Line Current Research Articles

Control Method of Step Voltage Regulator on Distribution Lines with Distributed Generation

Generally, utilities regulate the voltage on the long power distribution line within a permissible range by using a step voltage regulator (SVR), which is located around the middle of the line and operates according to the condition of the line current. However, as large-scale distributed generations (DG’s) are interconnected into distribution lines, it is difficult to maintain the line voltage properly owing to bi-directional power flow or reverse power flow. Therefore, this paper proposes a novel SVR tap-changing algorithm to solve the problem, considering line load conditions and reverse power flow. Its validity is verified through the PSCAD/EMTDC software tool and simulations.

Dimensional reduction of a 3D thermoelectric model to create a reliable and time-efficient 2D model representing an aluminum electrolysis cell

In order to optimize the use of their assets, aluminum smelters aim at increasing the line current. In addition, the fluctuations of the energy market push the industry to exploit power modulation strategies. These variable operating conditions directly affect the thickness of the protective layer (ledge) inside the electrolysis cell. Continuous monitoring of the energy balance is therefore of utmost importance, although physical measurements are difficult due to the hostile environment. The 3D thermoelectric slice digital model is the most frequently used to solve the energy balance, but its high computation time prevents it from being used in a control system. For the first time, the creation of a novel 2D model from an existing 3D slice model is proposed for online thermal diagnosis and process control purposes. The creation starts with a detailed dimensional reduction protocol, which is a significant industrial contribution since it can be applied to any industries relying on 3D physical models. The rigorous methodology developed involves the identification of adjustable parameters that are inserted in the 2D model to minimize deviations caused by physical differences between models and ensure its reliability. Once calibrated, the 2D model is tested using a ± 5% input current variation. The results show minor differences (below 1%) between 3D and 2D models for the predicted behavior of the heat losses distribution and generated heat distribution. Side ledge profiles are very close in both models, the largest difference (1.8 cm) being in an acceptable range for this key operational parameter. During all simulations, the 2D model computing time was 40 to 60 times faster than the 3D model. With the 4.0 era, this is a reliable and time-efficient tool for industries wishing to turn to digital twins.

A backup protection scheme for the transmission lines based on the directional traveling wave and waveform similarity

The voltage source converter based high voltage direct current (VSC-HVDC) system is a practical solution to improve renewable energy integration, flexible consumption, and wide-range complementarity. Because the DC line fault must be cleared within a few milliseconds, a reliable and rapid identification of faulty lines and poles is crucial. Unfortunately, the traditional backup protection cannot respond to internal faults fast enough and is sensitive to fault resistance. What is worse is that a double-pole-to-ground fault with unequal positive-pole and negative-pole fault resistances cannot be identified by most of the existing protection methods. The paper proposes a pilot backup protection based on directional traveling wave and waveform similarity for VSC-HVDC transmission lines to compensate for the drawbacks. The fault characteristic of 1-mode directional traveling waves and the waveform similarity of instantaneous currents of DC lines are elaborated theoretically. Based on the analyses, the protection is established, and the four-terminal VSC-HVDC grid is modeled in PSCAD and OPAL-RT. Detailed theoretical analyses and extensive simulation/experiment results have proven that the proposed protection scheme has higher reliability, selectivity, rapidity, and sensitivity. Compared with the existing backup protections, the proposed protection scheme has superiorities in fault resistance and distance tolerance (2,000 Ω and the entire length of DC lines), fault differentiation of double-pole-to-ground fault with unequal positive-pole and negative-pole fault resistances, immunity from signal noise and measurement error interference, and quicker fault judgment (less than 4 ms).© 2017 Elsevier Inc. All rights reserved.

Modelling and design of low-power, non-conventional current sensors based on smart materials for high-voltage transmission lines

This paper presents a new technique for measurement of current based on magnetic shape memory (MSM) smart alloys. MSM alloys undergo shape changes when exposed to magnetic fields. The non-conventional instrument transformer (NCIT) proposed in this paper utilises this property to measure current. There is a correlation between the magnetic field produced by a current and the shape change of an MSM material (MSM sensor). By exploiting this correlation, we have shown that it is possible to measure alternating currents (a.c.) in high voltage overhead transmission lines. A change in the length of the MSM element causes voltage output in a linear variable differential transducer. The design of the NCIT was optimised for transmission lines. Several designs of its magnetic circuit were simulated using finite element package ANSYS APDL. Several key parameters were investigated to evaluate their effects on the sensitivity of the NCIT. Results are presented as the relationship between the current in the conductor and strain (linear elongation) of the MSM element. A commonly used conductor in high-voltage transmission lines was modelled together with the MSM element and the magnetic circuit. Recommendations have been made on the design of NCITs considering various parameters. In addition, analyses of errors in ANSYS models for the magnetic circuit have been presented. The developed methodology and obtained results are verified by comparing them to the results obtained through an experiment done by a manufacturer of MSM materials.

Decoupling Control of Triple Active Bridge-Power Flow Controller in Meshed Bipolar DC Distribution Grids

Due to the lack of freedom in power flow (PF) control of the meshed bipolar dc distribution grid, the PF of some lines becomes uncontrollable. Besides, there is a coupling between the poles due to the unbalanced current and line resistance, which may deteriorate the dynamic performance of the control system. To solve the above problems, this article proposes an unbalanced PF regulation methodology based on a triple active bridge-power flow controller (TAB-PFC) and studies the decoupling control of TAB-PFC. This article analyzes the influence of receiving-end voltage on positive and negative PF and line losses. The expressions of output voltage and line current of TAB-PFC under constant power control are also derived, and the small-signal model of a meshed bipolar dc distribution grid with TAB-PFC is established. On this basis, the decoupling matrix is introduced between the positive and negative control loops to improve the dynamic performance of the control system. A simulation model of meshed bipolar dc distribution grid with TAB-PFC is established in MATLAB/Simulink, and an experimental platform is built. The simulation and experimental results verify the effectiveness of TAB-PFC in restraining unbalanced PF and the effectiveness of the decoupling matrix.

A Parallel-Connected 24-Pulse Rectifier Using Hybrid Harmonic Reduction Method at DC Side

This letter proposes a parallel-connected 24-pulse rectifier using a hybrid harmonic reduction method at the dc side. The proposed rectifier contains an unconventional interphase reactor (UIPR) with a double-tapped primary winding and secondary winding connected pulsewidth modulation converter. The proposed method is a combination of the passive and active harmonic method. From the viewpoint of total harmonic distortion (THD) of the input line current, ripple load voltage, and kilovoltampere rating of the active auxiliary circuit, the UIPR and active injecting current are optimally designed. An experimental prototype is developed to verify the theoretical analysis. The experimental THD is about 2.87%, which shows good performance of harmonic reduction.

Electromagnetic Energy Harvester Design for Power Transmission Line

Parallel to the increase in energy demand, the importance of energy transmission lines is also increasing. In order for energy transmission to be carried out correctly, transmission lines must operate uninterruptedly. Therefore, it is important to inspect and monitor the lines and to intervene immediately in case of any fault. The use of Unmanned Aerial Vehicles (UAV) for the inspection and monitoring of these lines provides advantages in terms of both safety and cost. In the study, the design and analysis of electromagnetic energy harvesters have been made in order to provide energy to UAVs. Firstly, the mathematical modeling of the core has been presented. Electromagnetic energy harvesters designed based on this model have been analyzed using the Finite Element Method (FEM). The performances of toroidal magnetic energy harvesters have been investigated according to core size, material properties, air gap and line current. According to the results obtained from the analyzes, the harvester with the best size/energy density has been determined

Single phase to ground fault location method of overhead line based on magnetic field detection and multi-criteria fusion

The distribution network has many branches and complex structure, and the existing protection is difficult to achieve accurate fault section location. Moreover, the monitoring device based on electrical quantity has some problems, such as high cost, power failure in installation and maintenance, and the information collected can not meet the requirements of distribution network for operation monitoring. Therefore, fault location is still a difficult problem in the existing distribution network. The magnetic field (MF) under the overhead line can reflect the change of line current, locate the fault position between two pole towers, and the magnetoresistive sensor has the advantages of low cost and uninterrupted installation and maintenance. Therefore, in recent years, the non-contact fault feeder detection and location method based on MF has begun to develop rapidly. Based on the similarity analysis of MF waveform under overhead line from different aspects by using five criteria: relative entropy, Frobenius norm, maximum mean discrepancy, cosine similarity and correlation coefficient, it proposed a multi criteria fusion fault location method based on D-S evidence theory, which makes full use of the advantages of each criterion and overcomes the limitations of single criterion. Simulation results show that the method is not affected by conductor sag, galloping and data window length, and it fit for the noise interference and high impedance fault (HIF). Compared with the other MF method, the method has stronger adaptability.

Multiple open-switch faults detection for grid connected inverter

This paper proposes a new multiple open-switch faults (OSFs) detection method for grid connected inverter. Frequency deviation and changing loads of the grid interference are mainly concerned. Only line currents of the inverters are used as the inputs. The method uses extended Kalman filter (EKF) to extract the fault current features and random forests (RF) to recognize the features for fault detection. Because the EKF is a time varying method and load variation does not change the shape of current waveforms, the features can be extracted without influence of load changes. By taking fundamental frequency into the filter model, impact of frequency deviation of the grid on fault detection is also discussed and solved. OSFs detections in bias current and overcurrent situation are also discussed. Motor load influence is discussed too. Results of simulation and experiment demonstrate that the new method can be used on multiple open- switch faults within 2 cycles delay in grid in various situations including load changes, bias current, overcurrent, frequency deviation and motor load interference of the grid.

State and Parameter Estimation of Non-Uniform Transmission Line Using Kronecker Product Based Modeling

The distributed parameters of a non-uniform transmission line (NTL) are assumed to depend on a finite set of unknown parameters, for example, their spatial Fourier series coefficients. The line equations are set up taking random voltage and current line loading into account with the random loading being assumed to be white Gaussian noise in the time and spatial variables. Bypassing over to the spatial Fourier series domain, these line stochastic differential equations (SDEs) are expressed as a truncated sequence of coupled stochastic differential equations for the spatial Fourier series components of the line voltage and current. The coefficients that appear in these stochastic differential equations are the spatial Fourier series coefficients of the distributed line parameters and these coefficients in turn are linear functions of the unknown parameters. By separating these complex stochastic differential equations into their real and imaginary part, the line equations become a finite set of SDEs for the extended state defined to be the real and imaginary components of the line voltage and current as well as the parameters on which the distributed parameters depend. The measurement model is assumed to be white Gaussian noise corrupted versions of the line voltage and current sampled at a discrete set of spatial points on the line and this measurement model can be expressed as a linear transformation of the state variables, i.e, of the spatial Fourier series components of the line voltage and current plus a white measurement noise vector. This work uses sparse matrix factorization using Kronecker product to implement the Fourier unitary transform. This work implements perturbation theory for non-uniform transmission line. The extended Kalman filter (EKF) equations are derived for this state and measurement model and our real timeline voltage and current and distributed parameter estimates show excellent agreement with the true values based on MATLAB simulations. The use of sparse matrix factorization using Kronecker product ease the mathematical complexity and provides compact representation.

Edge Computing Based Fault Sensing of the Distribution Cables Based on Time-Domain Analysis of Grounding Line Current Signals

Due to the complex structures and fault conditions, accurate fault sensing of the underground distribution cables is a difficult task. The distribution automation system (DAS) and fault indicator system (FIS) are only applicable to the severe faults. In addition, the current DAS and FIS are all based on the centralized data processing system (CDS), increasing the communication pressure and data processing time of the main station. Considering the advantages of edge computing, this paper investigates using the grounding line currents to sense the fault in distribution cable networks. First, an edge computing based fault sensing system (ECS) is designed. The equivalent time-domain distribution cable models considering multi-conductor mutual couplings are then established in both normal and fault cases. The time-domain features of the cable grounding line currents prior to and after the fault are analyzed to create the fault detection criterion. Combining the network topology, locations of the measurement units (MUs), and direction factor of the ground line current, a space matrix based algorithm is created to achieve fault cable section location at edge. Finally, a distribution cable network is created by PSCAD/EMTDC. Various simulations are conducted to validate the effectiveness and robustness of the algorithm.

Decentralized Mutual Damping Control of Cascaded-Type VSGs for Power and Frequency Oscillation Suppression

This paper reveals the power and frequency oscillation mechanism of the islanded cascaded-type virtual synchronous generators (VSGs), and proposes a decentralized mutual damping suppression method. In the proposed scheme, the mutual damping term is constructed via the decentralized manner with transmission line current. It contributes to increasing damping and accelerating the frequency convergence of cascaded-type VSGs in the dynamic process. As a result, the power and frequency oscillations are restrained, and the system dynamic performance is improved. Further, the system stability based on the Lyapunov energy function method is proved. Finally, the effectiveness of the proposed decentralized mutual damping control is verified by simulations and experimental results.

Analysis and Demonstration of Control Scheme for Multiple Operating Modes of Energy Storage Converters to Enhance Power Factor

Rectifiers are required by the devices connected to the distribution end of the electrical power networks for AC/DC conversion. The line current becomes non-sinusoidal when a capacitor with a significant value is used to mitigate the output voltage ripple. This type of converter emulates a non-resistive impedance to the grid, due to which a bend occurs in the shape of the line current, which results in high total harmonic distortion and a low power factor. For perceiving sinusoidal current, power factor correction techniques are required. A digital controller for parallel-connected buck-boost power factor correctors is presented in this article to maintain a constant output voltage and to deal with circulating currents amongst parallel-connected converters. The proposed digital supervisory controller also regulates the input and line currents to keep them sinusoidal according to the input supply voltage to maintain the high power factor of the system. In this paper, using the differential equations of a buck-boost converter, the duty cycle calculations are performed for both Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM), which are responsible for providing a unity power factor. A supervisory controller encompasses a feed-forward control algorithm for tuning model parameters for eliminating the harmonics from the line current. The proposed scheme helps calculate duty cycles which provides a unity power factor and minimizes the circulating currents. The proposed method was simulated in MATLAB/Simulink and their digital-hardware validation testing was also performed using C2000 MCU Launchpad.

Model Predictive Phase Control for Single-Phase Electric Springs

In this paper, model predictive control (MPC) is proposed for single-phase electric springs (ESs) with the help of the existing δ control, which is realized by controlling the instantaneous phase angle of the predefined sinusoidal reference of a certain controller. System modeling is analyzed first to get differential forms of state variables. The discrete-time state space model is obtained through first-order approximation. Critical load (CL) voltage can be predicted by the prediction of ES voltage and line current. The operating modes of ESs can be determined and the reference signal for CL voltage can be provided by δ control. As a result, cost function is obtained as the absolute value of the error between predicted CL voltage and its predefined reference. Two typical operating functions such as pure reactive power compensation mode and power factor correction (PFC) mode are selected and simulated to validate the proposed control and analysis. It is revealed that both control objectives can be achieved with the proposed MPC and δ control. Additionally, the total harmonic distortion on the critical load is limited to about 0.5%, which is better than other existing methods.

A Three-Phase Sensitivity-Based approach for smooth Line-Switching in islanded networks

This paper deals with a new smooth line-switching method that facilitates the network reconfiguration of islanded networks. Its distinct features include the ability to handle network asymmetries and to minimize the line current during the switching action. This is attained by developing a three-phase sensitivity-based method to determine the operating set-points of distributed generators (DGs) that minimize the current of the candidate line participating in the switching action. These set-points correspond to the positive-sequence powers as well as to the negative- and zero-sequence currents of DGs. Furthermore, the network constraints such as voltage limits and power limits of DGs are always satisfied. Simulations are performed in a balanced 33-bus islanded network as well as in the unbalanced IEEE 8500-node network to evaluate the performance of the proposed method.

On-Line Core Losses Determination in ACSR Conductors for DLR Applications.

Dynamic line rating (DLR) is a method that focuses on dynamically determining the maximum allowable current of power lines, while ensuring they operate within safe limits. DLR needs to monitor the temperature and current of the line in real-time, as well as the weather variables in the surroundings of the power line. DLR approaches also require determining the AC resistance of the power line conductors, which is a key parameter that enables it to determine Joule and core losses. This paper presents an approach for an on-line alternating current (AC) resistance estimation of aluminum conductor steel-reinforced (ACSR) conductors to determine the DLR capability of such conductors from real-time conductor and meteorological parameter measurements. For this purpose, conductors with one, two and three layers of aluminum strands are analyzed in detail. Based on the experimental results presented in this paper, two possible approaches are proposed.

Observers for Differential Algebraic Equation Models of Power Networks: Jointly Estimating Dynamic and Algebraic States

Phasor measurement units (PMUs) have become instrumental in modern power systems for enabling real time, wide-area monitoring and control. Accordingly, many studies have investigated efficient and robust dynamic state estimation (DSE) methods in order to accurately compute the dynamic states of generation units. Nonetheless, most of them forego the dynamic-algebraic nature of power networks and only consider their nonlinear dynamic representations. Motivated by the lack of DSE methods based on power network’s differential-algebraic equations (DAEs), this article develops a novel observer-based DSE framework in order to perform simultaneous estimation of the dynamic and algebraic states of multimachine power networks. Specifically, we leverage the DAE dynamics of a power network around an operating point and combine them with a PMU-based measurement model capable of capturing bus voltages and line currents. The proposed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathcal {H}_{\infty }$</tex-math></inline-formula> observer, which only requires detectability and impulse observability conditions that are satisfied for various power networks, is designed to handle various noise, unknown inputs, and input sensor failures. The results obtained from performing extensive numerical simulations on IEEE 9-bus and 39-bus systems showcase the effectiveness of the proposed approach for DSE purposes.

Smooth Integration of Rectifier-Battery Banks Operation in Real-Life Isolated Microgrids Based on Renewable Sources: Theory and Application

Isolated microgrids need dispatchable energy sources to ensure continuity of service during lack of the renewable energy sources. One common source is the diesel generator which is used as a backup and for battery charges. Practical experiences have shown that microgrids have operation problems when diesel generators and rectifiers work together to charge battery banks. This is because of resulting diesel generator voltage distortions caused by the interaction of line currents harmonics with equivalent series reactance of the AC source and the notches introduced during the current commutation between 2 SCRs of the bridge rectifier. These distortions eventually inhibit the rectifier operation, causing malfunctioning of the system. One solution is to use generators with rated power greater than the rectifiers, which results in oversized installation and high operating costs. This work proposes a solution to avoid generator oversizing by using resonant filters, notch filter and a capacitive reactive compensation circuit to avoid self-excitation of the generator. Soft-start operation of the rectifier, and disturbance decoupling in the generator are also proposed. The design allows the use of generators and rectifiers of similar power, reducing the installation and operation costs of microgrids. It has been used successfully in two real-life microgrids.

Evaluation of Different Methods for Voltage Sag Source Detection

This paper compares and evaluates three different methods for voltage sag source detection. First method (method I) is based on the assumption, that the energy flow at the monitoring point increases during downstream events and decreases during upstream events. Second and third method (methods II and III) are both based on the assumption that currents measured at the monitoring point increase during downstream events and decrease during upstream events. The slope of a current-voltage trajectory is investigated in method II, while a real current component is observed within method III. Both current-based methods (II and III) require fundamental harmonic components of sampled voltages and currents, which are extracted using discrete orthogonal series expansion, such as Fourier or Walsh. Algorithms of this type are especially appropriate for studying steady-state and periodically repeating conditions. Voltage sags are, on the contrary, transient disturbance events. Thus, usage of the discussed algorithms may not be appropriate. Furthermore, criteria within methods II and III are checked for each phase individually. In the cases of asymmetrical voltage sags exact interpretation of the obtained results, therefore, might not be possible. Method I is, on the contrary, based on instantaneous values of line voltages and currents, while three-phase criterion is used. An exact interpretation of the results obtained by this method is, therefore, also possible in cases of asymmetrical voltage sags. All the discussed methods for voltage sag source detection have been tested by applying extensive simulations and field tests. The results for ground faults, asymmetrical voltage sags, upstream events and motor starting have been analyzed in order to evaluate all the discussed methods. The obtained results show that all discussed methods are very successful in cases of heavy motor starting and other symmetrical voltage sags. In cases of asymmetrical voltage sags the methods II and III do not work well, especially for those originating from the upstream side, while the method I is not successful only in particular cases of voltage sags due to upstream ground faults. Based on the performed evaluation it can be concluded, that further development is still needed to increase the degree of confidence in the discussed methods.

Testing and validation of a modified bride-type nonsuperconducting fault current limiter

As the amount of distributed generation (DG) integrated into the distribution network keeps increasing, this leads to an increase in the levels of fault current in the network, and this will result in the network having a fault current above what the existing protecting devices can handle. Hence, an upgrade of protective devices will be necessitated. A lot of techniques have been developed to mitigate against high fault currents. Some of these techniques with their limitations include current-limiting fuses that need constant replacement after an operation; circuit breakers that are very expensive for high current applications; isolation transformers that result in additional network power losses; and current-limiting and air-core reactors that impede the voltage stability of the network. Following these shortcomings, fault current limiters saw the light as the paramount solution to restrain fault currents in the distribution network. This research work is a follow-up of previous research that was on the design of a modified bridge-type nonsuperconducting fault current limiter (MBNSFCL) for application in the distribution network. In this paper, the designed MBNSFCL is tested and validated on two standard IEEE distribution networks, which are the IEEE 13 and the 33 node test networks. The simulation was done using PSCAD/EMTDC. In the case of the IEEE 13 node, the occurrence of a 3 phase to ground fault on the swing bus leads to the source current shooting from an amplitude of 0.602 kA on phase A, 0.335 kA on phase B, and 0.356 kA on phase C to 34.996 kA on phase A, 35.126 kA on phase B, and 34.983 kA on phase C. The fitting of the MBNSFCL into the test network restrains the fault current below the nominal line current, a level at which the circuit breaker can comfortably clear. The virtue of the MBNSFCL is also established when tested on the IEEE 33 node test network. This reveals the reliability of the proposed MBNSFCL for application in the distribution network.