The May-June 2002 issue of INMR contained an article about a new compact tower design for 400 kV lines whose relevancy seems only to have increased since that time. As more and more power utilities worldwide are examining ways to upgrade existing lines or build new lines with minimal environmental impact, that research project from the early 1990s (successfully applied for over a decade on a line section passing a community near Oslo, Norway) is worth another look.
Most engineers would agree that designing a 420 kV line that meets all the strict criteria to be accepted in urban and suburban neighborhoods is a growing challenge. These criteria include: a relatively narrow corridor; much less visual impact in terms of structure, size and height; aesthetic appeal; as well as reduced electromagnetic fields and audible noise – all the while meeting basic performance requirements such as sufficient lightning protection.
By the early 2000s, precisely this challenge had been considered for over 10 years by TSOs in the Nordic countries of Europe, who faced a difficult task of obtaining new right-of-ways. These network operators wished to identify alternatives to laying expensive cable when upgrading transmission lines that pass through sensitive population centers.
STRI researchers and engineers at Statnett were part of the team that conducted some of the early development and testing work beginning in the early 1990s. At the time, a pre-study looked at various types of conductor arrangements that could be employed in a compact design while still meeting the 45 dB maximum audible noise limits existing in Finland, Norway and Sweden. Corona testing performed on a 3-phase 420 kV test span helped verify that the calculations of audible noise were indeed correct.
The first prototype of a compact structure developed and built to support this conductor arrangement was dictated largely by the types of insulators available at the time. This structure was an interesting but unwieldy design that placed reliance on tension strings employing composite long rod insulators. The upper phase relied on 5 insulators while the remaining two phases used 4 each. The prototype had a total height of 19 meters with an additional 2.5 m added by the insulator at the top of the pole. Since earth wires had to be eliminated to achieve the desired compaction, a transmission line arrester with a polymeric housing was designed into the structure.
Ultimately, while this design met most of the basic criteria, it also became obvious that such a structure had too many parts and therefore was not suitable from an aesthetic viewpoint. In addition, it would be quite costly to build since each string span would have to be tensioned individually.
During this time period, developments in hollow core composite insulators were progressing rapidly and soon reached the point where custom-designed composite post insulators having high mechanical strength and with relatively low weight could be supplied. The updated compact design that became a reality on a 400 kV test span at STRI in 2001/02 incorporated a trio of hollow core insulators that utilized special foam to fill the space within their inner FRP tubes. Given the horizontal configuration of these insulators, it became necessary to increase the original optimized conductor phase spacing of 5 m to either 5.4 m or 6.4 m in order to account for the modified air gap between conductor and structure.
The hollow core composite post insulators utilized in this design were of two types: a vertical unit with mechanical strength of 148 kN and total creepage of 8600 mm; and two identical horizontal post insulators of 50 kN and circa 8000 mm creepage. The reason for the significantly higher mechanical bending strength required of the vertical unit was that this type of design had to withstand relatively high bending forces whenever the line changes direction. The other undergo only tension or compression.
While these post insulators were relatively expensive, each one was intended to replace several tension insulators, meaning that their higher cost could be justified. Equally important was the fact that construction costs with this design were significantly reduced since the entire insulator and arrester section could be assembled on the ground and then hoisted into place as one unit. Also, only one flange was needed to attach the entire assembly to the supporting structure – in this case a steel pole but which could be any other type of desired structure.
The modified design, by eliminating the uppermost support insulator of the original prototype, offered a total structure height of only 19 meters, by any measure a low visual impact for a 400 kV line. It was designed to handle a maximum ice load of 4 kg/m, considered acceptable for most service environments – even Norway where high ice accretion is always a risk. Span length of the new design was between 200 and 250 m, depending on service conditions such as wind and ice. The shorter range would apply more to areas where prevailing wind conditions present greater risk of conductors being brought too close to one another.
Reviewing the advantages and drawbacks of this innovative compact design, it was not intended to replace conventional long distance transmission lines but rather for application in areas considered sensitive from an environmental perspective or where restrictions exist in terms of the available line corridor. It was also much less costly than cables for suburban areas where permission could not be obtained to build conventional lattice structures. Another benefit was that electromagnetic fields in this design are only some 40 per cent of what is normally encountered on conventional lines at this voltage. Similarly, right-of-way needed was only one-third of the 18 m width generally required – a reduction of 12 m. Finally, simple installation with mounting of the entire pre-assembled superstructure onto the pole offered definite savings.
This design offered the same outage rate as a conventional line when utilized over shorter segments, achieved by employing polymeric-housed TLSAs in place of earth wires. These arresters were mounted using a special device so that if any energy surge encountered is too great, they simply disconnect from the conductor. Visual line inspection could then readily determine that they needed to be replaced, even though the line remains in service. In spite of this feature, the design was not seen as an ideal solution for areas with heavy lightning problems.
Compact design required only one-third right-of-way of conventional structures (background) with significantly less visual impact as well.