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The effects of furfural generation and partitioning on the aging assessment of oil-paper insulation of power equipment were investigated. Oil-paper samples with various furfural contents were prepared using an accelerated thermal aging experiment. The mass of the furfural generated during the aging of insulation paper was calculated and used to establish the quantitative equation between the accumulated furfural and the degree of polymerization (DP) of the insulation paper. Results showed that the reciprocal of the DP value was linearly related to the amount of furfural generated by the insulation paper. Effects of temperature, moisture, and temperature on the furfural partitioning ratio between oil and paper were analyzed using a furfural partitioning experiment, and a furfural partitioning model was established. Results showed that the furfural partitioning ratio in oil was positively correlated with temperature and moisture but negatively correlated with the DP of the insulation paper. A method for assessing the age of an insulation paper by integrating the furfural generation and partitioning models was proposed and applied on a 500-kV reactor that was returned for factory maintenance. The application result indicated that the method could accurately assess the overall aging condition of the oil-paper insulation of the power equipment. [ABSTRACT FROM AUTHOR]

Oil-paper insulation ageing of transformer has characteristics of spatial locality due to uneven temperature distribution. The transformer is sealed, it is difficult to obtain the local ageing state of oil-paper insulation by sampling directly. Currently, there are no effective detection method to detect the insulation state of local regions. The oil-immersed paper resistivity inversion method can nondestructively assess the local state of oil-paper insulation. However, the current inversion methods still have shortcomings and need to be improved. In this paper, combining the back propagation neural network (BPNN) and Newton downhill method (NDM), an improved inversion method is proposed to optimize the current inversion method, so as to reduce the effect of initial value on it and make calculated results converge better. Finally, a 3D 10 kV transformer model is established to verify the effectiveness of the proposed method. The resistivity calculated by the improved method show that the method can provide initial resistivity values for technicians and improve the convergence and accuracy of current inversion method, which is more suitable for extending to the engineering practice. [ABSTRACT FROM AUTHOR]

In this paper, we present an update to the models used to calculate the remaining life of transformer paper insulation. A drawback to the current IEEE method is that it does not take into account the availability of water and oxygen on the life expectancy of Kraft paper insulation. We therefore set out to test an algorithm which does take these into account. For our investigation, we loaded three test transformers and ran them to near their end of life. Kinetic equations were applied to model the fall in the degree of polymerization of paper. Our model showed better agreement with the test results than that given from using the IEEE standard. The IEEE standard gives life expectancy for Kraft paper being aged in minimal oxygen under dry conditions, which is not necessarily representative of an old transformer. Wet paper and using high levels of oxygen can age nearly 40 times faster. The IEEE method needs to take the synergistic effect of water and oxygen on increasing the rate of paper aging into account. [ABSTRACT FROM PUBLISHER]

Moisture is one of the key variables that must be considered when determining the load profiles that can be safely applied to a transformer. The presence of high amount of water accelerates the aging rate of the transformer's solid insulation, shortening the life expectancy of the equipment. Moreover, the dynamic processes of moisture migration between paper and oil must be considered in order to avoid a potential cause of failure, such as the formation of bubbles in the paper–oil interface, or the moisture saturation and the subsequent formation of liquid water in oil. These processes are linked to the changes in temperature; however, within the transformer, both processes have very different time constants, which complicates the analysis. In this paper, a mathematical model is presented, which allows us to study the moisture dynamics in a transformer. The model considers a multi-physical approach incorporating a thermal module and a moisture dynamic module that makes it possible to analyze the behavior of the moisture for a certain load profile. Some application cases are included in this paper to illustrate the model operation. [ABSTRACT FROM AUTHOR]

UHV DC wall bushing is the only channel connecting the valve hall and the DC electric field in the converter station. For the epoxy - SF6 gas composite insulation wall bushing, the main insulation material is the epoxy/crepe paper core and SF6 gas. The transient process of establishing the DC electric field for the wall bushing is impacted by the temperature gradient. In this paper the 3-D temperature field of the wall bushing has been simulated considering the effect of heat conduction, convection and radiation. The simulation results have been confirmed using a temperature rise experiment. The transient process of the wall bushing under the actual operation condition has been simulated using the gas saturation current density method. The temperature impact on space charge density in the core, the surface charge on the interface, and the electric field distribution on the bushing is investigated. In this way a theoretical basis is developed for the optimal design of the bushing that is conductive to the safe operation of the bushing. [ABSTRACT FROM AUTHOR]

This paper addresses the frequency oscillation problem in a parallel-inverter-based grid-connected system. Angular frequency interactions between inverters and the grid exhibit various numbers of complicated characteristics that seriously threaten the connected power system's stable operation. This paper proposes a unified frequency oscillation method analysis to investigate the oscillation problem in a multi-VSG grid-connected system. In this method, the power control model of each grid-connected inverter is equivalent to a two-terminal network by analogy method, which can be described as a “current source” connected in parallel with a reciprocal of “admittance” and then joined in series with a joint of “admittance.” Next, considering the influence of grid impedance, the model of the whole grid-connected system is established to reveal the mechanism of multi-VSG angular frequency oscillation. Subsequently, the influence factors of frequency oscillation and its variation law are analyzed through transfer functions, which provide an essential theoretical method for solving the frequency oscillation problem in the current multi-inverter grid-connected engineering application. Finally, we validate the effectiveness and correctness of the small-signal model and the frequency oscillation characteristics by building the MATLAB/Simulink simulation. [ABSTRACT FROM AUTHOR]

This paper addresses procedures for power cable time-dependent rating calculations. Such calculations are required to obtain cable emergency ratings and also to observe the conductor temperature variations with time when subjected to variable loading. The calculation procedure is described in the IEC Standards 60853-1 and 2. A real difficulty in applying those procedures lies in an ability to take into account variation of the electrical resistances of the cable metallic components with temperature. The standard gives an approximate equation, developed by Goldenberg. However, this equation is applicable to a single current step applied to the cable. The paper shows that it cannot be easily applied to multiple current steps and proposes a new approximate algorithm providing an analytical solution to this problem. A comparison with a complex iterative procedure is also offered. [ABSTRACT FROM AUTHOR]

This paper deals with stabilization of an ideal transformer model (ITM) circuit, used as an interfacing algorithm for hardware-in-the-loop simulations. The ITM allows for the distribution of simulation model computations over multiple processors or can be used in power hardware-in-the-loop simulations. When specific impedance conditions of the partitioned circuit are not met, this interfacing algorithm becomes unstable. This paper first presents a new theoretical approach for representing the ITM interfacing algorithm that is based on delay differential equations. With a stability analysis of the obtained time-delay system, a stabilization method is proposed, which increases the stability boundaries of the ITM algorithm. Simulation examples, including the scenario of a two-level inverter system, are provided. [ABSTRACT FROM AUTHOR]

DC-DC converters are needed for the future development of high voltage direct current (HVDC) grids, as they allow to interconnect lines with different voltages and topologies. The dc-dc converters can increase the grid controllability adding power flow control, voltage regulation and/or fault blocking capability. The dc modular multilevel converter (dc-MMC) is a non-isolated solution proposed to interconnect HVDC systems with the same line topology. This paper proposes a new dc-MMC with a control strategy, which allows the converter to interconnect different line topologies (e.g., rigid bipole connected to a symmetric monopole). The paper presents the different line topologies in HVDC installations. Then, a mathematical model with a variable transformation is proposed for the new dc-dc converter. A control structure is proposed and implemented in Matlab/Simulink using an average arm model and simplified dc grids. The results validate the control in normal operation, fault blocking capability and post-fault scenario (degraded mode). [ABSTRACT FROM AUTHOR]

The Rogowski coil transducer's integrator will amplify the transmission error, which may lead to mal-operation of the relay. In this paper, a novel method of line relay based on the differential output of Rogowski coil is proposed. Besides, an improved fast distance relay based on the differential output of Rogowski coil is presented. Based on the equal transfer process of transmission lines(ETPTL), the voltage signal should be processed by the virtual Rogowski coil to solve the inconsistent transfer problem between voltage and current. Furthermore, this paper also presents the reconstruction method of voltage at the fault point and the least-square algorithm to solve the R-L differential equation, so as to improve the algorithm accuracy. Furthermore, the equal transfer process error of the virtual Rogowski coil is analyzed in the paper, and the design principle of low pass filter is also proposed. Simulation results and experiment results demonstrate that the improved fast distance relay scheme has high operation speed, which is signed prior to other distance relay schemes. The proposed method also provides a valuable reference for other types of transmission line relay based on Rogowski coil transducer. [ABSTRACT FROM AUTHOR]

A modular multilevel converter control system, based on converter energy storage, is proposed in this paper for two different control modes: active power and dc voltage. The proposed control system decouples the submodule (SM) capacitor voltages from the dc bus voltage. One of the practical applications is the management of active redundant SMs. A practical HVDC system with 401-level MMCs, including 10% redundancy in MMC SMs, is used for validating and demonstrating the advantages of the proposed control system. This paper also presents a novel capacitor voltage balancing control based on \max–\min functions. It is used to drastically reduce the number of switchings for each SM and enhances computational efficiency. [ABSTRACT FROM AUTHOR]

Forced ventilated underground installations are challenging to design due to the complex mathematics of turbulent fluid flow. Previous methods have tackled this problem, but only recently, the effect of confinement on a single cable has been considered. In this paper, the effect of cable confinement on the practical case of ventilated trefoil installations is investigated. The complete Computational Fluid Dynamics (CFD) problem is solved using Finite Element Method (FEM) simulations and then validated by laboratory experiments using pipes. FEM simulations are then used to vary the current, the wind velocity, and the distance of the trefoil from the tunnel wall to get a comprehensive picture of the phenomena. This data is used to create a new empirical equation for the Nusselt number for a wide range of operating conditions. The principle of similitude is validated for a scaled-down model of a large tunnel and then used to analyze the thermal rating of tunnels. The new equation improves the IEC standard equations by accounting for the effect of cable confinement on trefoil installations. An important finding of this paper is that a lower maximum permissible current rating is obtained, which if ignored can present a serious risk for thermal damage. It was also found that the cables must be installed three cable diameters away from the wall to prevent the derating effect of confinement. [ABSTRACT FROM AUTHOR]

This paper proposes an accurate impedance model for line commutated converter (LCC) in the stationary frame. The commutation overlap process caused by the transformer leakage inductance complicates the modeling for LCC. Traditional impedance models are derived in positive sequence and negative sequence frame based on the simplified commutation overlap process, which degrades the modeling accuracy and limits the description of frequency coupling. This paper systematically analyzes the effects of the commutation overlap process on the impedance model, and the impedance model with the accurate commutation overlap process is established in the dq-frame. For clearly revealing the frequency coupling phenomenon of the LCC, the complex space vector is employed to extend the impedance model in the dq-frame to the complex αβ-frame. A closed-loop model on PSCAD/EMTDC is used to verify the proposed impedance model in the time-domain and the frequency-domain. The consistency between the simulation results and theoretical analysis effectively verifies the accuracy of the proposed model. [ABSTRACT FROM AUTHOR]

This paper contributes to the accurate and efficient computation of control system equations in electromagnetic-transient simulation tools. The proposed approach is based on the formulation of a reduced rank Jacobian matrix in a fully iterative Newton method. In addition, this paper compares the accuracy and efficiency for the proposed method's variants, including a hybrid approach with a fixed-point solution. [ABSTRACT FROM PUBLISHER]

Filter capacitors are among the most serious noise sources in HVDC converter stations, and there are still no satisfactory noise reduction measures. This paper proposes the use of the mass tuning (MT) method to adjust the vibration frequency response characteristics and to realize vibration control and noise reduction. First, the theoretical model of the MT structure is developed based on continuum mechanics. Based on the theoretical model, it is revealed that inserting an MT component into the capacitor element package will cause a decrease in the resonant frequencies and antiresonant frequencies. Furthermore, the movement degree of the resonant frequencies increases as the position of the MT component becomes closer to the package end, while the movement degree of the antiresonant frequency is the largest when the MT component is located in the middle of the package model. The vibrations and the noise levels of an example MT capacitor are then compared with the results of an original capacitor. The comparison results show that the MT scheme designed in this paper can achieve a noise reduction effect of 12–15 dB(A). The proposed reduction method is proven to be both effective and efficient. [ABSTRACT FROM AUTHOR]

The IEC Standard 60287-1-1 contains the formulae for the calculation of the sheath and armor loss factors of a 3-phase cable system. Sheath circulating current losses are computed for single-, multiple-, or cross-bonded systems. This paper deals with cross-bonded cable systems. For unequal minor section lengths, the standard gives a formula allowing calculation of the circulating current losses when the lengths of the minor sections are known. The origin of this formula is not documented; hence, this paper gives its derivation. When the lengths of the minor sections are not known, the standard suggests using a factor of 0.004, which multiplies the circulating current loss factor computed for a multi-point bonded system. In this paper, the cable sheath circulating loss factor was found using different methods with the aim of checking the applicability of the 0.004 multiplier to practical cable systems with differences in the length of minor sections and system voltage. Three different cable systems are studied; the first is a 132 kV circuit with known lengths of minor and major sections. The second circuit is a 220 kV system where the lengths of minor sections are unknown and the third is a 220 kV real system with known, highly irregular section lengths. The calculated values of sheath circulating current loss factor, applying the formulas from the IEC 60287-1-1, were compared to the values calculated based on practical measurements of sheath induced voltages and currents. The effect of taking the measured and IEC values on the continuous current rating (ampacity) is also presented for the first two circuits and the comparison of the unknown and known section lengths for the third circuit is offered. [ABSTRACT FROM AUTHOR]

Network parameters error may seriously affect the accuracy of fault location results, even making them unacceptable in extreme cases. This paper proposes a new method that incorporates these errors in wide area fault location problem using phasor measurement units to estimate inaccurate network parameters employing robust state estimation. Our proposed method is composed of two stages. The first stage or the primary stage provides an initial point for the second stage, which is the main contribution of this paper. Additionally, the set of measurements contaminated with bad data could be obtained from the first stage, and its intersection with leverage measurements are then eliminated from the measurement set before feeding it into the second stage. Finally, fault point and faulted line parameters are estimated by the main (second) stage of the method. The proposed method has been applied to two test systems, namely Institute of Electrical and Electronics Engineers (IEEE) 14-bus and IEEE 57-bus systems, to evaluate its accuracy. Simulation results indicate high accuracy and consistency of the proposed method. [ABSTRACT FROM AUTHOR]

This paper utilizes the Newton Raphson (NR) iterative algorithm to solve a detailed heat balance equation for overhead transmission conductors, for the purpose of calculating a Dynamic Line Rating or temperature-dependent impedance using real-time or forecasted weather data. Provided ambient weather conditions, conductor parameters and electrical current passing through the conductor, a starting guess is formed and the iterative solution method arrives upon a solution within a preset solution tolerance. A simple method to approximate the Rayleigh Number / Nusselt Number and Reynolds Number / Nusselt Number relationships is proposed to ensure successful convergence. The paper performs a thorough, numeric electrothermal conductor simulation to demonstrate proper solution convergence. This exercise identifies a minimum conductor temperature rise necessary for the solution method to converge successfully. Error related to the minimum starting conductor temperature rise, and error introduced by the approximated air property functions is illustrated. The temperature solution method is compared with several well-known thermal models. [ABSTRACT FROM AUTHOR]

The degradation of the electrical contact in a gas insulated switchgear (GIS) is closely related to overheating at the contact. However, the disconnector contact is one of the hot contacts in GIS. Therefore, in this paper, the finite element method was used to solve the three-dimensional multiphysics coupling temperature rise field and flow field in the Single Phase in One Tank (SPIOT) type GIS disconnector under different operating conditions. From the perspective of temperature rise field and flow field, this paper analyzed the temperature rise characteristics under different working conditions and explored the feasibility of the direct application of air, CO2 and C4F7N/CO2 mixed gas to an electrical device. The disconnector temperature rise experiment platform was then built to verify the simulation. The results showed that the temperature rise field and the flow field distribution were similar in four gas environments. SF6 gas had the smallest temperature rise, followed by C4F7N/CO2 mixed gas, and the temperature rise of CO2 and air was relatively large. Therefore, from the perspective of the temperature rise field and flow field, it was feasible to directly apply C4F7N/CO2 mixed gas to an electrical device in order to meet insulation requirements. [ABSTRACT FROM AUTHOR]

Optical current transducers (OCTs) have clear advantages over conventional current transformers. To better understand the OCT performance, a basic OCT model is developed using finite-element analysis. To model an open-core OCT, the Faraday rotation and detection is implemented in the COMSOL Multiphysics. This paper introduces two experimental setups, which can be used for studying OCT performances under ac and dc excitations. The measured data agree with simulation results and validate the proposed model. The presented model can be developed to study different nonideal effects, in order to identify and mitigate noise sources in OCTs. [ABSTRACT FROM PUBLISHER]

This paper presents the results of ampacity studies and proposed remedial actions for the situation where a steam pipe and several distribution circuits cross a duct bank with HV transmission cables. The studies involved application of a novel solution using a gravitational water cooling system. The system and the corresponding mathematical model are described in detail in the paper. The system was implemented in the BC Hydro network in downtown Vancouver. Practical concerns of BC Hydro engineers involving safety and public utility regulations as well as the measurement results obtained before and after installation are also discussed in the paper. [ABSTRACT FROM AUTHOR]

The modular multilevel DC converter (M2DC) exploits the interleaving between the three legs of an MMC to realize a promising uninsulated DC/DC converter to interconnect HVDC grids. This paper details a current and energies decoupled model of the M2DC. The major idea proposed in this paper is focused on the full energy control generating optimal current references to minimize the internal currents magnitude. The energy sum and difference models are fully detailed. Both current and energy control loops are based on the model inversion principle in order to control all the state variables. The proposed control is based a dynamic control developed with the model inversion principle associated with an optimization of the current magnitude deduced from a quasi-static analysis. All dynamics of the system are then explicitly controlled, which guarantee a good dynamic behavior during the transient. Therefore, current and energy controls are presented in detail. Simulation results show the dynamic behavior of the converter for various operating points. [ABSTRACT FROM AUTHOR]

This paper presents an FPGA-based Digital Real-Time Simulation (DRTS) environment for the analysis of Electromagnetic Transients (EMTs) of power systems that include multiple Power Electronic Converters (PECs). The main features of the proposed DRTS platform are that it i) employs two-value resistor switch model for PECs and thus avoids inaccuracies of Associate Discrete Circuit (ADC) model and ii) does not require separation of PECs by transmission lines (stub lines) and thus accurately represents PECs in close electrical proximity of each other. These features respectively result in a time-variant and large system admittance matrix which significantly increases the computation burden in each simulation time-step and can render it infeasible for real-time applications. To overcome this problem this paper introduces i) Switching-Network Partitioning (SNP) method which is tailored for PECs representation in FPGA and ii) Reformulated Modified Nodal Analysis (RMNA) method to enable computational parallelism between the solution of network and component models in FPGA. SNP enables solution-level partitioning of the PECs and the rest of the system. The developed DRTS platform enables sub-microsecond simulation time-step and provides a fixed hardware irrespective of the system configuration. Performance of the platform is evaluated for a study system and verified versus off-line simulation studies. [ABSTRACT FROM AUTHOR]

The ultimate objective for transformer winding fault diagnosis is fault location and even severity evaluation. The issue has been studied in many works, based on frequency response analysis, using numerical indices or mathematical correlation between ladder network and winding. This paper utilizes Gauss–Newton iteration algorithm (GNIA) to diagnose winding faults for its efficient search of optimal parameters and discrimination of linearly-dependent vectors. Three key problems are handled for the application of GNIA in this paper. The first problem is the establishment of connection between ladder network components and frequency response function in GNIA. Another issue is the method to identify the faulty section to locate the defective winding position. The assessment of fault severity with GNIA data is another challenge. GNIA is applied on an air-cored model coil and a customized transformer successfully, with satisfactory diagnosis on deformation location and severity. [ABSTRACT FROM AUTHOR]

Identifying sources of subsynchronous resonance (SSR) is crucial for understanding and mitigating incidents. Recent studies have shown that SSR impedance and power are effective indicators that can identify an SSR source. However, the calculation of two indexes requires SSR voltage and current phasors as the inputs, which are not directly available from existing online monitoring systems. To address this issue, this paper presents a novel method to calculate SSR impedance/power based on synchrophasors provided by a wide-area monitoring system. As a result, the online detection of SSR sources is enabled. Through mathematical derivation, it is revealed that a synchrophasor under SSR is dominated by four modes: two from the fundamental frequency and two from the SSR frequency. The problem thus can be transformed to a modal parameter extraction (MPE). Two classic MPE algorithms, Prony analysis and the matrix pencil method, are employed in this paper to extract the SSR parameters and calculate the SSR impedance/power. Their performances are verified using synthetic signals and electromagnetic transient simulations. Furthermore, the method is applied to an actual SSR incident involving 23 wind farms. The findings suggest that 17 of 23 wind farms did not contribute negative damping to the oscillation. [ABSTRACT FROM AUTHOR]

Offshore wind farms pose the challenge of transmission of produced energy in an economic and effective way over long distances, justifying investments in High Voltage Direct Current (HVDC) systems. A solution to surmount the need for an expensive and large power transformer station is to connect each wind turbine to a DC/DC converter in which a Medium Frequency Transformer (MFT) is utilized to reduce the size of the station. Further, the DC outputs of the converters of the wind turbines can be connected in series to create a high DC voltage. In this solution, the MFTs are required to be insulated against a high DC voltage to ground in a limited compartment. This contribution presents a prototype of a 50 kW oil-paper insulated MFT operating at biased 422/4500 V/V AC 5 kHz and 125 kV DC test voltage, which has been manufactured and subjected to verification HV tests. The dielectric design of the prototype MFT based on finite element solution of non-linear Maxwell-Wagner equations is introduced and the acceptance criteria and practical design considerations are discussed. [ABSTRACT FROM AUTHOR]

Power flow (PF) flexibility in the multi-terminal HVDC (MT-HVDC) grids is a thought-provoking issue. It leads to employing the active DC power flow controller (DC-PFC)s. Hence, this paper examines the static average model (AM) and power injection model (PIM) of an interline DC-PFC (IDC-PFC). It is to provide a suitable base for DC PF studies and easy embedding of the DC-PFCs into MT-HVDC grids’ PF equations. In this regard, this paper proposes a new DC PF solver (DC-PFS) for the IDC-PFC compensated MT-HVDC grids within the well-accepted Newton-Raphson (NR) framework. It requires a few modifications in the main structure of the system's Jacobin (J) matrix compared to the uncompensated MT-HVDC grid. Also, the system's admittance matrix and its symmetry are preserved. In the proposed concept, the IDC-PFC cooperates with other MT-HVDC grid's state variables to satisfy the predetermined control objective(s). Furthermore, this paper proposes a new solution procedure (SP) to handle various system's limitations during the processes of solving the DC PF problem. Meanwhile, there is no need to modify the related J matrix. The effective and accurate performance of the IDC-PFC's models, as well as presented NR-based DC-PFS and SP, are verified by performing several simulations on the 8-bus CIGRE MT-HVDC grid. [ABSTRACT FROM AUTHOR]

Thermal stress plays a prominent role in the reliability of bushings and contributes to the reliability of power transformers, especially during overload conditions. Thus, exploring the temperature distribution in the bushings is essential. This paper proposes a new thermal model to estimate the hotspot temperature (HST) of oil-impregnated paper (OIP) bushings, based on a modified thermal-electrical analogy model. The proposed model is developed based on the finite element method (FEM) to accurately model all fluid flow and internal convection as well as the thermal conduction mechanism. To this end, convection thermal resistances are defined, and their nonlinear characteristics are calculated for different overloading conditions. The proposed approach is applied on a 245 kV, 800 A OIP bushing to analyze not only the normal loading condition but also short-term overloading beyond the rated current. In addition, the oil flow condition with different load currents and transformer top oil temperatures (TTOT) are investigated. The results show a consistent temperature rise with the typical test conditions with no identified problematic situation. However, during overloading, the temperature rise exceeds beyond the permissible limits recommended by IEEE C57.19. Hence, the overloading of transformers may deteriorate the bushing insulation and reduce its lifetime. [ABSTRACT FROM AUTHOR]

Real-time dynamic line rating forecasting can be classified into two critical steps of evaluating the operation risk, and avoiding the overload operation of transmission lines. The dynamic capacity of the transmission line is generally predicted for scheduling power networks in the day-ahead market. However, the forecasting error motivates researchers to propose real-time forecasting models to correct the day-ahead scheduling results based on the accurate data close to real-time values in the balancing market. Among the methods presented for real-time dynamic line rating forecasting, the single-point estimation type has been considered more than other forecasting types. Despite its safety, and efficiency, a single point estimation type suffers from several significant drawbacks. For instance, this forecasting type cannot indicate the probabilistic distribution function of forecasted value, which is used in novel balancing methods of market scheduling. The main purpose of this paper is to present a density forecast method to predict real-time dynamic line rating for covering the balancing market requirements. Dynamic stochastic general equilibrium (DSGE) is applied to achieve this aim. Due to incorporating stochastic volatility in the proposed real-time forecasting model, evaluating simulation results, and reference models highlight a significant improvement in the performance of the real-time density forecast of DLR. [ABSTRACT FROM AUTHOR]

The Under Frequency Load Shedding (UFLS) is the last automated action to restore power system frequency stability when a severe contingency causes a significant drop in the frequency of interconnection or islanded areas. In this paper, a novel probabilistic algorithm that considers the wind turbine frequency response is proposed to design and optimize the UFLS scheme. The uncertainties of power systems such as wind speed, hourly load, inertia time constant, and generation losses are taken into account. The Monte-Carlo Simulation (MCS) method is used to model the power system uncertainties. Additionally, a novel method is developed to calculate the power system frequency response and accelerate the multistage UFLS scheme optimization processes. In this method, a time-independent formulation for power system frequency response is utilized to reduce the computational complexity. More than Twenty different UFLS plans are designed using the proposed probabilistic algorithm. Besides, some time-domain simulations are carried on the modified IEEE 39-bus test system to investigate the performance of the designed UFLS schemes. Results indicate that designed UFLS schemes provide stable frequency response and improve the power system frequency stability. [ABSTRACT FROM AUTHOR]

Electromagnetic Transient (EMT) simulation tools are typically developed using conventional procedural programming languages. On the other hand, modern high-level and equation-based programming languages, such as Modelica, are currently available. Modelica allows formulating models that are easy to develop, maintain and understand by expressing what needs to be computed without stating how it should be computed. This paper presents a Modelica-based simulator for electromagnetic transients. It is demonstrated that this approach offers significant advantages for developing sophisticated models. Computational performance and accuracy are compared to a conventional EMT-type simulation tool. [ABSTRACT FROM AUTHOR]

This paper proposes a novel and applicable approach to calculate the reactive power loss of converter transformer under dc bias. By using this proposed method, only general information of the converter transformer and the network is needed for the calculation within a certain dc current range. Specifically, a three-dimensional model of ± 800 kV converter transformer model for simulation test is established by the finite element method. Through simulation test, the electromagnetic characteristics and reactive loss characteristics of converter transformer under dc bias can be obtained, and the relationship between the bias current and the reactive loss of ± 800 kV converter transformer is quantified. Simulation results validate the feasibility of proposed algorithm. [ABSTRACT FROM AUTHOR]

The difficulty in acquiring fault label data is a major obstacle to the application of data-driven fault isolation in DC microgrids. To remove this barrier, this paper introduces an approach of generating synthetic data with the line currents measured during normal operation as the substitute for fault label data in training an ensemble model, which is aimed to isolate line short-circuit faults in DC microgrids. Based on a high-frequency model of DC microgrids, we prove that the line currents during the closing of unloaded DC lines have similar high-frequency features as those during a line short-circuit fault. On this basis, the synthetic data are obtained through performing discrete wavelet packet transform on the line currents measured during the circuit breaker operation. With normal operating data and synthetic data, an ensemble model is trained as the fault classifier. In the verification tests of different fault scenarios in a three-terminal DC microgrid model, the detection rates of the proposed ensemble model are over 90% while the false positive rates are below 0.5%. These results prove the effectiveness of using synthetic data as the fault labels in the ensemble learning-based fault isolation in DC microgrids. [ABSTRACT FROM AUTHOR]

This paper proposes a time-domain method of characteristics to simulate the impulse corona with background space charge, and derives the ordinary differential equations of the space charge generated by the impulse corona along the electric field line. The corresponding boundary conditions, the selection of time step and space step, and the update strategy of space charge are given. The calculation result of the impulse corona onset voltage is consistent with the test result, proving that the background space charge has almost no effect on the impulse corona onset electric field which is obtained by Kaptzov's assumption. The q-u curve obtained by simulation is compared with the test results to verify the effectiveness of the simulation method. The simulation method is used to obtain the development process of space charge during the application of the opposite polarity impulse voltage. By analyzing the space charge distribution during the process of impulse corona, and the corresponding microscopic charge distribution and macroscopic charge amount at different corona states, the characteristics of impulse corona are explained. [ABSTRACT FROM AUTHOR]

Free metallic particles are the main factor inducing flashover incidents and threatening the safety of gas-insulated switchgear (GIS) and gas-insulated transmission line (GIL) devices. Acoustic diagnosis is widely used to detect free metal particles due to its high sensitivity, but it generally fails under multiple particle conditions because of the overlap of acoustic signals. This paper proposes a new method of acoustically recognizing multiple free metallic particles. The pattern of Δt-Normalized N that can be used to distinguish single/multi-particles is determined first. Second, each particle's neighboring collision signals are extracted from raw data by the established physical relation between adjacent collision signals. Wire, lamellar, and spherical free metal double-particles’ mass values are calculated as per the numerical connection between adjacent collision signals. The geometry of the particles is assumed to be known, so the density of the spherical particles can be effectively estimated. The proposed method is validated in a scale concentric hemisphere platform. The results of this work may provide a workable reference for GIS free metal multi-particle monitoring. [ABSTRACT FROM AUTHOR]

Reliability-centered maintenance (RCM) as a subset of the reliability assessment problems, is a powerful tool for enhancing reliability, and subsequently achieving the customers’ higher satisfaction. However, deterministic RCM models could scarcely guarantee global optimality and feasibility of their solutions with real-world uncertain failure rates. In this paper, a robust model is proposed to consider the impact of failure rate uncertainty in the RCM scheduling problem, followed by a two-stage robust model development. The two-stage robust model is solved using Benders decomposition (BD) and outer approximation (OA) algorithms. The proposed model and solution method are applied to both educational RBTS2 system and real-size Finnish Distribution System, followed by demonstration of a higher financial and reliability performance, compared to that of the deterministic model. Sensitivities of the costs, available budget, uncertainty interval and computational time are analyzed and discussed, to conclude the paper. [ABSTRACT FROM AUTHOR]

This paper presents a novel two-terminal fault location algorithm without using line parameters. The algorithm is formulated using unsynchronized voltage and current phasors of the fundamental frequency at both ends. The novelty of the proposed method lies in the fact that it takes into account different angles of synchronism mismatch in pre-fault and fault conditions while considering distributed parameter line model and its solution is in closed form. Simulation results and performance comparison with existing methodology are presented under different fault conditions. The simulations were performed on EMTP-RV platform. The algorithm has also been tested with practical data of Power Grid Corporation of India Limited (PGCIL). The test result shows the validity and accuracy of the developed fault location algorithm. [ABSTRACT FROM AUTHOR]

This paper introduces a novel frequency domain technique to globally evaluate the accuracy of electro-magnetic transient simulations. It is shown that simulation accuracy at low frequencies can sometimes be poorer than at high frequency. A modified approach which quantifies accuracy from a driving point as a function of frequency is also introduced that uses the Bilinear Transformation and Norton equivalents, to produce a “simulation accuracy spectrum”. This approach can be applied to large systems without explicitly forming state space equations. It also permits the accuracy analysis of networks with distributed components such as frequency dependent transmission lines. Two examples are used to verify the proposed technique, a small network with a frequency dependent transmission line modeled by the universal line model; and the IEEE 39 bus system connected with an LCC-HVdc system. [ABSTRACT FROM AUTHOR]

The paper presents the results of experimental registrations and numerical simulations of the transient overvoltages caused by single phase intermittent arc earth fault in 35 kV network with isolated neutral. Experimental laboratory investigation is performed in order to implement appropriate arc fault model for arcs with low and discontinuous current. Single phase to earth arc fault is created in field in 35 kV network. Numerical model of the system is created in EMTP-ATP software. Overvoltage waveshapes and overvoltage factors are calculated and compared to measured values. Good agreement between the measured and calculated results is achieved. [ABSTRACT FROM AUTHOR]

Multi-terminal modular multilevel converter (MMC) power transmission is the current development trend in DC transmission. However, owing to its fast control and dynamic characteristics, it may interact with the other inductive devices or the grid and cause stability issues. This paper describes the use of the impedance analysis method in the frequency domain to analyze system stability. First, the multi-harmonic linearization method is used to model the impedance of a three-terminal MMC system. To make the model more accurate, the modeling considers the effect of frequency coupling caused by an asymmetric controller and the AC sampling site position. This paper then proposes an equivalent method of the grid impedance for the three-terminal system that can accurately judge the stability of the grid-connected system when the AC side is coupled. This work provides a theoretical basis for the prediction and suppression of the oscillation of MMC grid-connected system with coupling on the AC side. Finally, a three-terminal MMC model is built in Matlab/Simulink and the Nyquist stability criterion is used to verify the correctness of the analysis. [ABSTRACT FROM AUTHOR]

The randomness and time variability of power load are the main problems in the load modeling, in which the model error and measurement noise greatly affect the modeling accuracy. In this paper, an adaptive fuzzy logic strong tracking based load modeling method is proposed. The problems of load characteristics and parameter identification of load model are transformed into a real-time tracking of load. The optimal model set is first selected from the initial load model set, and according to the mixing probability, the state estimation and error covariance are mixed as the input of the filter. In order to reduce the model error in the parallel filtering, the strong tracking filter (STF) is constructed based on the orthogonality principle and the filter gain is adjusted by introducing a time-varying fading factor. The adaptive fuzzy logic strong tracking filter is constructed by combining the fuzzy logic with the STF to adjust the covariance matrix of the STF according to the change of measurement noise. Moreover, the weights of each sub model are updated in real time by the parallel filtering, and the sub models are fused to get a comprehensive load model which can reflect the actual load characteristics. The effectiveness of the method is verified by the simulation of IEEE 39-bus system. This method can establish an accurate load model and has good robustness. [ABSTRACT FROM AUTHOR]

This paper presents the load-flow modeling and steady-state capabilities of the bundle-controlled line impedance modulator (LIM). This distributed flexible ac transmission system device is under development for the management of power flows. In particular, this paper presents the back-to-back interconnection of two bundle-controlled line segments. The LIM configuration is very interesting from an economic and operational point of view: it enables continuous control of the transmission-line impedance from its standard value up to 1.64 p.u. and, as a result, the power flow can be reduced smoothly. [ABSTRACT FROM AUTHOR]

Stray voltages have always been a concern to utility companies and customers. But methods to troubleshoot and monitor nonroutine stray voltage problems are very limited. This paper presents a passive, measurement-based method to identify the contributors to the common cause of stray voltages, the neutral-to-earth voltage (NEV) rise at the service entrance point. The paper shows that the NEV can be decoupled into two components: those related to the customer under investigation and those outside the customer facility. A method to quantify the relative contributions of the two components is proposed. A new concept called current return ratio is introduced to facilitate the decoupling and to quantify the grounding conditions of the system. This paper presents the background, circuit model, concept, and theory of the proposed method. [ABSTRACT FROM AUTHOR]

This paper investigates the applicability of averaged-value models (AVMs) for modular multilevel converters (MMCs) operating in a voltage-sourced converter-based-high-voltage dc (VSC-HVDC) grid. The AVM models are benchmarked by comparison with a detailed electromagnetic transient model of the grid, including a fully detailed MMC model. Analysis results show that the AVM is only effective as long as the capacitors are large enough to maintain nearly constant voltage across each MMC submodule. This paper also shows that previously developed MMC averaged models are not able to accurately simulate the transients under dc fault conditions. This paper introduces topology changes to a previously proposed averaged model that results in much improved simulation for such conditions. This paper also shows that the model can be used effectively to study HVDC grids with significant time savings. [ABSTRACT FROM AUTHOR]

On transmission lines where switching surges are not mitigated with closing resistors and/or surge arresters, high-speed reclosing on a line with trapped charge will produce high overvoltages that have been measured above 3 p.u. Careful simulations of these switching events using available electromagnetic transient (EMT) programs consistently produce significantly higher voltages than the measurements. This indicates a phenomenon present on the line that acts to reduce the switching surge magnitudes but is not typically modeled. This paper aims to identify the required simulation practices in reproducing the field measured overvoltages in EMT simulations and investigates the sensitivity of results to modeling approaches and electrical parameters. Field measurements from a switching surge test of a 230-kV line without surge mitigation have been used for model comparisons. Variations in frequency-dependent line modeling, ground resistivity, skin effect, shunt conductance, parallel lines, and source-side detail have been tested in an unsuccessful attempt to decrease the difference between the field measurements and the higher simulation overvoltages. It is demonstrated that even though the pattern of the transient voltage waveforms can be reproduced very well using frequency-dependent line models, the magnitude of the maximum overvoltage is significantly overestimated unless the effect of corona is considered. Two types of corona models are tested, and both demonstrate that corona is the primary factor that allows the simulations to correctly reproduce high peak overvoltage measurements. [ABSTRACT FROM AUTHOR]

Ice accretion on the overhead contact lines of the electric railways may cause severe inconveniences to train traffic. In order to avoid such problems, methods are presently applied that are characterized by high cost and/or some risks as described in this paper. This paper presents new simple techniques for the deicing of the overhead contact lines in the ac and in the dc electric railways that avoid any inconvenience. They consist in rearranging the system scheme in special configurations and inject a proper current into the circuits obtained. Under these configurations, the line continues to be protected against the faults. Experimental results have proved the viability of these techniques. The limits of applications of these techniques are discussed in this paper. [ABSTRACT FROM AUTHOR]

This paper describes the development of a thermohydraulic radiator model. The setup of the hydraulic as well as the thermal model is explained. For the thermal calculations, a method to predict the air velocity as a function of fan characteristics and radiator geometry is established. The coupling of both models is explained as well. A large part of this paper has been dedicated to the validation of the radiator model. This shows the power of the new model and gives new insights into radiator-fan cooling. [ABSTRACT FROM PUBLISHER]

Frequency Response Analysis (FRA) is a comparison based method and is useful to identify the mechanical fault in transformer winding only when the ‘healthy’ or ‘reference’ magnitude response is available. But, in practice, ‘healthy’ response may not be always available and the subsequent FRA measurement alone also cannot detect the fault. This is one serious limitation for conventional FRA method and the desired solution is still far away. To this effect, this paper presents an innovative way of measuring the frequency responses of transformer winding with two equal and opposite polarity sinusoidal excitations from two ends. For viability test, the present work is limited to fault-detection only in a single isolated uniformly-wound transformer winding followed by verification on an actual single-phase autotransformer. The two equal and opposite polarity excitations for such winding are likely to impose a virtual ground at the centre and hence the two admittances measured from the two source-ends are likely to be identical. Two admittances also get dissimilar for a fault in transformer winding and therefore the fault could be identified simply by measuring and comparing these two responses in absence of ‘healthy’ response. Pertinent simulation and experimental results are also included in the paper. [ABSTRACT FROM AUTHOR]

Several models were successfully developed to reproduce the dynamic behavior of ZnO surge arrester, but the accuracy and reliability of these models are only evaluated for residual voltage calculations. This paper presents a model developed with the specific goal of achieving reliable and accurate modeling of the energy absorbed by a ZnO surge arrester during impulsive transients. To develop and test the model, an experimental database was built, composed of responses of ZnO varistors to current impulses. The database contains the response of varistors of different classes and manufacturers, subjected to current impulses with different waveforms and amplitudes. Then, using system identification techniques, a mathematical model was proposed and validated. The proposed model was successfully tested on the same circumstances that traditional models are used and its performance was compared with the most relevant existing ZnO model. The results indicate that the developed model is an overall valid model for ZnO surge arresters and that energy calculations performed with it are more accurate and reliable. [ABSTRACT FROM AUTHOR]

This paper proposes a secondary Model Predictive Control (MPC) based architecture to provide DC voltage and DC power control to MT VSC-HVDC networks interfacing wind power generation. The proposed architecture places the controller at a supervisory level, thus providing coordination amongst existing converters’ local droop controllers. Droop control is a type of decentralized DC voltage/power control. Hence, in case of secondary controller failure, droop control acts as a contingency control scheme. A simplified linear dynamic model of the MT VSC-HVDC network is utilised for the MPC's design, thus minimizing computational effort needed to compute secondary control action. Updates of droop gains and offshore and onshore “AC” variables, in the dq domain, are explicitly considered as measured input disturbances within the formulation of the MPC controller, thus, ensuring appropriate control response in order to minimise adverse impact of their variation on the overall system's performance, particularly under wind power variations. Simulation results show that MPC, whose design is based on the system's simplified linear model, is capable of delivering satisfactory performance when applied to the high fidelity non-linear full order model of a six-terminal VSC-HVDC network simulated in PSCAD. [ABSTRACT FROM AUTHOR]