Heli Ni

Alternating streamer propagation in mineral oil under bipolar oscillating impulse voltage

This study aimed to clarify the basic process of streamer propagation in mineral oil at bipolar oscillating impulse voltage. Shadow images and light signals of streamers showed that under bipolar oscillating impulse, positive and negative streamers propagated in an alternating manner: after polarity reversal, new streamers with opposite polarity were initiated and propagated first through the gaseous channels left behind by former streamers and then toward the ground electrode. The velocity of positive streamers was found nearly an order of magnitude higher than that of negative ones; thus, positive streamers are primarily responsible for the insulation failure of mineral oil. Negative streamers played the role of maintaining gaseous channels and facilitating positive streamers initiation due to their strong heat effect. High oscillation frequency and large damping factor decreased the durations and amplitudes of positive peaks, which restrained positive streamer propagation and further resulted in the increase in the breakdown voltage. Experiments on dielectric behavior of mineral oil were conducted to verify above inferences.This study aimed to clarify the basic process of streamer propagation in mineral oil at bipolar oscillating impulse voltage. Shadow images and light signals of streamers showed that under bipolar oscillating impulse, positive and negative streamers propagated in an alternating manner: after polarity reversal, new streamers with opposite polarity were initiated and propagated first through the gaseous channels left behind by former streamers and then toward the ground electrode. The velocity of positive streamers was found nearly an order of magnitude higher than that of negative ones; thus, positive streamers are primarily responsible for the insulation failure of mineral oil. Negative streamers played the role of maintaining gaseous channels and facilitating positive streamers initiation due to their strong heat effect. High oscillation frequency and large damping factor decreased the durations and amplitudes of positive peaks, which restrained positive streamer propagation and further resulted in the in...

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.

Extension of the empirical formula for pulsed electric strength of transformer oil

Discharges in liquid obey time effect, pressure effect and scale effect. Traditional formulas are usually applied in a narrow range to keep the formula form consistent with Martins formula. In this paper, we have developed a new empirical formula for the electric strength of transformer oil depending on pulse duration, hydrostatic pressure and gap geometry in form Ebr = κe^at^-b_eff P^γ A^-β d^-ξ to be applied in a wider range. In this formula, the time dependent pressure effect is represented by constant y, which can be expressed as γ = a_pe^-b_pt^-c^p_eff. Time effect, area effect and distance effect are represented by e^at^-b_eff, β and ξ, respectively. For transformer oil, κ=0.42, a=0.53, b=0.23, a_p=0.14, b_p=0.93, c_p=0.41, β=0.1 and ξ=0.25±0.06, with E_br, t_eff, F, A and d in MV/cm, μs, MPa, cm² and cm, respectively.

Effects of electrode chemical reactions on SF6 discharge characteristics in extremely inhomogeneous electric fields

SF6 is widely used in the gas-insulated metal-enclosed switchgear or the corona-stabilized gas spark switch applications as a gas dielectric. It is generally believed that the discharge characteristics are only related to the electric field distribution and gas molecular density; however, the electrode chemical reactions can indeed markedly affect the SF6 discharge characteristics in the extremely inhomogeneous electric fields under the steady-state voltage. In this study, we used a needle-plane electrode system to build an extremely inhomogeneous electric field and examined the discharge characteristics within it including U-p characteristics, corona appearance, and corona current. We also analyzed the micro-region characteristics of the electrode surface, including the surface morphology, elemental composition, and chemical state to fully qualitatively determine the role of the electrode chemical reactions in discharge behavior. We found that the N-shaped U-p curve widens, the filamentous leader channels disappear, and the corona current drops suddenly as the duration of the electrode chemical reactions increases. Varying the surface morphology, elemental composition, and chemical state was observed on the electrodes of different polarities through micro-region analysis. The metal fluoride or metal sulfide film on the electrode surface may serve as a resistive coating due to its low electrical conductivity, which obstructs the leader discharge while enhancing the streamer discharge, suppressing the transition from the streamer to leader discharge and altogether significantly altering the discharge characteristics. The resistive coating produced is the primary cause of the electrode chemical reaction effects on the discharge characteristics, primarily as it depresses the transition from the streamer to leader discharge. The results presented here may provide useful guidelines for further research on SF6 discharge under inhomogeneous electric fields.SF6 is widely used in the gas-insulated metal-enclosed switchgear or the corona-stabilized gas spark switch applications as a gas dielectric. It is generally believed that the discharge characteristics are only related to the electric field distribution and gas molecular density; however, the electrode chemical reactions can indeed markedly affect the SF6 discharge characteristics in the extremely inhomogeneous electric fields under the steady-state voltage. In this study, we used a needle-plane electrode system to build an extremely inhomogeneous electric field and examined the discharge characteristics within it including U-p characteristics, corona appearance, and corona current. We also analyzed the micro-region characteristics of the electrode surface, including the surface morphology, elemental composition, and chemical state to fully qualitatively determine the role of the electrode chemical reactions in discharge behavior. We found that the N-shaped U-p curve widens, the filamentous leader channel...

On polarity effects in streamer discharge of mineral oil under bipolar oscillating impulse voltage

As a supplement to our previous studies on alternating streamer propagation in mineral oil under bipolar oscillating impulse voltage [Ni et al., Phys. Plasmas 25, 072125 (2018)], this study was conducted to further analyze waveform-dependent 50% breakdown voltages and polarity effects. Contrary to breakdown under unipolar impulses, breakdown voltages under positive bipolar impulses are generally higher than those under negative impulses, except for when the damping factor approaches 1. The polarity effect under bipolar impulses, per alternating streamer propagation in mineral oil, is attributable to the characteristics of positive and negative streamers as well as their roles in the prebreakdown process. The existence of polarity effects under bipolar impulses is also dependent on the arrangement of electrodes.

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 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%.