Tonglei Wang

The oil-paper insulation breakdown characteristics under non-standard lightning impulse voltages

The non-standard lightning impulse voltages have variable front time, duration time and may have high-frequency components superimposed on the waveforms due to the reflection in the substation and other factors. These surges can propagate to the transformer winding and may even lead to the breakdown of the winding insulation. To maintain the high reliability in insulation performance of the oil-immersed transformer and offer reference to the design of the insulation in the transformers, it is necessary to obtain the oil-paper insulation characteristics under non-standard lightning impulse voltages (non-standard LIWs). In this paper, two electrode models are established to simulate the insulation configuration of windings in oil-immersed transformers. The experiments were carried out on the breakdown characteristics of the electrode model in oil. The breakdown voltages of oil-paper insulation for different front time of impulses, oil-gap distance and insulation structure were measured and then compared with the breakdown voltages under standard lightning impulse waveform (SLIW). In the examined range, the dielectric breakdown values under steep front non-standard lightning impulse waveforms were much higher than those under standard lightning impulse waveforms in all cases. Breakdown voltage of turn-to-turn model under oscillating lightning impulse waveform (OLIW) is much higher than that under SLIW. The experiment results clearly showed that the same insulation structure could withstand higher steep front non-standard LIWs and OLIWs, and these findings may offer a reference to clarify the insulation margin between non-standard LIWs or OLIWs and SLIWs.

Measurement and analysis of transient overvoltage distribution in the low-voltage winding of 1000 kK power transformer without oil immersion

High frequency voltage oscillations will be generated when transient overvoltages intrude into power transformers. These oscillations may cause damage to the insulation of transformers. In this paper, a capacitive coupling sensor array was presented for measuring voltage distributions and voltage gradient distributions in low-voltage winding of a 1000 kV power transformer. A variable impedance transmission line between impulse generator and winding was also designed to avoid distortion of the applied impulses. The results indicate that under the lightning impulse voltages, voltage distributions have been restrained below applied voltage at sandwich-interleaved section but exceed the applied voltage in continuous coils in the first half of the winding and lead to a very nonuniform voltage distribution. Longer duration time will result in a more nonuniform distribution. The amplitude of the transient voltage under steep front square wave reduces faster downward the winding than that under lightning impulse voltages. A shorter rise time will lead to a much worse distribution. In addition, a dominated oscillation of 63 kHz is found under both impulse voltages and steep front square waves.

Measurement method of transient overvoltage distribution in transformer windings

This paper presents a new method for measuring the very fast transient overvoltage (VFTO) distribution in the windings of transformers. Classical methods usually have direct electrical connections with the windings, which can influence the transient process. The method here is based on the coupling capacitive sensor, and wide frequency band measurement of transient overvoltage can be realized. The transformer winding used in this experiment is the low voltage winding of a 1000kV grade UHV power transformer. Three kinds of coils (sandwich-interleaved coil, interleaved coil and continuous coil) are used in the winding. Thirty-six sensors are distributed by equivalent distance from the first to last disc, and a mathematical conversion is used to get the corresponding disc transient overvoltage. The voltage and voltage gradient distributions of different discs are mapped and studied. The effects of rise time and fall time on the distributions are studied. By analyzing the experiment result, several conclusions can be drawn: the longer the rise time, the more homogeneous the voltage distribution, but the fall time has little effect. The voltage gradient is large in the first several discs and the interface of the two different structures of the windings.

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.

Experimental study on reverse recovery characteristics of high power thyristors in HVDC converter valve

The high power thyristor is the basic unit in HVDC converter valve, and its transient performance has a tremendous influence on the reliability, stability of the whole power system and designs of thyristor control unit. For this reason, each thyristor needs to be tested routinely before the operation of the power system. However, there is no specification for the routine test of thyristor valve. The test methods of thyristors are associated with the relationship between reverse recovery characteristics and external operating conditions. Therefore, in order to provide a reference and basic datum for the routine test of thyristor valve, its necessary to investigate the reverse recovery characteristics of high power thyristor in converter valve. In this paper, we focus on the evaluation of thyristor reverse recovery time by forward current, namely forward current amplitude, conduction pulse width and rate of change of commutating current. The influences of thyristor reverse recovery time have been investigated to determine the parameters of thyristor testing based on testing the reverse recovery process of thyristor and analyzing the influence mechanisms. The results show that the reverse recovery time mainly depends on peak forward current and commutating di/dt, where reverse recovery time decreases with the increase of the commutating di/dt and is directly proportional to peak forward current; peak forward current relative to the commutating di/dt is the main influence factor of reverse recovery time; the ratio of turn off time and reverse recovery time is less than 1.2, which is particularly significant for the evaluation of thyristor reverse recovery ability by impulse test in thyristor testing.

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.

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.

The influence of temperature on Partial Discharges and wormhole effect of oil-paper insulation under DC voltage

Partial Discharges (PDs) are one of the most severe threats to electrical insulations, while the failure of which turns out to be the main cause of power equipment outages. PD activity is related to different mechanisms, and out of which the change of temperature will influence it greatly. In order to study the effect of temperature on PD characteristics, a needle-plane oil-paper insulation model was adopted. Via changing oil temperature, PD inception voltage, PD repetition number and breakdown voltage of the model were recorded and further analyzed. The experimental results show that PD inception voltage and breakdown voltage of the insulation model decreases with the rise of temperature, whereas PD repetition rate tends to increase when temperature rises. Based on the facts an equation set is raised to analyze the phenomenon and according to which temperature influences the resistivity of dielectrics and therefore the discharging behavior. Besides, at higher temperature fibers are easier to gather around the needle electrode on the surface of the pressboard. The final breakdown process also exhibit a wormhole effect when oil temperature is high enough. Due to the wormhole effect, breakdown of oil-paper insulation at a high temperature happens easily. Therefore oil temperature should be strictly controlled not only to avoid PD activity, but the wormhole effect as well.