Transmission

Otter Tail Power Company's first transmission line, between Dayton Hollow and Wahpeton, was completed in 1909. It was rated at 22,000 volts or 22 kv. That was considered a high voltage at that time. Today anything lower than 115 kv is called sub-transmission.

When the company started to expand in 1913 and 1914, all main transmission lines were designed and built to operate at 41.6 kv. That remained the company's basic transmission voltage for more than 35 years.

After World War II Otter Tail Power Company's load grew so rapidly that the 41.6-kv system was no longer adequate. It was overloaded. The company then made plans to overlay it with a 115-kv system. The capacity of a transmission line is proportional to the square of the voltage.

The first 115-kv line was built between Fergus Falls and Wahpeton and it was completed in 1950. The structures were a wood pole, H-frame design. That design was used widely by other electric utilities at the time because it was economical to build and it had a good operating record.

In 1951 another 115-kv line was built between Morris and Ortonville. That was followed by a third line between Fergus Falls and Morris the next year. At that point one company officer remarked that with the company's three main generating stations connected - Wahpeton, Hoot Lake, and Ortonville - there would be no need to build anymore 115-kv lines. That prediction didn't last long. The next year 115-kv lines were built from Ortonville to Canby and from Canby to Granite Falls, and during the next 30 years the company built almost a 1,000 miles of 115-kv and 230-kv lines.

In 1959 Otter Tail Power Company discovered conductor fatigue damage on some of its 115-kv lines, mostly in the Red River Valley. That was alarming because conductor failures could lead to serious outages.

When a strong, steady wind blows across the conductors it creates standing waves, called aeolian vibration, which subjects the conductor to bending stresses at the support point. If those stresses are high enough and if they persist long enough, some of the strands in the conductor will break. If enough strands break, the conductor will part and fall to the ground.

Aeolian vibration attracted the attention of other electric utilities all over the country, but nowhere was it as severe as it was on the Otter Tail system. That's because Otter Tail operates in the northern part of the country where the winters are cold. Cold temperatures increase the tension in the conductors, and higher tensions lead to more vibration.

The transmission lines in the Red River Valley were especially susceptible to conductor vibration because the terrain is flat and open. The winds are strong and steady with a smooth, laminar flow. There are no hills or trees to create turbulence and break up the standing waves.

Otter Tail Power Company started an inspection and repair program on the transmission lines with conductor damage that lasted several years. Workers repaired the support points with broken strands and installed stockbridge dampers  to reduce vibration. All transmission lines built after that were designed with lower conductor tensions and less rigid armor rods.

In the early 1960s it became apparent that within five years the 115-kv system would be overloaded because of the projected load growth. It would have to be overlaid with a 230-kv system for four times as much capacity.  When engineers mapped the proposed 230-kv lines, they noted that some of the new lines followed the same route as existing 115-kv lines -- an expensive duplication that would burden landowners.

So the company studied the practicality of uprating existing 115-kv lines to 230-kv. The studies determined that the uprate made sense and construction work started in 1965.

To uprate the existing 115-kv structures, the crossarms and the shield wires were raised, and one insulator was added to the existing seven to make a string of eight. Then the conductors were replaced with a larger size. Other transmission engineers around the country viewed this design with skepticism. They said there would be frequent flashovers and the operating performance would be unacceptable. They probably had reason to be skeptical. The standard insulation for 230-kv lines at that time was 13 insulators per string.

Otter Tail Power Company energized the first uprated line at 230 kv in 1967, following with others during the next few years to bring the total to 160 miles. That was 40 years ago, and those uprated lines have an excellent operating record.  Why have they performed so well? The answer is geography. Otter Tail Power Company operates in a region that has clean air. Those lines would have failed in an industrial area where there is a lot of air pollution or near the ocean where there is a lot of salt in the air.

In 1978, after some engineering studies and field trials, Otter Tail Power Company uprated its first 41.6-kv line to 115 kv, increasing the capacity of the line by a factor of eight. That line performed so well that other 41.6-kv lines were uprated in later years.

Written by Myron Broschat, Otter Tail Power Company retiree