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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 modular multilevel converter (MMC), owing to the presence of a large number of submodules (SMs) and energy storing elements, requires a large footprint and initial investment. To address these issues, this paper proposes the switched modular multilevel converter (SMMC). It consists of two major elements; the chain-links (CLs), consisting of cascade connection of SMs as in MMC, and the single-pole-triple-throw (SPTT) switches, which can be realized using either the T-type or NPC-type switch strings. The SMMC works on the principle of connecting all the phase-legs in series and their respective positions are mutually interchanged. It offers a compact structure as the SMs count is reduced to 33% of MMC, which results in a much lower energy storage requirement. Furthermore, it does not have inter-phase circulating current problem and hence the CL inductors can also be avoided. The SMMC structure, operating principles, control, and modulation schemes are explained in detail. A detailed comparison of SMMC with the other major existing converters is carried out to highlight the important features of SMMC. The efficacy of SMMC is validated using PSCAD simulation studies with a 19-level grid-connected SMMC model. A 3-phase 19-level SMMC laboratory prototype is developed to further validate its feasibility. [ABSTRACT FROM AUTHOR]

In this paper, the dc biasing magnetomotive force (mmf) in presaturated core fault current limiter (PCFCL) is replaced with permanent magnet (PM) to overcome the well-known drawbacks of the dc biased PCFCLs. These drawbacks are excessively induced voltage across the terminals of the dc coil during fault condition and high total power losses during the steady-state condition. The novel contributions of this paper are the introduction of the selection criteria of PM material and its representation by actual nonlinear $B$ – $H$ hysteresis loop of Jiles-Atherton method, and the use of PM alone as an equivalent to the dc biasing coils for three-phase fault current limiter (FCL), contrary to work reported in earlier papers. Single- and three-phase PCFCLs are modified by replacement of dc saturation coil by the equivalent PM through time-domain finite element (FE) simulation of COMSOL Multiphysics package. Experimental verification shows that the implementation of accurate nonlinear representation of PM demagnetization curve with FE simulation gives a realistic performance of the PM biased PCFCL, especially in the fault condition. The performance of PCFCL can be evaluated through the voltage drop and total power losses during the steady-state condition and the fault current clipping ratio during fault condition of the grid. Furthermore, a comparative investigation of dynamic behavior is considered between dc and PM biased PCFCLs. It is found that the PM biased PCFCL has enriched capability of limiting any type of fault current with leading advantages of reduced voltage drop and power losses significantly together with risk elimination of the high induced voltage across dc coil terminals. Moreover, the governing parameters of PM biased PCFCLs are subjected to performance analysis to study their inherent effects. [ABSTRACT FROM AUTHOR]

This paper investigates the dependence of the supercapacitor Peukert constant on its terminal voltage, aging condition, and operating temperature. Recent studies show that the charge delivered by a supercapacitor during a constant current discharge process increases when the discharge current decreases if the discharge current is above a certain threshold, i.e., Peukert's law applies. By conducting extensive experiments using three supercapacitor samples with different rated capacitances from different manufacturers, this paper reveals that the Peukert constant increases when the initial voltage of the constant current discharge process is lower, the supercapacitor is more heavily aged, or the operating temperature is lower. The physical mechanisms accounting for the Peukert constant dependence are illustrated by analyzing an $RC$ ladder circuit model. When the supercapacitor terminal voltage is higher, the aging condition is lighter, or the operating temperature is higher, more charge is stored in the supercapacitor. Consequently, when the same discharge current is applied, the discharge time is longer and the branch capacitors are more deeply discharged. Therefore, the relaxation effects of the slow branches are reduced and the supercapacitor behaves more like a single capacitor rather than a distributed capacitor network, which ultimately leads to a lower Peukert constant. [ABSTRACT FROM AUTHOR]

In this paper, a new non-isolated two-input single-output dc–dc converter with high voltage gain is proposed. In the proposed converter, there are two three-winding coupled inductors used to achieve a high voltage gain and ripple-free input currents. The proposed converter has two switches, and their control pulses are the same as the control pulses of the two-phase interleaved converters with a phase shift equal to $\text{180}^{\circ} $. The voltage stresses of switches and diodes are reduced and are always lower than the high output voltage. The proposed converter is extendable to a four-input single-output converter in which the switches have the control pulses the same as those in four-phase converters with phase shifts equal to $\text{90}^{\circ} $. In this paper, the proposed topology is analyzed in all operating modes, and the values of current and voltage stresses of all switches and diodes, voltage gain, and required conditions for input current ripple cancelation are calculated during a switching period. Finally, the accuracy performance of the proposed converter is reconfirmed through experimental results for 225- and 298-W prototypes. [ABSTRACT FROM AUTHOR]

High-frequency overvoltages are most dangerous for the insulation of transformer coils. It is because high-frequency pulse voltage creates an uneven distribution of voltage across the windings. The coil's first turns receive a disproportionately high voltage drop, leading to the inter-turn circuit. The paper analyzes the types of lightning pulses and proposes a new suppression method based on the anomalous skin effect in a multilayer material and the design in the form of a coil. The developed device is simulated, and the results of overvoltage monitoring at the substation with the installed high-frequency overvoltage suppression device are analyzed. [ABSTRACT FROM AUTHOR]

This paper proposes advanced control strategies to enhance fault ride-through capability of modular multilevel converter-based high voltage direct current (MMC-HVDC) connected doubly-fed induction generator (DFIG)-based wind farms. A stepwise voltage drop control is proposed for the sending-end MMC to suppress dc line voltage fluctuations by demagnetizing DFIGs. Meanwhile, a modified transient current control is proposed for the DFIG, where a voltage-dependent active current control is integrated into the rotor-side converter to improve synchronization stability of the converter, and a feedforward transient stator current control structure is designed to eliminate dc component in the stator current. The proposed control strategies are validated by comparing with three typical methods through time-domain simulations. [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]

This paper proposes a multi-agent deep reinforcement learning-based approach for distribution system voltage regulation with high penetration of photovoltaics (PVs). The designed agents can learn the coordinated control strategies from historical data through the counter-training of local policy networks and centric critic networks. The learned strategies allow us to perform online coordinated control. Comparative results with other methods show the enhanced control capability of the proposed method under various conditions. [ABSTRACT FROM AUTHOR]

Conventional droop control methods for power-sharing in a multi-terminal high voltage DC (MTDC) grid lead to voltage deviation from the nominal value. Moreover, the power-sharing is inaccurate in the droop-controlled MTDC system. This paper proposes a secondary controller with a distributed architecture to compensate the voltage deviation and to achieve equal power sharing automatically among the converter stations. A distributed observer is used to estimate the average voltage of the converter stations. The proposed distributed controller and observer use local measurements of output powers and voltages, and only communicate with neighboring stations. Therefore, the requirement of the global information in the centralized secondary control schemes is eliminated, which reduces the communication requirement and improves the reliability of the MTDC grid. The case studies illustrate that the proposed distributed controller regulates the stations’ average voltage and shares the power mismatch accurately. Impact of the communication time-delays and the control gain on stability of the MTDC grid are also investigated using the Lyapunov-KrasovskiiLMI condition. It is shown that reducing the control gain can stabilize the MTDC grid in the case of large communication time-delays. [ABSTRACT FROM AUTHOR]

DC–DC converters are expected to interconnect High Voltage Direct Current (HVDC) grids with different configurations or voltage levels, which will increase reliability and operational flexibility. The dc Modular Multilevel Converter (DC-MMC) is suggested as a potential non-isolated topology for HVDC applications. This paper presents an energy-based closed-loop control for the DC-MMC. Such control structure provides a full control of the current components, while ensuring energy balancing in normal and fault operation. First, the converter configuration is described and the fundamental equations are presented. Then, the normal operation of the converter is explained from the steady state conditions and the energy and current components. A closed-loop control is designed to ensure a proper dynamic response, balance the internal energies of the converter, and minimize the ac circulating current. Also, the control structure is modified to ensure dc fault ride through capability. Simulation results in MATLAB/Simulink are used to validate the converter control under normal operation and DC faults. [ABSTRACT FROM AUTHOR]

Surface charge accumulation on epoxy insulator installed in gas insulated switchgear (GIS) or gas insulated transmission line (GIL) has been recognized as an important factor for electric field distortion that results in reduction of surface flashover voltage. This paper reports on surface charge accumulation behavior on a actual epoxy insulator for 100 kV GIL prototype with multi-arc surface profile. A coaxial electrode system is used to investigate the charging behavior under DC voltages with metal particles positioned at various positions of the surface so as to estimate its influence on the charge accumulation behavior using a Kelvin type probe. The results show that positive charges are accumulated mainly on the nonplanar region around the high voltage electrode in the shape of speckles, and the average charge distribution along the radial direction presented a double-peak feature. Moreover, the charge accumulation is enhanced by metal particles contacting to the grounded electrode. Using a numerical simulation method, it is suggested that the charge accumulation is mainly determined by gaseous ionization which is enhanced by the metal particle induced electric field distortion. [ABSTRACT FROM AUTHOR]

This paper presents an adaptive self-configurable rectifier to increase the range of vibration excitation that a piezoelectric energy harvesting power management circuit (PMC) can operate with. The proposed circuit configures itself as a voltage doubler (VD) to increase the low voltage of piezoelectric energy harvesters to meet the minimum operating voltage of the PMC. This allows energy harvesting from low voltage range that is not viable using a full-wave bridge (FB) rectifier. When the piezoelectric voltage increases and causes the voltage amplified by the VD to approach the maximum operating voltage of the PMC, the proposed circuit reconfigures itself to an FB rectifier that does not amplify the voltage. This allows operation at a higher voltage range that is restricted by the VD topology. Two different reference voltages each for the VD topology and the FB rectifier topology with low-pass filtering are used to ensure correct switching of the rectifier topology. The proposed control circuit can cold start from a piezoelectric voltage of 1.2 V in the VD topology to amplify the voltage to allow an existing PMC that needs a minimum input voltage higher than 1.2 V to operate while consuming 240–410 nW of power. With the two-reference-voltages control method, 20% more energy can be transferred through the PMC than the one without it. [ABSTRACT FROM AUTHOR]

Improvements in the method of determining voltage instrument transformer (IT) errors are described in this paper. Two auxiliary ITs are used in series and separately to test the standard voltage IT under test on the voltage of U and 2U to measure the errors variation between these two voltages. The experimental results showed that the measurement uncertainty of the method was below $5 \times 10^{-6}$ and $4~\mu $ rad at an applied voltage of 110/ $\surd 3$ kV. This method is used to finish the traceability of the standard ITs up to 1000 kV. [ABSTRACT FROM AUTHOR]

With the development of high-voltage direct-current (HVDC) transmission, DC gas insulated lines (GIL) with SF 6 /N 2 gas mixtures as insulating medium have been successfully adopted. Space is typically the weak link of equipment insulation where surface discharge is one of the common discharge forms in power equipment. Therefore, research on the surface discharge characteristics of SF 6 /N 2 under DC voltage can contribute to insulation design of SF 6 /N 2 insulated DC GIS, GIL and other power equipment. In this paper, a DC high-voltage experimental system is established to study partial discharge (PD) along insulation surfaces. The characteristics at the initial discharge and the discharge development stages are analyzed in detail. Additionally, the synergistic effect of mixed gas is investigated based on the discharge characteristics and the synergistic effect coefficient is introduced. The experimental results show that the synergistic effect of mixed gas partial discharge inception voltage (PDIV) under high pressure is more obvious compared to low pressure and the discharge quantities of mixed gas is smaller than that of pure SF 6 at the discharge development stage. This study on the multiple influencing factors of synergistic effect is expected to provide important insight for exploring the SF 6 /N 2 discharge mechanism and helps decreasing the usage SF 6. [ABSTRACT FROM AUTHOR]

Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) connected offshore wind farms could suffer serious challenges: DC overvoltage during a fault and post-fault DC voltage restoration. In this paper, to mitigate DC overvoltage, a two-stage voltage drop control scheme is proposed for offshore MMC to realize a fast active power reduction, by coordinating with a voltage-dependent active current control of individual wind turbines. The voltage drop depths are designed by considering the offshore de-loading efficiency and the stability of WTs. To improve the post-fault DC voltage recovery performance, an adaptive voltage rise control scheme is proposed to coordinate the recovery of onshore and offshore active power. The recovery trajectory of DC voltage is formulated and further employed to design the onshore active power ramp rate for uninterrupted operation of WTs. Case studies have been conducted on two test systems to validate the proposed method. [ABSTRACT FROM AUTHOR]

This paper presents a modification of the Vienna rectifier (VR) to obtain more than three voltage levels at the DC line connection, while preserving the unity power factor at the AC grid side. The proposed unidirectional multilevel there-phase rectifier, called the “Negev rectifier”, has a reduced device count and utilizes a unique novel Time Division Multiplexed based control scheme, which makes the challenging N-level VR expansion possible. The Negev rectifier can serve as a grid interface for supplying dc voltage to multilevel inverters with 4 up to 7 voltage levels, eliminating the burden of inverter-side voltage balancing control. The Negev hardware can be constructed by two complementary topologies, making it suitable for either high current or high voltage applications. [ABSTRACT FROM AUTHOR]

This paper presents a voltage multiplier topology that is a hybrid between a Cockcroft–Walton multiplier and a Dickson charge pump. The Cockcroft–Walton structure exhibits significant output voltage drop under load as the number of multiplier stage increases. This is because all coupling capacitors are connected in series. Dickson charge pump mitigates this issue by connecting all capacitors in parallel. But this solution comes at the expense of large capacitor voltage stress at the last multiplier stage. The proposed hybrid structure arranges some capacitors in parallel and others in series, thereby achieving low output voltage drop and low capacitor voltage stress at the same time. We develop a model that predicts hybrid multiplier's performance and validates it experimentally. We also demonstrate a 60–2.25 kV dc–dc converter based on a 16-stage hybrid voltage multiplier which achieves a voltage gain of 12.8 while keeping the highest capacitor voltage stress to 660 V. [ABSTRACT FROM AUTHOR]

This paper presents a simple dc-link voltage modulation scheme to minimize the commutation torque ripple in a permanent magnet brushless dc motor (PMBLdcM). Maintaining a constant current in the noncommutating (NC) phase helps to minimize the torque ripple, which can be achieved by boosting the motor input voltage whenever two phase currents are undergoing commutation. At low speeds, since the required ac voltage at commutation is also less than nominal, the standard pulsewidth modulation operation of the inverter with nominal dc-link voltage suffices. However, at higher speeds, a momentary (only during the commutation interval) boost in the dc-link voltage (above the nominal level) is needed to ensure a boost in motor voltage, and hence, a steady current in the NC phase. In the proposed scheme, the dc voltage boost at higher speeds is obtained by connecting two capacitors in series, which are normally connected in parallel, across the dc-link. The proposed scheme is verified for a 1-hp PMBLdcM drive through simulations and experiments in the laboratory. Results exhibit a notable improvement in reducing the torque ripple. [ABSTRACT FROM AUTHOR]

Negative- and zero-sequence internal currents of a modular multilevel converter (MMC) under unbalanced ac-grid conditions can increase the peak voltage of the submodule (SM) capacitors, which should be kept below a maximum voltage limit. This paper investigates the impact of offset pulsewidth modulation (OPWM) and zero-sequence voltage injection on reducing peak SM capacitor voltages of the MMC during unbalanced ac-grid conditions. It is demonstrated that OPWM extends the operating range of the MMC for a given set of restrictions, as it leads to a reduction of capacitor voltage in every phase by utilizing all the SMs in the arms. Zero-sequence voltage injection further reduces the maximum SM capacitor voltage among three phases under certain conditions. Simulation results from an 800-MVA MMC-based high-voltage direct current transmission system demonstrate the effect of OPWM in the operation of the converters and the efficacy of the method compared to standard modulation methods. [ABSTRACT FROM AUTHOR]

This paper proposes a novel isolated high step-down conversion circuit, called an integrated buck and modified push−pull (IBMPP) converter, which is able to lower the voltage level on the primary side of the ideal transformer, resulting in a lower turns-ratio and decreased leakage inductance. The IBMPP converter is able to prevent the duty cycle from operating in extremely low conditions. Furthermore, the proposed converter adopts active-clamp techniques to recycle leakage energy and to suppress voltage spikes, so that the conversion efficiency can be effectively improved. The advantages of the IBMPP converter are its simple topology, easy control mechanism (which requires only two signals with a 180-degree phase shift), high conversion ratio, low component counts for power switches, and less voltage stress on some of the switches on the high-voltage side. The operation principle, steady-state analysis, design considerations, and experimental results of the proposed IBMPP converter are presented in detail. The feasibility of the IBMPP is verified by hardware implementation. The full-load efficiency at 250 W is 81.44%, and the input and output voltages are 380 and 5 V, respectively. [ABSTRACT FROM AUTHOR]

This paper proposes an interleaved active-clamp forward (IACF) converter modified for high efficiency without any additional component. To employ high input voltage, the components on one active-clamp forward module are re-arranged from the conventional IACF converter. The proposed converter can significantly reduce the primary conduction loss with large turns-ratio of the transformer due to the higher input voltage of the modified module and the expanded duty ratio at the nominal input voltage. In addition, unlike the existing approaches, the power density is not affected because there is no additional component. Finally, the proposed converter is validated experimentally using a prototype converter with 36–72 Vdc input and 480 W (12 Vdc/40 A) output. [ABSTRACT FROM AUTHOR]

In this paper, a new high voltage gain step-up dc–dc converter is proposed for interfacing renewable power generation. The configuration optimally integrates both the coupled-inductor and switched-capacitor techniques to achieve an ultra-high step-up gain of voltage conversion with low voltage stress and high efficiency. It consists of a voltage boost unit, a passive clamp circuit, and a symmetrical voltage multiplier network. The structure becomes modular and extendable without adding any extra winding for ultra-high step-up voltage gain. The proposed topology not only reduces the voltage stress on the main switch but also maintains it steady for the entire duty cycle range. Furthermore, the reverse recovery issue of the diodes is alleviated through the leakage inductance of the coupled inductor. The operation principle and steady-state analysis are presented in detail. Experimental evaluation validates the claimed advantages and demonstrates a well-distributed efficiency curve and the peak of 96.70%. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC) is regarded as a topology with high potential for high-voltage direct current transmission system. However, the distributed capacitors lead to significant submodule capacitor voltage ripples, which have great influence on the terminal characteristics of MMC. In this paper, an equivalent model considering submodule capacitor voltage ripples and circulating current suppression control is established to explore the terminal characteristics of MMC. In the model, MMC is equivalent to the cascade of buck/boost converter and two-level voltage source converter (VSC) from a terminal perspective. The influence of capacitor voltage ripples is represented by the equivalent series impedances, whose characteristics are investigated in detail with harmonic linearization method. Then, the simplified equivalent series impedances are obtained. Based on the equivalent model, the study of terminal characteristics is greatly simplified by directly applying conclusions of two-level VSC and buck/boost converter to MMC. Furthermore, the control strategy design and impedance modeling of MMC have been carried out to show how the equivalent model can be used in practical applications. Finally, the effectiveness of the proposed model is verified by detailed simulation results. [ABSTRACT FROM AUTHOR]

High power pseudospark switches (PSSs) can control pulsed high voltage, high peak current discharges. In this paper, a PSS in series with a saturable inductor at the anode is investigated. The commutation process is analyzed with a time-resolved method. And the effects of the number of magnetic cores and anode voltages on discharge characteristics are studied experimentally. At the beginning of commutation process, when the anode voltage is 10.4 kV with four magnetic cores, the inductor is unsaturated, and there is a flat plateau of the forward voltage drop after the anode voltage drops rapidly to about 200 V, at the same time the current rise rate is 2.8 kA/ $\mu \text{s}$. When the inductor is saturated, there is a voltage spike that occurs at the end of the flat plateau, and the current simultaneously rises rapidly at the rate of 6.4 kA/ $\mu \text{s}$. The current sharp rise lags behind the voltage drop of the pseudospark gap, and the electrode erosion and energy dissipation reduce significantly. When the anode voltage is 15.6 kV, the commutation losses reduce by about 30% with seven cores in series at the anode. As a result, it is evident that the discharge process will speed up if the anode voltage is higher. And the trigger delay decreases by up to 8% when the anode voltage varies from 5.2 to 15.6 kV. Finally, in order to obtain a reproducible pseudospark discharge by the use of a saturable inductor, the number of magnetic cores has to be optimized for different anode voltages. [ABSTRACT FROM AUTHOR]

The right-half-plane (RHP) zero in a continuous conduction mode boost converter results in a significantly restricted closed-loop bandwidth (BW) for higher voltage gain and/or load current conditions. A compensating ramp is used for current-mode control (CMC), and a higher ramp slope degrades the BW. Variable-frequency digital CMC offers inherent current-loop stability and real-time tuning scope for higher closed-loop BW. However, the challenges are to select the sampling frequency and sampling instant of the output voltage with discontinuous ripple due to the effective series resistance of the output capacitor, and its impact on stability. This paper shows that an event-based current-mode constant off-time digital modulator achieves superior stability and performance in a boost converter along with the reduced RHP zero effect over other digital CMC techniques. Using a discrete-time framework, the fast-scale stability conditions and small-signal models are analytically derived for various digital CMC techniques, which are validated using SIMPLIS simulation. A boost converter prototype is tested, and the analytical predictions are verified experimentally. Further, the analysis is extended to a non-inverting buck–boost converter. [ABSTRACT FROM AUTHOR]

In this paper, a systematic and analytical study of non-isolated topologies belonging to the family of reduced redundant power processing (R2P2) converters is presented. Based on this study, all the non-isolated R2P2 topologies that can be implemented in reality are derived. Detailed examples clarify all possible cases of this analysis. In addition to this, the voltage ratio and the efficiency for each one of the R2P2 configurations are calculated, being further illustrated with examples. A comparison among configurations is also discussed with criteria the high step-up voltage gain and efficiency, aiming for high-power applications. One of these R2P2 converters is selected for further examination, as the most suitable for such applications. The theoretical analysis, as well as the calculations of the step-up voltage ratio and efficiency of the selected converter, is compared to experimental results conducted on a 2-kW laboratory prototype designed to operate in various step-up voltage ratios and in a high-power range, proving the effectiveness of the proposed investigation. [ABSTRACT FROM AUTHOR]

The efficiency improvement of the on–off controlled Class E dc–dc converter operated at 20 MHz switching frequency is investigated in this paper. It is found that with the on–off control, the input power of the converter during the on mode increases with the increase of the input voltage, and it can be reduced by increasing the switching frequency. With this discovery, a variable switching frequency (VSF) on–off control is proposed, which slightly increases the switching frequency when the input voltage increases, maintaining the input power during the on mode slightly higher than the rated output power over the entire input voltage range, and thus highly improving the efficiency of Class E dc–dc converter at high input voltage. A prototype of 9-V–18-V input, 10-W Class E dc–dc converter has been fabricated and tested in the lab. The experimental results show that the proposed VSF on–off control improves the conversion efficiency of the Class E dc–dc converter by 4%–10% compared to the constant switching frequency on–off control. [ABSTRACT FROM AUTHOR]

A nonisolated ultra-high step-down interleaved dc–dc converter with low voltage stress and common ground between the input and output ports is proposed in this paper. In order to extend the extremely narrow duty cycle, series capacitors and transformers are applied to the four-phase interleaved buck converter. In addition to extending the duty cycle, the series capacitors clamp the voltage stress across the switches and diodes, which results in using low voltage rating semiconductor components. Additionally, the main switches are turned on under a soft-switching condition. Hence, the conduction and switching losses are reduced and the overall efficiency of the designed converter is improved. Other important strengths of this topology are very low output current ripple due to interleaving and automatic uniform current sharing. The validity of theoretical analysis and converter operation is confirmed by the implementation of a 150 W prototype at 400 V input voltage and 12 V output voltage. [ABSTRACT FROM AUTHOR]

Recently, a star-channel modular multilevel converter (MMC) was proposed for zero/low-fundamental-frequency operation. However, it is companied by two potential issues in practices: the star-channel branch suffers from submodule capacitor voltage deviations and the entire-speed-range operation for motor drive is not confirmed. This paper investigates the control degrees of freedom, and proposes control algorithms for star-channel branch and main arms. These control algorithms cascaded with advanced motor controls form the overall control scheme for drive system. Simulation and experimental results show that the star-channel MMC along with the proposed algorithms successfully maintain the SM capacitor voltages, and achieves the variable speed operation from standstill to rated speed. [ABSTRACT FROM AUTHOR]

Envelope tracking (ET) is a promising power amplifier (PA) architecture for current and future communications systems, which uses dynamic modulation of the supply voltage to provide high efficiency and potentially very wide bandwidth over a large dynamic range of output power. However, the dynamic nature of the supply voltage can lead to a problematic variation in transistor gain, particularly in GaN HEMTs. This paper describes and analyzes this behavior and the detrimental effect it can have on ET PAs. Contributing factors and origins of gain variation are described in detail along with how, for the first time, meaningful comparisons can be made between different devices. Using these guidelines, gain variation is shown to be a widespread issue effecting most GaN HEMTs presented in literature. To allow an analysis of the intrinsic device behavior, an extended transistor model is developed that takes the effect of gate and source field plates into account. This model is refined using measurement data and used to demonstrate the fact that the parasitic gate–drain capacitance ($C_{\textrm {GD}}$) is the main contributor to the small-signal gain variation—a significant part of the overall gain variation. Based on this knowledge, possible strategies to reduce gain variation at the transistor technology level are proposed, allowing the optimization of GaN HEMTs specifically for ET PAs. One identified strategy involves reducing the length of the gate field plate and is shown to be a viable approach to reduce the gain variation in GaN HEMTs, albeit at an increased RF/dc dispersion. [ABSTRACT FROM AUTHOR]

In this paper, an expandable single-input three-output high step-up dc–dc boost converter is proposed. The voltage conversion ratio of the output ports of the converter can be increased by inductor-diode-capacitor and diode-capacitor cells. The proposed converter can be applied in renewable energy conversion systems, such as in photovoltaic and fuel cells. Also, due to the three output ports of the converter, it can be utilized in portable devices which require high voltage conversions. The main advantage of the proposed converter is supplying loads with different voltages and current ranges due to the expandable structure of the converter. Besides, the proposed converter has the merit of achieving high voltage gains at high frequencies without preselecting wide duty cycles. Furthermore, the proposed converter is able to achieve higher voltage ranges without having any transformer or coupled inductors in its structure; consequently, the related drawbacks are avoided. The switching pattern of the converter is simple and both switches are on and off simultaneously. In this study, a single switching period of the proposed converter, the voltage conversion ratios of the output ports and the expanded form of the structure are analyzed theoretically. Finally, to verify the theoretical results, the experimental results for 40-V input/200-V, 100-V and 360-V outputs with total 600-W output power are extracted and compared with each other. [ABSTRACT FROM AUTHOR]

A nonisolated bidirectional dc–dc converter with high voltage gain, low voltage stress, low component count, and soft-switching features is presented in this paper. In this topology, coupled inductors and voltage multiplier cells are merged to achieve high step-up/down voltage gain. Also, due to using active clamp circuits, the voltage stress of power switches is relatively low. Thus, the low voltage switches with low on-resistance can be employed to reduce the conduction losses. Furthermore, zero-voltage switching is accomplished in both high step-up and high step-down modes for all power switches, and due to zero-current switching operation of all antiparallel diodes, the reverse recovery losses are reduced. In order to verify the theoretical analysis and the converter performance, a 200 W prototype circuit of the proposed converter is implemented in the laboratory. [ABSTRACT FROM AUTHOR]

Several applications need a high power, high output voltage ac/dc converter. The structure includes a dc/dc resonant stage, which is fed from a three-phase line. Due to standards, rectified dc input voltage varies largely from 400 to 750 V affecting the adversely overall design and current levels. In fact, losses in the inverter become unbearable if both switching frequency and transferred power are desired beyond a certain level. In those cases, it is necessary to connect several switches in parallel. As an alternative, the new switches can configure a new modular structure instead, adding more flexibility in the control to compensate input voltage variations. As a result, the required circulating current is reduced to a half, with benefits in total power losses. Moreover, the nature of the topology allows a reconfiguration of the resonant net (LCC) for low power, reducing even further the minimum circulating current under those conditions (fluoroscopy, for instance). In this paper, the new multilevel resonant topology is mathematically modeled and proposed for radiography and fluoroscopy. An example illustrates the design procedure for a given application at 100 kW, 50–150 kV. [ABSTRACT FROM AUTHOR]

Impedance-source inverters using coupled inductors have been investigated as alternatives for providing high step-up voltages. However, leakage inductances of the coupled inductors have commonly led to lower overall effectiveness, in addition to generating high dc-link voltage spikes. The latter raises voltage stresses of switches, which in turn, may reduce the power levels of the inverters. A high step-up Y-source inverter has therefore been proposed in this paper to provide a high boost with a smooth dc-link voltage ensured by proper recycling of the leakage energy. These features have been verified by comparing simulation and experimental results of an existing Y-source and the proposed inverters. Factors compared are their respective boost ratios, voltage stresses, current stresses, and dc-link voltage spikes. [ABSTRACT FROM AUTHOR]

In this paper, a developed structure is proposed for high step-up nonisolated noncoupled inductor based multiport dc–dc converters. The voltage gain per number of devices of the proposed topology is higher than that of similar topologies. This means that, by using the same number of devices, the proposed topology can generate higher gains than other structures. In other words, the proposed topology utilizes less number of devices for producing the same voltage gain, which leads to less total cost, size, weight, and complexity. Despite high voltage gain, the normalized peak inverse voltage (NPIV) on switches/diodes is low. Higher number of input ports leads to higher gains and lower NPIVs on devices. The load energy can be absorbed and stored in first input unit. Modularity, simple structure, continuous input currents, and low input current ripple are the other profits of proposed topology. Low/medium power applications are suggested for the proposed structure. Proposed topology has been explained and operational modes along with steady-state analysis have been discussed. For better verification, the proposed topology has been compared with several multiport high gain converters. Comparison results confirm the advantages of the proposed topology. Finally, the proper performance of the proposed topology has been validated by experimental results. [ABSTRACT FROM AUTHOR]

The epoxy resin insulation in saturable reactors in a HVDC transmission converter valve is subjected to a bipolar exponential decay pulse voltages under normal operating conditions and its aging mechanism is still unclear. In this paper, an electrothermal aging experiment of epoxy resin was carried out under bipolar exponential decay pulse voltages at high temperature. The dielectric properties of epoxy resin before and after aging were measured using frequency-domain spectroscopy. To quantitatively study the aging characteristics, the double-relaxation Cole-Cole model was used to fit the dielectric spectra. As a result, an exponential function relationship between the relaxation time constant and the aging time of epoxy resin under various aging conditions was obtained. This relationship can be used as an empirical formula to evaluate the degree of aging. Furthermore, a comparison of the aging life characteristics of epoxy resin under pulse and sinusoidal voltages with the same peak value was performed. The results show that the electrothermal aging rate of epoxy resin under a pulse voltage is lower than under a sinusoidal voltage. The effect of the measurement temperature on the insulation life evaluation of epoxy resin was also analyzed. [ABSTRACT FROM AUTHOR]

Modular multilevel converters are increasingly being considered or used for various medium voltage applications. Multiple control methods have been proposed for the control of the direct to three-phase modular multilevel converter. They differ one from another in the way the capacitor voltage ripples are handled, i.e., either neglected, estimated, reconstructed by filtering or measured. This has implications on the performance level that can be obtained. This paper provides insights on the advantages and drawbacks of each control method, in inverter and rectifier mode, with a fair and thorough assessment supported by extensive simulations, with converter ratings that are realistic for medium voltage applications. Finally, this works highlights the impact of the higher dynamics for medium voltage direct current (dc) applications compared to high voltage dc ones on the choice of the control method. [ABSTRACT FROM AUTHOR]

The ultra-high voltage gain dc–dc converters are extremely required because of the low voltage level of many nonpolluting power resources. This paper proposes an ultra-high voltage gain converter based on an interleave technique and using three-winding coupled inductor and voltage multiplier cell. The interleave technique is useful for reducing the size of the filter components. In the proposed converter, high voltage gain is achieved without extreme duty cycle of solid-state switches. The voltage stress across the semiconductor devices can be reduced by increasing the turn ratio of the secondary of the coupled inductors. In addition, because of the implementation of the interleave structure, peak-to-peak variation of the main input current and switching losses are substantially reduced. The steady-state principles of the proposed converter in continuous conduction mode operation are given. Also, design equations are presented. Validity and performance of the proposed converter are surveyed by simulation and experimental results of a 120 W prototype. [ABSTRACT FROM AUTHOR]

High-temperature superconducting fault current limiter (HTSFCL) is an important application in the power industry. In high-voltage dc system, a polyimide (PI) is used as superconducting winding insulation in HTSFCL also withstands pulse voltage of lightning and operating voltage caused by line fault and switching. The combined dc-pulse voltage makes the electric field more deformed, which induces partial discharge and accelerates insulation failure. Direct fluorination is a good means of regulating the molecular structure and electrical properties of polymers. This paper studies the trap distribution and breakdown characteristics of direct-fluorinated PI film. The corona charging was implemented in an environment with a relative humidity of 40%, using a combined dc-pulse voltage. To calculate the trap distribution, the surface potential was obtained. The breakdown voltage was also measured in LN2 under different voltages. The experimental results indicate that moderate fluorination can effectively reduce the depth of trap level, accelerate charge detrapping, and improve the breakdown voltage of the PI film. [ABSTRACT FROM AUTHOR]

Although vacuum technology is commonly used in power systems, challenges still remain for vacuum circuit breakers (VCBs) to cope with the requirements of capacitor bank switching duties as the insulation performance can deteriorate by inrush current. In order to reduce the restrike probability, vacuum interrupters (VIs) in series are recommended. The objective of this paper is to investigate the field emission current I e characteristics and voltage distribution in series vacuum gaps after making inrush current. The results show that I e of the VI pre-stressed by inrush current is much higher than that of the new one and I e has a conditioning effect. It is found that the insulation performance of two pre-stressed VIs in series is not significantly improved compared to a single VI. But the insulation performance of a new VI and a pre-stressed VI in series is significantly improved. Moreover, the voltage distribution of two VIs in series can be estimated by I e. It is found that each VI only withstands about 50% of the applied voltage when two pre-stressed VIs are in series. However, the new VI can withstand >70% of the applied voltage when a new and a pre-stressed VI are in series. [ABSTRACT FROM AUTHOR]

When evaluating the risks resulting from the effects of electromagnetic interference (EMI) on buried pipelines running near ac electrified traction systems, the harmonic distortion in railway systems should be considered. This is because the tractive units enforce current harmonics on the interference source, in addition to the fundamental frequency. The presence of harmonics may exacerbate voltages accumulating on buried pipelines running in close vicinity to traction systems. Such voltages may cause danger to persons or may cause damage to the pipelines, in the long term. However, the technical standards on pipelines-traction systems’ separation disregard the impact of harmonics. They are based on the fundamental frequency induction. This paper uses state-of-the-art modeling techniques to holistically assess EMI on underground pipelines near ac traction systems. This is achieved through a realistic modeling of the infrastructure associated with a traction system and by accounting for the presence of harmonics in the contact line system (i.e., interference source). [ABSTRACT FROM AUTHOR]

A single-contact voltage sensor designed for accurate measurements of ac voltages across a pair of conductors is described. The sensor design is motivated by remote monitoring applications where accurate voltage measurement of high-voltage transmission lines is required. The body of the sensor is electrically and mechanically attached to a single conductor: either the neutral or high-voltage conductor. A capacitive sensing plate attached to the sensor creates a capacitive voltage divider by using the stray capacitance to the noncontacted line. A very high-impedance buffer is used to measure the voltage across the divider output and estimate the line voltage. An important part of this paper includes a method of calibrating the sensor such that blind voltage measurements can be made without knowing the exact geometry of the conductors. Other important aspects of the design include a two-stage voltage divider for retaining accuracy and increasing the voltage range of the sensor. The work is supported by extensive numerical simulation models which were used to determine the optimum design for the sensing plate and to evaluate the sensitivity to different configurations including conductor spacing and the height above ground. For calibration values which are accurate to 1%, the line voltage can be measured with an accuracy of 10%. The paper describes the theory, design, and experimental verification of the sensor up to a line voltage of 7.5 kVrms. [ABSTRACT FROM PUBLISHER]

The objective of this paper is to investigate the effect of a high-frequency high-voltage impulse conditioning technique on the inrush current interruption phenomena. In this paper, the inrush current interruption phenomena referred to the actual interruption and the currentless period following interruption. First, Six 7.2 kV vacuum interrupters (VIs) were conditioned by 400 batches of voltage impulses. The duration of each batch were set to 0.1 s. The peak value of each impulse could reach up to 100 kV. The repetition frequency of the impulse was set to 1000 Hz. The contact gap was 0.8 mm. The VIs were tested for the inrush current interruption performance with inrush current set to 2 kA and 5 kA, with a frequency of about 3800 Hz. The experimental results showed that there were two different types of inrush current interruption depending on high-frequency high-voltage impulse conditioning. The inrush current interruption type I referred to the inrush current interruption during the prestrike process before the contacts touched. This type inrush current interruption could be caused by the erosion of prestrike arc. Moreover, the high frequency voltage impulse conditioning influenced the occurrence frequency, the number of interruptions and the duration of the inrush current interruption type I. The inrush current interruption type II referred to the inrush current interruption during the contact bouncing process after the contacts touched. The inrush current interruption type II occurred only in the case of the VIs after conditioning. Furthermore, the occurrence frequency of the inrush current type II decreased with increasing inrush current. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is an emerging and highly attractive multilevel converter topology for high-voltage and high-power applications. This paper proposes the control method of parallel-connected modular multilevel converters (parallel-MMCs), which assumes that the multiple MMCs are directly connected at both ac and dc sides to effectively enhance the power rating as expected. Two key problems were first solved for the parallel-MMCs under the normal operation conditions: voltage balancing of submodules and mitigation of circulating currents, where the novel transformed third-order harmonic resonant controller in the synchronous reference frame was employed to mitigate the dominant second-order and fourth-order circulating currents and a sixth-order harmonic resonant controller is used to attenuate the zero-sequence sixth-order circulating current existed in all phase currents per MMC. Considering the high risk of switches fault in the parallel-MMCs, the fault-tolerant operation schemes were then proposed in this paper to address the major concerns of open-circuit and short-circuit switch fault in a submodule, respectively. Carefully controlling the healthy submodules and the corresponding phase arms, the parallel-MMCs can successfully maintain their balanced capacitor voltages and mitigate the circulating currents with the qualified output waveform obtained. In addition, the parallel configuration of MMCs provides the unique solution for the short-circuit switch fault operation which was seldom discussed in the published literature works with respect to the MMC fault-tolerant operation schemes. MATLAB simulations and the constructed experimental prototype have verified the performance of the proposed control strategy. [ABSTRACT FROM AUTHOR]

Due to the relatively low output voltage of photovoltaic (PV) source, a high step-up converter with high efficiency is needed when the PV source is connected to the power grid. A novel high step-up converter based on two switched capacitors and a coupled inductor is proposed in this paper. The operating principle is analyzed and the voltage gain is derived. A 100-W prototype is fabricated in the laboratory to verify the theoretical analysis, and the highest efficiency is 96.4%. [ABSTRACT FROM PUBLISHER]

A capacitor voltage balancing method for a modular multilevel dc (MMDC) converter is proposed. The MMDC inherits the desirable features of both a modular multilevel converter (MMC) and a dual active bridge, namely high voltage capability, fault tolerance, zero-voltage switching-on of all the power devices, and a wide operating range. These features are ideal for a dc transformer in any dc distribution system. One key MMDC control issue is balancing of submodule capacitor voltages. Conventional MMC balancing techniques cannot be used as arm currents of an MMDC change direction within each switching cycle. This paper establishes that the average capacitor voltage of each arm is self-balanced and proposes a method to balance voltages within an arm by assigning the gate signals with higher charge difference to the submodules with lower voltages and vice versa. Charge difference is obtained by the voltage difference, hence only the SM capacitor voltages are required for balancing control. Different from the existed methods, the proposed technique is not limited by voltage gain or load conditions. Simulation and experimental results show that capacitor voltages are balanced under different voltage gains, different output power, and also when the MMDC starts up, hence ensuring the stable MMDC operation. [ABSTRACT FROM PUBLISHER]

An innovative approach has been recently proposed for the voltage breakdown prediction in high-voltage systems, insulated by large vacuum gaps. This approach is based on complementary geometric formulations for electrostatics coupled to the analytical solution of the equations of motion for charged particles. In this paper, a goal-oriented local mesh refinement technique is introduced, which allows to increase the rate of convergence of the solution, enabling an effective voltage breakdown prediction also in large-scale systems with complex geometries. [ABSTRACT FROM AUTHOR]

High-voltage wide bandgap semiconductor devices such as the 15 kV SiC mosfet have attracted great attention because of their potential applications in high-voltage and high-frequency power converters. However, these devices are not commercially available at the moment, and their high cost due to expensive material growth and fabrication may limit their widespread adoption in the future. In this paper, a 15-kV 40-A SiC three-terminal power switch, the Future Renewable Electric Energy Delivery and Management (FREEDM) supercascode, is reported for the first time, which is based on a series connection of 1.2-kV SiC power devices. Compared with the monolithic 15-kV SiC mosfet, the FREEDM supercascode demonstrates obvious advantages in cost and thermal conductivity. The design and voltage-balancing mechanism of the FREEDM supercascode are introduced, and the performance including the voltage balancing, conduction characteristics over a wide range of temperatures, and dynamic switching performance, is analyzed. The FREEDM supercascode's low cost and excellent thermal dissipation capability will facilitate early applications of SiC in very high voltage and high frequency power converters. [ABSTRACT FROM AUTHOR]

A novel composite device structure named self-adjust conductivity modulation lateral insulated gate bipolar transistor (SCM-LIGBT), including normal LIGBT region (NLT region), the EM-nMOS region (ENM region), and the series diode region (DIO region), is proposed in this paper. By adding the enhanced-mode nMOS region and the series diodes region, the parasitic NPN transistor (NPN) bipolar structure in the normal LIGBT structure can be triggered in on-state and the conductivity modulation is dramatically enhanced, which leads to the improvement on the current capability and the forward voltage. In addition, due to the base voltage of the parasitic NPN bipolar structure in the proposed device can be clamped at the forward threshold of the series diodes, therefore the latch-up issues can be immunized to guarantee the forward-biased safe operating area. Numerous simulations and measurements are presented to investigate the electrical characteristics of the proposed structure. The length of the fabrication SCM-LIGBT is 78.5~\mu \textm , and the width of the DIO and NLT is 830 \mu \textm , while the width of the ENM is 4100~\mu \textm . The results demonstrate that the proposed SCM-LIGBT achieves a high-current capability ( JA) of 2428 A/cm2 at VG= 10 V, a low on-state voltage drop of 1.15 V at JA= 150 A/cm2 with its breakdown voltage of 590 V. [ABSTRACT FROM PUBLISHER]