Low-frequency Current Oscillations Research Articles

A Hybrid Power Sharing Control to Enhance the Small Signal Stability in DC Microgrids

Low-frequency power oscillations pose serious problems to the operation of Distributed Generations (DGs) equipped with droop control. This paper presents a new hybrid power-sharing control for damping low-frequency power and current oscillations in DC microgrids. The proposed control consists of a decentralized droop control together with a centralized average current sharing control. A small-signal equivalent model is analytically derived for the proposed hybrid control and is incorporated into the DC microgrid model. The parameters of the PI controllers involved in the average current sharing control units are tuned individually to achieve a satisfactory performance in a stand-alone operation of DGs with a local load. The contributions of control signals contained in the hybrid control signal to the damping characteristic are analyzed. The dynamic response of the DC microgrid under a variety of disturbance scenarios is investigated. The power-sharing accuracy of the proposed control is also examined under the investigated scenarios. The method performance is also investigated for the oscillations caused by constant-power loads. Sensitivity analyses are conducted to evaluate the impact of the loading level and line parameters on the damping ratio associated with the dominant low-frequency modes. The effectiveness of the control method is examined in case of losing a number of communication links. The impact of the delay in the communication links on the method performance is also examined. The simulations are conducted on a multi-source DC microgrid, and the results are verified in the OPAL-RT real-time simulator.

PWM Switching Delay Correction Method for High-Speed Brushless DC Drives

For high-speed brushless DC (BLDC) drives, the switching-to-fundamental frequency ratio is usually insufficient. As a result, the commutation errors caused by PWM switching delay are significant, which means the power switches will take action at inaccurate instants. This will cause low-frequency current oscillations and produce additional losses. In addition, the conventional commutation PWM method will cause accumulative errors in the sensorless operation and will further lead to instability issues in the high-speed region. In order to suppress these influences, a novel carrier-synchronized commutation PWM strategy is proposed in this paper. It shows that the proposed strategy can eliminate the commutation error caused by PWM switching delay. In addition, the instability phenomenon can be avoided and the current control performance is greatly improved. Finally, the proposed strategy is verified by simulation and experiments on a 78 000-r/min BLDC drive platform.

Spectral Analysis and Sideband Harmonic Cancellation of Six-Step Operation With Low Carrier–Fundamental Frequency Ratio for High-Speed Brushless DC Drives

High-speed brushless dc (BLDC) drives usually operate with low carrier-fundamental frequency ratio, which causes abundant sideband harmonic current components. Some sideband harmonics will be induced in low-frequency range, leading to low-frequency current oscillations and low-frequency torque ripples. To minimize these undesired sideband harmonics, a novel pulsewidth modulation commutation pattern, i.e., carrier-synchronized commutation (CSC), is proposed in this article. For comparison, two conventional commutation patterns, including regular-sampled commutation (RSC), and natural-sampled commutation (NSC), are analyzed. To reveal the characteristics of sideband harmonics caused by RSC and NSC patterns, an extended geometric wall model is introduced, which can be used for the Fourier decomposition of complex vectors. Theoretical, simulation, and experimental results demonstrate the conventional RSC and NSC patterns will produce additional sideband harmonic currents and torque ripples, while the proposed CSC pattern can considerably suppress these adverse influences and meanwhile presents high performance and robustness.

Non-Ohmic conduction in In2O3–Bi2O3 ceramics

The semiconductor In2O3–Bi2O3 ceramics exhibit unusual behaviour: The current quasi-saturation (current limiting) in dc current–voltage characteristic is accompanied by low-frequency (∼1Hz) current oscillations. In this paper some electrical properties of In2O3–Bi2O3 ceramics are studied and a mechanism of non-Ohmic conduction in this material is suggested. The electrical conduction is controlled by the grain-boundary potential barriers. Assuming that the barrier heights at different grain boundaries in a sample are not identical, the current quasi-saturation is explained qualitatively by capture of electrons at the interface states and respective increase in the height of key barriers. A mechanism of current oscillations is related to the current quasi-saturation. An increase in the height of key barriers leads to a raise of the voltage drop at these barriers and to Joule heating of barrier regions followed by decrease in the barrier height. The suggested mechanism of non-Ohmic conduction is confirmed by the obtained experimental data.

Low-Frequency Current Oscillations and Maximum Power Point Tracking in Grid-Connected Fuel-Cell-Based Systems

The study of a double-stage single-phase inverter for fuel-cell-based applications is proposed in this paper. A novel control strategy aimed at reducing the low-frequency oscillations of the fuel-cell (FC) current in order to guarantee the FC safety operating conditions is proposed. The reduction of such oscillations increases the FC lifetime and avoids high mechanical stress of the membrane and unnecessary consumption of reactants. Furthermore, it allows one to design a strategy for extracting the maximum power from the FC stack with a total control of the concentration losses. Simulation and experimental results confirm the effectiveness of the proposed approach.

Current limiting and negative differential resistance in indium oxide based ceramics

Indium oxide based ceramics with bismuth oxide addition were sintered in air in the temperature range 800–1300 °C. Current–voltage characteristics of In 2O 3–Bi 2O 3 ceramics sintered at different temperatures are weakly nonlinear. After an additional heat treatment in air at about 200 °C samples sintered at a temperature within the narrow range of about 1050–1100 °C exhibit a current limiting effect accompanied by low-frequency current oscillations. It is shown that the observed electrical properties are controlled by the grain-boundary barriers and the heat treatment in air at 200 °C leads to the decrease in the barrier height. Electrical measurements, scanning electron microscopy and X-ray photoelectron spectroscopy results suggest that the current limiting effect observed in In 2O 3–Bi 2O 3 may be explained in terms of a modified barrier model; the observed current limiting effect is the result of an increase of barrier height with increasing electric field, due to additional oxygen absorption. It is found that In 2O 3–Bi 2O 3–Co 3O 4–Cr 2O 3 ceramic exhibits current–voltage characteristics with negative differential resistance due to Joule microheating.

Odd periodic window and bifurcations on 2D parameter space of low frequency oscillations

Low frequency current oscillations have been usually investigated under the influence of the external applied voltage, temperature and illumination. A parallel applied magnetic field has been used in the present work in order to obtain odd periodic windows and bifurcations inside them in a two dimension parameter space for a semi-insulating GaAs sample grown by molecular beam epitaxy. Two kinds of parameter spaces have been used in order to stabilize the odd periodic windows and the bifurcations, namely, the external applied voltage versus the parallel magnetic field and the illumination versus the parallel magnetic field. We report on a successful observation of stable cycles of periodicity 3, 4, 5, 6, 7 and 8 inside a period-3 window. An example of bifurcation route is presented following the sequence from chaos to 4, 3, 6, 3, 5, 7, 5, and back to chaos in the parameter space of voltage versus magnetic field. For this bifurcation route we will show which branch of the cycle is bifurcating and which are coalescing with the control parameters.

A time-resolved laser induced fluorescence study on the ion velocity distribution function in a Hall thruster after a fast current disruption

The temporal characteristics of the Xe+ ion axial velocity distribution function (VDF) were recorded in the course of low-frequency discharge current oscillations (∼14 kHz) of the 5 kW class PPS®X000 Hall thruster. The evolution in time of the ion axial velocity component is monitored by means of a laser induced fluorescence diagnostic tool with a time resolution of 100 ns. As the number of fluorescence photons is very low during such a short time period, a homemade pulse-counting lock-in system was used to perform real-time discrimination between background photons and fluorescence photons. The evolution in time of the ion VDF was observed at three locations along the thruster channel axis after a fast shutdown of the thruster power. The anode discharge current is switched off at 2 kHz during 5 μs without any synchronization with the current oscillation cycle. This approach allows to examine the temporal behavior of the ion VDF during decay and ignition of the discharge as well as during forced and natural plasma oscillations. Measurements show that the distribution function of the axial component of the Xe+ ion does change periodically in time with a frequency close to the current oscillation frequency in both forced and natural cases. The ion density and the mean velocity are found to oscillate, whereas the velocity dispersion stays constant, which indicates that ionization and acceleration layers have identical dynamics. Finally, variations over time in the electric field are for the first time experimentally evidenced in a crossed-field discharge.

Transport in GaAs/Al x Ga1−x As superlattices with narrow forbidden minibands: Low-frequency negative differential conductivity and current oscillations

Current-voltage characteristics have been measured and low-frequency current instabilities have been studied for GaAs/Al x Ga1−x As superlattices with narrow forbidden minibands. At relatively low electric fields, a saw-like structure for current-voltage characteristics with alternating portions of positive and negative differential conductivity and spontaneous generation of low-frequency current oscillations with a complex frequency spectrum (varying from discrete to continuous) are observed. It is shown that the observed specific features of electron transport are caused by the spatial-temporal dynamics of electric-field domains (dipoles and monopoles). The effects of the bifurcation, hysteresis, and multistability of current-voltage characteristics are also observed. At high fields, regular features are observed and identified in the current-voltage characteristics; these features are caused by resonance tunneling of electrons between the levels of the Wannier-Stark ladders belonging to quantum wells separated by several periods.

Controlling chaos with magnetic field in semi-insulating GaAs

Chaos control has stimulated a large amount of work. We have studied the effect of an external parallel magnetic field on the low-frequency current oscillations observed on a molecular beam epitaxy GaAs sample grown at 265 deg. C, and we have shown that it can be efficiently used for chaos control. The study of the magnetoresistance indicates that the effect of the magnetic field on the charges of the hopping conduction mechanism induces changes in the low-frequency oscillations. Due to this, we have used the magnetic field to control chaos assessed through direct observation low-frequency oscillations, their attractors, and bifurcation diagrams. We also found that the magnetic field interferes indirectly with the Coulombian interaction between the free charges in the conduction band and the hopping carriers, as well as with the recombination mechanism of field enhanced trapping. Controlling the low-frequency oscillations in semi-insulating GaAs by means of an external magnetic field permits probing the interaction of the slow hopping carriers and the fast free carriers in the electric-field domains.

STUDY OF AN ACOUSTOELECTRIC INSTABILITY IN PIEZOELECTRIC SEMICONDUCTORS

The coupling between the acoustic transverse phonons and the electron drift current, in piezoelectric semiconductors, has long been known to give rise to a negative acoustoelectric current, thereby leading to a decreased conductivity with respect to the value derived from the Ohm law. Increasing the electric field above a threshold causes the differential conductivity to become eventually negative, which triggers off an instability. The latter has been characterized experimentally by a mm-wide, high-field soliton moving at transverse sound velocity and by low-frequency current oscillations. However it is shown here to consist actually of a high-amplitude acoustoelectric microwave-packet. Because the electric permitivity is inferred to be renormalized to zero, the properties of this solution, including its vibrational frequency and amplitude, are shown to be shaped by the nonlinear piezoelectric interaction. All experimental features turn out to be accounted for satisfactorily within the framework of the present analysis. The relatively long initial transient regime, exhibiting interesting observations by Brillouin scattering, is also addressed.

Mechanisms of current flow in p-TlGaSe2 single crystals

Temperature dependences of dark current, i(T), thermally stimulated current, TSC, and current–voltage, I–V, characteristics have been investigated in p-type TlGaSe2 single crystals having different technological origins, using different contacts, in a wide temperature range and at different heating rates. Two types of initially undoped crystals with high resistivity, differing in deviation from the stoichiometry (type 1 and type 2), have been investigated. It is shown that the mechanism of the current flow strongly depends on the technological origin of the sample. Negative TSC is observed in type 1 samples for the first time and attributed to the simultaneous release of holes and electrons from acceptor and donor states in the same temperature interval. In these types of crystals, the dark current appears due to the double injection of electrons and holes from the contacts and at high heating rates β⩾ 8–10 K min−1 has a thermally activated character. The parameters of subsequent impurities have been obtained using the TSC theory. The shallow donor states with extremely low capture cross sections were shown to contribute to the thermally activated processes in these types of crystals. In type 2 crystals low-frequency current oscillations, LFCO, have been observed for the first time in the temperature range 120–180 K. These types of crystals are characterized by nonlinear N-type I–V characteristics. Mechanisms are proposed to explain the nonlinear I–V characteristics and current oscillations.

Electret states and current oscillations in the ferroelectric semiconductor TlGaSe2

Low-frequency current oscillations (LFCOs) have been observed for the first time in the ferroelectric semiconductor TlGaSe2. It is shown that LFCOs can be observed in specially undoped high-resistivity single crystals and in crystals after annealing for some hours within the incommensurate phase. LFCOs are shown to be due to non-linear current–voltage (I–V) characteristics of the crystals. The origin of the non-linearity of the I–V characteristics is based on the formation of electret states due to the capture of carriers by deep impurity centres. The possible influence of electret states on the peculiarities of different physical properties observed in these crystals is discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of rare-earth doping on the electrical properties of InSe single crystals

The effect of rare-earth doping (R = Gd, Dy, Ho) to different levels (0, 10−5, 10−4, 10−3, 10−2, and 10−1 at %) on threshold switching and low-frequency current oscillations in single crystals of indium selenide, a layered III–VI semiconductor, has been studied in broad ranges of temperatures (77–400 K), wavelengths (0.30–3.50 μm), and illuminances (up to ∼102 lx). The results are interpreted in terms of the anisotropic chemical bonding in n-InSe and its response to rare-earth doping.

Blockade of free carriers by hopping carriers leading to the low-frequency current oscillations in semi-insulating GaAs

Semi-insulating GaAs samples show low-frequency current oscillations (LFO) under high dc voltage. These spontaneous oscillations are caused by traveling high-electric-field domains and can be explained based on the well-accepted model of electron trapping in deep levels by multiphonon emission processes. This paper aims to explain today the most important open question about LFO, namely, the cause of the very slow propagation of the domain resulting in the low frequencies. Our claim is that hopping conduction acts as a low mobility barrier for the free carriers leading to the formation of the electric-field domains. The barrier blocks, through a Coulomb shield, the flow of the high mobility free carriers leading to a charge accumulation, which produces the current pulse in the external circuit when the domain reaches the anode where it is destroyed. LFO occurs, therefore, only in the regime of band and hopping mixed conduction mechanisms.

Low frequency oscillations and bifurcation diagram in semi-insulating GaAs samples

We present an experimental study of bifurcation diagrams from low frequency current oscillations (LFO) measurements obtained from semi-insulating GaAs samples grown by low temperature molecular beam Epitaxy (LT-MBE). The considered growth temperatures were 215oC and 265oC. LFO are considered to be spontaneously generated oscillations under constant applied bias V. These oscillations were measurement and recorded in the form of time series. The bifurcation diagrams were obtained from the sequence of minima as a function of the applied bias. The standard measurement procedure was described elsewhere. As the control parameter, the bias allows the identification of a bifurcation route to chaos.

Theoretical time series analysis from electric field oscillations generated by rate equations of generation–recombination processes in n-type semiconductors

We have analyzed the nonlinear electric field time series theoretically generated by a set of three coupled first-order differential equations in order to interpret the chaotic behavior present in measurements of low-frequency current oscillations in semiconductor samples. In the present work, we carry out a direct investigation of the theoretical time series applying techniques of experimental nonlinear time series analysis. We have reconstructed the phase space of the system and calculated some parameters as the correlation dimension of the reconstructed attractors, the Lyapunov exponent's spectrum and the Kaplan–Yorke dimension. The conclusions are consistent with the theory of chaotic systems and with the available experimental results for semiconductors.

Bifurcation diagram, noise reduction and period-four cycle on low frequency current oscillations in a semi-insulating GaAs sample

Low frequency oscillations (LFO) on current I( V, t) curves were studied in a GaAs sample grown by molecular beam epitaxy. Our measurements were carried out at a temperature of 200 K and under infrared illumination. The nonlinear I( V) characteristics give rise to spontaneous oscillations under constant applied bias V. The attractors were constructed by the delay vector technique with the delay value given by minimal mutual information. A bifurcation diagram was constructed from the minima sequence as a function of the applied bias. A noise reduction algorithm applied to the raw data allowed the cleaning of the bifurcation diagram. We have identified a bifurcation route of periodicity following the sequence 0–1–2–4–2–∞ having the applied bias as the control parameter. Fourier transforms is a merit figure of the noise reduction procedure and also a qualitative proof of the chaotic regime.

Bifurcation diagram, noise reduction and period-four cycle on low frequency current oscillations in a semi-insulating GaAs sample

Low frequency oscillations (LFO) on current I ( V , t ) curves were studied in a GaAs sample grown by molecular beam epitaxy. Our measurements were carried out at a temperature of 200 K and under infrared illumination. The nonlinear I ( V ) characteristics give rise to spontaneous oscillations under constant applied bias V . The attractors were constructed by the delay vector technique with the delay value given by minimal mutual information. A bifurcation diagram was constructed from the minima sequence as a function of the applied bias. A noise reduction algorithm applied to the raw data allowed the cleaning of the bifurcation diagram. We have identified a bifurcation route of periodicity following the sequence 0–1–2–4–2–∞ having the applied bias as the control parameter. Fourier transforms is a merit figure of the noise reduction procedure and also a qualitative proof of the chaotic regime.

Model study of the influence of the magnetic field configuration on the performance and lifetime of a Hall thruster

A two-dimensional hybrid model of the discharge in Hall thrusters including the near outside region between cathode and exhaust plane has been developed. The topology of the applied magnetic field is calculated with a finite element software and used as input for the discharge code. In this paper, we examine the influence of the magnetic field topology on the thruster operation and properties, with emphasis on the thruster lifetime. Results show that a configuration with a zero magnetic field and a smaller region with large magnetic field tends to decrease wall erosion and low frequency current oscillations keeping a high level of performance.