NERC-FAC003-3The Use of Lidar in Transmission Vegetation Management

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NERC FAC-003-3 came into force on July 1st 2014. This article takes a look at how lidar-based workflows, provided by NM Group, can be used within a utility’s Transmission Vegetation Management Program (TVMP) to support compliance. It also demonstrates that utilities can combine lidar surveys for vegetation management with those carried out for engineering to update thermal line ratings in accordance with NERC FAC-008, and to preserve the initial investment in as-built models of the transmission network. These combined surveys serve to reduce any duplicated effort, while ensuring significant cost savings.

An Overview of NERC FAC-003-3
The FAC-003-3 reliability standard mandates that transmission lines that fall within the jurisdiction of the standard (all lines above 200 kV and lines below 200 kV that are elements of an Interconnection Reliability Operating Limit or a Major WECC Transfer Path) must be inspected to identify vegetation-related issues once per calendar year, with the interval between inspections not to exceed 18 months.

The standard requires vegetation to be managed to prevent encroachments into the Minimum Vegetation Clearance Distance (MVCD) due to grow-in from below conductors, fall-in from within and outside the right-of-way (ROW) and conductor blow-out into adjoining vegetation. The MVCD is the calculated minimum allowable distance between conductors and vegetation, for various altitudes and operating voltages.

The transmission operator needs to demonstrate that encroachment into the MVCD considers the movement of line conductors under all applicable line load and weather cases (i.e. the sag and sway of the conductor). While various inspection options exist within the utility forester’s toolbox, such as ground-based inspection and visual aerial inspection, this article examines the additional benefits of a lidar-based approach that provides an auditable solution to vegetation management that reduces costs, while maintaining high accuracy.

Lidar as a Practical Tool for the Utility ForesterCreating the Baseline
Lidar data, and its subsequent analysis, provides a rapid means to detect and report any critical vegetation-related issues that may affect the reliability of electrical supply, and allows these to be promptly addressed. This methodology provides high resolution coverage of the entire ROW and adjoining areas, and can identify clearance issues that would be undetected by other methodologies.

Aerial lidar consists of a laser scanner that emits hundreds of thousands of infrared pulses per second, accurately measuring structures, conductors, vegetation and any other hard and soft features. Lidar scanners achieve up to four returns for each pulse enabling the canopy and sub-canopy of vegetation, together with the ground surface, to be recorded. Shortly after the survey, the vegetation and conductors are rapidly analyzed and the location of vegetation that is infringing on a predefined radius from the as-surveyed conductor position is highlighted in order to communicate any imminent threats to line reliability.

The transmission line is then modeled at the maximum operating temperature and the effects of wind velocities on conductor sway. This enables further analysis to be performed against the MVCD and the buffer beyond the MVCD chosen by the utility. A baseline survey dataset therefore enables a thorough analysis of the area, the establishment of the MVCD considering sag and sway, and analysis of any encroachments within the buffer and MVCD zones. The data capture system can be further augmented through forward looking cameras for structure inspection and near-infrared (NIR) imagery for vegetation health indicators. Deliverables for baseline surveys include danger tree locations, vegetation height maps, vegetation health maps, vegetation risk maps, as-built PLS-CADDTM models and others.

Subsequent Vegetation Refresh Surveys and Reporting
The NERC Alert of 2010 led to the creation of mainly lidar-based as-built models for transmission infrastructure for the majority of the USA and Canada. These models incorporated the conductor positions under maximum sag and sway as described above. This existing information can now serve as a basis to confirm compliance with FAC-003-3. This means that subsequent lidar surveys for FAC003-3 compliance can be flown using a customizable and lean acquisition approach that utilizes fixed-wing aircraft, extended GPS baselines and rapid approaches for processing datasets. This has reduced the base cost of vegetation refresh surveys and is an auditable approach for keeping libraries up-to-date. It also allows for further data analysis, such as growth rates, as a multi-year data source is built up over time.

Ease of Access to Lidar Information across the Utility
The application of lidar to vegetation management on ROWs must be both cost-effective and practical. A key feature is the ability to make the results of a survey available rapidly to a utility’s forestry department in an easy-to-use format. In most instances, a utility will request adaptation to existing GIS and workflow systems. In some instances, the level of custom functionality necessitates the deployment of a forestryspecific system which may be supplied by either the lidar vendor or a third party. NM Group’s proprietary system, Caydence, delivers rapid and highly intuitive access to ROW information and areas of concern in a visually elegant package, for use on multiple platforms within forestry and across the business. It allows users to inspect and visualize specific circuits and spans and provides a means to record and review survey intelligence from the lidar program.

A Shared Investment? Enabling Engineering to Maintain Line Ratings for Continued FAC-008 Compliance
Thermal rating confirmation for all lines above 100 kV in order to comply with the NERC FAC-008 alert of October 2010 has now been completed by most utilities in-line with a compliance date of 31st December 2013. With 450,000 miles of lines across the USA and Canada, this has cost the industry somewhere in the region of $100-200 million using the preferred technique of aerial lidar and modeling in PLS-CADDTM.

What is less clear is how this large investment in the accurate modeling and engineering analysis of the network will be maintained and readily accessed going forward. New lines will be built, existing lines will be modified and potentially undetected changes to Rights-of-Way and facilities will continue to impact electrical clearances. Examples of the latter include heavy ice loading, storm restoration and undocumented under-crossings.

NM Group’s proprietary service, Asset Renew Right-of-Way Refresh (NM-ARROWTM), can be applied for intelligent change detection for lidar survey flights flown for the primary purpose of vegetation analysis and FAC-003-3 compliance. This service, drawing on proprietary algorithms and workflows, highlights areas of previously unknown change within the ROW that can be investigated further to check that line thermal ratings haven’t been compromised. Combined with survey flights to establish a new baseline for known changes to the network, this provides a mechanism by which investment in the existing library of as-built models and line thermal ratings can be efficiently maintained year-on-year. The Caydence system can be used to efficiently store and serve-up the library of year-on-year vegetation datasets and as-built models.

FAC-003-3 is clearly a game changer in terms of how transmission owners manage and protect their assets and it means that systems of vegetation management based on visual assessment will no longer suffice. Yearly inspections are now required and vegetation infringements within the MVCD can result in penalties, whether or not they result in sustained outages.

Aerial lidar has proved its worth to engineers and asset managers since the NERC 2010 alert. It continues to deliver clear benefits to utility foresters as a tool to provide accurate vegetation data while creating an auditable information library which can be further used for multi-year analysis purposes. By working together on cross-departmental surveys, wider business benefits can be obtained. Engineers can continue to comply with FAC-008 and protect their existing investment by ensuring their transmission infrastructure model library and thermal line ratings are up-to-date, and utility foresters can manage their work schedules in a timely and more precise way to comply with FAC-003-3.

Nick Ferguson is a business and product development professional with an academic background in the utilization of remote sensed datasets for environmental applications. Nick has five years of experience working with utilities in the US, Australasia, Europe, India and the Middle East.

Scott Ryder is a Senior Project Manager with over 42 years of experience for all phases of transmission project development and operation. He is a Registered Professional Engineer, Licensed Construction Supervisor, and member of the American Society of Civil Engineers.

Paul Richardson is the Technical Director of NM Group and a specialist in electrical transmission engineering and lidar surveys. He is a Chartered Civil Engineer and a leading authority on transmission line design and development.

A 1.612Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE