Nor Asiah Muhamad

Comparative Study and Analysis of DGA Methods for Transformer Mineral Oil

Dissolved gas-in-oil analysis (DGA) is a sensitive and reliable technique for the detection of incipient fault condition within oil-immersed transformers. The presence of certain key gases is monitored and quantified. There are a number of methods developed for analyzing these gases and interpreting their significance: Key Gas, Rogers Ratio, Doernenburg, Logarithmic Nomograph, IEC Ratio and Duval Triangle. This paper investigates the accuracy and consistency of these methods in interpreting the transformer condition. The evaluation is carried out on DGA data obtained from the local power utilities and from published papers. The data consists of 92 different cases. The key gases considered are hydrogen, methane, ethane, ethylene and acetylene. A MATLAB program was developed to automate the evaluation of the methods.

Effect of nanofillers on the polarization and depolarization current characteristics of new LLDPE-NR compound for high voltage application

Polymeric nanocomposites in which the nanosize fillers are evenly distributed in the polymer material attract attention as an insulating material due to their ability to enhance the materials performance properties of electrical and mechanical. For high voltage (HV) insulation application, one of the targets is to obtain new insulators with improved dielectric properties. This paper presents the outcome of an experimental study to determine the conductivity level of the linear low-density polyethylene- (LLDPE-)natural rubber (NR) compound, filled with different amount of SiO2 and TiO2 nanofiller by using the polarization and depolarization current (PDC) measurement technique. linear low-density polyethylene (LLDPE) and natural rubber (NR) with the ratio composition of 80u2009:u200920 are selected as a base polymer. The experiment was conducted to find PDC pattern and conductivity variations of each of the LLDPE-NR/SiO2 and LLDPE-NR/TiO2 samples. The results show that the addition of SiO2 filler exhibited less conductivity compared to TiO2 filler with certain percentage. From the study, it can be concluded that LLDPE-NR/SiO2 is a better insulator compared with LLDPE-NR/TiO2.

Faults identification of biodegradable oil-filled transformers based on polarization and depolarization current measurement (PDC) method

Polarization and depolarization current (PDC) measurements of insulation oil alone can be used for faults identification. From measurement of healthy oil conditions at different moisture levels and after faults, fault identification can be achieved by comparing the PDC pattern of an oil sample against the PDC fingerprints. Furthermore, a graph plot establishes a relationship between oil sample conductivity and moisture level as well as between oil samples measured capacitance and moisture level. These curves can be used to evaluate the oil moisture conditions and the fault type. This paper seeks to find the PDC pattern of the biodegradable oil (Envirotemp ® FR3™) at different moisture levels and after being subjected to different types of fault: partial discharge, arcing, and overheating. The experiment was carried out in the laboratory to obtain the PDC patterns generated by the oil. Changes in the PDC pattern of biodegradable oil from its fingerprint due to the increased moisture level in the oil or because of faults that occurred to the oil sample were identified. These can help in predicting the biodegradable oil-filled transformers condition and can give some indication of the maintenance work that should be taken on the transformer.

Dissolved gas analysis (DGA) of arcing faults in biodegradable oil insulation systems

Due to environmental concerns regarding the use of mineral oil, biodegradable oil was commercially introduced as an alternative for transformer insulation in 1999. Since then, the usage of this oil in the power industry has steadily increased and a lot of researches have been in progress to make biodegradable oil as compatible as mineral oil. This paper presents results of experimental tests performed on laboratory models of transformer windings immersed in biodegradable oil. The experiment involved an investigation of hydrocarbon gas products generated when arcing happened. The aim is to determine whether analysis based on existing DGA fault interpretation methods which were developed for mineral oil need to be modified when applied to insulation assessment of biodegradable oil.

Dissolved gas analysis of faults in biodegradable oil transformer insulating systems

This paper presents results of experimental tests performed on laboratory models of transformer windings immersed in bio-degradable oil. The investigations covered three common types of transformer faults: partial discharge (PD), arcing and overheating. Dissolved hydro-carbon gas products generated by these three fault types in bio-degradable oil are analyzed and compared with existing dissolved gas analysis (DGA) interpretation methods for mineral oil insulation. The PD and arcing tests were performed in small test cells using mineral and vegetable oils, with and without insulated pressboards. The overheating test was performed by heating the oils in the oven. At the end of each test, an oil sample was extracted for dissolved gas and moisture measurement. The results show that existing DGA interpretation techniques need to be modified if they are to be used for insulation assessment of bio-degradable oil.

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.

Application of PDC analysis to identify effect of overheating on dielectric response and conductivity of mineral insulating oil of in-services transformers

The diagnosis and maintenance of in-service power transformers is important for a safe and reliable of electrical power supply. Several testing and diagnostic measurement tools have been developed to detect ageing of insulation system due to thermal and electrical stresses as well as incipient faults which can increase the risk of transformer failure. The dielectric response measurement either in time and frequency domain were widely used presently to determine and assess the condition of insulation system. This paper presents the analysis result of dielectric response and conductivity of mineral insulating oil under overheating condition using time domain technique known as Polarization and Depolarization Current (PDC) measurement. The analysis results found that the insulating with overheating condition has higher polarization and depolarization current magnitude. In addition, the calculated conductivity demonstrated that overheating oil sample has higher conductivity level compared to normal oil sample.

Separation of corona noise from on-line partial discharge monitoring of power cables

The use of traditional maintenance scheme such as corrective maintenance and preventative maintenance are insufficient to prevent costly events triggered by failure of power system equipment. On-line condition monitoring is increasingly becoming popular. It is non-destructive and cost-saving as it can be carried out with the equipment kept in-service. The on-line monitoring of partial discharges (PD) in power cables in outdoor substation has lots of background noise, especially corona discharge. Thus, it is important to remove corona noise from the monitoring of PD in power cables. A new method to remove the corona noise was proposed in this paper. It involved the use of simultaneous multi-channel measurement, principal component analysis (PCA) and clustering analysis.

Dissolved gas analysis (DGA) of partial discharge fault in bio-degradable transformer insulation oil

Bio-degradable oil was commercially introduced as transformer insulation oil in 1999. Since then, the usage of this oil in the power industry has steadily increased and a lot of researches have been in progress to make bio-degradable oil as compatible as mineral oil. This paper presents results of experimental tests performed on laboratory models of transformer windings immersed in bio-degradable oil. The experiment involved an investigation of hydro-carbon gas products generated by partial discharge fault. The aim is to determine whether existing analysis techniques on partial discharge faults using DGA fault interpretation methods which were developed for mineral oil have to be modified when applied to insulation assessment of bio-degradable oil.

Comparison of partial discharge activity in mineral oil and bio-degradable oil

This paper investigates the partial discharge activity and characteristics in mineral oil and bio-degradable oil. Both types of oils tested are commercial products and commonly used in transformers. Tests were done on laboratory models using paper insulated conductor sections to simulate transformer windings and needle-plane configuration to simulate corona in transformer. The laboratory models are insulated with both types of oils, with and without moisture. The overall partial discharge activity was analysed in terms of the phase resolved distribution. Also the individual partial discharge waveforms were analysed in the time and frequency domain.