Chongxing Zhang

Explanation and analysis of oil-paper insulation based on frequency-domain dielectric spectroscopy

In this paper, to effectively distinguish between the aging and moisture of oil-immersed paper (OIP), and to enhance the accuracy of the nondestructive diagnosis of the insulating state for oil-paper insulation, we perform an in-depth study of the frequencydomain dielectric response theory. First, we present a theoretical explanation for frequency-domain spectroscopy (FDS) based on investigations into the mechanism and characteristics of conductivity loss and polarization loss. Secondly, we explore the classical loss characteristics by performing a systematic analysis of the relationship between the complex dielectric permittivity and the frequency based on the measured FDS curves. Finally, we study the influences of the aging condition and moisture content on FDS. We extract some of the characteristic parameters that are related to the insulation condition. The results obtained indicate that at low frequencies, the FDS characteristic was mainly influenced by the conductivity loss, which may be explained by the Dyre model. At high frequencies, the characteristic was mainly influenced by the polarization loss, which may be explained by the H-N model. In addition, the characteristic at mid-range frequencies was due to both the conductivity and polarization loss. The moisture content significantly affects only the conductance process. Therefore, the characteristic parameter, namely the DC conductivity σ 0, which is closely related to the conductance process, can be used to quantitatively describe the moisture content of OIP. Meanwhile, the characteristic parameters α, β, and τ, which are associated with the polarization process, can be used to quantitatively describe the aging status of OIP.

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.

Electrochemical and Infrared Absorption Spectroscopy Detection of SF6 Decomposition Products

Sulfur hexafluoride (SF6) gas-insulated electrical equipment is widely used in high-voltage (HV) and extra-high-voltage (EHV) power systems. Partial discharge (PD) and local heating can occur in the electrical equipment because of insulation faults, which results in SF6 decomposition and ultimately generates several types of decomposition products. These SF6 decomposition products can be qualitatively and quantitatively detected with relevant detection methods, and such detection contributes to diagnosing the internal faults and evaluating the security risks of the equipment. At present, multiple detection methods exist for analyzing the SF6 decomposition products, and electrochemical sensing (ES) and infrared (IR) spectroscopy are well suited for application in online detection. In this study, the combination of ES with IR spectroscopy is used to detect SF6 gas decomposition. First, the characteristics of these two detection methods are studied, and the data analysis matrix is established. Then, a qualitative and quantitative analysis ES-IR model is established by adopting a two-step approach. A SF6 decomposition detector is designed and manufactured by combining an electrochemical sensor and IR spectroscopy technology. The detector is used to detect SF6 gas decomposition and is verified to reliably and accurately detect the gas components and concentrations.

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.

Partial discharges triggered by metal-particle on insulator surface under standard oscillating impulses in SF 6 gas

The characteristics of partial discharge (PD) triggered by a metal particle on an insulators surface were studied experimentally under standard oscillating lightning impulse (OLI) and standard oscillating switching impulse (OSI) conditions. Light emission detection was carried out to assist in PD measurement. Experiments showed that the largest number of successive PDs occurred during the rise time of each oscillation period of applied impulses and that a small number of reversed-polarity PDs occurred during the fall time. The composition of the PD sequence was determined by the position of the particle on the insulators surface. Specifically, when a needle particle detached from the plate electrodes, the PD sequences consisted of both negative point PDs and positive point PDs. Two types of negative discharge, three types of positive discharge, and reversed-polarity discharge were differentiated by their current and light pulse waveforms. In these experiments, detectable partial discharge inception voltages (PDIVs) of the three particle positions were investigated at gas pressures from 0.05 to 0.35 MPa. The light emission inception voltages (LEIVs) were lower than PDIVs under OLI, OSI, and AC voltages. These inception voltages of different particle positions could be sorted as follows: LEIV <; PDIV+ (d = 30 mm) <; PDIV- (d = 0 mm) <; PDIV+/- (d = 15 mm). We demonstrated that the OLI voltages could excite PDs from defects at lower equivalent voltage levels than for OSI and a 50 Hz AC voltage, and the PDs under OLI were more active in terms of occurrence frequency than those under OSI and AC voltages. A simple model was proposed to explain PD inception and propagation under oscillating impulses. The rapid change of background electric field and the formation and accumulation of charges on the insulators surface were considered the main causes of the greater efficiencies of OLI and OSI at exciting PDs compared with the AC voltage.

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.

Antenna factor computation of UHF dipole antennas used for partial discharge detection in power equipments

The ultra-high frequency (UHF: 300MHz-3GHz) detection has been presented as a promising and effect way to diagnose the partial discharge (PD) activities in power equipments in recent years. The characteristics of UHF detection in the near field when the receiving antennas arranged and worked in the near electromagnetic (EM) field radiated from PD source are different from the far field detection. In this paper, the receiving characteristics of UHF dipole antennas both in the near and far field detection are investigated by using finite difference time domain (FDTD) method. The near and far field antenna factor (AF) of three UHF dipole antennas are computed and compared by coupling the FDTD method and total-field/scattered field (TF/SF) technology together. The spherical and plane waves are employed as near and far field incident EM waves, respectively. It is revealed that, the PD detection can be considered as the far field detection when the received antenna arranged about 0.1-1m from PD source corresponding to the UHF band. The simulation results show that, the multiple odd resonances of dipole antennas can be observed in both near and far field AFs. The lowest AF values in the far field appear near their designed frequencies and the AF values at the resonant frequency are in good agreement with the factory designed data. However, the frequency with the lowest AF values in the near field and their AF values are higher and lower than that of designed data, respectively, indicating the sensitivity of dipole antennas working in the near field is lower than that working in the far field PD detection. The AF values difference of dipole antennas between the near field and designed data decreases gradually with the increment of resonant frequency. This research is helpful for people to select and design the antennas and analyze detection data in UHF PD diagnosis.

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.