Zhipeng Lei

Partial discharges of cavities in ethylene propylene rubber insulation

The aim of this paper is to investigate the characteristics of partial discharge (PD) within dielectric-bounded and electrode-bounded cavities which are embedded in Ethylene Propylene Rubber (EPR) by using plane to plane electrode. The theoretical analysis is carried on the electric field strength and PD inception voltage of cylindrical and ellipsoidal cavities in different dimensions. The samples with the different dimensions of artificial spherical cavities in EPR are made and the PD characteristics in the cavities, such as PD number, amplitude and phase, are measured by professional instrument in laboratory. It has been found by the experiments that the symmetric PD distribution changes from “rabbit-like” to “turtle-like” shape with the time of applied voltage. With the increase of applied voltage on the samples, the PD inception phase becomes prior to the zero crossing of positive and negative half-cycle. Meanwhile, PD pulse repetition rate has an increase trend with increasing the experimental voltage and the amplitude of PDs occurred in dielectric-bounded cavities changes from a higher value to normal during a short time. However, no obvious amplitude change of PDs occurred in electrode-bounded cavities has been observed in the experiment. As a result, internal PDs occurred in cavities is one of the prime reasons resulted in the degradation and corrosion of EPR.

Slot discharge pattern of 10 kV induction motor stator coils under condition of insulation degradation

In the manufacturing process of the 10 kV induction motors, a semi-conductive layer with certain resistance covering the linear part of the stator coil is often used to suppress the partial discharges (PDs) happened in the slot. However, due to electromagnetic force while the motor is running, slot discharge is still one of the main causes of stator insulation degradation. In this paper, an adjustable width mockup core slot is designed using silicon steel sheet. Several 10 kV real machine stator coils subjected to varying aging degrees are adopted as test coils. The phase resolved partial discharge (PRPD) patterns of slot discharge are analyzed using a PRPD analyzer and the variation law of slot discharge patterns with voltage variation is illustrated. The investigation result has been applied to the online monitoring and pattern recognition of the insulation for a 10 kV induction motor in a coal mine and it has great application significance.

Effects of vapor with different chemical properties on corona partial discharges of stator windings

The effects of vapor with different chemical properties on corona partial discharges (PDs) at the junction section of the semi-conductive tape and the stress grading tape of stator windings were investigated. The phase resolved partial discharge (PRPD) patterns were collected during the experiment to obtain the characteristics of the PDs. The result has confirmed that relative humidity (RH) of the vapor with different chemical properties can cause corona discharge to change significantly. The acidic vapor can suppress corona discharge effectively. When RH exceeds 60%, the corona discharge would be hardly induced even though the voltage is boosted up to twice the operating voltage. When RH is over 80%, corona discharge will develop into phenomenon of creeping on the surface of the ground wall insulation and can hardly be collected through the capacitive coupler.

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.

Influence of cavities on the dielectric properties of Ethylene Propylene Rubber insulation

Cavities are one of the typical defects leading to the degradation of Ethylene Propylene Rubber (EPR) insulation in extruded power cables. It had been observed by previous experiments that ageing phenomena on the surface of dielectric bounded cavities (DBC) and electrode bounded cavities embedded in EPR insulation was different. The surface of DBC is corroded by partial discharges (PD) which leads to the decrease of surface conductivity. To describe this phenomena, it is essential to analyse the dielectric properties of EPR insulation. So this paper is concerned on the investigation of EPR dielectric behavior which involves permittivity and dissipation factor in frequency domain (0.01 Hz ~ 1 MHz). It has been found by the experiments that the dielectric loss of pure samples are increased with the increase of temperature in low frequency region. The real permittivity of pure EPR have the temperature dependence only in lower frequency region. The real parts of complex permittivity of samples with DBC and EBC is larger than that of EPR without cavities. Moreover, the dielectric loss of samples with DBC is more sensitive to frequency in low frequency region. Thus, the study of dielectric properties is effective for assessment of EPR insulation caused by cavities. The corrosion can be observed on the surface of DBC aging under AC voltage.

Influence of thermal aging on AC leakage current in XLPE insulation

Cross-linked polyethylene (XLPE) has been widely used as cable insulation material because of its excellent dielectric properties, thermal stability and solvent resistance. To understand the influence of thermal aging on AC leakage current in XLPE insulation, all XLPE specimens were aged in oven in temperature range from 120 °C to 150 °C, and a series of tests were conducted on these XLPE specimens in different aging stages to measure the characteristic parameters, such as complex permittivity, leakage current and complex dielectric modulus. In the experiments, the effects of thermal aging, temperature and frequency on the AC leakage current in XLPE insulation were studied by analyzing complex dielectric constant and dielectric relaxation modulus spectrum, the change of relaxation peak and activation energy. It has been found that the active part of leakage current increases sharply with the increase of aging degree, and the test temperature and frequency have an influence on AC leakage current but the in...

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.

AC flashover performance and polarization current of ethylene propylene rubber under different temperature

In order to evaluate the influence of AC flashover on the insulation performance of Ethylene Propylene Rubber (EPR) at different temperature, the characteristic indicators were studied, based on the polarization and depolarization current (PDC). In the paper, the partial discharge inception and extinction voltage and the flashover voltage were firstly measured under AC voltage at the ambient temperature and 90 °C, respectively. Before the occurrence of flashover, the partial discharge distribution was discussed under different voltage amplitude. Then, PDC was measured after the flashover test. The fit lines of PDC were drawn through the empirical formula for explaining the mechanism of AC flashover over the surface of EPR.

Effect of surface electrical tracking of ethylene propylene rubber on return voltage

Electrical tracking is one of the main reasons for the deterioration of insulation surface. According to the problem of the surface electrical tracking erosion of insulation used in the high-voltage mining cables, the Ethylene Propylene Rubber samples with different electrical tracking development stage were selected in the paper. The relationship between the surface electrical tracking erosion degree of Ethylene Propylene Rubber and return voltage were analyzed. Characteristic parameters, which including the maximum value of return voltage, the constant of dominant time constant and the initial slope of return voltage were calculated based on the return voltage curves. It has been found that the maximum value of return voltage, the constant of dominant time and the initial slope of return voltage are closely related to the electrical tracking development stage. The constant of dominant time is significantly reduced with the increasing of the surface electrical tracking erosion degree. Research results demonstrate that the severity degree of electrical tracking erosion can be discriminated with Return Voltage.

Characteristics of flashover and discharge over the surface of Ethylene Propylene Rubber

Defects of Ethylene Propylene Rubber (EPR)cablesare the source of partial discharges, which will lead to insulation degradation. The flashover voltage and discharge properties along the surface of EPR were measuredunder edge to edge electrode, needle to edge electrode and rod to ring electrode. Experimental results show that the flashover voltage magnitude over EPR depends on the type of voltage. The inception voltage magnitude of surface dischargerelates to the formation and development of electron avalanche and streamer. The surface discharge distributions in different electrode configuration are different. Moreover, the applied voltage drops to low values when the spark discharge orarc discharge is taking place.