Ken W2KB wrote:With all due respect, you analysis of what is transmission and what is “sub-transmission” is wrong. And I state that as a 33-year transmission design engineer employed by the largest electric utility in the country. We operate 5000 miles of transmission lines up to 765kV. At my company, and at most companies I deal with (and that is most companies), 69kV is and always has been a transmission class voltage.prr60 wrote:By the way, at most utilities 69kV is defined as a transmission voltage. Sub-transmission is usually 34.5kV when used on a network, not radial line....However, "most utilities" makes the above excerpt accurate because so many utilities are relatively small and may consider their 69kV as transmission, and indeed for them it may perform such function to at least some extent. But "most" in terms large majority percentage of utility infrastructure ownership, define transmission as "everything above 100kV" relegating 34.5 and 69 to subtransmission. The Federal Energy Regulatory Commission agrees, as its two major annual report requirements for transmitting utilites define transmission as 132kV or greater (Form 1) and 100kV or greater (Form 715), and in both cases, network, not radial facilities.
Of course, causing confusion, is that transmission service is sometimes provided over subtransmission, or even distribution, lines, most commonlly in the case of interconnection of a small generator to those classes of lines. But there is a distinction between "transmission service" and "transmission facility."
The reason for this is simple. The engineering, tools, and skills required to design and maintain a 69kV line are dramatically different than the requirements for a 34kV line. The requirements set by the National Electrical Safety Code (NESC) change as you move above the 34kV voltage level. The power transmission at that level typically requires larger conductors which require more rigorous structural analysis. The structures are bigger, taller, and the code requirements are more severe. While 34kV and lower lines are usually installed using cookbook standards and minimal engineering, 69kV and higher typically are engineered on a line by line basis with custom structures, full line surveys, and far more detailed specifications and drawings. Then, when all is said and done, the line is dispatched by the transmission board, not the distribution board, and is maintained by transmission line crews, not distribution crews.
The term “sub-transmission” is typically used to describe lower voltage network lines (not radial feeds) that are designed, installed, and maintained by the distribution side of the T&D operation. Basically these are distribution lines performing a transmission function. Lines at 69kV rarely fall into that category and when they do it inevitably turns out to be a colossal blunder. I say that from painful personal experience.
One further clarification. The Federal Energy Regulatory Commission does not specify a voltage in its regulatory definition of “transmission line”. To FERC, the definition is purely functional. If a line moves power from one point to another with the intent to move the power to still another location, then that line is a “transmission line” and must be placed in the FERC transmission asset category. If, on the other hand, the line only moves power to end users (radial feeds to distribution substations, for example), it is by FERC definition a “distribution line”. A line may be 765kV or 2.4kV: it does not matter. What matters for FERC accounting is what it does. We have FERC-designated "transmission lines" as low as 13kV. And, we also have FERC-designated “distribution lines” operating at 230kV. This is strictly a regulatory and accounting nuance. It has nothing to do with our design, maintenance or operation. To us up to 34kV is distribution, and 69kV and up is transmission. That is also, by the way, the legal definition prescribed by Pennsylvania Utility Code.
FERC Form 1, the annual report filed by most utilities, is a statistical summery of a utility’s assets and operation. For that form, on Page 422, FERC wants a listing of all FERC-designated transmission lines constructed for operation at 132,000 volts or higher. Note that it is the design voltage, not the actual operating voltage that determines inclusion in FERC Form 1 (a line designed for 132kV but presently operating at 69kV is included). This does not mean that they do not consider voltages lower than 132kV as "transmission". They certainly do. All it means is that for the purposes of that particular report they are only requiring the utility to list those transmission lines designed for 132kV and up.