Hidayat Zainuddin

Investigation on the surface discharge phenomenon at the oil-pressboard interface

Surface discharge on the oil-pressboard interface is considered to be serious fault condition. This phenomenon leads to the formation of a conducting path on the surface of an insulation material as a result of surface currents. The surface current is effectively the drift of charge under the action of an electric field which may be either the general electric field or a locally enhanced field developed around a space charge. Sustained surface discharge can eventually result in equipment failure. A needle-bar electrode configuration has been used to investigate the surface discharge phenomenon at the oil pressboard interface. This paper presents the partial discharge (PD) patterns corresponding to the surface discharge events.

Characteristics of leakage current during surface disharge at the oil-pressboard interface

Surface discharge at the oil-pressboard interface leads to the development of a conducting path that is generally accepted as white marks due to localized heating. The visible white marks are believed to be due to partial discharge (PD) energy that is high enough to dry out the pressboard through vaporizing the moisture and breaking oil molecules to generate gases in the pressboard pores. A long period of electrical stress applied to a surface discharge experiment using a needle-bar electrode configuration has led to the formation of full-gap white marks on the pressboard surface that bridged the needle tip and earth bar. During the surface discharge, leakage current may occur intermittently that sometimes is visible in the form of full discharge of arcing at the oil-pressboard interface bridging the needle tip and earth bar. This condition is expected to be similar to that of dry-band arcing from inclined-plane tracking experiments for outdoor insulation. This paper explains the surface discharge experiment and details the obtained leakage current characteristics in terms of waveforms and frequency spectra. Correlation between leakage current measurements using a shunt resistor and PD signal detection has been undertaken in order to identify the useful information for condition monitoring of surface discharge in a large transformer. An equivalent circuit to represent the leakage current occurrence at the oil-pressboard interface without considering the superimposed pulses due to individual PD events is also proposed. The effect of moisture on the experimental results is also discussed.

Application of response surface methodology for optimizing the oxidative stability of natural ester oil using mixed antioxidants

Natural ester insulation (NEI) oils have been developed since the early 1990s due to the increasing environmental, health and safety concerns from the public. NEI oils are well-known for having high fire and flash points, great moisture tolerance, good biodegradability and non-toxicity. However, NEI oils also have several drawbacks compared to mineral insulation (MI) oils such as low oxidative stability, high viscosity, low pour point and low resistance towards lightning impulse. Previous researchers have discovered that the oxidative stability of NEI oils can be improved by the addition of mixed antioxidants. However, such studies are focused on only one criterion and they neglect the dielectric strength of the oils. On the other hand, the one-factor-at-a-time (OFAT) method is commonly used to determine the optimum concentration of mixed antioxidants and requires a large number of samples. In this study, response surface methodology (RSM) technique is used to determine the optimum concentration of mixed antioxidants which will enhance the oxidative stability of NEI oil. The dielectric strength of the NEI oil is evaluated based on the AC breakdown voltage (BdV). RSM is chosen in this study because it is capable of determining the optimum values of the parameters of interest, without the need for a large number of test runs which will incur high cost for experimentation. A regression model is developed in this study based on the AC breakdown voltage of the NEI oil, propyl gallate and citric acid antioxidant concentrations. The oxidative induction time (OIT) and partial discharge inception voltage (PDIV) are carried out to determine the effect of the oxidative stability and withstand voltage on the partial discharge, respectively. The results showed that there is enhancement in the oxidative stability and dielectric strength of NEI oil using the optimum concentrations of mixed antioxidants relative to the fresh NEI oil. It can be concluded that RSM is a feasible alternative to determine the optimum concentration of antioxidants for NEI oils, which in turn, helps improve the oxidative stability and dielectric strength of these oils.

AC breakdown voltage of natural ester mixed with Iron Oxide for oil-immersed power transformer application

Nowadays, nanoscale materials are increasingly popular in academic and industrial research. It is expected that the combination of nanoscale materials with other material can contribute more benefits. The use of nanoscale materials often require their dispersion in variety of liquids, to enable them to become part of the liquid product, also known as nanofluids. For example, one of the applications is the nanoparticles dispersed with transformer oil in oil-immersed transformer application. Traditionally, mineral oil is used as dielectric liquid and cooling medium in power transformer. In this paper, dielectric strength of ester oil when mixed with nanoparticles is investigated. For that purpose, AC breakdown voltage test is compliance with ASTM D1816 standard is performed. Statistical analysis and high dimension visualization have been done to analyse and compare the result between fresh ester oil taken from tank and the similar ester oil that mixed with nanoparticles. The results suggest that ester oil mixed with nanoparticles of 0.02 % by volume performs well compared to other samples.

Partial discharge characteristics of surface tracking on oil-impregnated pressboard under AC voltages

Creeping discharge at the oil-pressboard interface is a serious fault condition because it can lead to catastrophic failure under normal operating conditions of large transformers. Creeping discharge leads to the development of a conducting path that is generally accepted as white marks on a board surface due to localised heating. The visible white marks indicate a drying out process of the pressboard through moisture evaporation and the breaking of oil molecules to generate gases in the pressboard pores. The processes can continue from minutes to months or even years until failure. Characterising this type of faulty condition in a condition monitoring programme is important for transformer life management. This paper explains the behaviour of surface discharge observed during a surface discharge experiment at the oil-pressboard interface using a needle-bar electrode configuration. Correlation between partial discharge (PD) activities and measured data in terms of phase resolved partial discharge (PRPD) patterns have been undertaken. The experimental results also show a decreasing trend of the average discharge magnitude during the formation of the white marks that propagate towards the earth bar. This finding is qualitatively comparable to on-line problems reported previously, i.e. there was a falling trend and no obvious evidence of imminent failure from the monitoring data of PD activity of a large power transformer that eventually suffered from an unexpected fault.

A method for the measurement of leakage current due to surface discharge at the oil-pressboard interface

Surface tracking is the conducting path formed on the surface of an insulation material as a result of surface currents. Tracking has commonly been associated with solid insulation in air such as overhead line insulators but is now recognized to occur at the oil-pressboard insulation interface inside large transformers. The surface current is effectively the drift of charge under the action of an electric field which may be either the general electric field or a locally enhanced field developed around a space charge. A needle-bar configuration has been developed to promote surface tracking under controlled conditions at the oil-pressboard interface without surface flash-over or breakdown. This method is used to study the leakage current associated with surface discharge applied from a high voltage source on the oil-pressboard interface. This paper outlines the experimental set-up and presents results which compare the measured current against the corresponding partial discharges. Subsequently a mechanism describing the process by which leakage current is formed is proposed

Multi-response optimization of the properties of natural ester oil with mixed antioxidants using taguchi-based methodology

A Taguchi-based methodology is proposed in this study in order to determine the optimum concentration of antioxidants which will enhance the oxidative stability of natural ester insulation (NEI) oil while ensuring superior dielectric strength performance. Conventional experimental testing involves the procurement of materials and equipment, sample preparation, experimental set-up and testing the samples with a large number of test runs to determine the optimum design parameters, which is costly and time-consuming. In this regard, the Taguchi-based methodology is advantageous since it can predict the optimum design parameters with fewer test runs. This methodology can also be used for multi-response optimization by applying a desirability function. The number of test runs for the antioxidant mixtures is generated using the L9 orthogonal array with three levels and two factors. This methodology limits the need to deal with the uncertainties associated with the range of factors, unlike the one-factor-at-a-time (OFAT) method. The oxidative induction time (OIT) and AC breakdown voltage (AC BdV) are selected as the output responses in accordance with the ASTM E1858 and ASTM D1816 standard test method, respectively. Analysis of variance (ANOVA) is used to determine the percentage contribution of each factor as well as the significance level of the model developed from the Taguchi-based methodology. The desirability function is used to transform the two output responses and generate the optimum value which complements both of the responses. Repeated tests are conducted to verify the repeatability and reproducibility of the OIT and AC BdV results for the optimized NEI oil based on a 95% confidence interval. In general, there is a significant enhancement in the OIT and AC BdV for the optimized NEI oil compared to those for fresh NEI oil. Hence, it can be concluded that the Taguchi-based methodology is a feasible approach to determine the optimum concentrations of mixed antioxidants for NEI oils.

Modeling the inter-phase region of high voltage transformers

In large oil-filled power transformers, pressboard and paper in conjunction with oil are used throughout to provide electrical insulation. Microscopic views have shown that pressboard insulation is a fibrous and porous structure with a non-homogeneous surface. The porosity of pressboard can result in impurities within the oil being drawn into the oil-impregnated pressboard. The material properties and geometry of pressboard thus lead to a complex oil-pressboard interface. A 2-D model of oil-pressboard interface considering the porous and non-homogeneous of pressboard has been constructed using Finite Element Analysis software, COMSOL Multiphysics. This paper discusses the mathematical equations used to study the electric double layer (EDL) at the oil-pressboard interface where the oil is in a stationary condition. Subsequently, the simulation results of non-equilibrium charge density profile at the interface are also presented.

Development of Test Vessel for Gas Insulation Breakdown Test

Efficiency, reliability, high power quality and continuous operation are important aspects in electric vehicle attraction system. Therefore, quick fault detection, isolation and enhanced fault-tolerant control for open-switches faults in inverter driving systems become more and more required in this filed. However, fault detection and localization algorithms have been known to have many performance limitations due to speed variations such as wrong decision making of fault occurrence. Those weaknesses are investigated and solved in this paper using currents magnitudes fault indices, current direct component fault indices and a decision system. A simulation model and experimental setup are utilized to validate the proposed concept. Many simulation and experimental results are carried out to show the effectiveness of the proposed fault detection approach.The inverter with critical loads should be able to provide critical loads with a stable and seamless voltage during control mode change as well as clearing time. The indirect current control has been proposed for providing stable voltage with critical load during clearing time and seamless control mode transfer of inverters. However, the islanding detection is difficult since with the indirect current control the magnitude and frequency of voltage do not change when the islanding occurs. The conventional anti-islanding method based on the magnitude and frequency of voltage variation cannot apply to the indirect current control. This paper proposes an islanding detection method for the indirect current control. The proposed islanding detection method can detect the islanding using reactive power perturbation and observation when the frequency and magnitude of voltage don’t vary during clearing time. In order to verify the proposed anti-islanding method, the experimental results of a 600W three-phase inverter are provided.

A Review on the Reclamation Technologies for Service-Aged Transformer Insulating Oils

Zarina Mohd Noh*, Abdul Rahman Ramli, Marsyita Hanafi, M Iqbal Saripan, Ridza Azri Ramlee 1,2,3,4 Department of Computer and Communication Systems Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia 1,5 Fakulti Kejuruteraan Elektronik dan Kejuruteraan Komputer, Universiti Teknikal Malaysia Melaka (UTeM), Hang Tuah Jaya, 75450 Durian Tunggal, Melaka, Malaysia