The use of synchronous condensers, common through the 1950s, is making a comeback as an alternative to capacitors for power factor correction, and as a grid stabilizer in areas plagued by insufficient transmission capacity. Problems experienced by capacitors with harmonics, together with the high cost of new transmission capacity and the relatively low cost, simplicity and maintenance of synchronous condensers, is driving the comeback.
Several large utilities and independent producers have recently installed synchronous condensers for power support. Commonwealth Edison began considering the use of synchronous condensers at its Zion Nuclear Station in 1997 due to concerns about system reliability in the summer of 1998. Originally, the plans entailed converting Unit 1 to synchronous condensing, while Unit 2 would remain in generation mode. When ComEd decided to shut Zion down entirely, Unit 2 was also slated for conversion since it would no longer be available to for grid support.
“We opted for synchronous condensing rather than switched capacitors for two main reasons,” said Gene Stanley, vice president of nuclear operations for ComEd. “Synchronous condensers are more economical and they offer finer tuning control. With capacitors, you’re switching in all or nothing at a certain level; with synchronous condensers, we can control the excitation to match grid conditions.”
The single skid-mounted acceleration system, which serves both units, consists of an electric motor, a torque converter and a synchronous clutch to engage/disengage the generator. A new synchronization software package enables the generator speed to match the conditions on the grid; if grid voltage fluctuates, the software can adjust synchronization speed from 1800 to 1800.5 rpm, for example.
In the two years since going on-line, ComEd has operated one unit continuously in synchronous condensing mode; the other is brought on-line for the summer peak season and taken off-line in October. ComEd load dispatchers have been very pleased with the flexibility synchronous condensing offers, according to Stanley, since Zion can respond to requests to absorb or supply reactive power within seconds.
Farther north, in Alberta, Canada, ATCO Power recently brought the 43 MW Poplar Hill plant on-line in synchronous condensing mode. The Grand Prairie region of northwest Alberta had been experiencing many problems with grid stability, caused in part by the load growth associated with natural gas exploration and treatment. This load growth stressed the 144 kV transmission network. When the natural gas treatment facilities would trip off-line, sour gas rich in sulfur would be flared, raising environmental concerns.
The transmission administrator in Alberta explored various alternatives, including a transmission line extension from British Columbia into Alberta, but opted instead for a lower-cost solution from ATCO that incorporated synchronous condensing. ATCO could not get the clutch in time to meet the desired schedule, so Poplar Hill first came on-line in December 1998 in generation mode only, under a special permit exemption, using a GE LM6000PC turbine package from S&S Energy Products. In September 1999, during a 30-day outage window, the clutch package from SSS Clutch Company Ltd. was installed and the LM6000PC was replaced by the more advanced LM6000PD model, which features GE’s dry low emissions combustion system.
Since returning to service in late 1999, Poplar Hill has operated about 50 percent of the time in synchronous condensing mode, according to Harvey Kerslake, ATCO Power vice president of project development. The plant is operated by only one day-shift operator during the week; the rest of the time, it is operated remotely from a control center 250 miles away.
ATCO is compensated based on the avoided cost of the transmission line. “We get a fixed monthly payment from the transmission administrator, but we’re also able to generate revenue on the merchant market,” said Kerslake. ATCO capitalized on this arrangement several times during winter peak months when prices hit the capped level of $1,000/MWH, thereby shortening the payback period. Penalties are incurred if Poplar Hill is not available to the administrator within a certain amount of time to stabilize the grid.
Reliability and maintainability are not major concerns with synchronous condensing units, since they are similar in design to low-maintenance generators. Operators must monitor lube oil systems, operating temperatures and vibration levels, but the hardware that comprises a synchronous condensing application is fundamentally rugged in nature. The clutches, for example, are commonly found on ships with multiple engines, where high reliability is a must. ATCO’s Kerslake expects the clutch installed at Poplar Hill to last longer than the turbine.
With increasing economic and aesthetic constraints on transmission line construction, and the relatively low costs and quick installation of generation facilities, multi-function generation and synchronous condensing plants like Poplar Hill should become more popular in the future. It’s important to examine the pool prices in a given area, but in many cases, linking generation and synchronous condensing provides valuable operating flexibility and advances economic viability.
The future for Zion’s pure synchronous condensing units, on the other hand, is uncertain. “No one has figured out how this will be charged,” said ComEd’s Stanley. Although California is experimenting with markets for auxiliary service, there is not a universally accepted model. Further, with roughly 2,000 MW of additional generating capacity coming on-line in Zion’s service area, it’s unclear how long Zion will be needed in a synchronous condensing mode only.