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Predicting When Cables will Fail

image credit: www.baur.eu
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The East German utility company Netze Magdeburg (NMD) uses software as part of its cable testing regime to accurately evaluate a cable’s condition based on dissipation factor measurements

The urban area of Magdeburg in Eastern Germany has a geographical spread of around 200 sq km (80 sq miles). It has a considerable network of underground cables supplying electricity to the city's 240,000 population. The entire cable length totals over 2000 miles.

Ensuring the network is in functional condition is a challenge. To make the best use of its assets TSO NMD decided to invest in software diagnostics to survey the state of its cables remotely. The objective was to improve the overall reliability of its underground cable system, without incurring the large costs of digging up the street. NMD also has to contend with several different kinds of cables, some of them dating back to the period before reunification. These GDR (former East Germany) manufactured cables are not considered up to today's standards.

 

NMD uses three methods for evaluating the condition of its cables:

1. Partial discharge measurement to detect any damage to the insulation

2. Impedance measurement of each MV cable circuit

3. Dissipation factor measurement (tan δ) to assess the aging of the cable insulation.

The third method is the most reliable and effective in assessing the overall aging condition and therefore, the probable remaining lifetime of the cable. NMD makes the tan assessment by using a very low frequency (VLF) measurement with a 0.1-Hz sine wave. By using a specially-equipped cable-test van, the vehicle can move to the location needing testing. These special vehicles are produced by BAUR GmbH, and are also equipped for numerous other cable tests, such as fault location.

The testing scheme extends to all cables – the company found that merely testing those which were thought to be ailing did not pick up the sudden critical failures of others which had seemed functional.

 

Condition Evaluation Software

NMD implemented a database to store the measured values. However, because tan δ measurements alone do not provide any firm conclusions in terms of estimating remaining service life, the utility also rolled out BAUR statex software to maximize the benefits of the diagnostic measurement database and evaluate a cable’s condition. The statex software features procedures developed by South Korean grid operator KEPCO, which has used the software successfully for many years.

For cable condition evaluation, the solution requires tan δ measurements in accordance with BAUR’s VLF sine wave method, which uses statistical methods to analyze data. The software not only uses a utility's own measured values, but it also draws on more than 100,000 stored series of measurements. Additionally, the calculations can predict the following:

  • Remaining service life of a cable
  • Estimated speed of cable aging (aging index)
  • Recommendations based on these two predictions, for example, the frequency of tan δ measurements

In addition, the software can be used to document the condition of the cable population, enabling evaluation based on various criteria. Therefore, network-critical cable routes can be determined. This information is critical and can inform planning and managerial decisions on when to perform maintenance or replacement.

 

Deferred Replacement Improves O & M costs

The most significant advantage of the statex software is the ability to calculate an aging index and predict the remaining service life of cables. Based on this information, cable routes often can be used for several more years than would be expected otherwise. This results in cost savings for NMD, without jeopardising supply or safety.

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