Li Chengrong

Experimental study on the evolution of surface discharge for oil-paper insulation in transformers

Oil-paper is the main insulation of Oil-immersed power transformers. Reflecting the deterioration of insulation effectively, partial discharge (PD) has become one of the significant means to astimate the insulation condition of power transformers. Surface discharge is one of the most important type of PD. Studying the long-term process of this PD pattern, by the means of acquiring PD data and observing the length of discharge channel along the pressboard surface, this paper puts forward the descriptors and characteristics for each discharge stage, and lays the foundation for identifying the fault severity of surface discharge. In order to get reliable and objective PD data, we establish a temperature-controlled experimental platform, which can simulate the fault of surface discharge without corona under certain high voltage, and use ultra high frequency (UHF), ultra wide band (UWB), conversional Impulsive current(CIC) to study this PD evolution process. Voltage-elevating method is used to accelerate the discharge evolution, and the statistical characteristics of surface discharge process are extracted, which is from the initial discharge to the insulation breakdown. The observed data, e.g. the maximal amplitudes of PD (Urn), the number of PD (N) and the energy of PD (P), appear to have an increasing trend. The closer to breakdown, the better correspondence of the detected signals is.

Effect of Ageing on Insulating Property of Mineral Oil-based TiO 2 Nanofluids

A class of mineral oil-based nanofluid has been developed by modifying mineral oil with TiO2 nanoparticle to enhance their dielectric performance. In this paper, we present the investigation results of aged nanofluids, which have been subjected to accelerated aging condition of 130°C for 6 days. Both AC breakdown voltage and PDIV tests were carried out according to ASTM standards respectively. It was found that AC breakdown voltage of nanofluids has been largely improved before and after aging 6 days. The mean value of PDIV in aged nanofluid achieves an increase of 1.16 times than that of mineral oil. In addition, the results of statistical data indicate that 1% probability breakdown voltage of nanofluid is 78.34kV, which is 8kV higher than that of mineral oil. The experimental results demonstrated that both breakdown voltage and PDIV of mineral oil can be significantly improved by modifying with TiO2 nanoparticles.

Experimental study on the relationship between partial discharge and gas decomposition products in SF6 insulated electrical equipments

SF6 decomposition serves as an effective means for online fault detection and diagnosis of SF6 insulated equipments. The relationship between SF6 decomposition products and partial discharge (PD) in SF6 insulated equipments remains a significant theoretical and practical issue to be addressed. Based on the simulations of protrusion discharge, floating discharge and surface discharge in SF6 insulated equipments, the present experiment studied the impacts of differentiated PD patterns, PD severities and SF6 pressures on the concentrations of SF6 decomposition products as well as the change in concentrations over time. The experimental results indicate that three kinds PD pattern discharges produce SOF2+S02 and HF, the concentration of the SOF2+S02 being always higher than HF. Generation of both gases increase with voltage application time; The concentrations of SOF2+S02 and HF differ from differentiated PD discharge strength, and increase as discharge strength ascends; A negative relationship exists between SF6 gas pressure and the generation speed of SOF2+S02 and HF. It is concluded that SF6 decomposition products, SOF2+S02 and HF in particular, could be used as a means for on-line PD faults detection and diagnosis.

Characteristics of surface charge on oil impregnated pressboard under non-uniform DC stress

It is very important of the surface charge distributions on the oil impregnated pressboard under nonuniform DC electric field, for the design and application issue of a converter transformer, which is not reported in the literature. In order to have a better understanding of the characteristics of the surface charges on oil impregnated pressboard produced by the non-uniform DC electric field, we studied surface charge distribution on oil impregnated pressboard produced by the needle plate electrode based on an electrostatic capacitive probe measuring system. It is found that under the highly non-uniform electric field, the surface charge polarity on oil impregnated pressboard mainly depend on the polarity of applied voltage and the accumulation and dissipation characteristics of the surface charges under the positive stress are different from that under the negative stress.

Recent progress and future perspective on condition monitoring and diagnostic techniques for power equipments in China

Under the general backgrounds of fast development of economy, Chinapsila power industry is undergoing a new upsurge of infrastructure construction. With the urgent need of the whole society for reliable power supply, condition based Maintenance strategy is becoming the mainstream of the operation and management of the power system. In this paper, recent progress of condition monitoring and diagnostic technologies for major power apparatus are reviewed. Also, some statistical data on practical on-line monitoring applications in China are introduced. At last, the key problems and trends of CMD are discussed with a viewpoint future perspective.

Experimental study on the development characteristics of point discharge in GIS

Studying the characteristics of partial discharge development is conducive to make an accurate assessment of the severity of discharges and give a reasonable maintenance strategy for the equipment. Point discharge is a typical failure discharge fault occurred in GIS equipment, the differences of the characteristics of the PRPD patterns produced by UHF partial discharge enveloped signals in different development stage is studied and the variation with the test time of the average amplitude and number of discharges in each sampling period, the max discharge amplitude ratio and discharge frequency ratio of positive and negative half-cycle is discussed in this paper. It provides a basis for assess the severity of partial discharge. The results show that, with the extension of the test time, the average amplitude and number of discharges increase first, and then decreased, while the ratio of maximum discharge amplitude on the positive half-cycle to the maximum discharge amplitude on the negative half-cycle and the ratio of the number of discharges on the positive half-cycle to the number of discharges on the negative half-cycle increase gradually, and the decrease rate with the discharge amplitude of the number of discharges which is defined in this paper becomes faster and faster.

Multi-PD sources location by UWB RF detection on power transformers and a case study on-site

Ultra-wideband (UWB) RF sensor-array PD location is a novel technology based on antenna array detection and time-difference algorithm for power transformers. It has demonstrated high sensitivity and accuracy for single PD source by experiments in laboratory. In this paper, a further study has been carried out for multi-PD sources location using the UWB RF sensor array PD location method. Moreover, a practical location case is given on a real transformer. The results indicate that the method presented in the paper is effective for multi-PD source location and has promising future in real transformers.

Homogeneous dielectric barrier discharge in air for surface treatment

Dielectric barrier discharges in atmospheric pressure gas, especially in air, are usually made of a large number of filamentary micro-discharges, which may cause the ununiformity of the surface treatment. In this paper, a new method to obtain a homogeneous dielectric barrier discharge in air is proposed. The homogeneous discharge, which is of a nature in between those of the filamentary discharge and the glow discharge, is very efficient as a means of modifying the hydrophilicity of textile or non-woven and does not cause any damage to the specimen. The homogenous discharge occurs by using of a cylinder-plane electrode geometry. The electrical characteristics of the cylinder-plane dielectric barrier discharge were compared with those of a parallel-plate dielectric barrier discharge and a metallic cylinder-dielectric plane discharge. Compared to the parallel-plane dielectric barrier discharge and the metallic cylinder-dielectric plane discharge, the distribution of the current and temperature of the homogeneous discharge gap is more uniform. It is proved experimentally that the homogeneous discharge in ambient air is a promising surface treatment method for textile and non-woven.

The design of temperature monitoring system for power cable joint

Power Cable Joint is the area where the faults are often happening on the power cable. Real-time monitoring of Power Cable Joint temperature can discover the joint overheating, and can evaluate insulation aging degree, and then, potential safety hazard of the cables will be removed timely. In this paper, a temperature measurement method based on embedded sensors inner Power Cable Joint has been proposed, and a set of temperature monitoring system for Power Cable Joint has been developed. The temperature monitoring system was composed of embedded sensors, data acquisition unit, current induction power supply, data transmission based on combination of ZigBee and GPRS, data analysis software. The temperature measurement system has the advantages of high reliability, real-time and highaccuracy, the long-distance temperature measurement for Power Cable Joint has been achieved.

A novel sensor for UWB RF PD location and experimental investigation on a 220kV transformer

Ultra-Wideband (UWB) RF partial discharge (PD) location technique is a new method for PD source location on power transformers, which is based on multi-antenna PD detection and Huygens-Fresnel principle. In this paper, a novel sensor with 2GHz–8GHz bandwidth for UWB RF detection has been designed. A new location algorithm named Space-Grid-Search Algorithm has been proposed which greatly eliminate the divergent effect of analytical method. Four typtical artificial PD models are applied to a real 220kV transformer. Then experiments have been carried out for PD location on the transformer using UWB RF technique. The test results indicate that the average error of the pre-set PD sources was about 30 centimeters.