Multiple Taps on Long Distance Single Circuit Lines
Issue Whitepaper
DRAFT
Background
The Area Generation Connection Task Force (AGCTF) was tasked by the Market
Operations Policy Committee to develop and recommend policy to guide SPP Staff to
determine the optimum method of interconnecting generation given the complex
situations generally prevalent.
One issue identified by the AGCTF members is the difficulty in providing for wind
generation interconnection services at multiple points along a long distance 345kV lines
(generally those over 50 miles in length but varying depending on the steady state and
dynamic reactive support available at the connecting stations). While similar issues arise
at lower voltage levels, wind farm developers prefer to interconnect to the higher voltages
in order to accommodate larger amounts of wind generation investments.
This paper describes the technical challenges with multiple taps at single-circuit 345kV
lines and potential solutions to recommend to SPP in their responsibility to study and
guide potential wind generation development.
Description of Issue (Steve to provide)
Alternatives
The AGCTF members discussed several options for addressing the issue described above.
Following is a summary of the alternatives followed by general policy recommendations
consistent with the AGCTF charter.
Alternative 1:
The obvious option to address the limitation on the number of taps allowed is to direct
generation ties to be constructed to the end points of the existing 345kV line or to the
existing taps. This is consistent with the hub and spoke approach assumed in many
studies conducted by SPP in the EHV planning process.
Two problems with the hub and spoke assumptions exist:
1. Wind generation developers seek the least cost wind resources AND minimize the
cost of transmission to the EHV grid. The location of the hubs is typically at
existing load substations near populated areas where is more difficult to site a
wind farm and add new transmission lines (e.g., Tuco, Potter, Axtell, and
Woodward). Further, the hub concept relies on multiple transmission lines sited
into the hub which may be physically impossible to achieve due to limitation on
the number of circuits that can be routed into a single substation.1
2. The transmission costs associated with multiple radial lines into the hub will
likely be greater than the cost of multiple taps along the 345kV circuit route.2
Using more radial lines is also more land intensive. In other word, the number of
ROW miles associated with the hub concept is larger than if the 345kV line is
tapped.
Alternative 2
For all new lines, SPP could adopt a standard whereby all 345kV lines shall be
constructed with double-circuit structures even when one circuit is justified in the
planning studies. If one circuit is needed to meet the short-term system needs, then the
second one could be added for new electrical connections with generation (wind or other
technologies) whenever taping of the first circuit is not technically feasible. If new
generation developers do not requests interconnection along the length of the line, then
the open circuit can serve long-term system expansion needs when justifiable.
Figure 1 shows a set of scenarios for the evolution of a double-circuit capable 345kV transmission
line under alternative 2.
1
Possible recommendation is for SPP to conduct a review of the physical constraints when studying
collection hubs in the EHV planning processes, and limit the amount of generation at those sites
accordingly.
2
Arguably generator developers are responsible for gen-tie cost and therefore uplifted costs are the same
under the hub concept. However, the societal costs are greater since consumers pay for all transmission
costs vis-à-vis generation supply costs and transmission uplifts.
Additional licenses to construct the second circuit are not required (is this true in all
states where SPP operates?). Double-circuit 345kV towers, in most configurations, do
not require wider ROWs than single-circuit H-frame towers thus improving the use of
transmission corridors.
The additional cost for a double-circuit capable 345kV over single-circuit is estimated
between XX and YY% (Bojorquez will provide this data).
A strong consideration for those single-circuit 345 kV projects with recent SPP NTCs is
to review these NTCs to allow for double-circuit capable structures even if the
engineering and routing has been completed.
Alternative 3 (others?)
Policy Recommendations (to be added)