Background of Model
Qualitative details on transmission
ReEDS considers the availability of capacity on existing transmission lines, the cost of accessing and using those lines, and the cost of building new transmission lines for new generation (e.g. dedicated to new wind or CSP farms) when existing lines are not available. To determine how much wind or CSP can access existing transmission lines and the cost of building a line from the wind site to the grid, we use a geographic information system (GIS) database to develop a four-step supply curve for each class of wind/solar in each supply region that presents the amount of capacity that can access the grid at each of four different costs. (The supply curve is formed of discrete steps, with each step represented by a different variable within the linear program.)
The costs increase with increasing distance from the resource to an existing transmission line that has adequate remaining capacity available to accommodate the generation. Although the lines are usually carrying generation from other sources, at any given instant, they may or may not have the capacity to transmit additional power from new wind or CSP generators. It is practically impossible at the national level to assess the capacity available at any given time on each line in the country. Thus, ReEDS requires that the user input the fraction of the capacity of each line that will be available for wind or CSP; the default fraction is set at 10% for all lines. This transmission availability constraint severely limits the amount of wind or CSP that can be transmitted on existing lines, well below that found in previous studies (Parsons and Wan 1995) that required only that the wind resource be within 20 miles of an existing transmission line.
In addition to the cost of building a line from the wind/CSP site to the grid, ReEDS also allows the user to input a cost for the use of the grid. That cost can be based on the distance the power is transmitted or on the number of power control areas that the electricity must pass through (called a "pancake rate").
ReEDS also verifies that the existing transmission lines crossing the border of a supply/demand region have enough capacity to carry the wind and CSP generation into and out of the region. In addition, all generation (that from both renewable and conventional generators) is constrained from flowing between any two balancing authorities in each time-slice by the capacity of lines that connect the two balancing authorities. ReEDS does not account for loop-flows, contingencies, etc. that could further restrict transmission on existing lines.
While new transmission lines dedicated to renewables are not constrained by the remaining transmission capacity available, they do have additional cost. For lines built to serve remote sites, the entire cost of constructing and maintaining a new line is attributed to the wind or CSP capacity at that site. This means that the lines are used only when the wind is blowing (or sun is shining), and their costs must be amortized over that intermittent power. The costs of new transmission lines can vary significantly based on terrain, congestion, labor costs, etc. Currently, ReEDS assumes a single cost for new lines expressed in $/MW-mile, which is increased for rough terrain and population congestion. In the future, we anticipate modifying ReEDS to vary the new transmission line cost per mile with the length of the transmission line and the amount of renewable capacity potentially available within the supply region.
New transmission lines dedicated to wind or CSP can be built either between supply/demand regions as described above or within a region. Dedicated inregion transmission lines are assumed to transport the electricity generation directly from the wind/CSP site to a load center within the region, bypassing the transmission grid and connecting to the distribution system within the load center. As with the construction of lines connecting renewables to the grid described above, the GIS is used to develop supply curves for each resource class in each supply region for the cost of building these intraregional transmission lines directly to load centers.
New transmission lines are also built in ReEDS to transmit power from one balancing authority to another. These lines can be accessed by either conventional or renewable generators. ReEDS builds these lines when it is cost-effective and there is a need for more transmission capacity between the balancing authorities in one or more of the 16 time-slices in each year; or when it is needed to ensure capacity reserve margins are met in the different balancing authorities, NERC regions, or interconnection regions.
Transmission losses are modeled in ReEDS as a linear function of the distance the power is transmitted. These losses apply to the transmission of both renewable and conventional generation, and are currently specified in terms of the fraction of power lost per MWh-mile.
This section includes:
Qualitative model description
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