Lightning breakdown voltages of refined bleached and deodorised palm oil under needle-plane configuration

In recent years, vegetable oil such as Palm Oil (PO) has been identified as a potential alternative dielectric insulating fluid for transformers. It is biodegradable, non-toxic and has high flash and fire points. In this paper, a study on the positive lightning impulse breakdown voltages of PO under non-uniform field is carried out. The testing was carried out using needle-plane electrodes configuration at gap distances of 25 mm and 50 mm. Rising voltage, 1 and 3 shots per step testing methods were used and 3 types of Refined Bleach and Deodorized Palm Oil (RBDPO) and Mineral Oil (MO) were examined. It was found there is no significant effect on the breakdown voltages of all samples. The breakdown voltages of all RBDPO at 50% probability are comparable with MO. At 1% probability and gap distance of 50 mm, the breakdown voltages of all RBDPO are lower than MO

Top-oil temperature model for transformers based on nonlinear thermal resistance, lumped capacitance and thermal-electrical analogy

Top-Oil Temperature (TOT) is one of the basic components to estimate the Hot-Spot temperature (HST) of the transformers. This paper presents an alternative TOT model based on the heat transfer theory that utilises Nonlinear Thermal Resistance (NTR) and Lumped Capacitance (LC) approaches. It is applied in a thermal-electrical analogy and the heat transfer equivalent equation is determined. This model is tested on a measured TOT of 250 MVA ONAF and 400 MVA ONAF transformers obtained from IEC 60076-7 and previous research. A comparison of TOT is carried out with the existing models IEC 60076-7 exponential and IEEE Loading Guide clause 7 methods. It is found that the thermal model based on the NTR and LC approach could determine the measured TOT closer than the existing methods available in the standards

Lightning study and experience on the first 500kV transmission line arrester in Malaysia

Being in one of the highest region of lightning is truly a big challenge to the utilities operating transmission and distribution lines where lightning related cause of tripping has become the major concern every year. Tenaga Nasional Berhad (TNB) in Malaysia is one of those utilities facing the huge challenge in ensuring the line performance is maintained so as to conform to the National Grid requirement. In 2011, the management has addressed their major concern which was the line outages due to lightning on 500kV lines. This is an extraordinary case as commonly lightning caused tripping normally affects lower voltages with lesser insulation. Extensive studies were conducted on several selected 500kV lines and solutions were being proposed in the studies. Besides technical evaluation using software simulation and historical analysis, cost effect in applying possible solutions were also studied. This is to ensure that the best line performance will be achieved with the most cost effective solution to the company

Development of palm - based neopentyl glycol diester as dielectric fluid and its thermal aging performance

The potential of palm-based neopentyl glycol diester as dielectric insulating fluid was investigated. The details of the transesterification of high oleic palm oil methyl ester (POME) with neopentyl glycol (NPG) with the final product yield of more than 90 wt% of NPG diester were discussed. The thermal aging performance of NPG diester was compared with conventional mineral insulating oil at 90, 110 and 130 °C. This paper focused mainly in the effects of aging to chemical, physical and electrical properties of NPG diester. Apart from being fully biodegradable and non-toxic, the synthesized diesters exhibited high flashpoint and the breakdown voltage was comparable to mineral oil. The result indicated that throughout the aging period, NPG diester exhibited lower acid value than mineral oil and no significant change in viscosity was observed. The study on mechanical properties of insulating paper aged in NPG diester shows higher tensile strength than paper aged in mineral oil. The synthesized esters have shown great potential to be used as transformer oil

Investigation on the effect of moisture on AC breakdown voltage of refined, bleached, and deodorized palm oil

The effect of moisture on the AC breakdown voltage of palm oil was studied in this paper. The type of palm oil used in this study was Refined, Bleached and Deodorized Palm Oil (RBDPO) olein. RBDPO with different moisture contents were prepared by oven drying method. The AC breakdown voltage measurement was carried out based on ASTM D1816 at a gap distance of 1 mm. The AC breakdown voltage data were analyzed statistically by normal, lognormal and Weibull Cumulative Distribution Function (CDF). It is found that the AC breakdown voltage of RDBPO decreases almost exponentially as the moisture content increases. Weibull CDF can represent most of the AC breakdown voltage data of RBDPO at different moisture content

Assessment of transformer health index using different model

Transformer failures lead to interruption of power supply. Therefore, asset management is important to monitor the efficient functioning of transformers. An important approach in asset management is condition assessment whereby the health status of the transformer is assessed via a health index. There are many methods in determining the final value of a health index. This paper examines how different assessment methods can be used in order to come up with the final health index and output of final health index. The output trend shapes are almost the same for Assessment Model A, B and C except for Assessment Model D. There is no strong correlation between the health index and age of the transformer. Generally, the value of health index of the transformer is reflected by its operation and loading history .This paper hence examines the assessment steps and results that will guide the development of a new approach to determine health index value

Effect of soil resistivity on magnetic field in the case of lightning strike to a tall structure

When a tower is struck by lightning, an induced voltage can generate in conducting objects within its vicinity. This phenomenon is due to the interaction between the electromagnetic field of the lightning with the object. The magnetic field is part of this field. Previous studies have shown that the magnetic field is affected by ground reflection coefficient factors that are usually taken as constants. However, the ground reflection coefficient factors have correlation with tower and ground impedance whereby the ground impedance can be determined based on soil resistivity and grounding system. This paper investigates the effect of soil resistivity on the value of magnetic field for a case of lightning strike on tall structures. Lightning current has been modelled accordingly and a set of equations is proposed to evaluate the magnetic field. Also, the effects of soil resistivity on the value of ground reflection coefficient factor consequently of the magnetic peak have been investigated. The results indicate that the magnetic field changes substantially with a variation of the soil resistivity. The magnetic field peak also has a strong correlation with the changes of soil resistivity as well as the fast front time of the channel base current. The outcome of the result of this paper will be beneficial to utility companies in determining the appropriate level of lightning protection since the magnetic field will be a result of induced voltage when have interaction with power lines

Effect of the grounding system arrangement on the lightning current along tall structures

The behaviour of lightning current along tall structures is an important case to be evaluated. The behaviour is influenced by a few factors of which one is the ground or earth reflection factor. Previous studies have assumed that the ground reflection factor is a constant value and this value is usually used when evaluating the lightning current along a tall tower. This study presents the behaviour of lightning current along a tower, including variation in the value of the ground reflection factor. The simulation work is done by selecting the sum of two Heidler functions as a current channel base and a distributed source representation current is used to simulate the lightning current along the tower and channel. The ground reflection factor is calculated based on the correlation between tower and ground impedances whereby the ground impedance is dependent on a circle grounding system arrangement. The results indicate that by increasing the length of the rods, the value of the ground impedance decreases, but the peak value of the current along the tower is more dependent on the value of the ground reflection factor. It is shown that by increasing the length of the rods, the current peaks increase. Thus, the outcome of this study may be of benefit to Electrical Engineers to set an appropriate scheme of protection for the equipment which is installed along the tower, based on the effect of the grounding system arrangement for different conditions of soil resistivity

Investigation on the effect of beating on the physical and mechanical properties of untreated kenaf based insulation paper

This paper presents a study on the effect of beating on the physical and mechanical properties of untreated kenaf based insulation paper for application in transformers. The paper was developed based on kenaf bast fibers through soda pulping process. The pulps were subjected to beating process up to 9000 revolutions where the thickness, density and tensile index of paper were measured. It is found that the tensile index and the density of the paper increases while the thickness of the paper decreases with beating revolutions

Steady state performance of improvised ufer grounding practice

Ufer grounding is an effective method for electrical earthing, especially at highly resistive soil. The main material in ufer grounding is concrete which has moisture-dependent resistivity. In previous work, mixing concrete with Bentonite was done to observe slight improvement during short term measurements. In current study, the detailed results of extended period of measurements have been presented. It was found that the improvement of the proposed mix over the standard concrete mix has increased with time. In addition, mechanical strength of the proposed mix was found to be only 10% lower than the standard mix. Therefore this suggests that the proposed mix may be applicable as the mixture for building foundation as in the case of ufer grounding