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For extra-high-voltage high-temperature superconducting (HTS) cables, not only high electrical insulation performance but also low dielectric loss is required. Therefore, we focus on a porous synthetic paper with lower dielectric loss, e.g., Tyvek polyethylene nonwoven fabric, than that of polypropylene laminated paper (PPLP), which has been used for a conventional HTS cable insulation system. In this paper, we compare partial discharge inception characteristics for different liquid nitrogen/synthetic paper composite insulation systems. The partial discharge inception electric field strength in the composite insulation system depends on the electric field strength in the porous area of synthetic paper. [ABSTRACT FROM AUTHOR]

Oil-paper insulation of higher voltage winding and traction winding in on-board traction transformers frequently withstand large AC fluctuation voltages mainly combined with AC and inter-harmonic components. This paper presents the characteristics of partial discharge (PD) of oil-paper insulation under superimposed inter-harmonic AC voltages. The experimental results show that the addition of the inter-harmonic to the fundamental sinusoidal component results in variation of peak voltage and dv/dt along the phase. This superimposed inter-harmonic AC voltages has a significant impact on PD including partial discharge inception voltage (PDIV), breakdown voltage, PD repetition rate, and phase resolved partial discharge (PRPD). In addition, a dynamic characterization method of PD statistical parameters is proposed to determine the developing trend. The critical values and inflection points of PD are obtained. Besides, the theoretical mechanism is expounded to better interpret and understand the influence of inter-harmonic component on PD properties. These methods will provide the dynamic datasets and more information for insulation condition assessment of on-board traction transformer. [ABSTRACT FROM AUTHOR]

The breakdown and streamer behavior in double-layer oil-paper insulation under impulses of varying front time were investigated in this study to determine the prebreakdown process in oil-paper insulation. The relationships between front times and breakdown characteristics were investigated and impulse-polarity-dependent variations were observed. The complete impulse breakdown process and precise role of front time were assessed as per streamer initiation and streamer propagation based on emitted light signals and shadow images. The re-initiation of streamers in non-first oil gaps was determined to be the key factor in breakdown as-dominated by applied voltage and space charge accumulation. The initiation and propagation times of streamers were observed to find that the streamers tend to propagate at phases with higher voltage amplitude as front time increases, which facilitates streamer re-initiation and thus decreases the breakdown voltage. Equivalent applied voltages during streamer propagation were further calculated to quantitatively investigate the effect of front time on streamer propagation. [ABSTRACT FROM AUTHOR]

It is state-of-the-art to describe the dielectric behavior of an insulation material by its permittivity and its specific electric conductivity in order to estimate the dielectric stress of an insulation system. Thus, the electric field at DC voltage stress is determined according to the stationary electrical conduction field with the electric conductivity of the insulation materials. However, at oil-insulated arrangements a higher field strength in front of bare metal electrodes at high DC voltage stress occurs, which is not explainable with this model. Therefore, a charge carrier-based approach is presented to describe the dielectric behavior of the oil-paper insulation. It describes the movement of charge carriers and their effect on the electric field strength. Their drift leads to an accumulation of charge carriers in front of electrodes which results in a higher field strength in these areas, which can be calculated numerically using the Poisson-Nernst-Planck equation system. Compared to the conductivity-based model fundamental differences can be shown. Breakdown experiments qualitatively confirm the expectations according to the charge carrier-based approach. The results show that the charge carrier-based field distribution has to be considered for modelling the electrical field strength distribution at high DC voltage stress. They also show, that the dielectric behavior of these arrangements cannot be explained according to the state-of-the-art model. [ABSTRACT FROM AUTHOR]

The main insulation system in HVDC transformer consists of oil-paper insulation. The formation of space charge in oil/paper insulation system can not only affect the dielectric strength, but also relate to the stability and security of HVDC transformer. Moisture has detrimental effect on the electrical breakdown strength and insulation life of oil-paper insulation. Thus a better understanding the influence of moisture on the charge distribution in multi-layer oil-paper insulation system becomes increasingly practical importance. In the present study, the space charge dynamics in three layers oil-paper insulation system with three different moisture concentrations (0.28, 1.32 and 4.96%) were investigated using the PEA technique under 6 kV. The influence of moisture on the charge distribution in multi-layer oil-paper insulation system was provided. Results show that the higher the moisture content of the oil-paper sample, the higher the mobility of the trapped charges, and the more obvious the positive charge injection and negative charge injection were observed. The negative charges can inject across the paper-paper interface near to the cathode when the moisture content increases to 4.96%. The higher moisture content of the oil-paper sample, the faster the dissipation rate of the charges trapped in the sample. Moreover, the higher mobility of the trapped charges leads to less slow moving charges trapped in the sample with higher moisture content. [ABSTRACT FROM PUBLISHER]

Power transformer is one of the most expensive equipments in a high voltage AC power system. Adequate monitoring system for power transformers can help in reducing the failure rate and thereby enhancing system reliability and economic efficiency. The failure of power transformer has serious consequences on system reliability. Long term and comprehensive testing cannot be done in working transformer, so prorated model of transformer is preferred. In the proposed work, a new dual temperature test cell is developed based on IEC 62332 and IEEE C57.100. The cell provides for a current carrying conductor wrapped with insulating paper and further insulation is provided with high-density pressboard as per arrangements in power transformer. In this paper a new mathematical model is developed using activation energy and preexponential constant calculated from experimental results at different temperatures. The expected life based on degree of polymerization (DP) value, shows nonlinear aging of insulation based on investigation carried out on prorated model. [ABSTRACT FROM AUTHOR]

This paper presents and verifies a new mathematical model to explain the dynamic behavior of characteristic gases in the oil-cellulose insulation of high voltage devices like power transformers. The model is based on the diffusion process. Parameters of the model are already quantified from experiments and presented in previous publications. The mathematical model and assumptions are presented here. The model solution is obtained analytically and gas concentration change inside the paper insulation as a function of time is simulated. The results are converted to the concentration change in the oil using the principle of conservation of mass and validated with experimental measurements. The presented model can be used to reduce the error of Dissolved Gas Analysis due to the migration of characteristic gases inside a healthy power transformer. [ABSTRACT FROM AUTHOR]

High voltage (HV) transients in electrical power systems are mainly caused by lightning strikes and switching operations. The dielectric properties of a particular electrical insulation at the instant of interaction with a high voltage transient would determine a level of the degradation of an electrical insulation. This work investigates how high voltage impulses would change Phase Resolved Partial Discharge (PRPD) patterns in test samples consisting of a cavity deliberately introduced between the sheets of oil-impregnated paper used in power transformer bushings. It also investigates how the change in PRPD patterns is related to the degradation level of oilimpregnated paper. In order to accomplish these objectives, the experiments were set in such a way that the effect of HV impulses and an early stage PD activity at an elevated AC stress, HV impulses in combination with a prolonged PD activity at an elevated AC stress, and a prolonged PD activity alone at an elevated AC stress could be investigated separately. The experimental results presented in this paper indicate that HV impulses below the impulse breakdown stress following an early stage AC PD activity would not cause a significant change in PRPD patterns, and would not damage oil-impregnated paper to a level which can be noticed by a visual observation. On the other hand, a prolonged AC PD activity alone can cause the change in PRPD patterns, but cannot quickly damage the oil-impregnated paper as it would do when it is combined with HV impulses. However, the combination of both, HV impulses and a prolonged AC PD activity caused a high drop in the PD parameters (total PD charge and a repetition rate). The study found that the decrease of the PD parameters can be considered as a sign of severe degradation of oil-impregnated paper. [ABSTRACT FROM AUTHOR]

Copper-in-oil dissolution and copper-on-paper deposition may affect the insulation of oil-filled power transformers, shunt reactors and high voltage bushings. Dissolved copper increases the dielectric losses in oil, and copper deposition can dramatically increase insulating papers' conductivity. The available literature does not report researches where the mechanisms of dissolution of copper in oil and copper deposition on the surface of solid insulation (Kraft paper) have been elucidated. This study is aimed at the investigation of the tendency of some commercially available unused mineral oils of different compositions to dissolve copper and to deposit it in the cellulose phase. Different conditions of reaction, presence of oxidation inhibitor (of the hindered-phenols family), addition of triazole-type compound (used as corrosion inhibitor) and general oil properties were studied to investigate their influence on copper concentration in oil and onto paper after artificial aging in controlled conditions. [ABSTRACT FROM AUTHOR]

Dielectric parameters of insulation used in High voltage equipments like power transformers are dependent on insulation size and geometry. As a result parameters evaluated from the model of one insulation system finds limited use in predicting the condition of other. Poles calculated from branch parameters of insulation model, on the other hand are independent of insulation geometry. However such technique requires accurate modeling of dielectric response function. Conventional Debye model does not consider the effect of temperature gradient that exists across the length of the insulation during its operation. In the present paper it is shown that for more accurate modeling and better understanding of dielectric response of oil-paper insulation, a new model which considers the effect of temperature gradient on insulation properties, is necessary. The present paper reports a methodology that is capable of identifying such a model using PDC measurement data. It is also shown that use of conventional Debye model overestimates the pole values and the error increases with usage of the insulation. [ABSTRACT FROM AUTHOR]

Voltage rise caused by high levels of distributed generation is manifesting as voltage regulation challenges for many electricity network service providers. In this environment it would be ideal to reduce supply voltage magnitudes, however, many network operators are hesitant to do so due to concerns related to consumer appliance performance at reduced supply voltage magnitudes. Voltage regulation requirements are defined by network standards and network service providers must ensure voltages remain within specified limits. Through an evaluation of domestic appliance performance when supplied at various voltage magnitudes, this paper examines the impact of varying voltage levels on residential appliances. Equipment energy demand, operation and actuation were monitored for each applied voltage magnitude. While no equipment failures were recorded, appliance behaviour varied significantly with applied voltage magnitude. Individual appliance conservation voltage reduction (CVR) factors have also been established. The results highlight the importance of good voltage regulation and provide substantiated appliance performance figures for future studies. The outcomes of this paper allow electricity network service providers to understand the implications of supply voltage magnitude on domestic appliance performance, whether it be understating of the impact of higher voltage magnitudes caused by distributed generation or implications of reducing voltage magnitudes to provide headroom for distributed generation integration. [ABSTRACT FROM AUTHOR]

Test methods for evaluating flashover voltage of ceramic and nonceramic insulators under ice, snow, and cold-fog conditions are recommended. The paper describes the procedure to be followed at various stages of an evaluation test including preparation of the test object, insulator precontamination, ice, snow, and cold-fog deposits as well as voltage applications and flashover voltage evaluation. Some relevant parameters for ice accretion, flashover voltage evaluation, artificial pollution, and power source requirements are taken from IEC 60129, IEC 60507, and IEEE Std.-4-1995 on standard techniques for high-voltage testing. [ABSTRACT FROM AUTHOR]

In recent years, there is an increased use of passive filters in both transmission and distribution systems due to the proliferation of power electronic devices. However, the problem of filter design has not been well addressed in the past and one example is the application of high-pass filters in a filter package. Three types of HP filters are widely used to mitigate multiple harmonics, i.e., 2nd HP filter, 3rd HP filter and C-type filter. There is still a lack of research to clearly reveal the characteristics of each filter, which makes it difficult for the designer to select the optimal filter topology under different circumstances. The main goal of this paper is to solve the problem of how to select a proper configuration among three HP filter candidates for a given harmonic problem. Unlike the conventional optimization-based filter studies, this paper investigates three HP filters in an analytical way. As a result, the paper provides a deep insight into the inherent characteristics of the filter and the conclusions drawn in the paper are universal; thus can be used in different applications, ranging from low voltage levels to high voltage levels. The research results lead to a recommended application scope of each HP filter, which is very useful to guide the design of filter packages and help designers to evaluate the design results. [ABSTRACT FROM AUTHOR]

The iso-margin method based on the classical analytical equations is widely used in the development of extra high voltage (EHV) bushing. Therefore, its effectiveness and feasibility have been fully validated. However, with the theoretical formula, it is difficult to evaluate the influence of stray capacitance and temperature on the internal electric stress of the ultra high voltage (UHV) bushing. It?s necessary to further improve the traditional iso-margin method. Firstly, the basic principle of iso-margin method has been briefly described. After that, the E-field distribution of condenser bushing was investigated on the Finite Element Method (FEM) computing platform. Then, the mathematical model of the advanced equal margin design method was established. Furthermore, this paper presents the program flow of optimization which combines the simulation of electro-thermal coupling process and the particle swarm optimization (PSO) algorithm. The proposed methodology was applied to the design of the prototype of the UHV resin impregnated paper (RIP) oil-SF6 bushing, which realizes the uniform axial E-field distribution of the bushing condenser. Meanwhile, the partial discharge margin between adjacent foils can be equal. Moreover, the hot-spot temperature is lower than the operation limiting temperature of the RIP material. The bushing condenser was fabricated according to the optimal structure design. The prototype of bushing has passed through all the type tests. In this paper, the FEM electro-thermal coupling simulation and the advanced equal margin design method were applied to the inner insulation structure optimization of the UHV RIP oil-SF6 bushing. Meanwhile, the proposed methodology provides some theoretical guidelines for the future research and development of other types of bushing on the UHV level. [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]

We are pleased to announce that the August 2017 issue of the IEEE Transactions on Dielectrics and Electrical Insulation (TDEI) will be a special issue on Power Modulators, Repetitive Pulsed Power and High Voltage topics. [ABSTRACT FROM PUBLISHER]

Describes the above-named upcoming special issue or section. May include topics to be covered or calls for papers. [ABSTRACT FROM PUBLISHER]

The converter transformers are reported to fail generally on the thyristor-valve side, where complex alternating voltage waveforms superimposed with dc voltages would occur. The effect of these waveforms on the failure and endurance of the valve side winding insulation of the transformers is not yet understood, which is investigated here. In this paper, the voltage waveforms occurring in a converter transformer are obtained through PSCAD/EMTDC for a typical double pole ±500 kV, 1000 MW DC-link HVDC transmission system based on the CIGRE benchmark system. The waveforms are then amplified using a High Voltage amplifier for experimental breakdown investigations. Stepped-stress damage equalization method has been used for obtaining the lifetime characteristics from failure data. A comparison with life curve under pure sinusoidal and actual alternating voltages with superimposed dc and harmonic voltage has been made to understand why the converter transformers generally failed on the valve side. The results first time give fundamental reasons for the possible failure of the converter transformers and put forth important design considerations. [ABSTRACT FROM AUTHOR]

The commutation failure (CF) is the most common fault in line-commutated high voltage direct current (LCC-HVDC) systems that may lead to the transient overvoltage in the sending-side system. In the worst condition, the CF may lead to large-scale wind turbine tripping. To resolve this problem, the mathematical relationship between the reactive power consumed by the rectifier and DC voltage, DC current is derived. Then, a transient overvoltage calculation method is proposed in this paper. Furthermore, the mechanism of transient overvoltage caused by the CF is analyzed; it is revealed that the reason for three times transient overvoltage is the rapid decrease of the reactive power consumed by the rectifier during the CF and the recovery period from the CF. This paper proposes a constant reactive power control (CRPC) to inhibit transient overvoltage of the sending-side AC system. The proposed CRPC can increase the reactive power consumed by the rectifier, reduce the exchange reactive power between AC and DC systems, and suppress the transient overvoltage. A simulation model in PSCAD serves to verify the proposed CRPC on the transient overvoltage suppression in the situation of different fault types, fault duration, fault severity and short circuit ratio (SCR). [ABSTRACT FROM AUTHOR]

Standard TRIAC-based dimmers introduce power quality issues especially for a large-scale lighting network. Other existing dimming protocols involve additional wiring systems and/or additional controllers to light emitting diode (LED) drivers. This paper proposes a central dimming system for a large penetration of LED lamps. The dimming system is remotely controlled through a webpage or a desktop application. Dimming is achieved while maintaining high voltage and current quality waveforms, which results in a high power factor and a low input current harmonic distortion. The system does not require additional wiring or specific adjustments to commercial dimmable LED drivers. The system allows scheduling a dimming profile to endorse energy saving. In the proposed dimming system, dimming function is achieved by connecting a voltage source converter (VSC) between the grid and the LED lamps. An advanced feature is added to the VSC dimmer to remotely send/receive messages between the system and the user through a graphical user interface. Thus, the user can communicate with the VSC dimmer by sending commands and receiving feedback information. The influence of communication delay on system stability is analyzed by using small signal models. A VSC dimmer prototype (500 VA/120 V) has been built with a communication module to provide remote control. Experimental results and comparisons between utilizing the TRIAC-based dimmer and the VSC dimmer for dimming function are discussed in the paper. [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]

While much attention was given in the literature to improve the fault ride through (FRT) capability of a doubly-fed induction generator (DFIG)-based wind energy conversion system (WECS) during various faults at the grid side, not much attention was given to investigate and improve the performance of the DFIG-WECS under ferroresonance conditions. This paper investigates the impact of ferroresonance on the overall performance of a DFIG-WECS. The paper also presents a new technique to improve the high voltage ride through capability of the investigated system under such condition. The proposed technique relies on incorporating a superconducting coil within the dc-link of the DFIG grid-side and rotor-side converters. Energy exchange between the coil and the system is controlled through regulating the duty cycle of the dc-chopper interfacing the coil with the dc-link. In this regard, a simple but yet effective proportional-integral (PI) controller is employed to regulate the dc-chopper duty cycle. Parameters of the PI controller are optimized using elephant herding optimization algorithm. Simulation results verify the effectiveness of the proposed controller in improving the overall performance of a DFIG-WECS during ferroresonance events. [ABSTRACT FROM AUTHOR]

For most mass-impregnated (MI) paper insulated HVDC cables, the dielectric strength is found to be weakened when the cable cools, resulting from the creation of dielectric cavities. To better understand the cavity creation, two mechanisms are frequently compared, based on the imperfect impregnation process and sheath plastic deformation respectively. This paper presents an analytical calculation of cable internal pressure, which is applicable to both cavity creation mechanisms. It demonstrates that the two mechanisms can jointly place constraints on the MI cable rating, by limiting the overall rating-related thermal expansion. [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 design, construction, and operational characteristics of a low-impedance and low pulse drop transmission line transformer (TLT) are described in this paper. The multicore coaxial transmission line used for this transformer is different from traditional coaxial transmission line since its inner conductor consists of several high-voltage cables that are connected in parallel by using two metal disks placed at both ends of them. Simultaneously, the impedance of the coaxial transmission lines with ten cables designed in this paper is perfectly reduced from 60 to $13.5~\Omega $. Moreover, the transmission lines can withstand a high voltage of several tens of kilovolts with 13.9 mm in diameter. The transmission lines were then used to construct TLTs. The transformers are wound inductively in separated cylinders made of polypropylene to enhance the isolation from the input to the output of the transformers. Consequently, the pulse drop is substantially reduced, minimizing pulse distortion in the transformer. A three-stage TLT, whose input and output impedances are 4.5 and $40.5~\Omega $ , respectively, was developed with the transmission lines. The experimental results show that the three-stage transformer based on the low-impedance and low pulse drop design transmission lines can achieve a voltage gain factor of almost 3.0. [ABSTRACT FROM AUTHOR]

This paper has introduced two important indexes, turn-off surge voltage (TSV) on power switch and peak positive pole potential (PPPP) of storage batteries, which can be used to evaluate switching transient process of high-voltage energy storage system. Research on them is of great significance to power switch choice and high-voltage isolation design in engineering applications. Power switch response process at the turning off moment and transient process in freewheel stage have been analyzed by the established small-signal model of single power switch; then, a power switch test platform for hybrid energy storage system was built to verify it. Taking pulsed-power supply charging device with a novel inductor of low resistance and high inductance as a research object, this paper has analyzed dynamic process of the TSV and the PPPP at the turning off moment through establishing state space model of the whole system. Comparison shows that the numerical curves match the simulated curves well. Finally, the exactness of the employed model has been further verified in two pulse capacitor charging experiments. [ABSTRACT FROM AUTHOR]

This paper proposes a novel emulation system for emulating the power system in the fuel cell electric vehicle. The major purpose of the proposed emulation system is to test the dc–dc converter between the fuel cell and high-voltage powertrain. The bidirectional buck–boost type dc–dc converter is applied in the emulation system as the basic elements. A modularized approach is presented for easily scaling up and down the power level of the emulation system. Meanwhile, an energy loop structure is implemented in the proposed emulation system for power capability boosting purpose. Because of the proposed energy loop configuration, the emulation system offers long-term running capability with high power rating without the grid connection. A neutral current control mechanism is applied in this paper to further enhance the long-term testing capability as well as extending the lifespan for the battery packages. The simulation studies are proposed in this paper to validate the control mechanisms of the emulation system. A 10 kW hardware prototype has been built and the experimental tests are presented for validating the functionalities of the emulation system. [ABSTRACT FROM AUTHOR]

The Ultra High Voltage (UHV) converter transformer may generate serious inrush current during closing. It will cause commutation voltage distortion and easily result in commutation failure of converters on the same commutation bus. Due to the change of flux amplitude and phase caused by closing resistance in the Ultra High Voltage Direct Current (UHVDC) system, it is difficult to determine the closing time of the converter transformer when considering the influence of residual flux. Therefore, this paper first analyses the existing closing resistance strategy, and constructs the magnetic bias function according to the influence characteristics of closing resistance on the flux and phase. Then an inrush current suppression strategy based on magnetic bias simulation is proposed for star connection and delta connection of UHV converter transformers, which can accurately calculate the ideal closing time of mutual cancellation between the residual flux and bias flux during closing. Finally, the no-load closing model of the UHV converter transformer is established by PSCAD/EMTDC for simulation analysis of the feasibility, real-time verification is carried out using the RTDS closed-loop experimental platform. The results show that the proposed inrush current suppression strategy can effectively suppress the inrush current under different residual flux conditions. [ABSTRACT FROM AUTHOR]

This paper widens the application of a testing protocol for overhead line insulators introduced by TERNA. Testing results relevant to three groups of Room Temperature Vulcanized pre-coated cap-and-pin insulators removed from High Voltage Alternate Current and Extra-High Voltage Alternate Current Italian lines after different years of service are reported and statistically processed; the third group is treated here for the first time and new results for the other two groups are included. Among the various tests prescribed in the protocol, focus is on the Quick Flashover test, with the aim to examine and compare the flashover voltages of all three groups as a function of service time. The dependence of flashover voltage on electric field is also analyzed and a model correlating flashover voltage with field and time is proposed for the first time. It is confirmed that the flashover voltages provided by the Quick Flashover test can be considered as a diagnostic property of Room Temperature Vulcanized insulators, as they drop with the ageing time and the electric field. [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]

Four-level hybrid-clamped inverter is a newly proposed topology that can operate under a wide voltage range without switches connected in series. However, when it is applied in medium voltage high power conversions, the flying capacitors in each phase will occupy a huge volume and a high switching frequency is required to restrain the voltage ripples. In order to overcome this drawback, a four-level active neutral-point clamped inverter is discussed in this paper, which consists of only six switches and no diodes or flying capacitors are required. In order to balance the neutral-point voltages under the full power factor and modulation index range, a capacitor voltage balance method based on carrier-overlapped pulsewidth modulation is proposed in this paper. The upper and lower dc-link capacitor voltages are balanced by zero-sequence voltage injection and the central dc-link capacitor voltage is balanced by adjusting the duty cycles of switching signals slightly. Simulation and experimental results are presented to confirm the validity of this method. [ABSTRACT FROM AUTHOR]

With the rapid development of photovoltaic and fuel cell stacks, high voltage gain converters are urgently demanded. Conventional boost is limited by the exaggerated duty cycle, it is impossible to obtain a high voltage gain while ensuring efficiency. Therefore, a zero-voltage switching (ZVS)-modified SEPIC converter that combines a voltage multiplier cell with traditional SEPIC is proposed in this paper. With detailed parameters design process, this presented converter works at zero voltage switching (ZVS), while obtaining high voltage gain without extremely high duty cycle. Besides, voltage stress is also reduced and low-voltage component with lower parasitic parameters can be used, thus efficiency is maintained. In order to further improve the power density, an integrated inductor scheme is adopted in this paper without changing intrinsic working states of the two inductors. A 36-W prototype is built, and experiments are conducted to verify the advantages of the proposed converter. The efficiency is up to 91.5% at full load. [ABSTRACT FROM AUTHOR]

The benefits of employing multilevel pulsewidth-modulated (PWM) waveforms for diagnostic purposes compared to sinusoidal- and square-shaped voltages are presented in this paper. Examples on how the high voltage circuits are designed to enable these studies, as well as results, are discussed. To demonstrate the approach, it is investigated what influence the choice of PWM levels has on the partial discharge (PD) characteristics for two similar wire specimens insulated by different materials. Here, to resemble a motor winding, two different twisted pair test objects insulated by either pure polyamide-imid (PAI) material or with added conductive Cr2O3 material were exposed to voltages with ultrashort rise times. The test objects were fed from either two-, three-, five-, or seven-level inverter waveforms of similar shape as in actual applications. To compare the performances, measurements of the PD characteristics were conducted. Specifically, the total number of PDs, as well as their average maximum amplitude per cycle, was obtained together with the phase-resolved PD pattern for each case. The experimental results presented in this paper show that the total peak and summed PD magnitude (exposure) drop considerably more for the Cr2O3 insulation compared to PAI when applying a five-level or higher level inverter. This reduction is more significant for the insulation with added Cr2O3 material and the usage of higher inverter levels reveals this. As more information can be obtained using higher inverter levels, the evaluation of new material constitutions is preferably done utilizing at least a three-level inverter or higher. [ABSTRACT FROM AUTHOR]

The staircase modulation strategy is the best choice for asymmetric cascaded H-bridge multilevel inverter (ACMI) because it makes the output voltage to present a high number of levels and makes one of the ACMI modules to operate at low frequency. However, the staircase modulation is usually employed without giving its deserved attention. This paper presents a clear understanding of the staircase modulation strategy and its applications in both isolated and grid-connected ACMI.  The main novelty of this paper is the obtaining of the switching function for each module. The principle of operation is carefully described, also indicating how to command the ACMI transistors. The output voltage of the two superior modules is evaluated for different values of iterations. Special attention is given to the power distribution among the ACMI modules when the staircase modulation is applied. It is demonstrated on how the power is processed in each module considering an ACMI with three modules. The analysis is presented showing the staircase modulation within a closed-loop control strategy for isolated and grid-connected modes of operation, showing that the content of this paper is valid for both modes. Experimental results clarify and show the efficacy of the content presented in this paper. [ABSTRACT FROM AUTHOR]

A high-frequency high voltage gain discontinuous conduction mode coupled-inductor boost LED driver based on planar component is presented in this paper. GaN switch is adopted with lower conducting resistance and higher switching speed. Zero voltage switching (ZVS) is achieved at the same time to reduce high switching loss caused by high switching frequency. Compared with the previous work, the voltage gain is higher and efficiency is higher because of ZVS. This paper focuses on high efficiency; therefore, resonant gate driver is adopted and efficiency is increased by 0.6%. In terms of the coupled-inductor, planar component is used and parameter optimization is also proposed to realize lower loss, including wingding loss and fringing loss caused by the air gap. A 500 kHz 36 W prototype has been designed to demonstrate theoretical analysis. In addition, comparison of drive loss between the resonant gate driver used in this paper and conventional driver implemented by Si8271 is made, revealing advantages of the resonant driver. Obtained efficiency was up to 92.1% at full load. [ABSTRACT FROM AUTHOR]

This paper analyzes synchronous generator self-excitation during power system restoration, caused by a large capacitive load and the impact of generator voltage control on the voltage rise. The paper analyzes measurements from a recent self-excitation incident in the Finnish 400-kV transmission system, models the system using an electromagnetic transient model and a linear d-axis generator model, validates the models using the measurements and analyzes the impact of generator voltage control during self-excitation. Self-excitation is rare during a normal system operation but may occur in a weak system during system restoration after a blackout. Since high voltages caused by the self-excitation may delay the restoration or even damage the equipment, self-excitation should be taken into account in restoration planning. This paper shows that the voltage controller and especially the under-excitation limiter settings of the generator have significant impact on the voltage during self-excitation. This paper also shows that details in the excitation system response influence the simulations significantly and reference measurements from the specific generator are required for detailed modeling. However, all measured incidents with digital static excitation systems are important references for simulations. In addition, special attention should be given to protection relays during the restoration. [ABSTRACT FROM AUTHOR]

DC fault protection is the bottleneck that restricts the development of modular multilevel converter based high voltage direct current (MMC-HVDC) system. Superconducting fault current limiters (SFCL) is an ideal protection device that can significantly limit short circuit current and enhance the fault tolerance ability of MMC-HVDC system. At present, several topologies of SFCLs have been proposed, but there is no systematical study of their current-limiting characteristics in MMC-HVDC. This paper studies the development process of DC short-circuit faults in MMC-HVDC system, and analyzes its demand for fault current limiting. Then, resistive type, saturated iron core type and hybrid type SFCL are modeled and their limiting effect of critical fault current under DC pole-to-pole fault is tested. By comparison, the current-limiting characteristics of three types of SFCL are concluded. Results show that resistive SFCL has best performance in steady state, while saturated iron core SFCL has best performance in transient state. The current-limiting characteristic of hybrid type SFCL is between the other two types. [ABSTRACT FROM AUTHOR]

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

The dc-dc converters with high step-up DC-voltage gain play a vital role in integrating low voltage DC sources. Though several converter topologies are reported in the recent past, attempts have been made to reduce the components, especially the switching devices, passive elements, converter losses, etc., of the converter. A novel DC-DC converter topology, viz., single switched impedance network (SSIN)-based converter with n-stage is proposed in this paper. The operation of the SSIN based converter in continuous and discontinuous conduction modes are discussed. The effect of parasitic elements on DC-voltage gain and efficiency of the SSIN is carried out. The small-signal model of the SSIN converter is derived. The performance of the SSIN converter is compared with the similar converter topologies. The 500 W prototypes of the SSIN with n = 1 and n = 2 are fabricated, and the experimental results are presented in this paper. To improve the converter performance, SiC-based semiconductor devices are used. The SSIN converter with two-stage has low switch loss at 500 W load, improved efficiency during high power and high output voltage operation, the switch stress of Vo/2, etc. [ABSTRACT FROM AUTHOR]

This paper presents a high-voltage emission driver chiplet for inorganic micro-pixelated light-emitting-diode ($\mu $ LED) displays in a standard 0.18- $\mu \text{m}$ CMOS technology. Different from the conventional driving scheme of the active-matrix organic light-emitting-diode display, which places the scan drivers and emission drivers at the panel bezels, the CMOS driver chiplets are proposed to sit on the glass substrate among $\mu $ LEDs. The high-voltage swing buffers in the CMOS chiplet are used to drive the low-temperature poly-silicon thin-film transistors on glass backplane to enable the emission control of 300 $\mu $ LEDs. A single CMOS chiplet has 20 output channels, each offering 16M-color depth with 256 grey levels realized by an 8-bit digital-to-analog converter. The maximum channel current is 247 $\mu \text{A}$ and the TFT-gate-driving voltage is 15 V. [ABSTRACT FROM AUTHOR]

When a fault occurs on the dc side of the voltage source converter based high voltage direct current (VSC-HVDC) transmission system, the fault current rises quickly due to the low damping of the dc system. Using a dc fault current limiter (DC-FCL) to limit the rise of the fault current can significantly reduce the breaking requirements of the DCCB. A topology of a bidirectional active dc fault current limiter (BA-FCL) based on coupled inductor is proposed in this paper. No expensive full-controlled power electronic devices are used in this topology. Only a few half-controlled power electronic devices are used to achieve active control of the current limiting unit. Compared with many topologies of current limiter at present, the fault current limiting effect of BA-FCL is improved significantly while the cost, volume, topology complexity, and control complexity are reduced. The stress analysis and parameter design of the proposed current limiter are carried out. Finally, the simulation case study and hardware experiment are implemented to verify the good economy and current limiting performance of the proposed topology. [ABSTRACT FROM AUTHOR]

Hybrid high voltage direct current systems utilized for bulk-power transmission have recently attracted more attention. The overhead lines (OHLs) are mainly used for the implementation of such systems due to their outstanding economic advantages. However, OHLs are vulnerable to temporary faults, jeopardizing the safe and reliable operation of the network. The resiliency to the line faults is of considerable significance to the power transmission. This paper proposes a fault property identification strategy based on the Pearson correlation coefficient for the LCC-MMC hybrid HVDC system which can differentiate temporary and permanent faults. To achieve this, a voltage signal is injected into the faulty line by decreasing the firing angles of the LCC-based rectifier. Based on the established OHLs model and the injected signal, an estimated backward travelling wave can be accurately calculated. The fault property is identified by comparing the wave difference of the real backward travelling wave and the estimated backward travelling wave. The proposed strategy has a high accuracy since all influencing factors including the fault resistance, fault distance, and line parameters have been taken into account. Finally, to verify the efficacy of the proposed algorithm, several simulations have been performed using PSCAD/EMTDC. The simulation results show that the proposed algorithm can effectively distinguish the fault property. [ABSTRACT FROM AUTHOR]

With the emergence of the hybrid multi-infeed high voltage direct current (HMIDC) system, there is complex dynamic interaction between the AC/DC system and converter stations, while the interaction mechanism of the HMIDC system is unclear. For this purpose, this paper establishes the HMIDC linearized model based on the motion equation concept. Then, after decoupling and reconstructing the interaction path of the HMIDC system, the damping coefficients are derived. The AC system strength and voltage source converter (VSC) terminal voltage controller parameters are selected to analyze the impacts on damping coefficients, and the interaction mechanism of the HMIDC system is analyzed. In order to solve the potential instability issue, an additional damping controller in the VSC terminal voltage loop is proposed to enhance the voltage stability of HMIDC system, which is realized by adjusting the reactive power/amplitude coupling damping coefficient. Finally, the RT-LAB-based hardware-in-the-loop experiments verify the accuracy of the interaction mechanism analysis of the HMIDC system and the effectiveness of the proposed controller. [ABSTRACT FROM AUTHOR]

The increased power and voltage ratings of semiconductors fosters the use in medium voltage components, such as ac–dc converters. With a bipolar medium frequent rectangular (MF RECT) voltage, the converter size can be reduced and the efficiency increased. Possible insulating materials for medium voltage transformers are oil and insulation paper. The breakdown voltage of insulation materials is depending on the frequency and the shape of the voltage curve. Until today, only a few investigations of the breakdown voltage of insulating materials at sinusoidal and RECT voltages up to a few tens of kilohertz have been published. To design and optimize MF transformers at RECT voltages, further investigations are necessary. Therefore, a test bench for measuring the breakdown voltage at RECT voltages in the frequency range between one and ten kilohertz up to several tens of kilovolts is developed. The test bench is inspired by a Marx generator, which is a high voltage source for impulse voltages. By substituting resistances and spark gaps by IGBTs and adding galvanically insulated power sources, a medium frequent bipolar voltage can be generated. The test bench is characterized with respect to slew rate, input power, frequency, and voltage amplitude. Its usability is shown in first breakdown voltage measurements of insulation papers, used for transformer insulation. [ABSTRACT FROM AUTHOR]

To improve the reliability of power supply, reclosing schemes are required after transient faults, which commonly occur in overhead line based high voltage DC (HVDC) systems. However, in the event of permanent faults, the auto-reclosing scheme may cause a severe strike. To avoid the severe impacts caused by permanent faults, the fault type should be discriminated before activating the reclosing scheme. Therefore, an adaptive reclosing scheme based on phase characteristics is proposed in this paper. Firstly, the modulation of a periodic voltage by actively controlling the hybrid DC circuit breaker (DCCB) is introduced. Then, a cascaded π equivalent model and its decoupling algorithm are presented to analyze the frequency-domain characteristics of the measured impedance of the coupled overhead lines. From the frequency-domain characteristics, the frequency of the periodic detecting voltage is determined to analyze the phase features of the measured impedance at primary frequency. The permanent or transient faults can thus be accurately identified by using these different phase characteristics, with negligible influence on the healthy lines. In addition, the proposed scheme is robust to various fault resistances, leading to improved reliability. The effectiveness of the proposed scheme is verified in PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

Changing the current loading of mass-impregnated nondraining (MIND) power cables causes the pressure in the insulation to vary over a wide range. It is generally recognized that under certain conditions—typically after a large load reduction—the pressure becomes so low that shrinkage cavities (voids) form in the insulation and that partial discharges (PDs) may damage the insulation ignite. A finite-element model for temperature, electric field, and pressure dynamics throughout the insulation layer has been developed. Comparisons with previously obtained measurements on full-scale high voltage direct current (HVDC) subsea cable samples subjected to a variety of loading patterns, external pressures (sea depths), and ambient temperatures show that the model replicates the pressure behavior of the insulation reasonably well. Most importantly, the complicated viscoelastic properties of the lead sheath, which have a profound effect on the internal pressure dynamics and thereby on the prospects of forming potentially harmful cavities, appear to be sufficiently accurately modeled. [ABSTRACT FROM AUTHOR]

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]

The missing capability of DC fault current limitation is an inherent disadvantage in the typical half-bridge modular multilevel converter (MMC), which restricts its application in long-distance high voltage direct current (HVDC) systems. The full-bridge submodule (FBSM) and clamped double submodule (CDSM) topologies allow for fault current self-clearing, but also suffer from additional hardware cost and power losses. In this paper, an improved submodule topology utilizing the new reverse-blocking insulated gate bipolar transistor (RB-IGBT) is proposed to improve system economy without compromising its performance. The operating principles and DC fault characteristics of the proposed topology are analyzed in details, including comparison with various topologies in terms of cost, losses, fault current clearing speed and thermal. Compared with the traditional CDSM topology, the proposed RBSM can clear DC faults faster with reduced cost of switching devices. A DC fault protection strategy for MMC-HVDC system is also presented. Simulation results under normal operating, permanent and transient fault conditions are carried out to verify the effectiveness of the proposed topology. [ABSTRACT FROM AUTHOR]

It is a great challenge to differentiate partial discharge (PD) induced by different types of insulation defects in high-voltage cables. Some types of PD signals have very similar characteristics and are specifically difficult to differentiate, even for the most experienced specialists. To overcome the challenge, a convolutional neural network (CNN)-based deep learning methodology for PD pattern recognition is presented in this paper. First, PD testing for five types of artificial defects in ethylene-propylene-rubber cables is carried out in high voltage laboratory to generate signals containing PD data. Second, 3500 sets of PD transient pulses are extracted, and then 33 kinds of PD features are established. The third stage applies a CNN to the data; typical CNN architecture and the key factors which affect the CNN-based pattern recognition accuracy are described. Factors discussed include the number of the network layers, convolutional kernel size, activation function, and pooling method. This paper presents a flowchart of the CNN-based PD pattern recognition method and an evaluation with 3500 sets of PD samples. Finally, the CNN-based pattern recognition results are shown and the proposed method is compared with two more traditional analysis methods, i.e., support vector machine (SVM) and back propagation neural network (BPNN). The results show that the proposed CNN method has higher pattern recognition accuracy than SVM and BPNN, and that the novel method is especially effective for PD type recognition in cases of signals of high similarity, which is applicable for industrial applications. [ABSTRACT FROM AUTHOR]

Power Electronics applications for the metal industry deal with high voltage and currents. The heat dissipation of the power losses of the static switches should be the most appropriate in terms of reliability, power density, and cost. This paper discusses an oil immersion heat dissipation application. An explanation of the thermal circuit for static switches and most used cooling methods are presented. A case study aiming to design an immersion tank for heat dissipation is demonstrated. Experimental results of air and oil. [ABSTRACT FROM AUTHOR]