Power Line Maintenance Inspection

The clearance report explained

The Clearance Report is a document divided in sections, each section dedicated to a particular analysis or purpose.

The basic form is a HTML file with the sections presented in sequence, but the same information can be transmitted on spreadsheet or database formats, with links to photos and graphs.

In applications including multimedia servers, it is also possible to add links to video and audio links to review the original data and user input.

The following is an introduction to each section of the report.

Notes

• You can also review the sample report on a separate page.
• In order to keep the example untraceable, the line name and the inspection date have been suppressed and the localisation coordinates are relative to the starting point.

1.Inspection descriptors

This section includes all general descriptors of the inspection:

• Type of inspection: clearance inspection
• Name of thie installation (suppressed in the sample report)
• Date of inspection
• Time of inspection
• Line characteristics:
  • Design voltage and exploitation voltage
  • Number of cables
  • Line geometry

See the sample on this page. See the sample on a separate page.

Comments

The design voltage, the number of cables and the line geometry are used by the algorithm to automatically detect the conductors and towers on the LiDAR data.

The exploitation voltage is used to evalueate clearance distances.

The number of wires distinguishes the number of phases, ground wires and separate optical fibre.

In case there is more than one geometry, the default or most common type is used.

The automatic detection and clearance can be overriden in post-processing for a local section of the line (or the total line) in case the actual parameters differ from the design or in case there are multiple identical lines within the LiDAR field of scan.

2. Summary of points of interest

This section includes a list of all points of interest (PoI). These include clearance anomalies and/or location of towers. Each PoI is depicted by some or all of the following data:

• Scan count
• Time
• Description of the PoI: level of severity and clearance distance or number and or type of tower
• Latitude and Longitude of the PoI
• Latitude, Longitude and Altitude of the aircraft
• Line span

In some implementations the global list of PoI is preceded by a short list of critical PoI (example: list of towers, list of the three most critical clearance issues).

See the sample on this page. See the sample on a separate page. The sample report does not include all the above mentioned data.

Comments

The scan count is an internal variable used for quick reference of all multimedia data (filenames, links, etc.) It refers to the index count of the laser scans.

The description of the PoI is dependent on the inspection specifications. On the sample report, PoI include only clearance inspection defined with three levels of severity - Critical (red), Urgent (orange), To be scheduled (yellow) - and the clearance distance measured in metre.

The two sets of localisation coordinates reflect the distance between the aircraft, whose coordinates are measured from the GPS, and the PoI, whose coordinates are estimated from the absolute GPS combined with the relative coordinates computed from the LiDAR and other sensor data.

The localisations are most often presented in geographical coordinates since these are readily available in any GPS handset and software. However, cartesian coordinates can be used instead.

The line span and the tower count number (not depicted in the sample report) are auxiliary indexes, used in some cases to maintain compatibility with previous inspection and maintenance procedures. It allows the inspector to locate any PoI in the absence of localisation in geographical coordinates (obtained from GPS).

3. Overview graphs

This section includes application specific graphs intended at managers requiring a quick overview of the over-head line condition and/or inspection procedures. A wide range of graphs have been developed answering to the needs of different clients:

• Minimum clearance at each scan with alert thresholds
• Minimum vegetation clearance
• Clearance hazards indexes per span
• Altitude profiles: aircraft, over-head line, terrain
• Trajectory of the over-head line with Points of Interest and quality audit variables
• Velocity over ground
• Distance of aircraft to the ground
• Distance of aircraft to the over-head line

The first four graphs are relevant for condition assessment and clearance management. The later four graphs are mainly intended as internal quality assessment tools.

See the sample on this page. See the sample on a separate page. The sample report does not include all the above mentioned data.

Comments

The minimum clearance at each span (shown in the example) depicts the variation of clearance along the line. The three colour thresholds show the region where clearance is below the specified levels.

The minimum vegetation clearance is a similar graph; it illlustrates distances to trees, bushes and other natural objects around the line. The purpose of this graph is to measure differences in consecutive inspections and estimate local growth rates.

The clearance hazards indexes (shown below) is a tool for vegetation managers. It is used to prioritise and distribute vegetation maintenance resources along a line and across a grid.

The graph shows the span sequence on the vertical axis and the hazard index on the horizontal axis. In this example, the critical spans are from tower 8 to 10 and from tower 42 to 44. Since the index has above linear growth, focusing maintenance on these two sections yields a higher safety margin at minimum costs. For more information read the January 2008 newsletter.

The altitude profiles, shown below provide an overlook of the vertical profile of the ground, the over-head line - with its towers - and the aircraft along the line.

4. Points of Interest detailed

This last section features a list of all Points of Interest [PoI]. For each PoI, a set of variables is shown:

• # Scan: the scan count since the inspection start. It is a unique identifier to index all issues found along the line.
• Time: the time of the PoI detection.
• Descriptor: the nature of the PoI.
• Level/Type: the variable associated with the PoI. For a tower it is the tower number, for a clearance anomaly it is the most severe distance in a cluster of neighbouring measurements.
• Lat: latitude of the PoI
• Lng: longitude of the PoI.
• GPS Lat: latitude of the GPS sensor when the PoI was detected.
• GPS Long: latitude of the GPS sensor when the PoI was detected
• Altitude: altitude of the GPS sensor above mean sea level when the PoI was detected.
• Span: the span count is determined from the numbering of previous and following tower.
• Max dist (for clearance PoI only): the less severe distance measured within the cluster.
• Mean: the average clearance measured for all the individual PoIs within a cluster
• # Anom: the number of anomalies included in each cluster.
• Std. Dev.: standard deviation of the clearances grouped in one cluster.
• Notes: a text field to register the users notes or comments on the PoI.

In addition to the numerical and text fields, each PoI is represented by a laser-based distance diagram (in case of clearance PoI, the distance is on the top right corner) and a video snapshot for documentation.

The actual set of variables depends on the particular implementation and the nature of the PoI.

The above example shows a snapshot of a sample PoI description, which can be viewed in full size at the sample on this page. You can also see the sample on a separate page.

Comments

PoI coordinates differ from the GPS coordinates since the PoI is located at a distance from the inspection aircraft. The localisation of the aircraft is estimated from GPS - hence the name GPS Lat, GPS Long - while the latitude and longitude of the PoI are based on GPS readings combined with signal processing algorithms. The altitude of PoI is not required since the operator is expected to be localising it from the ground.

Clearance PoI are clustered since obstacles often take more than one scan. Otherwise, a single tree or building would rise tens or hundreds of anomalies, all regarding the same object. To prevent this effect, clearance objectes near a line are clustered within a given distance, and the overall cluster is represented by the closest measurement detected.

The size of the cluster depends on the severity of the shortest clearance. In case this clearance is critical, the clusters are smaller to prevent that two nearby objects (such as two trees) could be regarded as a single object and elude trimming during maintenance. On the other hand, moderate clearance instances may spread along multiple obstacles as they are handled during scheduled maintenance procedures.

The two images (the laser distance and the video snapshot) are available as standalone files to facilitate the immediate transmission through e-mail in case the PoI refers to an urgent issue.

The Home link under the PoI description send the user back to the top of the report where he can scan readily the list of issues. Moving back and forth between the list on section two and the detailed descriptions in section four is an efficient way to browse through a long report while in the field with a portable device.