Lingyu Zhu

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.

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.

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.

A Noise Level Prediction Method Based on Electro-Mechanical Frequency Response Function for Capacitors

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.

Partial discharge development in needle-plane configuration of oil-paper insulation under AC voltage

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.

Vibration Characteristics of Filter Capacitors Used in HVDC Converter Stations

Audible noise of can-type filter capacitors in high- voltage direct current converter stations originates from the vibration of capacitor cases, so it is meaningful to investigate the vibration characteristics. However, no experiment under practical excitations or thorough theoretical analysis was presented for the vibration characteristics. This study theoretically and experimentally investigated the vibration characteristics of can-type filter capacitors with stacked elements parallel to bottom. The vibration amplitude was found to be proportional to voltage squared. If the frequency spectrum of excitation voltage included several harmonics (more than three), vibration amplitude on some specific frequencies was related with the phases of these voltage components. Each capacitor surface had a concavo-convex vibration shape and the vibration of the bottom surface and the side surfaces were in opposite phases. The vibration amplitude of the bottom surface was the largest. Moreover, frequency characteristics of capacitor vibration were studied. The frequency response of the bottom surface was bigger than that of narrow and broad side surfaces for lower frequencies, while was smaller for higher frequencies. It was inferred that bottom surface vibration was mainly caused by elastic forces from direct contact and side surfaces vibration mainly came from damping forces from dielectric liquids.

Primary Differential Pulse Method for Partial-Discharge Detection of Oil-Immersed Inverted Current Transformers

This paper proposes the primary differential pulse method (PDPM) for the live partial-discharge (PD) detection of oil-immersed inverted current transformers. The PDPM is based on the differential current measurements that are taken at the high-voltage side of the current transformer under test. The proposed technique overcomes the problem of an internally grounded test tap and identifies the internal PD based on the polarities of current pulses. The PDPM detection system is composed of current sensors, a data-acquisition module, a fiber-optics-based data-transmission module, and an operation mechanism with an insulating rod. The testing results prove that the designed system is effective for the live PD detection and can also effectively differentiate between PD and outside interference for oil-immersed inverted current transformers.

Initial Features of the Unintended Atmospheric Pressure DC Arcs and Their Application on the Fault Detection

Unintended atmospheric pressure dc arc is very common and harmful in many power applications. There is still no effective detection method for series dc arcs. In this paper, the initial features of current and electromagnetic signal of series dc arc are investigated on an arc generation platform. It is found that the current varies abruptly at the starting moment of the arc, and then a series of stochastic pulses can be observed. Corresponding to the current pulses, electromagnetic pulses are also measured by antenna simultaneously. Based on the abrupt current variation and unique pulse-time pattern features of the series dc arc, a fault detection algorithm is proposed. The proposed algorithm is proved to be robust and effective.

Decomposition characteristics of SF 6 under three typical defects and the diagnostic application of triangle method

Detection of decomposition products is a widely studied method for condition diagnosis of SF₆ 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 SOF₂, SO₂F₂, CO2, and SO₂. Concentration ratios including Φ(SOF₂+SO₂F₂+SO₂)Φ(CO2) and Φ(SOF₂+SO₂)/Φ(SO₂F₂) 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, CF₄, CO, and CS₂, 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.

Audible Noise Characteristics of Filter Capacitors Used in HVDC Converter Stations

This paper presents a detailed investigation of audible noise characterization of filter capacitors by investigating the noise radiation characteristics and analyzing the effect of voltage frequency, phase angle, and magnitude on a single capacitor unit. First, the noise radiation for capacitor surfaces was characterized theoretically using finite rectangular plate model and experimentally measured in a semianechoic room. Second, voltage frequency impact on the noise distribution was investigated through simulation. The simulation results show that noise directivity increases with increasing frequency. The excitation frequency impact on the radiation ratio and the noise level was characterized through a sweep-frequency experiment. Our investigations show that the noise frequency response below 500 Hz is quite low, while from 500 to 2000 Hz, it is significant and increases slowly. Third, the impact of the voltage phase angle on the noise level was investigated. The results show a noise-level difference of 3.9 dB for different phase angles. Finally, we present the voltage magnitude impact on the noise level. Our theoretical analysis and experimental study show a linear relationship between the sound power level and the logarithm of the voltage magnitude with a slope of 40.