Improving IEC thermal model for oil natural air natural transformers using optimised parameters based on dynamic simulation

Accurate assessment of hot-spot temperature is essential for the safe operation of power transformers.Existing dynamic thermal models cannot estimate hot-spot temperature accurately since some input parameters are roughly determined by transformer capacity and cooling mode while ignoring the effect of winding structure,tank dimensions,and material physical properties.To improve the accuracy of temperature assessment,empirical parameters of the IEC thermal model including thermal constants,winding and oil exponents are optimised with the help of numerical simulation in this article.Based on energy conservation and heat transfer theory,a computational fluid dynamic(CFD)model of a transformer in oil natural air natural(ONAN)cooling mode is established.This CFD model simulates the entity's structure,sizes,and multi-stage heat dissipation processes realistically,so it can more precisely calculate the dynamic hot-spot temperature.According to the simulated temperature curves at different operating conditions,the thermal constants and oil exponent are estimated using non-linear regression,and the winding exponent is optimised using linear regression.A case study is conducted on an ONAN transformer.It shows the improved IEC model with optimised parameters can more accurately evaluate hot-spot temperature,and the absolute error is decreased by 2.4 K(38.7%)compared with traditional thermal models.

Experimental and simulation study on luminous characteristics of Trichel discharge in air at picosecond time resolution

The optical morphology of discharge is the main characteristic parameter describing the physical process of the discharge,and the luminescence behavior is inextricably linked to the motion behavior of charged particles.Since the Trichel discharge in air is a weak ionization event occurring on the sub-nanosecond time scale,micron space scale and accompanied by weak luminescence,the current experimental observation means have a low spatial and temporal resolution,which is insufficient to study the details of its luminescence properties clearly.To address this problem,the intensity of light generated by Trichel discharge in air is first measured and resolved in time and space using a constructed ultrafast optical counting and imaging system.At a very high temporal resolution of 55 ps,it is found that both its light emission and current pulse showed a trend of first rapid growth and then slow decline on the time scale of tens of nanoseconds.Later,in order to further investigate the connection between the microscopic behavior of charged particles and photons,a two-dimensional hydrodynamic simulation model with three components(electrons,positive ions,and negative ions)is developed and the current simulation results are in good agreement with the experimental results.The results show that the size of the ionization region and the interaction between particles in the discharge process are the main factors affecting the Trichel pulse and its photoemission.Electrons play a dominant role in photon generation during the rise time process,while it is the recombination process of positive and negative charges,which contributes the main photons during the fall time process.Finally,it is proposed that the expression of photon flux should be amended to include an additional term responsible for the photons generated due to recombination.The study helps to advance the understanding of the luminescence characteristics and the discharge mechanism of Trichel discharge.

Method of GIL partial discharge localization based on natural neighbour interpolation and ECOC‐MLP‐SVM using optical simulation technology

Partial discharge(PD)is one of the main causes rendering the deterioration of the insulation state in gas‐insulated transmission line(GIL).Accurate and timely localization of the PD source is essential to ensure the safe and stable operation of the GIL.At present,optical PD detection technology shows advantages in terms of high sensitivity and strong anti‐inter-ference performance.Therefore,an optical PD localization method based on optical simulation fingerprint database is proposed.The introduction of simulation conquers the difficulty of obtaining a PD fingerprint database in field experiments.This method con-structs a simulation fingerprint database by performing PD simulation in a GIL simulation model of the same size as the actual GIL.The natural neighbour interpolation algorithm is applied to expand the simulation fingerprint database to cover all locations in the GIL.The two‐level localization method proposed is to match the PD fingerprint to be tested with the expanded fingerprint database,which can reduce the amount of calculation while main-taining the localization accuracy.The experimental results show that the average localiza-tion error of this method is only 9.7 mm,and the localization time is reduced by about 11 times compared with the normal one‐level localization method.

Experimental research on the influence of temperature on the discharge signal of void defects in GIS

Temperature is an important environmental factor during the operation of gas‐insulated switchgear(GIS),affecting the evaluation results of the GIS equipment to increase the risk of the power system.However,the influence of temperature on the partial discharge detection signal of GIS is still unclear.Aimed at the common void defects in GIS,the law of change on the number of ultrahigh‐frequency(UHF)pulses,the UHF amplitude,the characteristic value of the UHF map,and the maximum apparent charge of a single pulse with temperature are obtained using the UHF method and IEC60270,and corresponding theoretical analysis is carried out.The results show that an increase in temperature leads to a decrease in the void discharge delay time,causing an increase in UHF pulses and a decrease in the apparent charge of a single discharge pulse in the experiment.The in-crease of temperature makes the void discharge current rise quickly so that the induced UHF amplitude increases.In the range of 40-70℃,the maximum pulse amplitude in-creases by approximately 30%for every 10℃ increase,and the average pulse amplitude increases by approximately 12%.The result of UHF signals affected by temperature obtained in this study has research significance for the realisation of a comprehensive evaluation of the insulation state of GIS equipment considering temperature.

Intelligent learning approach for UHF partial discharge localisation in air-insulated substations

To achieve comprehensive insulation deterioration motoring of power equipment and early fault warning in air-insulated substations,a data-driven partial discharge(PD)source localisation method employing noisy ultra-high frequency(UHF)received signal strength indicator(RSSI)and particle filter is proposed in this study.Compared with the existing UHF time-difference-based techniques,UHF wireless sensor arrays and RSSI-based methods provide an economical and high-adaptability solution.However,owing to the multi-pathing and shadowing effects,UHF signal attenuation cannot be modelled.Therefore,a Kalman filter was employed to smoothen the RSSI signal.Furthermore,a semi-parametric regression model is proposed to achieve a more accurate relationship between the RSSI and the transmission distance.Finally,in contrast to traditional localisation algorithms directly based on the RSSI ranging model,a particle filter was used to achieve higher accuracy.It predicted the best distribution of the position of PD by learning and considering all the system states of the previous moment.The laboratory test was performed within an area of 6 m×6 m,and the results demonstrate that the mean PD source localisation error was 1.16 m,which gives a potential application for the identification of power equipment with insulation deterioration in a substation,while the accuracy is still needed to be verified further by field tests.

Penetrating power characteristics of half-wavelength AC transmission in point-to-grid system

With the proposed concept of global energy interconnection, half-wavelength alternating current transmission(HWACT) technology becomes of more interest.HWACT lines can be adopted to establish a pointto-grid system, in which the penetrating power(PP) is produced between receiving terminals, having a significant effect on the power flow distribution.In order to investigate this phenomenon, the PP characteristics of the HWACT system are researched in this paper.First, the mathematical relationship between the transmission power and terminal bus voltages of a single HWACT line is derived using the equations of a distributed parameter model.The research indicates that the relationship between power and terminal voltages shows ‘‘reverse characteristics"opposite to those of regular short transmission lines.Then, the concept and definition of PP in a point-to-grid system with two receiving terminals are proposed, and the corresponding relationship between PP and the terminal bus voltages is derived.Simulations are carried out to validate the theory under different conditions, so that the accuracy and adaptiveness of the theoretical analysis can be proved.In addition, the results demonstrate that selecting the location for a HWACT system has demanding requirements in order to control the value of PP.

Application of multi-source information fusion based on D-S evidence theory in insulation defect identification of DC XLPE cable

During the installation and laying process of cross-linked polyethylene(XLPE)cable,it is difficult to avoid the influence of external harsh environment,which leads to insulation deterioration and failure.The partial discharge and leakage current play an important role in formation mechanism,characteristic law and identification tech-nology of DC XLPE cable defect.However,the defect identification accuracy of single source data is limited.The correlation between partial discharge and leakage current of typical defect is analysed here.Based on the shape characteristic param-eters of typical DC partial discharge spectrum and the energy characteristic param-eters of leakage current wavelet decomposition node,a weighted Dempster-Shafer(D-S)evidence theory is used to fuse and identify different information sources.The results show that the classification rate of the proposed method can reach 88%,which can effectively identify insulation defects.

An array virtual extension method for power equipment acoustic emission direction of arrival estimation based on high‐order cumulants

The performance of array‐based power equipment acoustic emission(AE)source direc-tional of arrival(DOA)estimation methods is decided by not only algorithm performance,but also the array hardware parameters,that is,array aperture,sensor number,and so forth.Improving the estimation performance by hardware approaches always leads to the increase in cost.In this study,an array virtual extension method based on high‐order cumulants is proposed which can improve the sensor array performance without the need for hardware change or extra investment.Compared to low‐order cumulants,the high‐order cumulants established from Taylor expansion contain higher order terms which are omitted in low order cumulants.Therefore,they have more detailed directional information about the signal source to improve the DOA estimation performance.Moreover,because high‐order cumulants contain information only for the non‐Gaussian components,the array after extension shows better anti‐Gaussian noise performance.Depending on the establishment method of the high‐order cumulant,two types of extension including aperture extension and sensor number extension are proposed for different application scenarios.Simulation and field tests results show that the array directional resolution is improved around 40%by aperture extension and the spectrum sidelobe level is reduced about 14 dB by sensor extension.