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Contamination of paper tapes by corrosive sulfur in insulating oils may cause shorting faults between turns. Typically, this occurs at higher temperature in the upper portions of the windings of shunt reactors and power transformers. In many of the tested oils, high amounts of dibenzyl-disulfide (DBDS) were found. [ABSTRACT FROM AUTHOR]

Weather-related or other types of faults can negatively impact the continuity of the electricity delivery to the consumers and, thus, lead to poorer reliability scores. 90% of customer outage-minutes are due to events which affect local distribution systems. The radial configuration is simple and has a low cost design; however, when a fault occurs it often causes power outages beyond the location of the fault itself. Therefore, it is beneficial for utilities to automatically isolate the fault and, thus, restore power to as many consumers as possible. The benefits of fault isolation and service restoration include, but are not limited to: enhanced reliability and resiliency, reduced loss of revenue to the utility, and enhanced comfort and satisfaction for the utility’s consumers. This paper provides an algorithm to quickly isolate a fault and automatically restore power to as many nodes of the distribution system as possible by opening and closing switches already placed within the system. The expert model and centralized algorithm that are proposed in this paper guarantee an optimal solution and a short run-time regardless of the size of the distribution system. A mathematical proof of the optimal nature of the solution is provided. In addition, the model is validated in practice, solving two different-sized systems. [ABSTRACT FROM AUTHOR]

The paper presents an experimental validation of a method to locate partial discharges (PDs) on power distribution and transmission networks. The method is based on electromagnetic time reversal (EMTR) theory, and it uses a Transmission Line Matrix (TLM) model to describe the propagation of the PD signals in the reversed time. Since PDs are regarded as a symptom of insulation degradation, on-line PD location is considered an important approach to monitoring the integrity of a power distribution network, with the aim of detecting and preventing faults and improving network reliability. In this paper, the EMTR-based method is described and its effectiveness in PD localization using only one measurement point is demonstrated in three real 33 kV power lines. Its effectiveness is proved with and without an on-line electromagnetically noisy environment, and its accuracy is evaluated with respect to different signal-to-noise ratio (SNR) levels of the networks. The validation shows that the method is able to locate PDs with an error of 0.14% with respect to the total length of the line in the absence of noise, and with an error that is always lower than 0.5% for an SNR down to −7 dB. [ABSTRACT FROM AUTHOR]

This paper proposes an adaptive self-tuned current controller design for an LCL-filtered LC-tuned single-phase shunt hybrid active power filter (HAPF), which offers adaptability and flexibility of the control system to filter parameters changes. System modelling and online identification of parameters are presented. Then, a systematic self-tuned controller (STC) design is proposed to calculate the controller parameters and obtain the final control law adaptively. The exact HAPF parameters design is described and the overall closed-loop stability evaluation with considering digital control delay is discussed in this paper. To confirm the theoretical achievements, simulations and experimental tests on a real prototype system are performed. The results are in good accordance and demonstrate the effectiveness and superior performance of the suggested control technique. Furthermore, a comprehensive comparison between the proposed control technique and other controllers is provided to show the superiority of the proposed system from different aspects. [ABSTRACT FROM AUTHOR]

Ground wires (GW) struck by lightning are often exposed to transient impulse voltages in the MV range. This results in intense negative streamer-type corona, causing a considerable increase in the effective wire capacitance and a corresponding reduction in its surge impedance. Such variables have a significant influence on the tower top potential and on the electromagnetic coupling between the ground wire and phase conductors. Presently there is considerable uncertainty regarding the impulse corona parameters to be used in back flashover calculations. In the present paper a new method is formulated for determination of GW impulse corona parameters. The new approach is compared with two existing methods and with available experimental results. It is found that the method usually used within the IEEE has little physical justification and is excessively conservative. It is also shown that such corona parameters significantly depend on the GW height above ground, which has been totally ignored in the CIGRE method. This and other unjustified simplifying assumptions explain the surprisingly large discrepancy between values of the negative impulse corona constant recommended by CIGRE and IEC. [ABSTRACT FROM AUTHOR]

This paper presents a novel methodology to estimate the saturation curve parameters of single-phase transformers, using minimum information from inrush and no-load tests. Only the value of the first current peak and the decay time are used from a unique inrush test, and only the third and fifth current harmonics from a unique no-load test. The proposed methodology is based on the existence of a signature for each transformer which can be easily obtained and characterize all possible inrush currents. This is the main contribution of the paper. Different transformers with different parameters have been tested in the laboratory. Experiments and simulations are carried out to validate the methodology. First, the respective no-load and inrush tests are made. Second, the parameters of each transformer are estimated. Third, the performance and accuracy of the proposed methodology are verified by a close agreement between the measured and the estimated saturation curves and between the simulated and the recorded inrush waveforms. Some remarkable advantages of the proposed methodology are: (a) considers the deep saturation, (b) avoids the need to know the residual flux (or to demagnetize the transformer) and the switching angle from the inrush test, and (c) it is computationally straightforward. [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]

Examines the effective approach for evaluating the condition of insulation in aged power transformers. Information on the criteria for judging the most effective techniques; Comparative assessment of the effectiveness of the available techniques; Correlations between various techniques for measuring degradation.

Dynamic thermal rating for underground cables with balanced phase currents has been extensively studied in the literature. Paradoxically, distribution feeders operate inherently unbalanced and no publications are available for unbalanced thermal rating. Because of the lack of technology, utilities operate with very conservative margins using the maximum of the three-phase currents to determine the line loading. This practice leads to the significant underutilization of underground cables. To fill the gap this paper presents a physically sound and accurate transient electro-thermal model of unbalanced three-phase distribution cables considering variations in the load and environment. Moreover, the model proposed in this paper allows harvesting the large hidden capacity of distribution systems trapped in the thermal inertia of underground cables. A convenient visual tool is developed to uncover hot spots and underutilized energy routes. This facilitates real-time network reconfiguration based on predicted load graphs and dynamic thermal conditions. The proposed model is also useful to study load growth, perhaps eliminating or deferring capital investments needed to upgrade the capacity of the existing underground feeders. The IEEE 37-bus distribution network is used as a case study. The results show that the proposed methodology has a high potential to defer investments, for instance, one could save $5M for a small system like the IEEE 37-bus distribution network which would have been necessary under current conservative practice. [ABSTRACT FROM AUTHOR]

When lightning strikes a transmission line tower or shield wires, electromagnetic waves propagate through the tower back and forth, increasing the voltage across insulator strings. This can eventually lead to a back-flashover (BF), which may cause damage to equipment or costly power outages. To calculate the over-voltages and predict the probability of a BF, an accurate model of the tower and its grounding system is needed in electromagnetic transient (EMT) type simulators. There are a number of theoretical models for the equivalent circuit of a transmission tower. However, they either are not accurate enough or they are derived for a certain type of transmission tower, which limits their applicability. Numerical electromagnetic analyses have less simplifications compared to the theoretical solutions and are by far less expensive than field measurements. They also have the flexibility to analyse any type of tower. In this paper, the direct method for the measurement of tower impedance is implemented by NEC4 and applied to a 400-kV double circuit tower with all its details. The process of obtaining the wire network of the tower used in this paper is completely automated and it can be applied to any other type of transmission tower. The results of the numerical simulations are compared to those obtained with existing tower models. The developed model in this paper is capable of considering all the details of the tower and including the finite resistance of the ground and grounding electrodes. [ABSTRACT FROM AUTHOR]

Condition monitoring of circuit-breaker (CB) operating mechanisms is one the most important aspects of predictive maintenance. This paper proposes a contact force monitoring scheme for 10 \sim 35-kV vacuum CB. With specially designed force sensors and a signal-processing method, we can accurately find the instants of contact touch and separation. With the indirectly measured contact touch-and-separation instants, we can measure the opening speed, closing speed, and open gap over the travel of each phase. The accuracy of our online monitoring scheme is compared with the conventional offline methods that can directly obtain the instants of contact touch and separation by measuring the connectivity of the contacts. The results show that the accuracy of the scheme proposed in this paper is satisfactory and it can meet the engineering requirements of online condition monitoring of CBs. By introducing a set of new force sensors, some additional information that is useful for overall condition assessment of CB operating mechanism can also be extracted; this will help us to find new ways to improve our CB condition monitoring systems. [ABSTRACT FROM AUTHOR]

The nature and causes of corrosive sulfur induced failures are examined in oil-filled transformers and shunt reactors. Copper sulfide, which is formed when the corrosive sulfur in a mineral oil reacts with the copper conductors, is likely to diffuse into the paper tapes insulating the conductors. Since copper sulfide is partially conducting, the dielectric losses of the contaminated oil-impregnated-paper tapes are markedly increased; paper tapes in close proximity to the copper conductors are found to attain tan 5 values > 1.0 even at room temperature. It is highly likely that thermal instabilities develop at those sites at operating temperatures, leading to increased loss currents and, ultimately, short circuits between the turns. This sequence of events is substantiated by evidence from the field, which indicates large areas of thermally degraded insulations and charred breakdown regions along the coils, the extent of which becomes more pronounced at higher operating temperatures (toward the top of the windings). [ABSTRACT FROM AUTHOR]

Presents a series of classified lists of power system relaying references from 1927 to 1999. References to the subjects of service; Publication of Electrical and Electronics Abstracts; Presentation of relaying algorithms.

These instructions give guidelines for preparing papers for this publication. Presents information for authors publishing in this journal. [ABSTRACT FROM AUTHOR]

In the last 20 years the use of ester fluids as an alternative to mineral oil for transformer insulation has been an active field of research and development. These liquids have a much lower environmental impact than mineral oils, besides reducing the transformer’s fire risk. Although the use of natural and synthetic esters is nowadays frequent for certain applications, as for distribution transformers in highly populated areas, railway transformers or off-shore windmill transformers, the experience on large and medium sized units is still reduced. One of the critical aspects that must be assessed to use esters as dielectric fluids for large size transformers is the dielectric design of the equipment. The permittivity of esters and ester-impregnated cellulose are different from those of mineral-oil-cellulose systems, what has an impact on the electric field distribution in the transformer. Additionally, in some cases, the dielectric strength of ester fluids differs from that of mineral oil. This paper presents a study about the dielectric design of ester-based power transformers. The insulation system of a 400/138 kV, 280 MVA transformer was modelled using a finite element tool and a comparison of the field distribution under 50 Hz AC power voltage of both insulation systems is provided. [ABSTRACT FROM AUTHOR]

With the increase in number of VSC based HVDC systems, it is inevitable that there would be interconnections with weak ac grids. The paper presents the analytical basis for identifying coupling in the power loops which is dependent on grid inductance and operating point by utilizing the impedance based modeling technique. The formulation is used to define an inverse-based decoupling outer-loop controller for VSC-HVDC system. The concept is extended to HVDC grids and it is shown that the gains obtained using simple inverse may lead to instability. Robust controller tuning with pole-placement and minimization of $H_\infty$ norm is presented for HVDC grids, which form a restricted feedback system. The results show that the proposed method of decoupling results in effective decoupling of the power loops over a large range of grid inductance values as compared to that with traditional vector control. The system stability is also maintained over a large range of grid inductance values and PLL gains when a sudden variation of parameters is considered. The stability margins can be enhanced by pole-placement. The observations from the small-signal models are confirmed using detailed time-domain simulations. The proposal is expected to be useful in controller design for single VSC as well as HVDC grids, especially, during planing stage. [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]

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 hybrid LCC/VSC cascaded HVDC (HC-HVDC) scheme, in which the LCC is adopted as rectifier, and one LCC in series with multiple paralleled VSC based MMCs is adopted as inverter, combines the advantages of both LCC and VSC-MMC technologies. Presently, a HC-HVDC project is planned to deliver the hydro power from Baihetan to Jiangsu in China. One of the major challenges is to suppress the DC overcurrent issue of VSC-MMCs especially under commutation failure condition of LCC at inverter side. Based on Baihetan-Jiangsu project under construction, the model of HC-HVDC system is developed in PSCAD/EMTDC. In the system, the rectifier employs a dual 12-pulse LCC, while the inverter adopts a hybrid scheme, i.e., one LCC converter in series with three paralleled VSC-MMC converters. This paper addresses the main cause and feature of the DC overcurrent, then proposes an overcurrent suppression control method for HC-HVDC system based on fuzzy clustering and identification approach. And the system transient performances with the proposed control approach are evaluated under AC fault conditions. The results show that the proposed approach can not only suppress the DC overcurrent following the commutation failure to a certain extent, but also improve the post-fault recovery performance of the overall system. [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 proposes to administer a multi-step artificial intelligence approach with an ensemble of adaptive neural networks (NNs) trained on 50000 samples to identify partial discharge (PD) diagnostic measurements for in-service medium voltage (MV) power cables. To evaluate the performance of the algorithm, a case study was performed on cables deliberately selected to contain both uncomplicated measurements and disruptive irregularities representative of conditions during field testing. Experimental test results prove that the proposed cable-specific adaptation improves PD identification accuracy, with further increment through the NN ensembles. The main contribution of the approach is in both the cable-specific adaption and the NN ensemble being applied to MV cable field measurements. [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]

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]

This paper highlights an immediate need for renewable generation developers, manufacturers, utilities and industry standardization institutes to jointly address a challenge of capacitor switching transient immunity in inverter-based wind or solar farms. The need was identified from wide-area operation records which showed partial or full generation loss in coincidence with routine utility-owned capacitor switching. The lack of a clear industry guideline has caused confusion to all involved parties. This paper recommends all entities to address this challenge collaboratively through enhanced equipment specification, design, and test before a well-defined industry guideline becomes available. [ABSTRACT FROM PUBLISHER]

Dielectric response measurement has recently been adopted by utilities for evaluating moisture content in cellulose insulation (paper and pressboard) of transformers. Moisture distribution is highly dependent on temperature. Since the temperature inside a transformer may change during the dielectric response measurement, the moisture in the transformer’s cellulose and oil insulation can hardly attain an equilibrium state. Instead, moisture dynamics exist inside the transformer: 1) cellulose absorbs (desorbs) moisture from (to) oil with the changes in temperature and 2) moisture migrates inside cellulose due to a moisture gradient. This paper investigates moisture dynamics and its effect on the dielectric response of a transformer’s cellulose insulation. It proposes a distributed parameter model to reveal the correlation between moisture distribution (under nonequilibrium conditions due to thermal transients) and dielectric response parameters (dielectric losses and permittivity) of cellulose insulation. It then estimates these parameters under moisture nonequilibrium conditions. The accelerated ageing and moisture diffusion experiments are conducted on a prototype transformer to verify the proposed model. The methodology developed in this paper can help the proper interpretation of dielectric response measurement of field transformers under thermal transients. [ABSTRACT FROM PUBLISHER]

This paper yields a systematic scientific foundation for the evaluation of responsibility of utility and customer to the harmonic distortions at the point of common coupling (PCC). In this regard, this paper proposes a technical process for the evaluation of harmonic distortions. This technical process provides a way to answer some technical questions related to harmonic issues based on the following aim: identifying the dominant side, determining the responsibility of each side to harmonic distortions, determining the influence of the utility on harmonic current distortions at the PCC, and determining the influence of the customer on harmonic voltage distortions at the PCC. This paper deciphers these technical questions in the concrete “relevant factors” for the evaluation of harmonic distortions. These “relevant factors” are systematically classified based on the equivalent circuits of utility and customer side for harmonic studies. In this regard, six well-defined “relevant factors” for the Thevenin equivalent circuit and six analogue “relevant factors” for the Norton equivalent circuit are formulated. Some of these “relevant factors” are proposed, for the first time, in this paper. Furthermore, this paper explains the relations between the “relevant factors” to ascertain their interdependence. In addition, the usefulness of the “relevant factors” to answer the technical questions related to the harmonic issues is illustrated using a calculation example. [ABSTRACT FROM AUTHOR]

Inverter-Based Resources (IBRs), including Wind turbine generators (WTGs), exhibit substantially different negative-sequence fault current characteristics compared to synchronous generators (SGs). These differences may cause misoperation of customary negative-sequence-based protective elements set under the assumption of a conventional SG dominated power system. The amplitude of the negative-sequence fault current of a WTG is smaller than that of an SG. This may lead to misoperation of the negative-sequence overcurrent elements 50Q/51Q. Moreover, the angular relation of the negative-sequence current and voltage is different under WTGs, which may result in the misoperation of directional negative-sequence overcurrent element 67Q. This paper first studies the key differences between the WTGs and SG by comparing their equivalent negative-sequence impedances with SG's. Then, simulation case studies are presented showing the misoperation of 50Q and 67Q due to wind generation and the corresponding impact on communication-assisted protection and fault identification scheme (FID). The impact on directional element is also experimentally validated in a hardware-in-the-loop real-time simulation set up using a physical relay. Finally, the paper studies the impact of various factors such as WTG type (Type-III/Type-IV) and Type-IV WTG control scheme (coupled/decoupled sequence) to determine the key features that need to be considered in practical protection studies. The objective is to show potential protection misoperation issues, identify the cause, and propose potential solutions. [ABSTRACT FROM AUTHOR]

This paper presents an evolution of control systems of Modular Multilevel Converters (MMCs) focusing on the internal voltages and currents dynamics. MMCs have passive components inside the converter that create extra dynamics compared to conventional VSCs. Some control schemes that do not consider these internal dynamics may still stabilize the system asymptotically thanks to the linearisation in the modulation step. However these control schemes are less robust because they are prone to poor damped oscillations on the dc side of the converter. The MMC circuit and energy relationships are presented in this paper. Along with a gradual development of the energy based control, the important roles of each internal dynamics are clearly demonstrated. Experimental results are presented to show the impacts of the linearisation in the modulation step on the system behaviour. [ABSTRACT FROM AUTHOR]

This two-part paper proposes a revised method for calculating high voltage (HV) cable sheath currents, and for fault location of cross-bonded HV cable systems. The equivalent circuit and the cable impedance per unit length under short-circuit fault conditions are revised from those under non-fault conditions. Part 1 of this paper proposes a revised phasor domain mathematical model for the sheath current calculation under fault conditions, following demonstration that short-circuit faults can cause changes in the equivalent circuit of the cable metal sheath, and cause changes in the cable impedance per unit length. The calculation results, as given in a case study, show that there exists a unique relation between the fault position and the sheath fault currents, making it possible to locate a fault by analyzing the characteristics of the sheath currents during a fault. Part 2 of the paper focuses on the fault location method via sheath current monitoring. [ABSTRACT FROM AUTHOR]

The majority of interruptions in the network of a distribution system operator (DSO) occur at low voltage (LV) levels. Although the number of affected customers is less than that for medium voltage level interruptions, contribution to the annual customer minutes lost is considerable, and the yearly costs of solving the outages are high. Underground LV cables cannot be visually inspected, and monitoring systems are still experimental. In order to improve asset management (AM) for LV cables, it is beneficial for DSOs to be able to perform condition assessments using historical data, in combination with asset and environment data. In this paper, survival analysis is performed using the Cox proportional hazard model. The results of this analysis can be used to identify those variables that predict a relatively high failure probability and to estimate the relative risk of failure for cables. This enables the improvement of AM strategies, such as the preventative replacement of cables. The method presented in this paper shows promising results, allowing for greater insight into the causes of failures. [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]

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]

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]

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]

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]

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]

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]

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]

The objective of this two-part paper is to provide clarity to physical concepts used in the field of transformer modeling, to dispel common misconceptions regarding numerical instabilities, and to present unified modeling techniques for low-frequency transients. This paper focuses on proper modeling of nonlinearities (magnetizing branches) since these components are critical to determine the low-frequency behavior. A good low-frequency model should properly represent: normal operation, inrush currents, open and short circuit, out-of-phase synchronization transient of step-up transformers, geomagnetic-induced currents, ferroresonance, and harmonics. This paper discusses the derivation of electrical dual models from the equivalent (magnetic) reluctance networks and the direct application of the principle of duality. It is shown that different dual models need to be derived for different transformer geometries and the advantages and disadvantages of each method are discussed. This paper also compares double-sided versus single-sided dual models, and shows that the double-sided model is a more general approach. The mathematical equivalency of several leakage models (negative inductance, mutual coupling, and BCTRAN) is demonstrated for three-winding transformers. It is also shown that contrary to common belief, a negative inductance is not the source of numerical oscillations, but they occur due to the use of noncorrect topological models for representing the core. [ABSTRACT FROM PUBLISHER]

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]

Over the years, there have been a number of cases where users experienced higher noise levels of transformers on-site versus what was measured in the factory acceptance tests. In some of these cases, measurements have shown that a power transformer can have significantly different noise characteristics (both noise level and frequency spectrum) on-site compared to what was measured in the factory. This difference can range from a few decibels to more than 10 dB in some cases. Some of these differences are found to be caused by local environmental factors that influenced the on-site measurements. Others were caused by operating conditions on-site that are different from those of the factory tests. This paper presents results of investigations and measurements performed, over the past several years, on the main contributors to this difference. For each of these contributors, data are presented that demonstrate the impact of this contributor as well as quantify typical magnitudes of this impact. It is hoped that the material presented in this paper will assist users in: 1) understanding some of the pitfalls of on-site measurement of transformer noise; 2) how operating conditions contribute to the noise performance of transformers on-site; and 3) how significantly higher on-site noise levels can, in some cases, indicate an issue with the transformer or any of its components/accessories. It is also hoped that the material in this paper will contribute to industry standards/application guides on noise determination and abatement. [ABSTRACT FROM PUBLISHER]

Investigates the degradation processes observed in the insulation from aged transformers. Description of the analysis of accelerated aged insulation samples; Performance of short term accelerated ageing experiments; Techniques used to examine the condition of aged insulation samples; Details on studies carried out on other electrical properties and pressboard samples; Indication of the results.

This paper aims at providing reliable methods of assessing moisture in oil-paper-insulated power transformers. The traditional method of moisture evaluation, oil sampling with the subsequent application of an equilibrium diagram, suffers from severe errors resulting in an overestimation of water concentration. To improve this, moisture sorption in cellulose (paper, pressboard) and moisture solubility in insulation oils were theoretically described and represented as sorption isotherms and equilibrium diagrams under the special consideration of aging. Several steps lead to the key recommendation of this paper, which is to use water saturation in oil and in cellulose to describe water in power transformers. The practical application and the use of vegetable oils are considered as well. [ABSTRACT FROM PUBLISHER]

Water is damaging for power transformers since it accelerates aging processes, reduces dielectric margin, decreases partial-discharge inception voltage, and, therefore, increases the probability of unexpected failures. Taking into account that power transformers are the most expensive equipment in an electrical plant and that the electrical supply guarantee is closely related to their reliability, it seems clear that transformer water content is a variable that must be kept under control. Since water is a byproduct of aging reactions in cellulosic insulation, the amount of water contained in a transformer usually increases from levels below 0.5% in weight, after its manufacturing, to levels of 6% in weight at the end of its life. However, it is not unusual to find high humidity levels in not so-old transformers, for example, in those that have been subjected to repairs in field. These days, some techniques are available to dry transformers in the field. Most of these methods achieve proper drying of transformer oil, but a great amount of water usually remains in cellulosic insulation and water in oil rises slowly some months after finishing the process. In these cases, drying treatments, which are very expensive; would have limited effectiveness. In this paper, a finite-element study is presented that analyzes different field-drying methods that are currently widely used. The efficiency of these methods will be studied and compared, and recommendations about drying parameters will be given. [ABSTRACT FROM AUTHOR]

This paper presents the induced voltage measured at the terminal of a 10 kV distribution line when triggered lightning taking place at a distance of 40 m. It is shown that the induced voltage waveforms exhibit three components including a slow increase preceding the initial negative peak, following a rapid polarity reversal and the attenuated oscillations. The waveform parameters are defined in this paper. The observed negative peak value ranges from −47.1 kV to −7.5 kV. The positive peak value ranges from 5.3 kV to 20.9 kV, and the peak to peak value ranges from 12.8 kV to 63.2 kV. The test results show that the negative peak value of the induced voltage at the terminal of distribution generated by nearby negative triggered lightning generally exceeds the positive peak value, and in some cases the negative peak value even reaches twice the positive peak value. The negative 10%-90% rise time ranges from 2.3 μs to 88.5 μs, and the average oscillation period is about 18 μs. Both negative and positive peaks of induced voltages show good linear relationship with the peaks of lightning return stroke current. [ABSTRACT FROM AUTHOR]

Point-on-wave (POW) is a single-event characteristic of voltage dips, influencing the performance of sensitive equipment, such as AC contactor and wind turbines. However, definition and calculation of POW are unsolved issues. The existing definition of POW is for single-phase dips; it does not consider three-phase dips. Moreover, existing POW calculation methods are inaccurate. This paper proposes a definition of POW as a single-event characteristic for three-phase voltage dips. Then, it proposes a POW calculation method based on space vector, including POW detection and angle calculation. The proposed method is based on the ellipse characteristics during the dip-stage and non-dip stage. The proposed method does not depend on zero crossing for angle reference. To verify the performance of the proposed method, a power-system simulation model is built to generate voltage dips with different origin. Furthermore, the accuracy of the proposed method is verified by 425 sets measured voltage dips, and compared with the published methods. [ABSTRACT FROM AUTHOR]

This paper deals with the modeling, hardware results and model validation by measurements of a VSC assisted resonant current (VARC) dc circuit breaker (CB) and the application within a future network by simulation. The newly emerging VARC dc CB can be used as a solution for the protection of offshore multi-terminal HVDC (MTDC) grids. In this paper, the proposed VARC dc CB is modeled in detail in a PSCAD environment, by taking into account dielectric strength of the vacuum gap, high-frequency current quenching ability and parasitic components. The PSCAD-model is then verified by data from the testing of a 27 kV VARC dc CB prototype with maximum current interruption capability of 10 kA. Additionally, the initial transient interruption voltage and current slope at zero-crossing during the interruption are analyzed. With respect to scaling to a higher voltage level, three types of series connected modules are presented and the performances are compared. The performance of the series connected modules is simulated in a model of a 4-terminal HVDC grid. The obtained results validate the VARC dc CB as a promising solution for the dc fault isolation in MTDC grids. [ABSTRACT FROM AUTHOR]