Ultra-high Voltage Transformer Research Articles

GÜÇ TRANSFORMATÖRLERİNDE MİNERAL YAĞLARA ALTERNATİF DOĞAL ESTERLERİN UYGULAMALARI

The electrical and environmental disadvantages of mineral oils, traditionally used as insulating fluids in power systems, lead to the search for alternatives. Natural esters produced from vegetable seeds are one of the most important alternatives. Natural esters with high dielectric strength can meet ultra-high voltage transformer design requirements. In addition, they can respond to environmental concerns caused by transformer oils with their biodegradability. This study first compares natural esters with other trans-former oils and explains why natural esters stand out. The basic properties of natural esters are defined in terms of advantages and disadvantages, and power system applications are exemplified. Esterification and synthesis of nanoparticle-added nanofluids, widely used in the literature to improve natural esters' electrical and thermal properties, are presented with experimental applications. Natural esters can have a better oxidation stability with the esterification process. The AC, DC, and lightning impulse (LI) break-down voltages of nanofluids based on natural esters also enhanced by 10% on average. This substation and experimental applications in which natural esters are used show that these environmentally friendly oils can provide insulation requirements in many power system equipment, especially transformers.

Analysis of harmonic characteristics of UHV AC / DC power network considering the influence of geomagnetic storm

The resultant harmonic effect of geomagnetically induced current (GIC) caused by geomagnetic storm has an impact on the safe and reliable operation of ultra-high voltage (UHV) AC / DC power grid in China, which in severe cases may lead to power grid breakdown or even large-scale blackout. Firstly, the harmonic effect model of converter transformer under the action of GIC is established based on the GIC calculation results of China's UHV power grid during the super-geomagnetic storm in November 2004. The harmonic voltage of transformer equivalent model port is described in phasor form. Secondly, the characteristic harmonic model of the converter is established based on the switching function modulation theory. By analyzing the working principle of the equivalent circuit of 12-pulse rectifier, the harmonic coupling model between DC magnetic bias harmonic voltage of transformer and converter is established, and the harmonic coupling characteristics of AC / DC power network under the action of GIC are analyzed. Lastly, the simulation model of ±800 kV UHV AC / DC transmission system in China is built by using PSCAD/EMTDC software as the simulation platform. The non-characteristic harmonic current and voltage on the AC side and DC side of the AC / DC power grid are calculated. The results show that the harm caused by GIC invading converter transformer is more serious than that caused by GIC invading UHV transformer. When both of them are invaded by GIC, the non-characteristic harmonics generated by mutual superposition and coupling have a greater impact on the power grid. The research results of this paper can provide a theoretical basis for space disaster prevention of UHV AC / DC power grid in China.

Retracted: Genetic Algorithm Integrated Fuzzy AHP-VIKOR Approach for the Investigation of W-Cut Insert Heat Exchanger for Cooling of Dielectric Fluid Used in Ultra-High Voltage Transformer

Retracted: Genetic Algorithm Integrated Fuzzy AHP-VIKOR Approach for the Investigation of W-Cut Insert Heat Exchanger for Cooling of Dielectric Fluid Used in Ultra-High Voltage Transformer

Topology Synthesis of Hybrid On‐Load Tap Changer in Ultrahigh Voltage (UHV) Transformer with Fast Arcless Switching

Topology Synthesis of Hybrid On‐Load Tap Changer in Ultrahigh Voltage (UHV) Transformer with Fast Arcless Switching

Retracted: High Efficiency Compound Vacuum Oil Gas Separation Technology Based on Integrated Oil Filter of Ultra-High Voltage Transformer

Retracted: High Efficiency Compound Vacuum Oil Gas Separation Technology Based on Integrated Oil Filter of Ultra-High Voltage Transformer

Seismic damage detection of ultra-high voltage transformer bushings using output-only acceleration responses

Seismic damage detection of ultra-high voltage transformer bushings using output-only acceleration responses

HFFB Test and Wind-Induced Vibration Analysis on 1 000 kV Transformer Frame

In order to propose a complete, wind-resistant design method for ultra-high voltage (UHV) transformer frames, the wind-induced vibration characteristics of a 1 000 kV transformer frame (TF1000) were studied using a high-frequency force balance (HFFB) test. Five section models and one whole model of the TF1000 were designed and constructed using 3D printing, and these were evaluated in a wind tunnel by means of HFFB tests for multiple loading scenarios. The finite element method (FEM) was used on the test data to analyze the wind-induced vibration on the TF1000. The results demonstrate that the shape factor of the TF1000 is significantly affected by the flow field type and solidity ratio; the minimum value occurs when the wind direction is between 30 and 45°. Moreover, all the shape factor values obtained by the test are larger than those established by the Chinese code. The wind-induced vibration analysis indicates that the most unfavorable wind direction for the TF1000 is approximately 60°, with a wind-induced vibration coefficient between 1,7 and 3,9.

Experimental Study on Influence of Soil–Structure Interaction on Seismic Performance of UHV Transformer

ABSTRACT Ultra-high voltage (UHV) transformer has a high seismic vulnerability. The transformers are usually directly connected to the shaking table to conduct an experiment to study its seismic performance, without considering the effect of the foundation soil on the upper equipment. In the present study, the transformer was scaled down based on the dimensional theory. Then, a soil box that can effectively eliminate the boundary effect is designed and a method that can simulate the site category is proposed. Finally, shaking table tests were conducted on rigid foundations and type I site for the scaled-down model, the seismic response of equipment under different site types is obtained. The results imply that under the rigid foundation, the transformer body response is larger with the seismic wave input in the X+Z direction compared to the seismic wave input in the X-direction; as the seismic acceleration increase, the acceleration and strain of bushings gradually ascend, and the response of bushing with Y direction earthquake wave input is significant than that with X direction earthquake wave input. Under type I site condition, with the earthquake excitation increase, the acceleration amplification effect for transformer body caused by the soil–structure interaction decreases gradually; the response of high voltage bushing under earthquake waves is more significant compared with medium and low voltage bushing; with the earthquake excitation increases, the acceleration of high voltage bushing is 1.13 to 1.40 times that of rigid foundation connection, and the strain response is 1.01 to 1.26 times that of rigid foundation connection. The research findings can be applied in the seismic field of UHV transformers to improve the level of seismic design.

High Efficiency Compound Vacuum Oil Gas Separation Technology Based on Integrated Oil Filter of Ultra-High Voltage Transformer

In order to improve the filtering effect of vacuum oil purifier, the author proposes the research and application of high-efficiency compound vacuum oil-gas separation technology based on electric ultra-high voltage transformer integrated oil purifier. The author studies under the condition of constant oil temperature and flow rate, the effect of vacuum degree on the filtering effect of transformer oil vacuum oil filter. Using the research results, a frequency conversion automatic vacuum oil filter is designed, which makes the processing process have the characteristics of simple operation, high degree of automation, and high work efficiency. The test results show that if the transformer oil processed by the variable frequency automatic vacuum oil purifier is new oil, its water content is less than 45 mg/kg and the gas content is less than 12%; when the breakdown voltage is greater than 25 kV, the transformer oil index can meet the following requirements after the equipment is filtered for one cycle: breakdown voltage ≥75 kV; water content ≤5 mg/kg; and gas content ≤0.1% (volume ratio). After one cycle (after 100 min), the variable frequency automatic (three-stage) vacuum oil filter can greatly shorten the time of transformer oil circulating and filtering. Conclusion. The designed frequency conversion automatic vacuum oil purifier is obviously better than other transformer oil vacuum oil purifiers.

Genetic Algorithm Integrated Fuzzy AHP-VIKOR Approach for the Investigation of W-Cut Insert Heat Exchanger for Cooling of Dielectric Fluid Used in Ultra-High Voltage Transformer

This study proposes a novel W-cut twisted tape to evaluate and enhance the effectiveness of novel grooved tube heat exchanger. The experimental investigations are carried out with W-cut inserts, which possess two main control factors, namely, twist tape ratio (y = 3.5 to 6.5) and width tape ratio (WR = 0.43 to 0.6). The investigation results prove that in all cases the energy transmission and friction factor of W-cut insert along with the first law and second law efficiency are considerably higher than the conventional tube arrangement. The empirical model equations for friction factor, performance enhancement ratio, Nusselt number, and rational efficiency are developed, and they show good affinity with experimental data. Since the abovementioned factors have conflicting terms, it is essential to select the proper operating configuration of the heat exchanger in order to increase the thermal energy transfer rate with reduced consumption of pumping energy. The multi-objective genetic algorithm (GA) tool is used to identify the optimum operating conditions. Further, the hybrid multi-criteria decision-making model, FAHP-VIKOR, is applied to select the perfect model from the set of non-dominated solution. The ranking of alternatives is as follows: A4 > A11 > A2 > A9 > A21 > A17 > A1 > A24 > A11 > A23 > A7 > A18 > A13 > A10 > A20 > A5 > A19 > A14 > A16 > A22 > A25 > A6 > A15 > A3 > A8. The optimized configuration of W-cut insert is compared with the former inserts, and the optimal configuration exhibits supreme performance than other inserts.

Forecasting thermal parameters for ultra‐high voltage transformers using long‐ and short‐term time‐series network with conditional mutual information

Precise forecasting of the thermal parameters is a critical factor for the safe operation and fault incipient warning of the ultra-high voltage (UHV) transformers. In this work, a novel multi-step forecasting method based on the long- and short-term time-series network (LSTNet) with the conditional mutual information (CMI) is proposed for the UHV transformer. To improve the computational efficiency and eliminate the redundancy, the CMI-based feature selection algorithm is applied to analyse the correlation between the original monitoring parameters and construct the optimal feature subset. LSTNet, which is composed of a convolutional layer, recurrent layer and recurrent-skip layer, is utilized to capture both the short-term nonlinear characteristics and the long-term periodic characteristics. The LSTNet model is established to forecast the variation tendency of the oil and winding temperatures for different locations in the UHV transformer. The results show that the proposed method significantly enhances the accuracy in both one-step and multi-step thermal parameters forecasting and achieves better performance in terms of the RMSE and MAE compared with other existing methods.

Effects of particle erosion on the submersible pump in an ultra-high voltage transformer

The submersible pump is the only power component of the ultra-high voltage (UHV) transformer which plays a key role in the cooling effect of the transformer. Metal particles produced by erosion and wear of submersible pump blades and walls can pose a serious threat to the transformer. Therefore, it is necessary to analyze the wear law of submersible pumps. In this study, the particle erosion of the submersible pump for solid–liquid flow is investigated numerically. Results show that the main erosion area occurs on the upper edge of the blades, and as the size and concentration of metal particles increase, the average erosion rate of each component also increases. The erosion patterns of blades and volute show asymmetry under the same conditions, the average erosion rate of each blade is related to its initial position, and the most severely eroded area of the volute occurs when the curvature angle of the volute is about 120°. When the particle size reaches 350 μm and the volume fraction is 0.4%, the average erosion rate of the blade reaches its peak. The outcome of this study can provide valuable reference data for the design of impurity monitoring of submersible pumps and proposing fault warning schemes.

Analysis and Evaluation of the DC Magnetic Bias for 1000kV Ultra-High Voltage Transformer Caused by Changsha Rail Transit

In this paper, the stray current distribution network model of Changsha rail transit system is constructed from the perspective of the stray current induced by rail transit in the adjacent substation. We evaluated the degree of influence of the stray current of urban rail transit lines after the completion of the Changsha 1000 kV UHV substation, which provided a reference for the UHV transformer DC bias prevention experience.

Controlled Shunt Reactor for UHVAC System Reactive Power Control

Background: In this paper, a novel technique for reactive power management of the Ultra High Voltage (UHV) lines having voltage level of 1200 kV line and above is suggested. The UHV power grid has to frequently face the problems associated with the power- frequency and switching over-voltages. Methods: The technique involves the use of three-phase UHV Transformers as a Controlled Shunt Reactor (CSR) in combination with the use of a fixed reactor. The performance of the UHV AC transmission line is studied using PSCAD software and working of CSR is evaluated. Results: Standard parameters of the proposed Wardha-Aurangabad 1200 kV transmission line in India are considered for simulation model. The use of CSR has been found to be effective in mitigating system problems. Conclusion: The system can be used for maintaining voltage profile, resulting in enhancement of the reliability of the UHV transmission system.

Analysis of the DC Bias Effects on the UHV Autotransformer with Rated Load Based on a Reduced-Scale Model Experiment

The DC bias of transformers as a result of geomagnetically induced currents (GIC), monopole operation of high voltage direct current (HVDC) transmission and the increasing power electronic devices in the power grid, creates a high magnitude of fundamental and harmonic components in winding currents, reactive power, and vibro-acoustic noise. For transformers with large capacity, it is very expensive and difficult to carry out an in-depth DC bias effects investigation on a practical equipment, especially for the ultra-high voltage (UHV) transformer. In this paper, a reduced-scale model (RSM) based on the principle of identical saturation degree is proposed, and the equivalent relationship of DC bias current is obtained. Furthermore, the DC bias platform with the RSM is established, experiments have been conducted to analyze the characteristics of the transformer under DC bias. It turns out from the experiments that the current of high voltage side and excitation are less tolerated to the DC bias current, the distortion occurs during the first half of the cycle and peaking at the quarter. Besides, the no-load loss and total harmonic distortion (THD) increase significantly with an increasing DC bias. The results of the characteristics of RSM with load under DC bias may provide great services in protecting and reducing the DC bias effects on the UHV transformer in actual operation.

The impacts of nanotechnology on the improvement of liquid insulation of transformers: Emerging trends and challenges

The rise in transmission voltages levels, the demand for insulating reliability of the transformer is getting more and more critical. The mineral oil (MO) together with paper is the main insulation constituents in oil-immersed transformers. To enhance the insulation level of ultra-high voltage (UHV) transformers and to decrease its weight and size, it is highly required to increase the insulating conduct of transformer oil (TO) and oil-impregnated cellulose. Lately, a distinctive novel venture of application of nanotechnology with liquid insulation of transformer has been conducted and the results have displayed marvelous enhancement in the insulation properties of transformer oil. The transformer oil-based nanofluids research domain (i.e., NFs) has been attracting huge attention during recent years both in theoretical and practical fields. NFs may be applied as a potential substitute for MO in the near future. To recognize various brunts of NFs and show evolving developments and challenges, the study demonstrates a scientometric review established on 210 published features from 2004 to 2019 through co-author, co-occurring and co-citation investigations. The significance of the editorial lies in compiling former inquiries, categorizing research nature, motivations and forecasting impending research orientations. Lastly, some recommendations for future NFs research and application are suggested.

Generation of Impulse Voltage Waveforms for Tesing of UHV Transformer: A Study

According to IEC 60060-1, 2010 and IEEE 4-2013 standards, the standard lightning impulse (LI) voltage waveforms are needed for testing the insulation of large equipment. The generation of these impulse waveforms sometimes difficult due to presence of internal inductance (Ls) in impulse generator (IG) circuit, the level of test object capacitance (C2), and the level of test object inductance (Lt). Thus, a new procedure is proposed for identifying the limit of wavefront resistance (R1) to generate required impulse waveforms (i.e., rise time (tr ≤ 1.56 µs) and overshoot rate (β'≤5%)) when the parameters Ls, C2, and Lt of fourth order IG circuit are systematically varies. Another attempt has also made in this paper to determine the limit values of the test object parameters such as load resistance Rt and load inductance Lt for matching the response of fourth order to third order IG circuit. Further, the analysis performed in this paper will be helpful in generating the test impulse voltage waveforms for testing ultra-high voltage (UHV) transformer.

GIC Influence on UHV Power Grids Based on Kalman Filter and WAMS Data

Geomagnetically induced currents (GICs) flowing through transformers affect their normal operation and can lead to accidents in power grids. The reactive power loss caused by GICs is called GIC-Q. At present, geomagnetic data are usually used to calculate GICs in research. These methods do not have real-time characteristics and cannot be used for real-time research on GICs. Response mechanisms for GICs in 1000 kV ultra-high voltage (UHV) transformers and for the effect of GIC-Q on large power grids are urgently required to prevent damage. Based on the principle of Kalman filter, this paper presents a model and method of calculating and analyzing GIC disturbance using the reactive power measured by wide area measurement systems (WAMS). The proposed method exhibits superior characteristics for real-time analysis and solves the problem of real-time shortage in GIC research. The model provides a theoretical calculation of GIC-Q disturbance in a 1000 kV autotransformer. Further, this method was verified according to the measured reactive power in three 1000 kV substations in Shandong Province and the measured geoelectric field at the Anqiu geomagnetic observatory station on September 8, 2017. The calculation results show that the reactive power measured by phasor measurement units (PMUs) can be effectively used to calculate and analyze GIC-Q in the transformers, which is significant to prevent disasters in the power grid caused by magnetic storms through dispatch and operation.

Effect of Al2O3 nanorods on the performance of oil-impregnated pressboard insulation

The mineral oil (MO) in conjunction with paper is the main insulation components in oil-immersed transformers; their insulation properties play a significant role in the safe and stable operation of power transformers. To strengthen the insulation level of ultra-high voltage transformer and to reduce its size and weight, it is imminent to enhance the insulating performance of transformer oil and oil-impregnated cellulose. Recently, a unique novel effort of suspension of nanorods (NRs) into MO has been carried out and the results have exhibited improved insulation characteristics of transformer oil. The Al2O3 NRs with favorable features were prepared in our laboratory to develop transformer oil/paper insulation system with better insulation performance. The transformer oil-based nanofluids (NFs) were prepared with nanorod shape, oleic acid surface modification and 0.8 g/L concentration of NRs. The impregnated pressboards were prepared by impregnating them into dried oil and NF under vacuum beneath 1 kPa at 80 °C for almost 48 h to obtain the oil-impregnated pressboard (OIP) and nanofluid-impregnated pressboard (NIP), respectively. The interface at solid/liquid is considered weak link and is the main reason of oil-filled transformer collapse. The effect of Al2O3 NRs on creeping discharge and flashover traits of oil/pressboard (OP) interface under AC and impulse voltages were studied. Partial discharge and creeping flashover test of OIPs and NIPs were conducted. TSDC and PEA tests were applied to examine the space charge properties of OP interface before and after suspension of Al2O3 NRs.

A Validity Analysis of Extending Front Time in Lightning Impulse Tests for Ultrahigh-Voltage Transformers

High entrance capacitances of ultrahigh-voltage (UHV) transformers make it difficult to generate impulses with standard front time during the lightning impulse tests. In this paper, we discuss the feasibility of examining the longitudinal insulation design of a transformer via impulses with a front time longer than the standard value. We consider the following features: transient voltage distributions and insulation characteristics of longitudinal insulation. The transient voltages of two UHV windings are measured with a capacitive voltage sensor under impulses with different front times. The results demonstrate that the front time of the applied impulse has little effect on the gradient voltage distributions and the oscillatory component of the transient voltage, but it determines the front time of transient voltages. Therefore, the relationship between breakdown characteristics of the turn-to-turn insulation and the front time is studied. Experimental results revealed that whatever the insulation thickness, an increase in the front time of the applied impulse decreases the breakdown voltage, but this variation is within acceptable tolerance when the front time changes from 1.2 μ s to 6 μ s. Thus, as long as the front time is restricted within 6 μ s, we can ensure the validity of lightning impulse tests on the examination of longitudinal insulation designs.