For the Last 30 Years, Progress Energy's Principal Troubleshooting Method for faulted underground primary cable has been manual reset fault indicators. However, false trips, failures to trip and the failure of field personnel to reset the indicators after restoration made this method inconsistent and resulted in less-than-optimal usefulness of the indicators. Worse, it caused field personnel to revert to the cable-damaging method of refuse and blow until the correct section of cable was isolated.
In addition, using manual reset fault indicators requires the first responder to visit each transformer in many cases. This can be a tedious process at times, given the lack of accessibility to some of the transformers that are in backyards and behind fences, or otherwise inaccessible.
Time-domain reflectometer (TDR) technology has been in existence for some time and has been used in large cable-thumper units to reduce the detrimental effects of repetitive, high-energy cable thumping. TDRs measure the time between a pulse leaving the instrument and when a reflection is received. The time is then converted to a distance, which is displayed on a screen. It is usually accurate to ±5% of the total cable length.
TDR technology assists repair crews by getting them closer to a fault location, so the duration of the high-energy thump can be reduced. Fault-finding radar (FFR) units use TDR technology, but do not include a high-energy cable thumper. FFR units are compact and portable, and can be set up at any transformer location in the half loop, searching in either direction to determine the faulted section of the cable. A FFR unit is connected to the feed-through bushing at the transformer, along with a neutral return lead and an equipment grounding connection.
FAULT-FINDING RADAR USE
The operator will first sectionalize the cable or determine the location of each transformer relative to where the FFR unit is connected. To determine the transformer locations, the FFR unit sends a low-voltage pulse that compares impedance changes along the cable route. High impedance along the cable indicates elbow terminations (transformer locations), which appear on the FFR screen with relative footages from the hook-up point to each transformer. The operator will then try to locate the fault. This is accomplished by the FFR unit sending a 15-kV high-frequency pulse applied on the cable using arc-reflection technology to detect the distance to the flashover (cable fault). This allows the operator to see the distance to the fault location as well as the distance relative to each transformer location, thus enabling the operator to determine the specific section of cable to isolate in order to restore power to the remainder of the half loop.
Progress Energy uses FFR units as its primary means of troubleshooting single-phase underground-cable failures, thereby replacing its manual fault indicators, in both its Carolina and Florida service territories.
THE BUSINESS CASE
In moving toward this new technology, Progress Energy developed a business case to fund the project and to identify the potential benefits.
Purchasing FFR equipment is a one-time cost and allows for the elimination of recurring capital expenditures associated with fault indicators. FFR units reduce the number of steps required to troubleshoot underground primary loops, resulting in an estimated 20% reduction in restoration time for these types of outages. A reduction in unnecessary travel by two-man underground repair crews can be realized. Underground cable is switched out by a troubleman during the restoration process, but is not always tested.
Fault indicators are predominantly manually reset. For multiple reasons, the information fault indications provide can be misleading and inaccurate, because they can trip incorrectly, fail to trip or not be reset after restoration. FFR use is demonstrated to obviate these problems.
- Momentary interruptions and equipment damage
When fault indicators fail to perform, field personnel often will resort to testing sections of cable through refusing. This stresses the cable and equipment with more fault current, and causes additional momentary interruptions on the upstream device.
- Speed of restoration
Due to improved accuracy of the FFR units and the reduced steps required to troubleshoot underground loops, restoration times will be reduced. Trial sectionalizing of the loop will be eliminated, which will reduce the time required to isolate the failed section of cable. Having the FFR unit will allow the user to set up at the most accessible location in the half loop, thus eliminating the need to access transformers that may be in back lots, in gated areas prone to having dogs or in areas that present other obstacles.
Moving from current troubleshooting practices and considering how well the workforce would embrace the technology were major concerns from the onset of the project. Communication and training of all users and affected parties were critical to the successful implementation of this project. Buy-in for use of the technology had to begin at project inception. During evaluation of the FFR units, select troublemen throughout the company were given the opportunity to test the equipment in field conditions.
To complicate matters, Progress Energy was not only evaluating the FFR capability, but also comparing different equipment manufacturers. Troublemen had to keep records of FFR unit performance and other pertinent data documenting the specific events where they used the equipment. To ensure accuracy in performance tracking, the team would cross reference the Progress Energy outage database to reference the events described by the troublemen using the FFR units. Troublemen also were given direct contact information for technical support relating to equipment operation. This was to ensure the troublemen could have their questions about FFR operation answered during actual outage events.
In the end, feedback on the overall performance of the FFR was compiled, and each manufacturer's performance was graded. This information was used to develop a standard specification and prepare a request for quote for purchase of the FFR units.
The implementation plan's action items included:
FFR evaluation, including change management, was compared with current troubleshooting methods.
Safety, training and dispatch were subject to review. There was an emphasis on coordination and co-development of operating and switching procedures relating to the use of the FFR.
Coordination with fleet on storing and charging the units on the trucks.
Development of FFR specifications and evaluation of multiple manufacturers.
Development of a training module geared toward the end user. The training module was developed in conjunction with the manufacturer but taught by Progress Energy personnel.
Training topics included unit specifications, connections and unit care, arc-reflection technology, display indications, and troubleshooting options and situations. The training also consisted of several scenarios for the student to work through via a hands-on session. Instructors used a text box to create different scenarios for the classroom demonstrations.
As with any project that is implemented on a large scale, technical issues arose that had to be resolved. A number of units purchased experienced some quality issues and had to be sent back to the factory. Additional units on hand were used to assist with backfill of nonfunctioning units. Also, fine tuning of FFR unit settings is ongoing to allow for better accuracy in determining transformer locations. It also takes some time for the user to get a feel for what the FFR unit is trying to tell them.
Users of this equipment must know how to differentiate and interpret various waveform outputs of the FFR unit, such as open, cut, shorted and open-concentric cable. For example, a FFR unit that shows a different cable footage or fewer transformers than what is actually in the field could indicate that the cable has been cut in the clear, instead of having a pinhole fault. The operator must interpret the technology and realize that he is only seeing up to the point where the cable is cut in the clear.
Structured technical training is critical to the successful implementation of this technology. Refresher courses are encouraged to keep users up to date with radar applications and changing technology; they also provide a valuable opportunity for troublemen using this technology to teach each other.
Implementation of the FFR project has been completed at Progress Energy Carolina and will be completed at Progress Energy Florida by 2011.
Mark Danna is a lead engineer in the Distribution Standards Unit for Progress Energy Florida, where he is responsible for establishing standards and operational guidelines for various underground material items and equipment. Danna holds a BSEE degree from Auburn University, and he is a registered engineer and a licensed certified electrical contractor in the state of Florida.
David Johnson is a senior technical specialist in the Distribution Standards Unit for Progress Energy Florida. He is responsible for establishing standards and operational guidelines for the underground work methods, evaluating new tools and providing training for new tools and equipment. david.
Note: This article is based on a 2008 Southeastern Electric Exchange presentation.