JUNE 2008

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Analysing 2 months of regular work in Portugal
 
Lessons learned after 4000km

Power Line Maintenance Inspection

Analysing 2 months of regular work in Portugal

The Power Line Maintenance Inspection system [PLMI] developed by Albatroz Engineering has been regularly used by LABELEC since November 2007 to inspect transmission lines (REN) and distribution lines (EDP Distribuição). The system has now inspected more than 4000km lines and it is expected to achieve 10000km by the end of 2008.

To evaluate the system's overall efficiency after the initial training period (concluded in 2007), we have analysed, in detail, the statistical data of 2 months of regular work, between April 14^th and June th 5^th. At this time, all line inspection teams and pilots were completely familiar with the system. This analysis excludes the winter period, when the number of available days for inspection is small. The overall inspection data is described in Table 1.

Voltage level

Figure 1 presents the total length of inspected lines in terms of voltage level. It shows that almost all the inspections were performed on transmission lines (400kV, 220kV and 150kV), particularly on 150kV.

This is explained by the fact that, in November and December of 2007, most inspections covered 1025km of 220kV and 400kV and only 328km of 150kV lines.

The figure also shows that the 10-15kV and 30kV classes are empty, because EDP decided to inspect the 60kV high voltage lines first, since these are most critical for the overall quality of service.

Daily Inspection

Figure 2 shows the daily inspection lengths in km. Considering the total number of days analysed, the average daily inspection length is 83km. It is also clear that there was a period (end of April, beginning of May), where the daily inspection lengths were longer than 100km followed by a period where the total daily inspection length was shorter than 50km.

Figure 3 shows the distribution of km inspected per day. From an overall set of 39 working days (excluding weekends and holidays), in one third of the days there were no inspections and in another third, the total inspection length was in the 50- 99km range. In 80% of the cases, the total inspection length was shorter than 100km.

There are two main reasons for this distribution:

• May was exceptionally rainy, which limited the total time available for inspections. Therefore this data describes a more pessimistic scenario than expected.
• The transmission lines are longer and
  require less inspection manoeuvres, enabling easier and longer inspections. In the middle May, the inspected lines were mainly distribution lines (69kV), which are shorter and need longer ferries between each line.

Nevertheless, if we consider 250 annual working days, where the average inspection length is 83km, it is possible to inspect more than 20000km per year with one helicopter.

Since the clearance analysis with LiDAR is fully automatic and integrated, the on board inspection crew use most of their time for thermography and visual inspections and only occasionally take notice and review clearance issues.

Acknowledgement

Albatroz Engineering would like to thank LABELEC and HELIPORTUGAL for making available the data for the statistical analysis and all their collaboration towards the continuous improvement of PLMI.

Power Line Maintenance Inspection

Lessons learned after 4000km

Lesson #1: mother natrue rules (as always)

Despite all the technical and operational constraints defined before an inspection mission, a daily inspection plan has to be executed, with more or less variation, in conditions that are, most of the time, different from the ones expected. Whether this is the result of weather conditions or other operational reasons, the inspections must be done as long as it is safe to do so. Therefore, on-board systems have to be sufficiently robust to withstand a broad range of operational conditions.

Figure 4 to 6 illustrate an inspection done on June 23rd on a 60kV line in the mountains, where the maximum distance to the line and maximum inspection speed should remain below 50m and 30kt respectively. However, the sequence of peaks and valleys, aircraft dynamic constraints and air rarefaction forced the pilot to fly faster and farther from the line; a critical valley was encountered 1500m after the inspection began where the inspection speed was increased to 37kt and the distance to line approached 90m.

Failure to collect data under these circumstances is unacceptable. Thus the need for a robust and interactive system that tracks operational conditions and allows pilots and crews to make informed and timely decisions on whether to carry out an inspection, minimising risks and the use of resources, thereby avoiding unsafe and sub-standard inspections.

Section of 5500m metres of over- head line recorded in 8000 scans

Figure 4 - Altitude profile of the sensors (blue), the over-head line (red) and the ground (brown); scales in metres

Figure 5 - Distance to over head line in metres along 8000 scans

Figure 6 - Inspection speed in knots along 8000 scans