Investigations of ZnO microvaristor for stress control on polymeric outdoor insulators

This thesis is concerned with the investigation of the efficacy of Zinc Oxide microvaristor compound for stress control on polymeric outdoor insulators. The preliminary work has involved a comprehensive literature survey, followed by extensive computational modelling and simulation studies as well as laboratory works covering experimental investigations and fabrication of insulator prototypes. The literature survey reviewed stress-induced degradations as the cause of ageing and insulation failures, the determination of electric field distributions, considerations for outdoor insulator modelling, and field-optimisation techniques for achieving stress relief. An 11 kV polymeric insulator has been modelled and simulated under dry-clean and wet-polluted surface conditions in order to obtain electric field distribution along the insulator creepage path. The critical high field regions on polymeric surfaces were identified. In addition, clean fog solid layer tests were carried out to experimentally examine dry band formation and electric discharges. Experimental investigations confirmed the results previously achieved from theoretical simulations. A non-linear pollution model has been developed for simulating polluted outdoor insulators. The field-dependent conductivity was derived from layer conductance measurements in a non-standard low voltage test. The proposed model was used to simulate insulators under fog and light rain conditions which respectively represent a uniform and non-uniform wetting action in practice. It was demonstrated that the nonlinear pollution model yields a more detailed and realistic field distribution compared with results obtained with models using constant/linear conductivity. Short-length microvaristor coating, having a cone-shaped structure, was introduced at both insulator ends for controlling high field, particularly near the high voltage and ground terminals. The performance of field grading was evaluated through a number of simulation scenarios. The introduction of microvaristor material with an appropriate switching characteristic has led to a substantial improvement in the electric field and heat distributions along the insulator profile. The prototype of an 11kV insulator with microvaristor grading material was fabricated in-house for preliminary tests. Lightning impulse (1.2/50 μs) flashover tests were carried out using the ‘up and down’ method, and the flashover voltage was estimated by the 50% probability breakdown, U50. The results of the lightning impulse test have indicated a considerable increase in the flashover voltage up to 21% when using microvaristor-graded insulator. Favourable field distributions obtained in the simulation study have indicated a strong correlation with the experimental results.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Investigations of ZnO microvaristor for stress control on polymeric outdoor insulators

This thesis is concerned with the investigation of the efficacy of Zinc Oxide microvaristor compound for stress control on polymeric outdoor insulators. The preliminary work has involved a comprehensive literature survey, followed by extensive computational modelling and simulation studies as well as laboratory works covering experimental investigations and fabrication of insulator prototypes. The literature survey reviewed stress-induced degradations as the cause of ageing and insulation failures, the determination of electric field distributions, considerations for outdoor insulator modelling, and field-optimisation techniques for achieving stress relief. An 11 kV polymeric insulator has been modelled and simulated under dry-clean and wet-polluted surface conditions in order to obtain electric field distribution along the insulator creepage path. The critical high field regions on polymeric surfaces were identified. In addition, clean fog solid layer tests were carried out to experimentally examine dry band formation and electric discharges. Experimental investigations confirmed the results previously achieved from theoretical simulations. A non-linear pollution model has been developed for simulating polluted outdoor insulators. The field-dependent conductivity was derived from layer conductance measurements in a non-standard low voltage test. The proposed model was used to simulate insulators under fog and light rain conditions which respectively represent a uniform and non-uniform wetting action in practice. It was demonstrated that the nonlinear pollution model yields a more detailed and realistic field distribution compared with results obtained with models using constant/linear conductivity. Short-length microvaristor coating, having a cone-shaped structure, was introduced at both insulator ends for controlling high field, particularly near the high voltage and ground terminals. The performance of field grading was evaluated through a number of simulation scenarios. The introduction of microvaristor material with an appropriate switching characteristic has led to a substantial improvement in the electric field and heat distributions along the insulator profile. The prototype of an 11kV insulator with microvaristor grading material was fabricated in-house for preliminary tests. Lightning impulse (1.2/50 μs) flashover tests were carried out using the ‘up and down’ method, and the flashover voltage was estimated by the 50% probability breakdown, U50. The results of the lightning impulse test have indicated a considerable increase in the flashover voltage up to 21% when using microvaristor-graded insulator. Favourable field distributions obtained in the simulation study have indicated a strong correlation with the experimental results

Mixture of CO2-N2 as a potential to SF6 insulation gas: an overview

Each equipment in the high voltage (HV) system must retain an appropriate electrical insulation for protection and to avoid any of leakage current [1]. Thus, the choice of insulation material is essential. There are three types of insulation medium which is gases, liquids, and solids. Previous research claims that at normal temperature and pressure, gas insulation was an excellent insulator. An excellent gaseous dielectric material, however, required a long list of demanding properties, for example, having an utmost dielectric strength, good heat transfer, environmental-friendly, high thermal constancy, and available at low cost

Optimization of hot press forging parameters in direct recycling of aluminium chip (AA 6061)

This study introduces a new approach of direct recycling using the hot press forging process that eliminates the two intermediate processes of cold-compact and pre-heating. This method leads to low energy consumption without intervening the metallurgical processes. In this study, the optimum of machined chips from high speed milling is recycled by hot press forging. The mechanical properties and surface integrity of the different chips were investigated. The performance of recycled aluminium AA 6061 chips in the mechanical and physical properties were compared with the original aluminium billet. Response surface methodology (RSM) was used to develop mathematical model of the effects on pre-compaction cycle, holding time and suitable pressure significant to the process. It is hoped that, utilization of primary metal could be fully utilized by direct recycling technique (hot press forging) introduced in this study and at the same time developing a sustainable manufacturing process technology for future needs

Tensile strength of woven yarn kenaf fiber reinforced polyester composites

This paper presents the tensile strength of woven kenaf fiber reinforced polyester composites. The as-received yarn kenaf fiber is weaved and then aligned into specific fiber orientations before it is hardened with polyester resin. The composite plates are shaped according to the standard geometry and uni-axially loaded in order to investigate the tensile responses. Two important parameters are studied such as fiber orientations and number of layers. According to the results, it is shown that fiber orientations greatly affected the ultimate tensile strength but it is not for modulus of elasticity for both types of layers. It is estimated that the reductions of both ultimate tensile strength and Young’s modulus are in the range of 27.7-30.9% and 2.4-3.7% respectively, if the inclined fibers are used with respect to the principal axis

Lightning impulse breakdown voltage RBDPO with the presence of metallic particles under a uniform electric field

The performance of liquid insulation in power transformer is vulnerable to particles, especially the metallic particles. This paper presents the experimental study on the effect of metallic particles on the palm oil (PO) as dielectric insulating fluid under lightning impulse breakdown voltage. The type of PO used in this study is refined, bleached and deodorized Palm Oil (RBDPO) Olein. Two different types of metallic particles (copper and iron) with different concentrations (clean, low, medium, and high) were studied. The lightning impulse test has been carried out according to IEC 60897 standard and under the influence of a uniform electric field. For the comparative purpose, similar test has been carried out with mineral oil (MO). The presence of metallic particles reduces the average lightning impulse breakdown voltage of MO and PO but shows less significant effect to MO. This is because the streamers in the PO propagate faster and further than in the MO at the same voltage level. Hence, causing the breakdown voltage of PO lower than MO. Under negative lightning impulse, the breakdown voltage of MO is slightly higher than RBDPO. From the study, the increment of the number of particle level contamination will reduce the lightning impulse breakdown voltages of the PO

Analysis of the insulation characteristics of hexafluorobutene (C 4 H 2 F 6 ) gas and mixture with CO 2 /N 2 as an alternative to SF 6 for medium-voltage applications

This paper investigates C4H2F6, a promising environmentally friendly insulating gas that possesses high dielectric strength and a low global warming potential. The study focuses on examining the insulation properties of C4H2F6 when combined with CO2/N2, aiming to assess its suitability as a substitute for SF6 in gas-insulated applications. Finite element analyses are performed to evaluate the field utilization factor and electric field distribution in the proposed mixture. The properties of liquefaction temperature were examined in this study to determine the optimal mixing ratio for applications that require a minimum working temperature. Extensive experimental investigations were carried out to assess the dielectric strength characteristics of the gas mixture in both uniform and quasi-uniform electric fields. It was found that pure HFO-1336mzz (E) exhibits a dielectric strength approximately 1.2–1.6 times higher than SF6. Experimental results have revealed that the insulation performance of a 30% HFO-1336mzz (E)/CO2 mixture closely resembles that of SF6, with a matching efficiency of up to 90% in a weakly uniform electric field. This remarkable performance can be attributed to a positive synergistic effect between HFO-1336mzz (E) and CO2, combined with the gas mixture’s excellent self-recoverability property. These experimental findings are further supported by finite element analysis, which confirms the observed results. The 30% HFO-1336mzz (E)/CO2 gas mixture at 0.15–0.20 MPa pressure and constant 0.6 mm air gap reveal superior insulation tolerance and less sensitivity to the electric field, confirming its promising medium-voltage engineering applications. The associated results of this research provide a critical reference for the engineering application of the alternating (AC) and direct current (DC) insulation characteristics of the HFO-1336mzz (E)/CO2 gas mixture

The insulation performance of novel refrigerant gas as an alternative to SF6 for medium voltage switchgear

Gas-insulated systems are widely utilized in the electric power sector to transmit and distribute electrical energy. Sulphur-hexafluoride (SF6) has dominated gas insulation in high-voltage insulation technology since the early 60s. It is a greenhouse gas with a protracted lifespan in the atmosphere. This paper proposes an economical and comparatively more environmentally friendly R507 gas alternative to SF6 for medium-voltage applications. R507 has been analyzed experimentally through power frequency breakdown and lightning impulse testing to validate the performance and theoretical concepts. R507 has a very low liquefication temperature of −46.7∘C , but it must still be mixed with buffer gases such as CO2, N2, or dry air to meet the diverse liquefaction temperature applications. Various field utilization factors under AC and lightning impulse voltages are used in the experiments, along with different electrode geometries, including sphere-to-plane and rod-to-plane (i.e., quasi-homogeneous and inhomogeneous electric field distribution). For comparison, identical experiments are conducted with pure SF6. R507 gas was found to be a promising substitute for SF6 gas, with its dielectric strength being approximately 0.95 times that of SF6 gas. A positive synergistic effect is present between R507 and CO2, along with the good self-recoverability property of the gas mixture. The current research study serves as a fundamental resource for characterizing the R507/CO2 gas mixture insulation properties to be utilized in practical applications

Investigation of high voltage polymeric insulators performance under wet pollution

In this paper, a unique approach based on electrical characteristics observed from measurements of contaminated polymeric insulators was established to calculate the electric field distribution over their surfaces. A case study using two different 33 kV polymeric insulator geometric profiles was performed to highlight the benefits of the proposed modeling approach. The conductance of the pollution layer was tested to establish a nonlinear field-dependent conductivity for pollution modeling. The leakage current (LC) of the polluted insulator was measured in a laboratory under clean and wet conditions. Then, using the finite element method (FEM), the electric field and current density distributions along the insulator were computed. The results showed that the insulators experienced an increase in the electric field (EF) magnitude ranging from 0.3 kV/cm to 3.6 kV/cm for the insulator with similar sheds (type I) and 2.2–4.5 kV/cm for the insulator with alternating sheds (big and small, type II) under the high rain condition with a flow rate of 9 L/h. Meanwhile, the highest electric field under fog was 1.74 kV/cm for the insulator with similar sheds and 2.32 kV/cm for an insulator with alternating sheds. Due to the larger diameter on the big shed and the longer leakage distance on the insulator with alternating sheds, the EF on the insulator with alternating sheds is higher than the EF on the insulator with similar sheds. The proposed modeling and simulation provided a detailed field condition estimation around the insulators. This is critical for forecasting the emergence of dry bands and the commencement of flashover on the surfaces of the insulators