Nigel Hampton

Evidence of strong correlation between space-charge buildup and breakdown in cable insulation

Many processes have been considered over the years to explain the origin of breakdown in cable insulation. Such effects as space charge build-up, tree growth, charge injection, etc. have all been discussed. Various techniques are now available to measure, in a nondestructive way, space charge distributions in insulators. These techniques, for instance the pressure wave propagation (PWP) method, can be used under applied electric stress and thus make it possible to follow the development of space charge in selected regions of the insulators. In this paper we present new evidence linking space charge buildup, tree growth and breakdown in XLPE. We have used the PWP method to monitor the charge distribution as a function of time under dc stress in high insulating thickness cable. We show that for certain insulation systems the space charge buildup can increases the local field to a value which is more than 8/spl times/ the applied electric field, leading to breakdown. Post-mortem analysis followed by optical microscopy shows the presence of electrical trees, the breakdown channel being centered on one of them. The study of space charge evolution in practical insulations permits an understanding of the role of space charge in dc breakdowns. This understanding enables the development of technologies to suppress this effect and hence realize practical dc XLPE transmission cables.

Correlation between tan δ diagnostic measurements and breakdown performance at VLF for MV XLPE cables

The main contribution of this paper is to investigate the correlation between tan delta diagnostic measurements at 0.1 Hz (Very Low Frequency-VLF) and VLF breakdown performance for medium voltage (MV) cable samples through a laboratory test program. The cable samples used are a set of 15 kV, Cross-linked Polyethylene (XLPE), unjacketed cables removed from the field, the same service area, and having experienced similar operating conditions for almost four decades. The test program includes tan delta measurements at different voltage levels and a subsequent VLF extended step-withstand test. The VLF step test allows the evaluation of risk of failure during VLF tan delta testing and assessment of the ultimate performance of the cables. The tan delta diagnostic measurements are represented by the tan delta value and tip-up, which are considered the classical metrics. However, the paper also suggests the use of a new additional diagnostic feature that takes into account the scatter in the tan delta measurements for a particular test voltage level.

Characterization of Ageing for MV Power Cables Using Low Frequency Tan δ Diagnostic Measurements

This paper describes very low frequency (VLF) tan delta experiments performed on field-aged and non-aged distribution medium voltage (MV) cable samples. The field aged samples constitute a uniform set of Cross-linked Polyethylene (XLPE) of 15 kV unjacketed cables removed from the same service area having experienced similar operating and ageing conditions. The non-aged samples are a diverse set of crosslinked polyethylene (XLPE) and water tree retardant cross-linked polyethylene (WTRXLPE) cables of 15 kV and 25 kV and Ethylene Propylene Rubber (EPR) cable of 25 kV. The experiments are designed to contribute in understanding, time, voltage and discharge time dependence of Tan delta diagnostic measurements at VLF of 0.1 Hz. Results help in clarifying issues that arise when characterizing MV cable insulation by Tan delta diagnostic measurements. The issues include time-on-test, voltage level as a diagnostic tool, diagnostic features, and reproducibility and repeatability of the measurements. The paper shows that higher insulation losses, non-linearity, hyteresis, and variation in voltage and time of Tan delta diagnostic measurements at VLF are indicators that can be used to properly characterize the insulation and enhance the diagnosis.

Chemical crosslinking of polyethylene and its effect on water tree initiation and propagation

The water tree resistance of chemically crosslinked polyethylene and of low density polyethylene were compared in order to elucidate whether the crosslinking itself influences or not the water tree characteristics of polymeric cable insulation materials. For this purpose, water trees were grown in compression moulded disks, obtained from pellets of either thermoplastic or chemically crosslinked polyethylene. Three types of crosslinked polyethylene were evaluated: one containing only peroxide (XLA) and two others having, besides peroxide, a tree retarding additive system (XLB and XLC), and the results were compared with those obtained on their thermoplastic correspondents (A, B and C). Results were determined for both Water Tree Propagation and Water Tree Initiation. The results obtained for Water Tree Propagation indicate that there is generally a large difference between the types (A, B & C) yet only small difference between thermoplastic and crosslinked samples. However for two cases (A & B) these small differences between crosslinking are statistically significant. The complementary Water Tree Initiation results show both large and significant differences between material types and the crosslinking.

On Distribution Asset Management: Development of Replacement Strategies

The components of electricity networks are ageing. It is expected that within a horizon of 15 years, the performance will deteriorate significantly, while the costs for operating the networks will increase enormously. The main problem is that a significant part of the population of the assets is installed in the same period, resulting in a highly concentrated number of failures in a short time. The currently applied replacement strategy has to be revisited, in order to accommodate the effects of ageing assets: higher maintenance costs, high failure rates, and a steep increase of capital expenditure (CAPEX).

Underground Cable Systems

Underground cables have been used from the earliest time as integral parts of the power distribution and transmission system. Compared to their overhead analogues, they have been long regarded as the most critical of components due in part to their high total installed cost, their unique ampacity requirements, and the complexity of their installation. Thus, even from the very first technical papers, the sustainability, through reliability and longevity, has been of paramount importance. This contribution addresses the focus on sustainability today while maintaining a linkage to the lessons that have been already learned.

Control of water tree length and density in cable insulation polyethylenes

The goal of this paper is to study the influence of the chemical crosslinking of polyethylene on the water tree initiation and propagation in polymer insulation. For this, the water tree resistances of crosslinked and thermoplastic low density polyethylene were compared. Three types of crosslinked polyethylene systems were evaluated: one containing only peroxide and the other two having, beside peroxide, a free retarding additive system. The results were compared with those obtained on their thermoplastic correspondents. The data show that there are differences in both the water tree length and density that can be ascribed to the polyethylene systems. However, only differences in the water tree density could be ascribed to the material form (thermoplastic or crosslinked). The observed results are consistent with differences, on microscopic level, in permittivities and local breakdown strengths

Applying Diagnostics to Enhance Cable System Reliability (Cable Diagnostic Focused Initiative, Phase II)

The Cable Diagnostic Focused Initiative (CDFI) played a significant and powerful role in clarifying the concerns and understanding the benefits of performing diagnostic tests on underground power cable systems. This project focused on the medium and high voltage cable systems used in utility transmission and distribution (T&D) systems. While many of the analysis techniques and interpretations are applicable to diagnostics and cable systems outside of T&D, areas such as generating stations (nuclear, coal, wind, etc.) and other industrial environments were not the focus. Many large utilities in North America now deploy diagnostics or have changed their diagnostic testing approach as a result of this project. Previous to the CDFI, different diagnostic technology providers individually promoted their approach as the “the best” or “the only” means of detecting cable system defects.

How much does studying Polyethylene tell us about XLPE

Today technical publications often use the generic term “standard” polyethylene and cable. In this paper, we are trying to highlight the variations between different high pressure polyethylenes since the properties may vary between different resins. For example, one parameter that influences the properties of low density polyethylene is the type and content of the chain-transfer-agents that are used to control the chain length of low density polyethylene. They can be of different chemical nature and therefore strongly influence all properties of polyethylene. Moreover, the processing of the polymer during production of the cable has a significant influence on the morphology and thus the electrical properties. This is highlighted in various publications demonstrating a further factor that has to be taken into account when studying polyethylene. The morphology of the cross-linked polyethylene can depend on several factors that are outlined in this paper. We studied additionally the properties of cross-linked polyethylene, which is normally used in power cable applications for AC and DC applications. Since these cables are designed for a lifetime of more than 40 years, the influence of the different antioxidants that are used on the electrical properties are summarized, morphology and crystalline melting point were evaluated, highlighting a 8 K difference between virgin and stabilized polyethylene. Likewise, the history and construction of cables that are studied should be evaluated carefully, since the whole cable will influence the results and certain cable constructions are not used for higher voltages. The influence of tapes, for example, has not been studied anywhere in too much detail. In conclusion, we can say that a “standard” polyethylene and thus a “standard” cable does not exist and therefore care should be taken extrapolating data from one scientific study to a cable produced under different conditions.