Ageing effect of vegetable oils impregnated paper in transformer application

Vegetable oils are being considered as potential alternatives to mineral oil, due to their better environmental performance and for their high fire point. Although these liquids have been used in distribution transformer, it is still a significant step to adopt vegetable oils in power transformer due to high cost and high level of safety and reliability required in service for these units. Vegetable oils such as Palm oil (PO), Corn oil (CO), and Rice Bran oils (RBO) offer suitable alternative for mineral oil. It is anticipated that most of the un-aged oil could satisfy the minimum requirement for dielectric insulation liquids in transformer. However, since transformers in service could be subjected to heat and multiple environmental parameters, the oil could be subjected to ageing. The chemical properties of the oil may change and its performance could be affected by the presence of ageing-by-products such as moisture and acids. Therefore, considering the application of new dielectric insulation liquids such as PO, CO and RBO in transformer, it is crucial to first examine its ageing performances at laboratory level. This paper focused on the effect of ageing on the electrical and physicochemical properties of PO, CO, and RBO. Sealed ageing experiments were set at 90°C for 30 days, 90 days and 180 days. Before the ageing process, the samples were dried in a vacuum oven at pressure less than 0.8kP at 85°C for 48 hours in order to remove the moisture content in the oils. Then the oils were impregnated with the Kraft paper and continue to age for selected duration time. The electrical properties (relative permittivity, dielectric losses, resistivity and breakdown strength), mechanical properties (viscosity and tensile strength) and chemical properties (moisture and acidity) of the oils were measured throughout the ageing periods. It can be concluded that the laboratory accelerated thermal ageing experiment reveals that all vegetable oils in this study are resistant toward oxidation based on the stable viscosity and low acidity measurements of vegetable oils throughout the ageing duration even with the presence of oxygen. The AC breakdown voltages of vegetable oils can still comply with the recommended limit of new vegetable oil set by ASTM 6781 even after subjected to ageing. In general most of properties of vegetable oils are comparable with mineral oil

Examination on the lightning breakdown strength of biodegradable oil under quasi-uniform field

Among the alternative fluids considered for application in transformers are Palm Oil (PO) and Coconut Oil (CO). Among the important studies that need to be carried out before it can be applied in-service is the lightning impulse performance. This paper presents the experimental work on the lightning impulse breakdown voltage of different PO, CO and Mineral Oil (MO). The PO used in this is the Refined Bleached Deodorized Palm Oil (RBDPO) type. The test was carried out under quasi-uniform field using sphere-sphere electrode configuration with different testing methods (rising-voltage and up-and-down). It was found that the lightning impulse performance for some of RBDPO and CO were comparable to MO

Calibration and design of UHF partial discharge sensors using finite-difference time-domain modelling

Electric power supply involves three main stages which are generation, transmission and distribution. Properly maintained electrical equipment at each stage can assure the supply is continuous at all time to customers and it is important for safety requirements where fire and explosion accidents can be eliminated. The electrical equipment includes generators, motors, power transformers and switchgear. Electrical discharges (small sparks) which usually occur in defective insulation system of electrical equipment can cause equipment failure or even explosion if the discharges are not detected and treated in an appropriate time. The discharges generate electromagnetic waves which can be detected using ultra-high frequency (UHF) sensors. A lot of research have been done to design effective and low cost UHF sensors for different applications in high voltage systems. UHF sensor manufacturers are required to meet certain sensitivity standards so that the sensors are able to detect the minimum level of discharges and at certain frequency range. Manufacturers will fabricate a new sensor and then measure the sensitivity of the sensor using a calibration system. If the sensor does not achieve the minimum sensitivity standard, manufacturers will fabricate another one after changing certain parameters of the sensor that could improve the response and then the sensor will be characterised again. This repetitive and expensive process can be eliminated using numerical electromagnetic simulation to design and calibrate the sensor. Finite-difference time-domain (FDTD) method is a computational technique that can be used to design UHF sensors and predict the sensitivity of the sensors. In this study, a calibration system was modelled using FDTD technique to predict the responses of several existing UHF partial discharge sensors. The simulated calibration system was simplified to overcome the complex design of the experimental calibration system. However, the main function of the system was retained in the simulation. The performance of the simulated calibration system was validated by comparing the experimental responses of physical sensors with the simulated results. The percentage difference between the measured and simulated responses of the existing sensors is 7.65% on absolute average with standard deviation of 2.99. The factors that affect the responses of the sensors for examples, the electrical property of insulating materials and sensor sizes have been studied using this method. A new UHF partial discharge sensor was designed and modelled using FDTD simulation and then fabricated for testing. The measured and simulated responses of the sensor showed good agreement. Design variations of the same sensor were simulated to improve the sensor response. Then, the optimum simulated response and simplest design to manufacture was chosen as the final design of the sensor. The experimental and simulated results of the sensor also showed excellent agreement. The FDTD method can also study the characteristics of electromagnetic wave propagation generated by PD source in power transformer.Electric power supply involves three main stages which are generation, transmission and distribution. Properly maintained electrical equipment at each stage can assure the supply is continuous at all time to customers and it is important for safety requirements where fire and explosion accidents can be eliminated. The electrical equipment includes generators, motors, power transformers and switchgear. Electrical discharges (small sparks) which usually occur in defective insulation system of electrical equipment can cause equipment failure or even explosion if the discharges are not detected and treated in an appropriate time. The discharges generate electromagnetic waves which can be detected using ultra-high frequency (UHF) sensors. A lot of research have been done to design effective and low cost UHF sensors for different applications in high voltage systems. UHF sensor manufacturers are required to meet certain sensitivity standards so that the sensors are able to detect the minimum level of discharges and at certain frequency range. Manufacturers will fabricate a new sensor and then measure the sensitivity of the sensor using a calibration system. If the sensor does not achieve the minimum sensitivity standard, manufacturers will fabricate another one after changing certain parameters of the sensor that could improve the response and then the sensor will be characterised again. This repetitive and expensive process can be eliminated using numerical electromagnetic simulation to design and calibrate the sensor. Finite-difference time-domain (FDTD) method is a computational technique that can be used to design UHF sensors and predict the sensitivity of the sensors. In this study, a calibration system was modelled using FDTD technique to predict the responses of several existing UHF partial discharge sensors. The simulated calibration system was simplified to overcome the complex design of the experimental calibration system. However, the main function of the system was retained in the simulation. The performance of the simulated calibration system was validated by comparing the experimental responses of physical sensors with the simulated results. The percentage difference between the measured and simulated responses of the existing sensors is 7.65% on absolute average with standard deviation of 2.99. The factors that affect the responses of the sensors for examples, the electrical property of insulating materials and sensor sizes have been studied using this method. A new UHF partial discharge sensor was designed and modelled using FDTD simulation and then fabricated for testing. The measured and simulated responses of the sensor showed good agreement. Design variations of the same sensor were simulated to improve the sensor response. Then, the optimum simulated response and simplest design to manufacture was chosen as the final design of the sensor. The experimental and simulated results of the sensor also showed excellent agreement. The FDTD method can also study the characteristics of electromagnetic wave propagation generated by PD source in power transformer

Simulated and measured frequency responses of disk-yype UHF partial discharge sensors

Describes the simulated and measured frequency responses of disk-yype UHF partial discharge sensors

Evaluation of FDTD modelling as a tool for predicting the response of UHF partial discharge sensors

Ultra high frequency (UHF) partial discharge sensors are important tools for condition monitoring and fault location of high voltage equipment. There are many designs of UHF sensors which can detect electromagnetic waves that radiate from partial discharge sources. The general types of UHF PD sensors are disc, monopole, probe, spiral, and conical types with each type of sensor having different characteristics and applications. Computational modelling of UHF PD sensors using Finite-difference time-domain (FDTD) simulation can simplify the process of sensor design and optimisation, reducing the development cost for repeated testing (in order to select the best materials and designs for the sensors), and giving greater insight into how the mechanical design and mounting will influence frequency response. This paper reports on an investigation into the application of FDTD methods in modelling and calibrating UHF PD sensors. This paper focuses on the disc-type sensor which the sensor has been modelled in software and the predicted responses are compared with experimental measurements. Results indicate that the FDTD method can accurately predict the output voltages and frequency responses of disc-type sensors. FDTD simulation can reduce reliance upon costly experimental sensor prototypes and leading to quicker assessment of design concepts, improved capabilities and reduced development costs

Evaluation of FDTD modelling as a tool for predicting the response of UHF partial discharge sensors

Ultra high frequency (UHF) partial discharge sensors are important tools for condition monitoring and fault location of high voltage equipment. There are many designs of UHF sensors which can detect electromagnetic waves that radiate from partial discharge sources. The general types of UHF PD sensors are disc, monopole, probe, spiral, and conical types with each type of sensor having different characteristics and applications. Computational modelling of UHF PD sensors using Finite-difference time-domain (FDTD) simulation can simplify the process of sensor design and optimisation, reducing the development cost for repeated testing (in order to select the best materials and designs for the sensors), and giving greater insight into how the mechanical design and mounting will influence frequency response. This paper reports on an investigation into the application of FDTD methods in modelling and calibrating UHF PD sensors. This paper focuses on the disc-type sensor which the sensor has been modelled in software and the predicted responses are compared with experimental measurements. Results indicate that the FDTD method can accurately predict the output voltages and frequency responses of disc-type sensors. FDTD simulation can reduce reliance upon costly experimental sensor prototypes and leading to quicker assessment of design concepts, improved capabilities and reduced development costs

Characterizing the sensitivity of UHF partial discharge sensors using FDTD modeling

Ultra high frequency (UHF) partial discharge sensors are widely used for condition monitoring and defect location in the insulation systems of high voltage equipment. Designing sensors for specific applications often requires an iterative process of manufacture, test, and mechanical modifications. This paper demonstrates the use of finite-difference time-domain (FDTD) simulation as a tool to predict the frequency response of a UHF sensor design. Using this approach, the design process can be simplified and parametric studies can be conducted in order to assess the influence of component dimensions and material properties on the sensor response. The modeling approach is validated using a broadband UHF sensor calibration system, which uses the step response of the sensor to determine its frequency-domain transfer function. The use of a transient excitation source is particularly suitable for modeling using FDTD, which is able to simulate the step response output voltage of the sensor, from which the frequency response is obtained using the same post-processing applied to the physical measurement. Comparisons between simulation and measurement are made for three different sensors, demonstrating sensitivity agreement to within about 10%. Some examples of simple parametric studies carried out using the FDTD model are also presented

Investigation of the Electrical Properties of Mineral Oils with and without Carbon Nanotube Concentration under Different Magnetic Fields Applied in Transformer Applications

The increased voltage loading of transformers has led to research on improving transformers’ lifespans to meet demand. Insulation oil acts as cooling medium that can significantly affect the performance of a transformer. This paper discusses an experimental study on the influences of the doping of carbon nanotube (CNT) particles and magnetic fields on the electrical properties of mineral oil (MO). An analysis of electrical properties was conducted using AC breakdown tests, Tan Delta tests, Raman measurements, and simultaneous thermal analysis. Proper preparation was considered before starting the analysis of the electrical properties. The AC breakdown voltages before and after modification were measured. The experiment results indicated that the AC breakdown of mineral oil with a suitable amount of carbon nanotube particles (0.005 g/L) and a suitable magnetic field (0.45 T) gives the highest breakdown voltage. It was found that the proper treatment of nanofluid also greatly influences breakdown voltage. Additionally, Raman measurements analyzed the physical changes in the samples. From the results obtained, the addition of carbon nanotubes and the magnetic field of mineral oil leads to an improved performance of the transformer