Review of accumulative failure of winding insulation subjected to repetitive impulse voltages

The premature failure phenomenon that occurs in solid insulation under the application of repetitive impulse voltages is known as the accumulative effect of impulse voltages on a solid dielectric.The accumulative failure of winding insulation is considered to be a core problem resulting from this phenomenon.The main problems include the accumulative failure of motor winding insulation under pulse-width modulation impulses and the accumulative failure of transformer winding insulation under the application of overvoltages,and these problems represent new challenges in the safe and stable operation of power grids.With regards to the above problems,this study reviews the results of recent research on the accumulative failure characteristics and mechanisms of winding insulation when subjected to repetitive impulse voltages.The challenges faced are discussed at the end of this study.

Enhancement of Soil Discharge Channels on Potential Surrounding Buried Cables Under Impulsive Currents

Substations have a large number of signal transmission cables beneath the ground.Both the insulation safety and signal reliability of the cables are affected severely by the electromagnetic field.Under high-amplitude impulsive currents,the dispersion of currents can cause soil discharge and thus cause unexpected distortions in an electromagnetic field.This paper focuses on the distortions of the electric field.In general,soil discharge channels occur in the vicinity of the independent rod.Closer development of the channel might enhance the electric field distribution and the potential surrounding the outer insulation of the cables(i.e.the surface potential on the cable).Therefore,this paper establishes a platform for observing the soil discharge channel and measuring the surface potential.Direction characteristic of the channel is extracted from the captured image of soil discharge channels and the surface potential is obtained by the measured coupling capacitive current on the shield experimentally.This paper also presents an improved model considering a dynamic growing discharge channel for the transient analysis of the grounding electrode.Study results show the surface potential increases as the discharge channel approaches the cable.To quantify this enhancement effect,the ratio of the highest to the lowest value of surface potential in different directions is taken as the multiple of the surface potential increase.The calculated multiples of the surface potential increase are in the range of 1 to 1.64 times under different conditions by the improved model.Therefore,taking the soil discharge channel into account is helpful to accurately analyze the impulsive interference of buried cables.

Impact of time parameters of lightning impulse on the breakdown characteristics of oil paper insulation

At present,the insulation performances of electrical apparatus under lightning impulse voltages are always evaluated by the withstand test of standard lightning impulse voltage(1.2/50μs),which is recommended by IEC Standard.However,the actual time parameters of transformer invading lightning impulse voltage present an obvious dispersibility characteristics.The standard wave shape is only representative of one particular class of lightning waves,which is far from satisfactory to act as the guidance waveform in the design of power equipment insulation.On the basis of the above research background,the relationship between breakdown characteristics of oil paper insulation and time parameters of applied lightning impulse voltage was investigated in this study.The 50%breakdown voltage characteristics of oil paper insulation under lightning impulse voltage with different time parameters were determined.The test results indicate that a short wave front time could lead to relatively low 50%breakdown voltage,while when the wave tail time varied,the 50%breakdown voltage of oil paper insulation barely changed.At last the simulation model of oil paper insulation was developed,and the current flowing through the oil paper insulating system under lightning impulse voltage was calculated.On the basis of the simulating results,the relationship between current and time parameters was analysed.