Frequency Components Of Current Research Articles

Improvement of the Source Current Quality for a Shunt Active Power Filter Operating Using Hysteresis Technique with Stabilized Switching Frequency

Determining the current reference for a shunt active power filter (SAPF) can be carried out in many ways. Once the reference is determined, it can be shaped by SAPF switches with the use of pulse width modulation (PWM)/hysteresis control techniques. There are many variants of shaping the compensation waveform using these techniques. Nevertheless, regardless of the PWM/hysteresis technique adopted, a switching frequency current component appears in the system. It acts as a carrier used to inject a compensating current into the grid. Once the compensating current has been entered into the grid, the switching component should be reduced in it. This can be performed using RLC passive filters in various variants. The article discusses a variable/stabilized frequency hysteresis current control technique adapted for SAPF regulation with the use of current closed-loop control (source current direct control). For this technique, the passive filter should be placed outside the current control loop. The article focuses on examining the effectiveness of the interaction of the RLC filter with SAPF acting with such a control technique.

Noise Reduction Method for Fiber Optical Current Transformer Based on Mode Decomposition-Sample Entropy

When a fiber optic current transformer (FOCT) malfunctions, its output signal is usually accompanied by a large noise, which affects the subsequent fault diagnosis results of the transformer. In response to the above noise issues, this paper constructs a new noise reduction algorithm by combining Complete Ensemble Empirical Mode Decomposition Adaptive Noise (CEEMDAN) and Sample Entropy (SE). Above all, the output signal of FOCT is decomposed into multiple fault components using the CEEMDAN algorithm. Then, by comprehensively considering the frequency domain characteristics and sample entropy calculation value of each component, the characteristics of each component are determined, so the power frequency and low-frequency components are selected as the feature vectors representing the fault mode. Finally, the noise components with high sample entropy are eliminated, and the power frequency current components with medium sample entropy and the low-frequency fault feature components with low sample entropy are recombined and superposed to build a fault signal dataset after noise reduction.

Adaptive Control Approach-Based Isolated Microgrid System with Alleviating Power Quality Problems

In this article, a dual third-order generalized integrator-based phase-locked loop (DTOGI-PLL) and an additional degree of freedom-based frequency locked loop (ADF-FLL) control approaches are utilized on a voltage source converter (VSC) to control an isolated microgrid system under different operating conditions. The proposed adaptive approaches provide a fast dynamic response with a precise estimation of frequency, phase angle, and fundamental load current component compared to that of the TOGI-PLL and least mean square (LMS) algorithms. An ADF-FLL allows closed-loop poles to perform more optimally, resulting in faster dynamic performance without compromising filtering capability. Moreover, the proposed adaptive approaches with VSC provide power support, load leveling, harmonic eliminations, and enhance the isolated microgrid’s overall power quality during dynamics in load and energy sources. A solar PV (SPV) array with perturb & observe (P&O) MPPT control is interfaced at the DC link of a bidirectional controller (BDC) to draw maximum solar power. Through dc-link voltage control, the BDC with energy storage system (ESS) ensures the appropriate power flow and power equilibrium between energy sources and load. The Simulation and test results show the dynamic performance of the isolated microgrid is examined under different operational scenarios.

A method for determining arc breakdown in low-voltage electrical networks

In low voltage distribution networks, there is a high risk of fires caused by wiring faults that lead to short circuits. Arcing protection devices (ARCs) already exist, but experiments show that they also often do not disable such faults. The operation of most existing arc fault detection devices is based on the detection of high frequency current components and their comparison with load profiles. For the fast arc fault recognition low-frequency oscillations can be assessed, which serve as a marker of the arc fault presence in a LV distribution network. This method of arc fault recognition in LV networks is easier to implement practically. Using the windowed Fourier transform or Wavelet analysis, it is possible to determine the change in time of the harmonic components of the current, which will be a characteristic sign of the presence of an arc fault along with other signs.

A Power Circulation Method Using Two Frequencies in Motor Emulator System

When an inverter is tested with a motor emulator, most of the power circulates in a loop consisting of inverter, emulator, and DC-DC converter. Only the device losses are compensated from the grid, so they usually do not exceed 20% of the circulating power. Most commonly, an isolated bi-directional DC-DC converter is used between the back ends of motor emulator and inverter. In this work, the bi-directional DC-DC converter is replaced by a three phase high frequency system between two PWM (AC-DC) converters. Furthermore, two frequency current components are used independently based on two different reference frames: the 60 Hz is utilized for power trading with the grid, whereas 2 kHz is employed for power circulation. The high frequency link is especially beneficial, since it reduces the size of isolation transformer drastically. The effectiveness of the control algorithm is supported by computer simulation and experimental results.

МULTIFREQUENCY PROTECTING METHOD AGAINST EARTH-FAULTS OF PHASE IN THE COMPENSATED ELECTRIC NETWORKS

Introduction. A significant proportion of earth faults in 6 - 35 kV networks is a transient and short-lived process, which is followed by an electric arc. Problem. In such cases, earth-fault protection that responds to steady-state current and voltage is not able to operate properly. Also, the use of the Petersen coil to compensate for the capacitive earth fault current complicates the protection function because it significantly reduces the single phase earth fault current in steady state. Purpose. To develop selective single-phase earth faults protection algorithm using harmonic components that occur in zero-sequence currents and voltage in the transient process. Method. A mathematical model of the power supply system is applied to study the frequency components of currents and voltage of zero sequence in compensated electrical networks with phase-to-earth faults, and a mathematical model is used to test the operation of the developed protection algorithm. The results showed that, the reactive power for harmonic components of the frequency greater than 100 Hz, which are separated from the current and voltage of zero sequence in compensated electrical networks on the damaged feeder, is positive regardless of the degree of compensation of the capacitive current. That may be the basis of the principle of directional protection. Originality. Phase-to-earth fault selective protection algorithm has been developed. In that algorithm, first derivatives of currents and voltages of zero sequence are found, to reduce the influence of aperiodic components. And then, by using of the Fourier transform, a number of harmonic orthogonal components are extracted from them. Reactive power is calculated for each of frequency component and their total sum is found. If that sum excess of threshold, the relay will make a decision. The reliability of the developed protection algorithm is confirmed by the results of mathematical modeling and verification of the test sample at the laboratory stand and by means of field signals that were recorded by digital loggers at the substations.

Analytical compensation of harmonics caused by 60° flat‐top modulation

The 60° Flat-Top PWM modulation method (also called DPWM1, two-phase, or 60° bus-clamp) is widely used by the industry to reduce switching losses in three-phase inverters by connecting one of the three-phase legs to the positive or the negative DC rail continuously for 60°of the fundamental period. It is widely known that - compared to the most used SVM modulation - the Flat-Top modulation increases switching frequency current components. The actual amount is highly dependent on the modulation index. It is a lesser known fact that - at each switch-over happening at every 60°, because of the change of the phase in the switching frequency current component - low-order harmonics are produced as well. The effect of these can become especially significant, if one of the produced frequencies falls near to the resonant frequency of the LC or LCL filter. A modification of the original Flat-Top method has been developed, that corrects the duty ratios around the edges of the zero-sequence voltage signal. This way the filtered output voltage remains sinusoidal, while keeping the efficiency advantage of the Flat-Top. Experimental results on two different inverters show that the novel method significantly decreases the current THD.

Differential Frequency Protection Scheme Based on Off-Nominal Frequency Injections for Inverter-Based Islanded Microgrids

Protection is considered one of the major challenges associated with the operation of islanded microgrids with high penetration of inverter-based distributed generation (IBDG). As stated in the IEEE Std. 1547.4, due to the limited injected fault current levels by IBDG, conventional overcurrent relays might not be capable of detecting and clearing faults. Thus, this paper proposes a novel protection strategy for inverter-based islanded microgrids which relies on injecting an off-nominal frequency, through the DG interface, during fault conditions. A differential protection scheme that is based on the differences in the current frequency components, measured at both ends of a feeder section, is proposed to detect and isolate faults. This paper also presents the structure and control logic of the proposed relay for differential protection. The proposed protection scheme is implemented and tested on a distribution system equipped with IBDG, modeled to operate in an islanded mode. Simulation results are presented to highlight the effectiveness of the proposed protection scheme in detecting and isolating faults for inverter-based islanded microgrids.

Повышение точности одностороннего определения места «_» *_» 1 повреждения в электрической сети произвольной конфигурации

For clearing faults in power lines, the fault location procedure (FLP) is carried out. Aunilateral FLP based on the power frequency current and voltage components is considered. The existing calculation expressions for carrying out power line FLP are often applicable only for partial network configuration cases, and attempts to apply them for other cases result yield results containing a methodical error. A general fault location algorithm in which the superposition principle is used has been developed. A technique for compensating the effect of contact impedance based on the superposition principle is considered. A calculation expression for unilateral fault location written in the phase coordinates for an arbitrarily configured electric network is derived. The results from simulation experiments and calculations carried out using real time histories of emergency events have shown that the proposed method ensures high accuracy. Recommendation sonusing the developed method are given.

Mathematical Modeling of Arc Faults in Networks with Low Single Phase-to-Ground Fault Currents

The article comprises the results of the research defining open-flame arcs self-quenching conditions in the event of a single phase-to-ground fault in overhead medium-voltage distribution networks according to existing theories of arc extinguishing. The calculations included metallic and arc faults modeling in a network with low phase-to-ground fault current. The arc gap simulation based on the mathematical channel model of a cylindrically symmetrical upright arc stabilized by rising convective gas flow was carried out in ATPDraw software program. The single phase-to-ground arc fault calculations results indicated an increase in high-frequency currents’ attenuation rate during transient processes as well as a reduce reduction in the electric arc lifetime from 8 ms to 2 ms in case of the breakdown voltage decrease from the peak value to zero. Notably, in case of low single phase-to-ground fault current the arc extinguishing took place at the first high-frequency current zero. For the cases of nonzero breakdown voltages, the electric arc extinguishing was detected at the fundamental frequency current component zero-crossing instant. The maximum overvoltage ratio of K = 2.8 was obtained as athe result of the single phase-to-ground fault at the peak phase voltage.

A Novel Fault Detection and Location Method for PV Arrays Based on Frequency Analysis

This paper presents a fault detection and location method for photovoltaic (PV) arrays. One novelty of the method is the use of the different amplitude changes of the alternating current frequency component of the PV array output voltage. Another novelty is the technique used to generate the PV array output voltage. In this technique, the entire PV array is operated under dark conditions with red pulsed light only applied on one module of the PV array. Under these conditions, the presence of an open circuit fault and the variability in module impedances generate different increases in the total dynamic impedances of the PV array. As a consequence of these increases, the spectral component of the alternating current array output voltage also undergoes different increases, which facilitates fault detection. The discrete Fourier transform and statistical techniques are used to feature extraction and classification indication. The proposed method takes into account reliability and detection time and does not require the use of expensive or specialized equipment or the disconnection of or any modification to the PV array, which reduces the cost of the system. The earlier mentioned is achieved with the use of a pulsed light, which inputs a forward bias voltage into the array. The results of both the simulation and the experiment are presented in order to demonstrate the 100% effectiveness of the proposed method under different temperatures and PV array sizes.

Impact of an inverter‐based DG on a double‐ended fault location method

This paper describes the influence of Distributed Generation (DG) on a double ended fault location based on measuring the high frequency fault transients. The additional non-fundamental frequency current components from DG will influence the accuracy of an impedance based fault location technique based on non-fundamental frequencies. A double-ended impedance based fault location technique that utilizes the high frequency content (up to 5 kHz) is studied. The study showed that double-ended method is still able to locate a fault with a maximum error of 4% compared to the case without DG which showed a percentage error up to 2%.

Numerical study of the discharge properties of atmospheric dielectric barrier discharge by using 200 kHz/13.56 MHz excitations

In this work involving an atmospheric dielectric barrier discharge system, the feasibility of independence control of key plasma parameters through strategic modulation of applied dual-frequency excitation sources is demonstrated. In this paper, a one-dimensional fluid model with semi-kinetic method has been used to investigate the discharge properties of atmospheric dielectric barrier discharge by using 200 kHz/13.56 MHz excitations. Bearing good consistency and coherence with experimental results, the electrical characteristics and typical electron dynamics are studied numerically. It is revealed that the application of the high frequency excitation can restrain the increment of the low frequency current component and is effective in preventing the discharge from transitioning to a filamentary mode. This method then suggests and enables possible approaches to obtain discharges with good stability in described DBD systems.

A Current Frequency Component-Based Fault-Location Method for Voltage-Source Converter-Based High-Voltage Direct Current (VSC-HVDC) Cables Using the S Transform

This paper proposes a fault-location method for voltage-source converter (VSC)-based high-voltage direct current (VSC-HVDC) systems. This method relies on the current frequency components generated by faults in the cable, and requires the arrival time of the frequency components at two terminals. The S transform is a time–frequency analysis tool that is superior to the wavelet transform in some respects. Therefore, the S transform was employed to determine the arrival time in this paper. To obtain a reliable criterion, a novel phase-mode transform method for bipolar cables was developed, and the propagation characteristics of the current frequency components through out the cable were analyzed. A two-terminal VSC-HVDC system was modeled in power system computer aided design/electromagnetic transients including DC (PSCAD/EMTDC). Various faults under different conditions were simulated on the basis of this model, and the simulation results verified a high accuracy, robustness against fault-resistance, and noise immunity of the proposed method.

Low cost microcontroller based fault detector, classifier, zone identifier and locator for transmission lines using wavelet transform and artificial neural network: A hardware co-simulation approach

A low cost, fast and reliable microcontroller based protection scheme using wavelet transform and artificial neural network has been proposed and its effectiveness evaluated in real time. The proposed scheme, based on the hardware co-simulation approach performs all the functions of transmission line protection i.e. fault detection/classification, fault zone/section identification and location estimation. The fault detection/classification and zone identification algorithms use fundamental frequency current component to estimate a fault index. The fault location estimation module uses wavelet transform coefficients in hybridization with a parallel artificial neural network structure. For hardware implementation, a 8-bit ATmega microcontroller is used and interfaced with the simulated power system model using Integrated Development Environment (IDE). The scheme is tested on a power system model of 400kV, 50Hz three phase double circuit line with source at both the ends. Laboratory tests have been performed in real time for 20,000 fault cases including evolving faults with varying fault resistance, fault inception angle, fault distance, direction of power flow angle and its magnitude. The tests confirm the suitability and reliability of proposed scheme even with Current Transformer (CT) saturation. The implementation of the proposed approach on a low cost microcontroller with the lesser execution time, makes the prototype ideal for implementation on a digital platform (digital relay), thus leading to financial viability and sustainability of the protection scheme.

Single-Phase Grid-Connected LiFePO$_{\bf 4}$ Battery–Supercapacitor Hybrid Energy Storage System With Interleaved Boost Inverter

AC line integrated energy storage systems are attractive as they increase the system efficiency by reducing the number of required power processing stages. In this paper, operation of a recently proposed battery–supercapacitor hybrid energy storage system (HESS) comprising two DC/AC boost converters, battery, supercapacitors, grid connection, state of charge (SOC) estimation, and associated control systems is experimentally verified and further improved. The improvement is achieved by a phase-shifted interleaved operation of the boost converters. The proposed phase-shifted interleaved operation reduces the switching frequency current ripple component in both the battery and supercapacitor currents. Experimental results show that during the interleaved operation, the HESS operates as expected and allocates all fast current variations to the supercapacitor, while the battery responds to slow varying current demands. At the same time, the control system maintains the supercapacitor voltage at around a predefined value and the battery's SOC, estimated using an extended Kalman filter, is maintained within the specified SOC limits.

Applicability improvement and hysteresis current control method simplification in shunt active filters

Hysteresis current control method is widely used in Pulse Width Modulation (PWM) inverters because of simplicity in performance, fast control response and good ability in limiting peak current. However, switching frequency in hysteresis current control method with fixed bandwidth has large variation during a cycle and therefore causes nonoptimal current ripple generation in output current. One of basic problems in implementing hysteresis current control is its variable switching frequency that causes sound noise and increase in inverter losses and also high frequency current components injection to the source current. In this paper, a fixed frequency hysteresis current controller is proposed for grid connected inverters. In the proposed approach, the required bandwidth of the controller is calculated according to system parameters. Also, the derivative of the reference current is eliminated to solve the problems associated with practical implementation of the converter. Finally, a shunt active filter has been used for removing the current harmonic components generated by non-linear loads. Proposed method is simple to perform and reliable, and also has been simulated in MATLAB software environment.

Energy Storage System Based on Cascaded Multilevel Inverter with Decoupled Energy Balancing Control

This paper presents a three phase cascaded multilevel inverter based supercapacitor (SC) energy storage system with novel structure and control strategy to maintain the energy balance of between phases. Every two phases are coupled with a series LC filter. With the filter, SC cells in different phases could exchange energy with an auxiliary power flow at high frequency. The auxiliary power flow is orthogonal to the primary power flow. The phase difference between high frequency voltage and current components of each phase determines whether the energy is absorbed into or released from its SC cells. Unlike traditional energy balancing strategies, the proposed method is independent to the fundamental real power drawn by the energy storage system. Simulation results confirmed the effects of proposed theories.

Transformer Model in Wide Frequency Bandwidth for Power Electronics Systems

The development of the smart grids leads to new challenges on the power electronics equipment and power transformers. The use of power electronic transformer presents several advantages, but new problems related with the application of high frequency voltage and current components come across. Thus, an accurate knowledge of the transformer behavior in a wide frequency range is mandatory. A novel modeling procedure to relate the transformer physical behavior and its frequency response by means of electrical parameters is presented. Its usability is demonstrated by an example where a power transformer is used as filter and voltage reducer in an AC-DC-AC converter.

An analysis of the relationships between subthreshold electrical properties and excitability in skeletal muscle

Skeletal muscle activation requires action potential (AP) initiation followed by its sarcolemmal propagation and tubular excitation to trigger Ca2+ release and contraction. Recent studies demonstrate that ion channels underlying the resting membrane conductance (GM) of fast-twitch mammalian muscle fibers are highly regulated during muscle activity. Thus, onset of activity reduces GM, whereas prolonged activity can markedly elevate GM. Although these observations implicate GM regulation in control of muscle excitability, classical theoretical studies in un-myelinated axons predict little influence of GM on membrane excitability. However, surface membrane morphologies differ markedly between un-myelinated axons and muscle fibers, predominantly because of the tubular (t)-system of muscle fibers. This study develops a linear circuit model of mammalian muscle fiber and uses this to assess the role of subthreshold electrical properties, including GM changes during muscle activity, for AP initiation, AP propagation, and t-system excitation. Experimental observations of frequency-dependent length constant and membrane-phase properties in fast-twitch rat fibers could only be replicated by models that included t-system luminal resistances. Having quantified these resistances, the resulting models showed enhanced conduction velocity of passive current flow also implicating elevated AP propagation velocity. Furthermore, the resistances filter passive currents such that higher frequency current components would determine sarcolemma AP conduction velocity, whereas lower frequency components excite t-system APs. Because GM modulation affects only the low-frequency membrane impedance, the GM changes in active muscle would predominantly affect neuromuscular transmission and low-frequency t-system excitation while exerting little influence on the high-frequency process of sarcolemmal AP propagation. This physiological role of GM regulation was increased by high Cl− permeability, as in muscle endplate regions, and by increased extracellular [K+], as observed in working muscle. Thus, reduced GM at the onset of exercise would enhance t-system excitation and neuromuscular transmission, whereas elevated GM after sustained activity would inhibit these processes and thereby accentuate muscle fatigue.