Broadband responses under random excitations can reflect the modal characteristics of a structure. This paper investigates the applicability of operational modal analysis (OMA) to extract modal parameters of transformer windings under random excitations. Firstly, random vibration signals are extracted from measured vibration signals by estimating an accurate power frequency and eliminating vibration harmonics. Then, modal characteristics are obtained by applying OMA to the extracted random vibration signals. The numeric study on a winding axial vibration model demonstrates the effectiveness of the proposed method in obtaining its modal characteristics. Laboratory experiments show that the natural frequencies calculated by OMA are a subset of the actual natural frequencies of windings and the tank. The field test results of a 110 kV power transformer show that the number of natural frequencies increase when winding deformation occurs. OMA of transformer windings may pave a way for online winding condition monitoring.
The vibration of power transformer is closely related to the mechanical condition of transformer and current when the external short circuit occurs. In this paper, the vibration sensors based on piezoelectric effect are attached to the oil tank of transformer to acquire the vibration signals in short circuit condition, and then the transfer function is applied to study the relationship between the vibration and short- circuit current under multiple short circuit shocks. Compared with traditional frequency domain or time- frequency domain analysis, this method can normalize the vibration in different short circuit condition and clearly illustrate the relationship between vibration and current. The conclusion shows that this method is feasible to evaluate the mechanical condition of transformer by using the correlation analysis of amplitude- frequency characteristic of transfer function in different short circuit condition.
The measurement for the surface electric field intensity of insulators based on electroluminescent effect is a non-contact measurement. By spraying a coating of epoxy paint mixed with ZnS:Cu electroluminescence powder on the surface of insulators, the part with high surface electric field intensity would be the first to glow. As the applied voltage increases, the area of luminous and the brightness increase. Therefore, the surface electric field distribution can be deduced by photographing and analyzing the luminescence of insulator surface coating. This method has successfully measured the surface electric field intensity distribution of disc insulators in GIL. Without the use of electric field probe, this method eliminates the electric field distortion caused by the probe completely. Besides, the permittivity of the coating can be decided according to the permittivity of the insulator material, and so eliminate the electric field distortion caused by the coating. The measure results of this method are in good agreement with reality.
Detection of decomposition products is a widely studied method for condition diagnosis of SF 6 insulated equipment because of its anti-electromagnetic interference and high sensitivity. A 90 L discharge chamber with adjustable defect was designed to simulate typical failures including metal protrusion defect, floating potential defect and insulator defect. Primary products of both the former two defects are SOF 2 , SO 2 F 2 , CO2, and SO 2 . Concentration ratios including Φ(SOF 2 +SO 2 F 2 +SO 2 )Φ(CO2) and Φ(SOF 2 +SO 2 )/Φ(SO 2 F 2 ) could be used to distinguish these two defects which have different structures and discharge energy. Influence of essential conditions such as applied voltage, pressure, and moisture content has been investigated under metal protrusion defect. For the insulator defect, several extra carbon containing by-products, namely, CF 4 , CO, and CS 2 , are found to be critical characteristic decomposition products. Moreover, a triangle method utilizing composition of decomposition products is proposed. It is proved to be a graphical method which helps to follow the faults more easily and more precisely.
Two cold Argon atmospheric pressure plasma jets were generated based on the form of DBD configuration device. One device equipped with one powered ring electrodes as well as a grounded ring electrode, and the other's grounded electrode was a metal plate. The characteristics of two different plasma jets with mid-frequency sinusoidal power were measured in this paper. The waveforms of the applied voltage and discharge current, Lissajous figures, images and optical spectrum of DBD were acquired, when Ar was inlet. The transported charges, discharge power and electron excitation temperature were investigated through Lissajous figures. Moreover, the optical emission spectroscopy is used to measure the plasma parameters, of which the electronic excitation temperature is determined by the Boltzmann's plot method using seven characteristic lines of Ar. The experimental results indicate that, discharge power and electronic excitation temperature of two plasma jets increase with the applied voltage. Through investigating the two plasma jets' electricity and optical characteristics, it was found that, compared to one plasma jet generated by the device with a grounded ring electrode, the other plasma jet generated by the device with a grounded metal-plate electrode had smaller discharge power but higher electron excitation temperature when under the same applied voltage and Ar flow rate.
Nonthermal and low-temperature plasmas has been a subject attracting great interest of scientists engaged in different fields of science and application. In this paper, a cold Ar plasma jet is generated under atmospheric pressure by using a dielectric barrier discharge configuration device equipped with a copper ring electrode and an aluminous plate grounded electrode driven by a sinusoidal excitation voltage at 10 kHz. The distance ( d, for short) between the grounded electrode and the nozzle of the quartz tube is set at 10, 20, and 30 mm, respectively. The jet characteristics are investigated by electrical and optical diagnostics, including discharge images, voltage-current waveforms, Lissajous figures, and optical spectrum. The electric discharge and plasma parameters, such as discharge power, transported charges, electronic excitation temperature, etc., with different d are calculated and compared. Furthermore, the influence of the location of the grounded electrode on the characteristics of plasma jet is also studied. The experimental results show that the largest discharge power is generated with d=10 mm, while the electron density and electronic excitation temperature (EET) of plasma jet will be increasing for d=20 mm. Thus keeping reasonable distance between the grounded electrode and the nozzle of quartz tube not only can reduce the discharge power, but also improve the electron density and EET of the plasma jet, which contributes to practical application of this jet device.
Metallized film capacitors are important parts in modular multilevel converter based high-voltage direct current system (MMC-HVDC), which play the role of DC support and energy storage. The optimized square resistance design can improve the heating and self-healing characteristics. This project proposes a square resistance design that the resistance is distributed in the axial direction as a power function. Under the calculation of COMSOL Multiphysics, we find that the square resistance which changes with a high power distribution can reduce the maximum operating temperature. Meanwhile we derive the relationship between the self-healing loss area and the square resistance, find that the self-healing loss area is inversely proportional to the square resistance value. The square resistance changes with a high power function distribution can reduce the capacitor single self-healing loss area expectation. It provides a reference for the design of metallized film square resistance.
In series compensation stations of the UHV AC demonstration project in China, capacitive voltage transformers (CVTs) failed several times during the operation of disconnecting switches (DSs). The reasons for this failure are analyzed in this paper. The stray inductor, the stray capacitor between the platform and ground and the capacitor of the CVT forms a resonant circuit in series compensation stations, causing repeated oscillating overvoltage and impulse current acting on the CVTs during DS operation. A full-scale experiment indicates that the oscillating frequency is approximately 200 Hz and the repeating frequency is approximately 0.5 MHz. The simulation experiment in the laboratory indicates that the partial welding mode and the partial high electric field are the main causes of damage to the CVT element insulation. A mechanism of the insulation damage is proposed, and some suggestions are given based on our results.
The transformer windings will flow a large impact current in the event of an external short circuit, causing strong vibration, resulting in winding looseness and deformation. Therefore, it is of great significance to carry out the research on fault diagnosis under short-circuit impact. The sound signal can reflect the overall mechanical condition of the equipment. According to the characteristics of the impulsive sound signal in time-frequency domain, two fault parameters like sound entropy (SE) and odd-even ratio (OER) were proposed. It is found that the SE and OER increase when the winding loosens, while they decrease when the deformation occurs. The deviation of parameters can reflect the degree of mechanical failure. According to the proposed fault parameters, a diagnosis on winding failure through the impulsive sound of the transformer is proposed.
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