Rick Hartlein

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.

Ampacity derating factors for cables buried in short segments of conduit

Buried cables are often routed through short segments of conduit, and when this situation occurs, the ampacity must be reduced or the cable will overheat as a result of the high thermal resistance created at the location of the conduit. This problem is examined for extruded cables by using a finite element heat transfer software program to determine the derating in ampacity that cables in conduits must experience in order to remain below a maximum conductor temperature. The derating factors are provided as a function of conduit length, soil resistivity, burial depth and number of cables in the conduit. The results show that once the length of conduit exceeds about 20 times its outer diameter, then the ampacity of the circuit must be reduced to the value that it would have if the entire length were buried in the conduit. Factors that result in lower cable ampacities, such as high soil thermal resistivity and deeper burial depths lead to larger derating factors.

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.

Equipment Failure Forecasting Based on Past Failure Performance and Development of Replacement Strategies

When only partial information is available about equipment failures (installation date and amount, as well as failure and replacement rates), data on sufficiently large number of yearly populations of the components can be combined, and estimation of model parameters may be possible. The parametric models may then be used for forecasting of the system`s short term future failure and for formulation of replacement strategies. We employ the Weibull distribution and show how we estimate its parameters from past failure data. Using Monte Carlo simulations, it is possible to assess confidence ranges of the forecasted component performance data.

Ampacity of Electric Power Cables in Vertical Protective Risers

A method is presented to compute the ampacity of an electric power cable located in both vented and nonvented circular protective risers. While techniques for calculating the ampacity of underground cables and overhead lines are well established, techniques for rating the riser portion of a cable circuit have not been as thoroughly studied. Usually cable ampacities are based on the assumption that the underground portion of the circuit will be cooler than the portion located in the riser. Therefore, approximate derating factors are used to reduce the underground cable circuit ampacity when risers are present.

Some considerations on the selection of optimum location, timing, and technique, for diagnostic tests

The ageing of the electrical infrastructure is a growing concern for utilities, regulators and customers; and there is no doubt that addressing this problem will become an ever more important priority. Any solution will have to address three basic issues: firstly where is the optimum place and time to start, secondly what is the most appropriate suite of actions that can be taken and finally is the solution going to deliver the expected life. Diagnostic programs play an important part in the first and the third issues. In the first they may be able to guide the identification and prioritization of assets to be addressed; here they operate on the ageing population. The third issue benefits from diagnostics by using them as part of the assurance process that determines that the replacements / repairs have been effective. Addressing the ageing infrastructure is a large, complex and interacting challenge; this paper focuses on the first issue, namely how to select the appropriate locations, timing and technique.

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).

Determining routes for the analysis of partial discharge signals derived from the field

This paper discusses the experience of the National Electric Energy Testing Research and Applications Center (NEETRAC) with condition assessment of medium voltage power distribution cable systems using Partial Discharge (PD) measurements. Specifically, the main goal is to address the issue of selection of the appropriate PD diagnostic features. A large number of features for which a physical basis can be described are selected for analysis. The features are investigated using data obtained from field and laboratory tests. The investigation shows that two traditional diagnostic features are not enough to classify samples that are prone to fail in a four-year time window. Subsequently, cluster variable analysis is performed for a considerable number of PD diagnostic features. The analysis enables significant feature elimination and reveals that at least seven different types of PD diagnostic features are needed to achieve an acceptable level of dissimilarity between features. Future work, conclusions and remarks are also discussed.

Generating Cable Replacement Strategies Using Monte Carlo Simulation

The paper describes a simple replacement strategy developed on the basis of statistical failure parameter identification and a predictive replacement algorithm. The proposed approach is illustrated with examples showing both the effectiveness and limitations of the method

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.