The "roaring" 1920s were a time of major growth in the electric utility industry as the isolated small-town power plant (most of which were municipally-owned and/or DC) quickly gave way to more efficient transmission lines and substations served from centrally-located power plants, most of which were owned and operated by investor syndicates. One such group was headed by Henry Marison Byllesby and their flagship utility was Northern States Power. They had provided the needed capital to help establish the Sioux Falls Light & Power Co. in 1907 and had evidently considered purchasing the utility at Huron, SD (if that had happened, it is interesting to speculate as to how large NSP's presence in eastern SD would have been and whether Northwestern Public Service would have even existed).

At any rate, in the early 1920s, NSP began to acquire distribution systems in towns west of Sioux Falls and extended a 23.9kV system west from the Falls Park complex near downtown Sioux Falls - some remnants of this line are visible today:

The towns added during this phase were: Alexandria, Bridgewater, Dolton, Ellis, Emery, Marion, and Monroe (this town I know was added in 1923). A few substations were added later to serve rural residents along the main line. Photos of these substations as they are today:

Dolton: (likely last surviving original)
Monroe: (modern replacement)
Spring Valley rural:
Canistota rural:
(The Ellis 23.9kV system has long since been replaced with 13.8kV/7970V)

The 23.9kV system served NSP well for decades, but evidently by 1958, NSP decided that the 23.9kV system needed reinforcement and addressed this by tapping the 69kV system at at the 'Renner Junction' substation (now gone), extending a line west from Sioux Falls to the center point of the 23.9kV system, and building a 69/23.9kV substation at that point:

In June 1961, the last remaining segment of the former Central Gas & Electric system (then called Western Power & Gas Co.) was split up between NSP and NWPS, giving the company some new territory north of Canistota (between Salem and Forestburg). The original 23.9kV system did not have the capacity to serve these added towns or the 34.5kV (33kV) transmission line linking these towns, so NSP built an extension north from the Canistota Junction sub to Salem, at which point a pair of substations were built (one 69/4kV sub for Salem and one 69/34.5kV sub to serve the 34.5kV system) and which allowed the company to retire the original substation and the plant (which remained as their local shop). Since the new 69kV line passed right through Canistota, the substation was moved off the 23.9kV system and onto the 69kV line.

Canistota sub:
Salem subs:
Former Salem plant:

Later still, as utilities extended 115kV transmission lines across their systems and began to build interconnections for sharing power and backup service, NSP added a 115 / 69kV substation (in Grant township - hence its name) north of the Canistota Junction sub and added a backup connection to an East River 69kV line that crossed over the NSP line a couple miles east.

Grant substation:
East River tie:

Around 2002 or 2003, Xcel Energy (having succeeded Northern States Power in 2000) decided to reinforce the system in western Sioux Falls by remodeling the Cherry Creek 115kV switch station by converting it into a full substation with several 13.8kV and 34.5kV bays. Rather than try to acquire additional utility right of way easements, they decided to retire one of the two 69kV double circuit lines coming out of the Lawrence transmission substation and reuse its alignment for the new Lawrence - Cherry Creek 115kV line and a new 34.5kV distribution circuit.

This resulted in the retirement and removal of the Renner Junction sub and those parts of the affected 69kV line near the substation were downgraded to 34.5kV service.

Removal of first 7+ miles of Renner Junction - Canistota Junction 69kV line:
Eastern terminus of retired 69kV line:

After the 115/34.5kV conversion was done, the remaining 20.5 miles of this 69kV line (including sections that were rebuilt with posts and more recently, armless construction) was left in place as "banked ROW" in hopes that they could reuse the circuit should another line be needed at some point in the future. But it was not to be - in April 2013, a major ice storm hit southeastern SD and southwestern MN and which caused significant damage to Xcel's SD system (they put out mutual aid calls to KCPL, OPPD, OTP, MN Power, and their own crews across the system... NWPS was a notable absence).

Typical damage to 69kV line:

I guess Xcel took one look at this line and decided it was cheaper to finally remove this 69kV line (leaving the 23.9kV circuit which is now a rural distribution circuit) than continue to maintain it in hopes of reusing it "someday".

Start of line removal:

The contractors began demolition of the retired 69kV line on 7/19/13, reached the end of the section of the 23.9/13.8kV system on 9/5/13, and removed the final span on 9/23/13.