by Paul Richardson, Network Mapping
Increasing regulatory pressure in the U.S. electricity transmission industry as a consequence of large-scale blackouts during the past decade has prompted the North American Electric Reliability Council (NERC) to create reliability standard FAC-003.
This defines rigorous guidelines for preventing vegetation-induced electrical outages, including the establishment of a transmission vegetation management program (TVMP).
There has been much debate surrounding how electrical utilities can comply with NERC FAC-003 to maintain system reliability and avoid penalties. Aerial light-detection and ranging (LiDAR) technology adds value to TVMP-compliance efforts by reducing costs while ensuring comprehensive survey coverage of the transmission right-of-way with the production of actionable, auditable deliverables. This approach ensures all hazardous vegetation is removed and avoids environmental impacts of overcutting.
NERC FAC-003 is concerned specifically with vegetation management along transmission line rights-of-way. This is aimed at reducing outages caused by vegetation, such as the large-scale cascading failure and blackout during summer 2003, a consequence of higher than normal demand and inadequately maintained vegetation. The standard sets out the requirement for a utility to establish and maintain a TVMP for all lines operated at 200 kV and above and for any lines designated as critical to the reliability of the electrical system in the region by the regional reliability organization (RRO). The TVMP is intended to mitigate vegetation-related outages by:
- Preventing outages from vegetation on transmission line rights-of-way,
- Minimizing outages from vegetation adjacent to rights-of-way, and
- Maintaining clearances between transmission lines and vegetation on and along transmission rights-of-way.
Utilities are required to report to the RRO any sustained outages caused by vegetation for three categories:
- Category 1: Grow-ins–Outages caused by vegetation growing into lines from vegetation inside the right-of-way, outside the right-of-way or both.
- Category 2: Fall-ins–Outages caused by vegetation falling into lines from inside the right-of-way.
- Category 3: Fall-ins–Outages caused by vegetation falling into lines from outside the right-of-way.
To prevent vegetation-related outages, utilities with lines included within the jurisdiction of FAC-003 must create a schedule of rights-of-way inspections. They must identify and document clearances between vegetation and overhead lines while considering the following variables:
- Transmission line voltage,
- The effect of ambient temperature on conductor sag under maximum design loading, and
- The effect of wind velocity on conductor blowout.
Each utility determines clearance distances based on local factors and reference to existing National Electrical Safety Code (NESC) clearance requirements.
Aerial LiDAR is a rapid airborne surveying technique used to capture large swaths of topographic data and surface features as a digital 3-D model. The speed of survey is typically 100 km a day using a helicopter, and only one technician is required to operate a base station within each 20-km radius of the survey area. LiDAR collects high-accuracy and high-resolution data indiscriminately for the survey area with survey point densities in excess of 20 points per square meter. With adequate ground control, the absolute accuracy of a single georeferenced point will be less than 0.33 feet. The relative accuracy between two points will be even greater.
An aerial LiDAR system consists of a scanning laser mounted on an airborne platform that emits thousands of infrared pulses every second. The pulses are reflected by the ground surface and above-ground objects, and all returning pulses are recorded by the sensor. The timing between the emitted and returned pulse is then calculated to produce accurate range measurements. The position of the aircraft at the time of capture is determined by an onboard kinematic global positioning system (kGPS), which is post-processed with differential global positioning system (dGPS) readings from two or more ground base stations. This is combined with data from an onboard inertial measurement unit (IMU) tracking yaw, pitch and roll to produce a smoothed best-estimate trajectory for the flight path. This is used as a basis for processing the laser range measurements to form a 3-D point cloud, which is then projected in the local coordinate system and datum specified by the end user.
Aerial LiDAR technology has been applied extensively to electrical transmission capturing towers, conductors, vegetation and other objects within the right-of-way, as well as the terrain. An operator prepares the 3-D point cloud by classifying objects within the dataset. The classified dataset is used to build a 3-D engineering model of the line within PLS-CADD, including structural representations of the towers and conductors. The model is attributed with information pertinent to compliance with FAC-003. The line voltage is entered for each circuit together with the clearance distances required between the conductor and classified objects. The temperature of conductors for each span at the time of flight is calculated to IEEE standard 738 with in-flight meteorological data, ground weather station data and line load data. The conductors can be sagged to their maximum operating temperature and the infringement distances determined at this position. Weather cases can be defined so that vegetation clearance reports produced from the model include the effect of wind velocity on conductor blowout.
The PLS-CADD engineering models have been used hitherto for the design phase of capital expenditure projects, with examples including the simulation and evaluation of thermal upgrade or reconductoring alternatives to increase line capacity. They also are used for operational expenditure projects to detect violations of regulatory clearance distances between the conductors and objects such as buildings, undercrossing lines and vegetation.
The 3-D data acquired during the aerial survey is versatile and can be used to produce outputs in a range of formats, from KMZ files displayable in Google Earth to those for download onto mobile GPS units or hard copy plan sheets that display the exact location of vegetation violations for each span of transmission rights-of-way.
For surveys specifically aimed at vegetation management, flown infringement checks can be produced shortly after completion of aerial data acquisition. The vegetation and conductors are rapidly classified. An intelligent algorithm then is used to highlight the location of any vegetation points that are infringing a predefined radius from the as-surveyed conductor position. These are used to communicate immediately the vegetation conditions that present an imminent threat of a transmission line outage following the survey, as required by NERC FAC-003-1 R1.5.
Following the report of imminent vegetation threats, more precise plan sheets can be produced to comply with FAC-003-1 R1.2 by modeling the line within PLS-CADD. These checks are carried out with consideration of line voltage, the effect of ambient temperature on conductor sag under maximum design loading, and the effects of wind velocities on conductor sway.
The plan sheets incorporate the high-resolution photography taken during the survey to display grow-in infringements within the rights-of-way, falling tree infringements inside the rights-of-way and falling tree infringements outside the rights-of-way.
The introduction of NERC FAC-003 has stirred up much debate concerning how utilities successfully can comply with the standard and establish an effective TVMP. Aerial LiDAR already is becoming a valuable tool in support of NERC FAC-003 compliance programs and is seen as a step change from manual inspection and follow-up clearance estimates to highly precise measurements incorporated into engineering PLS-CADD models to identify infringements under all line operating conditions.
Paul Richardson worked as a transmission line designer with National Grid before being a founding member of Network Mapping in 2001. He is now technical director of Network Mapping, providing LiDAR survey and analysis to power utilities worldwide. Reach him at email@example.com.
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