary to this conventional paradigm, in this Letter we show that by operating the oscillator at special points in the resonator’s anharmonic regime we can overcome fundamental limitations of oscillator performance due to thermodynamic noise as well as practical limitations due to noise from the sustaining circuit. We develop a comprehensive model that accounts for the major contributions to the phase noise of the nonlinear oscillator. Using a nanoelectromechanical system based oscillator, we experimentally verify the existence of a special region in the operational parameter space that enables suppressing the most significant contributions to the oscillator’s phase noise, as predicted by our model.

Residential Distribution System Harmonic Compensation Using PV Interfacing Inverter

Abstract: The increased non-linear loads in today's typical home are a growing concern for utility companies. This situation might be worsened by the harmonic resonance introduced by the installation of capacitor banks in the distribution network. To mitigate the harmonic distortions, passive or active filters are typically used. However, with the increasing implementation of distributed generation (DG) in residential areas, using DG systems to improve the power quality is becoming a promising idea, particularly because many DG systems, such as photovoltaic (PV), wind and fuel cells, have DG-grid interfacing converters. In this paper, the potential for using photovoltaic (PV) interfacing inverters to compensate the residential system harmonics is explored. A system model including the residential load and DG is first developed. An in-depth analysis and comparison of different compensation schemes based on the virtual harmonic damping impedance concept are then carried out. The effects of the capacitor banks in the system are also studied. The effectiveness of the harmonic compensation strategies under different conditions is verified through analysis and simulations.

Harmonic Droop Controller to Reduce the Voltage Harmonics of Inverters

Abstract: In this paper, the load and/or grid connected to an inverter is modeled as the combination of voltage sources and current sources at harmonic frequencies. As a result, the system can be analyzed at each individual frequency, which avoids the difficulty in defining the reactive power for a system with different frequencies because it is now defined at each individual frequency. Moreover, a droop control strategy is developed for systems delivering power to a constant current source, instead of a constant voltage source. This is then applied to develop a harmonic droop controller so that the right amount of harmonic voltage is added to the inverter reference voltage to compensate the harmonic voltage dropped on the output impedance due to the harmonic current. This forces the output voltage at the individual harmonic frequency to be close to zero and improves the total harmonic distortion (THD) of the output voltage considerably. Both simulation and experimental results are provided to demonstrate that the proposed strategy can significantly improve the voltage THD.

Hybrid Voltage and Current Control Approach for DG-Grid Interfacing Converters With LCL filters

Abstract: This paper presents a hybrid voltage and current control method to improve the performance of interfacing converters in distributed generation (DG) units. In general, current-controlled methods have been widely adopted in grid-connected converters nowadays. Nevertheless, in an islanded system, the voltage control of DG units is desired to provide direct voltage support to the loads. Due to the absence of closed-loop line current controller, the voltage control scheme can hardly regulate the DG unit's line current harmonics. Furthermore, if not addressed properly, the transfer between the grid-connected operation and autonomous islanding operation will introduce nontrivial transient currents. To overcome the drawbacks of voltage- and current-controlled DG units, this paper develops a hybrid voltage and current control method (HCM). The proposed method allows the coordinated closed-loop control of the DG unit fundamental voltage and line harmonic currents. With the HCM, local harmonic loads of the DG unit can even be compensated without using harmonic current extraction. In addition, the HCM guarantees smooth transition during the grid-connected/islanding operation mode transfer. Simulated and experimental results are provided to verify the feasibility of the proposed approach.

High Power Wideband Gyrotron Backward Wave Oscillator Operating towards the Terahertz Region

Abstract: Experimental results are presented of the first successful gyrotron backward wave oscillator (gyro-BWO) with continuous frequency tuning near the low-terahertz region. A helically corrugated interaction region was used to allow efficient interaction over a wide frequency band at the second harmonic of the electron cyclotron frequency without parasitic output. The gyro-BWO generated a maximum output power of 12 kW when driven by a 40 kV, 1.5 A, annular-shaped large-orbit electron beam and achieved a frequency tuning band of 88-102.5 GHz by adjusting the cavity magnetic field. The performance of the gyro-BWO is consistent with 3D particle-in-cell numerical simulations.

Adaptive Selective Harmonic Minimization Based on ANNs for Cascade Multilevel Inverters With Varying DC Sources

Abstract: A new approach for modulation of an 11-level cascade multilevel inverter using selective harmonic elimination is presented in this paper. The dc sources feeding the multilevel inverter are considered to be varying in time, and the switching angles are adapted to the dc source variation. This method uses genetic algorithms to obtain switching angles offline for different dc source values. Then, artificial neural networks are used to determine the switching angles that correspond to the real-time values of the dc sources for each phase. This implies that each one of the dc sources of this topology can have different values at any time, but the output fundamental voltage will stay constant and the harmonic content will still meet the specifications. The modulating switching angles are updated at each cycle of the output fundamental voltage. This paper gives details on the method in addition to simulation and experimental results.

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

Abstract: 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.

Compact Filtering Power Divider With Enhanced Second-Harmonic Suppression

Abstract: This letter presents a compact power divider integrated with high-selectivity bandpass resoponses. A novel topology is proposed to integrate five resonators and a resistor to realize the dual functions of the power division and filtering. Two transmission zeros are generated near the passband edges and another one is created at the second harmonic frequency which enhances the rejection levels. For validation, a filtering power divider operating at 920 MHz is implemented with more than 20 dB isolation. The circuit size is 0.15 λg×0.14 λg, featuring compact size.

Optimal injection of harmonics in circulating currents of modular multilevel converters for capacitor voltage ripple minimization

Abstract: This paper proposes an algorithm to calculate the optimal amplitude and phase of the harmonic current components that can be injected in the circulating currents of a modular multilevel converter (MMC) to minimize the capacitor voltage fluctuations. An optimal second harmonic component and an optimal set of second and fourth harmonic components are proposed. Simulation results are obtained in MATLAB/Simulink environment to study the effectiveness of the calculated optimal currents. Selected experimental results have been obtained from an MMC laboratory prototype, testing the effects of the circulating currents. The reported results demonstrate the effectiveness of using a fourth harmonic component in the circulating current, which improves the effect of the second harmonic on reducing the capacitor voltage fluctuations.

Zeptosecond high harmonic keV x-ray waveforms driven by midinfrared laser pulses.

Abstract: We demonstrate theoretically that the temporal structure of high harmonic x-ray pulses generated with midinfrared lasers differs substantially from those generated with near-infrared pulses, especially at high photon energies. In particular, we show that, although the total width of the x-ray bursts spans femtosecond time scales, the pulse exhibits a zeptosecond structure due to the interference of high harmonic emission from multiple reencounters of the electron wave packet with the ion. Properly filtered and without any compensation of the chirp, regular subattosecond keV waveforms can be produced.

Tunable N-Path Notch Filters for Blocker Suppression: Modeling and Verification

Abstract: N-path switched-RC circuits can realize filters with very high linearity and compression point while they are tunable by a clock frequency. In this paper, both differential and single-ended N-path notch filters are modeled and analyzed. Closed-form equations provide design equations for the main filtering characteristics and nonidealities such as: harmonic mixing, switch resistance, mismatch and phase imbalance, clock rise and fall times, noise, and insertion loss. Both an eight-path single-ended and differential notch filter are implemented in 65-nm CMOS technology. The notch center frequency, which is determined by the switching frequency, is tunable from 0.1 to 1.2 GHz. In a 50-Ω environment, the N-path filters provide power matching in the passband with an insertion loss of 1.4-2.8 dB. The rejection at the notch frequency is 21-24 dB, P1 dB > +2 dBm, and IIP3 > +17 dBm.

Predictive sorting algorithm for modular multilevel converters minimizing the spread in the submodule capacitor voltages

Abstract: The modular multilevel converter is a suitable topology for bidirectional ac-dc conversion in high-voltage high-power applications. By connecting submodule circuits in series, a high-voltage waveform with excellent harmonic performance can be achieved with a very high efficiency and low switching frequency. The balancing of the capacitor voltages will, however, become increasingly difficult as the switching frequency is reduced. Although the capacitor voltages can be kept balanced over time even at the fundamental switching frequency, the spread and thus also the peak variation in the capacitor voltages will typically increase at lower switching frequencies. This paper presents a capacitor voltage balancing strategy which aims to combine a low switching frequency with a low capacitor voltage ripple. This is done by a predictive algorithm that calculates the amount of charge that must be stored in the submodule capacitors during the following fundamental frequency period. The converter is then controlled in such a way that the stored charge in the submodule capacitors is evenly distributed among all the submodules when the capacitor voltages reach their maximum values. In this way, it is possible to limit the peak voltage in the capacitor at switching frequencies as low as 2-3 times the fundamental frequency. The capacitor voltage balancing strategy is first validated by simulation results at 110 Hz switching frequency. It is observed that when the proposed method is used, the capacitor voltage ripple is 35% lower compared to the case when a conventional sorting algorithm is used. The capacitor voltage balancing strategy is also validated experimentally at 130 Hz switching frequency. The experimental results show that it is possible to combine the proposed method with previously presented circulating-current control methods.

Analysis and Control of Modular Multilevel Converters Under Unbalanced Conditions

Abstract: This paper investigates the contents of submodule voltage ripples, circulating currents, and internal converter voltage in modular multilevel converters (MMCs) theoretically. The operation of MMCs is studied under asymmetry of the upper and lower arm, as well as the unbalanced ac system. In three-phase MMCs, the analysis shows that the second harmonic circulating currents will be asymmetric under an unbalanced ac system and can be decomposed into positive-, negative- and zero-sequence parts. The positive- and negative-sequence components affect neither the ac side nor the dc side of MMCs while the zero-sequence components will flow into the dc side, aggravating the power fluctuations of the dc side. In order to solve this problem, a new controller based on the instantaneous power theory and proportional-resonant scheme is designed. Simulations with a detailed switching model on the PSCAD/EMTDC platform verify the theoretical analysis, and demonstrate that the proposed controller eliminates the active power fluctuations and suppresses the harmonic circulating currents as well.

Analysis and operation of modular multilevel converters with phase-shifted carrier PWM

Abstract: The modular multilevel converter is a suitable topology for high-voltage high-power applications since the cascaded submodules can generate high-voltage waveforms with excellent harmonic performance at low switching frequencies. Many publications have been presented on the modulation and control of this converter type, some of which are based on phase-shifted carrier modulation. This paper presents an analysis of how the switching frequency affects the capacitor voltages, circulating current and alternating voltage at phase-shifted carrier modulation. It is found that integer multiples of the fundamental frequency should be avoided as they can cause the capacitor voltages to diverge. Suitable switching frequencies are then defined for which the arm and line quantities will be periodic and symmetric in the upper and lower arms. The theoretical results are then validated by both simulations and experimental results.

A Novel Wideband Bandpass Power Divider With Harmonic-Suppressed Ring Resonator

Abstract: A simple stub-loaded ring resonator is presented as a novel wideband bandpass power divider with good in-band responses and out-of-band harmonic suppression. The first two resonances of the ring resonator are excited and employed to construct a wideband passband. By adjusting the length of loaded open stubs, four transmission zeros can be generated in the lower and upper stopbands. After installing three coupled-line sections at one input port and two output ports along the ring, additional two in-band poles and one upper-stopband transmission zero are further introduced. Subsequently, total five transmission zeros are utilized to improve the passband selectivity and suppress the high-order harmonic. In addition, a single resistor is properly placed between two output ports to ensure the isolation. Finally, a prototype power divider is fabricated and verified experimentally with attractive bandpass features.

Adaptive Harmonic Steady‐State Disturbance Rejection with Frequency Tracking

Abstract: This paper is concerned with the rejection of sinusoidal disturbances of unknown frequency acting at the output of unknown plants. Disturbance rejection is based on an adaptive harmonic steady-state (ADHSS) algorithm combined with a magnitude/phase locked-loop (MPLL) frequency estimator. The harmonic steady-state method assumes that the plant can be approximated by its steady-state frequency response. For high-order plants such as those encountered in active noise and vibration control (ANVC), this assumption greatly reduces the number of parameters and enables online estimation of the plant response using simple algorithms. The paper shows that when the MPLL is integrated with the ADHSS algorithm, the two components work together in such a way that the control input does not prevent frequency tracking by the MPLL, and so that the order of the ADHSS can be reduced. Thus, the addition of the MPLL allows disturbances of unknown frequency to be considered without significantly increasing the complexity of the original ADHSS. After analyzing the reduced-order ADHSS in the ideal case, the equations describing the complete system are considered. The theory of averaging is used to gain insight into the steady-state behavior of the algorithm. It is found that the system has a two-dimensional equilibrium surface such that the disturbance is cancelled exactly. A subset of the surface is proved to be locally stable. Extensive active noise control experiments demonstrate the performance of the algorithm, even when disturbance and plant parameters are changing.

Compact Microstrip Wilkinson Power Dividers With Harmonic Suppression and Arbitrary Power Division Ratios

Abstract: This paper describes a new Wilkinson power divider on a single-layer microstrip line that can reduce the occupied area, suppress the harmonic components, and/or provide the arbitrary power division ratios. It consists of two-section transmission lines, two inductors, and one isolation resistor. Four different designs have been conducted to investigate the capabilities of the structure. In addition, a compact divider along with harmonic suppression and a practical divider with a large power-dividing ratio has been constructed and measured. The simulation and measurement results are in good agreement with each other. This indicates that the structure can effectively be used as a power divider for miniaturized or arbitrary power division ratio applications.

Space-Vector-Modulated Three-Level Inverters With a Single Z-Source Network

Abstract: The Z-source inverter is a relatively recent converter topology that exhibits both voltage-buck and voltage-boost capability. The Z-source concept can be applied to all dc-to-ac, ac-to-dc, ac-to-ac, and dc-to-dc power conversion whether two-level or multilevel. However, multilevel converters offer many benefits for higher power applications. Previous publications have shown the control of a Z-source neutral point clamped inverter using the carrier-based modulation technique. This paper presents the control of a Z-source neutral point clamped inverter using the space vector modulation technique. This gives a number of benefits, both in terms of implementation and harmonic performance. The adopted approach enables the operation of the Z-source arrangement to be optimized and implemented digitally without introducing any extra commutations. The proposed techniques are demonstrated both in simulation and through experimental results from a prototype converter.

A Grid Fundamental and Harmonic Component Detection Method for Single-Phase Systems

Abstract: Single-phase grid-connected converters are widely used in many applications such as photovoltaics, fuel cells, active power filters, etc. An important topic for the development of their control schemes is ac signal detection, such as grid phase detection for grid-interfacing inverters, and harmonic detection for harmonic compensation devices. Since only one signal is available, the task is more difficult than in three-phase systems. Among the existing methods, the frequency-domain ones are known to have a one-cycle delay and heavier computational burden. Meanwhile, the time-domain methods often rely on phase-locked loop, quadrature signal generation, and complex filtering techniques; the resulted multiple-looped system may suffer from slow transients and stability issues. This paper proposes a new detection method based on anticonjugate harmonic decomposition and cascaded delayed signal cancellation. The method uses constant zero as the quadrature signal, and has a completely open-looped structure. The resulted detection system is very simple and robust. The fundamental and harmonic detection transients can be as short as 0.47 cycle in most cases, or 1.5 cycles for cases with considerable frequency variations. Meanwhile, zero steady-state error can be guaranteed in complicated harmonic scenarios, including all typical single-phase system harmonics. The performance of the proposed detection method is verified by experiments.

A grid-connected inverter with virtual synchronous generator model of algebraic type

Abstract: Summary This paper presents experimental results on a grid-connected inverter. The control of the inverter is based on a virtual synchronous generator model of algebraic type. When using the virtual synchronous generator model of algebraic type, there is no evidence for satisfactory operation with unbalanced load and nonlinear load, because the generator models are constructed under the assumption that the load is linear and balanced. By choosing the feedback control loop gain appropriately, satisfactory operation is achieved even if an unbalanced and nonlinear load is connected. Experimental results show that the power controls, active and reactive, are achieved preferably in grid-connecting mode. Smooth transitions are also achieved from the connected mode to the island mode. The inverter can operate satisfactorily when the unbalanced load is connected. Harmonic analysis is performed with a three-phase full-wave rectifier connected as a nonlinear load. The harmonic current is supplied by the inverter and its output LC filter. It is shown that the harmonic current is supplied mainly by the inverter and that the voltage waveform is acceptable for practical use. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 184(4): 10–21, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22428