63 research outputs found
Performance evaluation of modified turbulent models in simulating arc quenching process in circuit breakers
Full text linkGas circuit breakers rely on rapid gas expansion and turbulent mixing within a converging–diverging nozzle to effectively cool and extinguish arcs during current interruption. The transientarc interruption process involves coupled physical phenomena, including turbulent flow, electric current, convection and radiation. Selection of a suitable flow model is vital in obtaining simulation results matching the experimental data. In this study, the transient behaviour of the SF6 flow during the current ramp-down phase is investigated. Performance of the standard k-ϵ turbulence model is compared with modified k-ϵ models, in which the turbulence dissipation constant C1ϵ is systematically varied to identify the effect on the cooling process. Simulation results demonstrate that the adjustment of C1ϵ can improve the arc cooling process by enhancing turbulent kinetic energy, resulting in maximum rate-of-rise of recovery voltage (RRRV) estimation values similar to the experimental conditions.
The Effect of Gas Type on Flow Characteristics in a Circuit Breaker under Cold Flow Scenario
No full textThis paper presents first stage of supersonic flow modelling in gas circuit breakers without an arc. Flow characterisation focused on shock and flow separation phenomenon. Velocity deceleration caused by shock will play a significant role in determining arc cooling performance and will impact thermal interruption capability. Gas properties such as specific heat ratio, density, and viscosity influenced the flow characteristics including shock location, strength and the flow separation process
APPLICATION OF PRINCIPAL COMPONENT ANALYSIS FOR POWER TRANSFORMER ASSET MANAGEMENT
No full textMeasurements from insulating liquid are utilised to indicate the health of power transformer insulation systems. Due to various dynamics within the transformers, assessing the transformer health through condition monitoring data and their risk profile require input from transformer experts. This paper investigates the application of Principal Component Analysis (PCA) as an automated tool in ranking transformers based on their condition monitoring data with minimum expert input. Oil test data obtained from a transformer fleet over a five-year span was used for the analysis. For each transformer, a single value for each measurement was obtained by using the 90th percentile data from the five-year measurement span. Furthermore, multiple imputation was applied for transformers with missing data. The rankings obtained from PCA was compared against the ranking developed based on expert knowledge. Results indicated that PCA has potential to identify transformers with poor health condition with minimum expert input.</p
Effect of Applied Electric Field on Conductivity of Synthetic Ester Impregnated Kraft Paper
No full textSynthetic ester has emerged as an alternative to mineral oil due to its environmentally friendly and fire safety features. This paper assesses the applicability of synthetic ester in converter transformers from the perspective of conductivity of synthetic ester-impregnated paper, which affects electric field distribution and hence the design of converter transformers. Experimental studies unravel the effect of applied electric field on conductivity of synthetic ester-impregnated paper, i.e. a positive correlation exists between the short-term conductivity and applied electric field, whilst the long-term conductivity exhibits a non-linear trend. Key parameters describing space charge transport in synthetic ester-impregnated paper are identified and compared with those of mineral oil-impregnated paper. Simulations allow the intrinsic and injected charge carriers of synthetic ester-impregnated paper to be quantified, and reveal space charge phenomena under different applied electric fields and their effects on conductivity
Evaluation of Physicochemical and Dielectric Properties of Mineral oil – Natural Ester Mixtures under various Ageing Conditions:Implications for Retrofilled Transformers
No full textRetrofilling transformers with natural and synthetic esters has gained traction as a strategy to enhance the safety and performance of electrical systems because of the superior electrical and fire properties of esters. While prior studies have extensively analyzed the properties of fresh mineral oil - ester mixtures, they do not fully account for the aged conditions typical in retrofilled transformers, where mineral oil has undergone prolonged use and aging. This study investigates the dielectric and physicochemical properties of mineral oil and natural ester mixtures, including dielectric permittivity, dissipation factor, AC breakdown voltage, water content, and acidity. The findings provide valuable insight into the behavior of the liquid insulation of retrofilled transformers under various conditions, helping to optimize the retrofilling processes and improve transformer performance.</p
SIMULATION OF SUPERSONIC FLOW IN HIGH-VOLTAGE CIRCUIT BREAKERS
No full textHigh-voltage circuit breakers (HVCB) are critical protection and control equipment in the electric transmission and distribution system. Analysis of the arc quenching process in the nozzle through modelling and simulation is a key stage in the development process of the HVCB. This paper introduces the first stage of simulation in which the flow is simulated without an arc, which helps to investigate the characteristics of supersonic flow. A model built based on an experimental study was used in the study. Characteristics of the flow including the development of shock and flow separation from the nozzle wall and the electrode surface were observed. Furthermore, the investigation of Mach number and shock strength can help identify flow characteristics. The impact of inlet pressure and the location of the downstream electrode on the Mach number distribution and pressure distribution were analysed. The increase in inlet pressure resulted in an increase in the shock strength. Also, the results show that the position of the downstream electrode has a significant impact on the flow behaviour.</p
Simulation of Flow Characteristics in High-Voltage Circuit Breakers
No full textGas-insulated high voltage circuit breakers are used in the power network to protect the equipment and to isolate the system from the fault. Modelling and simulation are nowadays a key part of the HVCB development process, and understanding the arc quenching process in the nozzle through modelling is important. This paper investigated the first stage of arc quenching process. A comprehensive COMSOL Multiphysics model was developed, in which flow pattern in the nozzle and downstream is investigated without arc condition. Characteristics of supersonic flow including shock position, Mach number distribution and pressure distribution were studied. Results show that the flow behaviour is influenced by gas type, position of downstream electrode and the outlet geometry.</p
Effect of Electric Field on Conductivity of Synthetic Ester Immersed Kraft Paper
No full textRecent decades have witnessed rapid global application of the HVDC system in long distance electrical energy transmission or interconnection of different power networks. However, the failure rate of the HVDC system is higher than the conventional AC system. The high failure rate is mainly attributed to the malfunction of converter transformers under the AC plus DC working condition. Synthetic ester has emerged as an environmentally friendly alternative to traditional insulating liquids for their biodegradable/environmentally friendly nature and fire safety feature. Also, research indicated their potential of prolonging the lifetime of paper insulation. This paper assesses the applicability of ester in converter transformers from the perspective of conductivity, which is a crucial parameter for the design and maintenance of converter transformers. Experimental studies were designed to unravel the effect of electric field on the conductivity of the ester immersed Kraft paper. The test results indicated a positive correlation between short-term conductivity and the electric field, while long-term conductivity exhibited a non-linear trend.</p
Effect of Oil Regeneration on Improving Paper Conditions in a Distribution Transformer
No full textManaging a large fleet of ageing assets has become a technical challenge faced by many electricity utilities in developed countries. Asset managers are increasingly interested in techniques that can help extend the useful lifetime of a transformer. Oil regeneration is one of such techniques. In this paper, oil regeneration experiments were performed on a 6.4/0.4 kV retired distribution transformer to investigate the effect of oil regeneration on improving paper conditions. Oil regeneration was conducted in two stages, with the first stage aimed at ‘cleaning the oil’ and the second stage targeted at ‘cleaning the paper’. Oil samples were collected at regular intervals throughout the process and paper samples were obtained from the transformer before and after each oil regeneration stage. It was found that oil regeneration restores oil parameters, including moisture and acidity, similar to those of new oils at the end of stage 1. Analysis of paper samples indicated a reduction in paper moisture at the end of stage 2 by nearly 40%, while low molecular weight acids (LMA) in paper exhibited a reduction by around 30% on average. It is found that the extended oil regeneration period, i.e., stage 2, is necessary to improve the paper condition and hence to reduce the paper ageing rate
Ageing Assessment of Transformer Paper Insulation through Polarization-Depolarization Current Measurement
No full textCondition assessment of insulation systems is essential for reliable and safe operation of power transformers. Polarization-depolarization current (PDC) measurement is a non-destructive technique that has been applied for assessing the ageing condition of insulations. Modelling the dielectric response of insulations can provide better understanding of correlations between measured properties and dielectric processes occurring inside insulations. In this paper, a circuit model is proposed for condition assessment of liquid-impregnated Kraft paper based on PDC measurement. The model can represent the insulating materials separately and thus it can reflect the conditions of insulating liquid and Kraft paper. The model was applied on depolarization currents of aged Kraft papers of different degree of polymerization (DP) impregnated with fresh synthetic ester liquid. While calculated conductivity showed an inconsistent trend with ageing, the R-C parameters associated with large time constants deduced from the proposed model followed a trend with the ageing of paper insulation.</p
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