Attached is a letter received from Dave Shafer, the electrical engineer who staffed the CAI project for the Steering Committee, in response to questions raised at the Planning Commission hearings.
Let me know if you have trouble downloading the file and I can deliver it by fax or hard copy.
Honorable Frank B. Tiffany
City
of
Dear Mayor Tiffany:
Commonwealth
Associates, Inc. (CAI) is pleased to provide the following responses to your
e-mail request of
Xcel has finally applied
for a CUP from SFL, and we have had two of three scheduled public hearings to
process the application. At the
hearings, Xcel now states that they are only applying for Phase I – to double
circuit the line from Red Rock to
In my opinion, limiting the
project to Phase I would not change the need. This allows the utility to build the
project between Red Rock and
If only Phase I is completed, does it still constitute the best solution to resolving system deficiencies?
I have not been in contact with Xcel nor have I attended any of the public hearings, so I do not know the rational provided by Xcel for building only Phase I at this time. However, in my opinion, Phase I would still constitute the best solution since it does reinforce the power system into the Rogers Lake substation from the Red Rock 345 kV source and it does not preclude future expansion of the system in any way.
Is double-contingency planning really necessary?
If I were the utility executive, I would be very reluctant to reduce the reliability of the transmission system by moving from the past practice of double-contingency planning to a less reliable single-contingency planning criteria. Because of the high reliance on electrical power, the trend in the industry is to demand more reliable service, not less. The cost for an outage to a large commercial or industrial customer can run into the millions of dollars per hour. If the utility reduces reliability it can leave itself open to all sorts of litigation and risks. If I were the utility executive, I would want to have written guarantees from the public service commission, the reliability councils, the local cities, and the large customers that they will accept the risk of lower reliability and not hold the utility liable should outages occur.
Furthermore, it has been my
experience, that where utilities are vulnerable to inability to serve loads for
certain critical double contingencies, the utility typically implements a load
shedding program. In other words,
the utility would put into place controls to allow them to quickly disconnect
all or part of the loads at
How common is such planning in metropolitan
areas as large as or smaller than the Twin Cities Metro?
I have not done a survey, so I
can only speak in general terms. In
my experience, double contingency planning is only done for large cities, for
certain bulk power transmission, and for critical loads. Certainly, the metropolitan area of the
Twin Cities qualifies as a large city.
Especially, in this particular case where the Mall of America, the
Airport, and potential high-profile industrial loads served from
There appears to be an inconsistency in Volume I of the CAI report between the text in pages 3-1 & 3-2 and the tables which follow. In Case 1, single circuit, it is stated that "the 1999 loadings ranged from about 567 to about 658 amps at peak". In Case 2, double circuit, "each circuit
would carry about 284 to 329 amps". This is consistent. On page 3-2, next to last paragraph, "loading were set at 800 amps for the existing single circuit and 400 amps for each circuit when double-circuited". These figures were highlighted in Tables 3-4 & 3-5 for calculating EMF levels. However, Table 3-1 shows totals for both circuits ranging from 934 amps (normal) in 2001 to 1,304 amps (peak) in 2006, and as high as 1,854 amps in 2020. Which is correct? Obviously, this makes a huge difference in projecting future EMF levels.
Page 3-1, Case 1 existing line loading in 1999 is stated as 658 amps peak, which is consistent with Table 3-1 which also states 658 amps peak in 1999.
The future year loadings presented in Table 3-1 were provided by Xcel to the Minnesota Environmental Quality Board in 1999. We included that data for reference only. We did not attempt to independently verify this data. If memory serves me correctly, I believe I did ask at one time how the numbers were derived and the answer I received from Xcel was that the long range projection (i.e. from 2010 to 2020) was just an extrapolation using the earlier years growth rate.
I do not see any inconsistencies in the numbers in Table 3-1 and other parts of the report. Table 3-1 shows a 2020 peak loading of 927 amps per circuit. CAI Table 3-5 shows the magnetic field for a range of circuit loading. For example, at 1000 amps for each circuit, the magnetic field at 100 feet from the centerline of the circuit the magnetic field is 4.1 mG. You will note that this is actually a reduction from existing circuit configuration at 1999 loads. Refer to CAI table 3-4, for 600 amp loading of the existing circuit, at 100 feet from the centerline, the magnetic field is 8.4 mG.
If the present line can handle up to 1,600 amps using 795 kcmil SSAC conductors to transmit power from Red Rock to Rogers, why is a second circuit needed at all, given that both circuits would be on the same poles and equally subject to outages?
Two circuits on separate structures is preferred over a double circuit structure. However, right-of-way widths in congested areas preclude the use of separate structures. There is a level of reliability that is gained by using a double circuit structure over building a heavy-duty single circuit. Each circuit can be switched independently for maintenance and repair. Also, unless you have a catastrophic failure of the structure, most outage will be only one circuit, otherwise, you would be correct in that there would be no advantage to a double circuit. The widespread use of double circuit structures bears out the fact that they do provide a degree of reliability over a heavy duty single circuit structure.
In this particular case, we also gain the advantage of reduced EMF by phasing the two circuits to provide cancellation of the magnetic field.
Is it realistic to calculate EMF levels at 3’ and 12’ above ground when actual exposures in homes are likely to be 20’ above ground?
The industry standard used to calculate and compare magnetic fields is based on making the measurements at 3 feet above ground. We have included tables at 3 feet so that ready comparisons to industry standards can be made. We have provided calculations at 12 feet above ground as requested by the steering committee, see Table 3-3. Certainly, calculations can be provided for any height above ground as desired, though we question the rational for doing so. We believe a better approach is to make actual measurements in any homes where there is a concern. We have instrumentation that can record the magnetic field over a given period of time. This provides more practical data for a particular situation, than making random calculations.
Would using the latter height make a
difference in the reduction in EMF afforded by
double-circuiting?
It could. There are several factors that affect the calculation as you approach the conductor. For example, as you approach the conductor you will see more effect from the closest conductor and less cancellation from the others, just because you are closer to it. As you move away, you get more equal cancellation from the conductors. Also, in this particular case, the double circuit structure moves the conductors towards the center of the right-of-way and higher, as compared to the existing circuit, thus there may be some decrease in the field from this effect.
Yours truly,
David A Shafer, P.E.
Electrical Systems Engineer