Fundamental Ac Research Articles

A performance assessment of nine-stage ternary DC source mli through various PWM topologies

ABSTRACT In modern days, MLIs comprise a lot of concentrations in academia and industry. As they are altering interest in a feasible technology in supporting frequent applications of renewable energy conversion systems and drives, these huge power and medium/higher voltage applications and MLIs are extensively used as one of the sophisticated powers converter topography. The MLIs are divided into two types such as asymmetric and symmetric. MLIs of the asymmetric kind have more amount of output AC voltage stage with fewer DC input source voltage and switching components. In this research paper, asymmetric single-phase cascade full-bridge MLI is developed with carrier-based unipolar PWM techniques. This MLI can generate nine-stage of output AC voltage through eight switches and two input DC sources. It is tested through a variety of multicarrier unipolar PWM control combinations. The Phase Disposition (PD), Alternating Phase Opposition Disposition (APOD), Carrier Overlapping (COP) and Variable Frequency (VF) are all PWM control which included in support of preference of single-phase nine-stage ternary cascade multilevel inverters. With various modulation indices (ma), the harmonic substance of output AC voltage for each approach is observed and tabulated. The proposed design for generating nine-stage output AC voltage is demonstrated using MATLAB-SIMULINK and confirmed using a laboratory-based prototype model. The results show that the unipolar Alternating Phase Opposition Disposition PWM technique that provides a high-quality output voltage with less harmonic distortion. Besides, the performance of Carrier Overlapping PWM techniques is greater since it produces a superior fundamental RMS output AC voltage.

Multiobjective Vector Modulation for Improved Control of NPC-Based Multi-Source Inverters in Hybrid Traction Systems

The concept of a multi-source inverter (MSI) based on NPC topology was recently introduced for hybrid-electric powertrains to connect several dc sources to the traction load in a single conversion stage without magnetics. This work proposes a new modulation algorithm that integrates the tasks of dc currents and ac voltages control at the level of pulsewidth modulation by means of the space-vector formalism. The proposed algorithm allows the MSI to have full control of the dc input currents and fundamental ac output voltages, and enables a higher number of feasible operating modes, including load power sharing between sources and controlled recharging of one source from the other, both in motion and at standstill. Hence, improved control capability of the load and sources is achieved with respect to the state-of-literature modulation approach. To validate the proposed modulation under different operating conditions, steady-state and dynamic tests are performed on a small-scale traction system fed by a primary dc supply and a battery pack. The results are compared with those of the baseline control approach and show that the proposed technique improves the smoothness and flexibility of the control action and reduces the distortion of currents and voltages while ensuring the same converter efficiency.

Effect of viscous flux flow on magnetization and ac susceptibility in type-II superconductors

In this paper, the effect of viscous flux flow on magnetization and ac susceptibility in type-II superconductors is studied. The magnetization curves and ac susceptibility was further investigated by numerically solving both the Bean model and the viscous flux flow equations. In the unified picture, the fundamental ac susceptibilities of the slab as a function of temperature under different viscous flux flow velocities is derived, and some unique characteristics are reproduced. The results indicate that the critical state model can be well described by the viscous flux flow equation, which has a significant impact on magnetization and ac susceptibility.

Three-Level SOC Equalization Control Strategy for MMC-BESS Based on Feedforward Sliding Window Integral Method

The modular multilevel converter of the battery energy storage system (MMC-BESS) not only is suitable for the large-scale energy storage and dispatching of AC and DC grids, but also has a strong ability to suppress power fluctuations caused by the new energy output or grid failures. When an asymmetric voltage or a sudden change in DC load occurs in the AC grid, in order to compensate for the power difference between AC and DC sides, the energy storage submodule of the MMC-BESS will have a large unbalanced charging and discharging current, destroying the equalization state of SOC and seriously affecting energy storage capacity utilization and battery service life. In order to deal with the above problems, in this paper, the characteristics of the power difference between the MMC-BESS phase unit and the upper and lower bridge arms are analyzed. It is found that when considering the fluctuation of the submodule capacitor voltage, the phase unit power has the fundamental frequency AC circulating current component, and the power difference between the upper and lower bridge arms has the DC circulating current component. Therefore, the three-level SOC equalization correction control strategy is proposed based on interphase, upper and lower bridge arms, and submodules, and the feedforward sliding window integral method is introduced into the SOC equalization correction control layer of upper and lower bridge arms, so as to achieve the purpose of more balanced and accurate power distribution among phases and among upper and lower bridge arms of each phase. The simulation results show that the MMC-BESS has effective compensation ability when there is a large power difference in the AC and DC power grid. Under the unbalanced working conditions of the three-phase power grid, the three-phase AC current can quickly reach equalization, and the total harmonic content is 1.38%, and the unbalance degree is 2.4%. Under the same operating conditions, compared with the traditional optimal one-third average method, the SOC equalization correction control strategy proposed in this paper has smaller submodule capacitor voltage fluctuation rate, harmonic distortion rate, and three-phase system circulating current. And the SOC of each phase has a faster equalization convergence speed.

A High-Frequency MMC for DC–DC Applications Using a Three-Winding Transformer With DC Flux Cancellation

In this article, a novel high-frequency isolated modular multilevel dc–dc converter is proposed for large step ratio, medium voltage dc–dc applications. The proposed converter is based on the recently introduced class of converters termed the current shaping modular multilevel dc–dc converter (CS-MMC). In the CS-MMC, no string inductors are required and only a fraction of the voltage source modules are switched in each fundamental ac period enabling high effective frequencies to be achieved. The proposed converter builds on this earlier work by introducing a new topology featuring galvanic isolation using only a low voltage, high-frequency, three-winding transformer, and imposing dc flux canceling within the transformer. This article describes the converter architecture, its operating principles, and a control design. Experimental results are provided from a 1250V to 145V, 50 kHz laboratory-scale prototype with a peak measured efficiency of 96.8% at 2.8 kW.

Recurrent neural networks model based reliability assessment of power semiconductors in PMSG converter

To obtain accurate lifetime evaluation with acceptable simulation time for fulfilling the total life cycle design criteria, this paper proposes a Recurrent Neural Networks (RNN) based model with the replacement of the Simulink model. It starts with the establishment of the Averaged Switch (AS) model and Averaged Fundamental (AF) model of the Permanent Magnet Synchronous Generators (PMSG) to calculate accumulated damage. Then, under the same mission profile, the junction temperature, rainflow counting and accumulated damage of the AS and AF model are calculated and compared. It can be noted that the AS model is more accurate to calculate the reliability of components, since both the big thermal cycles caused by load variations and the small thermal cycles due to the fundamental AC current are considered. However, it consumes more time compared to the AF model. To this end, the RNN model is proposed to substitute the most time consuming part of the system reliability evaluation procedure. With aid of proposed model, the consumed time can be greatly reduced compared with the Simulink model. In the end, a 1-hour case study is applied to verify the efficiency of the RNN model. The Mean Absolute Percentage Error (MAPE) of the testing case is 0.51% and the time for getting results in the RNN model is less than 1 s. Besides, an annual case is also implemented to verify the RNN model, which has an averaged 0.78% MAPE for the whole year.

Reactive Power Compensation and Power Factor Correction by using Static VAR Compensator (SVC)

In this paper, a reactive power compensation system using static VAR compensator is presented. To confine on system stability and reliability, the reactive power compensation is the fundamental way forflexible AC transmission systems (FACTS). The variations of reactive power have an effect on thegenerating units, lines, circuit breakers, transformers, relays, and isolators. It can also cause effective voltage sags and increase losses. In the proposed system, the lead time between voltage pulse and curren pulse is measured and fed to the interrupt pins of the microcontroller where the program takes over to bring the shunt capacitors to the circuit to get the reactive power compensated. Back-to-back SCRs interfaced through optical isolation from the microcontroller are used in parallel for controlling the capacitor.

Common-Mode Insertion Indices Compensation With Capacitor Voltages Feedforward to Suppress Circulating Current of MMCs

In order to improve the efficiency of modular multilevel converters (MMCs) and reduce the capacitor voltage ripples, the circulating current between the phase-legs should be suppressed. In existing circulating current suppression control (CCSC) methods, multiple resonant regulators or dq-frame integrators are usually employed to reduce the harmonic components. Besides the complexity, it requires additional designs of controller parameters, which relies on accurate frequency and sequence information of the circulating currents. In this paper, a common-mode (CM) insertion indices compensation method is introduced to realize CCSC, which is based on feeding forward the capacitor voltages. This approach not only facilitates the analysis of circulating current mechanism with a deep insight, but also provides a simple approach to achieve a wide bandwidth circulating current suppression without considering the specific harmonic frequencies and sequences. The latter one makes it easy to be used even in the unbalanced, multi-frequency or variable fundamental frequency ac systems. In addition to these advantages, the proposed compensation method can maintain inter-arm capacitor voltages balanced without additional differential energy control loop, which is required in the existing capacitor voltage feedforward scheme. Analytical and extensive simulation results are given for validating the effectiveness of the proposed approach.

High-Frequency Voltage and Current Sense Circuits for Inductive Power Transfer Systems

This article proposes a voltage sense circuit and a current sense circuit for extracting the fundamental ac voltage and current in variable-frequency inductive power transfer systems. Particularly, the proposed circuits are designed for an electric vehicle (EV) wireless charger. A current source is generated via a high resistance and a current transformer in these two sense circuits, respectively. The main part of both sense circuits uses the “double resonance” and “critical damping” principles to gain a stable voltage gain and a linear phase response in the operating frequency band. The ac output voltage is processed by a rectifier for amplitude measurement and by a comparator for phase detection. Meanwhile, the harmonic components are effectively filtered due to the band-pass property of the sense circuits, hence their impact on the ac output voltage is negligible. The operating principle, parameter design method and parameter sensitivity analysis are discussed in detail. The feasibility of the proposed sense circuits is verified by calculations and experiments. (This article is accompanied by two videos demonstrating the performance of the voltage sense circuit.)

Fluorescence Fiber Based Identification of Partial Discharges in Liquid Nitrogen for High-Temperature Superconducting Power Apparatus

Partial discharge (PD) in liquid nitrogen (LN2) insulation is a key issue in cryogenic or high-temperature superconducting power apparatus. In this letter, we experimentally demonstrate PD identification in LN2 using a fluorescent optical fiber sensor and compare its sensitivity to the conventional ultrahigh frequency (UHF) technique. The Corona inception voltage (CIV) in LN2 is studied under fundamental ac and harmonic ac voltages with different values of total harmonic distortion (THD). It is observed that CIV reduces with the increase in harmonic voltage with higher THD. Phase resolved partial discharge pattern, peak-to-peak voltage, and the energy content due to corona discharges measured by both fluorescence-based and UHF techniques follows the same trend, indicating the viability of the new technique.

Students’ understanding of fundamental AC signal concepts

The aim of this study was to determine students’ understanding of fundamental AC signal concepts, such as frequency, amplitude, and phase difference. The participants in the study were 179 students (44 female and 135 male) attending Izmir Vocational School who were enrolled in the Analog Electronics Course. The data of the study were collected by means of three open-ended questions and semi-structured interviews. In the questions, students were given a graph of an AC signal and asked to draw a signal with equal frequency but higher amplitude, a signal with equal amplitude but higher frequency, and a signal with a certain phase difference. The main aim of the interviews was to provide a deeper insight into the difficulties detected in the analysis of the questions. The data analysis showed that the students confused the concepts of frequency and amplitude and had difficulties in interpreting the signals that exhibited a phase difference.

Control and Experimental Validation of the Series Bridge Modular Multilevel Converter for HVDC Applications

The series bridge converter (SBC) is a modular multilevel converter (MMC) recently developed to enhance power density in high-voltage high-power applications. The MMC is a well-established solution, widely researched, and exploited in practical HVdc connections, thanks to its high power quality and high efficiency. However, the main limitation of the MMC is the relatively large energy storage, also due to the fact that power ripples in the submodule capacitors include a component at the fundamental ac frequency. As a result, volume becomes critical in applications such as offshore or city center in-feeds, where space is restricted and expensive. The SBC offers a more compact footprint by exploiting a series connection on the dc side and by operating the submodules with rectified waveforms, thus moving the minimum component of the instantaneous power to twice the ac fundamental and reducing capacitors size. The drawback of the converter is a more complex energy control compared to the MMC. This paper proposes the first experimental validation of the SBC, using a 2-kW laboratory-scale prototype. Since the basic converter design has been discussed in previous papers, the focus of this paper is on converter control design and experimental validation.

Harmonic AC magnetic susceptibility analysis of FeSe crystals with composite morphology

The AC magnetic response of multi-domain FeSe crystals has been explored by means of fundamental and third harmonic AC magnetic susceptibility (ACMS) analysis. Our previous studies have revealed a complex morphology which especially modifies the mixed state properties. The effects were expressed by an additional ‘pseudo’ peak feature of the magnetic hysteresis and in the present study we analyze its vortex dynamics nature and irreversibility behavior in the context of ACMS. We find that the effect is detectable mainly with the sensitive third harmonic component and especially at high AC field frequencies. From the analysis, the temperature dependence of the ‘pseudo’ peak effect at different DC fields ranges was determined, confirming the previously established superconductor–normal metal–superconductor type links between the domains in the crystal. The irreversibility line, the most important parameter for high power applications, shows a typical glass/liquid transition in the vortex matter. In addition we have observed surface barrier effects and vortex avalanche activity, the behavior of which appears to be influenced by the superconducting and magnetic nature coexistence and morphology. The presented results show the effective application of the harmonic AC magnetic susceptibility technique for a versatile analysis of complex nonlinear phenomena in materials with a sophisticated AC magnetic response.

Analysis, design and experimentation with a four-element resonant immittance converter topology as a constant-current power supply

Resonant immitance converters (RICs) belong to a newly identified family of the resonant converters exhibiting unique characteristics with the help of which a voltage source can be converted to a current source and vice versa. Therefore, RICs are suitable for applications that demand a constant-current (CC) power supply. A fourth-order T-type RIC topology, T5, is analysed in this paper. Detailed steady-state analysis using fundamental frequency ac analysis is presented providing a detailed insight into the converter characteristics. Analytical results have been used to derive closed-form expressions for calculating the component values and ratings following a converter optimisation. The correctness of analysis is validated by developing and testing a 500 W, 109 kHz laboratory prototype converter.

Analysis, design and experimentation with a four-element resonant immittance converter topology as a constant-current power supply

Resonant immitance converters (RICs) belong to a newly identified family of the resonant converters exhibiting unique characteristics with the help of which a voltage source can be converted to a current source and vice versa. Therefore, RICs are suitable for applications that demand a constant-current (CC) power supply. A fourth-order T-type RIC topology, T5, is analysed in this paper. Detailed steady-state analysis using fundamental frequency ac analysis is presented providing a detailed insight into the converter characteristics. Analytical results have been used to derive closed-form expressions for calculating the component values and ratings following a converter optimisation. The correctness of analysis is validated by developing and testing a 500 W, 109 kHz laboratory prototype converter.

Enhanced Independent Pole Control of Hybrid MMC-HVdc System

This paper presents an enhanced independent pole control scheme for hybrid modular multilevel converter based on a full-bridge submodule and half-bridge submodule. A detailed analysis of the power distribution between upper and lower arms under asymmetrical dc pole voltages is presented. It is found that the fundamental ac currents in the upper and lower arms are asymmetrical. To enable operation under asymmetrical dc pole voltages, an enhanced independent pole control scheme is proposed. The controller is composed of two dc control loops, two ac control loops, and circulating current suppression control based on current injection. Six modulation indices are presented to independently control the upper and lower arms. With this controller, the dc voltage operating region is significantly extended. To ride through a pole-to-ground dc fault without bringing dc bias at the neutral point of the interface transformer, a pole-to-ground dc fault ride through strategy is proposed. The feasibility and effectiveness of the proposed control scheme are verified by simulation results using PSCAD/EMTDC.

Superparamagnetic nanoparticle detection system by using a fundamental mode orthogonal fluxgate (FM-OFG) gradiometer

A new magnetic nanoparticle detection system by using a fundamental mode orthogonal fluxgate (FM-OFG) gradiometer and ac magnetizing coil has been developed. The FM-OFG gradiometer has an active canceling coil on each of its sensor heads against the common magnetic field input to avoid the saturation of the amorphous wire core by a strong ac magnetic field. In addition, the ac magnetizing coil has an adjusting capability to make ac magnetic field strength affecting each of the gradiometer heads equal, which allows us to use a high gain amplifier at the latter stage. Two types of the gradiometer are tested: one is parallel configuration in which two sensor heads are placed in parallel side by side, the other is axial configuration in which two sensor heads are placed axially. Detectable distance was investigated using a 5μL (≈100μg in Fe atomic amount) magnetic nanoparticle sample. The maximum detectable distance for the parallel gradiometer is 17 mm, and that for the axial one is 18 mm.

Analysis of Non-Sinusoidal Steady Electric Field of ±500 kV Converter Transformer

The line side winding is under the fundamental frequency AC voltage, while the valve side winding contains not only fundamental AC voltage component, but also the DC voltage component, fundamental AC voltage component, and higher harmonic voltage components when the converter transformer is at its normal operating condition, and the electric field of converter transformer is a non-sinusoidal steady one. To analyze the non-sinusoidal steady electric field containing the DC component, fundamental AC and higher harmonic components, the voltage spectrum of the valve winding in a ±500 kV converter transformer is firstly analyzed, and the non-sinusoidal periodic steady electric field is obtained by the fast discrete Fourier transform. Different resistivity of the oil and oil-immersed paper is adopted to simulate the aging of oil paper insulation at operation, and get the non-sinusoidal steady electric field.

A quadratic polynomial signal model and fuzzy adaptive filter for frequency and parameter estimation of nonstationary power signals

Accurate estimation of amplitude, phase and frequency of a sinusoid in the presence of harmonics/inter harmonics and noise plays an important role in a wide variety of power system applications, like protection, control and state monitoring. With this objective, the paper presents a novel hybrid approach for the accurate estimation of dynamic power system frequency, phasor and in addition to suppressing the effect of harmonics/interharmonics and noise in the voltage and current signals. The algorithm assumes that the current during a fault occurring on a power system consists of a decaying dc component, and time variant fundamental and harmonic phasors. For accurate estimation of fundamental frequency, phasor, decaying dc and ac components in the fault current or voltage signal, the algorithm uses a quadratic polynomial signal model and a fuzzy adaptive ADALINE filter with a modified Gauss–Newton algorithm. Extensive study has been carried out to demonstrate the performance analysis and fast convergence characteristic of the proposed algorithm. The proposed method can also be implemented for accurate estimation of dynamic variations in the amplitude and phase angles of the harmonics and inter harmonics mixed with high noise conditions.

Analysis and Control of Modular Multilevel Converters under Asymmetric Arm Impedance Conditions

This paper presents a detailed analysis and improved control strategy for modular multilevel converters (MMC) under asymmetric arm inductance conditions. Unlike symmetric conditions, the fundamental ac current is not split equally between the upper and lower arms under asymmetric conditions and the dc and double-frequency components in the common-mode current also flow into the ac side. To solve these issues, a theoretical analysis of the effect of asymmetric conditions on MMC operation is carried out using equivalent circuits at different frequencies. Three control targets are then presented to enhance the operational performance. A control strategy providing the control of differential-mode current, common-mode current and power balance is designed. The feasibility and validity of the proposed analysis and control strategy are demonstrated by simulation results from a three-phase MMC system and simulation and experimental results from a single-phase MMC system.