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In this paper, a reliable methodology is proposed in order to implement and validate a Model Predictive Control (MPC) scheme on an actual Voltage Source Converter (VSC) integrated in a scale-down multi-terminal DC grid. The objective of the investigated MPC controller is to enable AC frequency support among two asynchronous AC areas through a High Voltage Direct Current (HVDC) grid, while considering physical constraints, such as maximum and minimum DC voltage. A systematic and accurate implementation strategy is proposed, based mainly on the Hardware In the Loop (HIL) and Power Hardware In the Loop (PHIL), leading to the real-life testing on VSC, controlled by a classical microcontroller. The technical problems during the implementation process, as well as the proposed solutions, are described in detail through this paper. This procedure is deemed valuable to bridge the gap between offline simulation and the actual implementation of such advanced control scheme on experimental test rig. [ABSTRACT FROM AUTHOR]

Utilities prefer to perform a condition-based assessment of power transformer insulation in the least possible shutdown time. It is essential to estimate the values of various insulation condition sensitive performance parameters with higher accuracy. In the present work, a technique for the estimation of performance parameters, moisture content of paper, dissipation factor of the entire oil-paper insulation, paper conductivity, and activation energy is proposed. The evaluation of these parameters using reported techniques requires complete measurement of polarization and depolarization current profile, which requires around 20000 s of measurement time. The present method uses a forecasted polarization current profile obtained from 600 s of measured polarization current data. Detrended Fluctuation Analysis is applied to the forecasted polarization current data obtained from various in-service power transformers to obtain a suitable parameter. It is shown that this parameter maintains well-defined relationships with parameters; moisture content of paper, dissipation factor of oil-paper insulation, paper conductivity, and activation energy. The proposed technique is beneficial for utilities as it eliminates the requirement of measuring depolarization current and return voltage measurement for estimating activation energy. [ABSTRACT FROM AUTHOR]

This paper proposes a novel hybrid AC/DC microgrid architecture incorporating a central energy storage system (ESS) for both the AC and the DC sub-grids. To ensure effective operation of the proposed hybrid AC/DC microgrid architecture, a coordinated control strategy between the central ESS and the inter-linking converter (ILC) is proposed in the paper. The proposed central ESS provides voltage and frequency control to the AC sub-grid by measuring voltage and frequency at the point of common coupling (PCC) of the AC sub-grid, while providing DC voltage support to the DC sub-grid by measuring DC sub-grid’s PCC voltage. Furthermore, per-unit frequency and the DC sub-grid PCC voltage are utilized to transfer power between the two sub-grids via the ILC. Therefore, two control strategies are coordinating with each other and maintain stable operation. The central ESS eliminates the use of multiple ILCs and complex control schemes for charge and discharge control to maintain same state of charge levels at each ESS. Effectiveness of the proposed coordinated controller is verified by applying various load disturbances, faults, and under different modes of operation. [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]

The Potential Transformer (PT) plays an important role in the measurement and monitoring of the power system. The accurate modeling of the PT is very important but not yet perfect. The stray capacitance will have a non-negligible effect on the performance of a PT. However, the influence of stray capacitance is rarely considered in existing models, which makes the broadband simulation performance weak and provides limited references in engineering. In this paper, a calculation method of PT stray capacitance is proposed to analyze the performance of the PT at different frequencies and extract the capacitive current of the PT port. This paper used a non-destructive method to obtain the test data of a PT, calculated the value of the stray capacitance, and drew the hysteresis loops of the iron core. A corresponding simulation model was established to predict the PT performance at different frequencies. The simulation results are in good agreement with the experimental data, which proves the accuracy of the modeling parameters. The data processing logic is clear and is easy to be simplified by programming, which makes it promising in engineering. [ABSTRACT FROM AUTHOR]

Conventional half bridge modular multilevel converter (MMC) system will suffer great over-current when there is a dc side fault. Therefore, topology study in sub-modules with dc side fault blocking capability have become one of the hotspots in MMC field. However, fault blocking topologies using conventional IGBT devices must employ more semiconductors than half bridge topology. The extra semiconductor devices employed in the topology will lead to the increase of cost and conduction loss. To reduce the conduction loss of the fault blocking topology, a novel sub-module topology is proposed in this paper. The proposed topology employs reverse blocking devices to form the low conduction loss path. Conduction loss analysis is conducted in this paper to show the priority of the proposed topology. Simulation and experiment results verified the dc fault blocking capability of the proposed topology. [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 neutral grounding method is one of the crucial technologies used to ensure the safe and stable operation of distribution networks. With the development of power electronics-based devices, the application of highly controllable inverters to grounding systems has become a new research direction. In this review paper, based on the summary and analysis for the research status, the flexible grounding system is systematically classified according to its control strategy and structure. And then, the following core technologies are summarized: faulted line selection, current component detection, high-resistance ground identification, ground parameter measurement, neutral point voltage control, and fault location. Subsequently, deficiencies of flexible grounding systems are summarized from the following four perspectives: reliability, accuracy, speed, and stability. Finally, some discussions and recommendations for future development and research direction are presented. Especially, the reliability-enhanced hybrid grounding system that combines active inverters and low resistance for high-resistance ground faults is recommended. This paper provides a systematic literature review of the crucial technologies for flexible grounding systems and can be used as a technical reference for future research. [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]

This paper analyzes the unsymmetrical operation of the dc/dc MMC caused by parametric uncertainty. The unsymmetrical operation is referred to as the condition in which parameters involved in ac power generation are not equal in the phase-legs of the converter. Because existing models in the literature cannot exhibit the unsymmetrical operation of the dc/dc MMC, new detailed and simplified steady-state models are developed in this paper. In the developed models, the connection between the operation of different phase-legs shows itself when the dc-link impedance is considered. Using the developed models, the parameters that can force the converter to operate in unsymmetrical conditions are identified, and the maximum variation that the converter can tolerate is found. It is shown that even a small parameter variation leads to the leakage of the ac circulating current into the dc-link, which is not acceptable. By conducting various case studies, the impacts of the dc-link length, the transmitted power, and the conversion ratio on the unsymmetrical operation of the dc/dc MMC are characterized. The results showed that the converter would be more sensitive to the parameter variation as these parameters increase. Comparative analysis and simulation results confirm the validity and effectiveness of the presented models and analysis method. [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]

The paper addresses an improved inner current reference calculation to be employed in the control of modular multilevel converters operating during either balanced or unbalanced conditions. The suggested reference calculation is derived based on the AC and DC additive and differential voltage components applied to the upper and lower arms of the converter. In addition, the impacts caused not only by the AC network’s impedances but also by the MMC’s arm impedances are also considered during the derivation of the AC additive current reference expressions. Another issue discussed in this article regards that singular voltage conditions, where the positive-sequence component is equal to the negative one, may occur not only in the AC network but also internally (within the converter’s applied voltages). Several different inner current reference calculation methods are compared and their applicability during the former fault conditions is analyzed. The paper presents a detailed formulation of the inner current reference calculation and applies it to different unbalanced AC grid faults where it is shown that the presented approach can be potentially used to maintain the internal energy of the converter balanced during normal and fault conditions. [ABSTRACT FROM AUTHOR]

This paper addresses the interactions among the control loops of voltage-sourced converters (VSCs) embedded in high-voltage direct current (HVDC) systems and the impact of these interactions on the performance of individual control loops. Furthermore, this paper proposes a method to design the individual control loops, rather than the interconnected control system, such that the overall system stability is ensured in the presence of interactions. This paper addresses three aspects of the impact of control loop interactions on the overall control system of the VSC including stability, transient response, and steady-state response. To illustrate the impact of interactions on the VSC small-signal stability, stability regions of the interconnected control loops are obtained and compared against those of the various individual control loops. Then, a novel formulation of the VSC stability criterion is presented, and the small-gain theorem is utilized to determine a range of stabilizing gains for the interconnected control system. Moreover, new indices using the infinity norm are introduced to evaluate the impact of interactions on the transient response of the VSC. Finally, the DC gain of a certain set of transfer functions is used to assess the impact of control loops interactions on the set-point tracking capability of the VSC. The introduced indices along with time-domain simulations show that the transient response of the d-axis control loops is not impacted by the interactions, whereas that of the q-axis control loops is significantly impacted by the interactions. The set-point tracking capability is proved to be independent of interactions. [ABSTRACT FROM AUTHOR]

Ladder network synthesis is very significant for winding fault diagnosis within transformer. To synthesize the network reversely based on the measurable frequency response analysis (FRA) data is the only applicable way. To date the iron core and non-tested winding, which result in the frequency-dependent network components and multiple ladders in some frequency ranges, are usually not considered in the reverse process and thus the synthesis precision is largely limited. This paper proposes to synthesize multiple ladder networks in different frequency regions to handle this problem. Firstly, the sensitivities and frequency-dependent features of network components are analyzed, based on which the topology and components of the networks in diverse frequency ranges are determined afterwards. Further, the network synthesis strategies in different frequency ranges are elaborately designed. Finally, the synthesis method is applied on a distribution transformer successfully, with the fact that the extracted network parameters meet all the constraints and its FRA curve matches well with the measured one. An intact method of multiple-network synthesis for transformer winding in wide frequency range, considering the iron core and non-tested winding, is offered in this paper. [ABSTRACT FROM AUTHOR]

Provides information on the publication `Fault and Disturbance Data Requirements for Automated Computer Analysis,' which describes the analysis objectives and the data necessary to facilitate automated analysis. Information on the application areas included in the publication; Listing of the types of conditions to be captured; Types of data required for analysis.

The measurement of water inside a transformer is essential to maintain its reliable operation, as the water content can increase as the transformer ages. This measurement is frequently performed by a utility as a part of its periodic maintenance activities. In this paper, the application of optical fiber sensors as an online method to measure water content of paper inside an operating transformer is presented. There are several advantages of using these sensors, including their size, lightweight, flexibility, multiplexing, no-electromagnetic interference, etc. A unique benefit of fiber optic technology is that sensors can be embedded into the winding structure and operate in areas of high electric field strength. For this work, fiber optic water and temperature sensors are investigated by comparing their measurements to a commercial water activity probe immersed in oil. The study found that the determination of water content by fiber optic sensors compared closely with other techniques, which lead to the conclusion that they can be an effective technology for water measurement in an operational transformer. [ABSTRACT FROM AUTHOR]

High voltage circuit breakers (HVCBs) play a critical role on providing the desired reliability, and resiliency in power systems. In order to extend their lifetime and predict the failures, various maintenance policies could be applied on these critical components. Amongst these strategies, condition-based maintenance (CBM) provides a satisfactory agreement with future smart environment. This paper aims to provide an insight into the relevant developments in this subject and to explore the viable visions compatible with future research stream. Accordingly, three directions, i.e., diagnostic signals, intelligent modelling and using monitoring data in asset management have been addressed in this paper. It presents challenges dealing with real-time assessment of the diagnostic signals relating to measurements, and analyses. Subsequently, the issues associated with using artificial intelligent (AI) and Machine learning for providing intelligent algorithms have been discussed. Finally, the connection between the monitoring data and the asset management approach is investigated. The latter is looking for the subjects including remaining lifetime estimation, prioritization, and health index definitions. This paper has attempted to make a bridge from past to future research trends in the failure diagnosis of HVCBs. [ABSTRACT FROM AUTHOR]

Voltage and current waveforms contain the most authentic and granular information on the behaviors of power systems. In recent years, it has become possible to synchronize waveform data measured from different locations. Thus large-scale coordinated analyses of multiple waveforms over a wide area are within our reach. This development could unleash a set of new concepts, strategies, and tools for monitoring, protecting, and controlling power systems and apparatuses. This paper presents an in-depth review and analysis of the advancements in synchronized waveform data, including measurement devices, data characteristics, use cases, and comparisons with synchrophasor data. Based on the findings, five strategies are proposed to discover and develop synchronized waveform based applications over multiple application areas. The paper also presents three complementary measurement platforms and two data screening algorithms for application implementation. It further discusses committee activities and standard developments useful to explore the full potential of the data. [ABSTRACT FROM AUTHOR]

Reliable voltage measurement is a pre-requisite for correct operation of a relay's voltage-dependent functions. The secondary circuit of a relay's voltage transformer (VT) is usually fuse-protected. The blowing of a VT's three-phase fuses prevents correct voltage measurement, commonly referred to as the loss-of-voltage (LOV) condition. Relays are equipped with different methods to detect the LOV conditions and avoid confusing them with grid faults. However, this paper shows that existing LOV detection methods are prone to misoperation in the presence of inverter-based resources (IBRs). The paper's findings are corroborated by hardware-in-the-loop testing of relays with the state-of-the-art LOV detection methods. This paper also develops a new LOV detection method for transmission system relays that addresses the unveiled problem. The proposed method considers the fault behavior of IBRs and detects LOVs by examining certain conditions in three stages, namely phase-voltage-check, phase-current-check, and current-transients-check. Simulation studies are used to verify the dependable and secure operation of this new method for different LOVs, faults, IBRs, and VT configurations. The obtained results demonstrate that this method performs successfully regardless of the type of generation units. Moreover, it is sufficiently fast to prevent relay misoperation during LOV conditions and does not require extra hardware. [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]

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]

In this paper, the discrete wavelet transform (DWT) and a chaotic system were combined with a convolutional neural network (CNN) and applied to the diagnosis of insulation faults in XLPE (cross-linked polyacetylene) power cables. First, four different types of insulation faults in power cables were constructed, including the normal state of the cable, the short outer semi-conducting layer, impurities in the insulation layer, and insulation layer damage, and a high-speed capture card (NI PXI-5105) was adopted to measure the partial discharge (PD) signal, which was then filtered through discrete wavelet transform. Then, based on the Lorenz chaotic system, a dynamic error scatter diagram was established as the feature of each fault state. Finally, the dynamic error scatter diagram was processed by CNN to recognize four different types of faults in the power cable. The test results show that the method proposed in this paper can quickly recognize the fault state of power cables and has excellent performance in terms of recognition accuracy, which reaches 97.5%. Therefore, the proposed method can effectively detect the fault signal changes of power cables and identify the fault state of power cables in real time. [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 presents improved indices to evaluate the contribution of reactive power compensators at HVdc converter terminals. The presence of such compensators impacts converter performance, such as the maximum available power (MAP) limit, susceptance to commutation failure, fault recovery time and temporary over voltage. Traditionally, the Effective Short Circuit Ratio (ESCR) has been used to indicate system strength. However, this index cannot be directly applied when power-electronics based converters are used to provide the reactive power support. The approach of this paper is to use an improved form of the ‘Apparent increase in Short Circuit Ratio’ (AISCR) index to quantify the impact of the compensator on the HVdc system. This index is evaluated by comparing the performance with the reactive power device with a system without the device, but having the same performance. The paper shows that a single AISCR index cannot quantify all behaviors and suggest modifications to AISCR for each of several HVdc phenomena. In this paper two types of compensators are considered – the Synchronous Compensator and the Static Synchronous Compensator (STATCOM). [ABSTRACT FROM AUTHOR]

The surface voltage of an insulator assumes importance for understanding tracking and flashover characteristics. Until now, determination of surface voltage of insulating surfaces is left to simulation software and such simulations are left unverified experimentally, leading to uncertainty of the results, which may sometimes be erratic with improper choice of boundary or initial conditions or inaccurate material properties, nonlinearities etc. The paper presents a novel method, using which the surface voltage of any insulating surface such as insulator strings or transformer bushings can be measured. Unlike a conducting surface, the measurement of insulating surface voltage using voltage dividers would be erratic as the measuring arrangement itself will vary the voltage division due to the local unknown impedance between the insulated surface and hv conductor. Recently a method for measurement of surface voltage of an insulated conductor using cylindrical strips was reported. In this paper, based on analytical derivations, a more generalized method of measurement, applicable to any point on an insulating surface of arbitrary geometry is proposed using circular-disc strips. The proposed experimental method is applied to insulator strings and transformer bushings and validated by simulation of the entire systems. The simulation and experimental results are in excellent agreement. [ABSTRACT FROM AUTHOR]

Protection of microgrids at the Point of Interconnection (POI) is a challenging task. Particularly, single-line-to-ground (SLG) faults on interconnection lines are difficult to detect using the fault current detection relays if the microgrid is ungrounded at utility side of its interconnection transformer; such microgrid contributes very low fault current, which disappears once the utility feeder breaker opens. Other common schemes for protection of microgrid interconnection line against SLG faults include Direct Transfer Trip (DTT) and over-voltage relay (59G). The main challenges associated with these methods are installation and maintenance costs of high-speed communication system, sensitivity of relay settings, and delayed fault clearing time due to selectivity requirements. To address such concerns, this paper proposes the use of a distance relay at the microgrid/LV side of the interconnection transformer to provide protection against SLG faults on the interconnection line at the utility/HV side. The designed distance relay executes an enhanced apparent impedance calculation using residual voltage compensation to correctly detect the fault, properly measure the fault location, and timely isolate the fault. In this paper, various scenarios are analyzed using ASPEN and PSCAD simulation tools to demonstrate the effectiveness of the proposed method for protecting the microgrid interconnection line under different fault scenarios. [ABSTRACT FROM AUTHOR]

IEC61850 is the mainstream of the development for substation automation. This paper presents a practical consideration and analysis for implementing a secure sampled measured value (SeSV) message in substation automation system. Due to the lack of security features in the standard, IEC Working Group 15 of Technical Committee 57 published IEC62351 on security for IEC61850 profiles. However, the use of authentication methods for SV based on IEC62351 standards are still not integrated, and computational capabilities and performance are not validated and tested with commercial grade equipment. Hence, this paper shows the performance of security feature enabled SeSV packets transmitted between protection and control devices by appending a message authentication code (MAC) to the extended IEC61850 packets. A prototype implementation on a low cost commodity embedded system has proved that the MAC-enabled SV message can fully secure the process bus communication in the digital substation with negligible time delay. [ABSTRACT FROM AUTHOR]

Grounding electrode is an indispensable part in HVDC transmission projects. During its design and safety evaluation, step voltage is a crucial aspect. By reviewing and sorting out the present standards involving step voltage measurement and its safety limit, this paper illustrates the contradiction between the measurement methods and the safety evaluation criteria. Reasons for the contradiction are explained by analyzing the principal of step voltage measurement and safety evaluation. The Open-circuit step voltage measurement method and the measurement method with a human body equivalent resistance paralleled are compared to find out their advantages and disadvantages. Suitable circumstances and corresponding safety criteria of these two methods are put forward. Method proposed in this paper can be applied to step voltage measurement and safety evaluation of HVDC grounding electrode, providing reference when revising the related standards. [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]

Aiming at the fault traveling wave on the LCC-MMC HVDC transmission line, this paper analyzed its reflection and refraction characteristics at the DC buses and the fault point, plotted its propagation grid diagrams, obtained the relationship between the polarity of fault traveling wave and the boundary elements, and drew the conclusion that when the impedance of DC filter is far less than the wave impedance of transmission line, the existing fault location methods that need to distinguish these two waves according to their polarities are not applicable to the LCC-MMC HVDC system. On these bases, this paper proposed a modal traveling wave arrival time difference-based location principle for pole-to-ground fault, and a fault traveling wave propagation path ratio-based location principle for pole-to-pole fault, these principles neither need to identify the wave polarity, nor need the wave velocity value. In order to better extract the wave heads and their arrival time, this paper pre-treated the sampling data to make sure all the wave heads are extracted from the same frequency band, then the wave heads and their arrival time can be extracted with the empirical mode decomposition. The simulation results verified the effectiveness of the proposed fault location principle. [ABSTRACT FROM AUTHOR]

This paper presents an asset health index (HI) prediction methodology for high voltage transmission overhead lines (OHLs) using supervised machine learning and structured, unambiguous visual inspections. We propose a framework for asset HI predictions to determine the technical condition of individual OHL towers to improve grid reliability in a cost-effective manner. The paper focuses on asset HI prediction and the selection of the most parsimonious model. Based on the technical specifications and HI data, our methodology allows for the prediction of a HI for OHLs without HI data, and models asset aging behaviour. Technical specifications and the HI as defined in this paper are taken from the Estonian TSO periodical visual inspections implemented in 2018. The case study successfully demonstrates that the proposed methodology can predict tower HI values for a single OHL with nearly 80 percent accuracy without the need for additional measurements. [ABSTRACT FROM AUTHOR]

This paper presents the possibilities of reducing emergency situations and reducing the time of failure in the power grid. The paper contains a detailed description of the method (algorithm) of operation in emergency situations on the example of a Gas-Insulated Switchgear (GIS) located in one of the system-forming (node) stations in the power grid. The level of detail contained in this paper, which has not been previously encountered in previous publications, favors an in-depth understanding of the possible solutions that result from these emergencies. The above method (algorithm) of activities has been developed based on three levels of research: simulation, theoretical and practical. This means that the simulation analysis that was performed based on theoretical information was experimentally tested as part of an emergency situation in one of the system-forming (nodal) stations in the power system. In order to achieve the result described in this article, one of the most promising types of devices used in power systems was selected - the GIS. This has been confirmed by numerous analyzes of the effectiveness of these devices included in this article. The paper contains a detailed description of the individual steps in the field of corrective actions and aimed at minimizing downtime. In addition, this article provides guidance to monitor possible emergencies that are designed to increase the reliability of the power grid. [ABSTRACT FROM AUTHOR]

Insulator pollution flashover occurs frequently and is harmful to the power transmission and distribution network. But the randomness of the pollution flashover and the complex influencing factors make it difficult to be predicted. The objective of this paper is to study the mechanism of the discharging on insulator surfaces and design a the calculation method which derives flashover probability. By calculating the distorted electric field in the initial streamer discharge quantitatively and combining it with the equivalent circuit of the insulator surface, the initial discharge voltage is derived. By analyzing the change in power input and dissipation in the process of the arc discharge, we propose the criterion on arc development and extinction, and the critical flashover conductivity of the insulator is obtained. Based on the probability density function, the behavior of the factors on the critical flashover conductivity is described, and the probability of insulator pollution flashover is calculated under different environmental conditions. In the validation experiment, the measured initial discharge voltage and pollution flashover ratio are close to the calculated values. The methods proposed in this paper could be used in the quantitative calculation of the initial discharge voltage and the prevention of the pollution flashover on insulators. [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]

This paper proposes a modeling approach for characterizing the probability of wildfire ignition caused by faults on power distribution systems. The proposed model serves as a starting point in research literature to illustrate, from an analytical perspective, the many factors that influence wildfire ignitions in power distribution systems. This paper presents the series of events that leads to power-system-related wildfire ignitions, and characterizes the wildfire ignition probability as a combination of the probability that a fault occurs along a power distribution line segment, and the probability that the fault results in the sustained ignition of a vegetation fuel bed surrounding the line. The proposed model integrates a variety of data including environmental conditions, power system protection settings, and power system line flows. A case study is performed on a test 33-bus distribution system using observed historical weather data from a high-threat fire district in California. The California case study is utilized to investigate the effects of three primary factors (wind speed, line congestion, and protection settings) on wildfire ignition probability. [ABSTRACT FROM AUTHOR]

The short-circuit response of inverter-interfaced distributed generators (IIDGs) is not adequately represented in many conventional protection studies. This paper presents an in-depth analysis of IIDG behavior during grid faults and proposes a more accurate reduced-order parameterized short-circuit current (RPSC) model of inverters. Typical inverter components are thoroughly investigated to identify those that play dominant roles during faults. The paper shows that the current limiter is the dominant factor for the steady-state fault-current of inverter; while the inverter filter along with the severity of the voltage disturbance largely determine the initial transient spike of inverter fault current. The proposed model is low-order and can be used in large scale simulations. The parameters of the proposed RPSC model can be extracted from laboratory experiments without requiring proprietary manufacturer information. The proposed fault-current model is analogous to the well-known synchronous machine model that segregates the inverter fault current into subtransient, transient, and steady-state fault currents. Experimental and simulation tests are presented to validate the model. [ABSTRACT FROM AUTHOR]

HVDC grid protection with adequate speed and reliability is required to minimise the impact of DC faults. In particular, fast breaker failure backup protection algorithms are needed to meet the expected reliability requirements of HVDC grids. In this paper, existing breaker failure backup protection algorithms are shown inadequate to detect partial failures like a single module failure of breakers with a modular structure. This paper proposes a backup protection algorithm which rapidly detects a DC breaker failure based on estimating the counter-voltage created by the energy absorption branch during an interruption. The performance of the proposed algorithm is evaluated using a four-terminal test network with both hybrid and mechanical DC breaker technologies. The simulation results show that the proposed algorithm is able to quickly detect both complete and partial failures of the two breaker technologies even considering measurement errors, noise and ageing of the energy absorption components. [ABSTRACT FROM AUTHOR]

This paper discusses the back-flashover (BFO) phenomena in a high-voltage (HV) vertical double-circuit transmission line by means of numerical simulations. The single- and multiphase BFOs are analyzed considering 24 cases of AC-voltage angles and nonlinear characteristic of flashovers, taking advantage of the circuit analysis method in time domain. The models used in circuit analysis are the TEM-delay model, which can take into account the non-TEM characteristics of tower and line, and the conventional models. Prior to the BFO analysis, the characteristics of circuit models are compared and discussed with the numerical electromagnetic analysis results. In BFO analysis, it is clarified that the occurrence probability of the multiphase BFO is heavily depending on the bias AC-voltages. The relation of measured BFO phases and AC-voltages are well explained by the TEM-delay model with the valid lightning current magnitude. The conventional circuit models could have underestimated the occurrence of BFOs in HV transmission lines. The analysis and discussions shown in this paper can be utilized for advanced evaluation of lightning performance of transmission lines that considers the seriousness of lightning accidents. [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]

The inrush current generated by power transformer energization (TE) causes voltage sag of the nearby bus, the typical residual voltage of the voltage sag caused by transformer energization (VSTE) is about 0.7 p.u.--0.9 p.u. and lasts for a longer duration, which may lead to the sensitive equipment trips. This paper proposes an estimation method of voltage sag frequency considering VSTE. Firstly, based on the maximum entropy method, the residual flux distribution is calculated by considering the relationship between the residual flux and closing angle at the moment of TE. The voltage sag waveform caused by TE is calculated based on the estimation of the probability distribution of residual flux, to evaluate VSTE objectively and comprehensively. Secondly, this paper proposes a non-rectangular voltage sag equivalent frequency statistical method based on multiple magnitude-duration (MMD) function, to avoid the overestimation of voltage sag frequency by the traditional methods. Furthermore, this paper proposes an assessment algorithm of voltage sag frequency. Finally, the simulation of IEEE-30 test system verifies the necessity and correctness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper analyzes high-frequency interactions between a Modular Multilevel Converter (MMC) and High Voltage Direct Current (HVDC) grid resonances by studying their effect on the system stability. The recent appearance of converter-related instabilities due to high-frequency oscillations at the converter's ac side raises concerns about whether similar interactions can also take place at its dc side. To determine the risks imposed by such interactions within an HVDC grid, this paper assesses the impact of the MMC internal dynamics and dc system resonances on the stability using an analytical impedance-based method. The effect of fault current-limiting inductors, grid topology changes and transmission line length is investigated, indicating that these parameters considerably influence the electromagnetic characteristics of the HVDC grid and consequently the system stability. Furthermore, a sensitivity analysis of the MMC internal controller dynamics on the converter's non-passivity, causing the instability, is performed. [ABSTRACT FROM AUTHOR]

Travelling wave based protection functions require significantly higher sampling rates than protection functions based on time-domain superimposed quantities or fundamental frequency phasors. Integrating these high sampling rates in digital substations leads to a significant increase of the communication load of process-level networks and causes high computational cost for centralized protection systems. This paper builds on a distributed signal processing approach, which allocates the filtering operations among standalone merging units (SAMU). In particular, the paper presents a decimation filter design to integrate signals with high sampling rates at the process-level. Thereby, signal sequences with high, medium and low sampling rates are provided according to the requirements of the respective protection function. The decimation filter structure is optimized with respect to time delay by the Remez exchange algorithm and with respect to computational cost by a multistage approach and a sparse filter design algorithm. In addition, the filter response is verified against the accuracy constraints defined in IEC 61869-6 and IEC 61869-13. The paper shows that it is feasible to distribute the filtering operations among SAMUs, while keeping the time delay below the maximum allowable processing delay of 2 ms. [ABSTRACT FROM AUTHOR]

Autonomous power systems with a single power station and a high-voltage (HV) network may be susceptible to overvoltage instability, especially during light-load conditions. Moreover, in low inertia systems, considerable frequency excursions may be observed after disturbances. To deal with voltage and frequency stability problems in such isolated power systems, several measures should be taken, such as rules for adequate automatic voltage control of the synchronous generators and load-tap-changers (LTCs), and/or adequate design of actions applied during emergency system conditions such as reactors switching, and redesigning underfrequency load shedding taking into account overvoltage problems. The effectiveness of the proposed countermeasures against overvoltage instability in autonomous power systems is illustrated in this paper. In 2016, the autonomous power system of the Greek island of Rhodes experienced similar voltage/frequency problems leading to a blackout. This event has been accurately reconstructed in this paper. The application of the suggested countermeasures during the simulation of the 2016 blackout event in Rhodes, further highlighted the effectiveness of these measures, providing a guideline for similar cases. [ABSTRACT FROM AUTHOR]

The presence of overvoltage can cause the insulation deterioration and excite the partial discharges (PD) in gas-insulated switchgear (GIS), and it is necessary to detect the overvoltage and PDs simultaneously to evaluate the insulation performance. A novel GIS internal composite sensor for PD detection and overvoltage measurement is proposed in this paper. The proposed sensor is mainly formed with a disc-type monopole UHF sensor and an overvoltage sensor. The effectiveness and sensitivity of the sensor are tested on a designed platform under power frequency voltage, switching impulse, lightning impulse, and oscillating lightning impulse, respectively. Experiment results show that the proposed sensor can be used to detect overvoltage and UHF signals effectively and is expected to help in the GIS condition evaluation. [ABSTRACT FROM AUTHOR]

Powerful solar storms emit plasma that may travel towards the earth. Interactions between the plasma and the earth magnetic field cause geomagnetic disturbances (GMDs), which in turn induce quasi-dc voltage along long conductors in power systems. The induced electric field can be calculated by magnetic field data measured by magnetometers at several stations. In this paper, a set of online processing methods are proposed to improve signal to noise ratio (SNR) of the calculated electric field signals. In the first step, magnetic field signals are denoised inside a sliding window by a wavelet transform. The sliding window width is optimized and the introduced transients are compensated. Time derivative of magnetic field signals are estimated by a continuous wavelet transform in the next step, and the size of the sliding window is minimized for optimal computational speed. Ultimately, a method is proposed to further enhance the SNR by updating the previously processed data points. The magnetic and electric field signals obtained by the online and offline processing methods are compared with each other. It is demonstrated that the online processing methods can decrease the noises to a level comparable to that obtained by the offline methods. The results of this study can be implemented to protect key elements of power systems against severe solar storms. [ABSTRACT FROM AUTHOR]

With the advancements of the battery energy storage systems (BESSs), reduction of their manufacturing costs and government subsidies, the BESS uptake is likely to increase rapidly in power distribution networks. This paper investigates the effects of residential BESSs on low-voltage (LV) networks using the actual household load profiles equipped with BESS and solar-photovoltaic (PV) systems. The electricity consumption data collected via smart meters (2200 households with PV/BESS, 1950 households with PV only and 1000 households without a PV or a BESS) at different solar-PV penetration levels and network types are used to simulate real network operating scenarios. A real LV distribution network in Australia is analysed in DIgSILENT PowerFactory under different scenarios, such as, customers with and without a solar-PV/BESS, with a solar-PV but without a BESS, and without a solar-PV, by using both the power flow and quasi-dynamic simulation studies under balanced and the unbalanced network loading conditions. According to the study, customers experience large voltage excursions from solar-PV power exports, which could be resolved by the household BESS, provided that the BESS charging is coordinated with the solar-PV production. Moreover, quasi-dynamic simulation shows that the BESS could reduce the violation of the over-voltage limit during the solar peak hours (midday) by lowering the worst-case feeder voltage by 3%. Finally, extreme-event (high solar PV generation scenario) simulation shows that the implementation of the BESS controller to facilitate charging BESS during afternoon solar-PV export may reduce the negative grid impact and will assist to avoid network upgrades. [ABSTRACT FROM AUTHOR]

This paper proposes the use of a LoRa like chirp spread spectrum physical layer as the basis for a new Power Line Communication modulation scheme suited for low-bandwidth communication. It is shown that robust communication can be established even in channels exhibiting extreme multipath interference, impulsive noise and low SNR (−40 dB), with synchronisation requirements relaxed compared to conventional LoRa. ATP-EMTP simulations using frequency dependent line and transformer models, and simulations using artificial Rayleigh channels demonstrate the effectiveness of the new scheme in providing load data from LV feeders back to the MV primary substation. Our results demonstrate error-free communication at SNRs of −40 dB, and can be further improved by trading-off data rate. We further present experimental results based on a Field Programmable Gate Array hardware implementation which match the simulated performance. [ABSTRACT FROM AUTHOR]

The stator ground fault location of large generators can guide rapid maintenance. In this paper, a location criterion without fault transition resistance is constructed by using the fundamental potential and the third harmonic potential of the faulty winding simultaneously. The above potentials of the faulty winding are corresponding to the fault position. To precisely locate the grounding fault, it is necessary to clarify the analytical relationship between the faulty winding potential and the fault position. However, the existing methods often take the coil potential as the unit to analyze the winding potential, which has theoretical errors for the generators with short-pitch winding. Aiming at this problem, a more precise analysis method for winding potential is established by taking the slot conductor potential as the unit. To solve the location criterion, the optimization model for each slot conductor of the faulty phase is constructed. The slot conductor with the smallest optimal objective function value is judged as the faulty slot conductor, and the corresponding decision variable is judged as the fault position. Simulation and experiments verify the effectiveness of the proposed method, which can still meet the needs of engineering applications under multiple adverse conditions. [ABSTRACT FROM AUTHOR]

This paper presents the DC fault transient study of a meshed DC grid with high-temperature superconducting (HTS) HVDC cables to integrate offshore wind power. A four-terminal meshed DC grid model with a ±100 kV DC voltage rating is developed. DC circuit breakers (DCCBs) are implemented in the DC grid to deal with DC cable faults. To conduct the fault transient study, the fault scenarios with different fault locations and protection delays are studied. The sensitivity study versus different DC fault locations reveals that the faults on the faulted cable will not cause the quenching of healthy cables. The sensitivity study considering different DC fault protection delays demonstrates that the HTS cables have a current-limiting effect and the long delay of DC fault protection does not result in large fault current and the quenching of healthy cables. Extensive simulations in PSCAD/EMTDC validate the founding. [ABSTRACT FROM AUTHOR]

It is well known that for high voltage transmission lines in the range of 245 kV–525 kV, particularly in regions of high ground resistivity, lightning back flashover problems could be a major concern. Line arresters have been used successfully to address the problem. Underbuilt ground wires have been proposed as a viable alternative. The present paper shows that the full potential of additional ground wires could be achieved by optimizing their position on the tower. Such optimization often results in a preferred position at the level, or higher than the attachment height of the lower phase conductors; therefore the designation Embedded Ground Conductor. An essential requirement is that such positioning does not infringe in any way on the dielectric integrity of the electrical clearances and, under the influence of induced charge, does not become a source of positive streamers that may lead to additional radio or audible noise. It is found that a streamer- inhibited conductor, when properly placed, fully satisfies the above requirements. It is determined that for 245 kV, 345 kV and 420 kV lines, a streamer-inhibited embedded conductor could improve the back flashover rate by a factor of 10 or more [ABSTRACT FROM AUTHOR]