DC Offset Component Research Articles

Three-Dimensional Structure Reconstruction System of Nasal Cavity Based on a Short-Source Acoustic Tube

The nasal cavity three-dimensional structure reconstruction is significant for the diagnosis and treatment of nasal and nasopharynx diseases. Currently, most systems for nasal cavity three-dimensional structure reconstruction are based on a long-source acoustic tube. However, the long-source acoustic tube has the weaknesses of low portability and high cost. To address these issues, a nasal cavity three-dimensional structure reconstruction system based on a short-source acoustic tube was designed in this research. The designed system comprises the lower computer of a nasal acoustic signal acquisition device and the upper computer of nasal acoustic signal analysis software. The lower computer of a nasal acoustic signal acquisition device consists of a signal processing unit, a detection unit, a microcontroller, and a USB communication interface. It is used to collect the original acoustic signal in the nasal cavity. The upper computer of nasal acoustic signal analysis software consists of the nasal cavity acoustic signal preprocessing method and the nasal cavity three-dimensional structure hierarchical reconstruction method. The nasal cavity acoustic signal preprocessing method is used to eliminate the DC offset component and suppress the high-frequency noise. Then, the nasal cavity three-dimensional structure hierarchical reconstruction method is used to establish the relationship between the cross-sectional area of the nasal cavity and the depth of the nasal cavity. In order to verify the effectiveness of the system designed in this research, the straight tube model and the simulated nasal cavity model were selected as test subjects, and the depth–cross-sectional area was selected as the evaluation indicator of the accuracy of the nasal cavity three-dimensional structure reconstruction result. The experimental results show that the root mean squared error and the mean relative error of the depth–cross-sectional area were reduced from 0.6284 cm2 to 0.0201 cm2 and 47.6467% to 1.8248%, respectively, under the straight tube model by using the nasal cavity acoustic signal preprocessing method. The root mean squared error and the mean relative error of the depth–cross-sectional area were reduced from 4.7730 cm2 to 0.1510 cm2 and 107.8128% to 3.3096%, respectively, under the simulated nasal cavity model. Meanwhile, this system can increase the effective measurement depth of the nasal cavity from 7 cm to 13 cm based on the preprocessing method. The results demonstrated that the designed system can not only reconstruct and visualize a nasal cavity three-dimensional structure with high precision but also generate comprehensive test reports. Therefore, this system can provide a basis for further auxiliary diagnosis of the disease and facilitate doctors in explaining the physiological structure of the nasal cavity to patients.

Enhancing Single-Phase Grid Integration Capability of PMSG-Based Wind Turbines to Support Grid Operation under Adverse Conditions

The proposed work delivers a robust control solution for a single-phase permanent magnet synchronous generator-based wind power conversion system (PMSG-WPCS) to enhance grid integration capability. The proposed control approach also offers an extended facility to fulfill low-voltage fault ride-through (LVRT) requirements under adverse grid conditions. Unlike the conventional observer-based PLL (O-PLL) approach, the proposed improved Lyapunov theory-based prefilter (ILP) is helpful in yielding a quadrature signal to solve the single-phase grid synchronization problem. Moreover, the proposed prefilter can leverage delayed signal operation, which improves the harmonic and the DC-offset component rejection abilities while eliminating the need for internal feedback-based submodule blocks for the case of an O-PLL. Consequently, the proposed ILP-PLL exhibits better dynamic behavior to rapidly synchronize a grid-tied power converter and can accurately track the fundamental amplitude information that is required for inverter control to meet the fault ride-through requirements. In addition, the suggested LVRT controller ensures smooth transition between the unity and non-unity power factor modes for superior converter control over reactive current injection into the grid to recover the grid from faults while maintaining a lower amount of total harmonic current distortions. The dynamic performance of the proposed control scheme is experimentally validated in view of the existing O-PLL approach for lower-rating wind-turbine-based PMSG-WPCS.

A synchronization technique for single-phase grid applications

The utility grid disturbances like dc offset and harmonic components can severely affect the estimated variables from the phase-locked loop (PLL), resulting in poor performance of the system relying on it. Therefore, there is an emerging need for well-designed PLL algorithms ensuring robust response against different operating conditions. This paper proposes a simple single-phase PLL algorithm with inherent dc offset and specific harmonic orders rejection capability. Utilizing adaptive time-delay fictitious signal generation. A full mathematical model of the proposed PLL has been provided. The proposed PLL is compared with other filter-based single-phase PLLs, to validate its simplicity and excellent performance.

Fault location in series-compensated transmission lines using adaptive network-based fuzzy inference system

In this paper a new and accurate method for fault location in series-compensated transmission lines (SCTLs) is presented using adaptive network-based fuzzy inference system (ANFIS). In the proposed approach, the desired features are obtained using the Least-Squares Estimation (LSE) from two cycles of voltage and current data in only one line terminal. After that, extracted features, including the decaying DC offset component of the current, the fundamental components of current and voltage, and the phase difference between current and voltage for three-phase current and voltage signals, are normalized and applied as inputs to the fuzzy neural network for fault location. The subtractive clustering technique has been utilized to design the basic ANFIS. This study has scrutinized different cases, including two different ANFIS arrangements, implementing a single network to perform fault location on the entire line and a separate network for each line segment. Moreover, the efficiency of feature selection has been analyzed using the RReliefF algorithm. Various training patterns and tests have been provided in a test transmission system for different system conditions, such as different fault inception angles, fault locations, fault resistances, and structural conditions. The results validate that the proposed approach has accurately located the fault under a wide range of system variations.

A Robust Half-Cycle Single-Phase Prefiltered Open-Loop Frequency Estimator for Fast Tracking of Amplitude and Phase

This article presents a fast phase-locking and amplitude detection scheme for single-phase applications. The proposed scheme employs a pseudo-rectification process in order to generate even harmonics from the grid signal. The dc-offset component present in the grid signal increases the sensitivity of the pseudo-rectification process, and the design of the prefiltering stage must, therefore, be completely focused on the rejection of dc-offset and the even-harmonics. The application of a half-cycle (HC) prefilter would consequently appear to be a highly attractive option as regards improving the dynamic response time and even harmonic rejection abilities. However, the elimination of the dc-offset component and the estimation of phase and amplitude information is still a very challenging task. The aforementioned issues are addressed by employing an HC comb filter and an HC nonadaptive Lyapunov orthogonal signal generator. The estimation of single-phase grid parameters is thereby made simpler by employing an improved open-loop frequency estimation, which is capable of providing accurate frequency deviation information with good precision. The research shown herein also reveals that the estimation of the amplitude and phase information differs slightly from the usual mathematical notations owing to the pseudo-rectification method adopted in the proposed work. Note that the prefiltering stage is nonadaptive in nature, which may lead to steady-state errors in the amplitude and phase information under off-nominal frequency conditions. An online mathematical correction approach dependent on frequency deviation is, therefore, employed in order to obtain the error-free amplitude and phase information. Finally, the robustness of the current proposal is validated by experimentally comparing several known grid parameter estimation schemes using a real-time controller.

A novel phase estimation method for renewable energy grid-connected equipments

When renewable energy is applied to the power grid, it will cause problems such as harmonic interference and changes in grid frequency, resulting in harmonic distortion of the inverter and other renewable energy grid-connected equipments output current. So the grid-connected environment of weak grid puts forward higher requirements on the performance of phase-locked loop (PLL). Therefore, these renewable energy grid-connected equipments require Phase-Locked Loop (PLL) with higher technical performance. Aiming at the control requirements of inverter and other grid-connected equipments in weak grid, an optimization method based on a novel PLL is proposed. The method uses a dual enhanced novel cascade second-order generalized integrator (DENCSOGI) to simultaneously filter out the fundamental voltage negative sequence (FVNS) component and the DC offset (DCO) voltage component, and it can speed up the dynamic response and phase-locked accuracy of the PLL. The proposed PLL is verified by the simulation experiment platform. The performance of the approach is excellent and the grid-connected power quality of the renewable energy grid-connected equipments is guaranteed.

A New Method for Analysis and Estimation of Microgrid Signals Under Fault Conditions

Accurate phasor estimation under various faulty conditions is one of the most critical issues in the monitoring, protecting, and controlling of a microgrid (MG). This paper presents a new phasor estimation algorithm (PEA) under fault conditions for MG applications. The main features of this method are to accurately phasor estimation of the signal combination of a decaying dc offset, a decaying fundamental frequency component, and harmonics. In this strategy, a Taylor series is replaced by the exponential functions of the fundament frequency and decaying dc offset component. Then, the time constants and the magnitudes of decaying components are estimated by the least square method. The proposed method is explained theoretically, and its performance is demonstrated by simulating various numerical signals in MATLAB and measured signals during fault conditions. The simulation results indicate that the proposed method estimates the dynamic phasor during fault conditions, accurately. Furthermore, a comprehensive comparison with the conventional methods is carried out to show the effectiveness of the proposed algorithm.

A Simple Approach to Reject DC Offset for Single-Phase Synchronous Reference Frame PLL in Grid-Tied Converters

Synchronous reference frame phase-locked loop (SRF-PLL) is widely used for grid voltage synchronization in single-phase grid-connected power converters. However, in the actual situation, dc offset component may be introduced in the input of the PLL, due to the transient fault of the power grid, sampling and measurement error, or A/D signal processing. A simple yet effective approach with additional all-pass filter based dc rejecter is presented for SRF-PLL, in this paper. Thereby, correct estimation and undesirable periodic ripple free can be achieved in SRF-PLL, when the input signal contains dc offset. The second order generalized integrator based PLL (SOGI-PLL) is first introduced, followed by the analysis on influence of the input dc offsets in SRF-PLL. The structure of enhanced-SOGI (ESOGI) with its analysis of dc offset rejection effects and performance have been then formulated in detail. Finally, experimental results are presented to demonstrate the effectiveness of the proposed ESOGI based PLL.

A Rotor Flux Observer of Permanent Magnet Synchronous Motors With Adaptive Flux Compensation

Due to the influence of current sampling and current reconstruction deviation, non-linearity of the inverters and parameter variation of the motors, DC offset component and high-order harmonic components exist in flux model, which makes the estimation of traditional rotor flux observer inaccurate and even divergent. In this paper, a rotor flux observer of permanent magnet synchronous motors (PMSM) with adaptive flux compensation is proposed. The DC offset component and the high-order harmonic components with the flux integrator observer can be eliminated by fed them back to the input of the flux integrator through PI regulator, so that the output of the flux integrator observer only contains the fundamental component, thus the rotor flux can be accurately estimated. Since the parameters of PI regulator are related with the estimated motor angular velocity, the control system has good robustness in a wide speed range. The flux observer is based on the voltage model in stationary coordinate system, so it is suitable for both surface and interior PMSMs. Moreover, the control method of the whole system has less computational complexity and fewer variables, it is easy to implement on a low-cost hardware platform. Finally, the experimental results verify that the flux observer has superior performance.

Islanded distributed generation system with feedforward PR‐EFLL control for AC‐electric vehicle charger

This study deals with the islanded distributed generation system (IDGS), which is used to feed different types of loads such as AC-EVs (electric vehicles) and non-linear home load. The IDGS consists of solar photovoltaic (PV) array with a storage battery. In IDGS feeding, the EVs and non-linear load, the currents contain high harmonics. Due to which, the generation of sinusoidal voltage with good quality at common coupling point (CCP) is very challenging. Therefore, a feedforward proportional resonant (PR) controller is proposed with enhanced frequency locked loop (EFLL) control algorithm to eliminate the CCP voltage harmonics and provides the high tracking capability of AC voltage as compared to the conventional proportional integral (PI) control. As a result, the voltage source converter control generates the balanced and sinusoidal CCP voltages even at unbalanced load condition. The proposed EFLL-based control algorithm is capable to reject the DC-offset component from the load current. Moreover, the DC-link voltage regulation and peak power extraction of solar-PV array are achieved through bidirectional DC-DC converter control. This BDC control is used to balance the active power flow in the islanded system. The effectiveness of the proposed IDGS is validated, through simulation as well as experimental steady-state and dynamic test results.

A Global Convergence Estimator of Grid Voltage Parameters for More Electric Aircraft

Phase-locked loops (PLLs) are widely popular in applications of grid synchronization for single- and three-phase utility power systems. However, these PLL-based methods are usually designed for constant frequency grids and these schemes only have fast dynamics and good accuracy in a small local region (i.e., phase-locked region). When applying these PLLs in the grids of more electric aircraft (MEA), in which the grid frequency varies in a wide range (360–800 Hz), both the dynamic response and the steady-state accuracy of PLLs reduce significantly. In light of this, this article presents a global convergence method to estimate the frequency, positive- and negative-sequence components of grid voltage in MEA power systems. By analyzing the relationship between the DC-offset component, the grid voltage, and their derivatives, a mathematic model with the unknown parameters, which are the DC-offset component and the grid frequency, is established. Accordingly, a global estimator is proposed to estimate the grid voltage parameters accurately. A Lyaponov-based argument ensures that the proposed estimator is global convergent and it has the capability to eliminate the DC offset effect completely. The experimental results are provided to verify the effectiveness of the proposed estimator.

Improved Reweighted Zero-Attracting Quaternion-Valued LMS Algorithm for Islanded Distributed Generation System at IM Load

Sudden start of an IM load and frequent change of nonlinear load in a standalone distributed generation system (DGS) cause the dip in ac voltage and frequency. Moreover, these loads distort DGS currents. Therefore, in this paper, an improved-reweighted zero-attracting quaternion-valued least mean square (I-RZA-LMS)-algorithm-based voltage source converter (VSG) control is proposed for voltage and frequency regulation, and power quality improvement in DGS. Moreover, the control algorithm of dc–dc bidirectional converter (BDC) is used for dc link voltage regulation and MPPT of the solar photovoltaic array at IM starting and nonlinear loading. In this DGS, the I-RZA-LMS-based control algorithm estimates the active and reactive component currents of distorted load currents for effective harmonics mitigation, reactive power compensation, and point of common coupling voltages regulation. The proposed control algorithm rejects the dc-offset component from load currents and gives the fundamental load component. The dc link voltage of VSC is regulated using BDC with a solar feedforward term, which is used to enhance the capability of dc link voltage control under solar power variation, IM starting, and unbalanced loading. The effectiveness of the proposed DGS is validated through the simulation as well as experimental results obtained on a developed prototype of DGS.

Adaptive Phasor Estimation Algorithm Based on a Least Squares Method

This paper proposes an adaptive phasor estimation algorithm based on a least square method that can suppress the adverse effect of an exponentially decreasing DC offset component in a phasor estimation process. The proposed algorithm is composed of three stages: a basic least squares model, a time constant calculation, and an adaptive least squares model. First, we use the basic least squares model to estimate the parameter of the DC offset component in the fault current signal. This model is designed to incorporate fundamental frequency, and harmonic and constant components. Second, we use the estimated parameter to calculate the time constant of the DC offset component. Third, we redesign a least squares model that incorporates fundamental frequency, harmonic components, and exponential function of the DC offset component. Since this model incorporates the exponential function of the DC offset component contained in the fault current signal, it estimates the phasor of the correct fundamental frequency component without influence of the DC offset component. We evaluated the performance of the proposed algorithm using computer generated signals and EMTP simulation signals. The evaluation results show that the proposed algorithm can effectively suppress the adverse influence of the exponentially decaying DC offset component.

Assessment of a Decaying DC Offset Detector on CTs Measurements Applying Mathematical Morphology

Power system protections are highly sensitive to decaying dc offset that could appear when a fault occurs. The consequences of this effect represent a challenge in the coordination of protection devices that have been widely studied. The elimination of the dc offset component of current transformers (CTs) measurements is the objective of this paper, including a new autonomous detector based on mathematical morphology. The idea enhances the digital signal processing to prepare the information before the protective device takes a decision. The detection and fast elimination of the decaying dc offset on current signals could improve the speed of protection, coordination, and selectivity. The paper presents the proposed algorithm and assesses the solution using alternative transient program. The model of the CT used for this validation is also presented. Results show the potential of the mathematical morphology for signal processing to delete the dc offset component.

A Multiple Improved Notch Filter-Based Control for a Single-Stage PV System Tied to a Weak Grid

In this paper, a control scheme based on improvement in a generalized integrator is implemented on a three-phase single-stage grid-tied solar photovoltaic system with the distribution static compensator capabilities under grid abnormal conditions of voltage distortion and voltage unbalance. The photovoltaic voltage-source converter system compensating for the reactive power consumed by nonlinear load at point of common coupling provides load balancing and mitigates harmonics. The proposed multiple-improved-notch-filter-based quadrature signal generator control approach extracts the load current fundamental component, independent of the grid voltage. This control has better dc offset and harmonics component rejection capability in comparison to a conventional second-order generalized integrator algorithm. The perturb and observe-based maximum power point tracking algorithm is applied for the extraction of maximum power from the photovoltaic array. The system is analyzed under different abnormal conditions of voltage distortions, voltages unbalance, voltage swell, voltage sag, load currents unbalance, and insolation change on a prototype developed in the laboratory. The system performance is found to be satisfactory, within limits as described in an IEEE-519 standard while feeding active power to distribution network and connected loads.

Coordinated control of distributed generation system with improved generalised enhanced phase locked loop control algorithm

Here, a distributed generation system (DGS) is proposed that allows the different distributed energy resources to operate in coordination with others to provide the electricity in rural areas. Two individual controls are used in the DGS: (i) solar maximum power point tracking (MPPT) and the battery protection control, and (ii) voltage source converter (VSC) control. The proposed VSC control algorithm is based on an improved generalised filter algorithm for power quality improvement of the DGS. This control strategy effectively regulates the active power flow and provides the harmonics mitigation, reactive power compensation, and load balancing in the DGS with low steady-state error and fast dynamic response under high impulsive noise with DC-offset component. The solar MPPT control is designed on the basis of battery state of charge (SOC). In this control, the solar PV array operates in limited power point tracking (LPPT) after the storage battery surpasses the maximum SOC limit. In LPPT control, the solar PV MPPT operates in the voltage source operating region of PV array characteristic, thereby, reducing the power generation and avoids the overcharging of the storage battery. The proposed DGS performance is validated through test results under dynamics and steady-state conditions.

Combining Mathematical Morphology and Unscented Kalman Filter for Digital Protective Relaying

The paper presents a digital filter algorithm, combining mathematical morphology (MM) and unscented Kaman filter (UKF), for microprocessor-based protective relaying. Firstly, the state and observation equations of UKF are designed based on the state variables of DC-offset component, fundamental angular frequency, and fundamental component of the fault signal. Approximation signals and detail signals are derived from the multi-scale morphological decomposition of the sampling signal. The approximation signals are employed to update the observations of UKF to improve the convergence speed of the filter algorithm, and the detail signals are applied to calculate the measurement noise variance to improve the convergence precision of the filter. The proposed MM-UKF algorithm is evaluated on the data collected from simulation model built in Matlab/Simulink.

An Efficient Hybrid Filter-Based Phase-Locked Loop under Adverse Grid Conditions

Synchronous-reference-frame phase-locked loop (SRF-PLL) is widely used in grid synchronization applications. However, under unbalanced, distorted and DC offset mixed grid conditions, its performance tends to worsen. In order to improve the filtering capability of SRF-PLL, a modified three-order generalized integrator (MTOGI) with DC offset rejection capability based on conventional three order generalized integrator (TOGI) and an enhanced delayed signal cancellation (EDSC) are proposed, then dual modified TOGI (DMTOGI) filtering stage is designed and incorporated into the SRF-PLL control loop with EDSC to form a new hybrid filter-based PLL. The proposed PLL can reject the fundamental frequency negative sequence (FFNS) component, DC offset component, and the rest of harmonic components in SRF-PLL input three-phase voltages at the same time with a simple complexity. The proposed PLL in this paper has a faster transient response due to the EDSC reducing the number of DSC operators. A small-signal model of the proposed PLL is derived. The stability is analyzed and parameter design guidelines are given. Experimental results are included to validate the effectiveness and robustness of the proposed PLL.

A Novel Hybrid Filter-Based PLL to Eliminate Effect of Input Harmonics and DC Offset

Synchronous reference frame phase locked loop (SRF-PLL) is widely used for grid synchronization and control in grid-connected applications. A major problem with SRF-PLL is how to improve its dynamic performance and filtering capability under complex grid conditions such as unbalance, distortion, and dc offset mixing without affecting its phase tracking performance and stability. In order to achieve this goal, a novel second-order generalized integrator (NSOGI) and a modified NSOGI (MNSOGI) with dc offset rejection capability based on NSOGI are proposed, and then, an effective hybrid filter is designed and incorporated into the inner loop of a SRF-PLL. The proposed hybrid filter is a combination of a dual MNSOGI and a moving average filter (MAF), it can block the fundamental frequency negative sequence component, dc offset component, and the rest of harmonic components in the SRF-PLL input three-phase voltages at the same time with a simple complexity. The proposed PLL in this paper has a faster transient response due to reducing the window length of the MAF. A small-signal model of the proposed PLL is derived. The stability is analyzed, and parameters design guidelines are given. The effectiveness of the proposed PLL is confirmed through simulation experimental results.

On-line parametric estimation of damped multiple frequency oscillations

Undesirable harmonic pollution is currently a reality in many electric power systems. Damped transients generated by the growing use of electronically controlled devices are an important source of harmonic distortion of voltage and current waveforms. Accurate real-time estimation of harmonics parameters during the on-line operation of some electric power system is thus an active and pertinent challenging research issue. In this paper a novel on-line algebraic parametric estimation method in time domain, for damped harmonic components on electric power system signals is introduced. Algebraic formulas to directly compute amplitude, damping, frequency and phase parameters and dc offset component into a small time window for variable multi-frequency oscillations are proposed. Parameter estimators enable the on-line and direct reconstruction in finite time of harmonic components and deviations of some measurable signal. Experimental results confirm the effectiveness of the algebraic parametric estimation on uncertain multi-frequency oscillating transient signals.