Changing the reversing switches installed at the 800-MW Jocassee pumped storage project allowed the plant to continue operating after many years in service and a generator uprating.
By Michael C. Luce, Mickey Simpson and Bruce Hack
Standing on the compacted road atop the South Carolina dam that closes off the two hills creating Lake Jocassee and separates it from Lake Keowee (pronounced like the fruit but with an “o” in the middle) is an experience in beauty, magnificence and just plain awesome. The dam is a 385-foot-high, 1,800-foot-wide manmade wall of local earth and quarried stone. Creating this engineering feat with machines and back-breaking toil in the early 1970s sealed off what used to be a small pass between these two adjacent hills.
Once upon a time, the waters from a few local streams with names like Horsepasture and Toxaway ran through that pass into what is now Lake Keowee. But by building this earthen dam, the waters of those local streams collected behind the new wall into one of the most beautiful scenes you can imagine. These waters, filling in that basin behind this newly created wall, formed what is now known as Lake Jocassee.
Map of Jocassee and Keowee lakes
Lake Jocassee has 13 square miles of surface area with more than 75 miles of shoreline. It encases the most incredibly clear, cold, pristine waters you’ve ever frozen a toe in. Jocassee offers recreation, first-rate fishing and bone-chilling diving for mile after gorgeous mile.
But as you stand on the top of this earthen barrier, glowing at the billions upon billions of gallons of water spread out before you, you slowly turn around to notice there is a near 400-foot sheer drop directly behind you! If you’re not one to get woozy from heights, this view is equally breathtaking.
This impressive, albeit a heck of a lot lower view, is Lake Keowee, 385 feet below the soles of your shoes and running away from the base of the dam for mile after blue-green mile.
Pop that dam open, and you’d flood an entire quadrant of the state. But let just a small fraction of those Jocassee waters flow to Keowee through a 33.5-foot-diameter pipe into a waterwheel connected to an electrical generator and there’s a whole lotta power in “dem dar” waters!
So far, this just describes a standard hydro project. But now comes the magic.
Remember when you were a kid and you or a friend used to like to drink chocolate milk by sucking it up through a straw and then letting it drop back down into the cup? Usually that would get you a tap on the head and a “Don’t play with your food.”
Well, plus/minus, that is exactly the idea behind a pumped-storage facility. In this case, the straw is a 33.5-foot-diameter, 1,383-foot-long tunnel running through the mountain. And the chocolate milk is the waters of Lake Jocassee running through four Voith turbines. Each of these turbines is direct-connected to a 34-foot-diameter 60-pole generator. And each generator is capable of pushing out about 200 MW of clean, carbonless, renewable electricity for the customers of Duke Energy.
Jocassee tunnel during construction
Lake Jocassee is so voluminous that this power station can run full bore for more than eight hours, offering a capacity of nearly 800 MW on demand with a very short dispatch (start-up) time. And the upper lake won’t drop more than 1.5 feet.
Again, if this were the end of the story, all we would be describing is a standard hydro facility. When Jocassee would run dry, the plant would be out of service, until the various streams filled Lake Jocassee up again. But, those turbine-generators whirring around have a neat little feature. They can also spin backwards.
Forward making megawatt-hours of electricity as the waters drop from Jocassee to Keowee. And reverse by swapping the electrical system’s phasing on the machines’ electrical terminals, turning these monsters into huge motorized pumps.
Running the machines backwards makes these multi-megawatt motor/pump combinations capable of pumping Lake Keowee back up into Lake Jocassee, “back up the straw,” refilling Jocassee for the next day’s needs.
At that moment you realize: You’re straddling one of the world’s largest man-made rechargeable batteries!
Jocassee upper yard with earthen dam behind
That gorgeous family recreation spot doubles as a reservoir for a few thousand megawatt-hours of “any time you want it” clean, dispatchable power. And you can run it and recharge it — more or less, as you like.
Lake Keowee has at its southern end another power station. This one is the 2,500-MW baseload nuclear plant Oconee. Lake Keowee is the headwaters for Oconee’s cooling water system. Oconee may not be large enough to supply all of Duke’s customers’ energy needs when demand is high. But at night and other times of lower energy use, Oconee is producing more electricity than the local clientele is requesting. So, what better time than this to use the extra, low-cost, baseload power from this 2,500-MW power station to “plug in” your rechargeable batteries? And that’s just what the Jocassee/Oconee combination can do!
By “recharging” (i.e. refilling) Jocassee, the Oconee/Jocassee combination can produce 30% more on-demand energy for its customers as soon as the need presents itself.
Now, electrically “flipping” around a 200-MW generator into a motor and back may look easy on paper, but the electrical and mechanical forces to achieve this feat mean the equipment required is orders of magnitude more “beefy” and rugged than your standard catalog-cut utility products. And in the case of the Jocassee system’s one-line (the drawing of the electrical path of the equipment), the quarterback of this “flipping” operation falls squarely on the medium-voltage, high-amperage, four-pole reversing switch. This specialized, highly customized reversing switch allows the electrical phase rotation present on the machine’s electrical terminals to be in either Generate Mode A-B-C or in Pump Mode B-A-C (Z-Y-X or Y-Z-X on the one-line).
The one-line drawing for the Jocassee system
The original General Electric reversing switch provided for this operation was a 15-kV, 8,500-amp, high-spring-pressure knife blade design. It served the system quite well for many decades. But like anything else mechanical, time takes its toll and the consistent numbers of repetitive operations wore away at moving joints, reduced spring contact pressure and ultimately created a condition of needing “a little more than a little love” or maybe major replacement.
Such 8,5000-amp switching devices were for the most part either completely custom or at least items with a very low production volume. So when the station’s electrical engineering/maintenance personnel sought parts from the OEM, they found the availability, lead times and costs to be sobering.
Fortunately, Earl Brinson, one of Duke’s more seasoned subject matter experts, had, over the years, worked with Jay Patel of Nimisha Inc., a consulting company in Blue Ash, Ohio, specializing in high-amperage Iso Phase Bus. Before opening Nimisha, Patel was the engineering product manager at the Westinghouse Iso Phase Bus division. Patel reminded Duke that Crown Electric Engineering and Mfg. of Middletown, Ohio, (a successor to the Westinghouse Iso Phase Bus division) were specialists in high-amperage Iso Phase Bus and related electrical apparatus. Crown Electric had a product category of SD (Super Duty) disconnect switches specifically designed to meet the high amperage ratings for unique applications such as Jocassee.
As the parties started sharing information, Jocassee added a tidbit to the discussion. During recent years, the machines were upgraded/uprated and were now putting out about 10% more than their original nameplate amps. Duke and Crown agreed an engineering site visit would be prudent and offer value by measuring the existing enclosures, reviewing the electrical ratings, inspecting the existing bus work, verifying any ventilation for thermal considerations, taking a cursory look at the control scheme interfacing etc… and seeing if an upgrade/uprated switch employing newer technology could reasonably retrofit into the existing enclosure.
During a day when the plant would be able to de-energize and ground the exiting switch, Chad Shell, an owner and the engineering manager of Crown Electric, made a return visit to Jocassee with Patel. Access was provided such that critical, limiting dimensions, locations of internal structural supports, main copper bus runs, bolt hole patterns, control wireways, etc., could be gathered and compiled for an initial “sanity check” to confirm a retrofit’s possibility and practicality.
Extremely limited and tight access available for retrofill work
With agreement to move forward, a 3D rendering was created. Duke apprised Crown of the site’s work procedures and confirmed the dimensions of the egress/ingress paths to verify that gutting and rebuilding in place a medium-voltage, four-pole, 10,000-amp reversing switch could be accomplished. All this had to be done during an acceptable outage time frame of less than four weeks.
Crown Electric proposed its low-profile, high-amperage, telescoping disconnect switch design. Each pole would have its own interlocked drive mechanism with a manual emergency override. Optional integral, interlocked ground switches were discussed and determined to be unnecessary for this location in the system’s one-line.
Duke instrumentation and controls personnel provided Crown Electric with a full review of existing controls, protection, interlocking and annunciation schemes. Approval of submittals for a brand new control panel set in motion the component layout and fabrication of a panel that could fit within the old control cabinet and allow for wire drops that would land directly onto the existing terminal blocks.
New custom control panel designed for ease of retrofit and wire drops
Performing the work
With the plant entering its outage, Crown Electric mobilized a crew of the maximum size that could harmoniously access the tight work space. With a laptop in hand, loaded with 3D modeling Inventor software, Mark Meyer, Crown Electric’s senior project design engineer, supervised the on-site crew. This insured any unforeseen discoveries brought to light during the disassembly of the existing switch, which might impact the installation phase of the project, could be quickly addressed. Meyer could right there “on the fly” draw up critical areas of the installation with great resolution so that parts could be fabricated at Crown and shipped to Jocassee while the disassembly continued.
The total per-phase material weight of the original reversing switch coming out was well over 1,000 pounds. Each phase being mounted in its own very tight enclosure compartment (less than 2 feet wide) made laying hands on the equipment, or getting tools and torches into such sections, a highly choreographed chore for execution. Impromptu specialty hoists and slinging were essential for safely finessing the larger, heavy parts that had occupied this space for decades free and clear. And this was just the disassembly!
Once the switch chambers were clear and cleaned, the process turned to remaking each enclosure such that it could receive and secure its new SD disconnect switch phase by phase. No two sections were 100% identical, so welding of new custom bracket positions, holes, bolting points and wire ways were each accounted for in their own assembly drawings. Each section was to be executed by a lead mechanic and overseen by the project’s design engineer.
Rigging of each new phase was slow, methodical and very cautious. The crew ran practice simulations of the upcoming procedures to intercept shortcomings in the plan and ensure each person became versed in their responsibility.
SD Switch poles of this amperage and having their own motor drive are close to 4 feet long and weigh about 750 pounds or more. They needed to fit though an original door opening of about 17 inches wide and line up with an accuracy so as to allow a mechanic to slip a piece of 5/8-inch hardware into the associated mating holes.
Once in place, new custom 10,000-amp buswork was hand-fitted and installed to connect each phase to the closest original bus joint bolting points. Each switch’s wiring was trained to the new custom panel and landed point-to-point, picking up all the functionality, interlocking and annunciation required for safe, proper operation.
From left to right: The old main contacts off the removed switch, the new main reversing switch in the closed position and the new switch in the open position.
The mechanical override mechanism was ported such that loss of control power or a failed motor drive would not completely inhibit the ability to operate each of the pole assemblies.
Three weeks after the first gang boxes arrived, the Crown Electric crew performed mechanical and electrical operations checks for Jocassee instrumentation and controls and engineering.
The switch that controls the ability to switch from generator to motor and back — after 40 years — had been switched.
Michael Luce is Jocassee hydro station superintendent and Mickey Simpson is hydroelectrical lead engineer with Duke Energy. Bruce Hack is managing member with Crown Electric Eng. & Mfg. LLC.