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Predicting the insulation performance of SF6 substitute gases through gas molecular structures has been a popular topic worldwide. There is a lack of research on the relationship between the critical reduced electric field and molecular structure under low pressure. The critical reduced electric field intensity and boiling temperature (BT) of gas are used as inputs to extract the important molecular structure parameters (MSPs) of gas at low pressure by using the maximum information coefficient (MIC) method. Through multivariate nonlinear fitting, the structure–activity relationship model of important MSPs of gas is established. The model is verified by the critical reduction strength and MSPs of C4F7N, CF3SO2F, and SF6 gases. The results show that the model has reliable predictive ability. In this study, the micro mechanism of gas discharge under low pressure and a dc electric field is revealed at the molecular level, and the insulation performance of gas is quantitatively predicted, providing a reference for the analysis of SF6 replacement gas performance. [ABSTRACT FROM AUTHOR]

Nano-size positive temperature coefficient (PTC) ceramic particles and epoxy composites doped with 0, 0.5, 1, 5, 10 and 20 phr of PTC nanoparticles are prepared to suppress the decrease of electrical resistivity with temperature. Electrical resistivity under 30, 50, 70, 90, and 110 °C, DC breakdown strength, and trap characteristic are measured. DC electric field distribution under radical temperature gradient is simulated using a simplified bushing model insulated with prepared materials. The mechanism for the regulating of electrical resistivity-temperature characteristic of epoxy composites is discussed. The electrical resistivity at 110 °C, DC breakdown strength at 110 °C, and electric field distortion of the epoxy composite with 1 phr PTC nanoparticles is 454, 131 and 74%, respectively of the neat epoxy resin, which exhibits potential to be used as insulating materials in high voltage DC electrical systems. [ABSTRACT FROM AUTHOR]

In this paper, the insulation strength of C 4 F 7 N/CO 2 with temperature is obtained by AC breakdown tests and theoretical calculations. The experimental results with 9%C 4 F 7 N/91%CO 2 at 0.7 and 0.5 MPa pressures show that the breakdown voltage changes nonlinearly with temperature. Theoretical analysis shows that the nonlinear change is caused by liquefaction of C 4 F 7 N. The critical electric field of C 4 F 7 N/CO 2 is calculated based on the streamer discharge theory. A calculation model is established to calculate the usable pressure of C 4 F 7 N/CO 2 and the mole fraction of C 4 F 7 N at given minimum temperature, which can provide reference for its practical application. [ABSTRACT FROM AUTHOR]

C 4 F 7 N and mixtures have been intensively investigated around the world for the substitution of SF 6 in gas insulated transmission lines (GIL). In the present paper, the authors experimentally investigate the lightning impulse (LI) breakdown voltages of C 4 F 7 N/CO 2 in different electric field uniformities, temperatures and pressures. In a slightly nonuniform electric field, the insulation strength of 9%C 4 F 7 N/91%CO 2 can reach 80% strength of SF 6 at the same pressure. In a very un-uniform electric field, the pressure of 9%C 4 F 7 N/91%CO 2 must be increased to 0.7 MPa to keep 80% strength of the 0.4 MPa SF 6. Also, the polarity effect occurred in different electric fields. The breakdown voltage of 9%C 4 F 7 N/91%CO 2 versus gas pressure shows an N-curve, which may be utilized for the design of GIL. At the pressure of 0.35∼0.7 MPa, 9%C 4 F 7 N/91%CO 2 gas mixtures in ultra-high voltage platform has passed the LI withstand voltage tests. Based on various temperature breakdown tests and the Antoine equation calculation, 9%C 4 F 7 N/91%CO 2 could work at −15 °C. [ABSTRACT FROM AUTHOR]

The AC insulation properties of C 4 F 7 N/CO 2 mixtures in non-uniform electric fields were investigated. The optimized interval time between AC breakdowns for C 4 F 7 N/CO 2 mixtures was determined for a range of electric field non-uniformity f. At high value of f, the test results demonstrated some interesting dielectric behavior. The breakdown electric field strength of C 4 F 7 N/CO 2 was inversely related to f until the critical non-uniformity f 0. Once f reached beyond the f 0 , the breakdown electric field strength became constant. Also, the gas pressure was positively correlated with f 0. In addition, the trend of AC breakdown test in C 4 F 7 N/CO 2 mixtures and SF 6 demonstrated different N-curve phenomenon. The N-curve phenomenon, which has potential to be utilized for the design of gaseous insulation in electrical systems, could be attributed to the influence of corona and to the mean free path of electrons, which competes with each other. The breakdown voltage of 5∼9%C 4 F 7 N/CO 2 mixtures is more sensitive to the change of non-uniformity than SF 6. The AC insulation strength of the mixture 9%C 4 F 7 N/91%CO 2 is 80% that of SF 6. [ABSTRACT FROM AUTHOR]

C4F7N/N2 gas mixtures are considered as new candidate insulation gases to replace SF6. In this study, the Townsend first ionization coefficient α and the attachment coefficient η were measured in C4F7N/N2 gas mixtures using the steady-state Townsend method over a range of electric fields E/N from 200 to 500 Td. The C4F7N concentrations studied were 5.07, 7.00, 13.10 and 19.09%. In addition, the effective ionization coefficients and the critical electric field were also obtained. A comparison of the experimental results with those for SF6 gas indicates that 19.09% C4F7N/80.91% N2 provides better insulation performance with a higher normalized critical electric field of 395 Td. However, when the liquefaction temperature is considered, the C4F7N/N2 gas mixtures with C4F7N content varying from 7.00 to 13.10% may be an appropriate substitute gas for SF6 in electrical industry applications. [ABSTRACT FROM AUTHOR]

C 4 F 7 N/CO 2 is one of the most promising alternatives to SF 6 in high-voltage electrical equipment such as gas insulated switchgear (GIS) because of its excellent electrical performance and low environmental impact. However, the insulation characteristics of C 4 F 7 N/CO 2 is still unclear regarding the effect of conductor surface roughness. To obtain a figure of merit of C 4 F 7 N/CO 2 gas mixtures and to evaluate its sensitivity to local inhomogeneous electric field, the breakdown voltage of electronegative gas considering surface roughness is developed using the streamer discharge theory. A gas discharge experimental platform for C 4 F 7 N/CO 2 mixed gas is set up and the discharge test is carried out under gas pressures from 0.15∼0.5 MPa having a maximum protrusion height of 15 μm. The results demonstrate that when the volume fraction of C 4 F 7 N is 10%, the figure of merit of C 4 F 7 N/CO 2 mixture is slightly higher than that of SF 6 gas. Therefore, at the same pressure, the standard of conductor surface roughness for C 4 F 7 N/CO 2 equipment can refer to the original SF 6 gas equipment. [ABSTRACT FROM AUTHOR]

Cross-bonded three-phase single-core Cross-linked Polyethylene (XLPE) cables are widely used in power transmission and distribution systems. A great deal has been published on on-line monitoring of HV cable systems to prevent failure. Relative Dielectric Losses (DL) monitoring reveals the general condition of the cable insulation. Generally, DL measurement relies on measuring operating voltage and insulation leakage current. The main challenge to on-line monitoring of DL in Cross-bonded High voltage (CB HV) cables is the interconnectedness of metal sheath. Measured sheath currents are a combination of the leakage currents of each section and the imbalanced circulating current. Differentiating the leakage current of each interconnected section from the measured data is complex. The second challenge is that co-axial cables are used to connect cable sheaths to cable link boxes, for ease of installation and protection against moisture. As a result, any measured value contains currents from the two sheaths connecting at a joint. The third challenge is that it is hard to obtain the reference voltage synchronously and with high accuracy. Consequently, this paper investigates separation of leakage current components and analysis of DL based on leakage currents without reference voltage. A circuit model is proposed to describe the leakage current separation method for CB HV cables to allow on-line monitoring of DL, and the criterion required for assessing relative change of DL among different phases or sections of the cable for insulation deterioration are presented. Operational measurements from a cable system are analyzed and discussed. Results show that there is no significant deterioration in this monitored major section of a CB HV cable system. [ABSTRACT FROM PUBLISHER]

Basin insulators in gas insulated switchgear (GIS) are prone to failure by partial discharge (PD). Insulation diagnosis using gas analysis method has advantages of strong anti-interference ability and high detection precision. This paper reports a new characteristic gas Carbonyl Sulfide (COS) for the detection of PD in epoxy insulators of GIS. The generation of COS is verified by experiments with trace gas detection technique. Experimental results indicate that COS is generated only when the intensity of PD is strong enough to cause flashover on the epoxy insulators. The corresponding production mechanisms of COS have been clarified using quantum chemistry calculations. To assess the PD level using COS, the relationship between discharge power and growth law of the concentration of COS is obtained from the experiments, which indicates that COS could be used as an effective characteristic gas to diagnose epoxy solid insulation of GIS concerning high energy discharge. [ABSTRACT FROM PUBLISHER]

Contaminant composition affects the discharge characteristics of external insulation. This paper studies the intense discharge phenomenon at low relative humidity (77%) in an AC 500 kV substation in Southern China. Due to farming and the soil feature in the sugar cane field around the substation, aluminum phosphate is formed in the soil, and accumulates on the external insulation surface owing to blowing dust. The porous structure and four-coordinated aluminum atom lead to strong moisture absorption capacity of aluminum phosphate. Then electrical performances of aluminum phosphate as a contaminant on insulation surface were investigated by means of discharge test in a constant humidity chamber. The experimental results show that, compared to conventional contaminant, aluminum phosphate contamination insulator has lower discharge inception voltage and stronger discharge intensity when relative humidity is above 70% and the aluminum phosphate content exceeds 20% in the non-soluble deposit. Aluminum phosphate is the main cause for the discharge phenomenon in the substation under study. It needs to be considered for insulation design and maintenance in ferralsol areas where phosphate fertilizer is heavily used. The moisture absorption of contaminant merits special consideration for further research. [ABSTRACT FROM AUTHOR]

Contaminant is the main factor affecting the insulation performance. The contaminant composition determines the inception voltage and the intensity of insulation discharge. This paper studies the intense discharge activity at low humidity (77% RH) in AC 500 kV Gangcheng substation where the contaminant contained glucose, and this phenomenon was rarely explained in previous publications. Electrical and chemical performances of glucose as a contaminant on insulation surface were investigated by means of discharge test (below 80% RH), artificial pollution test (100% RH), molecular dynamics simulations, and quantum chemistry calculations. The experimental results show that due to stronger moisture absorption capacity compared to non-glucose contaminant, glucose contamination sample has lower discharge inception voltage and stronger discharge intensity at 75% RH as the content of glucose exceeds 0.10 mg/cm2. Quantum chemical calculations reveal that the glucose molecules can decompose to generate free radicals with the unpaired electrons under the strong electric fields. The surface carbonation enhances the electron transfer rate and strengthens the discharge intensity. Moreover, the competitive adsorption of glucose and water molecules cause the low moisture discharge phenomenon, and it needs to be considered for insulation design and maintenance in areas producing sugar. [ABSTRACT FROM PUBLISHER]

This paper presents a novel Partial Discharge (PD) localisation method, based on PD pulse Rise-Time and Transfer Function (RTTF), for use in on-line cable PD monitoring systems. This is based on the change in rise time as a function of cable length along which a PD pulse has propagated. A comparison with four other well-known location techniques, i.e. Time-Domain Reflectometry (TDR), Phase difference, Arrival Time Analysis (ATA) and Amplitude-Frequency (AF) mapping has been made and presented. Analysis shows that the TDR and Phase difference methods, although requiring a single measurement point, are only applicable to short length cable systems as both incident pulse and reflected pulse should be observable when it reaches the detection point. On the other hand, the ATA and AF mapping, which can be applied to longer cable systems, need to install detectors at multiple points and detect synchronously. In comparison, PD localisation by the proposed RTTF method, which requires detection of the incident pulse at only one measurement point, can be used to localise a PD source which is further away from the PD detection point than other methods and can be used in on-line PD monitoring systems. As on-line PD monitoring sensors commonly comprise of a high frequency current transformer (HFCT) and a high-pass filter, the rise time of detected PD pulses depends on the attenuation of the cable, the HFCT used and the filter applied. Simulations of pulse propagation in a cable and PD monitoring system are performed, based on analyses in the frequency domain using the concept of transfer functions. Results of RTTF from the simulations have been verified by laboratory experiments and on-site PD measurements, which provide evidence that the RTTF method is a very valuable and useful PD localisation technique. [ABSTRACT FROM PUBLISHER]

This paper addresses the detection and localization of partial discharge (PD) in crossbonded (CB) high voltage (HV) cables. A great deal has been published in recent years on PD based cable insulation condition monitoring, diagnostics and localization in medium voltage (MV) and high voltage (HV) cables. The topic of pulse propagation and PD source localization in CB HV cable systems has yet to be significantly investigated. The main challenge to PD monitoring of CB HV cables is as a result of the interconnectedness of the sheaths of the three single phase cables. The cross-bonding of the sheaths makes it difficult to localize which of the three phases a PD signal has emanated from. Co-axial cables are used to connect cable sheaths to cable link boxes, for ease of installation and protection against moisture. A second challenge is, therefore, the coupling effect when a PD pulse propagates in HV cable joints and the co-axial cables, making PD detection and localization more complex. The paper presents experimental investigations into PD pulse coupling between the cable center conductor and the sheath and the behavior of PD pulse propagation in CB HV cables. It proposes a model to describe PD pulse propagation in a CB HV cable system to allow monitoring and localization, and also presents the knowledge rules required for PD localization in CB HV cable systems. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) based condition monitoring has been widely applied to power cables. However, difficulties in interpretation of measurement results (location and criticality) remain to be tackled. This paper aims to develop further knowledge in PD signal propagation in power cables and attenuation by the PD monitoring system devices to address the localization and criticality issues. As on-line or in-service PD monitoring sensors commonly comprise of a high frequency current transformer (HFCT) and a high-pass filter, the characteristics of detected PD pulses depend on the attenuation of the cable, the HFCT used and the filter applied. Simulation of pulse propagation in a cable and PD monitoring system are performed, based on analyses in the frequency domain using the concept of transfer functions. Results have been verified by laboratory experiments and using on-site PD measurements. The knowledge gained from the research on the change in pulse characteristics propagating in a cable and through a PD detection system can be very useful to PD denoising and for development of a PD localization technique. [ABSTRACT FROM AUTHOR]

The epoxy spacers in SF6 Gas Insulated Switchgear (GIS) are vulnerable to discharge. Once the surface of the epoxy spacers is deteriorated under discharge, insulation break down may occur subsequently. From a number of experiments carried out on 110 kV GIS, a new gas CS2 was detected when creeping discharges occur on the epoxy spacer surface. In order to verify the correlation between CS2 and the degradation of epoxy spacers, the quantum chemistry analysis was used to investigate the mechanism of CS2 development. Based on the results, the relation between CS2 and spacer insulation degradation was determined. [ABSTRACT FROM PUBLISHER]

This paper presents a series of experimental investigations involving the use of ultraviolet (UV) detection of corona discharges from a rod-plane air gap subjected to high AC voltages. To establish a criterion to identify the beginning of dangerous levels of corona discharges, using UV images, coronas from air gaps of various lengths were observed using a UV camera located at a fixed position, and coronas from an air gap with a fixed gap length were recorded from various observation distances. The corresponding UV images were then analyzed via an image-processing method developed by the authors. In the processed UV images, the area ratio of the UV discharge zone to the total region of interest was defined as an indicator of the intensity of corona discharges. Test results revealed that for different corona discharges, their developing stages could be uniquely described using attenuation in the intensity of the corona UV image with observation distance. Based on the findings, a criterion is proposed for the detection of corona inception. An onsite application of the proposed method and criterion is provided and discussed. [ABSTRACT FROM AUTHOR]