DC Current Component Research Articles

English

Solar energy technologies have gained much importance in the recent scenario due to their ability to produce clean, reliable, useful power. Grid connected Photovoltaic system requires conversion from DC to AC to harness the useful energy produced. A Photovoltaic inverter directly connected to the grid can cause, besides the generation of several current harmonics, a DC current component injection. Excessive DC current injection into the AC network can result in problems such as increased corrosion in underground equipment and transformer saturation. The paper aims at evaluating the output DC-current injection in grid connected inverter used for a 100kW solar power plant installed at Amal Jyothi College of Engineering, Koovapally, through experimental analysis. A simulation based study on output DC current injections in inverters with two different multilevel topologies is also conducted.

A Single Phase Doubly Grounded Semi-Z-Source Inverter for Photovoltaic (PV) Systems with Maximum Power Point Tracking (MPPT)

In this paper, a single phase doubly grounded semi-Z-source inverter with maximum power point tracking (MPPT) is proposed for photovoltaic (PV) systems. This proposed system utilizes a single-ended primary inductor (SEPIC) converter as DC-DC converter to implement the MPPT algorithm for tracking the maximum power from a PV array and a single phase semi-Z-source inverter for integrating the PV with AC power utilities. The MPPT controller utilizes a fast-converging algorithm to track the maximum power point (MPP) and the semi-Z-source inverter utilizes a nonlinear SPWM to produce sinusoidal voltage at the output. The proposed system is able to track the MPP of PV arrays and produce an AC voltage at its output by utilizing only three switches. Experimental results show that the fast-converging MPPT algorithm has fast tracking response with appreciable MPP efficiency. In addition, the inverter shows the minimization of common mode leakage current with its ground sharing feature and reduction of the THD as well as DC current components at the output during DC-AC conversion.

AC/DC Current Transformer With Single Winding

Dc currents in power grids are created by induction from geomagnetic variations and newly also by transformerless power inverters installed in solar power stations. These currents may not only cause serious errors of current transformers (CTs) measuring the consumed energy but also cause overload of distribution transformers that may result in blackout. Installing dc current sensors into the grid would be very expensive. We have shown that the existing CTs in the grid can be used in the fluxgate mode to simultaneously measure dc current. Utilizing this mode, the CT ac error is inevitably increased. We analyze the sources of these errors and show techniques to reduce them. The experiments were made on the off-the-shelf 500 A/5 A CT. We show that the uncompensated 5 A dc current without compensation increases its ac amplitude error from 0.1% to 1%. We also found that the dominant additional error was caused by the increased burden. With the optimized design of the excitation circuit, it is possible to sense a dc current component with 10% accuracy and reduce the additional ac error from previously reported 1.5% to 0.1% when the dc current component is compensated. This technique can be widely used to improve the ac accuracy of the existing CTs.

Compensation Strategy to Eliminate the Effect of Current Measurement Offsets in Grid-Connected Inverters

For the digital control of systems such as grid-connected inverters, measuring inverter output currents accurately is essential. However, current measurement offsets are inevitably generated by current measurement paths and cause DC current components in real inverter output currents. Real inverter output currents with DC components cause the DC-link capacitor voltage to oscillate at the frequency of a utility voltage. For these reasons, current measurement offsets deteriorate the overall system performance. A compensation strategy to eliminate the effect of current measurement offsets in grid-connected inverters is proposed in this study. The validity of the proposed compensation strategy is verified through simulations and experiments. Results show that the proposed compensation strategy improves the performance of grid-connected inverters.

Active Filter for the Removal of the DC Current Component for Single-Phase Power Lines

Nonlinear loads and grid-connected converters can cause, in addition to the generation of several current harmonics in the grid current, a dc current component injection. A dc current component can cause the magnetic core saturation of distribution power transformers. Transformers operating under saturation conditions present increased power losses, overheating, and distorted current waveforms. Since a dc current component causes a small dc voltage component drop across the parasitic resistance of the distribution grid conductors, canceling the dc voltage component at the point of common coupling (PCC) implies the compensation of the dc current component injected/absorbed by electric loads or grid-connected converters connected at the same PCC. This paper proposes a low-cost nonlinear sensor for an accurate detection, free from offset problems, of the dc voltage component present in the grid voltage. The detection of the dc voltage component was used to realize an active filter of the dc current component. The proposed solution is outlined, and then the stability issue is addressed by means of a simplified model. Experimental results confirmed that the simplified model closely approximates the real system.

Detection Method of the DC Bias in Distribution Power Transformers

Nonlinear loads such as ac drives, switch-mode power supplies and grid-connected converters can cause, in addition to the generation of unwanted current harmonics, a dc current component injection into the grid. This dc current component can lead to magnetic saturation of the distribution power transformers, leading to higher current distortion and overheating. This paper presents a method to detect the dc current component flowing into the distribution power transformer with a precise measurement of the dc voltage component at the transformer winding. A magnetic sensor has been developed and implemented in a closed-loop control system to achieve a high sensitivity and guarantees a good linearity with a high rejection ratio to grid voltage variations. Simulation and experimental results confirm the effectiveness of the proposed approach.

Digital-Controlled Single-Phase Transformer-Based Inverter for Non-Linear Load Applications

The paper presents a new digital-controlled single-phase transformer-based inverter for non-linear load applications. A capacitive full-bridge circuit is added to provide instant current under non-linear load condition and thereby reducing the harmonics significantly to meet the required harmonic standard, IEEE 519-1992, even under non-linear load condition. The redundant capacity, cost, size and weight of line frequency transformer can therefore be dramatically reduced. Moreover, a new integrated controller for inverter control is proposed to eliminate both DC current component and steady state error even under heavy load condition. The proposed integrated controller consists of a Proportional (P) controller acts as voltage controller, DC offset canceller, an RMS compensator and non-linear load compensator.

A DC Offset Current Compensation Strategy in Transformerless Grid-Connected Power Converters

A line frequency transformer is usually employed in grid-connected power converters, from both renewable and traditional energy sources, in order to suppress the DC current component and the ground leakage current. Solutions employing a high frequency transformer or employing no transformer at all have recently been investigated in order to reduce size, weight and cost. As a consequence, unless a suitable remedy is adopted, a DC current component exceeding the limits enforced by international standards may be injected into the grid. This paper proposes a simple and cheap solution to reduce the DC current component injected into the grid in the case of a full-bridge, single-phase, transformerless converter. The proposed strategy is intrinsically insensitive to offset measurement errors and can be utilized as a robust and dynamic offset compensator for the current transducer. The simulation results have confirmed the theoretical behavior of the proposed solution, while the experimental ones, performed for different values of output power and for different current control architectures, have shown its effectiveness.

A nonlinear Reactor for DC Current Compensation in Single Phase Power Lines

AbstractNon-linear loads and grid connected converters, whose utilization is constantly growing, have been causing problems regarding the power quality. These problems comprehend the generation of harmonics in the current provided by the power distributor. Among different kinds of deterioration, there is the issue of the DC current component. As a matter of fact, a DC current is detrimental for the distribution power transformer, as it leads to saturation the magnetic core. The saturation of the power transformer increases the VAR absorption, leading to possible damage the magnetic component due to overheating. This paper proposes a compensator system based on a non linear sensor that acts as an active filter for the DC current component flowing in the power lines. The paper is organized as follows: the first part outlines the strategy, then the stability issue is addressed by means of a simplified model. A simulation verification showed that the simplified model closely approximates the real system.

DC Component From Pantograph Arcing in AC Traction System—Influencing Parameters, Impact, and Mitigation Techniques

Pantograph arcing in ac traction system generates transients, and causes asymmetries and distortion in supply voltage and current waveforms. These asymmetric voltage and current lead to a net dc component and harmonics that propagate within the traction power and signalling system and cause electromagnetic interference. This problem is enhanced during winter because of the layer of ice/snow on the overhead contact wire. The sliding contact becomes poor and a visible arc moves along with the pantograph. In this paper, it is shown how different parameters like traction current, line speed, power factor, and supply voltage influence the arcing, its characteristics, and the dc components. It is shown that the dc current component increases with increasing train speed and traction current, and reduces at a lower power factor. It is also discussed how the presence of an ice layer influences the arcing and the dc components. It is found that running the trains below the normal operating power factors is an effective choice to mitigate this problem. The findings presented in this paper could be beneficial to estimate the probable limit of the dc component at the planning stage so that proper precautions can be taken at the design stage itself.

Proximity-driven source of highly spin-polarized ac current on the basis of superconductor/weak ferromagnet/superconductor voltage-biased Josephson junction

We theoretically investigate an opportunity to implement a source of highly spin-polarized ac current on the basis of superconductor/weak ferromagnet/superconductor (SFS) voltage-biased junction in the regime of essential proximity effect and calculate the current flowing through the probe electrode tunnel coupled to the ferromagnetic interlayer region. It is shown that while the polarization of the dc current component is generally small in case of weak exchange field of the ferromagnet, there is an ac component of the current in the system. This ac current is highly spin-polarized and entirely originated from the non-equilibrium proximity effect in the interlayer. The frequency of the current is controlled by the voltage applied to SFS junction. We discuss a possibility to obtain a source of coherent ac currents with a certain phase shift between them by tunnel coupling two probe electrodes at different locations of the interlayer region.

Discharge characteristics of plasma sheet actuators

The electrical characteristics of a plasma sheet device used for subsonic airflow control are studied in this paper. Experiments are undertaken with a two-wire asymmetrical (different diameters, opposite polarity) electrode configuration connected to dc high voltage sources in the presence of a dielectric plate and under different gases (dry air, nitrogen and oxygen). For large distances electrode-plates it has been found that the discharge current consists of a purely dc component. The proximity of the plate reduces notably this dc current component until a limit situation for which the electrodes practically lay on the plate and a current pulsed regime is superimposed on the dc (small) component, thus establishing a plasma sheet regime. This regime could be reached only when the small wire was positive. This work establishes that the pulsed regime may be associated with a succession of positive streamers (cathode directed) which formation is promoted by different parameters of the gas and surface characteristics (thresholds of photoionization and photoemission, charge deposition, …). The dc component seems to be produced by a small number of electrons originated in the ionization region of the negative corona that are amplified in the ionization region of the positive corona. The charged particles produced during the streamer propagation could contribute appreciably to the ion momentum transfer to the gas. This transfer should be due very likely to the drift of the charged species present in the streamer channel during the streamer collapsing phase. The source of momentum transfer associated with the dc current would always persist with a magnitude that depends on the intensity of this current.

Current Mode Monolithic Active Pixel Sensor With Correlated Double Sampling forCharged Particle Detection

A monolithic active pixel sensor operating in current mode for charged particle detection is described. The sensing element in each pixel is an n-well/p-sub diode with a PMOS transistor integrated in an n-well. The drop of the n-well potential from the collection of charge modulates the transistor channel current. Each pixel features two current mode memory cells. The subtraction of distant-in-time samples frees the signal of fixed pattern noise (FPN) and of the correlated low-frequency temporal noise components, resulting in extraction of the particle footprint. The subtraction circuits are placed at each column end. A transimpedance amplifier, integrating in sequence two current samples and subtracting the results in an arithmetic operation, was adopted. The integrated version of the transimpedance amplifier, designed with a maximized conversion gain, is burdened by a risk of an early saturation, imperiling its operation, if the dispersions of the dc current component are too big. The degree of dispersions could not be estimated during the design. Some number of columns is available as a backup with the direct current readout. An external realization of the subtracting circuit, based on the same principle, is used to process direct output columns. The concept of the data acquisition setup developed, the tested performance of an array of cells, and the processing circuitry are described

多機系統における故障電流過渡直流分計算手法の精度の検討

Accuracy of the calculation method of transient DC component of three phase short circuit current is carefully studied. Using a multi-machine model system 4 different methods are compared, which are catenacci method, AIEE method, 4 parameters method, and the proposed method. Among all these four methods the proposed method gives the most accurate result, which gives almost identical wave form of the averaged wave obtained by the post processing of ATP-EMTP results. Compared to ATP-EMTP the proposed method is more convenient because it automatically gives the severest fault case while ATP-EMTP needs several computations.Main reason of the discrepancies of the conventional methods is based on the insufficient order of the internal models. In contrast to the proposed method, they use only a second order model at most. The usefulness of the proposed method is demonstrated using a test analysis of the saturation of a current transformer. Computing time is also discussed by comparing the results applied to two 10-machines systems and one 30-machines system.In conclusion, the proposed method is accurate and is ready to apply to multi-machine systems within a realistic computing time. The usefulness of the proposed method is confirmed by this study.

Auto-Calibrating DC Link Current Sensing Technique for Transformerless, Grid Connected, H-Bridge Inverter Systems

Power electronic inverters are commonly used for the interfacing of distributed generation systems to the electrical power network. These electronic inverters operate in a current controlled mode to inject unity power factor sinusoidal current into the network. To prevent possible dc current injection, a mains frequency isolation transformer is often employed at the inverter output. This isolation transformer is a costly component. An alternative approach is to use current sensing and control techniques to eliminate the dc current component. One method is to use a current controller to force the output dc current to zero. Current controllers are prone to errors associated with nonlinearity and offsets in the current transducers. This paper considers a novel auto-calibrating dc link current sensing technique that eliminates the errors associated with the current transducer, and helps avoid dc current injection into the grid when using a transformerless grid connect inverter system

Pumping in an interacting quantum wire

We study charge and spin pumping in an interacting one-dimensional wire. We show that a spatially periodic potential modulated in space and time acts as a quantum pump inducing a dc-current component at zero bias. The current generated by the pump is strongly affected by the interactions. It has a power law dependence on the frequency or temperature with the exponent determined by the interaction in the wire, while the coupling to the pump affects the amplitudes only. We also show that pure spin-pumping can be achieved, without the presence of a magnetic field.

Electrical study of DC positive corona discharge in dry and humid air containing carbon dioxide

As most part of the industrial effluents contain carbon dioxide and water vapour, it is interesting to study from an electric point of view ‘basic’ gas mixtures including air and various amounts of these gases, in order to precise how the corona discharge inception takes place in such mixtures. In a DC point-to-plane reactor, distinct parameters are varied, such as discharge current, partial pressure of CO2, and relative humidity. Gap voltage and streamer frequency are experimentally measured as functions of the discharge current. Current waveforms are recorded in various gas mixtures and electrical conditions. DC and impulsional current components, as well as streamer frequency are also plotted as functions of the CO2 partial pressure for dry and water saturated mixtures. For dry or wet mixtures, 5% CO2 in volume appears to be the proportion allowing the establishment of a stable corona discharge with the lowest energy cost.

JOSEPHSON-LIKE EFFECT IN A SPIN VALVE

The possibility of the Josephson-like effect in a spin valve system is discussed. It is argued that in the absence of the bias voltage, the system can have dc currents flowing through the tunneling junction under certain conditions when alignments of spins in two ferromagnetic (FM) layers separated by an insulating layer, are parallel. The system may have ac currents flowing through the junction when spins in the two FM layers are tilting aligned or antialigned, this being a result quite different from the Josephson effect in superconducting junctions. In the presence of the bias voltage, the tunneling current is, in general, alternating, but for the case of antialigned spins, it contains a dc current component when the bias voltage takes a specific value (a resonant condition). The case with an ac bias voltage is also addressed.

Low-voltage negative-resistance mixers of nano-meter SNS junctions

The current carried by the bound quasi-particles in the N region of a mesoscopic SNS junction has dc and cosine components. The dc current component is carried by the pair charge (-2e) transferred by a couple of Andreev reflections at both of the NS interfaces. The pair-charge transfers decrease due to reduction of the allowed number, 2/spl Delta//V, of Andreev reflections when the voltage, V, increases. Therefore, the low-voltage negative-differential-resistance is observed on the I-V curves of mesoscopic SNS junction, when the junction is driven by the low impedance voltage-bias source. The supercurrent carried by the quasi-particle in the N-region is sensitive to the external high-frequency fields. In a mixer experiment using the nano-meter SNS junctions of NbN, prominent IF signal peaks are observed at low bias voltage. Each IF signal peak corresponds to the negative differential resistance region at low bias voltage.

New transfer functions for an accurate estimation of harmonic distortion in AC/DC converters working under unbalanced conditions

In this paper, new transfer functions for the estimation of harmonic distortion in six-pulse AC/DC controlled converters working in continuous mode are proposed. One of the objectives of the paper is extending the use of transfer functions for the case of distorted and/or unbalanced supply voltages. Moreover, important changes in the structure of these transfer functions are proposed, with the aim of achieving higher accuracy in the estimations. With this technique, it is possible to predict the harmonic components of currents not only at the DC side, but also at the AC side, even for noncharacteristic harmonics. This paper deals only with the AC/DC transfer phenomenon. The new algorithm proposed in this paper is especially suitable to distinguish between the DC current harmonic components caused by the converter and those caused by the interaction of the load.