Two-terminal Transmission Research Articles

Transmission Line Fault Classification Using Conformer Convolution-Augmented Transformer Model

Ensuring a consistently reliable power supply is paramount in power systems. Researchers are engaged in the pursuit of categorizing transmission line failures to design countermeasures for mitigating the associated financial losses. Our study employs a machine learning-based methodology, specifically the Conformer Convolution-Augmented Transformer model, to classify transmission line fault types. This model processes time series input data directly, eliminating the need for expert feature extraction. The training and validation datasets are generated through simulations conducted on a two-terminal transmission line, while testing is conducted on historical data consisting of 108 events that occurred in the Taiwan power system. Due to the limited availability of historical data, they are utilized solely for inference purposes. Our simulations are meticulously designed to encompass potential faults based on an analysis of historical data. A significant aspect of our investigation focuses on the impact of the sampling rate on input data, establishing that a rate of four samples per cycle is sufficient. This suggests that, for our specific classification tasks, relying on lower frequency data might be adequate, thereby challenging the conventional emphasis on high-frequency analysis. Eventually, our methodology achieves a validation accuracy of 100%, although the testing accuracy is lower at 88.88%. The discrepancy in testing accuracy can be attributed to the limited information and the small number of historical events, which pose challenges in bridging the gap between simulated data and real-world measurements. Furthermore, we benchmarked our method against the ELM model proposed in 2023, demonstrating significant improvements in testing accuracy.

Transient current protection for transmission lines based on the Kalman filter measurement residual

This paper introduces a pilot protection method for two-terminal transmission lines based on the measurement residual of an extended Kalman filter designed to track alternate current signals. Transmission lines that interconnect generation plants to consumers are the primary equipment most susceptible to faults, and failing to isolate them correctly and promptly may lead to system collapse. In this context, the measurement residual allows for rapid fault identification and tripping. In addition, the proposed solution requires computing and communication resources typically available for pilot protection since it uses conventional sampling rates and has a low computational burden. This paper details the theory and application of the extended Kalman filter and the results of the proposed method when tested against different simulation scenarios. These scenarios were built on an extra-high voltage transmission system implemented in ATP/EMTP, varying fault impedance, inception angle, distance, phases involved, and fault type. It also presents a statistical analysis of these scenarios, including channel asymmetry, errors in the current transformers’ conversion rate, and noise. Besides, it describes an analysis of the proposed method regarding current transformer saturation, high impedance faults, zero-crossing faults, external breaker opening, and power-swing. Finally, it compares the solution with other algorithms in the literature. The results indicate that the proposed solution is fast, accurate, secure, and dependable.

Accuracy analysis using the EMD and VMD for two-terminal transmission line fault location based on traveling wave theory

This paper presents an accuracy analysis from the application of techniques for the extraction of characteristic data from current and voltage signals in transmission line fault location based on traveling wave theory. The techniques used were the empirical mode decomposition and the variational mode decomposition associated with Teager energy operator, which helps to identify the instantaneous energy of the first intrinsic mode function. The simulation of the power system was carried out using the MATLAB/Simulink® software, for a test system consisting of a 200 km long transmission line between two Thevenin equivalents. Data is obtained from both terminals. The fault resistance was fixed at 100 Ω, but the incidence angle, the sampling rate and the fault location were varied for all fault types. The numerical and graphical results proved that the techniques can extract the characteristic data of the current and voltage signals and estimate the fault distance to the terminal with high accuracy, depending only on the sampling rate adopted.

Stability analysis of two-terminal HVDC transmission systems using SISO open-loop gains

The ports of a three-phase voltage source converter have multiple frequency electrical parameters, which makes HVDC systems with these converters present multiple input multiple output (MIMO) characteristics. The generalized Nyquist criterion of the MIMO model is often used to analyze the system stability. However, this model is more difficult to obtain than the single-input single-output (SISO) model, and the stability margin and the oscillation frequency band are difficult to capture. To solve these problems, this paper took the two-terminal HVDC transmission system as the research object to derive a SISO model that can be measured by a port. Moreover, this study also proposed a method for analyzing the system stability through envelope SISO open-loop gains. Finally, the key port and parameters affecting the system stability were analyzed according to the stability margin, which provided theoretical support for the design of the system parameters. The established model and the proposed analysis method were verified in a MATLAB/Simulink simulation model.

Non-unit Protection for Asymmetrical DC Line Faults in Bipolar LCC-HVDC Transmission Systems

The faults in high voltage direct current (HVDC) transmission lines cause a sudden rise in DC, resulting in over-stressing of converter valves and transformer windings. DC line faults of longer duration, or permanent nature, will lead to a severe disturbance in the associated AC network as well. Thus, the HVDC line faults must be detected immediately to interrupt the fault current by initiating proper actions of control and protection. This paper implements a new method to detect the asymmetrical (pole-to-ground) faults in the conventional bipolar LCC-HVDC transmission system. The proposed method defines a Fault Indicating Parameter (FIP) based on the transient voltage and current elements of each pole. The transient elements are obtained based on single-end measurements. Single-end or non-unit measurements lead to the development of simple, cost-effective and fast protection criteria. Also, the method is applicable to faults close to the line boundary and of high resistivity. A two-terminal bipolar LCC-HVDC transmission system, based on ±500 kV, 1000 MW and 900 km length, is used to evaluate the performance of the proposed technique in offline mode, using MATLAB/Simulink software. Also, the results are validated using an OPAL-RT real-time simulator under divergent fault conditions. The simulation results prove the robustness, selectiveness and accuracy of the proposed scheme.

Current- and Voltage-Actuated Transmission Line Protection Scheme Using a Hybrid Combination of Signal Processing Techniques

This paper presents a current- and voltage-driven protection scheme for transmission lines based on a hybrid mix of Stockwell transform (ST) and Hilbert transform (HT). Use of both current and voltage waveforms to detect and categorize faults, improves the reliability of this protection scheme and avoids false tripping. Current and voltage waveforms captured during a period of fault are analyzed using ST to compute a median intermediate fault index (MIFI), a maximum value intermediate fault index (MVFI), and a summation intermediate fault index (SIFI). Current and voltage signals are analyzed via applying HT to compute a Hilbert fault index (HFI). The proposed hybrid current and voltage fault index (HCVFI) is obtained from the MIFI, MVFI, SIFI, and HFI. A threshold magnitude for this hybrid current and voltage fault index (HCVFITH) is set to 500 to identify the faulty phase. The HCVFIT is selected after testing the method for various conditions of different fault locations, different fault impedances, different fault occurrence angles, and reverse flows of power. Fault classification is performed using the number of faulty phases and an index for ground detection (IGD). The ground involved in a fault is detected by comparison of peak IGD magnitude with a threshold for ground detection (THGD). THGD is considered equal to 1000 in this study. The study is carried out using a two-terminal transmission line modeled in MATLAB software. The performance of the proposed technique is better compared to a discrete wavelet transform (DWT)-based technique, a time–frequency approach, and an alienation method. Our algorithm effectively detected an AG fault, observed on a practical transmission line.

Research on control and protection technology of flexible low-frequency AC transmission project

With the development of offshore wind power development to deep and far sea, offshore flexible low-frequency AC (LFAC) transmission technology has attracted more and more attention from academia and industry in recent years. The control and protection system is the brain of flexible LFAC transmission, which is directly related to the performance, safety and benefit of LFAC transmission. In order to improve its operation reliability and limit the impact caused by any control link fault, in this paper, the control system is divided into system level control strategy, converter station level control strategy and valve level control strategy, which is from high level to low level according to the stratification principle. For the protection system, a specific zoning protection scheme is proposed. Finally, a two-terminal flexible LFAC transmission system is built in PSCAD/EMTDC. The simulation results prove the validity and correctness of the proposed control and protection strategy.

Localization phenomena and electronic transport in irradiated Aubry–André–Harper systems

The role of light irradiation on electronic localization is critically investigated for the first time in a tight-binding lattice where site energies are modulated in the cosine form following the Aubry–André–Harper (AAH) model. The critical point of transition from delocalized-to-localized phase can be monitored selectively by regulating the light parameters that is extremely useful to have controlled electron transmission across the system. Starting with a strictly one-dimensional (1D) AAH chain, we extend our analysis considering a two-stranded ladder model which brings peculiar signatures in presence of irradiation. Unlike 1D system, AAH ladder exhibits a mixed phase (MP) zone where both extended and localized energy eigenstates co-exist. This is the fundamental requirement to have mobility edge in energy band spectrum. A mathematical description is given for decoupling the irradiated ladder into two effective 1D AAH chains. The underlying mechanism of getting a MP zone relies on the availability of two distinct critical points (CPs) of the decoupled chains, in presence of second-neighbor hopping between the two strands. Using a minimal coupling scheme the effect of light irradiation is incorporated following the Floquet–Bloch ansatz. The localization behaviors of different energy eigenstates are studied by calculating inverse participation ratio, and, are further explained in a more compact way by calculating two-terminal transmission probabilities together with average density of states. Finally, the decoupling procedure is extended for a more general multi-stranded AAH ladders where multiple CPs and thus multiple mobility edges are found. Our analysis may provide a new route of engineering localization properties in similar kind of other fascinating quasiperiodic systems.

Parameters-free non-iterative two-terminal measurements synchronization algorithm

This paper presents an innovative phasor-based algorithm to synchronize measurements taken from two-terminal transmission lines. The line capacitive effect is taken into account and only the analysis of steady-state voltage and current phasors is required, eliminating the need for settings based on line electrical parameters. To evaluate the proposed solution, several tests are performed using data from 138 kV, 230 kV, and 500 kV/60 Hz lines that belong to the Brazilian interconnected power grid. Real operating conditions of the Brazilian system are considered to assess the algorithm performance under different scenarios, including light, moderate and heavy line loading cases, and various power factors and line lengths. The obtained results show that the proposed solution is reliable, straightforward, accurate, being useful for practical two-terminal phasor-based applications when traditional common time reference sources are not available.

The Possibility of Enhanced Power Transfer in a Multi-Terminal Power System through Simultaneous AC–DC Power Transmission

The feasibility of power transfer enhancement, through simultaneous AC–DC power transmission in a two-terminal transmission network, has been proposed earlier by the authors, and the concept is well established. To meet the increase in demand for electricity, a new technique is proposed in this article to increase the use of existing transmission lines in addition to independent control of AC and DC power flow. This paper extends the concept to a three-terminal transmission network by considering a power tapping from the middle of the line. DC is also superimposed in the already existing three-terminal AC transmission system. In the proposed topology, a multi-terminal simultaneous AC–DC system is used, which is integrated with a zig-zag transformer and more than two voltage source converter (VSC) stations. Each terminal may represent an area of the power system. Anyone/two-terminal(s) may act as sending end, whereas the remaining two/one terminal(s) may act as receiving end. Power can flow in either direction through each segment of the transmission system. At sending end, VSC converts a part of AC to DC and injects it into the neutral of the zig-zag transformer. On receiving terminal, DC power is tapped from neutral of zig-zag transformer and fed to VSC for conversion back to AC. The concept is verified in the digital simulation software PSCAD/EMTDC.

Supermetal-insulator transition in a non-Hermitian network model

We study a non-Hermitian and non-unitary version of the two-dimensional Chalker-Coddington network model with balanced gain and loss. This model belongs to the class D^dagger with particle-hole symmetry^dagger and hosts both the non-Hermitian skin effect as well as exceptional points. By calculating its two-terminal transmission, we find a novel contact effect induced by the skin effect, which results in a non-quantized transmission for chiral edge states. In addition, the model exhibits an insulator to 'supermetal' transition, across which the transmission changes from exponentially decaying with system size to exponentially growing with system size. In the clean system, the critical point separating insulator from supermetal is characterized by a non-Hermitian Dirac point that produces a quantized critical transmission of 4, instead of the value of 1 expected in Hermitian systems. This change in critical transmission is a consequence of the balanced gain and loss. When adding disorder to the system, we find a critical exponent for the divergence of the localization length \nu \approx 1, which is the same as that characterizing the universality class of two-dimensional Hermitian systems in class D. Our work provides a novel way of exploring the localization behavior of non-Hermitian systems, by using network models, which in the past proved versatile tools to describe Hermitian physics.

A new fault location scheme for parallel transmission lines using one-terminal data

This paper proposes a fault location (FL) scheme for a two-terminal double-circuit transmission line using one-terminal current and voltage measurements. As the proposed scheme is formulated in the phase-coordinates, the parallel transmission lines are not required to be decoupled, and the proposed scheme can be applied for transposed or untransposed lines. In addition, the proposed scheme takes into account the line asymmetry and the potential couplings between the phases of the same circuit or different circuits. The derived FL equation is based on two concepts. The first concept is that the fault impedance is mainly resistive component. The second one is that the voltages of the similar phases in both circuits at each line terminal are equal to each other. Further, a new fault classification method is introduced to recognize the faulted phases, which are initially required to apply the proposed FL scheme. Moreover, the IEEE 39-bus system is deployed as a test case, and it is simulated on PSCAD/EMTDC software. The frequency-dependent phase model is used to emulate the parallel transmission lines. All fault types, including the cross-circuit faults, are tested considering different fault resistances and locations. In addition, the effect of the measurement errors on the fault location accuracy is investigated.

Fault location method on two-terminal transmission line using synchronized time information of traveling waves

Fault location method on two-terminal transmission line using synchronized time information of traveling waves

Fault location in overhead transmission line: A novel non-contact measurement approach for traveling wave-based scheme

Traveling wave fault location methods have gained considerable growth over the years, with the anticipation that power systems will be accurate and reliable everywhere. But the performance of classical traveling wave methods has focused only on traditional current transformer measurement in the literature till date. Therefore, in this paper, an innovative non-contact measurement of traveling wave formulation is proposed to achieve accurate fault location and time of arrival detection on a two-terminal overhead transmission line, since power cable naturally generates magnetic and electrical fields in the surrounding atmosphere. The proposed method solitary depends on the first two and successive time differences between the incident and reflective wave from the fault point of the transmission line. Thus, the proposed scheme does not depend on current transformer measurement that stimulates the accuracy and reliability of the conventional traveling wave fault location method. To ensure an accurate time of arrival waves detection, we exploit the magnetic field sensor array on each tower to measure the quality of current traveling waves generated on the transmission line and locate the transient disturbances as a result of a short circuit. By analyzing the line energization, a comprehensive fault analysis on a 500-kV line was simulated with PSCAD/MATLAB toolbox compared to the two-terminal classical method. The sensitivity analysis demonstrated that the proposed method is much accurate and reliable to be deployed and cost-effective, making it highly useful and efficient in comparison to the classical two terminals traveling wave method with an average error of 0.014%. The effectiveness of the proposed method was verified by laboratory experimental setup.

Research on Operating Characteristics of Two-Terminal Flexible Direct Transmission with Improved Deadbeat Control

Traditional DC transmission technology has obvious defects in power adjustment speed, loss reduction, protection and control, etc., and it has been unable to meet the diverse needs of electricity consumption. The flexible DC transmission system not only solves the above shortcomings, but also has the advantages of independently controllable power and power supply to passive network. It is widely applied in distributed power generation by new energy, micro grid and other fields. This paper takes the two-terminal flexible DC transmission system as the research object, establishes the mathematical mode of two-terminal flexible DC transmission system, and studies the basic operation mode of the system and the voltage source converter (VSC) control strategy on this basis. Considering the problems of control delay and poor responsiveness of current closed-loop control when the traditional deadbeat control is applied to VSC converters, an improved deadbeat control strategy is proposed to solve the above-mentioned defects by predicting the current sampling at k+2 moment. Finally, the co-simulation is used to verify the correctness and reliability of the above-mentioned method.

Time-Domain Fault Location Algorithm for Double-Circuit Transmission Lines Connected to Large Scale Wind Farms

This article introduces a new time-domain fault location algorithm for two-terminal parallel transmission lines connected to large scale wind farms. The proposed algorithm employs only a half-cycle data window of synchronized current samples at both line terminals to avoid inaccurate estimation of current phasors due to the generated sub- and inter-harmonics currents by the wind farms. The proposed algorithm does not need any transformation method to decouple the double-circuit transmission line. Moreover, it takes into consideration the effect of the line asymmetry and the potential couplings between the six phases. The fault location equation is deduced by equalizing the differential components of the calculated instantaneous voltages at the fault point, and then the fault distance is estimated directly without any iterative algorithm. The two-terminal parallel transmission line is emulated by PSCAD/EMTDC platform utilizing the frequency-dependent phase model, and the required calculations for fault location are conducted by MATLAB software. The proposed algorithm is tested for several fault resistances and fault locations, and all fault types, including cross-circuit faults. In addition, the effect of measurement, synchronization, and line parameters errors on the fault location accuracy is investigated. The obtained results confirm acceptable accuracy of the proposed fault location algorithm.

Determining alpha plane characteristics of line current differential protection in two-terminal high voltage transmission with different current transformer ratio

Determining alpha plane characteristics of line current differential protection in two-terminal high voltage transmission with different current transformer ratio

An Integral Fault Location Algorithm Based on a Modified T-Source Circuit Breaker for Flexible DC Distribution Networks

Flexible DC distribution networks have been widely developed because of their advantages of high efficiency, low cost, and easy access to renewable energy. However, as distribution networks have more ends than the two-terminal transmission line, it is more challenging to protect them and locate their faults. Therefore, this paper proposes a modified T-source circuit breaker to protect against DC faults. Based on the proposed circuit breaker, an integral fault location method using double-terminal asynchronous data is proposed. The accuracy of the algorithm is not affected by the fault resistance. There are no differential terms in the algorithm, which greatly reduces the sensitivity to noise. The integral algorithm can remain steady when the voltage and current are zero in oscillation and attenuation periods. This constitutes the main contribution of this paper. Finally, it is verified by simulation and experiment that the proposed circuit breaker can isolate the power source within a short time in the typical DC distribution network. Under a noise of 30 dB for simulation, the location error of the proposed algorithm is <0.4% when the fault resistance is 0.5 Ω–500 Ω.

Localization, transport, and edge states in a two-strand ladder network in an aperiodically staggered magnetic field

We investigate the spectral and transport properties of a two-arm tight-binding ladder perturbed by an external magnetic field following an Aubry-Andr\'e-Harper profile. The varying magnetic flux trapped in consecutive ladder-cells simulates an axial twist that enables us, in principle, to probe a wide variety of systems ranging from a ribbon Hofstadter geometry to helical DNA chains. We perform an in-depth numerical analysis, using a direct diagonalization of the lattice Hamiltonian to study the electronic spectra and transport properties of the model. We show that such a geometry creates a self-similar multifractal pattern in the energy landscape. The spectral properties are analyzed using the local density of states and a Green's function formalism is employed to obtain the two-terminal transmission probability. With the standard multifractal analysis and the evaluation of inverse participation ratio we show that, the system hosts both critical and extended phase for a slowly varying aperiodic sequence of flux indicating a possible mobility edge. Finally, we report signatures of topological edge modes that are found to be robust against a correlated perturbation given to the nearest neighbor hopping integrals. Our results can be of importance in experiments involving ladder-like quantum networks, realized with cold atoms in an optical trap setup.

Time-domain simulation analysis of combined suppression measures for subsynchronous oscillators

Subsynchronous oscillation (SSO) may occur in High Voltage Direct Current (HVDC) transmission systems during operation. It seriously affects the stability of the power system, and, if no reasonable control measures are taken, the shaft system of the unit can break. To study the AC side shafting of an HVDC system, the parameters of the first IEEE standard model of SSO were taken as the reference and combined with the CIGRE Benchmark two-terminal DC transmission model to build a complete set of HVDC systems for SSO research. The system SSO phenomenon was realized by setting the fault of a Supplementary Subsynchronous Damping Controller with wideband connection and a Supplementary Excitation Damping Controller with separate modes. They were designed at the network side to suppress the SSO of the system effectively, and the effectiveness of the suppression effect was verified by time-domain simulation.