Low Frequency Alternating Current Research Articles

Experimental Assessment of a Decentralized Control Strategy for a Back-to-Back Modular Multilevel Converter Operating in Low-Frequency AC Transmission

The Modular Multilevel Converter (MMC) has been widely used in high-power applications owing to its inherent advantages, including scalability, modularity, high-power density, and fault tolerance. MMCs have recently been used in Low-Frequency Alternating Current (LFAC) transmission, particularly in the integration of offshore wind power with onshore grids. However, LFAC applications produce significant voltage oscillations in floating capacitor voltages within the MMC. Early research efforts have successfully established and validated decoupled control strategies for LFAC-based MMC systems. However, validations are usually based on simulations or small-scale prototypes equipped with limited power cells. Consequently, this paper presents a decentralized voltage control strategy based on Nearest Level Control for an MMC-based LFAC system. Experimental results obtained with a 120-cell MMC prototype are presented to validate the effectiveness and operation of the MMC in LFAC applications.

Assessing the Discriminatory Capabilities of iEK Devices under DC and DC-Biased AC Stimulation Potentials.

There is a rising need for rapid and reliable analytical methods for separating microorganisms in clinical and biomedical applications. Microscale-insulator-based electrokinetic (iEK) systems have proven to be robust platforms for assessing a wide variety of microorganisms. Traditionally, iEK systems are usually stimulated with direct-current (DC) potentials. This work presents a comparison between using DC potentials and using DC-biased alternating-current (AC) potentials in iEK systems for the separation of microorganisms. The present study, which includes mathematical modeling and experimentation, compares the separation of bacterial and yeast cells in two distinct modes by using DC and DC-biased AC potentials. The quality of both separations, assessed in terms of separation resolution (Rs), showed a complete separation (Rs = 1.51) with the application of a DC-biased low-frequency AC signal but an incomplete separation (Rs = 0.55) with the application of an RMS-equivalent DC signal. Good reproducibility between experimental repetitions (<10%) was obtained, and good agreement (~18% deviation) was observed between modeling and experimental retention times. The present study demonstrates the potential of extending the limits of iEK systems by employing DC-biased AC potentials to perform discriminatory separations of microorganisms that are difficult to separate with the application of DC potentials.

Portable Electrochemical Impedance Spectroscopy Device Implementation for Alzheimer's Disease-Related Protein

Electrochemical impedance spectroscopy (EIS) is an electrochemical technique to measure the impedance varieties of a system based on the alternating current (AC) potential frequency. This technique is high accuracy and frequently used to distinguish the influences between different components. The charge transfer resistance and electric double layer phenomenon commonly occur in these components in an electrochemical system. A slight sinusoidal excitation with multiple frequency has been used to process the phenomenon to impedance values. These values can be simulated as an equivalent circuit to analyze the resistance, amplitude, and phase angle in the actual electrochemical cell [1].Alzheimer's disease (AD) is the most common form of amnesia. However, AD symptoms are difficult to be conscious of in the early stage, and current screening methods are hard to popularize. Current research generally agrees that AD has a considerable association with beta-amyloid (Aβ) precipitation [2]. Aβ is the proteolytic fragment derived from amyloid precursor proteins (APP). These proteins can be used to screen the pathogenesis of AD. Biological detection chips capture these proteins and then use commercial instruments for detection. The biological detection chips are fabricated using a screen-print-electrode (SPE) chip which contains three gold electrodes, and the fabrication steps show in Figure 1(A). 11-Mercaptoundecaoic acid (11-MUA) was used as a self-assembled monolayer (SAM) to functionalize the biological chip. 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC)/N-hydroxysuccinimide (NHS) have been used to activate the functional group on SAM to bond the antibody for specific detection of Aβ. Finally, block the residual activated groups with ethanolamine. After modifying the sensors, the protein was applied for measuring immediately.This research aims to popularize a portable device that can achieve rapid and accurate monitoring of early AD. In this study, an AD5933 chip was utilized as the main chip for a portable EIS device to compare with the commercial instrument CHI920D. This integrated circuit chip can be applied as an impedance meter. The AD5933 is embedded on a board that contains an analog front-end (AFE) circuit. This AFE circuit can bias the direct current (DC) output from 1.48 V to 0 V and AC output from 1.98V p-p to 0.1V p-p. Also, we applied a 25 kHz clock signal for AD5933 to sweep frequency in 10 Hz to 20 Hz with 1 Hz resolution. This portable device is controlled by NI Myrio1900, and NI LabVIEW system with a laptop, as shown in Figure 1(B).To analyze the signal gathered from the Aβ, a simulated actual electrochemical cell with an equivalent current is needed. The test frequency has been fixed between 10 to 20 Hz due to the limitation of AD5933. The original Randles circuit is the frequently used circuit applied to impedance calculation [1]. To optimize the current there are some conditions that can be ignored. For instance, the resistance of redox solution (RΩ), which the resistance is too small compared with the whole electrochemical cell. The Warburg element (Zw) gives a low impact on the frequency interval. Thus, the circuit can be optimized and simplified as a simple parallel resistor-capacitor (RC) circuit. To evaluate system performance, the same biological detection chips were used for both portable devices (Figure 1(B)) and CHI920D (Figure 1(C)). Both calculation method is sample charge transfer resistance (Rct) divided by blocking charge transfer resistance (Rct). The correlation coefficient R2 of these sensors is 0.97 (Figure 1(D)), which means the portable system result is identical to the CHI920D.In conclusion, our research accomplished the system with low frequency and low AC and DC measuring, indicating a higher sensitivity for early AD detection. The limitation of this research is the biological detection chips. These modified chips are time-sensitivities that lead to conservation becoming difficult. The experimental results suggest that this portable is a potential candidate for wide screening of AD, and arranging the portable device in pairs might accomplish the point of care of AD. Furthermore, this device can also develop with different biomarkers, depending on additional antibody modification. Bard, A.J., et al., Fundamentals and applications. Electrochemical Methods, 2001. 2(482): p. 580-632.Hardy, J.A., et al., Alzheimer's disease: the amyloid cascade hypothesis. Science, 1992. 256(5054): p. 184-186. Figure 1

Holographic superfluid with gauge–axion coupling

We have constructed a holographic superfluid with gauge–axion coupling. Depending on whether the coupling is positive or negative, the system displays metallic or insulating behavior in its normal state. A significant feature of the system is the appearance of a mid-IR peak in the alternating current (AC) conductivity in a certain range of parameters. This peak arises due to competition between explicit symmetry breaking (ESB) and spontaneous symmetry breaking (SSB), which results in the presence of a pseudo-Goldstone mode. Moreover, a dip in low-frequency AC conductivity is observed, stemming from the excitation of the SSB Goldstone mode. In the superfluid phase, the effect of gauge–axion coupling on the condensation or superfluid energy gap is only amplified in the presence of strong momentum dissipation. Notably, for the case with negative gauge–axion coupling, a hard-gap-like behavior at low frequency and a pronounced peak at intermediate frequency are observed, indicating that the evolution of the superfluid component is distinct from that of positive coupling.

Contact Resistance and Current Characteristics of NI HTS Coils in Low Frequency AC Method

The development of high-performance HTS magnets requires a method to correctly measure the contact resistance between turns of no-insulation (NI) coils. We have proposed a method to measure the turn-to-turn contact resistance of NI coils using low frequency alternating current (LFAC). In the LFAC method, AC current is used, and it is assumed that all the AC current flows in the radial direction of the NI HTS coil. To confirm the reliability of the proposed measurement method, AC current distribution characteristics were evaluated using four (quarter circle, 1/4 size) pickup coils in a previous study. The pickup coils were mounted under the NI test coils and the current bypass characteristics from circumferential to radial direction were evaluated as a function of frequency. However, the size of the fabricated four pickup coils was relatively large compared to the test coil, so its sensitivity and spatial resolution were not sufficient. Therefore, in this study, we prepared eight pickup coils smaller than the 1/4 size pickup coil used in the previous study. Two test coils with winding tensions of 0.5 and 1 kg were prepared, and the non-uniformity of the contact resistance of both NI coils was detected using eight pickup coils. We also identified locations in the test coil where the contact resistance changed due to multiple cooling cycles by liquid nitrogen.

Fine-Tuning the Characteristic of the Applied Potential To Improve AC-iEK Separations of Microparticles.

There is an immediate need for the development of rapid and reliable methods for microparticle and cell assessments, and electrokinetic (EK) phenomena can be exploited to meet that need in a low cost and label-free fashion. The present study combines modeling and experimentation to separate a binary mixture of microparticles of the same size (5.1 μm), shape (spherical), and substrate material (polystyrene), but with a difference in particle zeta potentials of only ∼14 mV, by applying direct current (DC)-biased low-frequency alternating current (AC) voltages in an insulator-based-EK (iEK) system. Four distinct separations were carried out to systematically study the effect of fine-tuning each of the three main characteristics of the applied voltage: frequency, amplitude, and DC bias. The results indicate that fine-tuning each parameter improved the separation from an initial separation resolution Rs = 0.5 to a final resolution Rs = 3.1 of the fully fine-tuned separation. The separation method exhibited fair reproducibility in retention time with variations ranging from 6 to 26% between experimental repetitions. The present study demonstrates the potential to extend the limits of iEK systems coupled with carefully fine-tuned DC-biased low-frequency AC voltages to perform discriminatory micron-sized particle separations.

In vivo peripheral nerve activation using sinusoidal low-frequency alternating currents.

The sinusoidal low-frequency alternating current (LFAC) waveform was explored recently as a novel means to evoke nerve conduction block. In the present work, we explored whether increasing the amplitude of the LFAC waveform results in nerve fiber activation in autonomic nerves. In-silico methods and preliminary work in somatic nerves indicated a potential frequency dependency on the threshold of activation. The Hering-Breuer (HB) reflex was used as a biomarker to detect cervical vagus nerve activation. Experiments were conducted in isoflurane-anesthetized swine (n=5). Two stimulating bipolar cuff electrodes and a tripolar recording cuff electrode were implanted on the left vagus nerve. To ensure the electrical stimulation affects only the afferent pathways, the nerve was crushed caudal to the electrodes to eliminate cardiac effects. (1) Standard pulse stimulation (Vstim) using a monophasic train of pulses was applied through the caudal electrode to elicit HB reflex and to identify the activated nerve fiber type. (2) Continuous sinusoidal LFAC waveform with a frequency ranging from 5 through 20 Hz was applied to the rostral electrode without Vstim to explore the activation thresholds at each LFAC frequency. In both cases, the activation of nerve fibers was detected by a HB reflex-induced reduction in the breathing rate. LFAC was found to be capable of eliciting an HB response. The LFAC activation thresholds were found to be frequency-dependent. The HB threshold was 1.02 ± 0.3 mAp at 5Hz, 0.66 ± 0.3 mAp at 10Hz, and 0.44 ± 0.2 mAp at 20 Hz. In comparison, it was 0.7 ± 0.47 mA for a 100 μs pulse. The LFAC amplitude was within the linear limits of the electrode interface. Damage to the cuff electrodes or the nerve tissues was not observed. Analysis of Vstim-based compound nerve action potentials (CNAP) indicated that the decrease in breathing rate was found to be correlated with the activation of slower components of the CNAP suggesting that LFAC reached and elicited responses from these slower fibers associated with afferents projecting to the HB response. These results suggest the feasibility of the LFAC waveform at 5, 10, and 20 Hz to activate autonomic nerve fibers and potentially provide a new modality to the neurorehabilitation field.

Digital Microfluidics-Powered Real-Time Monitoring of Isothermal DNA Amplification of Cancer Biomarker.

We introduce a digital microfluidics (DMF) platform specifically designed to perform a loop-mediated isothermal amplification (LAMP) of DNA and applied it to a real-time amplification to monitor a cancer biomarker, c-Myc (associated to 40% of all human tumors), using fluorescence microscopy. We demonstrate the full manipulation of the sample and reagents on the DMF platform, resulting in the successful amplification of 90 pg of the target DNA (0.5 ng/µL) in less than one hour. Furthermore, we test the efficiency of an innovative mixing strategy in DMF by employing two mixing methodologies onto the DMF droplets—low frequency AC (alternating current) actuation as well as back-and-forth droplet motion—which allows for improved fluorescence readouts. Fluorophore bleaching effects are minimized through on-chip sample partitioning by DMF processes and sequential droplet irradiation. Finally, LAMP reactions require only 2 µL volume droplets, which represents a 10-fold volume reduction in comparison to benchtop LAMP.

In-vivo application of low frequency alternating currents on porcine cervical vagus nerve evokes reversible nerve conduction block

BackgroundThis paper describes a method to reversibly block nerve conduction through direct application of a 1 Hz sinusoidal current waveform delivered through a bipolar nerve cuff electrode. This low frequency alternating current (LFAC) waveform was previously shown to reversibly block the effects of vagal pulse stimulation evoked bradycardia in-vivo in the anaesthetised rat model (Mintch et al. 2019). The present work measured the effectiveness of LFAC block on larger caliber myelinated vagal afferent fibers in human sized nerve bundles projecting to changes in breathing rate mediated by the Hering-Breuer (HB) reflex in anaesthetized domestic swine (n=5).MethodsTwo bipolar cuff electrodes were implanted unilaterally to the left cervical vagus nerve, which was crushed caudal to the electrodes to eliminate cardiac effects. A tripolar recording cuff electrode was placed rostral to the bipolar stimulating electrodes on the same nerve to measure changes in the compound nerve action potentials (CNAP) elicited by the vagal pulse stimulation and conditioned by the LFAC waveform. Standard pulse stimulation was applied at a sufficient level to induce a reduction in breathing rate through the HB reflex. If unblocked, the HB reflex would cause breathing to slow down and potentially halt completely. Block was quantified by the ability of LFAC to reduce the effect of the HB reflex by monitoring the respiration rate during LFAC alone, LFAC and vagal stimulation, and vagal stimulation alone.ResultsLFAC achieved 87.2 ±8.8% block (n=5) at current levels of 1.1 ±0.3 mAp (current to peak), which was well within the water window of the working electrode. CNAP showed changes that directly correlated to the effectiveness of LFAC block, which manifested itself as the slowing and amplitude reduction of components of the CNAP.ConclusionThese novel findings suggest that LFAC is a potential alternative or complementary method to other electrical blocking techniques in clinical applications.

Accuracy Improvement of Stator Inductance Identification Method Based on Low-Frequency Current Injection for Three-Level NPC Inverter-Fed IM Drives in Locked-Rotor Standstill Condition

In this paper, a stator inductance identification process is proposed. The process is based on a three-level neutral-point-clamped (NPC) inverter-fed induction motor (IM) drive with a standstill condition. Previously, a low-speed alternating current (AC) injection test for stator inductance identification was proposed to overcome practical problems in conventional identification methods for three-level NPC inverter-based IM drives. However, the low-speed AC injection test-based identification method has some problems if a heavy load or mechanical brake is connected, as these can forcibly bring the rotor to a standstill during parameter identification. Since this low-speed testing-based identification assumes the motor torque is considerably lower in low-speed operations, some inaccuracy is inevitable in this kind of standstill condition. In this paper, the proposed current injection speed generator is based on the previously studied low-speed test-based stator inductance identification method, but the proposed approach gives more accurate estimates under the aforementioned standstill conditions. The proposed method regulates the speed for sinusoidal low-frequency AC injection on the basis of the instantaneous reactive and air-gap active power ratio. This proposed stator inductance identification method is more accurate than conventional fixed low-frequency AC signal injection identification method for three-level NPC inverter-fed IM drive systems with a locked-rotor standstill condition. The proposed method’s accuracy and reliability were verified by simulation and experiment using an 18.5 kW induction motor.

Practical application of non-contact alternating current electric field mixing for reagent-saving in situ hybridisation of HER2

AimsHuman epidermal growth factor receptor 2 (HER2)-targeted agents are effective against HER2-positive breast cancers. However, their lack of survival benefit in HER2-negative patients as well as their toxic effects and...

Application of LFAC {16 2/3Hz} for electrical power transmission system: a comparative simulation study

Transmission of electrical power for long distance is one of the major challenges for electrical engineers over the years. The first and foremost target it is to ensure the transport of electric power with minimal losses over the distance. This paper proposes to employ the low frequency alternating current (LFAC) with (16 2/3Hz) instead of the traditional frequency (50Hz) for electrical power transmission based on a simulation study. Two transmission systems have been constructed using MATLAB/Simulink software namely: 50Hz transmission systems and (16 2/3Hz) transmission systems. In addition, three different distances of transport lines have been used to compare the efficiency of the two systems namely: short (50km), medium (150) and long (300km). The result demonstrated that the use of LFAC with (16 2/3Hz) is better than the traditional frequency (50Hz) in term of energy saving.

Topologies and control of low‐frequency alternating current for offshore wind farms based on modular multilevel matrix converter

As a new alternative to power transmission, low-frequency alternating current (LFAC) transmission system has a bright future for offshore wind power transmission. As a core equipment for LFAC system, modularised multi-level matrix converter (M3C) is a direct electric energy conversion device, which can achieve bi-directional power flow and provide any value of the power factor. In this study, the application of LFAC transmission system based on M3C (M3C-LFAC) for offshore wind farms is studied. For the characteristics of LFAC transmission system for offshore wind power, four types of LFAC offshore wind power grid-connected topological structure are proposed, and related control strategies are designed. By use of PSCAD/EMTDC to verify the feasibility of the proposed schemes, the advantages of the proposed topologies are verified through different working conditions.

Effect of low-frequency AC magnetic susceptibility of Ru/Co60Fe20V20 and Ta/Co60Fe20V20 films

In this study, the maximum alternating current (AC) magnetic susceptibility (χac) and optimal resonance frequency (fres) of (a) glass/Ru (X nm)/Co60Fe20V20(5 nm) and (b) glass/Ta(Y nm)/Co60Fe20V20(5 nm) films at room temperature (RT) were investigated through low-frequency AC magnetic susceptibility (χac) measurement, where X and Y each ranged from 5 to 10 nm. The variation of the highest χac at the optimal resonance frequency (fres), at which the spin sensitivity is maximal, was examined using an experiment. The results indicated that χac decreased as the frequency was increased. The maximum χac of glass/Ru(9 nm)/Co60Fe20V20(5 nm) and glass/Ta(8 nm)/Co60Fe20V20(5 nm) was 20.6 × 10−3 and 15.5 × 10−3 at fres 100 Hz and 1000 Hz, respectively. The maximum χac of glass/Ru(9 nm)/Co60Fe20V20(5 nm) was 1.3 times larger than that of glass/Ta(8 nm)/Co60Fe20V20(5 nm). This indicated that the effect of the Ru seed layer is apparently suitable for low-frequency magnetic recording applications.

Single-Walled Carbon Nanotubes Probed with Insulator-Based Dielectrophoresis.

Single-walled carbon nanotubes (SWNTs) offer unique electrical and optical properties. Common synthesis processes yield SWNTs with large length polydispersity (several tens of nanometers up to centimeters) and heterogeneous electrical and optical properties. Applications often require suitable selection and purification. Dielectrophoresis is one manipulation method for separating SWNTs based on dielectric properties and geometry. Here, we present a study of surfactant and single-stranded DNA-wrapped SWNTs suspended in aqueous solutions manipulated by insulator-based dielectrophoresis (iDEP). This method allows us to manipulate SWNTs with the help of arrays of insulating posts in a microfluidic device around which electric field gradients are created by the application of an electric potential to the extremities of the device. Semiconducting SWNTs were imaged during dielectrophoretic manipulation with fluorescence microscopy making use of their fluorescence emission in the near IR. We demonstrate SWNT trapping at low-frequency alternating current (AC) electric fields with applied potentials not exceeding 1000 V. Interestingly, suspended SWNTs showed both positive and negative dielectrophoresis, which we attribute to their ζ potential and the suspension properties. Such behavior agrees with common theoretical models for nanoparticle dielectrophoresis. We further show that the measured ζ potentials and suspension properties are in excellent agreement with a numerical model predicting the trapping locations in the iDEP device. This study is fundamental for the future application of low-frequency AC iDEP for technological applications of SWNTs.

Optimizing an integrated waveguide modulator for sensitive low-frequency alternating-current electric-field sensors

The use of integrated waveguide modulators is a common technique in electric-field sensing. However, the distortion in the modulated signal caused by high half-voltage Vπ and the difficulty in obtaining low-frequency responses are challenging issues for the use of low-frequency alternating-current (AC) electric-field sensors. This study investigates the use of an optimized segmented slot waveguide as the core of a sensor to determine the sensor features that produce useful frequency responses and sensitivity. The segmented slot waveguide is optimized in terms of periodicity and segment width to produce low Vπ and electrical bandwidth before testing the sensor sensitivity. The results show that reducing the segment width achieves a low Vπ of 0.32 V and a very low electrical bandwidth of 4.3 kHz. Our study provides evidence of the feasibility of using a segmented slot waveguide as the primary element for highly sensitive, low-frequency AC electric-field sensors.

Continuous Separation of DNA Molecules by Size Using Insulator-Based Dielectrophoresis.

Separation of nucleic acids has long served as a central goal of analytical research. Innovations in this field may soon enable the development of rapid, on-site sequencing devices that significantly improve both the availability and accuracy of detailed bioinformatics. However, achieving efficient continuous-flow operation and size-based fractionation of DNA still presents considerable challenges. Current methods have not yet satisfied the need for rapid fractionation of size-heterogeneous nucleic acid samples into specific and narrow size distributions. Dielectrophoretic (DEP) mechanisms integrated in microfluidic devices offer unique advantages for such applications, including short processing times, microscale reaction volumes, and the potential for low cost and portability. To facilitate such developments, we have adapted a microfluidic constriction sorter device to separate a wide range of nucleic acid analytes into distinct microchannel outlets. This work demonstrates selective and tunable deflection of DNA using alternating current (AC) insulator-based dielectrophoresis. We report conditions for size-based DEP sorting of 1.0, 10.2, 19.5, and 48.5 kbp dsDNA analytes, including both plasmid and genomic DNA. Applied potentials range from 200 to 2400 Vpp with frequencies ranging from 50 Hz to 20 kHz. These conditions result in sorting efficiencies up to 92% with a strong dependence on applied potentials and frequencies. In low-frequency AC fields, long DNA molecules form macro-ion clusters. This behavior is associated with an apparent shift from positive to negative DEP sorting behavior. Using a finite element model, we characterize the dynamics of sorting in the microdevice and link differential sorting to differences in dielectrophoretic mobility. We propose the use of a continuous-flow sorting strategy to facilitate future coupling to next generation sequencing approaches.

Experimental study of lean premixed CH4/N2/O2 flames under low-frequency alternating-current electric fields

In order to study the effect of the frequency and the intensity of low-frequency alternating-current (AC) electric fields on lean combustion, the effects of 100/80/60/40Hz AC electric fields on premixed CH4/N2/O2 flames with excess air ratio of 1.2/1.4/1.6 have been investigated in a constant volume combustion bomb. The results show that the frequency and the applied voltage of low-frequency fields have significant effects on lean combustion. Firstly, the mean flame propagation speed decreases with the electric field frequency, and meanwhile, the combustion peak pressure decreases and the peak time increases. Secondly, both the mean flame propagation speed and the combustion peak pressure increase linearly with the applied voltage, while the peak time decreases. What’s more, the developing degree of the ionic wind effect decreases with the electric field frequency and increases with the applied voltage, which can be used to explain the experimental results. The low-frequency AC electric fields can enhance the lean combustion effectively, which has a positive meaning in the further study of the electric field assisted combustion theory.

Stress Corrosion Cracking of X80 Pipeline Steel Under Various Alternating Current Frequencies in High-pH Carbonate/Bicarbonate Solution

The effect of various alternating current (AC) frequencies on stress corrosion cracking (SCC) behavior and mechanism of X80 pipeline steel was investigated in high-pH carbonate/bicarbonate solution by polarization curves and slow strain rate tensile tests. With decreasing AC frequency, the passivity of the steel degrades and the SCC susceptibility increases, especially at the low-frequency AC of 30 Hz. With or without various AC frequencies, the difference in the SCC behavior and mechanism of the steel in the solution is remarkable. In the absence of AC superimposition, the crack is intergranular and the SCC mechanism is anodic dissolution. In contrast, under various AC frequencies, the fracture mode is transgranular, and the mechanism is controlled by anodic dissolution and hydrogen embrittlement.

Analysis of alternating current driven electroluminescence in organic light emitting diodes: A comparative study

The alternating current (AC) responses of double-injection and double-insulated organic light-emitting diodes (OLEDs) were investigated and compared. To reveal the electroluminescent (EL) processes in these devices, the AC voltage and frequency dependence of the EL intensity and capacitive current were studied in the time domain with a focus on phase difference analysis. It was found that the voltage-dependent transit time and frequency-dependent carrier distribution were important for the AC-driven performance of the double-injection OLEDs. In contrast, although the double-insulated OLEDs shared some similarities with the double-injection OLEDs, they had some unique characteristics, which were the absence of resistive current and phase shift of EL profiles. It was revealed that the EL in the double-insulated OLEDs was driven by the displacement current generated by the ionization of the doped layers, which, however, formed space charge regions and undermined the EL emission. The space charge redistributed the electric field across the devices after the initiation of EL, making the EL maintain for a limited time interval. This effect was significant under low frequency and high AC voltage. Comparing the phase difference between both devices, it was indicated that the space charge effect was responsible for the observed EL phase shift and the asymmetric EL profiles at low frequency and high AC voltage in the double-insulated OLEDs. The proposed model was also of help to understand the EL saturation phenomena with AC frequency and voltage in those devices.