Cooper Power Systems’ NOVA-STS vacuum recloser installation with a 220-MHz radio antenna connected to a GE MDS SD2 radio. By the end of 2013, Southside Electric Cooperative will deploy 74 distribution automation locations.
As part of an organizational assessment performed in 2008, the reliability of Southside Electric Cooperative (SEC) was benchmarked against other cooperatives of like size and composition. SEC was consistently identified as being in the fourth quartile, or lowest 25% of performance. Prior to this assessment, there never had been any objective reviews of the coop’s reliability performance nor comparisons made to other utilities. The system reliability shortcomings made evident through the benchmarking process resulted in SEC’s board of directors and management team identifying reliability improvement as the No. 1 strategic objective for the cooperative.
In 2010, SEC completed a reliability improvement plan that established finite reliability improvement goals through 2015 for its system average interruption duration index (SAIDI), system average interruption frequency index (SAIFI) and customer average interruption duration index (CAIDI), as defined by Rural Utilities Service bulletin 1730A-119, Interruption Reporting and Service Continuity Objectives for Electric Distribution Systems, and IEEE standard 1366, IEEE Guide for Electric Power Distribution Reliability Indices.
This comprehensive plan created a framework for justifying future capital improvement projects, defined operational and maintenance practices focused on reliability improvements, and identified investments in technology that would directly impact system reliability performance. Through this reliability planning process, the coop identified the deployment of distribution automation (DA) as the single-greatest opportunity to improve performance at the quartile level.
Historically, during outage events, SEC had manually isolated faulted line sections and energized unfaulted line sections from adjoining circuits. The time required to manually transfer load was lengthy because of the size of the coop’s service territory. SEC operates 60 miles (97 km) of transmission, 34 substations, 120 distribution circuits and 8,149 miles (13,115 km) of line, serving 54,669 service locations. Reliability improvement planning revealed that 67% of the coop’s SAIDI minutes were generated by only 20 of its 120 total circuits. Of these 20 poorest-performing circuits, 19 had ties with neighboring circuits.
The reliability improvement plan called for the deployment of DA, specifically a fault detection isolation and restoration (FDIR) system, on the 20 poorest-performing circuits. During outage events, the FDIR system would restore service by switching unfaulted line sections automatically in minutes versus manually in hours.
For the FDIR deployment to be successful, SEC concluded the following critical components were necessary:
- Ties between distribution circuits
- Adequate system capacity, including voltage drop considerations
- Resources necessary to operate and maintain DA systems
- Intelligent electronic device (IED)-controlled reclosers at circuit midpoints
- IED-controlled switches at circuit tie points
- A robust supervisory control and data acquisition/distribution automation/distribution management software (SCADA/DA/DMS)
- A robust communications system to communicate with the IEDs.
SEC’s distribution system was an ideal candidate for the deployment of FDIR because 90% of the distribution circuits have ties to neighboring circuits. Also, the coop has followed Rural Utilities Service bulletin 1724D-101B, System Planning Guide, Construction Work Plans, which states that in the normal system configuration, primary conductors should not be loaded to more than 50% of the thermal rating for major tie lines during peak loading conditions.
Additionally, 78% of the distribution system is operated at 24.94 kV; the remainder, operated at 12.47 kV, is currently being converted to 24.94 kV. As a result, voltage drop during DA switching will not be a major issue. SEC was in a great position because circuit ties and system capacity are, by far, the most expensive components of a successful FDIR system, and these capital investments had been made already.
SEC had some valuable existing assets that could be leveraged for the new DA communications. It operates a 450-MHzvehicle radio system, a 900-MHz SCADA system and a 6-GHz microwave system, linking its four district offices and headquarters facility. To operate these radio-based communications systems, the coop owns 35 towers that have excess capacity and skilled telecommunication technicians on staff to provide maintenance support.