Yuqin Ding

Effect of hydrostatic pressure on the polarity effect of impulse breakdown characteristics of transformer oil

The aim of this study was to clarify the hydrostatic pressure effect on the initiation and propagation process of oil streamers under the application of pulsed voltage with different polarities. The statistical characteristics of the impulse breakdown voltage and time lag in transformer oil were studied over a wide range of hydrostatic pressure, and the statistical time lag and the formative time lag were calculated by the Laues pattern. The breakdown voltage distribution can be well fitted by 3 parameter Weibull distribution. The results show that the breakdown voltage increases with the hydrostatic pressure for both positive and negative polarity. However, the increase trend is different, linear for the negative polarity and saturated for the positive polarity. At a fixed voltage, when the pressure is increased, the breakdown time increases slightly for the negative polarity, but remarkably for the positive polarity. The statistical time lag for negative polarity is much larger than positive polarity, and the hydrostatic pressure has greater influence for negative polarity. The characteristics of formative time lag are quite different: for negative polarity, it decreases very slightly with the hydrostatic pressure, but increases remarkably for positive polarity. These differences are due to different initiation and propagation mechanisms, which are also discussed in the paper.

Capacitive voltage sensor array for detecting transient voltage distribution in transformer windings

Experimental investigation of transient characteristics is an important way to design transformer winding insulation structures. Of the previous measuring methods, there has been the problem of direct electrical connection with the winding, which may influence the transient characteristics. This paper presents a new non-invasive method for measuring the transient voltage distribution in transformer windings based on our previous study. The measuring method is based on the capacitive sensor with the stray capacitance between sensing electrode and transformer discs as the high-voltage arm and the film capacitance as low-voltage arm. An RC integrator and impedance adapter have been designed into the measurement circuit to enhance the sensor worked in the integrating mode and broaden its bandwidth respectively. The final measurement circuit can satisfy the need for measuring various kinds of waveforms, with 0.64 Hz lower cut-off frequency and 76 MHz higher cut-off frequency, respectively. The sensor array has been set up to measure the transient voltage distribution in a transformer winding. In this case, a mathematical conversion is put forward to decouple the influence of the adjacent discs on the corresponding disc to get the voltage distribution of different discs of the transformer winding.

Modified thermal circuit model for distribution transformers in three-phase unbalanced operation

The three-phase unbalanced operation of distribution transformer is unavoidable in practice. Under this situation, accidents arising from internal overheating occur easily. Therefore, the hot spot temperature analysis and calculation is of great concern. Thermal circuit model method is widely used to calculate the hot spot temperature of power transformer for the advantages of simple calculation and good accuracy. However, the existing thermal circuit model is a kind of single-phase model, ignoring the interphase thermal coupling effect. To make the model suitable for distribution transformer, we modify the single-phase thermal circuit model by analyzing the internal heat source, reconsidering the influence of environment factors, and implementing the temperature-rising test to calculate the interphase thermal coupling coefficient. The thermal circuit model presented in this paper can accurately calculate the hot spot temperature of distribution transformers both in balanced operating and unbalanced operating. This model can provide a reference for the safe operation of distribution transformers.

Gaseous characteristics of impulse breakdown initiation process in transformer oil

The statistical characteristics of the impulse breakdown time lag in transformer oil are studied over a wide range of pulse width, and the statistical discharge time lag and the discharge formation time lag are calculated by the Laues pattern. Then a physical model for the discharge initiation process in quasi-uniform field is built. It is supposed that the field emission current generated by the small protrusion tips on the cathode surface could heat the liquid near the surface. When the liquid is superheated, nucleation sites would be generated. The nucleation sites would continue to expansion and elongate with the continuous heat supply until the gas in the bubble discharges. In this paper, the nucleation time is calculated based on the vaporization nucleation theory. A theoretical equation of the statistical discharge lag and the strength of the electric field is proposed based on the F-N theory. It is shown that the equation is consistent with the experimental data, indicating the accuracy of the proposed physical model.

Study on thermal circuit model of typical disconnector and current transformer

The internal overheating of power related equipment during dynamic rating is hard to avoid, which would accelerate the aging of insulation and even cause burn out accident. Therefore, thermal field analysis of the equipment is crucial for the safety operation of power system. Disconnector and current transformer are indispensable but easy to be ignored. The internal thermal defects of these devices are likely to become the bottleneck to restrict the development of dynamic rating. In this paper, we established the thermal circuit model of typical disconnector and current transformer, and use the model to calculate the hot spot temperature of these devices. The model takes account of meteorological parameters such as ambient temperature and wind velocity. Steady thermal field and dynamic thermal field are included in the model. The temperature rise test report of short circuit test is applied to modify the model in order to make it more applicable to engineering calculation. Though the calibration, we found that the model calculation results and the experimental data meet well as the deviation is within 10%, proving the accuracy of these proposed models. The thermal field analysis of disconnectors and current transformers will contribute to the safety operation of the entire power system.

Study on gas bubble formation in single-layer paper insulation

In oil-impregnated transformers, the paper insulation between turns in the winding often endures high temperature. During the dynamic short-time rating of the transformer, the temperature of the paper insulation over conductor rises at first. Regarding the old and wet transforms, fast rise of temperature leads to quick moisture migration from paper into oil. At a certain temperature, the migrated moisture can be vaporized in paper or at paper surface, and water vapor is present in the form of gas bubbles in the mixed phase. Bubbles have complex morphological evolution and migration patterns in liquid oil, which has detrimental effects on electrical insulation as they arrive at strong electric field regions. In this paper, we conducted the experiment on a single-layer insulation paper of 0.08mm thick. Results showed that certain temperature is the prerequisite for bubble formation. The bubble inception temperature decreases with the increase of temperature gradient, and the error of bubble initiation temperature is significant at high temperature with relative slow grow. During temperature cycling, the temperature of bubble formation is higher than that in previous cycling, and the duration of bubble phenomenon is shorter. Bubble inception temperature increases rapidly with decreasing moisture content in paper. Moreover, moisture saturation in system greatly influences the bubble inception temperature. When the relative gas saturation increases from 5% to 25%, a decrease of 20°C to 40°C in bubble inception temperature arises. With further increase of moisture content in the system, the decreasing speed of bubble inception temperature slows down, and the difference between different moistened samples gets closer.

Evaluation of the Martin empirical formulae for transformer oil: Statistical meaning of the time parameter

To study the statistical meaning of the time parameter t63% in the Martin empirical formula, we took Kunlun 25# transformer oil as the liquid insulating dielectric and utilized an impulse voltage generator, the impulse width of which could range from 50 ns to 1000 µs, along with the corresponding measuring equipment. The relationships between the breakdown voltage, the breakdown time lag and the discharge probabilities were acquired through experiments under impulses with quasi-uniform field. The results show that the discharge probabilities of different voltage levels under the fixed impulse waveform and the breakdown time lag under the same voltage level both could be well fitted to the three-parameter Weibull distribution functions, whose shape parameter was a constant only correlated to the electrode geometry. Meanwhile, on the basis of the Weibull distribution functions and corresponding impulse waveforms, we defined the effective duration and show the statistical meaning of the time parameter t63%.

Scale effect of the impluse breakdown in transformer oil

As an important dielectric liquid, transformer oil is widely used in power systems due to its higher breakdown strength than compressed gases. The key factor affecting the safe operation of power apparatus is decided by the steep impulse electric strength. In this paper, an impulse voltage generator with adjustable pulse width was set up to generate 0.12/50µs impulse voltage. Breakdown characteristics in transformer oil under pulsed voltages are studied over a range of electrode sizes and gap distances with a hemisphere electrode configuration of various geometric configurations. It is verified that both area and volume effects simultaneously lead to the degradation of the breakdown strength in transformer oil. The impulse breakdown strength is statistically analyzed by using Weibull distribution. By comparing two results of Weibull fitting function, it reveals that the 3-parameter Weibull distribution has a higher goodness-of-fit than the 2-parameter for impulse breakdown in transformer oil. The 3-parameter Weibull shape parameter m for the quasi-uniform field are estimated to be 2.4 to 2.58 and 2.79 to 4.11 under different electrode area and gap distance respectively. These effects are corresponding to the variation of breakdown initiation and propagation process when the dimension of electrode configuration changes. The relationships between the shape parameter m and electrode diameter as well as gap distance are in the forms of m(D)=0.004D+2.384(d=0.5mm) and m(d)=5.27−1.25d−0.423(D=30mm) and the field utilization factor η as a function of shape parameter m has been calculated in which case the relationship between electrode physical dimension and stressed area is established, which can be provided for insulation design directly.

Investigation on bubbling phenomenon in oil-paper insulation

Bubbling phenomenon usually arises from overheated cellulose insulation in oil, which is known for deleterious effects on the insulation strength of oil-immersed transformers. In this paper, the experiment was conducted on a self-heating conductor wrapped with insulation paper. The inception temperature of bubbling phenomenon negatively correlates to the rising rate of temperature and moisture content of paper, which is in accordance with the results of previous research. Besides, bubbles formed under different conditions could be different in morphology and locations. Thereafter, a method for temperature rise was proposed, based on which the models were heated for component research. It is found that the component changes in oil and blanket gas are significant in a short time interval under bubbling conditions. Comparison trials were then conducted on naked conductors and paper-wrapped conductors. Based on the theory of mass transport, it is revealed that the water vapor and carbon oxides contribute a major part in bubble composition, and their proportions vary with the bubbling temperature. This study concludes that bubbling phenomenon at temperature below 120 °C can be recognized as the vaporization of moisture in paper. Moreover, bubbles formed at temperatures around 160 °C can be attributed to the combined effect of water vaporization and gases from cellulose degradation.

Study on impulse breakdown characteristics in transformer oil with different electrode materials

The bubble mechanism based on field emission has long been considered to be responsible for breakdown in liquids under impulses with durations of microseconds. Hence, it is quite reasonable to associate the insulation failure with electrode materials. In this paper, the 50% impulse breakdown voltages and the discharge time delay with different metal electrode materials were acquired through experiments, along with the corresponding three-parameter Weibull distribution functions and the Laue plots. The results show that average breakdown electrical field with different electrode materials are quite distinct, with copper-tungsten being the largest, followed by stainless steel, copper and aluminum. Meanwhile, for that the shape parameters of the time-dependent Weibull distribution functions were all around 1, the average statistical time lag and the average discharge formation time were derived from the Laue plots and the differences were discussed. Furthermore, the appearance of the inflection point in Laue plot proved the existence of two parallel discharge processes. The work function and the etch pit were both taken into consideration to analyze the experimental results.