High-frequency Voltage Oscillations Research Articles

Applying a Multiple-Input Single-Output Interleaved High Step-Up Converter with a Current-Sharing Device Having Different Input Currents to Harvest Energy from Multiple Heat Sources

In this paper, a thermoelectric conversion system for multiple heat sources is proposed. For design convenience, the overall system employs only a single-stage converter. Such a converter uses coupling inductors and switched capacitors to increase the voltage gain. In order to reduce the high-frequency voltage oscillation of the turn-off of the main switches created from leakage inductors, two active clamp circuits with zero voltage switching (ZVS) turn-on are employed. By doing so, although the current-sharing device with interleave control is embedded in the proposed converter, the input currents can be unequal. Therefore, the inputs of the converter can operate under individual maximum power points and transfer the energy from different thermoelectric generators (TGs) to a single load. Furthermore, the main switches have low voltage stress during the turn-off period. As for the maximum power point tracking (MPPT) method, it utilizes a three-point-weighting method to improve the tracking stability. In addition, the number of inputs of this converter can be extended. The MPPT simulation is presented to verify the feasibility as well as several experimental waveforms to demonstrate the effectiveness. The field programmable gate array (FPGA) is used as a digital control kernel to control the thermoelectric conversion system.

ZVS Realization of H-Bridge Low-Voltage High-Current Converter via Phase-Shift and Saturable Control

A zero-voltage switching (ZVS) H-bridge phase-shifted low-voltage high-current converter with saturable inductors is proposed in this paper. The introduction of saturable inductors solves the short circuit problem caused by high-frequency on–off of the power tube, and effectively inhibits high-frequency voltage oscillation and voltage spikes of the rectifier tube. In addition, when the current flowing through the saturable inductor does not change rapidly, it exhibits a low impedance and consumes very little power. The detailed design process of main parameters in the converter is presented to provide design reference for power supply workers. To verify the effectiveness of design, a 3 kW(15 V/200 A) prototype converter is built. This low-voltage high-current prototype has around 90% efficiency, and can suppress high-frequency voltage oscillation and voltage spikes, avoiding the short circuit problem of power tube, all of which are verified by experimentation.

Analysis and suppression of voltage oscillation at DC circuit breaker terminal in DC fault transient process

High-frequency voltage oscillation occurs at the initial stage of DC short-circuit fault in the modular multilevel converter (MMC)-based high-voltage direct-current systems, which directly threatens the reliable operation of the secondary equipment in DC circuit breakers (DCCBs). Therefore, this paper focusses on the voltage oscillation characteristic at DCCB terminals, considered the stray parameters of connecting lines, the parasitic parameters of main equipment and the impedance of grounding grid, established equivalent circuit models of MMC-HVDC converter station at the initial stage of pole-to-pole and pole-to-ground fault, and obtained the analytical formula for the voltage oscillation frequency. Then, the comparison with the DC short-circuit test data of ±200 kV MMC-HVDC converter station verified the validity of the models and revealed the mechanism of voltage oscillation. Finally, the influence of the parameters of the converter station on the oscillation characteristics was discussed. It was proposed to install the DCCB at the MMC valve's terminal near the DC side, and the oscillation frequency is about 10–60 kHz, which is more than 5 times lower than that at the DC reactor's terminal near the DC pole bus, and the disturbance of oscillation to DCCB was alleviated.

Field-effect transistors as electrically controllable nonlinear rectifiers for the characterization of terahertz pulses

We propose to exploit rectification in field-effect transistors as an electrically controllable higher-order nonlinear phenomenon for the convenient monitoring of the temporal characteristics of THz pulses, for example, by autocorrelation measurements. This option arises because of the existence of a gate-bias-controlled super-linear response at sub-threshold operation conditions when the devices are subjected to THz radiation. We present measurements for different antenna-coupled transistor-based THz detectors (TeraFETs) employing (i) AlGaN/GaN high-electron-mobility and (ii) silicon CMOS field-effect transistors and show that the super-linear behavior in the sub-threshold bias regime is a universal phenomenon to be expected if the amplitude of the high-frequency voltage oscillations exceeds the thermal voltage. The effect is also employed as a tool for the direct determination of the speed of the intrinsic TeraFET response which allows us to avoid limitations set by the read-out circuitry. In particular, we show that the build-up time of the intrinsic rectification signal of a patch-antenna-coupled CMOS detector changes from 20 ps in the deep sub-threshold voltage regime to below 12 ps in the vicinity of the threshold voltage.

High-Frequency Voltage Oscillations in Cultured Neurons and Astrocytes

High-Frequency Voltage Oscillations in Cultured Neurons and Astrocytes

High-frequency voltage oscillations in cultured astrocytes.

Because of their close interaction with neuronal physiology, astrocytes can modulate brain function in multiple ways. Here, we demonstrate a yet unknown astrocytic phenomenon: Astrocytes cultured on microelectrode arrays (MEAs) exhibited extracellular voltage fluctuations in a broad frequency spectrum (100–600 Hz) after electrical stimulation. These aperiodic high-frequency oscillations (HFOs) could last several seconds and did not spread across the MEA. The voltage-gated calcium channel antagonist cilnidipine dose-dependently decreased the power of the oscillations. While intracellular calcium was pivotal, incubation with bafilomycin A1 showed that vesicular release of transmitters played only a minor role in the emergence of HFOs. Gap junctions and volume-regulated anionic channels had just as little functional impact, which was demonstrated by the addition of carbenoxolone (100 μmol/L) and NPPB (100 μmol/L). Hyperpolarization with low potassium in the extracellular solution (2 mmol/L) dramatically raised oscillation power. A similar effect was seen when we added extra sodium (+50 mmol/L) or if we replaced it with NMDG+ (50 mmol/L). The purinergic receptor antagonist PPADS suppressed the oscillation power, while the agonist ATP (100 μmol/L) had only an increasing effect when the bath solution pH was slightly lowered to pH 7.2. From these observations, we conclude that astrocytic voltage oscillations are triggered by activation of voltage-gated calcium channels and driven by a downstream influx of cations through channels that are permeable for large ions such as NMDG+. Most likely candidates are subtypes of pore-forming P2X channels with a low affinity for ATP.

Measurement and analysis of transient overvoltage distribution in the low-voltage winding of 1000 kK power transformer without oil immersion

High frequency voltage oscillations will be generated when transient overvoltages intrude into power transformers. These oscillations may cause damage to the insulation of transformers. In this paper, a capacitive coupling sensor array was presented for measuring voltage distributions and voltage gradient distributions in low-voltage winding of a 1000 kV power transformer. A variable impedance transmission line between impulse generator and winding was also designed to avoid distortion of the applied impulses. The results indicate that under the lightning impulse voltages, voltage distributions have been restrained below applied voltage at sandwich-interleaved section but exceed the applied voltage in continuous coils in the first half of the winding and lead to a very nonuniform voltage distribution. Longer duration time will result in a more nonuniform distribution. The amplitude of the transient voltage under steep front square wave reduces faster downward the winding than that under lightning impulse voltages. A shorter rise time will lead to a much worse distribution. In addition, a dominated oscillation of 63 kHz is found under both impulse voltages and steep front square waves.

Simulation Study on Insulated Gate Bipolar Transistor Turn-Off Oscillations

High-frequency voltage oscillation is occasionally observed in insulated gate bipolar transistors during a turn-off operation. The turn-off oscillation generation was explained by the plasma extraction transit time oscillation theory. Its basic mechanism was the negative resistance creation. A negative RF power, which meant that the insulated gate bipolar transistor showed a “negative resistance”, was generated by the trapezoidal current density induced by holes and the RF voltage applied to the insulated gate bipolar transistor. However, the author showed that the induced current density oscillates with an almost negative phase compared with the RF voltage oscillation. He carried out an insulated gate bipolar transistor turn-off oscillation simulation using simplified simulation models. In this work, he showed the generation of the negative resistance and the turn-off oscillation in detail. He also showed the common and different points between his results and the plasma extraction transit time oscillation theory.

Negative-resistance Generation in IGBT and Estimation of Frequency of IGBT Turn-OFF Oscillation

High-frequency voltage oscillation is occasionally observed in bipolar power semiconductors during a turn-OFF operation. The mechanism of generation of the turn-OFF oscillation is explained by the plasma extraction transit-time (PETT) oscillation theory. According to this theory, a packet of holes propagate through the drift region with constant velocity and induce current flow in the external circuit. The waveform of the corresponding current density is trapezoidal. Negative resistance is generated in the circuit by the current density and RF voltage applied to the semiconductors. The negative resistance is the origin of generation of the turn-OFF oscillation. The author investigates the generation of the IGBT turn-OFF oscillation by performing a 2D device simulation. The simulation results show generation of the negative resistance is different from that described in the PETT oscillation theory. The velocity of the packet of holes depends on the electric field in the drift region and is not constant. The simulated current density oscillates with an almost negative phase compared to the RF voltage oscillation. Further, on the basis of the simulation results, the author will propose the practical method to estimate the oscillation frequency in this paper.

Study of the Effect of a Vacuum Arc Current Interruption in a Nonuniform Magnetic Field

The effect of the magnetic field having axial and radial components on the vacuum arc stability and arc extinction was studied experimentally. It has been demonstrated that the magnetic field configuration affects the arc discharge channel shape. It has been shown also that there is a critical current value in the external magnetic field. With the current values below the critical value, high-frequency voltage oscillations appeared on the arc indicating the arc instability. The critical current value depends on the value of each of the magnetic field components. The effect of the magnetic field component values on the conditions of the vacuum arc current breaking was investigated. A physical model has been suggested, and a mathematical 3-D-model has been developed that describes the current channel shape in the external nonuniform magnetic field. The trajectories of ion motion in the vacuum arc current channel have been calculated. It has been demonstrated that when some part of ion trajectories go beyond the bounds of the anode, it can result in the arc burning instability.

Investigation on IGBT High-Frequency Plasma Extraction Transient Time Oscillation

High-frequency voltage oscillations occasionally appear between insulated gate bipolar transistor's (IGBTpsilas) collector and emitter terminals during turn-off operation. Based on the plasma extraction transit-time theory, the turn-off oscillations were investigated experimentally and analytically using real-sized high-voltage IGBT modules. Generating mechanism of the turn-off oscillations was clarified by 2D device simulation. In addition, a practical method for preventing the turn-off oscillations will be proposed in the paper.

High-frequency oscillations in low-dimensional conductors and semiconductor superlattices induced by current in stack direction

A narrow energy band of the electronic spectrum in some direction in low-dimensional crystals may lead to a negative differential conductance and $N$-shaped $I\text{\ensuremath{-}}V$ curve that results in an instability of the uniform stationary state. A well-known stable solution for such a system is a state with electric field domain. We have found a uniform stable solution in the region of negative differential conductance. This solution describes uniform high-frequency voltage oscillations. Frequency of the oscillation is determined by antenna properties of the system. The results are applicable also to semiconductor superlattices.

Instability and stable non-stationary state induced by current in stack direction in low-dimensional conductors

We have studied the problem of the current flow through the sample with narrow energy band when the average voltage corresponds to the negative differential conductance (NDC) region. We have found a stable solution in the NDC region which describes uniform high frequency voltage oscillations. Frequency of the oscillation is determined by antenna properties of the system. The results are applicable also to semiconductor superlattices.

High-frequency voltage oscillation in transformer windings and electrical breakdown properties of interturn insulation immersed in oil at VFT voltage

Switching operations in gas insulated switchgear (GIS) can generate very fast transient overvoltages (VFTO) which propagate in the GIS with little attenuation. When a transformer is directly connected to the GIS through a gas/oil bushing, the VFTO can enter the transformer and excite the voltage oscillation in the windings. In the paper, we firstly show a resonance phenomenon due to the superposition of the traveling waves which was found in the experiments using the coil windings of a 500 kV transformer. The fundamental resonance frequency ranged at about 2 MHz, and its voltage at the interturn (turn-to-turn) of the coil could reach 0.25 times of the applied voltage in the worst case. Secondly, the electrical breakdown properties of interturn insulation in oil were studied, applying the unipolar and bipolar voltages with 2 MHz oscillatory frequency. The breakdown voltages at the unipolar and bipolar VFT oscillatory stresses were about 1.25 and 1.40 times higher than that at the standard lighting impulse, respectively. We discuss the experimental breakdown stresses with the ones estimated from the analysis of VFTO in the transformer windings and also show the methods to cope with the VFTO. © 2000 Scripta Technica, Electr Eng Jpn, 132(4): 45–52, 2000

Experimental investigation of high frequency voltage oscillation in transformer windings

Very fast transient overvoltages (VFTO) comprising oscillating waveforms generate a voltage oscillation in transformer windings. The behaviour of the voltage oscillation in the winding is investigated experimentally on 500 kV class transformers using a pulse and a sinusoidal voltage. The interturn voltages are directly measured using an optical fiber transmission system. In the case of a pulse application, it is found that an interturn traveling wave is generated at either end of the coil and propagates along the winding. In the case of a sinusoidal voltage application, a resonance phenomenon, probably due to the superposition of the traveling waves, is found in the interturn voltage, which could reach 0.25 times the applied voltage. In the tested transformers, the estimated interturn voltage level at incoming VFTO can be 3 times higher than that in the lightning impulse voltage test.

Electrical resonance and Ca2+ influx in the synaptic terminal of depolarizing bipolar cells from the goldfish retina.

1. Whole-cell recordings and fura-2 measurements of cytoplasmic [Ca2+] were made in depolarizing bipolar cells isolated from the retinae of goldfish. The aim was to study the voltage signal that regulates Ca2+ influx in the synaptic terminal. 2. The current-voltage relation was linear up to about -44 mV. At this threshold, the injection of 1 pA of current triggered a maintained 'all-or-none' depolarization to a plateau of -34 mV, associated with a decrease in input resistance and a damped voltage oscillation with a frequency of 50-70 Hz and initial amplitude of 4-10 mV. A second frequency component of 5-10 Hz was often observed. In a minority of cells the response to current injection was transient, recovering with an undershoot. 3. Unstimulated bipolar cells generated similar voltage signals, driven by current entering the cell through a non-specific cation conductance that continuously varied in amplitude. 4. The threshold for activation of the Ca2+ current was -43 mV and free [Ca2+]i in the synaptic terminal rose during a depolarizing response. Simultaneous measurements of the fluorescence associated with the membrane marker FM1-43 demonstrated that these Ca2+ signals stimulated exocytosis. Regenerative depolarizations and associated rises in [Ca2+]i were blocked by inhibiting L-type Ca2+ channels with 30 microM nifedipine. 5. Depolarization beyond -40 mV also elicited an outwardly rectifying K+ current. Blocking this current by replacing external Ca2+ with Ba2+ caused the voltage reached during a depolarizing response to increase to +10 mV. 6. The majority of the K+ current was blocked by 100 nM charybdotoxin, indicating that it was carried by large-conductance Ca2+ activated K+ channels. A transient voltage-gated K+ current remained, which began to activate at -40 mV. High-frequency voltage oscillations were blocked by 100 nM charybdotoxin, but low-frequency oscillations remained. 7. These results indicate that the voltage response of depolarizing bipolar cells is shaped by L-type Ca2+ channels, Ca(2+)-activated K+ channels and voltage-dependent K+ channels. This combination of conductances regulates Ca2+ influx into the synaptic terminal and confers an electrical resonance on the bipolar cell.