Combined AC-DC Voltage Research Articles

Cascaded AC–DC Voltage Control to Provide Reactive Power Support for the PV‐Driven Grid‐Tied Synchronverter

ABSTRACTThe objective of this submission is to provide flexible reactive power regulation of a photovoltaic (PV)‐driven grid‐connected inverter. Here, inverter is realized as a synchronverter by employing frequency regulation using well‐established swing equation. However, reactive power regulation is accomplished by employing combined AC–DC voltage control in addition to traditional synchronverter control for ensuring smooth tracking of reactive power. Thus, the PV‐driven voltage source converter (VSC) will be injecting available active power as per the prevailing irradiation, temperature, and operator defined reactive power by employing proposed control. The performance of suggested control is evaluated and compared against existing control by considering realistic operational scenarios through a case study.

Influence of PD Between Needle and Plate Electrodes on Oil-Pressboard Aging Under Combined AC–DC Voltage

Influence of PD Between Needle and Plate Electrodes on Oil-Pressboard Aging Under Combined AC–DC Voltage

Effect of Electric-Field Components on the Flashover Characteristics of Oil-Paper Insulation Under Combined AC-DC Voltage

Flashover characteristics are closely related to the electrical field distribution on the surface of oil-paper insulation. In this study, the effects of electrical field and DC voltage components on the creepage flashover characteristics of oil-paper insulation under combined AC-DC voltage (CADV) are investigated using two electrode models. The results demonstrate that when the DC voltage component is large, the flashover voltage under the influence of strong parallel electric field component is approximately 1.2 times higher than that under the influence of strong normal electric field component. When the DC voltage component value increases under the influence of strong normal electric field component, the value of corresponding AC voltage component of the creepage flashover voltage exhibits a decreasing trend. It is believed that due to the strong normal electric field component, the electrons intensify the collision on the surface of the insulating pressboard. Under the influence of the DC voltage component and due to the presence of space charge on the insulating pressboard, the distribution shape of carbonized particles on the surface of the insulating pressboard after creepage flashover is different. Furthermore, the instantaneous energy release during creepage flashover under CADV is higher than that under AC voltage, and the greater the DC voltage component value, more severe is the damage to the fibers on the insulating pressboard surface.

Experimental Study on Corona Onset Characteristics of Small Curvature Electrode Under Combined AC–DC Voltages

An investigation into the corona onset characteristics of small curvature electrode with rough surface under combined ac–dc voltages was presented. The experimental platform under combined voltages with different electrode fillet radius and electrode spacing was built. The corona inception voltage of the small curvature electrode was obtained by collecting the corona pulse signal and changes in the number of ultraviolet photons. Based on the finite element algorithm, the corona onset electric field strength under combined ac–dc voltages was calculated. Results showed that the corona inception voltage increased with the increase of electrode fillet radius and electrode spacing. The corona inception voltage of positive dc–ac was higher than that of negative dc–ac. The linear superposition of ac and dc voltages when corona occurred was regarded as combined corona inception voltage. It increased first and then decreased slightly with the increase of ac voltage. The combined corona electric field strength on the electrode surface generally decreased with the increase of ac voltage. The research of this article can provide useful references for the maintenance of electrode surface and insulation optimization of converter valve.

Experimental and numerical study on corona onset threshold under combined AC-DC voltages in Wire-Plane electrodes

With the large-scale application of hybrid HVDC/HVAC transmission technology, the phenomenon of corona discharge is also intensified. This paper attempts to provide a comprehensive understanding of the variation of corona onset voltage under combined AC-DC voltages. In this paper, an experimental platform for measuring corona onset voltage and current pulses of wire-plane electrode under combined AC-DC voltages was built. A new corona model of wire-plane electrode under combined AC-DC voltages was also established by using the fluid model and the ion flow field model. The corona onset voltages of wire-plane electrode under different ratios of AC and DC voltages, conductor radius and conductor spacing were measured. It turned out that the conductor radius affected the intercept of the corona onset curve, while the conductor spacing affected the slope. In addition, the type of corona only affected the intercept of the corona onset curve, not the slope. With the same fixed voltage of non-corona conductor, the corona onset voltage of the DC-side positive corona was the largest, followed by the DC-side negative corona, then was the AC-side positive half-cycle corona, and the smallest was the AC-side negative half-cycle corona. The interaction between AC voltage and DC voltage was approximately linear. The calculations of the model illustrated that the residual space charges had a blocking effect on corona discharge. Therefore, the more residual space charge was, the higher corona onset voltage would be.

Electric field analysis of 66 kV and 110 kV SiR insulators under combined AC–DC voltages

Insulators are the most crucial part of power systems. The insulation performance of insulators is vital for the sustainability of power systems. Recently, silicone rubber (SiR) insulators are used frequently in all sections of the power systems. In this paper, a SiR insulator currently used in power transmission systems has been analyzed under combined AC–DC​ voltage using the finite element method. In the analysis, positive and negative DC voltages in different amplitude ratios were superimposed over the phase-earth operating voltage of the insulator. Analyses in the study were made in time-dependent. Only DC voltage was applied to the insulator for the first 60 s, AC + DC voltage was applied between 60 to 120 s. Thus, the electric field behavior of the SiR insulator under combined AC–DC voltage has been obtained. The change of electric field based on positive and negative DC components was investigated. As a result of the study, the effect of the polarity of the DC component in the combined voltage was observed. As a result, effect of the polarity of the DC component in the combined voltage on the maximum electric field intensity was observed.

Partial Discharge Simulation of Air Gap Defects in Oil-Paper Insulation Paperboard of Converter Transformer under Different Ratios of AC–DC Combined Voltage

A converter transformer is important primary equipment in a DC transmission project. The voltage on the valve side winding is complex when the equipment is running, including DC, AC, and AC–DC combined voltage. The insulation structure of the valve side winding of a converter transformer is an oil-paper insulation structure, which may have a variety of defects in the manufacturing stage and daily use, resulting in partial discharge. Therefore, it is the key to studying the partial discharge characteristics and mechanism of oil-paper insulation under AC–DC combined voltage. In this paper, we build a two-dimensional air gap model of oil-paper-insulated pressboard considering the actual particles and actual reaction based on the fluid model. The characteristics and evolution mechanism of partial discharge (PD) in pressboard under different AC/DC combined voltages are studied by numerical simulation. The results show that when the DC component increases, the polarity effect of partial discharge is more obvious, while the potential and discharge intensity in the air gap decrease. Further analysis revealed that the DC component in the combined voltage accumulated a large number of surface charges on the surface of the air gap, and the space charge distribution was more uniform and dispersed, which generated an electric field with opposite polarity to the DC component in the air gap and, then, inhibited the development of local discharge in the paperboard. The results of the simulation are consistent with the previous experimental phenomena, and the mechanism analysis of the simulation results also verifies the previous analysis on the mechanism of experimental phenomena. This will lay a theoretical foundation for the further study of partial discharge phenomenon of oil-paper insulation structures in practical operation in the future.

Pattern Recognition of Development Stage of Creepage Discharge of Oil–Paper Insulation under AC–DC Combined Voltage Based on OS-ELM

The recognition of the creepage discharge development process of oil–paper insulation under AC–DC combined voltage is the basis for fault monitoring and diagnosis of converter transformers; however, only a few related studies are available. In this study, the AC–DC combined voltage with a ratio of 1:1 was used to develop a recognition method for the creepage discharge development process of an oil–paper insulation under a cylinder–plate electrode structure. First, the pulse current method was used to collect the discharge signals in the creepage discharge development process. Then, 24 characteristic parameters were extracted from four types of creepage discharge characteristic spectra after dimensionality reduction. Finally, based on the online sequential extreme learning machine (OS-ELM) algorithm, these characteristic parameters were used to recognize the development stage of the creepage discharge of the oil–paper insulation. The results showed that when the size of the sample training set used in the OS-ELM algorithm is close to the number of hidden layer neurons, a high recognition accuracy can be obtained, and the type of activation function has little influence on the recognition accuracy. Four stages of the creepage discharge development process were recognized using the OS-ELM algorithm; the trend was the same as that of the characteristic parameters of the entire creepage discharge development process. The recognition accuracy was 91.4%. The algorithm has a high computing speed and high accuracy and can train data in batches. Therefore, it can be widely used in the field of online monitoring and evaluation of electrical equipment status.

Influence of Aging on Creepage Discharge Characteristics of Oil-Paper Insulation Under AC-DC Combined Voltage

There is a relative lack of research on the development of creepage discharge in aged oil-paper insulation subjected to AC-DC combined voltage in converter transformers. In this study, based on the physical and chemical properties of oil-paper, we propose a sphere-plate electrode model to study the creepage discharge parameters and characteristic spectra of aged oil-paper subjected to AC-DC combined voltage. The results show that the creepage discharge inception voltage of aged oil-paper is related to the AC voltage component of the AC-DC combined voltage. However, when the DC voltage component is 60 kV, the flashover phenomenon was found to occur in one of the severely aged samples before the AC voltage was applied. Despite being subjected to the same DC voltage, the flashover phenomenon did not occur in three other samples aged to different degrees. Large-amplitude discharges were found to appear earlier in the severely aged samples. At the same creepage discharge stage, as the aging time of the oil-paper increased, the number of large-amplitude discharges was found to rapidly increase, and the time interval between adjacent discharges was found to gradually decrease. At each stage of the creepage discharge, the small-amplitude discharge has a “hump” shaped distribution. The larger the DC voltage component, the shorter the endurance time of aged oil-paper, and the more severe the damage to the oil-paper. The DC voltage component was found to have an accelerating effect on the development of creepage discharge in aged oil-paper insulation.

The Influence of DC Component on the Creepage Discharge Paths in Oil-Pressboard Insulation Under AC-DC Combined Voltage

The main dielectric medium in converter transformers is the oil-pressboard. The valve windings of converter transformers are subjected to AC-DC combined voltage. Creepage discharge is a common defect of oil-pressboard insulation. The degree of the oil-immersed pressboard (OIP) damage is directly affected by different creepage discharge paths. In this study, a needle-plate model was developed to explore different creepage discharge paths in oil-pressboard insulation under AC-DC combined voltage. A high-speed camera was used to record the developmental process of white marks on the OIP. We explored reasons behind the different creepage discharge paths under different voltage types. Results revealed that the creepage discharge path and the degree of OIP damage were influenced by the DC component. Although the damaged OIP in converter transformers may cause catastrophic flashover inside the transformers, it is difficult to replace it. This implies that transformer insulation designers should pay attention to the influence of the DC component on OIP damage. In this study, we used pulse current method to detect discharge patterns. Discharge parameters were then extracted from discharge patterns. We observed that developmental process of white marks corresponded to developmental process of discharge parameters. This suggests that creepage discharges can be diagnosed based on discharge parameters. Based on these conclusions, important information was provided for the on-line monitoring design of transformer insulation status.

Experimental analysis of corona inception voltage under combined AC-DC voltages at various air pressure and humidity in rod to plane electrode

As the effect of climate plays a significant role on corona discharge under combined voltages, the variation of corona inception voltage with different air pressure and humidity is studied. The experimental platform based on rod-to-plane electrode is constructed with adjustable air pressure from 0.06MPa to 0.10 MPa and the relative humidity (RH) from 20% to 90%. The variation of ultraviolet (UV) photon count and corona inception voltage is obtained at various climate conditions under different applied voltages: the single alternate current (AC), single direct current (DC), and combined AC-DC. It turns out that all the corona inception voltages decline with the drop of air pressure and the rise of humidity under different applied voltages. The influence between different voltages mainly relies on space charge, as more AC component makes it easier to accumulate positive ions. The existence of AC makes ions move forward and backward while the existence of DC dominates the polarity of corona and general drift directions. At last the fitting formula of hybrid corona inception voltage combining air pressure and relative humidity is given.

Numerical analysis on space charge and AC-DC combined breakdown strength in polyethylene

In high voltage direct current (HVDC) power transmission systems, electrical equipment at the valve side usually withstands complex stresses like AC-DC combined voltages. Understanding the charge migration and accumulation characteristics under complex stresses and their influences on the breakdown strength are of great significance. This paper presents an in-depth simulation study on the AC-DC combined breakdown strength using a modified bipolar charge transport model, which takes the deterioration of materials into consideration. Firstly, the simulation results of charge profiles under AC voltages agree well with experimental results, which partly illustrates the appropriate selection of parameters. Then the AC breakdown strengths with different ramping rates and frequencies are studied. Charges accumulated relatively close to the surface of samples can cause severe distortion of the electric field after the change of voltage polarity. After that, the breakdown voltages with different ratios of the AC to DC component are calculated and analyzed, especially on how AC and DC component influence the charge migration and accumulation during the evolution of the aging process eventually leading to breakdown. The simulation results of AC-DC combined breakdown voltages show good agreement with previous experimental results.

Numerical analysis on space charge and AC-DC combined breakdown strength in polyethylene

In high voltage direct current (HVDC) power transmission systems, electrical equipment at the valve side usually withstands complex stresses like AC-DC combined voltages. Understanding the charge migration and accumulation characteristics under complex stresses and their influences on the breakdown strength are of great significance. This paper presents an in-depth simulation study on the AC-DC combined breakdown strength using a modified bipolar charge transport model, which takes the deterioration of materials into consideration. Firstly, the simulation results of charge profiles under AC voltages agree well with experimental results, which partly illustrates the appropriate selection of parameters. Then the AC breakdown strengths with different ramping rates and frequencies are studied. Charges accumulated relatively close to the surface of samples can cause severe distortion of the electric field after the change of voltage polarity. After that, the breakdown voltages with different ratios of the AC to DC component are calculated and analyzed, especially on how AC and DC component influence the charge migration and accumulation during the evolution of the aging process eventually leading to breakdown. The simulation results of AC-DC combined breakdown voltages show good agreement with previous experimental results.

Effects of temperature on creepage discharge characteristics in oil-impregnated pressboard insulation under combined AC–DC voltage

Due to the complexity of the valve side winding voltage of the converter transformer, the insulation characteristics of the oil-impregnated pressboard (OIP) of the converter transformer are different from those of the traditional AC transformer. The study on effect of temperature on the creeping discharge characteristics of OIP under combined AC–DC voltage is seriously inadequate. Therefore, this paper investigates the characteristics of OIP creepage discharge under combined AC–DC voltage and discusses the influence of temperature on creepage discharge characteristics under different temperatures from 70 °C to 110 °C. The experimental results show that the partial discharge inception voltage and flashover voltage decrease with increasing temperature. The times of low amplitude discharge (LAD) decrease and amplitude of LAD increases. Simultaneously, the times of high amplitude discharge (HAD) gradually increase at each stage of creepage discharge with higher temperature. The analysis indicates that the charge carriers easily accumulate and quickly migrate directional movement along the electric field ahead of discharging. The residual charge carriers are more easily dissipated after discharging. The ‘hump’ region of LAD moves to the direction of higher discharge magnitude. The interval time between two continuous discharges is shortened obviously. The concentration of HAD accelerates the development of OIP insulation creepage discharge. The temperature had an accelerating effect on the development of discharge in the OIP under applying voltage.

Influence of Combined AC–DC Voltages on Breakdown Characteristics of Epoxy Resin in LN2

In a dc superconducting cable system, the ripple waves contained lots of ac components that can superimpose dc voltage and current and are widely applied in rectifiers and other power electronic devices, which threaten the reliability and safety of a high-temperature superconducting (HTS) insulation system. In this paper, the partial discharge (PD) characteristics of the epoxy resin are studied with different ac/dc proportions in liquid nitrogen and room temperature. The results show that the PD inception electric field and breakdown strength of the epoxy resin increase as the dc component increases in both liquid nitrogen and room temperature. According to the phase-resolved PD distribution obtained in the inception stage, PDs show different behaviors at a higher dc component. The mean and total PD amplitude decrease as the dc component increases in both the inception stage and prebreakdown stage; they depend on the electric field modified by space charges. Compared with the PD characteristics in room temperature, they are weaker in liquid nitrogen, indicating the PD detection of insulation materials should be enhanced to eliminate the latent destruction in the HTS system. The results establish the foundation for diagnosis and assessment of insulation materials in liquid nitrogen with combined ac-dc voltages.

Phenomenon analysis and state classification of surface discharge on oil-impregnated pressboard under AC-DC combined voltage

Surface discharge may cause irreversible damage to turn-to-ground insulation in valve windings of converter transformer, where withstand with AC-DC combined voltage. This paper analyzes the phenomenon and characteristics of surface discharge on oil-impregnated pressboard (OIP) under AC-DC combined voltage, and develops a discharge state recognition method. The cylinder-plate discharge model was used to simulate surface discharge. The results showed that discharge development and OIP failure were significantly accelerated by white marks on OIP which were essentially gaseous channels. The discharge characteristics before and after white mark occurrence were both dominated by AC component because of its bigger contribution to electrical field distribution (EFD), and the DC component had obvious effect on accelerating OIP failure. A set of features representing discharge state after white mark occurrence was selected out by the entropy weight method (EWM), based on which the discharge process was classified into three states (stable, fast development and pre-breakdown state) by fuzzy C means clustering method (FCM). A support vector machine (SVM) classifier to recognize discharge state was trained and showed a good performance, whose average assessment accuracy was up to 91.98%. Moreover, the ratio between negative and positive discharge numbers could be used as an auxiliary indicator of pre-breakdown state.

Space Charge Modulated Electrical Breakdown of Oil Impregnated Paper Insulation Subjected to AC-DC Combined Voltages

Based on the existing acknowledgment that space charge modulates AC and DC breakdown of insulating materials, this investigation promotes the related investigation into the situations of more complex electrical stress, i.e., AC-DC combined voltages. Experimentally, the AC-DC breakdown characteristics of oil impregnated paper insulation were systematically investigated. The effects of pre-applied voltage waveform, AC component ratio, and sample thickness on AC-DC breakdown characteristics were analyzed. After that, based on an improved bipolar charge transport model, the space charge profiles and the space charge induced electric field distortion during AC-DC breakdown were numerically simulated to explain the differences in breakdown characteristics between the pre-applied AC and pre-applied DC methods under AC-DC combined voltages. It is concluded that large amounts of homo-charges are accumulated during AC-DC breakdown, which results in significantly distorted inner electric field, leading to variations of breakdown characteristics of oil impregnated paper insulation. Therefore, space charges under AC-DC combined voltages must be considered in the design of converter transformers. In addition, this investigation could provide supporting breakdown data for insulation design of converter transformers and could promote better understanding on the breakdown mechanism of insulating materials subjected to AC-DC combined voltages.

Breakdown Characteristics of Oil-Pressboard Insulation under AC-DC Combined Voltage and Its Mathematical Model

An AC-DC combined voltage is applied to the oil-pressboard insulation near the valve side during the operation of a converter transformer. To study the breakdown characteristics of an oil-pressboard insulation under such voltages, a typical plate electrode structure was employed in the laboratory to conduct a breakdown test on the oil-pressboard insulation. The electrical field distribution and the DC contents of the transformer oil and the pressboard in composite insulation under the AC-DC combined voltage were simulated by their dielectric parameters. The breakdown strength of the transformer oil decreases with the increase in the DC content of the applied voltage, whereas that of the pressboard increases. For the oil-pressboard insulation, the breakdown voltage increases first and then decreases. The electric field strength decreases in the transformer oil with the increase in the DC content, whereas it increases in the pressboard. And the DC contents of the transformer and the pressboard in composite insulation were different from that of the applied voltage. Finally, based on the above results, a mathematical model was proposed to describe the breakdown characteristics of the oil-pressboard insulation under the AC-DC combined voltage; the theoretical and experimental results were in good agreement.

Defect Pattern Recognition Based on Partial Discharge Characteristics of Oil-Pressboard Insulation for UHVDC Converter Transformer

The ultra high voltage direct current (UHVDC) transmission system has advantages in delivering electrical energy over long distance at high capacity. UHVDC converter transformer is a key apparatus and its insulation state greatly affects the safe operation of the transmission system. Partial discharge (PD) characteristics of oil-pressboard insulation under combined AC-DC voltage are the foundation for analyzing the insulation state of UHVDC converter transformers. The defect pattern recognition based on PD characteristics is an important part of the state monitoring of converter transformers. In this paper, PD characteristics are investigated with the established experimental platform of three defect models (needle-plate, surface discharge and air gap) under 1:1 combined AC-DC voltage. The different PD behaviors of three defect models are discussed and explained through simulation of electric field strength distribution and discharge mechanism. For the recognition of defect types when multiple types of sources coexist, the Random Forests algorithm is used for recognition. In order to reduce the computational layer and the loss of information caused by the extraction of traditional features, the preprocessed single PD pulses and phase information are chosen to be the features for learning and test. Zero-padding method is discussed for normalizing the features. Based on the experimental data, Random Forests and Least Squares Support Vector Machine are compared in the performance of computing time, recognition accuracy and adaptability. It is proved that Random Forests is more suitable for big data analysis.

Flashover performance of silicon rubber under combined AC-DC voltage and its improving method

Composite insulators in valve tower of UHVDC converter station withstand combined ac-dc voltage. The flashover performance of silicone rubber under combined voltage is investigated in this paper. The voltages on the post insulator of valve tower under different operating conditions are simulated in PSCAD/EMTDC, and the harmonic components of the voltages are analyzed. The simulation results indicate that the ac frequencies of the voltage on the post insulator are mainly 50 Hz, 3n and 6n±1 times harmonics. The flashover voltages of silicone rubber under different combinations of ac and dc voltages are tested, including different single frequency ac voltages superimposed on different dc voltages, and different combined harmonic voltages superimposed on different dc voltages. The test results indicate that different ratios of dc voltage to ac voltage and different percentages of harmonics have various influences on the flashover voltage of silicone rubber. Moreover, argon atmosphere pressure plasma jet (Ar APPJ) is applied to modify the surface of the silicone rubber under different modification times and voltages. The flashover voltages and the surface morphologies of the silicone rubber before and after surface modification are compared. Surface modification by Ar APPJ can improve the surface resistivity of the silicone rubber significantly, and the flashover voltage also increases.