A Huge Opportunity for Material, Time and Manpower Savings Exists in the reduction or elimination of substation copper control cables. This has inspired American Electric Power's (Columbus, Ohio, U.S.) interest in the process bus technology.

AEP decided to evaluate a next-generation distributed protection and control system with all process interfaces located in the switchyard, thus taking control cabling, with its associated material and labor costs, out of the design and replacing it with fiber-optic communication.

AT THE FOREFRONT

For decades, AEP has been at the forefront of power system protection and control technologies. In the 1970s, the utility participated in the introduction of digital relaying. In the 1980s, it took part in early research into optical instrument transformers. In the 1990s, it was an early participant of the Utility Communication Architecture (UCA) group, subsequently the UCA International Users Group, and the IEC61850 standard.

As the concept of a microprocessor-based relay matured and turned into practical products, AEP led the way with widespread adoption of the technology. Major improvements have been achieved in the areas of material cost savings, operational efficiency through remote access, control capabilities, multi-functionality, availability of data, simplification through integration of protection and control functions, and elimination of some auxiliary devices with associated panel wiring.

AEP envisions a possible future generation of protection and control systems with interface devices dispersed throughout the switchyard. The dispersed devices would provide the required input/output structure for existing and new apparatus: a simple, robust standard communication architecture and interoperable intelligent electronic devices performing traditional functions, working exclusively with communication-based inputs and outputs.

AEP encouraged the vendor community to pursue this vision. In 2008, GE Digital Energy (Markham, Ontario, Canada) developed the HardFiber system, a complete and commercialized solution designed to eliminate copper control cables from the switchyard. In the second half of 2008, AEP completed the installation phase of an evaluation retrofit project with the HardFiber product.

TECHNOLOGY AS A BRICK

The HardFiber process bus system is a remote I/O architecture for protection, control, monitoring and metering that allows designing out copper wiring for protection and control signaling within substations, replacing it with standardized optical-fiber-based communications. The system includes relays and fiber cross-connect panels, factory pre-terminated fiber cables and switchyard I/O interface devices known as bricks.

The bricks implement the distributed concept of an IEC61850 merging unit, expanded to optically connect relays with all types of I/O signals in the switchyard, not just instrument transformers. The bricks are interconnected to the relays in a simple point-to-point arrangement that does not involve other active components such as Ethernet switches.

The relays are GE Universal Relay series devices. The relay's current transformer/voltage transformer and contact I/O plug-in modules are replaced with an IEC61850 process card to allow optical rather than copper signal interface. The balance of the relay hardware, firmware, functionality, configuration software, documentation and user-setting templates are unchanged.

DEMONSTRATION INSTALLATION

The HardFiber demonstration installation is in AEP's Corridor Substation, a 345/138-kV transformer and switching station near Columbus that has been used for other new-technology trials. The HardFiber installation provides distance protection for the Conesville and Hyatt 345-kV line terminals and breaker-failure protection for breaker 302N, which connects these two lines in a breaker-and-a-half-like arrangement.

This portion of the station was considered typical and of sufficient size and diversity to demonstrate the HardFiber system technology. In addition, the lines already had existing universal relays installed. So, the existence of these devices enabled event and oscillography records to be easily compared to those from the HardFiber system. The trip/close control outputs of the HardFiber system are not connected at this stage of the evaluation.

A site survey was conducted early in the project with the manufacturer. The survey confirmed the viability of the scope described previously, the quantity and location of the equipment, and the lengths of the required pre-terminated fiber-optic cables.

Twelve bricks were necessary to provide fully redundant coverage: two bricks on each of the three circuit breakers, two on each of the two-line current-voltage transformers and two on the one free-standing current transformer in the zone. In each case, no space was found for mounting bricks inside the mechanism/marshalling boxes, so brick-mounting locations were selected either on the outside surface of the power equipment or on a supporting steel structure.

The fiber-cable routing for the 12 cables consists of a 200-ft (61-m) section in 6-inch (15-cm) duct, a section of up to 400 ft (122 m) in a pre-cast cable trench shared with conventional copper control cables, a direct-bury section of up to 150 ft (46 m) and an exposed section from grade to brick level. The factory-terminated cables required accurate cable-length measurements; a cable that was too short would have to be replaced and excess length would present slack management problems. Several length-measuring methods were tried, including use of site plans, time-domain reflectometry on existing spare conductors, a pulling tape with numbered foot markings and a measurement wheel. In the end, a surveyor's tape produced the best results. The cables were ordered with a 2% margin over the measured length.

Consistent with AEP's standard design practices, FT-style test switches were installed in the brick current-transformer circuits shared with in-service protection and the brick voltage-transformer circuits were fused.

ON-SITE INSTALLATION

Installation of the HardFiber equipment proceeded smoothly and did not reveal any obstacle to future deployments. Since the outdoor fiber cables were installed before the bricks were available, slack was left in the section between grade and the ultimate brick location. If sufficient slack was available, then a loop could be created in free space under the brick. This loop, not likely to be repeated in future installations, will increase the damage exposure in the evaluation installation, making the demonstration a more sensitive indicator of cable ruggedness. The bulk of the fiber cable slack was in the control house, where it was accommodated in an under-floor trench.

A transcription error made in transferring the measured cable lengths to the ordering system resulted in several of the outdoor fiber being made shorter than intended, but they could still be used by relocating the relay panel within the control house. A manufacturer's engineer visited the site to correct a minor patch panel problem, but otherwise installation and commissioning was completed entirely by AEP field staff.