Repetitive Voltage Pulses Research Articles

Preventing partial discharge in liquid metal polymer composites under steep voltage pulses

Liquid metal polymer composites (LMPCs) offer viable solutions for deformable electronics, including sensors and actuators. To ensure the longevity of the deformable technologies, partial discharge (PD) must be mitigated as it causes premature material aging and device failure. While studies on the dielectric properties of LMPCs have mainly focused on AC voltage, a large number of deformable applications will inevitably involve power electronics for higher efficiency and power density. Consequently, addressing PD in LMPCs under repetitive steep voltage pulses generated by power electronic switches becomes important. This study employs an electric field cancellation technique using electrets to reduce the PD magnitude and increase the partial discharge inception voltage (PDIV) of LMPCs under steep square voltage waves. We placed either a non-electret or electret thin film on top of LMPC samples and conducted PD experiments in flat and bent test coupon arrangements. The results show that electrets can reduce PD magnitude and increase PDIV in both test arrangements. Furthermore, the study provides deeper insights into the experimental results by conducting the electric field analysis of the LMPC experiments through numerical models.

Motor Winding Insulation Degradation Under Repetitive Voltage Pulses

Motor Winding Insulation Degradation Under Repetitive Voltage Pulses

Influence of Pressure on the PD and Induced Aging Behavior of Polyimide Insulation Under Repetitive Pulse Voltage

Reduction of air pressure can influence insulation material’s behavior with respect to resisting partial discharge (PD) generation and its induced aging process. This may bring threat to the insulation reliability of medium frequency transformers working under low pressure condition, considering they are exposed to high probability of PD inception caused by PWM voltage. Aiming at investigating the performance of insulation of medium frequency transformer being operated in an environment with a decreasing pressure, this paper introduces a test system that can conduct partial discharge and endurance lifetime tests under repetitive pulse voltage and controlled pressure. Using this test system and focusing on polyimide film, which is one of the main insulation materials used by medium frequency transformers, partial discharge and endurance lifetime tests are conducted under different pressures. The results show that partial discharge inception voltage decreases with decreasing pressure. However, the changing of partial discharge magnitude, time lag and its induced insulation lifetime do not change monotonously. Surface of the failed insulation samples show that the erosion area caused by partial discharge is larger under lower pressure. The mechanisms behind the changing of partial discharge time lag, discharge magnitude, endurance lifetime, along with the discharge eroded area with pressure are discussed in detail.

Study on the conversion mechanism of CO2 to O2 in pulse voltage dielectric barrier discharge at Martian pressure

CO2 constitutes around 95 % of the atmosphere on Mars, and its in-situ utilization is of great value. O2 is a significant target product transformed from CO2 that can support both human respiration and fuel combustion, its conversion mechanism remains unclear. In this study, using a dielectric barrier discharge (DBD) with parallel plate electrodes, simulation and experimental results are combined to discuss the discharge characteristics and CO2 conversion process, under Mars conditions. The DBD was driven by a repetitive frequency microsecond pulse voltage. According to the discharge current, the discharge process can be split into four stages, namely the positive (negative) discharge stage and the positive (negative) discharge extinguishing stage. With a contribution up to 98 % and primarily taking place in the positive discharge extinguishing stage, the recombination decomposition reaction of electron and CO+2 is the primary pathway of O2 synthesis. At the position near the edge of the cathode potential drop zone, electron impact ionization of CO2 ground state molecules dominates the formation of CO+2. Under the range of pulse parameters considered in this paper, increasing the pulse width can lengthen the time of recombination decomposition reaction of electron and CO+2 in positive discharge extinguishing stage, which is favorable for the formation of O2. Likewise, reducing the pulse's rising edge can increase the discharge intensity and CO+2 density, which is also favorable to O2 production.

High-Power LiNbO3 Domain-Wall Nanodevices.

Wide band gap semiconductors keep on pushing the limits of power electronic devices to higher switching speeds and higher operating temperatures, including diodes and transistors on low-cost Si substrates. Alternatively, erasable conducting walls created within ferroelectric single-crystal films integrated on the Si platform have emerged as a promising gateway to adaptive nanoelectronics in sufficient output power, where the repetitive creation of highly charged domain walls (DWs) is particularly important to increase the wall current density. Here, we observe large conduction of the head-to-head DW at an optimized inclination angle of 15° within a LiNbO3 single crystal that is 3-4 orders of magnitude higher than that of the tail-to-tail DW. The wall conduction is diode-like with a linear current density of higher than 1 mA/μm and an on/off ratio of larger than 106 under the application of a repetitive switching voltage pulse in time less than 10 ns and an endurance number of higher than 105. The high-power diodes can not only perform direct data processing in high-density nonvolatile DW memories in fast operation speeds and low-energy consumption but also function as sensors in compact electromechanical systems, selectors in phase-change memory and resistive random-access memory, and half-wave/full-wave rectifiers in modern nanocircuits in dimensions approaching the thickness of the depletion layer below which the tradition p-n junction malfunctions.

Improved Resistive Switching with Low-Power Synaptic Behaviors of ZnO/Al2O3 Bilayer Structure.

In this work, the resistive switching behavior of bilayer ZnO/Al2O3-based resistive-switching random access memory (RRAM) devices is demonstrated. The polycrystalline nature of the ZnO layer confirms the grain boundary, which helps easy oxygen ion diffusion. Multilevel resistance states were modulated under DC bias by varying the current compliance from 0.1 mA to 0.8 mA, the SET operations where the low resistance state of the memristor device was reduced from 25 kΩ to 2.4 kΩ. The presence of Al2O3 acts as a redox layer and facilitates oxygen vacancy exchange that demonstrates stable gradual conductance change. Stepwise disruption of conductive filaments was monitored depending on the slow DC voltage sweep rate. This is attributed to the atomic scale modulation of oxygen vacancies with four distinct reproducible quantized conductance states, which shows multilevel data storage capability. Moreover, several crucial synaptic properties such as potentiation/depression under identical presynaptic pulses and the spike-rate-dependent plasticity were implemented on ITO/ZnO/Al2O3/TaN memristor. The postsynaptic current change was monitored defining the long-term potentiation by increasing the presynaptic stimulus frequency from 5 Hz to 100 Hz. Moreover, the repetitive pulse voltage stimulation transformed the short-term plasticity to long-term plasticity during spike-number-dependent plasticity.

Voltage-driven gigahertz frequency tuning of spin Hall nano-oscillators

Spin Hall nano-oscillators (SHNOs) exploiting current-driven magnetization auto-oscillation have recently received much attention because of their potential for neuromorphic computing. Widespread applications of neuromorphic devices with SHNOs require an energy-efficient method of tuning oscillation frequency over broad ranges and storing trained frequencies in SHNOs without the need for additional memory circuitry. While the voltage-driven frequency tuning of SHNOs has been demonstrated, it was volatile and limited to megahertz ranges. Here, we show that the frequency of SHNOs is controlled up to 2.1 GHz by an electric field of 1.25 MV/cm. The large frequency tuning is attributed to the voltage-controlled magnetic anisotropy (VCMA) in a perpendicularly magnetized Ta/Pt/[Co/Ni]n/Co/AlOx structure. Moreover, the non-volatile VCMA effect enables cumulative control of the frequency using repetitive voltage pulses which mimic the potentiation and depression functions of biological synapses. Our results suggest that the voltage-driven frequency tuning of SHNOs facilitates the development of energy-efficient neuromorphic devices.

Pulse width modulation voltage source deadtime effect on partial discharge and lifetime of inverter‐fed motor insulation

This paper analyses the impact of square wave pulse voltage deadtime on the partial discharge (PD) and the lifetime of turn-to-turn insulation. A bipolar repetitive pulse voltage with a deadtime of 0–10 μs is produced using double half-bridge solid-state switches having push–pull technology controlled by a field-programmable gate array. The mechanism of the discharge process at rising and falling edges of the pulse voltage before and after deadtime is analysed in detail. The discharge amplitude and PD probability at the rising/falling edges of the voltage waveform increase as the deadtime increases from 0 to 10 μs due to the remanent charges. The number of PD and their intensity is higher at the first rising/falling edges of pulse voltage as compared to the second rising/falling edges for all deadtimes 0–10 μs. As the deadtime increases beyond 2 μs, the number of PDs increases and concentrates at a specific phase angle of rising/falling edges. These localise discharges degrade the insulation material and reduce its lifetime. This study helps to identify the inverter-fed motor insulation faults due to deadtime. It can provide guidelines to motor insulation designers to determine the limit value of deadtime to compensate PD and ensure the safer operation of such motors.

Spatio-temporal evolution of ion current extracted from pulsed dielectric barrier discharge

In the present study, the influence of various factors on the shape of the ion cloud extracted from the specific discharge setup was investigated. The overall installment presented a surface dielectric barrier discharge plasma in the air fed by impulse high voltage. The effect of the amplitude, polarity, and duration for single and repetitive voltage pulses were examined. The patterns of ion current distribution depending on the gas gap length and the average field strength were obtained, and the effect of pulse duration and frequency on the amount of the extracted charge were examined. It was shown that the extracted charge was non-uniformly distributed in the gap volume during single and periodic voltage pulses. The effect of volume charge accumulation in the gap at high pulse repetition rate was shown. Computer simulations demonstrated that the main role in the ion cloud shape distortion was caused by the surface charge deposited on the dielectric barrier.

Discharge Characteristics and Mechanism of Bubble Flow in FC-72 under Unipolar Repetitive Pulse Voltage

This article investigates the discharge characteristic of the bubble flow in FC-72 under unipolar repetitive pulsed voltage, which supports the insulation design of two-phase cooling in an HV power electronic device. The effect of thermal bubble flow on the insulation of FC-72 refrigerant was evaluated. According to the partial discharge (PD) characteristics, it is found that there are two stages in the discharge characteristics of the bubble flow as voltage rises. Combining the PD inception voltages of FC-72 vapor under unipolar pulse voltage and dc voltage, it reveals the discharge thresholds of the vapor inside the bubble under the transient electric field and the steady electric filed, respectively, which proves the mechanism of the staged PD evolution. Based on the motion calculation of the bubble, the PD probability of the bubble flow within a voltage period is analyzed. It simulates the phase-resolved PD pattern, which reveals the PD mechanism of the bubble flow. The research provides a reference for the insulation design for the two-phase cooling technology in power electronics.

A Method for Endurance Testing of Enameled Round and Rectangular Wires for Motors Controlled by SiC-based Inverters

The purpose of this article is to propose a method for investigating the influence of the impulse voltage waveform generated by inverters based on wide bandgap semiconductors on the insulation endurance of magnet wires. In this regard, endurance tests of enamel insulation are performed on a designed test bench, which generates repetitive voltage pulses with ns-level rising and falling times corresponding to modern silicon carbide-based inverters. Furthermore, we suggest a novel specimen's shape for testing enameled rectangular section magnet wires which find their application in electric traction motors. Considerations related to the deterioration of dielectrics subjected to impulse voltage with high slew rates and high switching frequency are given.

Electrical tree characteristics under AC and repetitive pulse voltages in wind turbine generator composite insulation

The composite insulation system in a wind turbine generator (WTG) is subjected to either the AC or the repetitive pulse voltage, which may lead to degradation in the insulation. In this paper, the effects of voltage amplitude, waveform, repetition rate and composite insulating material constitution on the electrical tree morphology and development are studied. The results indicate that higher AC voltage produces sparser branches, shorter initiation time, and higher growth rates in the impregnating varnish and along interfaces. Electrical trees are initiated and grow easier under repetitive pulse voltage than AC voltage. Increasing repetition rate of pulse voltage promotes the initiation and propagation of electrical trees. It has been found that the peak of the pulse overshoot on the rising edge plays an important role in the initiation and propagation of electrical tree, while the average of pulse voltage dominates the breakdown time of the composite insulation system.

Characteristics of analog memristor on thin-film Pt/Co0.2TiO3.2/ITO

In this work, thin films of Pt/Co0.2TiO3.2 (23 nm)/ITO were investigated showing their memristive capacity related to analog switching properties. A temporal analysis of current and voltage in nonlinear I–V graphs shows pinched hysteresis loop with gradual distribution in its electrical conductivity, typical of analog systems. The presence of trap centers such as oxygen vacancies suggests that the specific conduction mechanism is space-charge-limited current (SCLC). In addition, we study its performance under repetitive voltage pulses as well as at different durations. We further conducted data retention tests in the memristor high resistance state (HRS) and low resistance state (LRS).

Streamer characteristics of PEEK surface in nitrogen driven by positive repetitive pulse voltage

In this work, we present a detailed analysis on the observed streamer characteristics of PEEK (polyetheretherketone) surface in nitrogen at atmospheric pressure driven by positive repetitive pulse voltage, including the forward discharge streamer and the back discharge streamer. Fast oscilloscopic technique and digital photographic technique were employed to observe the streamers of surface discharges, and current pulse waveform analysis technique and image analysis technique were used to analyze the characteristics of the observed streamers. The experimental results show that the current pulse of forward discharge streamer has a shorter rise time, a higher current amplitude, a faster current rising rate, a shorter fall time, a faster current falling rate and a smaller charge quantity than those of back discharge streamer at the same voltage amplitude. Besides, the forward discharge streamer has a longer stopping length, a faster propagation velocity and a stronger maximum light intensity than back discharge streamer at the same voltage amplitude. Furthermore, the correlation between the electrical characteristics and optical characteristics of the streamer was analytically derivated, and the current rising rate of the streamer is proportionally correlated to the product of the streamer stopping length and the streamer velocity square.

Partial Discharge Analysis under High-Frequency, Fast-Rise Square Wave Voltages in Silicone Gel: A Modeling Approach

Wide bandgap (WBG) power modules able to tolerate high voltages and currents are the most promising solution to reduce the size and weight of the power management and conversion systems. These systems are envisioned to be widely used in the power grid and the next generation of more (and possibly all) electric aircraft, ships, and vehicles. However, accelerated aging of silicone gel when being exposed to high frequency causes fast rise-time voltage pulses that can offset or even be an obstacle for using WBG-based systems. Silicone gel is used to insulate conductor parts in the module and encapsulate the module. It has less electrical insulation strength than the substrate and is susceptible to partial discharges (PDs). PDs often occur in the cavities located close to high electric field regions around the sharp edges of metallization in the gel. The vulnerability of silicone gel to PDs occurred in the cavities under repetitive pulses with a high slew rate investigated in this paper. The objective mentioned above is achieved by developing a Finite-Element Analysis (FEA) PD model for fast, repetitive voltage pulses, which have been done for the first time to the best of our knowledge. By using the model, the influence of frequency and slew rate on the magnitude and rate of PD events is studied.

A Repetitive High-Voltage Nanosecond Pulse Generator

A generator of repetitive voltage pulses in the form of a damped sine wave with a period of ~6 ns, an amplitude of the first voltage half-wave of up to ~400 kV, and a pulse repetition frequency of up to 60 Hz is described. The generator is powered from a storage battery (7 A h, 36 V) and has several voltage build-up stages.

Electrical Tree in HTV Silicone Rubber With Temperature Gradient Under Repetitive Pulse Voltage

High temperature vulcanized silicone rubber (HTV SIR) is important insulation for high voltage direct current (HVDC) cable accessories. The pulse voltage in the HVDC system may initiate an electrical tree in SIR insulation. During the operation, there is a temperature gradient in SIR caused by the different temperatures of conductor and external environment. So it is necessary to research the electrical tree initiated by pulse voltage under the temperature gradient. In this paper, electrical trees in SIR with different temperature gradients were recorded. The inception voltage, tree length, and accumulated damage (AD) distribution were analyzed. The experiment results indicate that tree inception voltage decreases with the increase of temperature of the ground electrode when the needle temperature is 90 °C, it also decreases with the increase of needle temperature when the ground temperature is 90 °C. All the trees are in bush structure when the ground temperature is 90 °C, the structure changes from branch to bush with the increase of ground temperature when the needle temperature is 90 °C. When the needle temperature is 90 °C, AD distribution changes obviously with the increase of ground temperature. The conductivity of SIR under different temperatures and electric fields was tested. The effect of changing conductivity on tree inception was discussed. Surface potential decay (SPD) at different temperatures was tested. The trap characteristics and charge kinetic properties influenced by temperature may be the main reasons for the change of tree structures in the growth process.

The role of time-lag in the surface discharge inception under positive repetitive pulse voltage

Polyetheretherketone is used in the packaging of high voltage power electronics as a frame material, and the surface discharge behaviors of polyetheretherketone are the main concerns for the packaging design. Pulse width modulation voltage, which shows a variable duty cycle, is applied to polyetheretherketone at working conditions. The surface discharge behaviors of polyetheretherketone under a positive repetitive pulse voltage of 50 Hz with different duty cycles were investigated in this paper. It is generally considered that the surface discharge inception is mainly related to electric field distribution; however, the pulse width of voltage could significantly affect the surface discharge inception characteristics. In this study, the surface discharge inception voltage, time-integrated streamer images, and cumulative surface discharge patterns under a positive repetitive pulse voltage of 50 Hz were obtained. It was found that the surface discharge inception voltage decreased with the increase in the duty cycle. This relationship was explained by the role of time-lag. Time-lag distribution analysis was performed, and the time-lag distribution was mainly determined by stochastic time-lag. The mean stochastic time-lag was obtained by the Laue plot, and the relationship between the mean stochastic time-lag and applied voltage could be well fitted by the inverse power law. Furthermore, the relationship between the surface discharge inception voltage and duty cycle could be exactly fitted by the inverse power law, which was derived from the relationship between the stochastic mean time-lag and applied voltage. The closely related relationships demonstrate that time-lag plays a critical role in the surface discharge inception at different duty cycles.

30 kV Pulse Diode Stack Based on 4H-SiC

The paper reports on the studies of static and dynamic characteristics of 30 kV diode stacks based on 4H-SiC drift step recovery diodes (DSRDs). It was found that the optimal performance in terms of blocking voltage and switching speed can be achieved with 2 kV DSRD dies. Fifteen 2 kV DSRD dies were connected in series and sealed with molding compound. The stacks were dynamically tested in a special oscillator circuit. Repetitive voltage pulses of 30.5 kV with the leading edge of 1.6 ns were demonstrated.

System for detection and analysis of partial discharges under transient voltage application

A measuring system for detection and analysis of partial discharges (PD) in electrical insulation under fast repetitive pulse voltage application has been developed. Partial discharges can be triggered by transient voltages and are considered as being one initial cause of insulation failure.The experimental setup consists of a specially designed pulse generator for generation of fast voltage transients. The pulses are unipolar, with the peak variable voltage of up to 10 kV and with an adjustable frequency from 0 Hz to 1.7 kHz, burst frequency of up to 2 MHz.Partial discharge inception under pulse voltage application was determined by data acquisition by means of a standard oscilloscope and suitable data analyses in Matlab, using wavelet transform and denoising techniques. The measurement results are presented in a partial discharge classification pattern designed to be used in connection with fast repetitive pulses as originating from power electronics.The feasibility of the test system for insulation diagnostics was illustrated by application to twisted pairs of enamelled winding wire.The test system is suitable for achieving new results to get better knowledge of degradation mechanisms in insulating materials exposed to fast transients from eg. power electronics.