Jierui Zhou

Towards Optical Partial Discharge Detection with Micro Silicon Photomultipliers

Optical detection is reliable in intrinsically characterizing partial discharges (PDs). Because of the great volume and high-level power supply of the optical devices that can satisfy the requirements in photosensitivity, optical PD detection can merely be used in laboratory studies. To promote the practical application of the optical approach in an actual power apparatus, a silicon photomultiplier (SiPM)-based PD sensor is introduced in this paper, and its basic properties, which include the sensitivity, pulse resolution, correlation with PD severity, and electromagnetic (EM) interference immunity, are experimentally evaluated. The stochastic phase-resolved PD pattern (PRPD) for three typical insulation defects are obtained by SiPM PD detector and are compared with those obtained using a high-frequency current transformer (HFCT) and a vacuum photomultiplier tube (PMT). Because of its good performances in the above aspects and its additional advantages, such as the small size, low power supply, and low cost, SiPM offers great potential in practical optical PD monitoring.

Partial Discharge Monitoring on Metal-Enclosed Switchgear with Distributed Non-Contact Sensors

Metal-enclosed switchgear, which are widely used in the distribution of electrical energy, play an important role in power distribution networks. Their safe operation is directly related to the reliability of power system as well as the power quality on the consumer side. Partial discharge detection is an effective way to identify potential faults and can be utilized for insulation diagnosis of metal-enclosed switchgear. The transient earth voltage method, an effective non-intrusive method, has substantial engineering application value for estimating the insulation condition of switchgear. However, the practical application effectiveness of TEV detection is not satisfactory because of the lack of a TEV detection application method, i.e., a method with sufficient technical cognition and analysis. This paper proposes an innovative online PD detection system and a corresponding application strategy based on an intelligent feedback distributed TEV wireless sensor network, consisting of sensing, communication, and diagnosis layers. In the proposed system, the TEV signal or status data are wirelessly transmitted to the terminal following low-energy signal preprocessing and acquisition by TEV sensors. Then, a central server analyzes the correlation of the uploaded data and gives a fault warning level according to the quantity, trend, parallel analysis, and phase resolved partial discharge pattern recognition. In this way, a TEV detection system and strategy with distributed acquisition, unitized fault warning, and centralized diagnosis is realized. The proposed system has positive significance for reducing the fault rate of medium voltage switchgear and improving its operation and maintenance level.

Research on partial discharge monitoring system of switchgear based on wireless distributed TEV sensors

Metal enclosed switchgears, which have been widely used, play an important role in power distribution network. PD detection is an effective way to identify potential faults and inspect insulation defects for high-voltage switchgear. Occurring inside a switchgear, PD would emit electromagnetic (EM) wave that propagates and leaks to the outside, and it also excites the surface current on the metal wall of the switchgear. As a result of current and impedance of the material, it would produce a transient earth voltage (TEV) that could be detected by TEV sensor. Therefore, PD monitoring based on the non-intrusive TEV method has the great engineering application value for estimating insulation condition of switchgear. A wireless distributed on-line monitoring system based on TEV detection method is proposed in this paper. The results show that the TEV PD online monitoring system has the characteristics of high reliability, anti-interference ability, less maintenance.

Multispectral light pulse detection on partial discharges initiated by metal particle on insulator in SF 6 gas

Gas Insulted Switcher (GIS) is one of the most important equipments in power system. Partial discharges (PDs) detection has the vital significance to the condition diagnosis of GIS. In this paper, a multispectral optical PD detection system based on three photomultipliers (PMs) with corresponding light filters in the front is built to detect PD under power frequency voltage. The variations of the optical pulse parameters with increase of SF6 gas pressure and distance between PMs to PD source were investigated in various spectrum ranges. Average magnitude of PD light pulse, pulse repetition rate and the number of photons per unit time were proposed as the characteristic parameters. Phase-Resolved Partial Discharge (PRPD) pattern was employed as a tool for statistical analysis of PD light pulse at various applied voltage, as well as under various spectral bands.

Electrical conduction of aged oil impregnated paper

Oil impregnated paper is an important solid insulation dielectric in power transformers. Thus it is very significant to understand the dielectric properties of the oil impregnated paper. In this paper, a conductivity measuring system was built to explore the conduction mechanism of oil impregnated paper. This measuring system is able to measure the conduction current in the samples. Based on the system, the mathematical relationship between conduction current and field strength was studied firstly. The results show that the conduction current in oil impregnated paper is mainly caused by electrons injected from the electrodes. The Schottky coefficient ßs was calculated both in theoretical and experimental methods, and both results are in good agreement. The reasons for deviation between two results are analysed. The conduction current of aged oil impregnated papers were measured as a function of different electrical fields. The results show that with the increase of aging time, the conductivity of oil impregnated paper increases and the permittivity decreases.

High bandwidth measurement of partial discharge current pulses based on the optimized needle-plate electrode system

In order to obtain the nanosecond partial discharge pulse current signal of needle-plate electrode, a new measurement method of wideband PD pulse current is proposed in this paper. By the numerical calculation of current parameters, the optimized needle-plate electrode structure and the accurate finite element simulation, the experimental platform of the fast PD pulse current detection is set up and its bandwidth is up to 1.81GHz. The testing results indicate that, the established detection system has the some advantages, i.e., high sensitivity, good frequency response characteristics, strong anti-disturbance capability and virtually no waveform distortion, which lays the foundation for further researching the relation between the macroscopic PD pulse current waveform and the microcosmic discharge process.

Transformer oil-based nanofluids velocity measurement under DC voltage

The transformer oil-based nanofluids have significant improvements in their thermal conductivity and dielectric strength than traditional transformer oil, so they have a wide prospect of application. In order to explore the mechanism of the thermal dissipation enhancement of transformer oil-based nanofluids, the flow field distribution of them under the DC voltage were measured by particle image velocimetry (PIV), and the results were compared with that of the traditional transformer oil. The test samples were placed in an organic glass and two circular planar electrodes for 1cm apart were placed in the center of the glass in order to produce a uniform electric field. The tracer particles were hollow glass spheres of 10 μm diameters. The flow field measurement profile was the center of the electrode plate and the test samples were applied DC 10kV, 15kV and 20kV, respectively. The results show that the flow field distribution of transformer oil-based nanofluids is more regular and the flow velocity is faster than those of the traditional transformer oil, and its flow velocity is gradually slowed down from the positive plate to the negative plate. It is concluded that the flow field property of the transformer oil-based nanofluids is changed due to the addition of nanoparticles, which explains why the heat dissipation effect of transformer oil-based nanofluids is enhanced. The assumption of the electric double layer structure is reasonable. The investigation provides a new idea for exploring the reasons for the enhancement of the thermal dissipation of transformer oil-based nanofluids.

Measurement and analysis of the conduction current of the oil-paper insulation under high DC electric field

The conduction mechanism under high field has a close relationship with the pre-breakdown process of dielectrics. In this paper, a conduction measuring system was built to explore the conduction mechanism of oil-paper insulation under high field. Based on the established system, the conduction current of transformer oil, kraft paper and oil impregnated paper were investigated under various applied fields. The finding shows that the conduction mechanism of transformer oil can be divided into three stages according to its behavior. Especially, at high field, the current flowing through the transformer oil appeared to proportional to the square of applied electric field. Therefore, the conduction mechanism of transformer oil was satisfied with space-charge limited current theory in high electric field. In addition, the apparent carrier mobility μ in transformer oil was 1.1×10-9 m2/(V±s). As for oil impregnated paper, the obtained ln(I) and E1/2 characteristics behaved linearly, which could be explained by Schottky and Pullen-Frenkel mechanism; with the increase of aging time, the conduction current would decrease.

Partial discharge current pulse characteristics of needle-plate electrode based on the high bandwidth measurement method

In order to obtain the nanosecond partial discharge pulse current signal of needle-plate electrode, a new measurement method of wideband PD pulse current has been proposed in this paper. By the numerical calculation of current parameters, the optimized needle-plate electrode structure and the accurate finite element simulation, the experimental platform of the fast PD pulse current detection was set up and its bandwidth was up to 1.81GHz. Then the PD characteristics of needle-plate electrode in pure N2 and different SF6/N2 gas mixtures under power frequency voltage were investigated. The testing results indicate that, there are significant differences between PD current pulse when the gas atmosphere is changed and secondary electron emission process plays an important role for the PD current pulse waveform.

Influence of nanoparticle concentration on the frequency domain spectroscopy properties of transformer oil-based nanofluids

In order to explore the relationship between microstructure and macroscopic performance of transformer oil-based nanofluids, the transformer oil modified by silicon dioxide nanoparticles were prepared whose stability and moisture content were measured. Then the dielectric spectroscopy of the pure oil and transformer oil-based nanofluids were measured and the influence of nanoparticle concentration on the frequency domain spectroscopy of transformer oil-based nanofluids were investigated. The results indicate that compared with pure oil, two obvious polarization peaks can be observed in the imaginary part of the complex permittivity of transformer oil-based nanofluids in the whole measured frequency domain and the value of low frequency polarization peak is higher than the high frequency one. No obvious polarization peak is presented in the whole measured frequency range for pure oil. As the nanoparticle concentration increase, the value of two polarization peaks both increase, and the low frequency polarization peak has the tendency of moving to the high frequency. The electrical double layer theory was used to explain this phenomenon.