A&N Electric Cooperative's high-speed, decentralized feeder automation system isolates faults, transfers sources and restores service in less than 500 msec.
The A&N Electric Cooperative serves Accomack and Northampton counties on the Virginia Eastern Shore, a narrow 75-mile (121-km) peninsula surrounded by the Atlantic Ocean and Chesapeake Bay. Because of its seaward geography, the Eastern Shore is battered by frequent thunderstorms, tropical disturbances, hurricanes, flooding and, to a lesser extent, tornadoes and heavy, wet snowfall. These adverse weather conditions, along with numerous automobile crashes into pole lines, have negatively impacted A&N's operations over the years.
A&N's service area consists of vast rural areas broken up by small communities and is bisected by U.S. Route 13, a major highway connecting southern Virginia with Maryland and Delaware by the Chesapeake Bay Bridge-Tunnel. The cooperative serves its Northampton county members — which includes homes, schools, small businesses, government centers and Shore Memorial Hospital — with a 20-mile (32-km), 25/14.4-kV distribution feeder energized at both ends by substations located at Exmore and Bayview. For the most part, the feeder right-of-way alternates along busy U.S. 13 and a working railway, and also runs alongside some residential streets in Nassawadox.
Protecting Hospital and Community
Shore Memorial in Nassawadox is the area's only hospital; therefore, it is critically important to the Eastern Shore. To minimize the facility's need for backup power, A&N proposed to install an automated high-speed source-transfer system on the Northampton feeder to reduce outages that, at times, approached one hour due to the need to physically isolate faults and transfer substation resources.
To best serve the interests of the community, in addition to increasing the speed, the new system also had to offer maximum economy, reliability, upgradeability and sustainability, and benefit as many members as possible. Specifically, A&N determined the system should meet several requirements:
High-speed automation to minimize the impact of outages
Scalability to target specific problems over time in order of importance
Upgradeability to meet future needs with minimal outlay
Simplified system design and deployment using off-the-shelf components
Unattended operation through the application of decentralized automation
Increased system longevity through the use of modern industry standards
Rapid source-transfer capability
Flexibility to work with existing switchgear
Multiple human-machine interfaces (HMIs) for manual feeder control and quick assessment of feeder status from central and outlying points.
After surveying the available technology, A&N approached Siemens Energy, which had recently announced a high-speed feeder automation product — the Siemens SDFA-DC Distribution Feeder Automation System — that would reduce Fault Location, Isolation and Service Restoration (FLISR) time to less than half a second. The new offering met all of A&N's operational requirements, most importantly with respect to operating speed, which was rapid enough to avoid activating backup power at the hospital.
Traditional fault detection employs overcurrent methods to uncover line problems. While effective, this approach is sluggish, resulting in long restoration times. To achieve high operating speeds, this approach uses a novel means of fault detection wherein each sectionalizing device in the feeder is equipped with a smart relay and sensor that continuously supplies line current data to the relay in each adjacent section.
When a problem occurs along the feeder, the relay in each affected section receives information about the disturbance and compares it to the line current conditions in the adjacent sections. If a comparison indicates a fault condition, the affected relay issues a notification to all other relays in the system, thus identifying and isolating the fault. Using this approach, problems can be detected in less than 100 msec, which is sufficiently fast to permit the location of a fault before the protection and switchgear have time to disconnect the source from the faulted load.
Fault isolation, source transfer and service restoration tasks are performed according to simple sequential switching logic programmed into each relay using the Siemens DIGSI software tool. Thus, the system has flexibility to execute desired sequences based on central or systemwide operating modes, fault information and feeder status. Elapsed time to complete isolation and restoration steps is typically around 400 msec.
Component and Site Selection
To maximize availability, A&N elected to section the feeder into four protection zones using reclosers. An off-the-shelf-type SDR recloser from Siemens, which comes equipped with a recloser controller, and an automation controller were placed at each recloser site. To control the substation circuit breakers, protect the feeder and provide a secondary system of HMIs, a control and automation relay was installed at each substation.
Section lengths were apportioned according to load density and criticality. The section supplying the hospital measures 0.5 mile (0.8 km), while the others measure at 5 miles or 10 miles (8 km or 16 km) depending on local load characteristics. Because portions of the feeder run along a working railway with limited access, accessibility dictated the exact siting of the reclosers. To maintain access for line crews, the reclosers and related relay cabinets were attached to poles situated near the highway and a grade crossing.
Even though the automation system is fully capable of stand-alone operation, A&N chose to outfit the installation with both outlying and central HMIs. The automation system and feeder can be fully supervised, monitored and controlled — including operating breakers and reclosers, transferring sources, isolating line sections, locating faults, recognizing equipment lockouts and identifying hot line tags — from either the substations or control center. To ease operational tasks, A&N elected to deploy a Siemens' PC-based SICAM operator interface at the latter. This point-and-click HMI features an interactive feeder model diagram and color-coded dashboard to simplify operations and enable at-at-glance analysis of feeder and automation system status.
To prevent hazardous feeder operating conditions, critical system functions are interlocked to block conflicting or potentially dangerous commands automatically. Hot line tags can only be applied and released at the breaker and recloser relays. When a tag or equipment lockout is in place, the automation system is disabled automatically to prevent unwanted switching operations. However, the remaining untagged and unaffected switching devices will continue to operate in a stand-alone mode to maintain feeder protection and control.
Design and Installation
Although the installation was based on a standard Siemens protection and control scheme, the added automation made it the first of its type for the company. To test the concept, a working mock-up of the feeder, including all relays, breakers, reclosers, HMIs and communications devices, was configured, tested, analyzed and modified in Siemens' smart grid laboratory in Wendell, North Carolina, U.S.
A&N personnel conducted factory acceptance and performance tests at the laboratory to ensure all protection and automation sequences functioned according to plan. However, in spite of the testing, with no actual field experience on which to draw, project engineers could not promise the system would function exactly as intended. In fact, they fully expected changes and adjustments would be necessary to achieve desired operation once the equipment was actually installed and operating.
The feeder's load and physical characteristics presented several design challenges for the project engineers. In addition to typical load variations, the service area's seasonal ebb and flow in population results in wide demand swings. The high source impedance and extended length of the feeder create very low fault currents that complicate coordination, especially among fuses and upstream transformers. Wide variations in feeder topology created by the automation itself also had to be taken into account. These factors made coordination difficult, time-consuming and expensive.
To reduce system costs and construction time, A&N decided to interconnect the control relays using high-speed 3.65-GHz SHF WiMAX Ethernet links employing the International Electrotechnical Commission 61850 protocol and generic object-oriented substation event messaging. However, the 20-mile end-to-end system length made network planning difficult. To achieve reliable communications, A&N mounted three high-power RuggedCom WiN7237-5 base stations and related beam antennas on existing 200-ft (61-m) and 300-ft (91-m) radio towers to communicate with pole-mounted, directional RuggedCom WiN5237-5 subscriber units. Because the entire service area is relatively flat, there were no unusual propagation concerns outside of typical rural land obstructions such as old-growth trees.
The control center is located in Tasley, 15 miles (24 km) from the feeder. Fortunately, A&N recently had recently installed a broadband fiber system to interconnect its major facilities, which presented a convenient way to link the automation system with the control center. RuggedCom RS900-series Ethernet switches were specified to handle Ethernet switching and interface tasks throughout the communications network. The automation system and reclosers were installed and interconnected by A&N line crews with assistance from Stellar Communication Systems and Rock Creek Line Construction.
Test and Commissioning
Although personnel checked base station and subscriber unit performance before delivery, under actual field conditions A&N experienced difficulty in achieving reliable WiMAX link performance. With the assistance of Siemens and RuggedCom engineers, A&N personnel were able to achieve acceptable results by trimming obstructive trees to afford a better propagation path for the wireless signals, adjusting the attitude and elevation of the subscriber units, and altering base station and subscriber unit transmission power, bandwidth and frequency settings on a largely trial-and-error basis. To increase the elevation of two subscriber units, A&N installed a 50-ft (15-m) pole and a pair of mast extensions for mounting the units.
The WiMAX links were tested using the system's simulation mode wherein the switching functionality of the circuit breakers and reclosers is replaced by RS flip-flops. This allows the system to function normally up to the relay trip contacts, thus permitting in-depth analyses of system performance without operating any switching devices.
Once reliable links were established, A&N personnel were able to test actual recloser operations without interrupting service through the use of in-line bypass switches installed at each recloser. Following successful testing, the bypass switches were opened, placing the reclosers in circuit. The feeder's open point was then shifted using the control center HMI to check the system's reconfiguration and source-transfer functions. With this successfully accomplished, the system automation was activated to await an actual event to fully prove the system.
Within weeks of commissioning the system, a wave of severe electrical storms hit the Eastern Shore. During one such storm, project engineers at the control center were using simulation to analyze system performance when lightning struck the feeder near the Kellam substation. The engineers saw the fault notification appear on the model diagram and then observed the system as it reconfigured the feeder to isolate the faulted section and transfer the remaining viable zones to the Bayview substation.
In another particularly severe storm, the automation system failed because of a loss of communication. Using SICAM's extensive GPS time-stamped event-recording capabilities, A&N and Siemens engineers determined the feeder had sustained multiple lightning strikes, several of which knocked out the communications system along with other A&N plants. Because SICAM automatically uploads the fault data recorded by any affected relays, in-depth study of system performance is possible using the Siemens software-based SIGRA analysis tool. In the end, lightning damaged a relay communications card, a subscriber unit, an Ethernet switch and two power-over-Ethernet supplies interconnected to a base station and a subscriber unit. The latter units were subsequently returned to RuggedCom for repair.
Following repairs and the installation of diode-based surge protectors from Transtector Systems on all wireless-equipment Ethernet lines, it remains to be seen whether or not lightning strikes will continue to be a problem for the communications equipment. Given the severity of electrical storms in the area, further measures could be necessary to resolve the problem.
Project engineers will continue to monitor feeder and automation system performance, and make adjustments and improvements as data becomes available from additional operational events. The system's simulation mode and SIGRA analysis tools will be employed as effective and convenient means of testing and analyzing feeder operations and ideas for improvement. Any system configurations and settings developed as a result of the improvement process will continue to be recorded and entered into a database to facilitate future system design and commissioning.
As a cooperative, A&N Electric must focus on its member needs and work for the sustainable development of the community. The installation of this automated high-speed FLISR system is one way A&N demonstrates its commitment to the ideals of providing better, more reliable electric service for all.
Kelvin Pettit (firstname.lastname@example.org) joined A&N Electric Cooperative in January 1972. Pettit's 40-year career in the electric distribution industry began with the completion of the lineman apprentice program offered to returning Vietnam veterans. Currently, as vice president of system reliability, he is responsible for providing the cooperative's 35,000 members on the Virginia Eastern Shore with the most reliable electric service possible. His commitment to the use of leading-edge technology led to the development of software-driven maintenance programs for A&N's substations and line equipment. He led the team that was responsible for the deployment of A&N's first auto power restoration system in 1995.
A&N Electric Cooperative www.anec.com
Rock Creek Line Construction www.rockcreeklineconstruction.com
Siemens Energy www.energy.siemens.com
Stellar Communication Systems www.stellartowers.com
Transtector Systems www.protectiongroup.com/transtector