DC Component Research Articles

An innovative method to eliminate multiple exponentially decaying DC components based on the discrete Fourier transform in the numerical distance relay

Exponentially decaying direct current (EDDC) components in the faulted current of a power system can result in an imprecise estimation of the fundamental frequency component. Because of sources such as the faulted transmission line, the current transformer (CT), and the relay's internal CT, current signals tend to include multiple EDDC components. This paper presents an accurate model of the faulted current, including a new term of EDDC components that belongs to the relay's internal CT along with the transmission line and the CT response. Moreover, this paper proposes an innovative algorithm to eliminate multiple EDDC components. The feasibility and performance of the proposed algorithm were validated using several static, dynamic, and experimental tests. The static test was carried out using MathWorks’ MATLAB® software. For dynamic validation, the transient data of a power system (simulated in the commercial PSCAD© software) were employed to test the performance of the propounded method. The experimental test was performed on a distance protective relay manufactured by VEBKO AMIRKABIR Co. The main advantages of this method include the practical feasibility resulting from its simple structure, the ability to effectively suppress the adverse influence of multiple EDDC components (including the response of the transmission line, the CT, and the relay's internal CT), and the accuracy in estimating the fundamental frequency component.

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 Fourier Transform-Based Calculation Method of Wilting Index for Soybean Canopy Using Multispectral Image

The growth process of soybean plants needs a lot of water. The rapid detection of canopy wilting of soybean under drought stress is of great significance for soybean variety breeding, cultivation regulation and fine management. Aiming at the problems of cumbersome and time-consuming when the traditional chemical technology was used to determine soybean wilting index, a calculation method of wilting index for soybean canopy was proposed in this study based on multispectral images’ Fourier transform. Suinong 26, a northeast soybean variety, was taken as the object. First, four kinds of soybean multispectral images of green, red, red-edge and near-infrared channels were acquired by a Sequoia multispectral camera. Second, based on the multispectral reflection image preprocessed by median filter and mean filter, the target area of a multispectral image of the soybean canopy was extracted by the iterative threshold method and affine transformation algorithm, and the effective segmentation rate was 97.02%. In addition, Fourier transform was used to analyze the spectrum characteristics of the soybean canopy’s multispectral image. When the spectrum radius of each channel was 50, the energy reached more than 98% and was concentrated in the low-frequency region of the spectrum center. Finally, according to the difference between the low-frequency DC component and the proportion of total energy in the spectral radius of the multispectral images under normal and drought treatment, a calculation model of the soybean wilting index was constructed based on the energy spectrum of Fourier transform. The results showed that the difference of the wilting index between normal and drought treatment for the four channels (green, near-infrared, red and red-edge) was 2.38, 3.11, 3.56 and 4.11, respectively. The effectiveness of the wilting index was verified and analyzed by using the average leaf inclination angle. The determination coefficient R2 of the four channels between the calculated wilting index and the average leaf inclination angle was more than 0.85. This calculation method can provide a quantitative basis and technical support for the scientific regulation of ecological and morphological phenotypic traits of soybean plants under drought stress.

Using Constrained Square-Root Cubature Kalman Filter for Quantifying the Severity of Epileptic Activities in Mice.

(1) Background: Quantification of severity of epileptic activities, especially during electrical stimulation, is an unmet need for seizure control and evaluation of therapeutic efficacy. In this study, a parameter ratio derived from constrained square-root cubature Kalman filter (CSCKF) was formulated to quantify the excitability of local neural network and compared with three commonly used indicators, namely, band power, Teager energy operator, and sample entropy, to objectively determine their effectiveness in quantifying the severity of epileptiform discharges in mice. (2) Methods: A set of one normal and four types of epileptic EEGs was generated by a mathematical model. EEG data of epileptiform discharges during two types of electrical stimulation were recorded in 20 mice. Then, EEG segments of 5 s in length before, during and after the real and sham stimulation were collected. Both simulated and experimental data were used to compare the consistency and differences among the performance indicators. (3) Results: For the experimental data, the results of the four indicators were inconsistent during both types of electrical stimulation, although there was a trend that seizure severity changed with the indicators. For the simulated data, when the simulated EEG segments were used, the results of all four indicators were similar; however, this trend did not match the trend of excitability of the model network. In the model output which retained the DC component, except for the CSCKF parameter ratio, the results of the other three indicators were almost identical to those using the simulated EEG. For CSCKF, the parameter ratio faithfully reflected the excitability of the neural network. (4) Conclusion: For common EEG, CSCKF did not outperform other commonly used performance indicators. However, for EEG with a preserved DC component, CSCKF had the potential to quantify the excitability of the neural network and the associated severity of epileptiform discharges.

Improved experimental setup to reach a broad frequency domain in local electrochemical impedance spectroscopy measurements

We report a new experimental setup enabling local electrochemical impedance spectroscopy (LEIS) measurements over a wide frequency range, including the low frequency domain. First, an improved signal treatment was performed using a potential shifter to minimize the DC component of the measured potential, allowing for the local AC potential to be probed using an ideal electrochemical system. Then, a unique, three-electrode LEIS probe was implemented to study the influence of radial and lateral contributions on the LEIS response. The radial LEIS response is highest when the LEIS probe is positioned in the middle of the gold electrode substrate, whereas the lateral LEIS response is similar in both measured regions. Both experimental modifications were implemented to study the LEIS response over a corroding aluminum substrate, for which reliable results are obtained in the low frequencies. These tools could be used in future LEIS studies in other fields that require time-sensitive measurements.

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.

Stability problem in 3D multipole ion traps

We suggest a concept of ion trapping in three-dimensional multipole electric fields. In this work, we propose a vibromechanical interpretation of an ion localization stability problem in a three-dimensional octupole radio-frequency electric fields and have made a transition to an equivalent autonomous dynamical system. The conditions for stable ion localization in three-dimensional octupole trap in radio-frequency-only-mode and mixed-mode are determined. The ion's effective potential for the mixed-mode is calculated and the coordinates of the stable quasi-equilibrium points are obtained. The sensitivity of the effective potential configuration to the polarity of the DC component is estimated. The ion's dynamics in an original and autonomous systems is considered, and a method for optimizing the shape of trap's electrodes is proposed.

Discrimination of Microseismic Events in Coal Mine Using Multifractal Method and Moment Tensor Inversion

Discrimination of various microseismic (MS) events induced by blasting and mining in coal mines is significant for the evaluation and forecasting of rock bursts. In this paper, multifractal and moment tensor inversion methods were used to investigate the waveform characteristics and focal mechanisms of different MS events in a more quantitative way. The multifractal spectrum calculation results indicate that the three types of MS waveform have different distribution ranges in the multifractal parameters of ∆α and Δf(α). The results show that the blasting schemes also have a great influence on MS waveform characteristics. Consequently, the multifractal parameters of ∆α and Δf(α) can be used to discriminate different MS events. Further, the focal mechanisms of MS events were calculated by seismic moment tensor inversion. The results show that an explosion is not the dominant mechanism of deep-hole blasting MS events, and the CLVD and DC components account for an important proportion, indicating that some additional processes occur during blasting. Moreover, the coal-rock fracture MS events are characterized by compression implosion or compression/shear implosion mixed focal mechanisms, while the overburden movement MS events are tensile explosion or tensile/shear explosion mixed focal mechanisms. The focal mechanisms and nodal plane parameters have close relationships with the inducing factors and occurrence processes of MS events.

On the Use of Supercapacitors for DC Blocking in Transformer-Coupled Voltage Amplifiers for Low-Frequency Noise Measurements

When performing low-frequency noise measurements on low-impedance electron devices, transformer coupling can be quite effective in reducing the contribution of the equivalent input noise voltage of the preamplifier to the background noise of the system. However, noise measurements on electron devices are usually performed with a biased device under test. A bridge configuration must be used to null the DC component at the input of the transformer. Unfortunately, using a bridge results in a complication of the set-up and degradation of the system’s sensitivity because of the noise introduced by the nulling arm. We propose an alternative approach for blocking the DC component that exploits the fact that supercapacitors with capacitances in excess of a few Farads are nowadays easily available. Actual measurement results in conventional and advanced measurement configurations are discussed that demonstrate the advantages of the approach we propose.

Direct Torque Control of Induction Motor with Stator Flux Estimation Based on an Improved Voltage Model

Using sensors to measure the stator flux for induction motor presents many drawbacks such as far less reliability, noisy signals, vibration, and additional cost. Thus, it is essential to synthesize an estimator (also called a software sensor) allowing to replace the hardware sensor, for the flux estimation. In general, this estimation is done by integrating the back electromotive force (EMF) of the stator. However, the presence of the pure integrator presents problems due to the inevitable DC component and the initial condition error in the back EMF. To overcome these problems, a low pass filter has been used in the literature. Nevertheless, this solution does not solve the problem of undesirable high-frequency noise, especially at low speed. In this paper, we present a solution to improve the stator flux estimator by proposing a fractional or- der integrator to estimate the flux at low speed. This solution does not require any hardware device; it involves low calculation time and provides accurate practical results. The proposed solution has been implemented using the dSPACE 1104 platform.

Investigation of using dynamic phasors for improving the speed of distance protection

The application of dynamic phasors (DPs) in overhead line protection was investigated. Equations to estimate the fault loop inductance and resistance were derived using the principles of DPs. A systematic amplitude comparator-based line distance protection scheme was developed using the equations derived to estimate the fault loop inductance and resistance. A least-squares-based DP extraction method that mitigates the effect of decaying dc components was used to extract the DPs and their derivatives from time-domain signals. The performance of the algorithm was investigated using a power system simulated in PSCAD/EMTDC. Testing under different fault scenarios with different fault locations, fault resistance, and fault inception angles showed that the proposed line distance protection scheme is capable of operating successfully for forward and reverse faults, short circuits and high impedance faults, evolving fault conditions, and under a wide range of source impedance ratios. The operating time of the proposed scheme was compared with the standard quadrilateral relay, and the results showed that the operating times of the proposed scheme are faster for close-in phase-to-ground faults but not significantly improved for phase-to-phase and three-phase faults compared to the standard quadrilateral distance relay.

Study on the effect of different pre-excitation factors on the output characteristics of MCSR

This paper takes the BKFSZT-30000/400 three-phase eight-limb magnetically controlled shunt reactor as the research object. Firstly, an electromagnetic transient simulation model is established based on the parity principle to verify the operating characteristics of the MCSR during direct energization and pre-excitation energization. On this basis, the influence of the pre-excitation capacity and the target capacity on the output characteristics of the MCSR is investigated. The results show that during the pre-excitation process the currents of working and control winding produce a DC component and a harmonic component dominated by the second harmonic, with the size of both DC components mainly influenced by the pre-excitation capacity and the attenuation determined by MCSR; the variation of the working harmonics is mainly influenced by the target capacity, while the size and attenuation of the control harmonics are determined by the pre-excitation capacity and the target capacity respectively. The results of the study clarify the output characteristics of the MCSR when energization with pre-excitation and provide a reference for further analysis of the interaction with the grid. Finally, to solve the problem of no reliable external power supply for pre-excitation, a fast excitation energization method with short-connected of the compensation winding is provided and verified by simulation.

Two modified DFT‐based algorithms for fundamental phasor estimation

The discrete Fourier transform (DFT) is a powerful phasor estimation algorithm conventionally used in digital relaying applications. The DFT-based phasor estimation is usually accompanied by oscillatory error due to the presence of the decaying DC (DDC) components in fault current signals. Accordingly, two modified DFT-based phasor estimation algorithms are proposed in this paper. The proposed algorithms accurately estimate the exact fundamental phasor by extracting the DDC components using successive outputs of the conventional DFT algorithm. The performance of the proposed algorithms is validated by the mathematically generated and PSCAD-simulated signals and is compared with previous algorithms. The results demonstrate that the proposed algorithms in this paper are more robust than other ones. Moreover, they are applicable in the DFT algorithm with different data windows.

Detecting Serial Arcs in Aeronautical Applications Using Inductive Sensors

There is a clear trend in the aircraft industry to use more electrical systems in propulsion and electromechanical and electrohydraulic actuators to comply with environmental sustainability and to improve reliability and maintenance processes. The increase of electrical consumption requires an increase of the rated voltages to supply power to these so called More Electrical Aircraft (MEA). Unfortunately, this voltage rise, currently up to 540 Vdc, can lead to ionization processes within the electric wiring due to the lower air density at high altitudes. As a consequence, a degraded insulation can create extremely hazardous events in flight such as arcs between wires (serial) or between wires and fuselage (parallel). Serial arcs in DC bus circuits are specially dangerous for the aircraft operation, so its detection within fractions of seconds is required to ensure a reliable operation. However, during a sustained serial arc the rated current can be passing through air deceiving the protections and avoiding their tripping. In this work, it has been found that arcing introduces high frequency current pulses superimposed to the DC component that can be identified to detect the occurrence of an arc. The frequency components of these pulses depend on the line characteristics and can be detected with inductive sensors. This manuscript also designs and tests a light and inexpensive sensor for arc detection in aircraft applications.

Broadband Dynamic Phasor Measurement Method for Harmonic Detection

A large number of nonlinear loads and distributed energy sources are connected to the power system, leading to the generation of broadband dynamic signals including inter-harmonics and decaying DC (DDC) components. This causes deterioration of power quality and errors during power measurement. Therefore, effective phasor estimation methods are needed for accurate monitoring and effective analysis of harmonics and interharmonic phasors. For this purpose, an algorithm is proposed in this paper that is implemented in two parts. The first part is based on the least square method in order to obtain accurate DDC component. In the second part, a Taylor–Fourier model of broadband dynamic harmonic phasor is established. The regularization optimization problem of the sparse acquisition model is solved by harmonic vector estimation method. Finally, the piecewise Split-Bregman Iterative (SBI) framework is used to obtain the estimated value of the harmonic phasor measurement and to realize the reconstruction of the original signal. Through simulation and performance test, the proposed algorithm significantly improves the accuracy of the phasor measurement and estimation, and can provide a reliable theoretical basis for the PMU measurement.

Spin pumping from antiferromagnetic insulator spin-orbit-proximitized by adjacent heavy metal: a first-principles Floquet-nonequilibrium Green function study

Motivated by the recent experiment (Vaidya et al 2020 Science 368 160) on spin pumping from sub-THz radiation-driven uniaxial antiferromagnetic insulator (AFI) MnF2 into heavy metal (HM) Pt hosting strong spin-orbit (SO) coupling, we compute and compare pumped spin currents in Cu/MnF2/Cu and Pt/MnF2/Cu heterostructures. Recent theories of spin pumping by AFI have relied on simplistic Hamiltonians (such as tight-binding) and the scattering approach to quantum transport yielding the so-called interfacial spin mixing conductance (SMC), but the concept of SMC ceases to be applicable when SO coupling is present directly at the interface. In contrast, we use a more general first-principles quantum transport approach which combines noncollinear density functional theory with Floquet-nonequilibrium Green’s functions in order to take into account: SO-proximitized AFI as a new type of quantum material, different from isolated AFI and brought about by AFI hybridization with adjacent HM layer; strong SO coupling at the interface; and evanescent wavefunctions penetrating from Pt or Cu into AFI layer to make its interfacial region conducting rather than insulating as in the isolated AFI. The DC component of pumped spin current vs. precession cone angle of the Néel vector l of AFI does not follow putative , except for very small angles for which we define an effective SMC from the prefactor and find that it doubles from MnF2/Cu to MnF2/Pt interface. In addition, the angular dependence differs for opposite directions of precession of the Néel vector, leading to twice as large SMC for the right-handed than for the left-handed chirality of the precession mode.

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.

Embedded micro-probe fiber optic interferometer with low nonlinearity against light intensity disturbance.

A novel low-nonlinearity Michelson microprobe fiber interferometer against light intensity disturbance for high-precision embedded displacement measurements is introduced. To analyze the influence of light intensity disturbance on the microprobe and measurement accuracy of the integrated fiber interferometer, an equivalent model of micro-probe sensing with the tilted target is established. The proposed PGC demodulation and nonlinearity correction method with simple principle helps avoid DC component varying with light intensity. The experiments show that residual displacement errors of the micro-probe fiber interferometer are reduced from 4.36 nm to 0.46 nm, thus allowing embedded displacement detection with sub-nanometer accuracy under low frequency light intensity disturbance.

Energy balance, efficiency and operational limits of the dynamo type flux pump

In rotating flux pumps, a rectified voltage, with non-zero DC component, is obtained at the terminals due to the combined effect of the distributed AC electromotive force, produced by one or more permanent magnets in circular motion and the non-linear resistivity of the superconductor. Overcritical currents are continuously induced in the tape during operation, giving rise to the DC voltage and producing, at the same time, dissipation. In this paper, the energy behavior of the flux pump is numerically investigated. It is shown that induced currents interact with the rotating magnet(s), producing a resistant torque that is little affected by the output current of the flux pump. Due to this interaction mechanism, a significant part of the mechanical power supplied to the rotor is converted into Joule heating within the tape. The paper also explores the operational limits of the flux pump, showing that the generator operation, involving an electric power delivered to the load combined with a mechanical power supplied to the rotor, can only be achieved in a restricted range of current and voltage at the terminals and that the maximum power transfer and efficiency are reached at the middle of the generator range. Under no conditions, the mechanical torque produced on the rotor can be reversed, reaching the motor mode involving an electric power absorbed at the terminals combined with a mechanical power produced on the rotor. A revised equivalent circuit comprising, besides the effective resistance reported in the literature, a further intrinsic resistance is proposed in the paper for taking all the dissipation mechanisms into account. It is shown that this equivalent circuit can predict the energization of an RL load both concerning the final steady values and the full time-domain behavior of the current (including ripples).

Four-wavelength quadrature phase demodulation technique for extrinsic Fabry-Perot interferometric sensors.

In this Letter, we report a four-wavelength quadrature phase demodulation technique for extrinsic Fabry-Perot interferometric (EFPI) sensors and dynamic signals. Four interferometric signals are obtained from four different laser wavelengths. A wavelength interval of four wavelengths is chosen according to the free spectrum range (FSR) of EFPI sensors to generate two groups of anti-phase signals and two groups of orthogonal signals. The linear fitting (LF) method is applied to two groups of anti-phase signals to eliminate the dc component and ac amplitude to obtain two normalized orthogonal signals. The differential cross multiplication (DCM) method is then used to demodulate the phase signal from these two normalized orthogonal signals. The proposed LF and DCM (LF-DCM) based four-wavelength quadrature phase demodulation overcomes the drawback of the traditional ellipse fitting (EF) and DCM (EF-DCM) based dual-wavelength demodulation method that it is not suitable for weak signal demodulation since the ellipse degenerates into a straight line, which makes the EF algorithm invalid. Moreover, it also avoids the assumption that the dc component and ac amplitude of interferometric signals are identical, which is widely used in three-wavelength demodulation. An EFPI acoustic sensor is tested to prove the four-wavelength quadrature phase demodulation and experimental results show that the proposed phase demodulation method shows advantages of large dynamic range and wide frequency band. Linearity is as high as 0.9999 and a high signal-to-noise ratio (SNR) is observed from 1 Hz to 100 kHz.