S. J. Dodd

Effect of side chains on the dielectric properties of alkyl esters derived from palm kernel oil

Alkyl ester derivatives were synthesized from laboratory purified palm kernel oil. The steps in the synthesis involved transesterification of palm kernel oil to produce a methyl ester, followed by epoxidation and then the grafting of side chains by esterification with propionic and butyric anhydride. The dielectric and thermal properties of the ester derivatives were analyzed and compared with the methyl ester. The melting point of the ester derivatives were found to reduce with side chain attachment and antioxidant improved its thermal stability. The dielectric loss was dominated by mobile charged particles and the chemical modification appeared to increase the rate at which electric double layer was formed at the electrode-liquid interface. The esters possessed excellent breakdown strengths suggesting that the processing to optimize their physical properties did not have a negative influence on their electrical breakdown strength. This product may prove useful as an insulation fluid in Electrical Power Transformers.

Pulse sequence analysis on PD data from electrical trees in flexible epoxy resins

Pulse Sequence Analysis (PSA) was carried on PD data from electrical trees grown in flexible epoxy resins. The samples used for the electrical tree experiments were conditioned in environments with different relative humidities in the range 15-100% prior to the electrical tests with the corresponding moisture content in the samples between 0.1 and 6.9%. The electrical treeing experiments were carried out at different temperatures in the range 20-70°C. The details of PD dynamics during the electrical tree growth have been found to change significantly with temperature and absorbed moisture. In this paper, it is shown that PSA could be successfully used to discriminate between PD data from electrical trees with different shapes and runaway tree growth.

Dielectric spectroscopy measurements on very low loss cross-linked polyethylene power cables

The principles of dielectric spectroscopy are reviewed and the techniques in both time and frequency domains are explored in search of appropriate methods for measurement on low loss XLPE cables. By combining the techniques of frequency response analyzer, transformer ratio bridge and discharging current measurements, some preliminary tests results on homopolymer XLPE model cables have been presented and analyzed, in a wide frequency range of 10-4Hz~2×104Hz. Dielectric loss mechanisms of XLPE cables are discussed based on the measurement results.

Electrical trees in silicone gel: A combination of liquid and solid behaviour patterns

Electrical trees in liquids (streamers) occur on the nano-second timescale and produce structures that dissipate on voltage removal and sometimes even during voltage application. In AC fields the structures are a combination of fine filaments from the positive half-cycle and spheroidal cavities from the negative half-cycle. In contrast electrical trees in solids are permanent filamentary structures with a fractal geometry that grow on timescales of hours or longer at typical field values. Here we present the results of tree formation in a silicone gel under an AC applied electric field. These grow on timescales of minutes and possess a fine branched filamentary structure as well as spheroidal cavities. As in liquids, the cavities can collapse during tree growth. In contrast the filamentary structure is permanent as in solids. However the whole tree contracts following the removal of the applied voltage. The observed stability of the partial self-healing tree features are discussed in terms of the mixed liquid and solid features of the gel structure.

Electrical treeing in silicone rubber/organo-montmorillonite

Electrical treeing is one of the main reasons for failure of polymeric materials used in high voltage applications. Treeing is observed to originate at points where impurities, voids, defects, or conducting projections cause excessive local electric field stress in the dielectric. It has been proposed that nanofillers could be employed as an electrical tree inhibitor in polymeric insulating materials. With development of nanotechnology, polymer nanocomposites have drawn much attention, because they are expected to have improved mechanical, thermal and electrical properties over the neat polymer. In this paper, modified nanoclay was used to examine the initiation and propagation of electrical treeing. Effects of electrical treeing on nanocomposites insulating material filled with 1 wt% and 3 wt% of Organo-Montmorillonite (OMMT) were used in this work. The results have shown that electrical tree growth was suppressed in silicone rubber nanocomposites compared with neat silicone rubber which allowed a faster growth of electrical treeing. However, more studies should be carried out with different nanofiller/silicone ratios to see in further enhancements in tree inhibition could be achieved.

Space charge measurements on different epoxy resin-alumina nanocomposites

Recently a number of papers have reported enhanced electrical (ageing and breakdown) performance of nano-composites when compared to the properties of the base resin material. The physical reasons for this improvement are the subject of many current studies and have yet to be fully formulated. In this work, the build-up and decay of space charge in epoxy resin materials, containing nano-particles of alumina of various concentrations, 0.5% to 2% was investigated during the application of DC voltage and after voltage removal. Space charge profiles in thin film samples were investigated using the Pulsed Electro-Acoustic (PEA) technique. The results were compared with the space charge behavior of the unfilled resin and for the 1% nano-filled resin with differing levels of absorbed moisture. The results demonstrate that the space charge behavior is dominated by the properties of the host resin. However, the accumulation of positive charge at the anode is affected by the concentration of filler particles. The results also demonstrate the importance of controlling the amount of absorbed moisture in the samples as this governs the dynamics of the charge build-up and decay in the samples.

Feasibility of using a flat bottom electrode for PEA space charge measurement on mini-cables under high temperatures

Pulsed Electro-Acoustic (PEA) apparatus has been developed to measure the space charge distribution inside XLPE based mini-cable insulation. The technique relies on the acoustic contact between the cylindrical cable surface and a flat aluminium plate to couple the acoustic waves to the sensor. This work addresses the feasibility of using this technique over a temperature range of 25 °C to 70 °C. The results demonstrate that increasing the temperature under constant clamping force causes the cable insulation to soften and distort increasing the contact area and signal from the PEA apparatus. However on subsequent cooling to 25 °C the mechanical distortion remains frozen-in with little change to the contact area. For reproducible space charge measurements it is necessary to first condition the cable sample by taking the cable and PEA through a temperature cycle before application of the applied voltage.

Electrical breakdown strength characteristics of palm kernel oil ester-based dielectric fluids

Natural ester fluids have been synthesized from crude palm kernel oil for consideration as an alternative to mineral oil based insulating fluid. Chemical modification of the oil enhanced the physico-chemical properties of the fluid. This paper presents the statistical analysis of the AC electrical breakdown strength of the synthesized esters in comparison with the crude palm kernel oil sample. The breakdown test was carried out in accordance with ASTM 1816 test method using a bespoke test cell designed for small sample volume. The estimated characteristic breakdown strength of the esters, defined as the 63.2% cumulative failure probability, is significantly higher than the BS148 mineral oil. The slopes, indicating the shape parameters, are similar. The results suggest that, at least in this regard, the synthesized esters may serve as an alternative to mineral oil as a transformer fluid.

PD Pattern Recognition Using ANFIS

An application of an adaptive neuro-fuzzy inference system (ANFIS) has been investigated for partial discharge (PD) pattern recognition. The proposed classifier was used to discriminate between PD patterns occurring in internal voids. Three different void shapes were considered in this work, namely flat, square and narrow. Initially, the input feature vector used for classification was based on 15 statistical parameters. The discrimination capabilities of each feature were assessed by applying discriminant analysis. This analysis suggested that some of the features possess much higher discriminatory power than the others. As a result, a simplified classifier with reduced feature vector has been obtained. The results demonstrate the importance in identifying and removing redundancy in the input feature vector for reliable PD identification.

Electrical properties of ester dielectric fluids from palm kernel oil

This paper presents characteristics of dielectric fluids prepared from chemical modification of palm kernel oil ester as an alternative to mineral oil for use in HV electric equipment such as transformers. The palm kernel oil-based alkyl ester dielectric fluids were synthesized from palm kernel oil through epoxidation and esterification reactions. The functional groups in the alkyl esters were identified using FTIR spectroscopy. The rationale for this processing and its effect on the physical properties, such as melting, are presented. The AC breakdown voltage, volume resistivity, relative complex permittivity, and tan δ of the esters were measured. The dielectric response results indicate that the relaxation processes correspond to ionic conduction and electrode polarization phenomena. The breakdown strength of the alkyl esters were found to have a significant improvement compared with the crude oil sample.