Voltage Oscillations Research Articles

Dynamic Modeling Framework for Evaluating Electromagnetic-Electro-Thermal Behavior of Power Conversion System During Load Operation

Abstract This study introduces a modified dynamic multiphysics modeling framework to characterize the electromagnetic-electrothermal (EET) coupled behavior of a power conversion system during a long load operation. The modeling framework extends the prior model with more comprehensive analysis and enhanced computational efficiency and modeling simplicity. This framework incorporates a fully integrated electromagnetic circuit (FIEC) model for extracting parasitics, including self and mutual inductances and also exploring their effect on the switching characteristics and power losses, and a dynamic power loss-temperature thermal (PTT) model for describing the temperature-dependent instantaneous electrical behavior and power loss. Moreover, a simple resistance-capacitance (RC) snubber circuit design is applied to prevent overvoltage and diminish voltage oscillations and spike value during the operation, and their power losses are also assessed and considered in the dynamic EET coupled modeling. Furthermore, the proposed PTT model employs an equivalent thermal RC network to calculate the chip junction temperature with a given power. Additionally, a simple power-temperature relationship derived from the FIEC cosimulation is applied for modeling simplicity and computational efficiency. This framework is tested on a three-phase inverter operating with a 180-deg conduction mode. The proposed FIEC cosimulation and computational fluid dynamics thermal models are validated by double pulse (DPT) and infrared thermography experiments, respectively. Moreover, the PTT model is validated compared with the conventional dynamic coupled electrothermal model. Finally, a design guideline for enhanced thermal performance of the tested power conversion system is sought through parametric analysis.

Fuzzy control based virtual synchronous generator for self-adaptative control in hybrid microgrid

Maintaining the stability of low-inertia microgrid becomes a key challenge in the presence of high penetration of renewable energy sources. However, in such systems, the virtual inertia values are often fixed constants, and the choice of their values will significantly affect the frequency and voltage stability of the microgrid. Higher frequency and voltage oscillations may occur due to improper selection of fixed virtual inertia values. Therefore, virtual inertia-based control has attracted a lot of attention. In this paper, an adaptive virtual inertia control system using a fuzzy system is proposed by setting fuzzy logic rules and affiliation functions to provide adaptive inertia control for the system to ensure the frequency and voltage stability. In the proposed adaptive control strategy, the virtual inertia values are automatically adjusted according to the signal deviation and rate of change of the actual system, avoiding the selection of inappropriate inertia values and providing fast inertial response. Simulation and experimental results show that the proposed adaptive control algorithm, by combining the advantages of large inertia and small inertia, enables effective improvement of the dynamic response of the system voltage and frequency in both rectifier and inverter modes. The effectiveness of the proposed control strategy is verified.

Photovoltaic System for Microinverter Applications Based on a Non-Electrolytic-Capacitor Boost Converter and a Sliding-Mode Controller

This paper presents a photovoltaic (PV) system designed to reduce the DC-link capacitance present in double-stage PV microinverters without increasing the capacitor interfacing the PV source. This solution is based on a modified boost topology, which exhibits continuous current in both input and output ports. Such a characteristic enables the implementation of PV microinverters without electrolytic capacitors, which improves the reliability in comparison with solutions based on classical converters with discontinuous output current and electrolytic capacitors. However, the modified boost converter exhibits different dynamic behavior in comparison with the classical boost converter; thus, design processes and controllers developed for the classical boost converter are not applicable. This paper also introduces a sliding-mode controller designed to ensure the stable operation of the PV microinverter around the maximum power point. Moreover, this solution also rejects the voltage oscillations at double the grid frequency generated by the grid-connection. The global stability of the complete PV system is formally demonstrated using mathematical analyses, and a step-by-step design process for both the power stage and control system is proposed. Finally, the design process is illustrated using a representative application example, and the correct operation of the PV system is validated using realistic circuital simulations. The results validate the accuracy of the theoretical equations proposed for both the design and control of the novel PV system, where errors below 4.5% were obtained for the ripple prediction, and below 1% for the prediction of the dynamic behavior.

An inertia‐emulation‐based cooperative control strategy and parameters design for multi‐parallel energy storage system in islanded DC microgrids

This paper proposes an inertia-emulation-based cooperative control strategy for the multi-parallel energy storage system (ESS) to meet the requirements of state-of-charge (SoC) balance, inertia enhancement and zero-steady-state voltage deviation. The inertia emulation loop (IEL) is constructed by analogy with DC motors to dampen voltage oscillation, while the secondary voltage recovery loop is derived from the circuit equivalence of an inductor to indicate the system stiffness. Moreover, to equalize SoCs of energy storage units (ESUs) dynamically, a SoC self-balance algorithm is developed. The redefined SoC mismatch degree and balance speed adjustment factor k are introduced into the droop resistance, adjusting the SoC self-balance rate and eliminating the SoC deviation among ESUs. The dynamic performance of the SoC self-balance algorithm is analyzed and the small signal model of the DC microgrid (DC-MG) with proposed strategy is established. Based on eigenvalue analysis and step response, the system stability is assessed, and the influence of control parameters on transient characteristics and stability margin is investigated. Considering power constraint, voltage deviation constraint and dynamic stability constraint, the optimal design method of k is given. Finally, simulation and experiment verify that the proposed control, without modifying hardware, performs better dynamic and static characteristics and can equalize SoC among ESUs in charge and discharge mode.

A Double-Side Feedback Pulse Train Control for the Output Voltage Regulation of Two-Stage Wireless Power Transfer System

Pulse Train (PT) control is a nonlinear voltage regulation method with favorable characteristics of quick response time and simple structure. In this paper, a PT-based feedback control strategy is proposed for stabilizing the output voltage of two-stage wireless power transfer (WPT) systems. Low-frequency voltage oscillations can be observed in PT controlled front-stage power converters, which significantly degrades the output power quality of the WPT system. To solve this problem, by using feedback variables sampled from both output sides of a power converter and WPT, a capacitor current and output voltage feedback PT (CC&OV-PT) controlled two-stage WPT system is further devised in this paper. A discrete time model of the two-stage WPT system is established, and the performance of low-frequency voltage oscillations suppression for the double-side feedback CC&OV-PT controlled WPT system is analyzed. Simulation and experimental verification have been conducted, both of which show that the CC&OV-PT controlled WPT system achieves fast response with effective low-frequency oscillation suppression.

Numerical study of various scenarios for the formation of atmospheric pressure DC discharge characteristics in argon: from glow to arc discharge

In this work, on the basis of a unified model from the point of view of describing the gas discharge gap and electrodes for 2D geometry, numerical calculations were carried out to study various modes of direct current discharges in argon at atmospheric pressure. The influence of the cooling conditions of the electrodes on the current–voltage characteristic of the discharge is shown: the transition from normal glow to arc discharge with the formation of an abnormal glow mode and without it. It is shown that, depending on the cooling conditions of the electrodes, two forms of arc discharge can be obtained: with a diffuse or contracted current spot. In the low-current mode, current and voltage oscillations were obtained during the transition from the Townsend to the normal glow discharge.

Improved crowbar protection technique for DFIG using fuzzy logic

<span lang="EN-US">A doubly-fed induction generator is the most widely used as a wind turbine generator. Due to its drawbacks, doubly fed induction generator (DFIG) is extremely sensitive to grid disturbances, and the fragility of some components which are costly to the producer. Also, its acquisition value is very high in terms of maintenance time or component cost, causing substantial harm to both the energy production and power supplier. It is required that the DFIG components must be protected, especially power electronics devices and DC-Link capacitor. Therefore, this paper presents an improved crowbar strategy for DFIG. This method is based on the AI technique concept of utilizing a fuzzy logic controller. The main goal of this project is to improve the system performance by reducing the dangerous oscillations of electromagnetic torque, DC-link voltage, and rotor current during fault. This work consists of replacing the hysteresis control for the crowbar with fuzzy logic to realize crowbar-FLC. The proposed crowbar is based on free light chain (FLC) depending on rotor currents and DC-link voltage measurements. The control strategy is simulated in the MATLAB Simulink platform to evaluate the efficiency of the suggested technique.</span>

Investigation of Subsonic to Supersonic Transition of a Low-Pressure Plasma Torch Jet

The plasma jet emanating out of a thermal plasma torch operating at low pressure ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 30$ </tex-math></inline-formula> mbar) has been investigated experimentally. Different anode exit diameters are chosen, and the torch is operated at various gas flows and discharge current and their thermodynamic outputs have been calculated. Current–voltage characteristics (CVC) of plasma torch with different anode geometries are explored highlighting their differences. Interesting correlations are found between the anode exit diameter and the threshold limit for subsonic to supersonic transition of the jet. The spectral analysis of the voltage fluctuations reveals the presence of different frequencies and also yields the threshold. The kinetic energy (KE) component of the plasma jet has a direct correlation with the oscillations in the arc voltage. In addition to restrike mode and Helmholtz mode, an acoustic mode of oscillations is also investigated. A new resonator model explaining the different oscillations occurring in the low-pressure operation is also presented. The Helmholtz mode oscillations are validated by comparing the experimental values with the theoretical calculations. The power associated with the Helmholtz mode and acoustic mode has direct correlation with the plasma velocity and acts as a signature of the transition from subsonic to supersonic regime.

Sequential Model Predictive Fault-Tolerance Control for T-Type Three-Level Grid-Connected Converters With LCL Filters

Recently, fault-tolerance control (FTC) methods have been proposed to further improve the reliability of T-type three-level converters. However, the application of model predictive control in FTCs is rare, and the neutral point (NP) voltage imbalance before an open-switch fault occurs has not been solved. This paper focuses on these two problems and proposes two FTC control methods: sequential model predictive control (SMPC) and sequential model predictive tolerance control (SMPTC). Both methods are implemented by designing a sequential predictive control that first considers the NP voltage balance and then grid current tracking. Through the staggered operation of the two methods, a T-type grid-connected converter with an LCL filter can operate normally after an open-switch fault occurs. Moreover, both methods can achieve the NP voltage balance and excellent grid current outputs, eliminate the time-consuming selection of weighting factors, and reduce the number of calculation loops compared with the traditional model predictive tolerance control, thereby improving the control flexibility. The performance and comparison of the open-switch FTC with respect to the NP voltage oscillations, total harmonic distortion of the grid current, and pole voltage were demonstrated using numerous simulation and experimental results.

Effect of the Dynamics of the MTDC Power System on DC Voltage Oscillation Stability

Ignoring parts of the dynamics of a multiterminal DC (MTDC) power system can simplify its stability analysis but may miss some instability risks in the analyzed results. This study analyzed the stability of DC voltage oscillation by considering comprehensive dynamics of an MTDC system in a DC voltage time scale to reveal the stability mechanism of the DC voltage oscillation accurately. It demonstrated that a DC voltage oscillation loop is first formed by the DC voltage control of the voltage source converter (VSC). Subsequently, it is affected by the dynamics of the remaining DC system (RDCS) through two paths: (1) the equivalent conductance of the RDCS is equal to an additional damping added to the DC voltage oscillation loop, and (2) the equivalent susceptance forms a mirrored subsystem and results in an open-loop modal resonance, which always induces negative effects on DC voltage oscillation stability. These conclusions are also correct in parts of the MTDC system using DC voltage droop control. Moreover, a method was proposed to quickly determine the stable operating region of the RDCS. Finally, accuracy of the stability mechanism of the DC voltage oscillation and effectiveness of the proposed method are verified based on the SIMULINK platform.

DC-Link Voltage Stability Analysis of Grid-Tied Converters Using DC Impedance Models

With the integration of renewable energy sources into the power grid, a number of power electronic converters need to be connected together in parallel. Due to this interconnection among the power converters with a common DC bus, the equivalent impedance of the DC network, i.e., DC network impedance (DCNI) of these parallel converters, may vary and can cause oscillations in the DC link voltage (DCLV). In the literature, impedance models of grid-tied converters (GCs) based on the AC side are well reported without including these variations in DCNI. In addition, the dynamics of a phase-locked loop (PLL) play a significant role in GC system stability. To evaluate these stability issues, this paper proposes small signal impedance models viewing from the DC side of a three-phase GC operating under different control modes considering the PLL dynamics and the DCNI variations. Using the proposed DC impedance models (DCIM), DCLV stability analysis is evaluated for a GC. It is verified through bode plots that the interaction between the proposed DCIM and DCNI leads to unstable operation of the closed-loop converter near the PLL bandwidth when the phase difference between DCIM and DCNI is more than 180 degrees. Finally, the analytically developed models are validated using hardware in-the-loop (HIL) testing.

Baseline for Split DC Link Design in Three-Phase Three-Level Converters Operating with Unity Power Factor Based on Low-Frequency Partial Voltage Oscillations

The study sets a baseline for split DC link capacitance values and voltage set points in three-phase three-level AC/DC (or DC/AC) converters operating with unity power factor. In order to equalize the average values of partial DC link voltages, the controller generates a zero-sequence containing DC components only while employing neither dedicated DC link capacitance balancing hardware nor high-order zero-sequence component injection. Such a baseline is required in order to evaluate the effectiveness of different DC link capacitance reduction methods proposed in the literature. Unlike most previous works, utilizing neutral point current based on cumbersome analytical expressions to determine neutral point potential oscillations, the instantaneous power balance-based approach is employed in this paper, resulting in greatly simplified and more intuitive expressions. It is demonstrated that while the total DC link voltage is low-frequency ripple-free under unity power factor balanced AC-side operation, split DC link capacitors absorb triple-fundamental frequency power components with one-sixth load power magnitude. This yields significant opposite phase partial voltage ripples. In such a case, selection of DC link capacitances and voltage set points must take into account the expected values of AC-side phase voltage magnitude and split DC link capacitor voltage and current ratings. Simulation and experimental results validate the proposed methodology by application to a 10 kVA T-type converter prototype.

A Novel Image Encryption Algorithm Based on Improved Arnold Transform and Chaotic Pulse-Coupled Neural Network.

Information security has become a focal topic in the information and digital age. How to realize secure transmission and the secure storage of image data is a major research focus of information security. Aiming at this hot topic, in order to improve the security of image data transmission, this paper proposes an image encryption algorithm based on improved Arnold transform and a chaotic pulse-coupled neural network. Firstly, the oscillatory reset voltage is introduced into the uncoupled impulse neural network, which makes the uncoupled impulse neural network exhibit chaotic characteristics. The chaotic sequence is generated by multiple iterations of the chaotic pulse-coupled neural network, and then the image is pre-encrypted by XOR operation with the generated chaotic sequence. Secondly, using the improved Arnold transform, the pre-encrypted image is scrambled to further improve the scrambling degree and encryption effect of the pre-encrypted image so as to obtain the final ciphertext image. Finally, the security analysis and experimental simulation of the encrypted image are carried out. The results of quantitative evaluation show that the proposed algorithm has a better encryption effect than the partial encryption algorithm. The algorithm is highly sensitive to keys and plaintexts, has a large key space, and can effectively resist differential attacks and attacks such as noise and clipping.

Pulsed hydraulic-pressure-responsive self-cleaning membrane.

Pressure-driven membranes is a widely used separation technology in a range of industries, such as water purification, bioprocessing, food processing and chemical production1,2. Despite their numerous advantages, such as modular design and minimal footprint, inevitable membrane fouling is the key challenge in most practical applications3. Fouling limits membrane performance by reducing permeate flux or increasing pressure requirements, which results in higher energetic operation and maintenance costs4-7. Here we report a hydraulic-pressure-responsive membrane (PiezoMem) to transform pressure pulses into electroactive responses for in situ self-cleaning. A transient hydraulic pressure fluctuation across the membrane results in generation of current pulses and rapid voltage oscillations (peak, +5.0/-3.2 V) capable of foulant degradation and repulsion without the need for supplementary chemical cleaning agents, secondary waste disposal or further external stimuli3,8-13. PiezoMem showed broad-spectrum antifouling action towards a range of membrane foulants, including organic molecules, oil droplets, proteins, bacteria and inorganic colloids, through reactive oxygen species (ROS) production and dielectrophoretic repulsion.

Parasitic Parameter Extraction and Identification Method for HFT Based on DC-DC Converter in EV Application

Dual active bridge (DAB) resonant converter is one of the most commonly used soft switched converters of the new electric vehicle (EV) energy system. One of the most essential parts of the DAB resonant converter, the high-frequency transformer (HFT) includes certain parasitic parameters that caused voltage and current oscillation, or ring to the DAB converter. These precise parasitic parameters of the HFT provide accurate information and are useful to the DAB converter control system design. However, they are not provided by the manufacturer. This study focuses on this prospect of information, proposing a novel parasitic parameter identification method for the HFT based on DAB converter. Moreover, a 40 kHz full-bridge resonant DAB converter prototype based on SiC MOSFETs and HFPT laboratory prototype is constructed to verify the effectiveness and feasibility of the proposed method. Simulation and experimental results verify that the proposed method is more cost-effective than the conventional way to measure via impedance (IMDP) analyzer, requires shorter simulation time than the traditional finite element analysis (FEA) method, while it is also more accurate has fast convergence than the traditional forgetting factor recursive least square (FFRLS) identification algorithm.

An AC Fault Ride Through Method for MMC-HVDC System in Offshore Applications Including DC Current-Limiting Inductors

The current-limiting inductors (CLIs) are commonly used to suppress DC fault current to safely trip DC circuit breakers in MMC-HVDC systems, but it significantly jeopardizes DC dynamics and causes instantaneous power mismatch between AC and DC sides of islanded rectifier station (RS), exposing MMC internal states to large disturbance. This paper firstly provides a detailed analysis of the influence of CLIs on AC fault, establishes RS mathematical representation including AC side dynamic, and reveals the mechanism of DC voltage oscillation, modulation saturation and PCC waveform distortion. The analysis shows that the AC and DC fault characteristics are decoupled under unsaturated AC modulation, but coupled under excessively low submodule capacitor voltage, where AC modulation is saturated and PCC waveforms are distorted. To solve this issue, a novel RS AC fault ride through method for MMC-HVDC system with CLIs is proposed to stabilize RS DC voltage, desaturate AC modulation and decouple fault characteristics by regulating the zero-sequence modulation of circulating current suppression. In addition, a current reference disturbance and an AC energy dissipation device coordinated control are designed, which adaptively recovers the submodule capacitor voltage. Finally, the feasibility of the proposed method is verified by simulation results.

A DC Solid-State Circuit Breaker Based on Transient Current Commutation

This article proposes a new dc solid-state circuit breaker (SSCB) based on silicon carbide (SiC) MOSFETs. There are two main contributions. First, a transient current commutation (TCC) is obtained in the main switch during current interruption. To achieve this, a compact and fast active injection circuit (AIC) is employed. TCC brings two main benefits: 1) it eliminates the effects of parasitic inductances in circuit connections and removes voltage oscillations on the gate of the main switch, which prevents spurious turn-on and 2) it eliminates the transient power shock on the switch during turn-off, which extends the lifetime of SSCB and increases its reliability. Second, the voltage on the employed metal oxide varistor (MOV) is reduced to zero during the OFF-state. This extends the maximum allowable dc bus voltage on the SSCB and solves the MOV reliability issue in the SSCBs. The presented SSCB employs a time-sequence control and needs no real-time detections of the dc current, which simplify the design. LTspice simulations of 380 V/80 A and 600 V/80 A are presented, and the results are compared with MOV- and MOV resistor–capacitor–diode (MOV-RCD)-based SSCBs. Also, experiments of a 380 V/80 A prototype validate the effectiveness of the proposed SSCB in practice where the voltage on the SSCB is clamped to 721 V with a fixed response time of 9.4 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> .

Modeling Li-Ion Battery Voltage Oscillations and the Impact of Exchange Current Density on Oscillation Properties

Modeling Li-Ion Battery Voltage Oscillations and the Impact of Exchange Current Density on Oscillation Properties

Performance Enhancement of Doubly Fed Induction Generator–Based Wind Farms With STATCOM in Faulty HVDC Grids

In this study, an investigation of different faults for a wind turbine–based doubly fed induction generator (DFIG) system is studied and the performance using a static compensator (STATCOM) is observed. The DFIG network is connected to a voltage source converter high-voltage dc link with a fault occurring near the wind generator network. The ride through capability of DFIG is promising with STATCOM using the proposed control strategy. The ac and dc voltage and torque oscillations are damped effectively, and improved power flow is observed. The low voltage AC grid fault occurs for an HVDC transmission, and the DFIG performance without and with STATCOM is compared, where the DFIG converter control schemes are developed using the proposed improved field-oriented control (IFOC) method. In this, the reference rotor flux value alters to a new synchronous speed value or a slighter value or a standstill depending on the stator voltage dip due to grid disturbance. This speed variation leads to introducing rotor current at that new rotor slip frequency as there is a change in the rotor speed because of the fault, which further decreases the stator flux dc component. Hence, this dc-offset constituent in the stator flux is alleviated and decays rapidly in scheming the divergence of the speed of the rotor to a new orientation speed with decay in the rotor flux. This operation is done in the inner control scheme of the rotor converter, which is quicker in response to the faults. Apart from this, the stator’s real and reactive power also changes accordingly based on the lookup table mechanism–based closed-loop control action of the pulse generator, and this power change is done in the outer loop. The analysis for DFIG and HVDC operation is verified under different faults without and with STATCOM.

A New Perspective for Relating Virtual Inertia With Wideband Oscillation of Voltage in Low-Inertia DC Microgrid

Virtual synchronous generator (VSG) has been a grid-friendly integration control technique for the integration of grid-connected inverters. However, the emulated inertia and damping of VSG control technique can also be used in the field of dc systems. In this article, a virtual synchronous control is proposed to dampen the wideband oscillation of dc voltage in a dc microgrid. The proposed control strategy contributes to maintaining the synchronous operation of dc converter with the network. Besides, the relationships among damping, inertia, wideband oscillation, rate of change of voltage (RoCoV) as well as DC voltage nadir (DCVN) are studied. It is concluded that the RoCoV and DCVN are similar as the oscillation frequency and fluctuation ranges of poorly-damped oscillation, respectively. A unified concept is proposed by connecting the oscillation-related stability with the inertial transient response originated from the imbalanced powers/mismatched currents. Besides this, the inertia plays the same role as damping because the inertia contributes to maintaining the original state and damping to impeding further change. A new feedback analytical method is proposed to clarify the important role of RoCoV and DCVN on the motion of dc voltage. Finally, the theoretical results are compared with simulations and experiments.