AT THIS POINT, there is no telling when the Department of Energy will issue its final report detailing the cause(s) of the August 14 blackout. However, according to an initial blackout report by the U.S./Canada Power Outage Task Force, inadequate levels of reactive power at the points where it was needed contributed to the voltage collapse.
If reactive supply is not provided at the end of a power line, the voltage can fall precipitously. When this happens, the transmission system can no longer transfer electric power from distant generation to energy users at load centers. This was the problem that the City of Key West faced when it decided to import most of its power rather than generate it locally.
The city, located at the southwestern end of the Florida Keys, has its own generating facilities located on Stock Island. The plant has three GE Frame 5000 gas turbines and three high-speed peaking diesels. Another six diesels are also available for emergency power. Unfortunately, generating the power is costly. Because of shallow waters and coral reefs in the area, the fuel has to be brought in on small barges, which adds to its expense.
As a result of the high cost of generating their own electricity, and the increasing need for increased electricity, the city found that it was cheaper to import its power through dual 138 kVA lines from a Florida Power and Light plant. However, the plant is located on the mainland, more than 150 miles away. As a result there are potential power losses and voltage fluctuations. The reactive power needed for voltage control cannot travel long distances because it meets considerable resistance over the transmission lines. Therefore, reactive power sources need to be close to the point of reactive power demand/load centers.
“We are at the end of a radial system so we didn’t have good voltage control,” says David Gerstenkorn, electrical supervisor for Key Electrical System (KES). KES is Key West’s public utility. The utility calculated that this was costing them $4 million dollars per year. To reduce these costs, KES examined several methods of bringing the transmission lines, which were voltage limited, up to their thermal limit.
“We took a close look at synchronous condensing as a means of creating the reactive close to our load centers,” Gerstenkorn says. The Key West public utility determined that the best solution was to use a hydraulic motor and a clutch to disconnect the generator from the motor once it had reached synchronous speed.
KES had decommissioned a 37 MW steam turbine unit at its Stock Island Plant. Although the turbine was not expected to be utilized again, the hydrogen cooled, 44.8 kVA, 13,000 kV generator could be put back on line as a synchronous condenser. This would provide up to 34.1 MVA lagging power and 22.5 MVA leading power. The goal was to bring the voltage limited transmission lines up to their thermal limit.
Key Electric System’s acceleration system: Hydraulic motor, turning gear and SSS clutch connects synchronous condenser.
The utility contracted with General Electric Company and Hydro-Mechanical Systems, Inc. to design a two-stage startup package for the generator. The first stage consists of a small hydraulic motor that provides the torque necessary to disconnect the generator from the motor and then accelerate it up to 60 rpm. At that point a 350 HP hydraulic motor takes over which increases the generator’s speed to 3600 rpm needed to synchronize it with the transmission line. A 600 HP, 5500 psig hydraulic pump, with a variable displacement axial piston, controlled by a GE Mark V controller running customized software, feeds both of the motors.
Since the turning gear was removed along with the turbine, a new turning gear had to be installed. A new thrust bearing to locate the generator axially until it is synchronized, and to handle the generator thrust when it operated off its magnetic center, was also installed.
The motors transmit the torque through a 48T SSS Encased Clutch from SSS Clutch Company, Inc. A Babbitt thrust bearing is incorporated into the clutch to axially locate the generator rotor. During startup, the generator is accelerated to 102 percent of its rated speed; excitation is then applied followed by controlled deceleration to bring the generator into synchronization. As the hydraulic motor decelerates below the speed of the generator, the clutch automatically disconnects. It takes ten to twenty minutes to bring the generator up to speed and synchronize it to the incoming power.
Once the new synchronous condenser was put into service it improved stability and boosted the transmission capacity of the line by up to 34 MW. Although the conversion cost was relatively low, $500,000, it saves the utility about $4 million per year in transmission losses. “Adding an SSS clutch to the generator has stabilized our voltage and our control is much better. We can also import more power from the mainland,” says Gerstenkorn.
Based on this success, KES has converted a gas-turbine generator so that it can be used as a backup for either generation or condensing in the event something happens to the main generator. In this case, an SSS clutch has been installed between the turbine and the generator. The utility’s goal is to have the capacity to generate at least 60 percent of its load locally.