WHAT DO YOU DO when you have 18 MW of hydroelectric generation but no way to get it to market because your generators operate at 40 Hz in a 60-cycle world? This was precisely Great Lakes Hydro America's (GLHA) dilemma at its 18-MW Weldon plant, one of the last vestiges of a 40-Hz hydroelectric generation system built back in the 1920s.

PROJECT BACKGROUND

Weldon Station is just one of several hydroelectric facilities built on the Penobscot River in northern Maine by Great Northern Paper Co. (GNP). GNP built the original 127-MW hydroelectric generation system to serve an electrically independent and physically isolated paper company load in what was then an undeveloped region in the northern part of the state — before the 60-cycle standard was established in this country.

GNP originally selected the area, now known as Millinocket, as a prime location for a paper mill because of the tremendous hydroelectric potential from the Penobscot River and the proximity to a vast region of virgin forest. The operation grew steadily over the years from its initial beginnings as a two-paper machine mill in Millinocket to include several higher-speed machines and a second mill located a few miles down river. As the mill expanded, so did the power system, which grew to meet the demands of the increased production. At one point, the paper mill had almost 400 MW of installed hydroelectric and steam capacity.

As 60 Hz became the de facto standard for power systems, GNP upgraded parts of its system to be compatible with the new higher frequency. But a substantial portion of the mill complex remained at 40 Hz. In the 1980s, GNP embarked on a program to gradually convert the remaining generation and load from 40- to 60-cycle operation. An economic downturn in the paper industry, however, halted the conversion program before it could be completed.

Needing a means to transfer power between the two systems, GNP commissioned the construction of a solid-state frequency converter, which connected the 40- and 60-cycle systems and allowed for the transfer of power between the two. The successful operation of the frequency converter, however, requires a sufficient quantity of short-circuit capacity available on both power systems, and as increased economic pressures forced the shutdown of less efficient equipment, short-circuit capacity diminished, threatening the proper operation of the frequency converter. The last straw was a bankruptcy in 2000, which effectively ended GNP's power system improvement plan and resulted in the sale of the hydroelectric generating assets to GLHA.

GENERATING A PLAN

When GLHA took over the operation of GNP's hydroelectric system, it set an ambitious goal to reconfigure the power system in order to sell energy to the New England market. Part and parcel of this goal was finishing the frequency conversion process GNP had started.

GLHA set to work developing a rough idea of how to make the conversion a reality. Since wholesale changeover of the station would result in the loss of valuable production for an extended period of time — and was therefore not an option — GLHA had to develop a more measured approach.

Any approach had to satisfy three criteria. One, GLHA would have to maintain production at Weldon to the greatest extent possible to maximize its investment in the hydroelectric facility. Two, GLHA would have to schedule the transmission line work in the late summer season when river flow would be at its historical low. And three, GLHA would have to coordinate the Powersville Road Substation work and its accompanying outage to coincide with an abbreviated outage from a customer who is served from the Powersville Road Substation.

Of all the hydroelectric facilities GLHA owns, Weldon Station is the furthest downriver facility and the most remote. It is connected to the GLHA grid via a 7-mile (11-km), 34.5-kV transmission line that terminates at the East Millinocket Substation and then continues north to the Dolby Hydro Station via a 40-cycle bus. The new terminus for this line would need to be the recently constructed Powersville Road Substation, which abuts the transmission line right of way and is located somewhat closer to Weldon Station than Dolby.

Engineers at GLHA formulated a plan to convert the Weldon hydroelectric units to 60-cycle operation one at a time, taking advantage of the station's design, which permitted separation of the generator bus into two sections (each bus connected to two generators). The idea would be to separate the buses in order for one to operate at 40 cycles and the other at 60 cycles. The next step would be to tear down the first unit for conversion while the remaining three continued to operate at 40 Hz.

After converting the first, GLHA would convert the second unit on the same bus, operating the first unit at 60 cycles. The remaining two units would continue to operate at 40 cycles. When the second unit was completed, it too could be operated as a 60-cycle unit. Under this scenario, it would be possible to operate two units at 60 cycles: one unit operating at 40 cycles with one unit undergoing conversion. When the third unit's conversion was complete, the temporary substation would be decommissioned and the transmission line (designated Line 7) would be rerouted from Dolby Station to Powersville Road. This work would all need to take place within a narrow window of opportunity in order to coincide with the customer outage and the reduced river flow. With a rough plan in mind, GLHA called on POWER Engineers to help work out the details.

PUTTING PLAN INTO ACTION

Fortunately, there were several things working in the project's favor to facilitate this plan. First, a Bangor Hydro Electric 46-kV line was located in close proximity to the Weldon Station, meaning a new line would not be inordinately expensive. Second, Bangor Hydro allowed GLHA use of a 34.5-kV/46-kV power transformer and circuit breaker, which it had in inventory. Third, the Weldon Station power system was built with redundant generator step-up (GSU) transformers and a split-bus system that allowed for the units to be paired and operated independently. And finally, because 40-cycle transformers can operate on 60-cycle power adequately, there was no need to change out the GSUs or metering transformers.

The next challenge involved the construction of the temporary substation. Because of its proximity to the Penobscot River, oil containment was a concern. Compounding the complexity of the situation was the fact that the environmental permits did not allow for any substantial excavation at the site, thus the containment system had to be designed to work with the existing grade. POWER designed an above-grade containment system using an oil-impervious liner laid on-grade with dunnage around the perimeter to provide an above-grade dyke system. This type of design requires minimal excavation and can be removed at the conclusion of the project with little remediation. The containment volume is adequate to handle any transformer oil spill as a quantity of fire fighting and storm water. Also, because this area is an active construction site, maintenance is not an issue because workers can drain any accumulated water before it reaches overflow volume.

To further mitigate any potential environmental impact, the temporary substation was built of wood, which could be easily disassembled and reused later. The 34.5-kV/46-kV power transformer was placed in the containment area with the temporary substation constructed immediately adjacent. A temporary line structure was built in order to interconnect with the Bangor Hydro line. Potential transformers, metering units and a 34.5-kV circuit breaker completed the arrangement.

With the transformer and high-side power circuit breaker in place, the two generator step-up units were electrically separated at the bus to allow for dual-frequency operation. Each GSU was originally equipped with its own independent protection and control schemes with high-side circuit breakers, which remained operational. The new protection and control scheme for Line 7 was adapted to protect the temporary substation transformer and was installed in such a way to allow it to be quickly converted from transformer protection to line protection.

Completion of the temporary substation occurred simultaneously with the conversion of the first generator unit. Because the import capability of the local utility line is limited to approximately 9 MWs, only two units' output could be accommodated at any one time. Consequently, the balance of the project had to be completed before the third unit was ready to operate at 60 Hz.

While the work was underway at the Weldon Station, preparations were taking place to re-route the northern terminus of Line 7 from its current location at Dolby Hydro Station (which operated at 40 Hz) to a new breaker position in the Powersville Road Substation (which operated at 60 Hz). With the new circuit breaker position ready, GLHA coordinated a 12-hour shutdown of Powersville Road's 34 kV with that of the local customer. Taking advantage of this brief window of opportunity, linemen circuited Line 7 into Powersville Road.

The temporary substation at Weldon was now de-energized, and the line taps dropped from Bangor Hydro's 46-kV line. The 46-kV/34-kV transformer was completely disconnected and the existing GSUs were reconnected to the overhead bus back on Line 7. Termination of Line 7 into Powersville Road was completed and the protection schemes were tested.

Now came the moment of truth: rolling the first unit at Weldon with the new tie. After testing and confirming correct phase rotation, GLHA brought Weldon into full available capacity on the two completed 60-cycle units. The third and fourth units will be ready for completion in time for high water season this winter.

To date, the Weldon Station project has been a great success. GLHA has saved time and money by using existing equipment and minimized any harmful effects on the environment by using nonintrusive construction techniques. By performing prudent planning and using innovative construction practices, GLHA choreographed and commissioned a complicated conversion in a short window of time.

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Brian Wiley has been in the power and electrical distribution business for 36 years, starting with engineering/design at Stone and Webster Engineering in 1969 and then joining Great Northern Paper in Maine in 1973 as an electrical engineer for both the paper mill and company-owned hydro and steam generation facilities. In 2002, when the hydro utility assets were acquired by Brascan Corp., Wiley moved with the assets to Great Lakes Hydro America, a subsidiary of Brascan Power. He is now an electrical engineer for Brascan Power New England, where he is responsible for many of the company's power/distribution engineering in its New England facilities. He earned his ASEE degree from Maine Technical Institute in 1967. brian.wiley@brascanpower.com

William Crowell has been in the electric utility business for 24 years working for several electric utility companies, major engineer-procure-construction contractors and now for POWER Engineers in Yarmouth, Maine, as a system design consultant. He earned his BSEE degree from the University of Maine in 1981. wcrowell@powereng.com