Gulf Power Co., like many utilities, used herbicides for vegetation control decades ago. Some of the chemicals were later determined to be hazardous. To prevent future problems, Gulf Power Co. (Pensacola, Florida, U.S.) removed all traces of the herbicides.

The company innovatively used distribution equipment to actuate the remediation effort undertaken at its Pensacola City Station. This substation was located on the site of an earlier generation plant.

The most effective clean-up method is to remove the contaminated soil. However, this has proven to be a nearly impossible task around energized equipment and bus. An alternate method, bio-enzyme remediation, is expensive and time-consuming. Bio-remediation efforts at the City Station were predicted to take four years and cost US$2.5 million.

Distribution engineers recognized an opportunity to save money for the company while updating the switching station. This station is a crucial component of the distribution system, which is the source for the three-feeder network underground system serving Pensacola's downtown area. All of this could be accomplished by replacing the station with state-of-the-art distribution equipment.

About City Station

Pensacola City Substation once had 12/4-kV transformers that served a 4-kV distribution system. The 4-kV distribution circuits have long since been converted to 12 kV, and this station no longer has any transformers. The 12 kV for the network is provided by two 12-kV overhead feeders from a substation 2 miles (3.2 km) away. The replacement of the Pensacola City Substation equipment was simplified because transformation was not part of the new design. The traditional substation breakers, relays, regulators and control house could be replaced with new pole-mounted distribution equipment, clearing the entire station site for easy excavation of the contaminated soil.

The concept, if proven feasible, was a win-win situation for all involved. The environmental group could use traditional excavation techniques and remove all soil at one time. The substation group would eliminate an old installation that was deteriorating rapidly in the salt spray environment of the coast, and increasingly taking more of their maintenance dollars to keep up to standards. Power-delivery engineers could improve reliability to the downtown customers with new equipment with better coordination characteristics. The city, in the process of major redevelopment of the waterfront, would appreciate the new, better-looking equipment. The initial replacement is outside the footprint of the substation and is considered temporary construction. The construction uses overhead distribution equipment, but the city has eagerly entertained a later conversion strategy to install pad-mounted underground equipment, with the city willing to share the cost.

Pre-existing Circuit Scheme

The existing network service point was configured with relays and substation equipment. It was fed from two 12-kV feeders tied together through two old oil circuit breakers (OCBs) with directional reverse current relays at City Station. Between these OCBs was a common regulated bus. Finally, off of the common bus, three similar vintage OCBs fed each of the three 12-kV underground network circuits. A differential scheme was used inside the five OCBs. Communications, data collection and control equipment were located in a concrete block house and reported back to the company's distribution operations center. Duplicating this scheme with new-technology distribution equipment required a steep learning curve for the project engineers.

The majority of protective equipment and many of the voltage regulators recently installed on Gulf Power Co. distribution lines had been supplied by Cooper Power Systems. The substation design learning curve was reduced by starting with existing Cooper equipment and expertise.

Installing New

Cooper's Form 6 recloser control provided the functionality needed to mimic the existing scheme at City Station. The familiar Cooper VWE recloser was the initial choice for interrupters. However, the company had three of Cooper's Nova solid dielectric reclosers in stock.

Two additional reclosers and controls, plus one set of spares, were ordered. Gulf engineers considered the solid dielectric Novas, without oil and with their high current ratings, provided distinct advantages over the older oil-filled models. The Form 6 controls offered reverse current sensing and loss of voltage sensing not provided by Form 5 or Form 4C. Voltage regulators at 150 kVA were chosen for installation on the primary common bus of the network. These are small but acceptable regulators for the network because a significant loss of load has been seen dating back to the 1960s, as downtown commercial businesses migrated to the suburbs. Today, load on the downtown network is a small fraction of the overall system capability. Downtown redevelopment is beginning to provide some immediate growth, with potential for continued load growth in the future.

Facilities are Tight

City Station, as its name suggests, is located downtown where real estate is a premium. Clearing the area inside the station fence required Gulf Power Co. to consider unusual construction practices. The new layout had to be constructed within an 8 ft (2.4 m) perimeter outside the 100 ft by 125 ft (30 m by 38 m) fence line. Gulf Power crews had to install within this small footprint 10 poles, five electronic reclosers with controls, three voltage regulators, three single-phase potential transformers (PTs) for the network, a three-phase wye PT bank for voltage sensing, and two single-phase PTs for voltage sensing and control power. Two 477 all-aluminum conductor (AAC) circuits and a common bus connected this equipment. A communications system reported status and provided control for each piece of equipment.

Traditionally, Gulf Power Co. had used transducers, hard wire, and RTUs to transmit data. The parallel source feeders and the redundancy of the network system provided a low-risk environment to install and become familiar with serial communications. The Cooper Form 6 provides the status (open/close position, targets, low dc voltage, ac fail, lockout, local/remote mode), analog (amps, volts, watts, vars, dc voltage), and control points (open/close, reset targets, reclosing, ground trip) through a DB9 port. The communications protocol is Distributed Network Protocol 3 (DNP3). Gulf Power Co. chose to transmit data over fiber-optic cables through fiber transceivers from the controls to the DAQ Electronics Inc. IoG modules for reliable communications and electric isolation. A wide-spectrum radio was used between the substation and the input/output of the energy management system.

Details on the Control Circuits

The application at Pensacola City switching station requires sophisticated relaying and control and must handle overcurrent, directional logic and voltage sensing. Proper sensing of input parameters required PTs for both sources. It required the load-side common bus wired to specific screw terminals in the controller. The voltage phasers from the PTs had to match the current phasers from the recloser bushing CTs.

Three-phase voltage was being used for metering on the common bus, and also as voltage sensing input into the controllers. The common bus and the three PTs were the on-load side of the source reclosers. Gulf Power Co.'s phase rotation of 3-2-1 had to be set relative to the actual phase connections through the Nova's bushings. Unlike reclosers in series, G-262 and G-264 face each other. This made the phase attachments of the horizontal construction to the recloser bushings opposite of each other. Three-phase voltage-sensing input on the Form 6 called for source voltage. Wiring connection adjustments had to be made for this. The complex settings were facilitated by the engineers at Cooper Power Systems along with the Cooper ProView software. In addition, this software provided protection simulation along with diagnostics.

Directional relaying looking back on the two source feeders is provided at Pensacola City Switching Station by the Form 6 controllers on recloser G-262 and recloser G-264, which tie the source feeders together. These two reclosers consider the load common bus as the forward direction. They are set up to monitor overcurrent in the reverse direction by looking for faults back on each of their respective source feeders. This ensures that the non-faulted source feeder does not lock out for a permanent fault on the faulted source feeder. That required a lot of coordination between various group. It saved Gulf Power Co. major environmental costs and met all the other objectives of the project.

Stanley Nurnberger is a senior power-delivery engineer with Gulf Power Co. and is project engineer for the City Station. Nurberger is a professional engineer and is a graduate of the Georgia Institute of Technology. His career spans 35 years in power delivery with various utilities.
slnurnbe@southernco.com

Sophisticated Programming of Relaying Circuitry at the Pensacola City Substation

During the initial programming of the two source controllers, engineers were not aware that through the Work Bench module of ProView, overcurrent could be monitored in both directions by the Form 6. Engineers thought it was necessary to select one direction or the other. A compromise work-around allows a fault on the common bus to operate the breakers on both source feeders. Unfortunately, this will blink and interrupt the service to other customers on the source feeders. The source feeders automatically reclose in five seconds. When both of the source feeders initially operate and open the voltage on the City Station, common bus will go to zero. The two reclosers, G-262 and G-264, are set up to trip on low voltage. They will open and lockout after a 2-second delay if there are 10 V or less on the common bus. The 2-second delay means that G-262 and G-264 will open and lockout before the 5-second reclose on the two source feeders that tripped on a common bus fault. A temporary fault on the common bus will also cause G-262 and G-264 to lockout, dropping the downtown network. Fortunately, the exposure of the common bus is only 180 ft (55 m), protected by overbuild of the source feeders. There is no reclosing for a low-voltage trip, so both breakers must be closed manually at the control panel or remotely by the distribution operations center to return the system to normal operation. This programming will be simplified with the installation of underground equipment in the near future.

Each of the three network underground circuits feed off of the common bus. Each circuit is protected with a Nova recloser. The Form 6 controls are set for over-current protection only with one trip to lockout (no reclose). An earlier model control like the 4C or Form 5 would easily provide the protection necessary for the network feeders. However, they do not have the voltage inputs to provide the metering and power calculations that report back to the company's energy management system. Additionally the cost of the Form 6 to Gulf Power is equivalent or below the cost of the earlier vintage controls.

Newly mounted on the riser frame of each of the network underground feeders is a potential transformer. A warning light is connected across the secondary terminals to provide an indication of back-feed from the network feeder. When any of the network feeder breakers are open, the network protectors on the secondary side of the affected feeder's associated transformers should open and prevent any back-feed voltage through the transformer to the 12-kV feeder. If the potential light is burning, it indicates that at least one of the protectors did not open and 12-kV voltage is on the feeder. This addition was a safety enhancement.

The scheme at Pensacola City Station has been operating as expected for 13 months and is providing reliable service to the downtown network system. The environmental clean up was completed in a three-week time frame at a fraction of the cost of working around energized substation equipment. The obsolete substation equipment was replaced with state-of-the-art distribution equipment. Gulf Power anticipates that the city of Pensacola will request the company to replace this overhead system with pad-mounted equipment in early 2004.