DC Current Value Research Articles

Transient overvoltage suppression of LCC‐HVDC sending‐end system based on DC current control optimisation

AbstractThe receiving‐end system AC fault of the line‐commutated‐converter‐based high voltage direct current (LCC‐HVDC) will lead to commutation failure of the inverter side. During the fault and its recovery, AC transient low voltage and transient overvoltage (TOV) will occur in the sending‐end system. The TOV has the risk of triggering the disorderly off‐grid of the nearby renewable power generations. Besides, in a serious situation, it will threaten the power system to maintain a secure and steady operation. Therefore, the authors analyse the mechanism involved in the AC transient voltage during the AC fault and the recovery period first. It reveals that the key factor causing the TOV of the sending‐end system is the setting of the DC current reference value. Then, a DC current reference value limit method based on the AC TOV sampling value is proposed, which is used to accelerate DC current recovery and suppress the TOV of the sending‐end system. Finally, the effectiveness of the designed control method has been confirmed through electromagnetic transient simulations using the CIGRE HVDC benchmark model and a ±800 kV HVDC transmission system model situated in Northwest China.

Comparative research on the speed performance of Axial flux Permanent Magnet Generator (PMG)165 with manufacturing specification and experimental for implementation in the Low-Speed Wind Turbine System

This paper is on working on energy assessment of generator’s voltage, current and power output based on the generator speed. Today, demand of power is increasing because of technology advancement and increasing electrical devices being manufactured and installed. For this reason, the generator applied in renewable energy is recommended. It must be improved because it is an important component inside the energy system in producing enough power. This research is used the DC Motor function as prime mover and direct shaft to the shaft connection of AFPM165. The parameters of the investigational research are generator speed (rpm), DC current value form generator output (Amp), and voltage value (V). The speed of DC Motor has been gradually increases, from 0 ∼ 300 rpm by adjusted speed adjustment knob. The series circuit 15-ohm resistor and 5 Watts lamp to make it total 19-ohm constant as a load. The result is presented that the more speed of generator, the more voltage, and current it can be inducted. The findings of the study are presented that there is a difference between the factory specification and the experiment where the experimental value is more than the factory specification value which is 11.43% variation.

A Dynamic Nonlinear VDCOL Control Strategy Based on the Taylor Expansion of DC Voltages for Suppressing the Subsequent Commutation Failure in HVDC Transmission

Subsequent commutation failure in high-voltage DC transmission systems seriously emphasizes the safe and stable operation of power systems. Via analyzing the mechanism of commutation failure and the principle of voltage-dependent current order limiter (VDCOL), this paper proposes a dynamic nonlinear VDCOL control strategy based on the Taylor expansion of DC voltage for suppressing subsequent commutation failure. To solve the problem of fluctuating DC current command value caused by a large drop in DC voltage, this paper constructs a nonlinear VDCOL control that can be dynamically adjusted according to the AC bus voltage level, and Taylor expansion of DC voltage is used to obtain its first-order and second-order differential components. Different scales of differential elements are chosen to predict the DC voltage compensation value while balancing sensitivity and accuracy. The compensated DC voltage, used as the starting voltage of VDCOL, is input to the VDCOL control constructed in this paper to suppress the subsequent commutation failure of the transmission system by reducing the fluctuation of the current command value. Finally, the standard test model of HVDC is established based on the actual parameters, and the simulation results show that the Method proposed in this paper has an effective suppressing effect in the case of single-phase or three-phase faults of different severity and is conducive to the restoration of the system power transmission.

フライングキャパシタ変換器をベースとした非絶縁形DC-DC変換器の電流制御法比較

This paper presents a bidirectional non-isolated dc-dc converter based on three-level flying-capacitor converters. The converter uses multiple auxiliary converters, each of which is based on cascaded chopper cells and an inductor, which enables active control of the inductor current. This paper compares the following two current control methods: conventional sinusoidal-wave current control method and proposed trapezoidal-wave current control method, in terms of ac-current peak values, dc-current values, current root-mean-square (RMS) values, voltage ripples in dc-capacitor voltages, current ripples in dc currents, and converter efficiency. The validity of the comparison and trapezoidal-wave current control method is verified in a simulation using MATLAB and experimentally using a 2-kW downscaled model.

Fast modeling approach of large-scale non-inductive HTS coils under different current supply

This paper presents a fast-integral modeling approach for the design and characterization of large-scale non-inductive high temperature superconducting (HTS) coils made of first-generation HTS tape. The integral modeling is combined with a strategy of far tape approximation allowing to model a reduced set of consecutive tapes instead of the entire coil. It makes it possible to estimate the losses and the current density distribution in the coil, under different current supply. This leads to a considerable reduction in the computation time. The calculated AC losses show a good agreement with the Norris ellipse formula and measurements, highlighting the importance of considering the elliptical section of the superconductor in the tape, in the modeling. The developed modeling approach is in particular useful in the AC + DC mode, where Norris’s formulas cannot be used to determine the losses. Besides, some interesting phenomena have been observed, related to the current distribution in the tapes at different AC and DC current values.

Fault Security Region Modeling and Adaptive Current Control Method for the Inverter Station of DC Transmission System

Line-commuted converter-based high-voltage direct current (LCC-HVDC) has undergone extensive development in power systems. Commutation failure (CF) in LCC-HVDC poses a considerable threat to the safe and stable operation of power systems. The coupling of electrical quantities, including DC current, DC voltage, AC voltage, and extinction angle (EA) of an inverter station, is a critical factor that makes CF difficult to suppress effectively. Existing methods adopt preset fixed control parameters and a single control target. The demand of CF recovery typically cannot be satisfied. Control even becomes the driving force of subsequent CF (SCF). Accordingly, a new concept of a fault security region (FSR) in an inverter station under the effects of multi-electrical quantities is proposed. The security range under the coupling effects of electrical quantities is described by the active power and exchanged reactive power of the inverter station. EA, active power, and exchanged reactive power are coordinated through the adaptive regulation of DC current. Thus, SCF can be prevented under the condition that the effect of varying active power and reactive power of LCC-HVDC on a power system is balanced. First, the coupling characteristics of multi-electrical quantities of the inverter station are analyzed. Second, the modeling method for the FSR in the inverter station is proposed. Third, an adaptive current control method is developed by analyzing the evolution characteristics of the FSR. Moreover, a calculation method for the DC current order value is presented. Finally, the effectiveness of the proposed method is verified using the CIGRE HVDC standard system.

A novel low-voltage current-limiting control strategy with improved voltage compensation

By optimizing the design of the conventional low-voltage current limiting controller (VDCOL), the continuous commutation failure on the receiving end of the traditional LCC-HVDC transmission system is effectively suppressed. In order to overcome low insensitivity of conventional VDCOL under mild and moderate faults, the dc current deviation value is superimposed into the starting voltage of the VDCOL in the form of a compensation voltage to enhance sensitivity of the VDCOL to the dynamic changes of the dc current during mild and moderate faults. In the face of severe faults, the conventional VDCOL adopts a linear adjustment method with a fixed slope, which is bad for suppressing continuous commutation failure. Aiming at this problem, a nonlinear VDCOL strategy is adopted, which the dc current command and the dc voltage are a nonlinear function relationship with a variable slope with the increasing or decreasing of the dc voltage. In the PSCAD/EMTDC simulation platform, the proposed control strategy is verified using the International Council on Large Electric Systems (CIGRE) standard model. Simulation results reveal that the proposed nonlinear VDCOL strategy with improved voltage compensation method can well suppress continuous commutation failures in the case of single-phase and three-phase short-circuit faults, and it is beneficial to restore dc voltage and transmission power and ensure the normal operation of the HVDC transmission system.

A Calculation Method of DC Current Reference Value for Suppressing Commutation Failure in LCC-HVDC System

Line commutated converter based high voltage direct current (LCC-HVDC) transmission system is prone to suffer commutation failure, which can lead to the increase of DC current and the interruption of DC transmission power. Furthermore, the increase of DC current might lead to subsequent commutation failure. Based on the reactive power balance of the inverter AC bus, this paper studies the functional relationship between DC current and commutation voltage. The DC current reference value is obtained by using the DC current acquired from the above functional relationship, and combining with the DC current order given by voltage dependent current order limiter (VDCOL). This calculation method can adjust the DC current reference value in time according to the commutation voltage, thus limiting the increase of DC current. In order to verify the commutation failure suppression ability of the proposed method, several simulations are conducted based on CIGRE benchmark HVDC model in PSCAD/EMTDC. Simulation results show that the proposed method can not only suppress commutation failure successfully, but also improve the fault recovery performance of the LCC-HVDC system.

Primary frequency regulation scheme applicable to LCC – VSC series hybrid HVDC considering AC voltage stability at receiving end

The penetration of high proportion renewable energy generation in the load center reduces the inertia of the power system and causes frequency stability problems. Series hybrid HVDC (e.g., BaiHeTan-JiangSu) with landing point at load center should play an important role in primary frequency regulation. Thus, this paper proposes a control scheme that coordinates line-commutated converter (LCC) and voltage-source converter (VSC) operation of the series hybrid HVDC (SHHVDC) and couples inertia emulation control with droop control (CIEDC). The CIEDC absorbs or releases the energy stored in the augmented capacitor of VSC for inertia emulation through regulating DC voltage. Then the DC voltage of LCC is coupled with the DC voltage of VSC through the voltage distribution coefficient. Meanwhile, the DC voltage of the inverter side is used to transmit the frequency information to the rectifier side to change the DC current order value without communication-based. Next, The equivalent DC input resistance based on communication-free is introduced to obtain the reactive power variation caused by LCC, which is transmitted to the reactive power control link in VSC to suppress the AC side converter bus voltage fluctuation. Finally, The CIEDC control scheme is verified by case studies in the event of slight AC fault and step load variation compared with the Fixed-Inertia Emulation Control Scheme (FIEC) and conventional droop control (CDC) in AC bus voltage stability, frequency deviation, rate of change of frequency (RoCoF).

Intrinsic Influence of Dynamic Resistance on Current Distribution of Two HTS Parallel Branches Without Terminal Contact Resistance

If a superconductor carries DC transport current and exposes to an alternating current (AC) magnetic field whose amplitude is more than its threshold field, a DC resistance so called dynamic resistance occurs on the superconductor because of the dissipative interaction between the DC transport current and the external magnetic field, which possibly appears in many kinds of high temperature superconductor (HTS) apparatus, such as linear synchronous motors. Occurrence of the dynamic resistance significantly affects the current distribution of the HTS components connected in parallel such as cables and conductors except for increasing the AC loses. In this paper, we employ a partially slit second-generation (2G) HTS tape with two branches to study the influence of dynamic resistance on current distribution such that the terminal contact resistances can be avoided. In experiment, DC transport current is applied to the whole 2G HTS tape and branch 1# is only exposed to an applied AC magnetic field. In addition, we use an analytical formulation of the dynamic resistance of branch 1# based on Bean critical state model and voltage calculation of branch 2# relies on E-I curve to build analytical model by using Newton Iteration. It is shown that DC current value of branch 1# decreases and branch 2# increases with the increasing of the dynamic resistance of branch 1#. The results are important for HTS DC devices with carrying ripple current or exposed to AC magnetic field.

Analyses and Comparisons of Generic and User Writing Models of HVDC System Considering Transient DC Current and Voltages

Analyses and comparisons of generic models and a novel modeling method of line-commutated converter (LCC)-based high voltage direct current (HVDC) systems are presented for the power system simulator for engineering (PSS/E) simulation tool. The main purposes are to describe the dynamics of the DC voltage and current of HVDC system and to improve estimation of the maximum values using the generic and proposed HVDC models when AC line-to-ground faults are occurred. For the generic HVDC models, the characteristics and limitations as well as parameter investigations are conducted. Three modules are also are developed for the proposed HVDC model, which are composed of (a) AC/DC conversion; (b) controller selection; and (c) DC line model. Case studies were conducted considering the real-operated HVDC system in Korea under the PSS/E and power systems computer aided design (PSCAD) simulation environments. The simulation results were compared with measured data from the real-operated HVDC system and the results from the PSCAD HVDC models considering single- and three-phase line-to-ground faults. The case study results show that a specific generic HVDC model in PSS/E can simulate the dynamics of the DC voltage and current after the AC line-to-ground faults. The case studies also showed that the proposed modeling method is effectively improves the estimation of the maximum DC voltage and current values for the AC line-to-ground faults.

Total loss measurement and simulation in a REBCO coated conductor carrying DC current in perpendicular AC magnetic field at various temperatures

In many high-temperature superconducting (HTS) applications, HTS coated conductors carry DC currents under external AC magnetic fields. There are two AC loss mechanisms in this situation: magnetization loss due to the external magnetic field and dynamic loss due to the interaction between the DC current and the external magnetic field. The sum of these two loss components is referred to as total loss. In this work, the total loss in a 4 mm wide REBCO coated conductor is measured under perpendicular AC magnetic fields up to 105 mT at 77 K, 70 K, and 65 K, with reduced DC current level, i (I dc/I c0), from 0.025 to 0.98, where I dc is the transport DC current value and I c0 is the self-field critical current of the coated conductor at each temperature. The experimental results show a good quantitative agreement with an analytical equation for each loss component, as well as 2D finite element modelling (FEM) results from H -formulation. For any given temperature, we observe that the total loss is mostly dominated by magnetization loss at i< 0.2, while dynamic loss makes a comparable, even greater contribution to total loss at i > 0.5. Electromagnetic analysis from the FEM modelling shows the evolution process of total loss, where the dynamic loss region and magnetization loss region vary across the conductor width at high magnetic fields or high DC current level. The simulation results also reveal the superposition of (positive) DC current and the anti-parallel (negative) shielding current, which occurs at high DC current level. The superposition drives the current density of one conductor edge to subcritical stage, and it leads to one-sided loss generation in each half-cycle. Our results provide a valuable reference for total loss behaviours in REBCO coated conductors.

Optically detected spin–orbit torque ferromagnetic resonance in an in-plane magnetized ellipse

Time-resolved scanning Kerr microscopy has been used to perform optically detected, phase-resolved spin–orbit torque ferromagnetic resonance (SOT-FMR) measurements on a microscale CoFeB ellipse at the center of a Pt Hall cross subject to RF and DC current. Time-resolved polar Kerr images revealed localized dynamics with large amplitude at the center and weaker amplitude at the edges. Therefore, field swept SOT-FMR spectra were acquired from the so-called center mode to probe the SOTs active at the center of the ellipse, thus minimizing non-uniform edge contributions. When the magnetic field was applied at 30° from the hard axis of the ellipse and a DC current was applied, a marked asymmetry was observed in the amplitude and linewidth of the FMR peaks as the applied field was reversed. Both absorptive and dispersive parts of the spectra were in good agreement with a macrospin calculation. The damping parameter (α) and the Slonczewski torque parameter were determined to be 0.025 and (6.75 ± 0.75) × 10−7 Oe A−1 cm2, respectively. The hard axis SOT-FMR linewidth was found to be almost independent of the DC current value, suggesting that the SOT has a minimal influence in the hard axis configuration and that thermal effects were insignificant. This study paves the way for spatially resolved measurements of SOT probed using localized modes of microscale devices that go beyond the spatially averaged capability of electrical measurement techniques.

Cold Cathodes Based on an Assembly of Microchannel Plates for Low-Power X-Ray Tubes

t—A simple and economical method for manufacturing a cold cathode (electron generator) for a miniature and low-power X-ray emitter is described. The autoelectronic generator contains an assembly of two microchannel plates (MCPs) of the chevron type with an emissive carbon layer of Graphite 33 conductive glue, which contains carbon nanostructures (CNSs). The operating voltage does not exceed 2000 V. The dependence of the electron-generator output current on the distance between the MCPs in the assembly is determined. Thus, when the gap is increased from 0.2 to 0.63 mm the maximum DC current value increases from 0.65 to 4 μA; when the gap is further increased to 1.4 mm, the current increases to 4.5 μA. It is established that the maximum current of the electron generator in the pulsed mode of 53 μA is achieved if the first MCP in the assembly with a CNS layer at the input end is supplied with a pulse voltage of 800 V, a frequency of 2.3 kHz, and an off-duty ratio of 28, while a constant voltage of 1200 V is fed to the second MCP. In this case, the gap between the MCPs in the assembly is 1.4 mm.

A Study to Define Critical Current Limit for Covetic Al Formation

This paper presents the production steps of Al covetic material, as well as a margin for the critical current in its production process along with stress-strain mechanical measurement results. As the need for energy is increasing every day, it is getting important not only its production but also its transmission in power grids. Covetic metals, one alternate to superconductors, show high electrical conductivity property for that purpose. During covetic production, a metal matrix is first melted and then mixed with Carbon Nanotubes (CNTs) before a high DC current is applied to the molten material. The role of this DC current is important in covetic material production and still under investigation. In the course of study, covetic samples have been produced for four different current values, namely 100A, 200A, 300A and 400A, applied for 10 minutes, and one sample without a DC current, all having CNT-Al mixing ratio of 1% by weight. Another sample was also produced without CNT doping for comparison purposes. Obtained results have showed that covetic material formation takes place for DC current values greater than or equal to 200A based on conductivity comparison of the samples. It is also observed that covetic materials show increased flexibility.

The measurement of electrochemical noise of a Li-ion battery during charge-discharge cycling

For the first time electrochemical noise of a commercial Li-ion battery was measured during discharge via a constant value resistor for 1500 cycles. Spectral and statistical analyses were carried out. Both the power spectral density frequency dependence level and noise amplitude were increasing during battery cycling. Distribution diagrams, skewness and kurtosis values demonstrated unchanged normal noise distribution during cycling. Linear dependencies of the current power spectral density values on cycle number were found in the low-frequency band. Dependencies of the power spectral density on the DC-current value were measured at different stages of charge–discharge cycling. The total progress of electrochemical noise growth during cycling was divided into three stages with different prevalent battery degradation types: film growth, active material exfoliation, and structural changes.

Real-Time Calculation Method of DC Voltage and Current to Eliminate the Influence of Signal Delay

In this study, the influences of the signal delay duration between rectifier and inverter in ultra-high voltage direct current (U-HVDC) transmission systems have been analyzed. The simulation results with different signal delay time under the same fault condition show that the signal delay time has a significant influence on the control system response during the fault and recovery periods. In the worst condition, it may lead to a commutation failure in the inverter. To eliminate the influence of the signal delay time on the DC control system, a fast calculation method for computing the voltage and current has been proposed by establishing the equivalent capacitance and inductance of the DC line. A voltage dependent current order limiter is added in the rectifier control system, which can calculate the DC current order value. Based on this concept, this paper proposes an improved control strategy to eliminate the influence of signal delay. The analysis and simulation results show that the calculation method can effectively improve the response speed and shorten the fault recovery time of the HVDC systems.

Relaxation electrochemical noise of Li/SOCl2 and Li/MnO2 primary batteries

Electrochemical noise measurements were carried out on commercially available Li/SOCl2 and Li/MnO2 primary batteries during relaxation following removal of load. Another type of experiment was performed with different values of DC current. Power spectral density dependencies on the load current were obtained in this way for both battery types. On the basis of the measurement results, a physical interpretation of noise-generating processes was proposed for the investigated battery types. Two distinct SOC evaluation techniques were tested for each battery type—one based on spectral characteristics of electrochemical noise for the Li/SOCl2 system and the other based on the relaxation time length of the electrochemical noise for the Li/MnO2 system.

Application of electro-Fenton process for treatment of composting plant leachate: kinetics, operational parameters and modeling.

Composting plant leachate is considered as one of the highly polluted wastewaters which is necessary to be treated by simple, economic, fast and environmentally compatible methods. In this study, treatment of fresh composting plant leachate by electro-Fenton (EF) process was investigated. The effect of various input variables like pH (2-7), DC currents (1.5-3 A), H2O2 concentrations (theoretical ratio H2O2/COD: 0.1-0.6), TDS changes (4-6%), feeding mode, and BOD/COD ratio at the optimal point were studied. The settling characteristics of the waste sludge produced by the treatment (sludge volumes after 30-min sedimentation: V30) were also determined. Artificial neural network (ANN) approach was used for modeling the experimental data. Based on the results, the best removal rate of COD was obtained at pH: 3, 3 A constant DC current value, 0.6 theoretical ratio H2O2/COD and the feeding mode at four step injection. BOD/COD ratio at the optimal point was 0.535 and the maximum COD removal was achieved at TDS = 4%. In the optimal conditions, 85% of COD was removed and BOD/COD ratio was increased from 0.270 to 0.535. The data follow the second-order kinetic (R2 > 0.9) and neural network modeling also provided the accurate prediction for testing data. Results showed that EF process can be used efficiently for treatment of composting plant leachate using the proper operating conditions.

Optical properties of silver nanoparticles embedded in dielectric films produced by dc and rf magnetron sputtering

In this work it is reported the experimental production of silver-dielectric nanocomposites by the rf and dc magnetron sputtering techniques, as well as, the evolution of the plasmon resonance in silver nanoparticles as a function of the size of silver nanoparticles in the nanocomposites. Our nanocomposites are constituted by the intercalation of dielectric (Teflon, SiO2, TiO2 or a-C) layers made by rf magnetron sputtering with layers of silver nanoparticles made by dc magnetron sputtering. Size of silver nanoparticles was modulated by changing the values of dc current of the plasma during deposition and keeping constant all the other parameters. High resolution transmission electron microscopy was used to observe the shape and size of nanoparticles. The chemical composition and crystalline structure of the materials were analyzed by X-ray photoelectron spectroscopy and X-ray diffraction, respectively. UV-visible spectroscopy in the 350-1100 nm range was used to study the plasmon-resonance evolution. Dipolar and Quadrupolar Plasmon resonance were observed and interpreted in terms of the Mie theory.