Direct Current Flow Research Articles

Down-hole magnetometric resistivity inversion for zinc and lead lenses localization at tobermalug, county Limerick, Ireland

The magnetometric resistivity (MMR) method consists of measuring the variation of magnetostatic field associated to current density variations. It is based on the principle that direct current flow between two electrodes takes the path of least resistance through the earth. Consequently, conductive or relatively conductive ore bodies such as massive, semi-massive to disseminated sulphides are preferentially energized, which increases the current density in their vicinity. MMR is more efficient than electromagnetic methods in the case of weak conductivity contrasts.To improve the signal to noise ratio the magnetic field can be measured in boreholes rather than on the surface. Down-hole magnetometric resistivity measurements have been conducted in Tobermalug prospect in County Limerick, Ireland. The survey was used as an alternative to down-hole electromagnetic to delineate subhorizontal zinc/lead mineralization lenses that can be poorly conductive. Two survey areas were investigated, DHMMR1 and DHMMR2.Interpretation is based on the regularized least-squares inversion of MMR data, in which the MMR forward problem is solved with a finite volume discretization of the electrostatic and magnetostatic equations. Inversion of synthetic data generated over two conductive horizontal discs model shows that conductive structures are well positioned but their extensions is biased along current electrodes orientation.Inversion of the field data allowed localizing a few conductive elongated targets. At DHMMR1, the conductivity is weaker and seems to be associated to disseminated mineralization. DHMMR2 contains a higher conductivity and more elongated target. It appears to be associated to semi-massive sulphides.

Influence of Renewable Energy Sources on Transmission Networks in Central Europe

This paper focuses on the influence of increased wind and solar power production on the transmission networks in Central Europe. To assess the exact impact on the transmission grid, the direct current load flow model ELMOD is employed. Two development scenarios for the year 2025 are evaluated on the basis of four representative weeks. The first scenario focuses on the effect of Energiewende on the transmission networks, the second one drops out nuclear phase-out and thus assesses isolated effect of increased feed-in. The results indicate that higher feeding of solar and wind power increases the exchange balance and total transport of electricity between transmission system operator areas as well as the average load of lines and volatility of flows. Solar power is identified as a key contributor to the volatility increase; wind power is identified as a key loop-flow contributor. Eventually, it is concluded that German nuclear phase-out does not significantly exacerbate mentioned problems.

A modeling and simulation framework for the reliability/availability assessment of a power transmission grid subject to cascading failures under extreme weather conditions

Electrical power transmission networks of many developed countries are undergoing deep transformations aimed at (i) facing the challenge offered by the stochastically fluctuating power contributions due to the continuously growing connections of renewable power generating units and (ii) decreasing their vulnerability to failures or malicious attacks and improving their resilience, in order to provide more reliable services, thus increasing both safety and profits. In this complex context, one of the major concerns is that related to the potentially catastrophic consequences of cascading failures triggered by rare and difficult to predict extreme weather events. In this work, we originally propose to combine an extreme weather stochastic model of literature to a realistic cascading failure simulator based on a direct current (DC) power flow approximation and a proportional re-dispatch strategy. The description of the dynamics of the network is completed by the introduction of a novel restoration model accounting for the operating conditions that a repair crew may encounter during an extreme weather event. The resulting model is solved by a customized sequential Monte Carlo scheme in order to quantify the impact of extreme weather events on the reliability/availability performances of the power grid. The approach is demonstrated with reference to the test case of the IEEE14 power transmission network.

Fabrication of 200 mm Diameter Sintering Body of Skutterudite Thermoelectric Material by Spark Plasma Sintering

Filled skutterudite is a promising material for thermoelectric power generation because its ZT value is relatively high. However, mass production of high-performance thermoelectric materials remains a challenge. This study focused on the sintering process of thermoelectric materials. Large-diameter n-type (Yb or La, Ca, Al, Ga, In)0.8(Co, Fe)4Sb12 skutterudite sintering bodies with a small thickness were successfully produced by the spark plasma sintering (SPS) method. When direct current flows through the thermoelectric sintering body during the SPS pulse, the Peltier effect causes a temperature difference within the sintering body. To eliminate the Peltier effect, an electrical insulating material was inserted between the punch (electrode) and the sintering body. In this way, an n-type La-filled skutterudite sample with a diameter of 200 mm, thickness of 21 mm, and weight of 5 kg was successfully produced. The thermoelectric properties and microstructures of the sample were almost the same throughout the whole sintering body, and the dimensionless figure of merit reached 1.0 at 773 K.

Optimal transmission conversion from alternating current to high voltage direct current transmission systems for limiting short circuit currents

The excess of short circuit currents (SCC) has threatened the security of electrical energy transmission and become one of the most critical problems in power grid operations. At present, high voltage direct current (HVDC) transmission technology is a practical and applicable option to limit short circuit currents by separating a synchronous power grid into several asynchronous power grids. During the transmission expansion planning, the short circuit currents can be effectively limited by strategically converting alternating current to high voltage direct current transmission systems, improving transmission capacity as well. Due to the high construction cost of high voltage direct current transmission systems, improper conversion would influence the economy of power system planning and the effect of short circuit currents reduction. Therefore, an approach of optimal transmission conversion (OTC) for limiting short circuit currents is proposed in this paper. To solve the problem with the strong nonlinearity of short circuit currents, a novel linearization method based on direct current power flow is developed. The case study based on a 22-bus power system and IEEE 118-bus system are performed for illustration and validation. The simulation results demonstrate that the short circuit currents are effectively reduced and the security of electrical energy transmission is guaranteed by applying the proposed approach and model. The major contribution of this paper is to provide a new method for limiting short circuit currents. Hopefully, the proposed approach can provide new insights for the planning and operations of large-scale hybrid power systems.

Temperature Distribution of Three-Dimensional Photovoltaic Panel by Using Finite Element Simulation

The low electricity performance of a photovoltaic (PV) panel has been concerned in the PV application system. The effect of environmental and operating condition was affected the performance of the PV panel. In this research work, the main objective is to perform a three-dimensional geometry model of monocrystalline silicon PV panel with and without cooling system by using finite element method. In the case of a cooling system, the effect of the Direct Current (DC) fan flow rate on the temperature distribution of PV panel was investigated. The electrical behaviour of this PV panel is obtained based on the average temperature of the PV panel obtained and average solar irradiance from site location. According to the experimental results, PV panel with cooling system can be significant to provide better performance than the PV panel without cooling system in the same environmental condition. For the effect of flow rate of DC fan in the PV panel with cooling system, the performance of this PV panel has been improved as increasing in flow rate of DC fan.

Influence of Currents from Electrostatic Charges on WEC Formation in Rolling Bearings

ABSTRACTWhite etching crack (WEC) early bearing failures occur when the rolling contact is subjected to a so-called additional load such as an electrical current flowing through the bearing, in addition to the pure rolling load (pHz). Tests on rolling bearings showed that a low electrical direct current flow, such as that resulting from electrostatic charges, can lead to WEC failures in oil-lubricated roller bearings and greased ball bearings.The WEC formation in the performed tests was dependent on the current, electrical polarity, load type (rotating or stationary ring load), and bearing load. A black oxidation of the WEC critical bearing ring led to a significant increase in lifetime. Based on the findings, the failure hypothesis “cathodic WEC fatigue” for electrical direct current-initiated WEC failures was established.

Australian National Electricity Market Model - Version 1.10

This working paper provides details of the Australian National Electricity Market (ANEM) model version 1.10 used in the research project titled: An investigation of the impacts of increased power supply to the national grid by wind generators on the Australian electricity industry. The paper provides a comprehensive reference of the ANEM model for the other project publications that use the ANEM model to analysis the sensitivity of four factors to increasing wind power penetration. The four factors include (1) transmission line congestion, (2) wholesale spot prices, (3) carbon dioxide emissions and (4) energy dispatch. The sensitivity of the four factors to wind power penetration is considered in conjunction with sensitivity to weather conditions, electricity demand growth and a major augmentation of the transmission grid of the Australian National Electricity Market (NEM) called NEMLink (AEMO 2010a, 2010b, 2011a, 2011b).The sensitivity analyses use 5 levels of wind power penetration from zero wind power penetration to enough wind power to meet the original 2020 41TWh Large-scale Renewable Energy Target. The sensitivity to weather is developed by using half hourly electricity demand profiles by node from three calendar years 2010, 2011 and 2012. The sensitivity to growth is developed by incrementing the nodal demand profiles over the projection years 2014 to 2025.Five inputs of the ANEM model are:(1) Half hourly electricity 'total demand' for 50 nodes in the NEM;(2) Parameter and constraint values for 68 transmission lines and 330 generators, albeit incorporating the de-commissioning of Generation plant occurring over the period 2007-2014;(3) Carbon price, which is assumed zero in this project;(4) Fossil fuel prices; and(5) Network topology of nodes, transmission lines and generators.Five outputs of the ANEM model are:(1) Wholesale spot price at each node (half hourly),(2) Energy generated by each generator (half hourly),(3) Energy dispatched (sent out) by each generator (half hourly),(4) Power flow on each transmission line (half hourly), and(5) Carbon dioxide emissions for each generator (daily).ANEM uses a Direct Current Optimal Power Flow (DC OPF) algorithm to determine optimal dispatch of generation plant, power flows on transmission branches and wholesale prices.

Analyzing the Impacts of Constraint Relaxation Practices in Electric Energy Markets

Reliable and economic deployment of a generation fleet to satisfy demand is a complex problem. Independent system operators (ISOs) and regional transmission operators (RTOs) solve complex unit commitment and economic dispatch models to determine appropriate resources to deploy at various time stages. Due to the complexity of power systems, several approximations are made within optimization models, including approximations of the transmission network with a linearized formulation known as the direct current optimal power flow (DCOPF) instead of the more realistic formulation known as the alternating current optimal power flow (ACOPF). Furthermore, approximations occur in these models by relaxing specific constraints in the model, i.e., the constraint is allowed to be violated based on a predetermined penalty price. By doing so, the ISO/RTO receives several benefits, including the ability to manage prices, clear the market, as well as the potential to obtain gains in social welfare (market surplus). This paper describes the constraint relaxation practices of ISOs in their unit commitment models and analyzes the corresponding market implications based on these practices.

A Stochastic Transmission Planning Model With Dependent Load and Wind Forecasts

This paper introduces a two-stage stochastic program for transmission planning. The model has two dependent random variables, namely, total electric load and available wind power. Given univariate marginal distributions for these two random variables and their correlation coefficient, the joint distribution is modeled using a Gaussian copula. The optimal power flow (OPF) problem is solved based on the linearized direct current (DC) power flow. The Electric Reliability Council of Texas (ERCOT) network model and its load and wind data are used for a test case. A 95% confidence interval is formed on the optimality gap of candidate solutions obtained using a sample average approximation with 200 and 300 samples from the joint distribution of load and wind.

Enhancement of Do-Not-Exceed Limits With Robust Corrective Topology Control

In recent years, the penetration of renewable resources in electrical power systems has increased. These renewable resources add more complexities to power system operations, due to their intermittent nature. As a result, operators must acquire additional reserves in order to maintain reliability. However, one persistent challenge is to determine the optimal location of reserves and this challenge is exacerbated by the inability to predict key transmission bottlenecks due to this added uncertainty. This paper presents robust corrective topology control as a congestion management tool to manage power flows and the associated renewable uncertainty. The proposed day-ahead method determines the maximum uncertainty in renewable resources in terms of do-not-exceed limits combined with corrective topology control. The day-ahead topology control formulation is based on the direct current optimal power flow; therefore, topology control solutions obtained from these algorithms are tested for AC feasibility and system stability. The numerical results provided are based on the IEEE-118 bus test case and the Tennessee Valley Authority (TVA) test system.

Impact of Carbon Prices on Wholesale Electricity Prices and Carbon Pass-Through Rates in the Australian National Electricity Market

This paper investigates the effect of a carbon price on wholesale electricity prices and carbon-pass-through rates in the states comprising the Australian National Electricity Market (NEM). The methodology utilize an agent-based model, which contains many features salient to the NEM including intra-state and inter-state transmission branches, regional location of generators and load centres and accommodation of unit commitment features. The model uses a Direct Current Optimal Power Flow (DC OPF) algorithm to determine optimal dispatch of generation plant, power flows on transmission branches and wholesale prices. The results include sensitivity analysis of carbon prices on wholesale prices and carbon pass-through rates for different states within the NEM.

Power System Voltage Stability Assessment Based on Branch Active Powers

We prove that the active power through a branch (referred to as branch active power) in a power system is a continuous function of the bifurcation parameter in the closed interval from an initial parameter value to the bifurcation-point value of a saddle-node bifurcation (SNB) or limit-induced bifurcation (LIB) of the power flow equation (PFE). Then we show that generally there is a sequence of branch active powers meeting maxima when the parameters are equal to intermediate values in the closed interval. These results can be used to qualitatively evaluate and classify the state of power system operation conditions in terms of voltage stability. Also, they well explain the limitation of the direct current (DC) power flow methods on stressed power systems. Numerical simulations of the IEEE 118-bus and 57-bus systems are used to illustrate the studies and their applications.

Alongshore and cross-shore circulations and their response to winter monsoon in the western East China Sea

An array of four bottom-mounted acoustic Doppler current profilers (ADCPs) were deployed during the winter of 2008 (28 December 2008 to 12 March 2009) along a cross-shelf section in the western East China Sea to investigate the winter circulation and its response to wind. During the observation period, the observed subtidal currents exhibit coherent spatial structure and temporal variation in terms of their mean (seasonal), trend (intra-seasonal), and synoptic variability. The subtidal currents are polarized roughly in the alongshore direction parallel to local isobaths, and the weak cross-shore current is closely linked to the alongshore component. The temporal variation of the currents follows the rhythm of wind stress, sea level, and sea level difference at the synoptic scale.The mean currents are basically composed of two anti-parallel currents in the alongshore direction: the East China Sea coastal current (ECSCC) flows southwestward along the inner shelf and the Taiwan warm current (TWC) flows in the opposite direction along the outer-shelf. The strongest current occurs over the mid-shelf as a coastal jet. The intra-seasonal currents exhibit an expansion and intensification of the ECSCC along with shrinking and weakening the alongshore component of the TWC. There is a significant increase in onshore current particularly over the mid-shelf.The fluctuations of synoptic currents show a significant positive correlation with wind stress, and the fluctuations are negatively correlated with sea level and sea level difference. The coherent spatial structure of the currents indicates that the depth-independent column motion is related to the sea level difference through a barotropic pressure gradient. The vertical shear of currents is related to the density-related baroclinic pressure gradient in the whole water column and to the friction within the surface and bottom boundary layers.

Adjustable risk-based direct current optimal power flow

In order to reflect operators' risk-averse preference to the worst-case contingency, an adjustable risk-based direct current optimal power flow is proposed in this paper. This problem is formulated via a two-stage min-max-min programing, in which the first-stage optimizes the base case preventive dispatch, while the second-stage determines the best corrective actions to deal with the worst-case contingency. A Benders decomposition-based iterative algorithm is developed to solve the problem. Numerical studies based on a six-bus system and a modified RTS-96 indicate that the proposed approach mitigates risk of the worst-case contingency and allows the flexibility of adjustable preventive/corrective solutions. Copyright © 2014 John Wiley & Sons, Ltd.

Influence of inner diameter and position of phase adjuster on the performance of the thermo-acoustic Stirling engine

A thermo-acoustic Stirling engine can convert thermal energy into useful acoustic power without moving parts, which can be used as driving power for the refrigerators and the generators. However, the low efficiency of the thermo-acoustic engine limits its commercial application. In this paper, to promote the performance of the thermo-acoustic engine, a phase adjuster (PA) is introduced by narrowing the cross-sectional area on the basis of the previous studies. The influences of the position and inner diameter of PA on the resonant frequency, pressure amplitude, acoustic power, sound direct current (DC) and efficiency are studied systematically. It is found that the maximal pressure amplitude, acoustic power and efficiency with optimal PA could be increased by 3%, 4.5% and 10% respectively compared to those without PA. In addition, the effect of PA on DC flow is also discussed in detail. Results show that the sound DC decreases with the farther position and smaller inner diameter of PA. Therefore, the suitable inner diameter and position can improve the efficiency of the engine, which needs to be weighed when designing or optimizing PA.

Reduction of PTDF matrix and its application in DC optimal power flow

SUMMARY The paper presents an approach where traditional Power Transfer Distribution Factors (PTDF) matrix is reduced in size so that it reflects sensitivities of branch flows to changes in nodal power injections at generator buses only. The advantage of the reduced PTDF matrix is in both reductions of decision variables in the optimization procedure and number of columns in branch flow constraints matrix. Furthermore, the number of rows is also reduced on the basis of a simple check, which effectively detects non-binding branch flow limits allowing corresponding rows of the matrix to be omitted. The reduced PTDF matrix was successfully applied in solving two direct current optimal power flow problems: cost minimization and total transfer capacity calculation. Copyright © 2014 John Wiley & Sons, Ltd.

Optical second-harmonic generation induced by electric current in graphene on Si and SiC substrates

We find that the flow of direct electric current (dc) through graphene on substrate enhances surface optical second-harmonic generation (SHG) from the graphene/substrate system. The current can enhance surface SHG by about 300% for a chemical-vapor-deposition (CVD) graphene monolayer on a SiO${}_{2}/\mathrm{Si}(001)$ substrate, and by about 25% for an epitaxial four-layer-graphene film on a 3.5\ifmmode^\circ\else\textdegree\fi{}-miscut vicinal SiC(0001) substrate. The enhancement in both the CVD and epitaxial graphene samples is due to electric field-induced SHG, which is produced by the current-associated vertical electric field at the SiO${}_{2}/$Si interface or at the graphene/SiC interface. Measurements of rotational-anisotropy SHG (RA-SH) from both samples revealed that the current-induced SHG varies strongly with the measurement location along the current flow direction. By measuring RA-SH from the vicinal SiC(0001) substrate, we determined all three second-order susceptibility tensor elements (${d}_{33}$ = \ensuremath{-}52.0 pm/V, ${d}_{15}$ = 20.0 pm/V, and ${d}_{31}$ = 18.7 pm/V) that characterize the SHG response of hexagonal SiC at the fundamental wavelength of 740 nm. We further determined the three effective susceptibility tensor elements (${d}_{33}$ = \ensuremath{-}135.8 pm/V, ${d}_{15}$ = 18.5 pm/V, and ${d}_{31}$ = 14.6 pm/V) that characterize the surface SHG from the graphene/vicinal-SiC(0001) sample and finally showed that the current-dependent tensor element ${d}_{33}$ can be enhanced to a large value of ${d}_{33}$ = \ensuremath{-}199.0 pm/V by electric current in epitaxial graphene.

DC corrective optimal power flow based on generator and branch outages modelled as fictitious nodal injections

This study deals with a new formulation for the direct current corrective optimal power flow. The formulation is based on the outage of generators and/or branches modelled as fictitious injections of active power. By including that fictitious injections in the optimisation problem, the injections are adjusted to the post-contingency state as a consequence of the corrective actions carried out to bring the system back to its normal state. So, when the analysis of contingencies is performed, the classical topological analysis and the subsequent analyses are avoided with this approach. This new formulation uses the sensitivity matrix between branch power flows and powers injected in a power system. An important feature of this matrix is to remain constant during the contingency analysis performed for the generation-load scenario (base case) of each period of time to be analysed. The approach proposed is illustrated in the IEEE-RTS of 24 buses. The results obtained in this distribution network demonstrate that the proposed methodology can assess the impact of contingencies with an acceptable accuracy and a short computation time.

Accuracies of Optimal Transmission Switching Heuristics Based on DCOPF and ACOPF

This paper considers optimal transmission switching (OTS) to reduce generation cost by removing lines from service. A mixed integer program (MIP) has been proposed to solve the OTS problem, based on the linear direct current optimal power flow (DCOPF) model. Because of excessive computation times for large, real systems, the MIP model has been followed by some heuristics, also based on the DCOPF, to obtain near-optimal solutions quickly. However, the approximations in the DCOPF model may lead to poor choices of lines to remove from service. We assess the quality of line removal recommendations that rely on a previously published, DCOPF-based heuristic, by estimating actual cost reduction with the exact ACOPF model, using the IEEE 118-bus and 300-bus test systems with several demand levels. We also extend this heuristic to be based on the ACOPF and compare the quality of its recommendations to those of the DCOPF-based heuristic. The DCOPF-based heuristic performs very poorly in several cases, even leading to cost increases sometimes. There is a need for approximations to the ACOPF which are accurate enough to produce reliably good results for OTS heuristics, but fast enough for practical use.