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The ins and out of adding new power Strandberg explains the complexities of adding new hydropower to the Alaska Railbelt’s stranded and fragile electricity grid Alan Bailey Petroleum News
At first blush it might appear that connecting a new hydropower plant into the Alaska Railbelt electricity grid would just be a question of wiring up a transmission line or two from the plant to the nearest point on the grid infrastructure.
Well, not quite.
As the Alaska Energy Authority moves forward with a potential plan to build a 600-megawatt hydropower dam on the Susitna River, south of the Alaska Range, the authority has been communicating with the six Railbelt power utilities, starting discussions on how to ensure reliable, stable power supplies while also maximizing economic efficiency, if the proposed power plant comes to fruition, AEA project manager James Strandberg told the Law Seminars International Energy in Alaska conference Dec. 7.
A Susitna hydropower plant would represent by far the largest single piece of power generation kit in the entire Railbelt. And, so, how can the stability of the Railbelt power supplies be assured, especially given the extreme isolation of the Railbelt grid from any other North American power system?
Three centers The Railbelt utilities synchronize their power delivery arrangements across the grid using three power dispatch centers, two in Anchorage and one in Fairbanks, Strandberg said. Dispatchers at each center have access to information about all power generation systems on the grid, with the ability to make decisions about which generators to use in support of the constantly fluctuating power demand.
A new hydropower plant would not just have transmission lines connected into the grid: It would also have control circuitry connected to those dispatch centers, moving information about the plant operations into the synchronized computer-based systems that control the entire grid, thus enabling dispatchers to control the output from the plant’s huge water turbines.
And dispatch decisions about how much power to generate from each power plant on the grid become a question of balancing economic and practical issues relating to the engineering and financial characteristics of each power source.
To explain this, Strandberg considered three potential power sources on the grid: a modern, simple-cycle gas turbine, a more complex combined-cycle gas-fired turbine system, and a large hydropower system, such as is envisaged for the River Susitna.
Control responses A simple-cycle gas turbine is relatively cheap to install and its lightweight construction allows its power output to be very rapidly altered, to match continuously varying levels of power demand, Strandberg said. And the operational cost of a turbine fluctuates along with the power output, as the rate of the gas burn varies.
On the other hand, a vertical-shaft, hydro turbine generator is at the other end of the power generation spectrum, with heavy machinery that is sluggish to respond to any change in its control inputs. Huge volumes of water typically flow through the turbine, and the relatively high mass of the water has an inertia effect that further slows the responsiveness of the generator to any request to change the power output level.
“The power and inertia of a moving water column is very different from the power and inertia of a moving air column,” Strandberg said.
In addition, although a typical hydropower plant has a very much higher up-front capital cost than a gas turbine system, once the hydropower system is working the operational cost is almost constant, regardless of how much power the system is generating.
And a combined-cycle gas-fired system has operational characteristics somewhere between a simple-cycle turbine and a hydropower system, Strandberg said.
Base-load power Taken together, the differences in generator control responsiveness and cost characteristics across a power grid that has a choice of power sources tend to lead dispatchers to favor the use of hydropower at maximum output as the prime source of base electrical power, using gas-turbine powered generators to adjust the power supply to match the fluctuating power demand.
However, the need for a dependable power supply across the grid brings another set of complications into the power dispatch equation. Essentially, the grid requires spare generation capacity that can be brought into play rapidly, to ensure continuity of supplies should a generator or transmission line fail. And dispatchers need to keep in their back pockets enough reserve generation capacity to cover those potential hiccups in the generation system.
“The reserve capacity is maintained so that the largest unit that’s operating, if that’s clicked off, the other units could pick it up,” Strandberg said.
And that’s a particular concern in the isolated Railbelt grid with its lengthy and flimsy transmission lines connecting the main population centers — in the Lower 48, by comparison, there is vast network of robust transmission lines connecting huge numbers of power plants, so that if one power generation system fails there may be no noticeable impact on the grid as a whole.
In Alaska, what would replace the perhaps 150 megawatts of power emanating from a Susitna hydropower system were, say, a safety system at the plant to trigger a shutdown?
Varied sources needed An integrated resource plan study commissioned by AEA recommended that a broad, diversified set of power generation systems, including hydro, gas turbine generation, coal-fired power, wind power, geothermal energy and energy from municipal waste, would be especially appropriate for the Alaska Railbelt, Strandberg said.
But a large hydropower system, coupled with an upgraded power transmission system, could provide a stable backbone for base-load power in the grid.
“A large-scale project like this, with very heavy inertia, is very beneficial to the long-term stability of the network,” Strandberg said.
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