DC Voltage Stress Research Articles

Control and Protection Strategy Study of VSC Station Fault at Hybrid HVDC Transmission System

LCC-VSC UHV hybrid cascaded DC transmission system will face over-voltage problems caused by power surplus when receiving terminal VSC station fault occurs. An over-voltage suppression strategy based on the action input of a high-voltage controllable arrester and the stepwise blocking of non-fault VSC at the receiving end is proposed, and a dynamic simulation test system is built to verify that the stepwise blocking protection scheme can effectively reduce the DC voltage stress of fault VSC by about 25.9%. Based on the test results, the effectiveness of the over-voltage suppression scheme of VSC at the receiving end in practical projects can be demonstrated by analogy.

Rainer Waser – A Pioneer of Fundamentals of Resistive Switching Memories

Rainer Waser – A Pioneer of Fundamentals of Resistive Switching Memories

Partial Discharge Behavior of Epoxy-Mica Insulation System under Superimposed AC and DC Voltage Stress

Recent technology advances in wind energy conversion systems include a new type of permanent magnet synchronous generator with a segmented stator that allows lightweight concepts with high efficiency and superior controllability. Each stator segment is driven by a separate power converter. The various cascaded power converters can be connected in series and in parallel, which ultimately leads to the superposition of a DC and an AC voltage stress in the insulation of the stator coils. The resulting DC-AC mixed voltage is an entirely new electrical stress for the epoxy-mica insulation system that is typically utilized as main insulation of rotating electrical machines. In order to resemble this stress, typical dielectric tests such as partial discharge (PD) measurements and long-term aging of model insulation systems have to be be carried out under DC-AC mixed voltage stress. The mentioned investigations are presented in this contribution. The results indicate that the AC voltage is dominant with regard to the PD inception and influences the PD parameters such as the apparent charge and the repetition rate substantially when compared to the DC voltage proportion. The accelerated aging tests with significantly higher AC-DC mixed voltage stress than is expectable under operating conditions revealed no measurable PD deterioration of the investigated insulation systems.

Comparative Study of Various DC-DC Converter Topologies for PV Powered EV Charging Stations

There is a drift in automotive industry from conventional internal combustion engines (ICE) to Electric Vehicles (EVs). This drift from ICE to EVs counts to the reduced carbon emission and thus reducing environmental pollution. EVs also find a solution for increasing fossil fuel costs. When it comes to renewable energy sources, typically to solar energy, it is affluent and reliable. The usefulness of solar energy is maximized by the incorporation of advanced power converter topologies along with their advanced controls. This paper aims to compare some of the boost converter topologies that are used in EV applications with solar photovoltaic powered charging stations. The comparative study is conducted on various parameters, such as DC voltage gain, duty-cycle, efficiency, voltage stress, merits, and demerits. Simulation results are analyzed and compared using MATLAB/Simulink platform.

An Asymmetric Switched-Capacitor Multicell Inverter With Low Number of DC Source and Voltage Stress for Renewable Energy Sources

Asymmetric multilevel inverters generate high-quality output voltage using the same number of components as symmetric multilevel inverters. The main drawback of these topologies is that they require many DC voltage sources, and the power switches must endure high voltage stress. In this paper, a switched-capacitor sub-module inverter topology is proposed to reduce the number of DC voltage sources and the voltage stress on the switches of asymmetric multilevel inverters. The proposed sub-module inverter can generate 15 voltage levels by using two DC power supplies and a capacitor. The voltage of the capacitor can be automatically charged at half of the input DC power supply without the need for any sensors. In addition, the capacitor charging operation does not produce an inrush current because it is charged by the direction of the output current; this is an advantage over switched capacitor multilevel inverters. A modular topology is also presented based on the proposed sub-module inverter to achieve high voltage levels while reducing the number of elements. A comprehensive comparison between the proposal and other multilevel inverter topologies is performed to validate the design of the proposed inverter. In addition, thermal and loss distribution simulations of the proposed sub-module inverter are performed. Finally, the performance, efficiency, and accuracy of the proposed inverter are confirmed through laboratory prototyping.

Evaluation of Erosion Discharge Characteristics in Inclined Plane Tracking and Erosion Tests on Silicone Rubber under AC and DC Voltages

This paper investigates partial discharge (PD) characteristics that lead to the erosion of silicone rubber (SR) polymer under AC stress and DC stress of both polarities. The experiments are performed on high temperature vulcanized (HTV) SR material samples. The inclined plane test apparatus, constructed in accordance with IEC 60587 requirements, is employed to produce the surface partial discharges and the resulting accelerated aging of the specimens. Two commercial instruments are used to obtain the PD data. A concurrent analysis of visual observations of discharge and PD data is performed to classify discharges based on the severity of material degradation that each type causes. Three types of PD are identified and characterized using diagrams of charge magnitude versus time and a signal processing technique called time-frequency mapping. Individual pulse waveshapes of each type of discharge are also analyzed. PD pulse waveforms are analyzed according to their amplitude, energy, pulse width, and frequency spectrum. These pulse waveform parameters are evaluated and compared for the eroding discharge pulses under AC and DC voltage stresses. It is found that the stages of material degradation during IPT are related to the variations in discharge magnitude and the location of pulse clusters on the time-frequency maps.

Detection of Partial Discharge in Oil Filled-Tank using Acoustic in Audio Frequency

In this paper, the acoustic emission of partial discharge (PD) in oil contained in the tank was detected by using microphone sensor. PD was generated by point-to-flat electrodes under DC high voltage. The given variables were the DC voltage stress, the electrode gap distance, and the location of PD and sensors. The acoustic and high frequency current were recorded along with the increased voltage. The acoustic signal was then transformed using the fast Fourier transform (FFT) to obtain the frequency spectrum. Furthermore, the acoustic waveforms, the acoustic frequency spectrum, and the PD current were analyzed and correlated. The results showed that the PD could be detected by its audio frequency signal. The impulse-shaped acoustic signal was recognized as a transition condition from normal to PD. The acoustic sensor attached to the outer wall of tank (contact type) was more sensitive than sensor that had a distance from the tank (non-contact type). The perfection level of impulse waveform may indicate the PD location in the oil-filled tank. The signal of acoustic and current did also not affect each other.

Experimental validation of nine‐level switched‐capacitor inverter topology with high voltage gain

SummaryThis paper proposes a new switched‐capacitor nine‐level (9L) inverter with reduced switch count. In the proposed topology, floating capacitor (FC) is employed as a voltage booster, and it does not need any additional sensors to maintain the voltage across the FC. Due to additional FC, the number of dc sources and voltage stress on switches is reduced. Moreover, the proposed topology can be cascaded to achieve more voltage levels. Various parameters are considered in the comparison of the proposed topology with other recent switched‐capacitor topologies. Simulation and experimental results demonstrate the performance with different load and modulation index variations.

Feasibility of C 3 F 7 CN/CO 2 gas mixtures in high‐voltage DC GIL: a review on recent advances

With the rise of renewable energy resources, especially offshore wind power plant, the DC transmission concept has attracted more and more attention. The selection of eco-friendly insulating gas, as most critical insulating part of gas-insulated equipment is top priority for developing highly reliable system. Moreover, it is acknowledged that eco-friendly C3F7CN/CO2 gas mixtures have become a potential alternative to SF6 gas due to its excellent performance. This study reviews the basic physical properties of C3F7CN gas/gas mixtures and insulation properties including the gas gap breakdown and surface flashover performance at DC, as well as lightning impulse voltage. Investigation about the gas stability is presented, including the compatibility of gas with solid materials in gas-insulated transmission line (GIL), and the decomposition characteristics under long-term electrical and thermal stresses. The important progress on the charge accumulation characteristics of gas–solid interface in the novel gas environment under DC voltage stress is analysed. Finally, the key issues that need to be paid attention to and further followed through regarding the application of this novel gas in GIL are summarised and put forward. This study hopefully can provide a complete reference for the development of eco-friendly DC gas insulation equipment with C3F7CN/CO2 gas mixtures.

Partial Discharge Behaviour of a Protrusion in Gas-Insulated Systems under DC Voltage Stress

High reliability, independence from environmental conditions, and the compact design of gas-insulated systems will lead to a wide application in future high voltage direct current (HVDC) transmission systems. Reliable operation of these assets can be ensured by applying meaningful and robust partial discharge diagnosis during development tests, acceptance tests, or operation. Therefore, the discharge behavior must be well understood. This paper aims to contribute to this understanding by investigating the partial discharge behavior of a distorted weakly inhomogeneous electrode arrangement in sulfur hexafluoride (SF6) and synthetic air under high DC voltage stress. In order to get a better understanding, the partial discharge current is measured under the variation of the insulation gas pressure, the gas type, the electric field strength, and the voltage polarity. Derived from this, a classification of the different discharge types is performed. As a result, four different discharge types can be categorized depending on the experimental parameters: discharge impulses, discharge impulses with superimposed pulseless discharges, discharge impulses with superimposed pulseless discharges, and subsequent smaller discharges and pulseless discharges. Concluding suggestions for partial discharge measurements under DC voltage stress are given: recommendations for the necessary measurement time, the applied voltage and polarity, and useful measurement techniques.

The Modular Multilevel DC Converter With Inherent Minimization of Arm Current Stresses

The modular multilevel dc converter (M2dc) is a partial power processing dc-dc converter that is gaining popularity for medium-voltage and high-voltage dc (HVdc) grid applications. However, internal ac current stresses go up as the step-down dc voltage ratio increases, leading to increased cost and losses, and ultimately renders the M2dc impractical for some applications. The HVdc autotransformer (AT) (HVdc-AT) is another class of the partial power processing dc-dc converter that circumvents this issue by using a transformer for interarm ac voltage matching, although the core must tolerate a very large dc voltage stress between windings that leads to increased magnetics size and weight. Interestingly, the M2dc does not suffer from interwinding dc voltage stresses. This article presents a new class of the partial power processing dc-dc converter that uses an integrated center-tapped transformer to merge the best traits of the M2dc and HVdc-AT. Comparative analysis reveals the proposed converter can minimize ac current stresses at all operating points while also achieving a significant reduction in transformer area product relative to the HVdc-AT. A dynamic controller is proposed that regulates dc power transfer while ensuring balanced capacitor voltages. The converter operation and dynamic controls are validated by simulation and experiment.

Balanced-Bridge Method for the Characterization of Dielectric Charging in RF MEMS Cantilever Switches

This paper presents a balanced-bridge method for the characterization of dielectric charging effect in RF MEMS (radio frequency micro-electromechanical system) cantilever switches. The method measures the relative change of up-state capacitance caused by dielectric charging. Compared with previous methods, this approach employs simple and inexpensive test equipment. The test system consists of two symmetric capacitive switches, two adjustable resistors, a function generator, and a voltmeter. Measurement results indicate that it can achieve a resolution of 0.1 fF/ $\Omega $ . By this test system, the slight variation of up-state capacitance can be converted to the significant change of the resistance. Analysis of the experimental data shows that dielectric charging under the DC-voltage stress mainly occurs in the first 1200 seconds. The mechanisms resulting in capacitance variation can be divided into two groups: dielectric polarization and charge injection. Moreover, in contrast to the DC voltage, the RF signal does not have a significant influence on the charging process.

Studying the Aging Evolution of Oil-Paper Insulation Comprising of a Gas Cavity Under the Stress of AC and DC Voltage Depending on Partial Discharge and Dissolved Gases Measurements

The oil-paper insulation system is widely used for the insulation purposes of power transformers as a result of its ability to withstand high thermal and electrical stresses. The presence of a gas cavity between impregnated papers layer is a serious problem. When localized breakdown takes place within this cavity, partial discharge (PD) occurs and ages the rest of the insulation system. This aging phenomenon declines the expected life time of the insulation system and subsequently the life time of the power transformers. Dissolved gas analysis is a recommended technique that is used in order to assess the state of the insulation system inside the transformers. In this paper, PD is recorded in the case of gas cavity presence between the layers of the impregnated papers under the effect of AC and DC voltage stress. This record is extended from the instance of PD inception until the breakdown of the whole insulation system. Furthermore, the concentration of the gases that dissolve in the insulating oil due to the aging of the impregnated paper is measured eight times during the record of the PD. Finally, the degradation of the insulating papers is studied optically through the use of a microscope at different stages of the aging test. Depending on PD measurements, dissolved gas analysis (DGA) results, and the optical study of the paper aging, the evolution of the aging of oil-paper insulation system as a result of internal PD is declared when AC and DC voltage stresses are applied. It is obtained that the aging evolutionary process totally differs with the alteration of the applied voltage.

Overview of development, design, testing and application of compact gas-insulated DC systems up to ± 550 kV

The development of high-voltage direct current gas-insulated switchgear assemblies (DC GIS) of rated voltages up to ±550 kV has been completed. DC GIS provide a compact technical solution with a high functional density, optimized for projects with limited space as in offshore HVDC converter platforms, onshore HVDC converter stations and transition stations between different transmission media. Up to now, no standards for testing of gas-insulated DC systems are available, although pre-standardization work is in progress within CIGRE. Some tests can be performed as required in AC GIS standards. Special aspects of DC voltage stress, like the electric field distribution of insulators influenced by the accumulation of electrical charge carriers and the operation-related inhomogeneous temperature distribution, must be considered by additional electric and thermoelectric tests. For DC GIS, the experience of long-term performance is limited today. Although ageing is expected to be of lower importance, tests are recommended. This contribution summarizes the physical and technical background to design and develop compact DC switchgear assemblies using gas-insulated technology. It explains the developed modules of the substation and gives an overview of the performed tests. Furthermore, it provides an insight in the on-going standardization activities and describes applications in converter and transition stations, showing its space-saving characteristics.

Dielectric modeling of oil-paper insulation systems at high DC voltage stress using a charge-carrier-based approach

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.

Proton-irradiation Effects on Charge Trapping-related Instability of Normally-off AlGaN/GaN Recessed MISHFETs

We investigated proton-irradiation effects on the charge trapping-related instability of normally-off AlGaN/GaN recessed MISHFETs. The reduction of drain current (ISUBD/SUB) by 57.3 % and the shift of threshold voltage (VSUBth/SUB) by 1.5 V were induced by 5 MeV proton irradiation with a total dose of 5ⅹ10SUP14/SUP ㎝SUP-2/SUP. Short-term DC voltage stress tests were carried out before and after irradiation in order to investigate how temporary instability caused by the charge trapping could be affected by the proton irradiation. The increase in the interface trap density was probed by capture emission time (CET) map and conductance method, whereas the breakdown voltage was increased after proton irradiation. TCAD simulation using Silvaco Atlas showed the reduction of lateral electric field of gate edge at drain side after proton irradiation as well as the increase of lateral electric field in the access region where charge trapping mostly occurs. Radiation effect can worsen the device instability by increasing interface traps and electric field in the access region.

Charge generation and electrical degradation of cross‐linked polyethylene

Cross‐linked polyethylene is a chemically stable polymeric dielectric material with high dielectric strength, low dielectric constant and a small dissipation factor. Because of these excellent dielectric properties, cross‐linked polyethylene is used as an insulating material for electric power cables. Based on the results of much research working as an engineer of a cable manufacturer and in academia, this article describes the outline of charge generation under DC voltage stress and electrical degradation occurring in the material under AC voltage stress. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Voltage Polarity Reversing-Based DC Short Circuit FRT Strategy for Symmetrical Bipolar FBSM-MMC HVDC System

Fault ride-through (FRT) control and protection are of vital importance for a modular multilevel converter-based high-voltage direct current (MMC-HVDC) system when dc-side pole-to-pole (PTP) or pole-to-ground (PTG) fault occurs. In this paper, a FRT strategy by reversing dc voltage polarity is proposed for symmetrical bipolar full-bridge submodule-based MMC-HVDC system. It is suitable for both PTP and PTG faults. By adopting this method, the dc voltage stress in PTG fault scenario and dc current stress in PTP fault scenario are eliminated. Meanwhile, the capability of transmitting rated active power is maintained during faults, which can minimize the impact of dc-side faults on ac grid. In addition, based on a deduced dc-side equivalent circuit, the dc line current oscillation phenomenon during FRT transient process is analyzed. An active damping control method is presented to suppress the transient oscillation. Finally, validity of the proposed FRT strategy is verified by PSCAD/EMTDC simulations.

(Invited) High-Speed Analog Resistance Change in TaO x Synthesized By Reactive Sputtering

Analog resistance change depending on the external voltage stress such as the number or height of the voltage pulse is accumulating considerable attention. This is because such characteristics are quite analogues to the signal transfer at the synapses between neurons and therefore it can be applied to the energetically conservative human-brain inspired information processing. Resistive switching device using the oxide material is one of the strong candidates due to the non-volatility of the resistance states and low power and high-speed resistance switching process. Previously, those excellent characteristics have been mainly applied to the digital-type non-volatile memory ReRAM (resistive random access memory) technology [1]. In this contribution, we successfully demonstrated the high-speed reproducible analog resistance change in the device using the TiN electrode and TaO x thin film synthesized by the reactive sputtering process. The present analog resistance change device has the stacking structure of TiN(TE)/TaO x -L/TaO x -H/TiN(BE). Here, TE and BE respectively denote top and bottom electrodes. TaO x -L and TaO x -H are both the reactive sputtering films of TaO x having different resistivity. The value of resistivity in TaO x -L is lower than that in TaO x -H. Henceforce, we represent our device as TaO x -RAND (resistive analog neuromorphic device). Fig. 1(a) and 1(b) are the optical microscope and cross-sectional transmission electron microscope (TEM) images of TaO x -RAND. As shown in Fig. 1(a), the load resistor (LR) in series with TaO x -RAND was fabricated using TiN. Two kinds of LR having different resistance values, 1 and 3 kOhm, were fabricated by adjusting the length of the TiN wire. The role of LR is to suppress the transient current during the rapid resistance decrease process. The diameter of the concave region in Fig. 1(b) is 100 nm (designed value), which corresponds to the device size. The current-voltage (I-V) curve and response to the voltage pulse were measured by applying the external voltage to TiN(TE) with the TiN(BE) grounded. Figure 2(a) and 2(b) are the I-V curves for TaO x -RAND with LR = 1 and 3 kOhm respectively. In order to change the resistance of TaO x -RAND after the reset process, the value of the current compliance (CC) was varied from 75 to 200 uA. In all the cases, the resistance decrease process (set process) is observed when the positive voltage is applied to TE. On the other hands, the resistance increase process (reset process) is observed when the negative voltage is applied to TE. This voltage polarity dependence of the resistance change is quite reasonable when we consider the oxygen ion movement and consequent redox reaction inside the device. It should be noted that the negative differential resistance (NDR) during the reset process is unclear in the present TaO x -RAND compared with the previously reported resistance switching devices with precious metal electrodes such as Pt. The unclear NDR is expected to cause the gradual resistance increase. Also, the reset process is influenced by the relation between the resistance values of TaO x -RAND and LR. By connecting the larger LR in series with TaO x -RAND, the maximum current during the reset operation decreases. The resistance-increase-starting voltage (V RIS) values during the reset process indicated by the arrows in Figs. 2(a) and 2(b) are strongly influenced by LR. The larger value of LR results in the increase of V RIS when we compare the data obtained under the same CC condition. On the contrary, corresponding current at V RIS is almost independent on LR. These results indicate that a certain amount of current is required for the present reset process. The reset and set voltage pulse width dependences of the resistance in TaO x -RAND are plotted in Figs. 3(a) and 3(b), respectively. Although the magnitude of the resistance change induced by the voltage pulse is smaller than that induced by the DC voltage stress in Figs. 2, the analog resistance change behaviors in both the reset and set processes are clearly observed. Figure 4 is the voltage pulse number dependence of the resistance change in TaO x -RAND. In this case, 20 sets of reset and set voltage pulses are alternatively applied to the device. The analog control of the resistance by voltage pulse number was successfully demonstrated. It should be emphasized that the analog resistance changes were realized by the considerably high-speed voltage pulse having the pulse width of 500 ns. This switching speed is much faster than the actual signal transfer process at the synapses [2]. [1] H. Akinaga, and H. Shima, Proceedings of the IEEE 98, 2237 (2010). [2] H. Jo et al, Nano Lett. 10, 1297-1301 (2010). Figure 1

About the influence of temperature and humidity level on pulse shape parameters of positive DC corona discharges in air

The increasing importance of HVDC transmission systems requires the development and evaluation of suitable basic approaches regarding the partial discharge diagnosis. Although diverse promising approaches are identified, recent research is focused on standard atmospheric conditions. Due to the fact that this is just partly consistent to real operating conditions, additional research is required. Focusing on the time domain analysis of corona discharges, occurring under positive DC voltage stress in air, a measurement method for investigating the influence of varying atmospheric quantities is presented. Measurements are carried out for five different relative humidity levels in the range of 20 % to 95 % and for four different temperature levels in the range of 20 °C to 65 °C. As characterizing pulse shape parameters, the rise time, the pulse width and the fall time are determined as well as the apparent charge. The gained values are compared to each other and reconciled with physical processes.