Shengchang Ji

A study of the sweep frequency impedance method and its application in the detection of internal winding short circuit faults in power transformers

Undetected short circuit faults are a significant problem in power transformers and can eventually develop into catastrophic faults. At present, frequency response analysis (FRA) is one of the well-recognized diagnostic tools for the detection of winding faults, but it has some limitations, such as a low signal-to-noise ratio (SNR) and instability caused by changes in the measuring voltage. In this paper, a novel method called sweep frequency impedance (SFI) is proposed to address the difficulties that arise from FRA. Based on the mechanism of this new method, a nondestructive testing system was established to demonstrate the advantages of SFI measurements. The SFI test system has a better stability, repeatability, and SNR by comparing it with the FRA test system. Moreover, FRA and SFI curves obtained under the same conditions was symmetrical about a specific straight line above 10 kHz, and the SFI value at 50 Hz is equivalent to the short circuit impedance (SCI) value of a transformer. These results indicate that the existing criteria of FRA and SCI methods can be used in the SFI method to detect transformer faults. Finally, the experiments on a special oil-immersed testing transformer demonstrate that the SFI detection system is feasible, sufficiently sensitive to detect short circuit faults and able to quantify the level of the fault.

Study on the Influence of Dielectric Barrier Materials on the Characteristics of Atmospheric Plasma Jet in Ar

In this paper, an Ar plasma jet is generated under atmospheric pressure using a DBD configuration device equipped with a copper-ring powered electrode as well as an aluminous plate ground electrode, which is driven by a sinusoidal excitation voltage at 10 kHz, and three types of dielectric barriers, i.e., quartz glass, epoxy, and PTFE, are employed to study the influence of the barrier materials on the characteristics of the plasma jet. The jet characteristics are studied by electrical and optical diagnostics, and the voltage-current waveforms, Lissajous figures, lighting emission images, and optical spectrum are obtained. The electric discharge parameters and plasma parameters, such as discharge power, transported charges, electronic excitation temperature, etc., are calculated and compared by using different dielectric barriers, and the mechanism of the influence of the barrier material on the jet discharge is discussed. The experimental results show that the influence of the barrier materials on the characteristics of the plasma jet is related not only to the relative dielectric constant but also to the desorption of surface electrons from them. Under the same operation conditions, more intensive jet discharge can be obtained by using PFTE as the barrier material, and the discharge power, transported charges, and electronic excitation temperature are higher than those of quartz glass and epoxy as dielectric barriers.

Electrical Model and Experimental Analysis of the Atmospheric-Pressure Homogeneous Dielectric Barrier Discharge in He

A dynamic electrical model for a plane-parallel configuration homogeneous dielectric barrier discharge (DBD) is investigated using the MATLAB Simulink. The electrical model is based on an equivalent electric circuit, in which the DBD is represented by a voltage-controlled current source associated to a resistance and a capacitance value. In addition, a plane-parallel DBD cell filled with He gas is used for the experiments, and a sinusoidal voltage of up to 5-kV peak value at frequencies of 10 kHz is applied to the discharge electrodes for the generation of homogeneous discharges. The electrical characteristics of homogeneous DBD under different operating conditions are studied using the electrical model simulations and experiments, and a comparison between them is conducted. It is shown that the simulated voltage and current waveforms and Lissajous figures are consistent with the experimental ones, which validates the functionality of the model. The dynamic behavior of the discharge parameters (such as gas gap voltage, discharge current, discharge consumed power, and transported charges), which are not measurable in the real process, is studied with the electrical simulation model, and the voltage-current curves are also obtained from simulation and used to analyze the evolution trajectory of the homogeneous DBD.

A Novel DC Arc Fault Detection Method Based on Electromagnetic Radiation Signal

Because of lack of current zero, dc arc is hard to be extinguished, which has become the main reason causing faults in dc system. To find an effective method to detect dc arc faults is of great significance. In this paper, a method for detecting dc arc fault based on electromagnetic radiation signal is presented. A dc arc generating device is designed to simulate dc arc faults caused by loose connection in dc systems. A fourth-order Hilbert curve fractal antenna is adopted to detect the electromagnetic radiation signals of dc arc. The amplitude and spectrums of electromagnetic radiation signals measured under different circuit currents are analyzed. A photovoltaic (PV) system is constructed and the dc arc generated in that system is measured. The test results show that, under the present experimental condition, the electromagnetic radiation pulses have an obvious characteristic frequency, which are in a range of 39–41 MHz. The characteristic frequency of electromagnetic radiation of dc arc generated in PV system is around 39 MHz. Moreover, the characteristic frequency of dc arc is compared with that of switch operation. The dc arc has higher characteristic frequency and longer interval of each electromagnetic radiation pulse than those of the switch operation. The results indicate that the characteristic frequency of electromagnetic radiation signals can be used as the detection parameter of dc arc.

SF 6 decomposition of typical CT defect models

Detection of decomposition products is a newly-emerging method for the condition diagnosis of SF6-insulated equipment. Based on the two grading structures that are commonly adopted in SF6-insulated current transformers (CT), three models consisting of the needle-to-plane, the ball-to-plane, and the plane-to-plane were established. Gas decomposition was investigated in a 90 L discharge chamber under operating conditions. Products were analyzed with gas chromatography (GC) and X-ray photoelectron spectroscopy (XPS). Typical gas decomposition products are CF4, SO2F2, CS2, SO2, and S2OF10. For the needle-to-plane with a gap of 30 mm, near-linear growths with time are observed in the concentrations of SO2F2, SO2, and S2OF10 when the applied voltage is set as 33 kV. Addition of the adsorbent suppresses the concentrations of all three products, especially SO2. Besides, CF4 and CS2 are also found to be the characteristic components of partial discharge involving polyester films. And their concentrations are higher when the electric field lines are parallel to the surface of the polyester. As for the polyester films, the atomic ratio of C decreases, the atomic ratio of S increases, and the atomic ratio of F increases significantly as the deterioration becomes more serious. Moreover, the concentration ratio of S2OF10/SO2F2 in the needle-to-plane is mostly within the range of 0- 2, whether with the addition of adsorbent or not. And the ratio is much higher (2-10) when polyester is involved in the discharge process. It could be used to diagnose the insulation condition of CT effectively.

Generation and Developing Process of Low Voltage Series DC Arc

The dc arc is unintended, self-sustaining high energy plasma discharge, threatening the safety of low voltage dc power systems. In this paper, a dc arc generator was designed and images of the generation and developing process of dc arc are presented. The shape of the arc during different phases, the emitting sparks, and the burning copper electrode is shown in the images, which is described and explained from the physical process point of view.

Effect of the Grounded Electrode on Cold Ar Atmospheric Pressure Plasma Jet Generated With a Simple DBD Configuration

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.

Experimental Investigations on Low-Energy Discharge in

The detection of gas decomposition products is widely used to diagnose the inner condition of SF6-insulated electrical equipment for its anti-interference ability and high sensitivity. A discharge chamber with Teflon coating inside was designed, with adjustable defect parameters and the volume nearly the same as real ones in gas-insulated switchgear. With the assistance of newly developed dual gas chromatography detection system, this paper investigated the influences of relatively low moisture on the discharge characteristics and gas decomposition under corona and mild spark in SF6. Considering notable hydrolysis reactions in the main gas volume and on the vessel walls, SO2F2, SO2, and S2OF10 were selected as typical gas decomposition products. The average discharge capacity and its fluctuation were found to decrease with the increase of moisture content within research scope, which is related to the electronegativity of water molecule. The concentration of typical decomposition products under various moisture conditions was observed in detail. The results show that in mild spark, O atom in S2OF10 mainly originated from oxygen rather than water vapor in SF6. To diagnose internal insulation condition by only monitoring, the magnitude of discharge capacity is proved unreliable. A new parameter, the concentration ratio of SO2/S2OF10, was proposed to group moisture content and discharge types.

{\rm SF}_{6}

The capacitors in high-voltage direct-current (HVDC) converter stations radiate a lot of audible noise which can reach higher than 100 dB. The existing noise level prediction methods are not satisfying enough. In this paper, a new noise level prediction method is proposed based on a frequency response function considering both electrical and mechanical characteristics of capacitors. The electro-mechanical frequency response function (EMFRF) is defined as the frequency domain quotient of the vibration response and the squared capacitor voltage, and it is obtained from impulse current experiment. Under given excitations, the vibration response of the capacitor tank is the product of EMFRF and the square of the given capacitor voltage in frequency domain, and the radiated audible noise is calculated by structure acoustic coupling formulas. The noise level under the same excitations is also measured in laboratory, and the results are compared with the prediction. The comparison proves that the noise prediction method is effective.

Under Low-Moisture Conditions

Condition of insulation materials can be obtained by analyzing partial discharge (PD) characteristics since features of PD are closely related to it. In this paper, electrical deterioration caused by partial discharge of oil-paper insulation under long-term AC voltage is studied. Feature characteristics of PD, such as phase-resolved partial discharge analysis and equivalent time-frequency, were recorded during the test process. Electrical deterioration is divided into four stages: initial, develop, stagnation and erupt stage. Four stages are separated by pulse repetition rate and discharge magnitude of PD. In different deterioration stage, statistical characteristics of PD change significantly. Surface properties of pressboard in different deterioration stage were studied. Test results indicate that the depolarization of cellulose in pressboard is the main reason for development of partial discharge, and discharges occur in different position of pressboard in different deterioration stages.