Converter Transformer Research Articles

Experimental Study on Surface Electric Field Distribution and Flash-over Voltage of Bushing Composite Insulator at High Altitude

At present, the high voltage bushing composite insulator is widely used in AC/DC system substation, and its structural parameters show the nonlinear growth trend with the increase of voltage level. Therefore, in 1000kV UHVAC and 800kV UHVDC system, bushing composite insulator has the typical characteristics of long insulation distance and large radial diameter. At present, with the proposal of high-voltage power equipment compact optimization design scheme, the radial diameter of the composite insulator for bushing can be effectively reduced by reasonable design of internal capacitor core or metal shield structure optimization design scheme, so as to realize its large aspect ratio compact design. On the other hand, the insulation distance of high voltage bushing composite insulator will be further increased when it is applied in high altitude area. The innovation of this paper is how to carry out flashover voltage test to fill the current test data gap. In view of this, this paper first establishes two typical structures of the high voltage bushing composite insulators, including the three-dimensional finite element models of the composite insulators for capacitive core converter transformer bushing and the double-layer metal shield through the wall bushing, obtains the distribution characteristics of typical voltage and electric field under the operating state, and analyzes the local high field strength and corona, the electric field distribution of composite insulators for bushing. Furthermore, the actual flashover tests between large fittings air metal grounding body and large fittings composite insulator metal grounding body are carried out in the laboratory environment, and the quantitative relationship curve between flashover voltage and space net distance is obtained. Based on the above theoretical analysis and test data, the size altitude correction and umbrella skirt contour topology optimization of composite insulator for the UHV AC/DC bushing are carried out. Finally, the DC withstand voltage test and one minute power frequency withstand voltage test are passed, which proves the rationality and effectiveness of the design of composite insulator for bushing at UHV level. In this paper, starting from the typical electric field distribution characteristics of all kinds of composite insulators for bushing, the flashover law is studied for the first time, and the flashover test is carried out in the laboratory to supplement the classical test data. The research results have good theoretical and practical significance for installation, design and operation maintenance of all kinds of composite insulators.

Flexible control strategy for HVDC transmission system adapted to intermittent new energy delivery

Intermittent new energy delivery requires increasing the flexibility of ultra-high voltage direct current (DC) power adjustment. Based on a converter steady-state model and a DC power model, the control angle constraints of a converter valve are relaxed for power regulation. In this paper, a flexible DC power control method based on a fixed tap changer position is proposed. The initial ratio of the converter transformer is optimized. The effects of the fixed-tap changer position control on the control angle, reactive power compensation, and commutation failure are analyzed. The new control method allows a DC system to operate at a large angle and increase the additional reactive power loss while improving the commutation security margin. Steady-state and electromagnetic transient simulations in the CIGRE test system verify the validity of the method proposed in this paper and the correctness of the analysis conclusions.

Effect of multi‐factors on heterocharges for oil‐impregnated paper in converter transformer using modified charge transport model

The heterocharge accumulated in the oil-paper insulation system of the converter transformer will enhance the local electric field and cause insulation deterioration. Numerical simulation is an effective method to analyze heterocharge. Currently, the numerical simulation of heterocharge is mainly based on the bipolar charge transport (BCT) model and the impurity ion transport (IIT) model. However, the effects of temperature, conductivity, and thickness of oil-impregnated paper on charge transport have not been considered in these models. In view of this, a modified charge transport (MCT) model by introducing the Maxwell–Wagner model and heat conduction equation is proposed in this paper based on the BCT model and IIT model. Then, the influence of multi-factors (temperature gradient, moisture content, and oil-impregnated paper thickness) on heterocharge accumulation is investigated by using the MCT model. The simulation results obtained by the MCT model are more consistent with the experimental results than the BCT model, which proves the accuracy and superiority of the MCT model. Moreover, the increase in temperature gradient, moisture content, and oil-impregnated paper thickness will enlarge the ion dissociation rate, conductivity, and trap density, and thus promote the generation of heterocharge. In that respect, the MCT model is expected to optimize the insulation design and provide a reference for space charge dynamic simulation.

Research on hidden dangers of oil-flow relay in HVDC converter transformer

HVDC converter transformer oil-flow relay plays an important role in realizing non-power protection of converter transformer on-load tap changer, which can act accurately and directly affect the safe and stable operation of the whole HVDC program as well as the converter transformer itself. In this paper, the research revolves the hidden dangers of HVDC converter transformer and its identification. The paper begins with the analysis of the converter transformer operating principles of two German companies: MR and EMB. Then, the current application status and related inspection requirements of oil-flow relay are expounded. Finally, pertinent hidden danger identification and modification schemes are provided based on analysis of the danger in actual operation of oil-flow relay and its causes. The feasibility and effectiveness of the scheme are verified by the actual operation on site.

Residual flux impact in controlled switching of HVDC converter transformer

Abstract Converter transformers are an interface point of HVDC frameworks and are basic hardware from activity and reliability view point. Converter transformers are turned off intentionally under planned support and un-deliberately during fault conditions. Random energization of HVDC Converter transformer delivers high inrush currents due to core saturation asymmetries and endanger equipment health. In this paper, charging approach has been proposed. By utilizing point on wave exchanging for moderation of charging drifters during charging of 888 MVA 400/325 kV Converter Transformer of ±800 kV HVDC Sub-station. The proposed procedure has been applied on HVAC electrical Circuit Breaker furnished with Pre-Insertion Resistor considering diverse degree of residual flux. Performance of proposed philosophy has been affirmed by field tests and reproduction in PSCAD programming package and is found very viable. Near investigation is done in this paper and shows that with the use of proposed system, the switching transient during stimulation and field tests of coupled three phase converter transformers is decreased to worthy level.

AC Side Current Harmonics Measurement and Analysis of UHVDC Converter Transformer

Current harmonics is crucial for converter transformer design considering temperature/loss and vibration/noise. For line commuted current high voltage direct voltage (LCC HVDC) system, harmonics has been analyzed thoroughly with assumptions for simplification (e.g. that transformer is ideal). However, the waveforms in practice, especially for UHVDC system, can be different due to nonideal conditions and they are rarely reported in literature. This paper designed a harmonics measurement system to extract the AC side winding current for a converter transformer in a ± 800kV UHVDC system. Harmonics characteristics is analyzed and simulation is performed for comparison. It is observed that with the increase of transmitted power, both the total harmonic distortion and single harmonic distortion decrease. The measured waveform are asymmetric with inrush spikes which is different from traditional theoretical analysis and the possible cause is given.

Load Flow Analysis in Hybrid AC-DC Transmission Network

Power-flow studies are of great significance in planning and designing the future expansion of power systems as well as in determining the best operation of existing systems. Technologies such as renewables and power electronics are aiding in power conversion and control, thus making the power system massive, complex, and dynamic. HVDC is being preferred due to limitations in HVAC such as reactive power loss, stability, current carrying capacity, operation and control. The HVDC system is being used for bulk power transmission over long distances with minimum losses using overhead transmission lines or submarine cable crossings. Recent years have witnessed an unprecedented growth in the number of the HVDC projects. Due to the vast size and inaccessibility of transmission systems, real time testing can prove to be difficult. Thus analyzing power system stability through computer modeling and simulation proves to be a viable solution in this case. The motivation of this project is to construct and analyze the load flow and short circuit behavior in an IEEE 14 bus power system with DC link using MATLAB software. This involves determining the parameters for converter transformer, rectifier, inverter and DC cable for modelling the DC link. The line chosen for incorporation of DC link is a weak bus. This project gives the results of load flow and along with comparison of reactive power flow, system losses, voltage in an AC and an AC-DC system.

Dielectric and Flash-over Characteristics of the ±1100kV Valve Side Converter Transformer Bushing with SF6 Gas Insulation

The ±1100kV high voltage capacitor filled SF6 gas bushing is an important equipment to realize the electrical connection between the converter transformer and the valve tower. The paper mainly discusses the design of the external and internal insulation structure. The research content is divided into four parts, the actual operating environment and the characteristics of the voltage and current waveform of converter transformer bushing are discussed. The design and analysis of the hollow composite insulator, corona ring and tail end of the bushing have been done. The main insulation structure of bushing has been designed by the equal margin method, and simulation and analysis of the multi-physical field have been done. The three dimensional electric field simulation model has been set up to calculate and analyze the external insulation performance of the bushing. The results show that the main insulation of the bushing is fit to the capacitance structure, the maximum axial field strength of the bushing is 0.62kV·mm−1, and the end of the bushing should be equipped with apple type corona ring. The insulation distance of the hollow composite insulator is designed as 7490mm. The insulation distance of the bushing tail is designed as 3110mm. In the actual operating environment, the maximum field strength of the corona ring appears on the lower end surface, and the value is 2386V·mm−1. The results of the paper can provide the theoretical basis for the design of the external insulation of the converter transformer bushing used in the HVDC transmission project.

Research on distribution characteristics of vibration signals of ±500 kV HVDC converter transformer winding based on load test

The converter transformer is important for the high-voltage direct current transmission system. The distribution characteristics of the vibration signal of the tank are significant for the operation status monitoring and the judgment basis for the fault condition of the converter transformer. However, most present research on vibration detection technology focused on ordinary power transformers rather than the HVDC converter transformers. Moreover, the influence of the special structure and the heat accumulation effect of the load current on the vibration signal distribution characteristics is not fully considered. To solve the above problems, this paper analyzes the intensity distribution of vibration signals at different positions on the tank surface and valve side bushings based on the offline load test. At the same time, to study the influence factors of vibration signal distribution, the influence of current heat accumulation on winding characteristics and transformer oil temperature is analyzed. The results show that vibration intensity is related to location and the load current. And the vibration intensity of the tank surface increases with the time of current flowing through the winding, nevertheless, the vibration intensity tends to decrease near the valve-side bushing. The test methods and research on distribution Characteristics in this paper can provide new thoughts for the optimization of the layout of converter transformer vibration detection sensors, and provide a research basis for the application of vibration signal fault diagnosis methods.

Investigation on magnetostrictive behaviour of a converter transformer influenced by dominant harmonics: AFEMandANNbased approach

The introduction of harmonics to the HVDC system is imminent, which affects the components of the power system from varied perspectives. The present work emphasises analysing the behaviour of the converter transformer concerning the harmonics in the input. The process involves designing a stepped configuration of a 240-MVA converter transformer using the finite element method (FEM). The designed model is analysed as per its structural dynamics to obtain its vibrational response for different ranges over the frequency spectrum. Furthermore, the module of acoustics and thermal analysis is coupled to the vibrational model. The coupled model serves as an efficient tool in analysing the impact of different orders of harmonics on the vibrational, acoustic and thermal performance of the core of the converter transformer. The computational effort taken in solving the coupled field problem is simplified by designing a neural network model and the prediction efficacy of the network is enhanced by using a Hybrid of Particle Swarm Optimisation and Gravitational Search Algorithm (HPSOGWO) as tuner of the internal parameters of the neural network. The significance of the designed model is also examined against other well-versed hybrid algorithms in the literature in terms of various statistical indices.

Multi-scale information fusion model for feature extraction of converter transformer vibration signal

The vibration signal feature extraction model of the converter transformer has the problem of low accuracy. The main reason is that the time-related information is easily lost during training. In recent years, convolutional neural networks (CNN) have been proven to break the limits of traditional classification models. CNN has achieved remarkable results in large-scale image recognition tasks. To solve the problem of difficulty in model building and loss of time-related information, this paper proposes a multi-scale fusion feature extraction model for converter transformer vibration signal. The model includes an image generation module and a multi-scale information fusion module. The image generation module converts vibration signals into time-domain, frequency-domain, and energy feature images by calculating the Markov Transform Field and Continuous Wavelet Transform. In the multi-scale information fusion module, the attention module is introduced into the multi-parallel convolutional neural network to fuse the feature images. The proposed method can make full use of the advantages of deep learning through generating images that can represent time series from multi-scale. To evaluate the effectiveness of the model in this paper, we establish a data set based on the measured vibration signals and tested by classification experiments. The result shows the accuracy of the model state recognition in this paper is 96.15%, which is better than classic time series processing networks such as LSTM and 1D-CNN.

Theoretical calculation model of insulation medium and eddy current heating under complex waveform of ±800kV convertor transformer RIP bushing used in valve side

Electrical and thermal properties of the converter transformer valve side RIP DC bushing interact with each other, so electrical insulation failure in operation is directly related to the thermal properties. It is essential to conduct quantitative analysis on the power loss of UHV converter transformer dry-type bushing. The total power loss of bushing consists of two parts, eddy heat from current-carrying structures and joule heat from bushing core. The actual current and voltage waveform imposed on the bushing contain many high frequency components. In this paper, the theoretical calculation model of bushing core joule heat and eddy heat is obtained from structure of HV convertor transformer dry-type bushing. Then the temperature and frequency properties of epoxy impregnated paper used by dry-type bushing core are acquired by the experiments. Furthermore, based on the frequency domain FEM, the numerical calculation method for dry-type bushing power loss is proposed. In the end, the numerical results are used to analyze the factors that may lead to convertor transformer drytype bushing insulation failure in operation. The research result obtained in this paper is appropriate to compute thermal characteristic of bushing and is of directive significance to the operation, protection and accident analysis.

Iterative Finite Element Method Applied to the Nonlinear Electric Field of ±400kV Converter Transformer Barrier System

The DC steady-state and the polarity reversal (PR) are typical states of composite insulation in converter transformer barrier system, and the insulation medium has the strong E-field and temperature nonlinear characteristic, so the DC steady-state and polarity reversal E-field will be affected by the variation of electric parameters under temperature gradient. The FEM was applied to the quantitative E-field analysis considering electric and temperature nonlinear factors simultaneously. First, the nonlinear relationship between conductivity with E-field and temperature of medium has been obtained by experiments. Then, the iterative FEM was proposed and verified by coaxial insulation with analytical solution. Finally, the full-scale model of ±400kV converter transformer barrier system has been established, the DC steady-state and polarity reversal E-field has been calculated with the proposed method. The results show that under linear and nonlinear conditions the E-field is quite different, and E-field non-linearity can inhibit the local high electric spot, temperature non-linearity can reduce the E-field in the high temperature area and increase that in the lower temperature area. Under the temperature gradient condition, the local high E-field spot is easy to occur in the PR process. The iterative FEM and calculation results can provide references for the design of converter transformer barrier system.

Performance Comparison of Ferrite and Nanocrystalline Cores for Medium-Frequency Transformer of Dual Active Bridge DC-DC Converter

This article reports an investigation into ferrite and nanocrystalline materials for the medium-frequency transformer of a dual active bridge DC-DC converter, which plays a key role in the converter’s efficiency and power density. E65 MnZn ferrite cores and toroidal and cut nanocrystalline cores are selected for the construction of 20-kHz transformers. Transformer performance is evaluated with a 1.1-kW (42–54 V)/400 V dual active bridge DC-DC converter with single-phase shift and extended phase shift modulations. The experimental results indicate that the toroidal nanocrystalline transformer had the best performance with an efficiency range of 98.5–99.2% and power density of 12 W/cm3, whereas the cut-core nanocrystalline transformer had an efficiency range of 98.4–99.1% with a power density of 9 W/cm3, and the ferrite transformer had an efficiency range of 97.6–98.8% with a power density of 6 W/cm3. A small mismatch in the circuit parameters is found to cause saturation in the nanocrystalline toroidal core, due to its high permeability. The analytical and experimental results suggest that cut nanocrystalline cores are suitable for the dual active bridge DC-DC converter transformers with switching frequencies up to 100 kHz.

Space Charge Characteristics in Epoxy Resin Impregnated Paper under AC and AC-DC Superimposed Electric Fields

In this paper, charge distributions in three-layered epoxy resin and epoxy resin impregnated paper samples under AC and AC-DC superimposed electric fields are measured by the improved high-speed pulsed electro-acoustic system. To exclude the interferences caused by the discontinuity of relative permittivity, a method to distinguish between net dipolar charges and free charges is proposed. Through analysis, the influences of frequency, AC component and DC component on the charge transport and trapping characteristics in multi-layered epoxy composites are obtained, which could provide important reference to the structural design and optimization of converter transformer bushings.

Research on operation characteristics of UHV converter valve hall based on intelligent image processing and 3D modeling technology

There are many power equipments in the UHV valve hall, including the converter transformer, converter valve tower, all kinds of bushing, tubular bus. The operation condition monitoring of the above main equipment mainly includes electrical performance parameter measurement, oil and gas insulation medium performance parameter measurement and other means. This paper mainly introduces a monitoring method for the operation status of converter valve hall from the perspective of image processing technology. The technology mainly includes: 1) the application of intelligent image processing technology to identify and classify the database of infrared thermal imager and ultraviolet imager; 2) the real-time on-line measurement for the insulation distance of typical fittings through Kalman filter technology, including the measurement of the hottest temperature The insulation distance between partial discharge point and zero potential grounding point; 3) the electric field simulation model of typical main equipment in valve hall is established based on three-dimensional modeling technology of finite element method, and the surface electric field distribution of key main equipment fittings is obtained. Combined with the image database information, insulation distance information and typical main equipment electric field distribution information, the operation state parameters are effectively obtained, and the intelligent algorithm is applied to automatically evaluate its operation state, discover the latent fault and locate the positive fault, so as to provide effective data support and the protection strategy for the main equipment operation and maintenance.

Experimental Research on Partial Discharge of High Voltage Direct Current

Under the action of long-term high-voltage DC voltage, the insulation of electrical equipment is inevitably aging, and it is necessary to monitor its insulation status. Since insulation defects of different nature damage electrical equipment to different degrees, the identification of defects is very important. To this end, a DC partial discharge detection system based on AC partial discharge detection equipment was built, focusing on key issues such as the choice of test power supply, the choice of test frequency, and the choice of pressurization methods. The successful completion of the on-site partial discharge test of UHV converter transformers has provided valuable practical experience for the future on-site partial discharge tests of UHV converter transformers, and provided important tests for the establishment of on-site partial discharge test standards for UHV converter transformers. in accordance with.

Electro-thermal comprehensive analysis method for defective bushings in HVDC converter transformer valve-side under multiple-frequency voltage and current harmonics

The insulation failure mechanism of the resin impregnated paper (RIP) valve-side bushings of HVDC converter transformer under high ambient temperature and strong electromagnetic field conditions is explored in this study via a novel electro-thermal comprehensive analysis method. Considering that the bushings are operating under Multiple-Frequency Voltage and Current Harmonics, a superposition analysis method is established for voltage and current with five frequencies based on a Fourier spectrum analysis of the rated voltage waveform and full-load current waveform. Calculation methods for the heat loss, temperature field, and electric field of RIP bushings at the converter transformer valve-side are then established under two defect conditions: (1) overheating defect caused by an abnormal spring contact at the copper-aluminium conductive tube transition joint and (2) electric potential suspension caused by disconnection between the outmost condenser’s foil layer and grounded flange. An overheating defect in a valve-side bushing with spring contact at the conductive tube is successfully identified by the proposed method to prove its effectiveness. This method can be used to optimize bushing insulation and current-carrying structures as well as to diagnose insulation failures in RIP valve-side bushings.

Operation State Identification Method for Converter Transformers Based on Vibration Detection Technology and Deep Belief Network Optimization Algorithm

The converter transformer is a special power transformer that connects the converter bridge to the AC system in the HVDC transmission system. Due to the special structure of the converter transformer, it is necessary to test its operation state during its manufacture and processing to ensure the safety of its future connection to the grid. Numerous studies have shown that vibration signals in transformers can reflect their operating state. Therefore, in order to achieve an effective identification of the operation state of the converter transformer, this paper proposes a method for identifying the operation state of the converter transformer based on vibration detection technology and a deep belief network optimization algorithm. This paper firstly describes the background, principle and application of vibration detection technology, using vibration measurement systems with piezoelectric acceleration sensors, piezoelectric actuators and data acquisition instruments to collect vibration signals at different measurement points on the converter transformer in states of no-load and on-load. By analyzing the time-frequency characteristics of the vibration signals, fast Fourier transform (FFT), wavelet packet decomposition (WPD) and time domain indexes (TDI) are combined into a fused feature extraction method to extract the eigenvalues of the vibration signals, so that the fused eigenvectors of the signals can be constructed. Considering the excellent performance of deep learning in classification, the deep belief network is used to classify the signals’ eigenvectors. To effectively improve the network classification efficiency, the sparrow search algorithm was introduced to build a mathematical model based on the behavioral characteristics of sparrow populations and combine the model with a deep belief network, so as to achieve adaptive parameter optimization of the network and accurate classification of the signals’ eigenvectors. The proposed method is applied to a 500 kV converter transformer for experimental verification. The experimental results show that the fused feature extraction method was able to fully extract the features of the vibration signal, and the deep belief network optimization algorithm had higher classification accuracy and better operational efficiency, and was able to effectively achieve accurate identification of the operation state of the converter transformer. In addition, the method achieved a precision response to the detection results of the vibration sensors, contributing to future improvements in converter transformer manufacturing technology.

Noise Source Identification for a ±800kV Converter Station Based on Beam-forming Acoustic Imaging Technology

In this paper, a beam-forming acoustic imaging technology is applied to identify the spatial position and frequency characteristics of the main noise source on the key position of a ±800kV DC converter station. The test results show that the main noise sources of the AC filter field are the reactor group and the capacitor group, the noise of the reactor group has obvious peak value at 600Hz, and the noise of the capacitor group has obvious peak value at 100Hz. The main noise of DC filter field is from the reactor. The main noise source in the converter transformer area is the cooling fan array arranged outside the transformer. The maximum noise of the water cooling system is generated by the spray water on the bottom water pan of the cooling tower.