Overvoltage Values Research Articles

Assessment of wind turbine transient overvoltages when struck by lightning: experimental and analytical study

This paper is aimed at presenting a numerical method for calculating the transient overvoltage across a wind turbine (WT) struck by lightning. The resulting overvoltage is determined at different points along the WT body using the proposed numerical method. The lightning strike has been simulated by injecting a current impulse to the tested WT. The equivalent circuits of WT components and the mathematical formulas to evaluate the circuit's parameters are presented. This makes it possible to develop π-equivalent RLC networks representing the WT components to write the nodal equations at each discrete time instant. MATLAB software package is used to solve the nodal equations and determine the transient behaviour of the WT. In the laboratory, a high impulse voltage is applied on a small-scale WT to corroborate the proposed method. The calculated overvoltage temporal variations are in good agreement with those measured at different positions along the WT for various grounding resistance values, demonstrating the validity of the proposed method. Further validation is also made by comparing the present simulation with that using PSCAD/EMTDC software package. The overvoltage values increase with the rise of the grounding resistance value. The obtained results are useful for designing WT lightning protection systems.

Перенапряжения от однофазных замыканий в сетях номинального напряжения 110 кВ

Overvoltages induced by single-phase ground faults in 110 kV networks operating in different neutral grounding modes are investigated. Owing to low values of resistance and high values of inductive and capacitive reactances, the transients in power lines and transformers that occur in 110 kV networks differ essentially from similar processes in 6 to 35 kV networks. This difference results in that the transient overvoltage free components have higher amplitudes, and that they have lower attenuation coefficients. It has been established that 110 kV networks cannot operate with the isolated neutral, because the overvoltage in this case may exceed seven times the nominal phase voltage, which would greatly complicate the transformer winding phase insulation. The use of a neutral grounded through resistance or inductive reactance with a value equal to the zero-sequence impedance of the considered circuit significantly reduces the level of ground fault currents and especially overvoltage. Neutral grounding through resistance leads to more efficient results, because the overvoltages and ground fault current have in this case fairly low values. Neutral grounding through inductive reactance results in somewhat higher overvoltage and ground fault current values. If the neutral is grounded through a nonlinear inductance, the overvoltages have still lower values, but the ground fault current approaches the value of the short-circuit current as in the case of the solidly grounded neutral. Comparison of the relevant results relating to various grounding methods of 110 kV networks has shown that the best results are obtained in case of using the neutral grounded through resistance with the value equal to the zero-sequence impedance of the considered network circuit. In this case, the overvoltage levels in the phases do not exceed 1.78 of the phase voltage value, and the ground fault current is much less than the short-circuit current in case of using the solidly grounded neutral. However, in choosing the neutral grounding method, it should be constantly borne in mind what particular task has to be solved: to limit the short-circuit current or to limit overvoltages. In the second case, the use of the neutral grounded through a fast-saturating reactor is a more acceptable choice.

Simulation and Visualization of Cell Performance Distribution of SOFCs

Introduction It has been reported for SOFC systems that high electrical efficiency beyond 70% could be achieved theoretically in a recent study[1]. It is important to understand the internal characteristics to design highly efficient SOFC. However, it is difficult to visualize internal reactions experimentally. From these reasons, it is desired to develop a simulation technique for visualizing various phenomena within fuel cell stacks. Here, we focus on numerical analysis on electrode kinetics. Exchange current density is an important phenomenological parameter in considering electrochemical reaction kinetics at SOFC electrodes and in simulating cell performance. Here, the exchange current density of fully pre-reformed methane-fueled SOFC anodes is measured and phenomenological equations of anode exchange current density are derived. Furthermore, we combine numerical analysis with the phenomenological relations to simulate the spatial distribution of anode overvoltages. Experimental conditions In this study, electrolyte-supported cells were used to measure the anode exchange current density. The electrolyte material was ScSZ (ScSZ:10mol% Sc2O3–1mol% CeO2–89mol% ZrO2) and the electrode prepared using NiO-ScSZ was applied to the anode. The electrode prepared using LSM（LSM: (La0.8Sr0.2)0.98MnO3）and LSM-ScSZ was applied to the cathode. By changing operation temperature, fuel utilization (Uf), and steam-to-carbon ratio (S/C), anode overvoltage was measured at each current density (0.25, 0.3, 0.35, and 0.4 cm2) for methane-fueled pre-reformed SOFCs by the current interrupt method. The anode exchange current density was then derived from the current density and anode overvoltage values. In simulation, three-dimensional cell model was created using a numerical analysis software. A parallel co-flow model was considered. The inlet gas temperature was 800°C and the anode gas composition was fixed with the S/C of 2.5. Results and discussion The equation of anode exchange current density based on hydrogen-fueled model[2] was revised to consider the equilibrium reforming reaction, the water gas shift reaction, and the conservation law of anode gas partial pressure. It has been reported that the electrochemical reaction, where the adsorbed hydrogen atom (Had) in hydrogen-based fuels is rate-limiting[2], an Arrhenius-type equation shown in Eq. 1 has been proposed based on the assumption that the electrochemical reaction of Had is rate- limiting: i 0,a = a·exp(b/RT)·PH2(A) 0.5·PH2O(A) 0.5 (1) The partial pressure terms were modified considering the equilibrium reforming reaction and the water gas shift reaction. Pre-exponential factor a and activation energy b were fixed using the experimental results. Fig. 1 shows the experimental results of anode exchange current density. The phenomenological values derived from Eq. 1 are shown in Fig. 1. As the fuel utilization varied, the exchange current density increased and reached the maximum at around 40% of Uf. The experimental results could be well fitted by Eq.1. The SOFC cell performance simulation was performed using the three-dimensional SOFC model. Fig. 2 shows the anode overvoltage distribution along the fuel flow direction from the left side to the right side. The overvoltage decreased from the inlet towards the outlet. This is partly because oxygen partial pressure on the anode increased to the outlet and the electromotive force and the exchange current decreased.

Estimation of Switching Surge Flashover Rate of a 1200 kV UHVAC Transmission Line due to Switching Overvoltages

The rapid increase in transmission voltages to fulfil higher demand of the transmitted power yields to put the switching surges as the governing factor in the insulation design process for EHV and UHV systems. Contemporary methods of insulation coordination utilise probabilistic approach and statistical methods particularly in the case of ultra-high voltage systems. This paper determines the statistical distributions of energization, re-energization, fault initiation and fault clearing overvoltages of the 1200 kV system modelled in PSCAD. Decisive figures like mean value, standard deviation and 2% statistical overvoltage values of switching overvoltage are estimated using randomness of the instants of closing with 526 operations for line energization and re-energization, and randomness of faulted phase, fault location and fault times with 3 faulted phases, 8 fault locations and 21 fault timings, 504 simulations for fault initiation and fault clearing. Ascertaining the investigations from the above simulations the stress on the towers is estimated. From the stress and insulation strength of the tower a systematic approach to calculate the switching surge flashover rate (SSFOR) is presented.

Characterization of SiPMs With NIR Long-Pass Interferential and Plastic Filters

In this paper, we report on the performances of silicon photomultipliers (SiPMs) with commercial long-pass interferential and plastic filters integrated on the detector's package for environmental light rejection. Several applications, including functional near infrared (NIR) spectroscopy or light detection and ranging, would benefit from the use of highly sensitive detectors like SiPMs with optimized electro-optical characteristics in NIR wavelength range. To this purpose, it is fundamental to reduce the absorption of environmental spurious light leading in application to a decrease of the detector's sensitivity, especially for very weak photon fluxes. We will show how the use of both types of filters has a relevant impact on the electro-optical performances of the bare detectors, in terms of cross talk reduction at high overvoltage values, detection efficiency, spectral response and capability to shield effectively the devices from the absorption of stray light, at the different light wavelengths used for the measurements.

High-Frequency Resonant Overvoltages in Transformer Regulating Winding Caused by Ground Fault Initiation on Feeding Cable

Transformer terminal equivalents obtained via admittance measurements are suitable for simulating high-frequency transient interaction between the transformer and the network. This paper augments the terminal equivalent approach with a measurement-based voltage transfer function model which permits calculation of voltages at internal points in the regulating winding. The approach is demonstrated for a single-phase three-winding transformer with its tap setting in the middle position. Three internal points in the regulating winding are included that represent the midpoint and the two extreme ends. The terminal equivalent modeling makes use of additional common-mode measurements to avoid error magnifications resulting from the ungrounded tertiary winding. The final model is used in a time-domain simulation where ground-fault initiation results in a resonant voltage build-up in the winding. It is shown that the peak value of the resonant overvoltage can be higher than during the lightning impulse test, with unfavorable network conditions. Additional measurements show that the selected tap position affects the terminal behavior of the transformer, changing the frequency and peak value of the lower resonance point in the voltage transfer between windings.

Temperature-compensated silicon photomultiplier

Silicon PhotoMultipliers (SiPM) have significant temperature dependence of important parameters such as gain, photon detection efficiency and optical crosstalk. Special temperature-controlled power supplies and off-line correction are typically used to mitigate this temperature dependence, though these methods are still not optimal for multichannel photodetectors exposed to significant temperature variations. We report on a design concept for a custom SiPM chip with an integrated thermo-compensation circuit that automatically adjusts an applied bias voltage thus maintaining a stable SiPM overvoltage value and gain over a wide range of temperatures. A prototype of the thermo-compensated SiPM chip was built using commercially available discrete components and tested in the temperature range from −30 to +30 °C. The measured temperature dependence of the thermo-compensated SiPM chip was reduced by factor 38 in comparison with the non-compensated device.

On iterative model of performance of micropixel avalanche photodiodes

This paper was devoted to the experimental verification of results of the iterative model for micropixel avalanche photodiodes. The avalanche processes initiated by single-photoelectron were studied for single-pixel avalanche photodiodes from two different manufacturers. It was found that the effective capacitance of the pixel calculated from the experimental charge–bias voltage dependence exceeds its terminal capacitance by a factor of two only at low values of resistance of the depletion region, while the maximum voltage drop on the pixel just after quenching of avalanche process equals twice the value of bias overvoltage. At high values of resistance of the depletion region, the voltage drop on the pixel equals the value of bias overvoltage and therefore the gain of the device is effectively reduced by a factor of two.

Electrocatalytic activity of Ni-Mo-Cu and Ni-Co-Cu alloys for hydrogen evolution reaction in alkaline medium

Tri-component Ni-Co-Cu and Ni-Mo-Cu alloys are electrodeposited depending on current density, temperature and copper ion electrolyte concentration. In both systems hydrogen evolution reaction (HER) is studied by polarization dependences and electrochemical impedance spectroscopy (EIS) in 6М КОН. It is shown that the incorporated copper quantity is significant for alloy catalytic activity. The lowest overvoltage and charge transfer resistor values for HER, and respectively, the best catalytic activity, is exhibited by Ni43Co37Cu20 and Ni57Mo27Cu16 alloys. The high electrode activity of Mo-alloys is associated with a well-developed surface area whereas in Co-alloys it is attributed also to Ni-Co synergism.

Electro-Optical Characterization of SiPMs With Green Bandpass Dichroic Filters

There is an increasing interest to use silicon photomultipliers (SiPMs) in medical applications like Photoplethysmography or Functional Near Infrared Spectroscopy, allowing the monitoring of physiological signs through non-invasive equipment. The use of large area SiPMs with embedded color filters for environmental light rejection could bring relevant advantages in these applications in terms of higher signal-to-noise ratio of the measurements. Here, we report on the first electro-optical characterization results obtained on n-on-p SiPMs with green interference bandpass filters glued on the top side of the detector SMD package. We will show how the use of the filter considerably increases, especially for large overvoltage values, the SiPM dark noise rate, due to the significant amplification of optical cross-talk effects (up to 150% of dark current increase at 50%-OV). This makes necessary a careful evaluation of the SiPMs performances in order to define the best operating conditions and layout characteristics of the detectors that will be used in the final applications.

Effect of Exchange Current Density on Current Distribution at Planar-Type SOFC Anodes

Introduction It has been reported for SOFC systems that high electrical efficiency of more than 70-80% could be achieved theoretically in a recent study [1]. However, their electrochemical performance would become lower due to an increase in ohmic and nonohmic resistances and possible fuel starvation. From these reasons, it is desired to develop a simulation technique for visualizing various phenomena in fuel cells. Here, we focus on numerical analysis on electrode kinetics. Since exchange current density is an important phenomenological parameter in considering electrochemical reaction kinetics at an SOFC electrode and in simulating the performance, the exchange current density of methane-fueled pre-reformed SOFC anodes was measured and compared with that of hydrogen-based fuel. Furthermore, the spatial distribution of current density was simulated for given average current density in case anode exchange current density i0 is either constant or variable. Experiment In this study, electrolyte-supported cells Ni-ScSZ/ScSZ/LSM-ScSZ (ScSZ:10mol%Sc2O3- 1mol%CeO2-89mol% ZrO2) were used. The electrode area was 8×8mm2 for i0 measurements. Pt mesh was used as a current collector. In this study, by changing operation temperature, fuel utilization (Uf), and steam-to-carbon ratio (S/C), anode overvoltage was measured at each (average) current density (0.25, 0.3, 0.35, and 0.4 cm2) for methane-fueled pre-reformed SOFCs by the current interrupt method. The anode exchange current density was then derived from the current density and anode overvoltage values. Results and discussion Anode exchange current density for hydrogen-fueled SOFCs may be described by Eq. (I) after Hosoi et al. [2]. Anode exchange current density could also be described by Eq. (II) in case vacant surface adsorption sites are predominant [2,3]. Figure 1(a) shows experimental values derived for the pre-reformed fuel in this study. The values derived from Eq. (i) are also shown with solid lines in Fig. 1(a) and (b), while the values derived from Eq. (II) are shown with dashed lines in Fig. 1(b). All these figures indicate gradual decrease in anode exchange current density in the high fuel utilization (Uf) region. i0,a = 3.5・exp(-6.2×104[Jmol-1] / RT)・(PH2 / PH2,ref)0.41・(PH2O/PH2O,ref)0.4 Eq. (I) i0,a = a’・exp(b’ [Jmol-1] /RT)・PH2 0.5 ・PH2O 0.5 Eq. (II) 3D-numerical simulation has been made using the software COMSOL to derive 2D current density of the cell shown in Fig. 1(c). Current density distribution for various average current densities is derived as a function of Uf and shown in Figs. 1(d) and 1(e) for constant i0 and variable i0 , respectively. While Fig. 1(d) suggests continuous decrease in current density from inlet to outlet, Fig. 1(e) suggests almost constant current density near the inlet and its gradual decrease near the outlet. The tendency of the latter is rather consistent with the dependence of anode exchange current density as a function of Uf in Figs. 1(a) and 1(b) for hydrogen-based fuels. Figure 1

Noise Reduction in Silicon Photomultipliers for Use in Functional Near-Infrared Spectroscopy

Silicon photomultipliers (SiPMs) have been recently proposed for different applications in medical imaging area, inclunding functional near-infrared spectroscopy (fNIRS), due to their single photon sensitivity, fast timing response, low operation bias, compactness, and low cost. Here, we report on the electro-optical performances of an SiPM technology suitably optimized for continuous wave (CW)-fNIRS application through the integration of a near-infrared (NIR) long-pass filter on the detector’s package. The use of the filter considerably decreases, especially for large overvoltage (OV) values, the SiPM’s dark noise rate, due to the significant reduction of optical cross talk effects (up to 90% of dark current reduction at 50%-OV). Moreover, the filter reduces also the absorption of environmental light of more than 85% in the SiPMs linear operation range above the breakdown, allowing at the same time a good photon detection efficiency in all the NIR range above 700 nm cut-on wavelength. These results open new perspectives for the employment of large area SiPMs operating with improved signal-to-noise ratio in advanced CW-fNIRS systems.

Influence of the Conditions of Electrode Position and Heat Treatment on the Structure and Properties of Metallic Coatings

Influence of electrodeposition conditions and heat treatment on the structure and properties of metallic coatings has been considered. It has been that at different values of overvoltage, metals crystallize with a cellular, subgrain or monoblock structure. The final formation of the structure occurs during annealing, in the process of which either polygonization or recrystallization develops in the coatings. Varying the conditions of electrodeposition and heat treatment allows obtaining coatings with functional characteristics for specific operating conditions.

Field Experiments on 10 kV Switching Shunt Capacitor Banks Using Ordinary and Phase-Controlled Vacuum Circuit Breakers

During the switching on/off of shunt capacitor banks in substations, vacuum circuit breakers (VCBs) are required to switch off or to switch on the capacitive current. Therefore, the VCBs have to be operated under a harsh condition to ensure the reliability of the equipment. This study presents a complete comparison study of ordinary and phase-controlled VCBs on switching 10 kV shunt capacitor banks. An analytical analysis for switching 10 kV shunt capacitor banks is presented on the basis of a reduced circuit with an ungrounded neutral. A phase selection strategy for VCBs to switch 10 kV shunt capacitor banks is proposed. Switching on current waveforms and switching off overvoltage waveforms with, and without, phase selection were measured and discussed by field experiments in a 110 kV substation in Chongqing, China. Results show that the operation of phase-controlled VCBs for 10 kV switching shunt capacitor banks is stable, and phase-controlled VCBs can be used to implement the 10 kV switching on/off shunt capacitor banks to limit the transient overvoltage and overcurrent. The values of overvoltage and inrush current using phase-controlled VCBs are all below those with ordinary VCBs.

New Equivalent Circuit of Transformer Winding for the Calculation of Resonance Transients Considering Frequency-Dependent Losses

In this paper, a new equivalent circuit for the transformer winding is presented by which the eddy current losses in the transformer can be represented accurately at high frequencies. The new model enables representing the frequency-dependent impedances of a coil system using the idea of mutually coupled series Foster circuits. An effective method for the determination of model parameters is also provided which involves using the vector-fitting (VF) method for the rational approximation of the system impedance matrix using a certain number of common stable poles. A set of nonlinear equations is then established using the VF results that can be solved with the method of fixed-point iteration to obtain the parameters of the equivalent circuit in a fast and accurate way. The new model can be used for the calculation of resonance transients within the transformer, the situation wherein the maximal value of overvoltages is highly dependent to the damping effects or the loss phenomena in the transformer. Since the model only contains constant lumped parameters, it can be used for transient calculations in the time domain directly using the electromagnetic transients programs, such as the Alternative Transients Program or Electromagnetic Transients Program.

FDTD simulation considering an AC operating voltage for air-insulation substation in terms of lightning protective level

The accuracy of lightning overvoltage evaluation of electrical equipment has been improved based on circuit analysis such as the Electro-magnetic Transients Program, but analyzing phenomena whose plane wave propagation (TEM) mode cannot be assumed remains unsolved. Since numerical analysis of electromagnetic fields such as the finitedifference time-domain (FDTD) method directly solves Maxwell’s equations, it is an appropriate means for solving this issue. In this paper, the lightning surge waveform due to the back-flashover is analyzed using the FDTD method; bearing detailed modeling of transmission towers and incoming substation lines in mind. An open end of a circuit breaker of a 500 kV air-insulated substation was selected as the subject of analysis, and ac operation voltage was considered as an analysis condition. Following FDTD analysis, a 150 kA lightning stroke to the first tower, which is the standard lightning stroke of the lightning protection design, does not cause back-flashover, but a 200 kA lightning stroke to the first tower and a 150 kA lightning stroke to the second did do so. The flashover occurred in the later timing than the result of conventional circuit analysis, and the rise of the lightning surge was also slower. The voltage waveform of the open end of the circuit breaker reached its peak after repeating reflections, and the overvoltage value became lower than the circuit analysis. According to the FDTD analysis, the required withstand voltage value after the waveform evaluation with a 200 kA lightning stroke is 1,337 kV, which is lower than the present lightning impulse withstand voltage (LIWV) 1,550 kV. The lightning stroke current 200 kA is the value adopted in the lightning protection design of the UHV system. Therefore, the existing air-insulated substation equipment is considered highly capable of withstanding overvoltage at the time of a back-flashover.

Corrosion characteristics of ZnNiCo ternary alloy electrochemically deposited by using different anodes

Corrosion characteristics, of ZnNiCo ternary alloy metallic coatings electrochemically deposited by using different anode shave been investigated. Analysis of corrosion characteristics was performed by using the polarographic methods and electrochemical impedance spectroscopy. Due to different values of oxygen overvoltage, each anode gives different cell voltage that increases in the order of MMO<Ti/Pt<BDD. Polarigraphic measurements showed the lowest corrosion rate (88 mm/year) of the coating deposited using Ti/Pt anode, while the corrosion rate for the remaining two coatings is approximately twice as large. The impedance measurements have shown two time constants that can be attributed to the dielectric properties of the layer of corrosion pro ducts at the coating surface and the coating layer itself. In concordance with polarigraphic measurements the resistance of both layers is approximately twice higher for the coating obtained using Ti/Pt anode.

Dynamic parameters of active rectifiers

The optimum motion trajectory in electric drives of position mechanisms operating under extreme conditions is determined by several separate segments formed from a set of phase trajectories with different target functions. It is shown that the electric drive optimization at one of the segments requires estimating the dynamic parameters of the “braking unit.” Different circuit designs of the “braking unit” of an electric drive are compared. The drawbacks of the most widely used braking circuit with a “drain transistor” (the kinetic energy excess in this circuit is dissipated at the active resistance) are noted. The dynamic parameters of “active rectifiers” operating in the generator mode and traditional circuits with a “drain transistor” are compared using the mathematical modeling methods. The dynamic parameters of “active rectifiers” were assessed quantitatively based on the dependence of the relative overvoltage value in the direct current link on the power returned to the mains.

Research on Commutating Over-Voltage Protection for High Power Converter

In the course of converter normal operation, large transient surges called commutating over-voltage may be generated internally during the commutating process of thyristor. The commutating voltage can effectively be reduced by the RC snubber circuit parallel to thyristor. In the RC snubber circuit design of high power thyristor, the reverse recovery process of thyristor must be considered in detail. Based on the analysis about reverse recovery transient of thyristor, the mathematical model of the converter thyristor reverse recovery current is created. Through the equivalent transformation of converter circuit during thyristor reverse recovery period, the analytical expressions about the relationship between commutating over-voltage and RC value are derived based on the Kirchhoff’s equation. Utilizing the analysis results, the optimal resistance and capacitance value can be selected by considering the reduction of peak over-voltage value and snubber power loss. It turns out that the proposed approach is available through the commutating over-voltage simulation and experiment results of high power converter.

An Approximate Formula for Estimating the Peak Value of Lightning-Induced Overvoltage Considering the Stratified Conducting Ground

In this paper, we present an improved and extended approximate formula for estimating the peak value of lightning-induced voltages in an overhead line, considering the horizontally stratified conducting ground. The approximate formula proposed in this paper is based on the return stroke transmission-line model (TL) and a trapezoidal lightning return stroke current waveform with the typical representative front time of 3.8 μs and the return stroke velocity of 120 m/ μs according to CIGRÉ and the IEEE Standard 1410 Guide. The extended approximate formula is validated by using the 2-D finite-different time-domain method and Agrawal field-line coupling model. The results show that the proposed approximate formula in this paper is simple and suitable for estimating the lightning-induced voltage peak value considering the horizontally stratified ground with satisfied accuracy.