gyan Lin

Breakdown performance and electrical aging life model of EPR used in coal mining cables

Ethylene Propylene Rubber (EPR) with excellent dielectric properties has been widely used as cable insulation specially to be used where cables need to be moved frequently, such as in coal mine. Usually, theses cables will be bending excessively, coupled with the electric stress and thermal stress, the aging of the insulation tend to accelerate and the strength of insulation could decrease, with the time prolonging, breakdown may be happened. So, it is necessary to make research on the breakdown characteristics of EPR. To understand the breakdown performance, a step-stress test method is used in the accelerated life tests of EPR and WEIBULL distribution is used to analyze the test dates. Finally, based on inverse power life model, the voltage life index of EPR have been obtained. Finally, the electrical aging life model of EPR have be obtained. Research on the breakdown characteristics and the voltage life index of EPR is significance for insulation structure design and residual life assessment.

Characteristic analysis of asynchronous motor fault signal based on wavelet packet decomposition

This paper presents characteristic frequency analysis of asynchronous motor fault signal based on wavelet packet decomposition. Asynchronous motors current and vibration signals are acquisition objects, Fault diagnosis is based on frequency information contained in motors signals. The wavelet packet decomposition (WPD) method is applied to extract their characteristic frequency, and then its necessary to ascertain WPD layers according to signal sampling frequency and the requirement of frequency resolution. A method is put forward, which characteristic frequency is located quickly to node in the WPD layer. It may promote the application of wavelet packet analysis method in motor fault diagnosis system.

Development of Intelligent Information Monitoring System in Greenhouse Based on Wireless Sensor Network

In order to solve the problems of complicated wiring, difficult maintenance and easy line corrosion in traditional greenhouse monitoring system, a greenhouse intelligent information monitoring system based on wireless sensor network was developed. In which ZigBee wireless sensors were used to collect field environmental parameters, such as indoor temperature, indoor humidity, light intensity, carbon dioxide concentration, soil humidity and etc. The information center interface was designed by Kingview, which can realize centralized monitoring, data display, data storage and data mining. The system can monitor and control greenhouse environment parameters, with four control modes, i.e. manual control, automatic control, intelligent control and remote control. The system has been applied in demonstration base for three years, and it has been shown that it is accurate, reliable, stable and friendly UI, which could meet the requirements in the modern management of solar greenhouse.

Partial discharge properties of electrical trees in XLPE

Cross-linked polyethylene is widely used as the insulation of power cables due to its superior insulation and mechanical properties. Electrical trees is one of the main reasons for the breakdown of cable insulation. In order to explore the growth process and partial discharge characteristics of electrical trees in cross-linked polyethylene, cuboid samples taken from a commercial cable and a needle-plane electrodes were used to develop electrical trees. In this paper, a real-time observation system was designed to record the growth picture of electrical trees and a partial discharge measurement system was used to obtain the partial discharge patterns during treeing. The experimental results show that the structure of electrical tree varied in stages. There are four stages during the growth of electrical trees, namely, initiation stage, slow growth stage, rapid growth stage and sustained growth stage. The outline of PD distribution patterns is bimodal shape. During treeing, the numbers of PDs in the two cycle are almost equal.

The effects of cavity on the growth of electrical trees in XLPE

Cross-linked polyethylene(XLPE) is widely used as the insulation of power cables due to its good properties. Nevertheless, some inevitable defects like cracks and cavities existing in cables will initiate electrical trees which is one of the main reason leading to the breakdown of cable insulation. Thus, it is necessary to conduct the study on the effects of cavity on the growth of electrical trees in XLPE. A needle electrode was inserted in XLPE to simulate the concentration of electrical field in this paper. Two kinds of samples were designed to explore the effects of cavities on the growth of electrical trees in XLPE. Samples with a cone-shape cavity underneath the point of the needle were named sample B, and other samples without cavity were sample A. The initiation and development of electrical trees under 50Hz ac voltage were investigated in this paper. The measurement system of electrical trees was designed to measure the partial discharge and to record the growth of electrical trees. The experimental results shows that tree initial voltage of XLPE decreases due to the existence of a cavity. A long time stagnation appears in the process of tree growth in samples with a cavity, which prolongs the tree growth cycle. The partial discharge phase width in the initiation period of sample B is wider than that of sample A.

Development of the space charge measurement techniques based on pulsed electric-acoustic method for HVDC power cables

At present, the space charge effect has become one of significant problems in the field of high voltage and insulation, which restricts the development of HVDC power cables. The pulse electric-acoustic (PEA) method has been accepted as a common non-destructive approach to measure the distribution of space charge in solid dielectrics. The basic principle of space charge measurement and characteristics of pulse electric-acoustic method is summarized in this paper. Then, the improvement of the space charge measurement system in recent two decades is introduced from the pulse injection mode, electrode configuration, and the recovery of detected space charge signals. Especially, the application of PEA method in coaxial power cables is expounded. Finally, the prospect of application in HVDC power cables aging evaluation based on the pulse electric-acoustic method is discussed.

Effect of water vapor on oil-contamination discharge of stator windings

It is a common phenomenon for high voltage (HV) motor that bearing bush leaks oil during operation. The oil contamination will etch stator windings and dissolve the ground-wall insulation, make the insulation and wire stratified, reduce the strength of insulation, and finally lead to partial discharge (PD). In addition, water vapor will make the condition worse. Contamination or water vapor covered on the end of stator winding will lead to an intense surface discharge and surface tracking. In this paper, the effects of oil-contamination and water vapor of different relative humidity (RH) on the surface PD taken palce in HV stator windings are discussed. The phase resolved partial discharge (PRPD) patterns were monitored during the aging period. The result confirms that water vapor has a significant impact on the oil contamination discharge. To a certain extent, water vapor can suppress the surface discharge, however, when the RH is higher than 80%, tracking will occur on the surface of the stator windings, and the insulation performance is degraded rapidly. Researching the oil-contamination discharge under different RH will provide a judging basis for the on-line monitoring and fault diagnosis of the 10 kV three-phase asynchronous motors.

Influences of humidity and temperature on oil contamination discharge of HV motor stator windings

The leaking oil, as one typical contamination in stator windings of high voltage motor, will corrode stator winding and be dissolved in the grounding insulation layer. The oil contamination could lead to the stratification of insulation, the reduction of dielectric strength and the occurrence of partial discharge. In order to investigate the influence rules of humidity and temperature on oil contamination discharge of stator windings, two discharge winding models with oil contamination were made. At twice the rated line-to-earth voltage of 6 kV, two windings were aged for 240 hours under different relative humidity and temperatures, respectively. The phase resolved partial discharge (PRPD) patterns were monitored and analyzed during the short aging period. The experiment results confirm that temperature and ambient humidity have a significant impact on the oil contamination discharge of winding. The PD intensity of oil contamination increases with the rise of temperature. The creeping phenomenon will take place at the higher relative humidity. In addition, it will provide a judging basis for the on-line monitoring and fault diagnosis of the 10 kV three-phase asynchronous motors.

Analysis of partial discharge of spherical cavities in EPR at different frequency

Cavities are one of the typical defects resulting in partial discharges (PDs) in extruded power cables. This paper focuses on studying the PD characteristics of arbitrary dielectric bounded cavities and electrode bounded cavities embedded in EPR insulation, such as PD distribution, PD inception and extinction voltage, under different type of testing voltage. During the experiment, the applied voltage is sinusoidal waveform at frequencies of 20, 50 and 200 Hz and damped AC waveform at frequencies of 20, 50, 200 and 500 Hz. The amplitude of applied voltage, the peak value, is 1.25, 1.5 and 2 times of the amplitude of PD inception voltage (PDIV). The diameter of cavities is 1.2, 1.5 and 2.0 mm. A batch of five simples has been tested during PDIV test. Each sample has been tested five times so as to obtain accurate data. It has been found by the experiments that the PD inception voltage increases with increasing frequency of applied voltage. The symmetrical PDs distribution can be observed in the time lag. It seems that the PD distribution depends on the frequency. The maximum PD amplitude at higher frequency is slightly larger than at lower frequency. PD distribution spreads more widely at higher frequency. It is also found that the cavity diameter and the amplitude of applied voltage influence the frequency dependence of PD distribution. The speed of degradation of EPR insulation is related with the frequency of applied voltage. PDs can also corrode the surface of dielectric bounded cavities embedded in EPR insulation, whereas there is not obvious change on the surface of electrode bounded cavities.