AC Component Research Articles

Indicator of the influence of a geomagnetic storm on the transmission electric system

Abstract Magnetic storms are severe disorders in the Earth’s magnetosphere caused by enormous solar activity. The magnetic storm induces a quasi-stationary geomagnetic current (GIC) in the wires of the very high voltage (VHV) lines. These currents cause the current and thus thermal overload of the VHV part of the transmission lines, especially the transformers of the system. In the submitted article described by a device that indicates these effects of magnetic storms on the transmission system and allows it to carry out its safe operation. The indicator is located on the VHV wire of the bundle conductor. Using the Hall probe, it senses the magnetic field of the current in the line. With a digital frequency low-pas filter removes the ac component of the indicated current and information about its dc component, ie about the GIC, wireless transmission to the workplace of the system operator. This will then instruct the transmission system protective regime.

Real-time computer-generated frequency-carrier Moiré profilometry with three-frequency heterodyne temporal phase unwrapping

A real-time computer-generated frequency-carrier Moiré profilometry with three-frequency heterodyne temporal phase unwrapping is proposed. While three sinusoidal gratings shared the same direct current(DC) component and the same amplitude but with designed different frequencies are projected onto the measured object, three corresponding deformed patterns can be captured by camera. Then, the shared DC component is extracted from the highest-frequency pattern. And based on it, three AC components of the captured deformed patterns can also be effectively extracted by normalization. From this way, the number of the needed gratings can be reduced from six to three, and the disturbance of the object’s nonuniform reflectivity can be avoided efficiently. By using computer-generated frequency-carrier Moiré profilometry(CGFCMP), the carrier frequency designed as pure sinusoidal and cosinusoidal gratings is optimized in order to effectively extract the computer-generated frequency-carrier Moiré fringes, and finally-three wrapped phases at different frequencies can be obtained. Then the wrapped phase with the highest-frequency grating can be unwrapped by three-frequency heterodyne temporal phase unwrapping algorithm. The simulation and experimental results verify the feasibility and flexibility of the proposed method. It is suitable to measure the complex surface with abrupt slope and multiple isolated objects, and has prospective application in real-time three-dimensional measurement.

Calculation of Heartbeat Rate and SpO2 Parameters Using a Smartphone Camera: Analysis and Testing.

Mathematical and signal-processing methods were used to obtain reliable measurements of the heartbeat pulse rate and information on oxygen concentration in the blood using short video recordings of the index finger attached to a smartphone built-in camera. Various types of smartphones were used with different operating systems (e.g., iOS, Android) and capabilities. A range of processing algorithms were applied to the red-green-blue (RGB) component signals, including mean intensity calculation, moving average smoothing, and quadratic filtering based on the Savitzky-Golay filter. Two approaches-gradient and local maximum methods-were used to determine the pulse rate, which provided similar results. A fast Fourier transform was applied to the signal to correlate the signal's frequency components with the pulse rate. We resolved the signal into its DC and AC components to calculate the ratio-of-ratios of the AC and DC components of the red and green signals, a method which is often used to estimate the oxygen concentration in blood. A series of measurements were performed on healthy human subjects, producing reliable data that compared favorably to benchmark data obtained by commercial and medically approved oximeters. Furthermore, the effect of the video recording duration on the accuracy of the results was investigated.

Proteus/Simulink Analysis of Rectifier based E-Vehicle Charger Circuit

In this paper, an AC to DC rectifier circuit has been discussed and the simulation results are provided. This converter is very useful for the upcoming development of the electric vehicles charging technology. For producing steady DC from a rectified AC supply requires a smoothing circuit or filter. The main purpose of the filter circuit is to remove the ripple or ac components from the output of the diode circuit. So, to make the output DC pure, the filter circuit is used. The filter circuit may be inductive (built with only inductors in series) or capacitive (built with only capacitors in parallel) or Inductive-Capacitive (built with both inductors and capacitors). A voltage regulator which is used to maintain the constant output DC voltage. Design of the controller is detailed for reference. The converter will operate in continuous conduction mode (CCM). Finally, some selected experimental results are presented to verify the proposed theory.

Improved equation to describe the induction motor currents during nearby three‐phase short‐circuits

AbstractThis article shows an improved equation to describe the induction motor currents during a three‐phase short‐circuit near to motor terminals. The proposed equation considers three details which are not in the equation shown in the IEEE Std. 551: (a) the equivalent frequency of these currents is not strictly equal to the grid frequency; (b) the magnitude of the DC component is not strictly equal to the magnitude of the AC component; (c) an angle is included to consider the instant of fault occurrence. The proposed equation was verified by experimental results, which were also verified by simulations. It is shown that the equivalent frequency of analysed currents is not exactly the grid frequency due to the sudden change of the machine speed (caused by the high braking torque originated by high transient currents during the short‐circuit). This effect is larger in smaller motors. The AC and DC components of the analysed currents have their own time constants, which are usually estimated from motor equivalent circuit. The application of the improved equation was useful to demonstrate that such a procedure does not lead to accurate results in case of the DC time constants of small motors. Finally, an example illustrates a case that cannot be well described by the equation from the IEEE Std. 551, whereas the result from the proposed equation is very similar to the measured current.

Indirect measurement of cutting forces during robotic milling using multiple sensors and a machine learning-based system identifier

Robotic machining has attracted great interest in the fields of aerospace and automotive manufacturing because of its advantages over conventional computer numerical controlled (CNC) machine tools; robotic machining is flexible and can fabricate components with complex (sculpted) geometries and of large dimensions. The principal challenge, however, is the poor structural rigidity of robots, leading to high-level vibration and chatter during machining. The cutting force must be monitored to detect and suppress chatter as well as optimize machining. A practical method is required to measure the cutting force during machining without impeding robot movement. This study presents an indirect method that accurately measures the milling force using an accelerometer located on the spindle and a flange-mounted capacitive force sensor. A Kalman filter was used to compensate for the dynamics between the robot tool center point (TCP) and the location of the accelerometer. In the context of pose-dependent modeling of the dynamics of the robot, a novel machine-learning (ML)-based identifier is presented that rapidly identifies the relevant dynamics (i.e., the modal parameters) without any initial human guesswork. Data from dynamic simulations and experiments were used to train and validate the ML-based identifier. The validation results showed that the identifier rapidly predicted the behaviour of physical systems with an average accuracy of 98.49 % compared to the modal analysis method. Next, the cutting forces were indirectly measured using the acceleration signals and a Kalman filter, developed based on the modal parameters. The forces measured by the proposed method were in good agreement with the reference forces, measured using a table dynamometer; the average peak-to-peak accuracy was 90.17 % for the AC component of the cutting force, and the average time-domain accuracy was 95.27 % when the DC component was also considered.

Circulating Current Generating Mechanism and Suppression Control of HMMC

Hexagonal modular multilevel converter (HMMC) is a full-bridge ac/ac converter, and it can achieve the interconnection between the ac grid and the low frequency system. However, the coupling of input and output on the branches causes the large fluctuation both in dc voltage of submodules (SMs) and branch currents. Moreover, the internal circulating current in branch current comprises large dc component and ac components, which causes an increase in current stress. The traditional circulating current control of HMMC adopts circulating current injection for dc voltage stability, which cannot decrease the current stress. Therefore, basing on dc voltage fluctuation analysis, the circulating current generating mechanism is analyzed. According to the influence mechanism of circulating current, a circulating current suppression control (CCSC) is proposed to decrease the current stress. Eventually, the CCSC is verified in the real-time (RT)-lab platform.

Analysis and Optimization of Energy Variations in MMCs With Variable DC Voltages

The modular multilevel converter (MMC) containing full-bridge submodules (FBSMs) can increase the modulation index to reduce the energy storage requirement. The MMCs containing FBSMs can also adjust the dc voltage to fit special applications. However, limited research explores the energy storage characteristics of MMCs with variable dc voltages, and few effective approaches can ensure low-energy storage requirements. This study proposes an approximate analysis method for the energy storage requirement in MMC with variable dc voltages. Results reveal that the energy variation under low dc voltages restricts the reduction effects of the increased modulation index on the energy storage requirement. Then, on the basis of the fact that the low ac component of the arm current under low dc voltages provides a large current margin, an optimal circulating current (OCC) is proposed to optimize the energy variation under low dc voltages. In addition, a practical curve-fitting approach is proposed to facilitate the calculation of OCC in real-time controllers. With these approaches, the energy storage requirement with variable dc voltage becomes similar to that with constant rated dc voltage. Simulation and experimental results verify the analysis.

Spin motive force induced by parametric excitation

Spin motive force generated by parametrically excited magnetization dynamics is numerically investigated. We calculate spin motive force in a permalloy disk under an ac magnetic field with twice the ferromagnetic resonance frequency parallel to the static magnetic field based on the Landau–Lifshitz–Gilbert equation. We found that large spin motive force originating from standing spin waves driven by parametric excitation appears in the system. The observed time dependence of the voltage shows a dc voltage with an ac component oscillating with twice of the resonance frequency. The estimated amplitude of the voltage due to the spin motive force is ∼μV. We also investigate spin motive force driven by different modes of standing spin waves. Our numerical results extend the way to generate spin motive force by making use of the magnetization dynamics with the steep spatial modulation created by nonlinear spin waves excitation, without a non-uniform magnetization structure such as a conventional magnetic domain wall and a vortex.

Simplified Approach for Acquisition of Submodule Capacitor Voltages of the Modular Multilevel Converter Using Low Sampling Rate Sensing and Estimation

The modular multilevel converter (MMC), used in high-voltage dc applications, consists of hundreds of submodules in each arm. The operation and control of the MMC require sensing of the capacitor voltage of each submodule. The transmission of the sensed data from hundreds of submodules over individual channels leads to wiring complexity. This work suggests that time-multiplexing can be used to transmit the sensed data of multiple submodules across a common serial interface bus. Time multiplexing, however, decreases the sampling rate. Since the submodule capacitor voltage has both ac and dc components, an adequate sampling rate must be maintained to avoid the aliasing of ac components. In this article, an estimation scheme is investigated for reconstructing the ac components of the submodule capacitor voltages within the processing platform, leaving just the dc component to be transmitted over the common bus. The mathematical analysis of the data transmission system and proposed estimation scheme is performed, and the influence on the closed-loop performance is studied. The proposed scheme is experimentally verified in the laboratory.

A High-Precision Sensor Based on AC Flux Cancellation for DC Bias Detection in Dual Active Bridge Converters

DC bias in a transformer is a threat to the safe and efficient operation of dual active bridge (DAB) converters. Detecting a dc component in milliamperes mixed with a large ac current is difficult. This article proposes a current sensor based on ac magnetic flux (MF) cancellation. The proposed sensor utilizes a current transformer (CT) to extract the ac component in the measured current. Then, a power op-amp copies the CT signal to an additional winding to generate an ac MF opposite to that of the main ac current. Hence, the sensor range required to measure the dc is greatly reduced for a much smaller absolute measurement error. Using only a CT and a simple op-amp circuit, the proposed solution features low complexity and power consumption. The relative error in a range of 15-A ac current is 0.038%, showing a five-fold improvement compared to off-the-shelf sensors. The proposed sensor is verified on a 1-kW DAB prototype, where the dc bias is suppressed to nearly null owing to the proposed sensor.

Thermal conductivity in fluorine-doped single-walled carbon nanotubes

Herein, we report on the axial thermal conductivity (κzz) of a non-degenerate fluorine-doped single-walled carbon nanotubes (FSWCNTs) under single and dual relaxation models. We derived the expressions for the carrier thermal current density (Q→), carrier thermal conductivity (κe), and simulated the lattice thermal conductivity (κℓ) using Callaway’s dual relaxation time model as a function of temperature field. The thermal conductivity was observed to be strongly nonlinear and purely phonon dominated. Moreover, the study investigated the effect of impurity concentration (no), and overlapping integrals for jumps (Δs and Δz) on κzz. Our results showed that, the axial thermal conductivity (κzz) and the operational temperature range of FSWCNTs could be tuned using doping (impurity) and overlapping integrals. The ac component of the electric field was observed to strongly energise the FSWCNT carrier gas and thus, caused the carrier contribution to the axial thermal conductivity to be non-significant but solely phonon dominated. Interestingly, using the Callaway’s model, κℓ was evaluated to be 107.201 W/mK, at 300 K. Thus, base on the FSWCNT’s low κℓ and its possibility of tuning to operate at high temperatures makes it a good choice for thermoelectric applications including chip level cooling, heat sinks and power generation.

Prediction Approach of Negative Rail Potentials and Stray Currents in Medium-Low-Speed Maglev

The problem of negative rail potentials (NRPs) appears in the line of the medium-low-speed (MLS) maglev. It is vital to forecast the NRP and the corresponding stray currents (SCs) in order to prevent safety hazards. This article proposes the prediction methods of NRP and SC for the first time for the MLS maglev. On the basis of developing a two-layer rail–earth circuit, a simplified prediction approach based on the analytical formula derivation is proposed to obtain the behaviors of NRP and SC under different steady running scenarios. Afterward, in order to accurately reveal the influence of vehicle-grid parameters, the ac component and the energy dispersal path of the maglev system are detailed, and the precise vehicle-grid model is developed to consummate the prediction approach using MATLAB. Under the premise of verifying the scheme effectiveness using the captured data in the on-site of the Chinese Xinzhu MLS maglev line, the scheme application studies are provided to investigate the behaviors of NRP and SC while considering the ac effect and the varying velocity of load current and rail–earth conductivity.

The electrical power quality indicator – interference power factor

Purpose. Substantiation of the methodology for calculating an indicator characterizing the pulsating current power distortion. Methodology. When analyzing the power of direct and alternating sinusoidal currents, the features of the ratio of a root-mean-square norm to its mean value, known as the invariance power factor, are noted. In this case, the root-mean-square power value acts as a normalizing parameter. Using a combination of direct and sinusoidal (pulsating) current, the dependences of the invariance power factor on the ratio of direct and alternating components are obtained. Findings. Taking into account the interaction of the current and voltage components of different frequencies, the corresponding power component is highlighted, called interference power. With its use, by analogy with the invariance power factor, the interference power factor is introduced. The interference power factor behavior for AC non-sinusoidal current circuit and DC pulsed current circuit of rectifier was investigated, as a result of which a difference was established in the interference power factor dependence in these circuits. Originality. The obtained dependences of the interfere power factor on the ratio of DC and AC components for current and voltage prove the versatility of its application for assessing power distortion in both DC and AC circuits, as proved by the example of a circuit with a single-phase controlled rectifier. Practical value. The results obtained can be used to assess the electrical power distortion level in electric complexes and systems of various kinds of current and kind of energy, including when it is taken into account. This is a prerequisite for the measures development to improve the electricity quality.

Fundamental frequency sequence amplitude estimator for power and energy applications

A grid-synchronization-based fundamental frequency positive-sequence (FFPS) and negative-sequence (FFNS) amplitudes estimation technique is proposed for unbalanced and distorted grid. In this technique, the sequence amplitudes are extracted by extracting the phase-angle of the FFPS and FFNS components. The extracted phase-angles have DC and double frequency AC components. The AC component is filtered out by using a Moving Average Filter (MAF) of appropriate window length. From the extracted phase-angle, the unknown frequency can be estimated by using a suitable controller. A frequency-fixed equidistant samples-based pre-loop filter is also applied to eliminate the effect of measurement offset. The proposed technique has a very simple structure and is easy to tune. Small-signal modeling-based stability analysis and gain tuning procedure are also provided. The proposed technique strikes a good balance between fast convergence and disturbance rejection capability. Comparative numerical simulation and experimental results with similar other techniques demonstrate the suitability and performance enhancement by the proposed technique.

Design and implementation of sequential excitation module for high fidelity piezoresponse force microscopy

The acquisition of accurate information through a contact resonance mode is critical for mapping weak electromechanical effect reliably by using piezoresponse force microscopy (PFM). However, it is very challenging to track resonance frequency shifting when the contact stiffness from the sample varies significantly. In this work, we have developed a sequential excitation (SE) module to enable high fidelity PFM. A customized discrete frequency sweep signal from an arbitrary waveform generator is used for drive excitation so that resonance frequency tracking is no longer necessary. Furthermore, the AC component of the piezoresponse is sampled by using an oscilloscope instead of using lock-in amplifiers. To accommodate high volume of data acquisition, a fast analysis method is also developed to fit the transfer function of the cantilever efficiently on the fly during scanning. Hardware implementation and data processing are described in detail. The capability of our SE module has been demonstrated on an ordinary PMN-PT film via first and second harmonic PFM, as well as a suspended freestanding MoS2 membrane that is very challenging to probe due to its substantial variation in contact stiffness.

Measurement of Small-Magnitude Direct Current Mixed With Alternating Current by Tunneling Magnetoresistive Sensor

Measurement of small-magnitude direct current (dc) mixed with alternating current (ac) is crucial in the power systems in which large-scale renewable energy and power electronics are used. This research proposes a method to design a nonintrusive surface-mount current sensor that can measure milliampere dc mixed with ac of different amplitudes based on the tunneling magnetoresistive (TMR) sensors. A sensor module configuration that uses a high-sensitivity TMR sensor to detect small-magnitude dc and ac was proposed. Moreover, because the high-sensitivity TMR sensor was susceptible to saturation, a medium-sensitivity TMR sensor was used to detect the large-magnitude ac. A signal processing framework was built, in which the mean and standard deviation of the detected waveforms were calculated to obtain the dc and ac components of the current under measurement, and the measurement error due to the TMR sensor saturation was also reduced by a compensation algorithm. The results showed that the sensor could accurately measure dc lower than 40 mA mixed with the root-mean-square ac ranging from 100 mA to 2 A.

A High Dynamic Range Vibration Radar Sensor With Automatic DC Voltage Extraction

One challenge of using a homodyne receiver to obtain accurate vibration information is the undesired DC voltage in a continuous wave radar. It reduces the output dynamic range and even saturates the signal path. In this paper, an adaptive DC extraction method with flexible clocks is proposed in a 5.8 GHz Doppler radar to separate the DC component from the input signal. Rather than simply eliminating the DC component, the proposed method digitizes the DC component as well as the input AC signal with a large dynamic range. It is accomplished by a mixed-signal feedback loop and features an excellent accommodation to large input DC variations and flexible settling time. In the loop, a closed-loop amplifier is used as the input stage and an ADC is followed to digitize the amplified quadrature signals from the radar. Afterward, a simplified digital filter is cascaded to extract the digital version of the DC component and feedback to the input stage. In this way, both the DC component and AC component in the input signal are separated and digitized without saturation. For verification, several simulations and experiments were conducted. The results show that a wide DC range can be tolerated and extracted. What’s more, the target motion with sub-millimeter amplitude can be successfully captured based on the proposed DC extraction loop.

Power Ripple Control Method for Modular Multilevel Converter under Grid Imbalances

Modular multilevel converters (MMCs) are primarily adopted for high-voltage applications, and are highly desired to be operated even under fault conditions. Researchers focused on improving current controllers to reduce the adverse effects of faults. Vector control in the DQ reference domain is generally adopted to control the MMC applications. Under unstable grid conditions, it is challenging to control double-line frequency oscillations in the DQ reference frame. Therefore, active power fluctuations are observed in the active power due to the uncontrolled AC component’s double line frequency component. This paper proposes removing the active power’s double-line frequency under unbalanced grid conditions during DQ transformation. Feedforward and feedback control methods are proposed to eliminate ripple in active power under fault conditions. An extraction method for AC components is also proposed for the power ripple control to eliminate the phase error occurring with the conventional high-pass filters. The system’s stability with the proposed controller is tested and compared with a traditional MMC controller using the Nyquist stability criterion. A real-time digital simulator (RTDS) and Xilinx Virtex 7-based FPGA were used to verify the proposed control methods under single-line-to-ground (SLG) faults.

Analysis and Compensation of Sampling-Delay Error in Single Current Sensor Method for PMSM Drives

In single current sensor (SCS) methods, different phase currents should be acquired sequentially under the duration time of specific voltage vectors within each switching period. The sampling asynchronization caused due to sampling delay will inevitably result in the increase of phase current reconstruction error and deteriorate the control performance. According to the fundamental expression formula of three-phase currents in permanent magnet synchronous motor drives, the sampling-delay error (SDE) occurring in the traditional SCS techniques is analyzed in this article. It is demonstrated that the error oscillates at the stator electrical frequency, with the amplitude affected by both the switching frequency and motor speed. Correspondingly, a novel compensation method based on a <i>&#x03B4;&#x2013;&#x03B3;</i> reference frame is presented. There is a simple proportional relationship between <i>&#x03B4;</i>- and <i>&#x03B3;</i>-axis error component after filtering the ac component by a notch filter. As a result, the SDE can be easily eliminated without extra complex compensation algorithms introduced. The prominent feature of the proposed method is that it is developed without necessity of motor parameters when compared with previous compensation techniques, making it a more robust solution. Several experimental results are provided to verify the effectiveness of the proposed method.