L13

Contents

• Overview
  • General data
• Tower forms
• Tower details

Overview

At the moment it remains impossible to say much about L13 as the information thus far is fragmentary and contradictory and very little has been officially revealed.

L13 is another replacement for L6, but it is also part of National Grid’s tower portfolio for new high-capacity links. See NG TS 3.04.29 - Technical Specifications - Specification for The Design of New 400 Kv Transmission Line Towers (L13) for details, although it would cost $184.00 to acquire a copy of this. The Purpose and Scope of this document notes:

In recent years, the non-availability of large section steel angles (305x305mm) has become critical in the event where a heavy angle tower (L6) required replacing due to condition or catastrophic failure.

Furthermore, recent growth in energy demand has led utilities including National Grid to install larger conductor systems to transmit the increased power, this in turn imposes larger loads on the existing towers. The design standards and legislative requirements that govern tower design have changed over the years and compliance with these requirements should be considered and applied in an efficient and economic manner. Retrospective modifications are considered impractical.

The design of a new tower series to be operated at 400 kV and capable of carrying the maximum electrical load needed by National Grid transmission system for a new Overhead Transmission Line (OTL) is now required.

This Specification covers the requirements applicable to the design of new double circuit (DC) tower structures, to be designated 'L13' for use on new lines and for replacement of existing L6 structures.

Some L13 towers are said to have already been constructed. There is visual evidence that L13 low-height has been designed, but no information to indicate that any yet exist.

General data

Tower forms

The introduction of L13 towers has been a chequered one, at one point being held to be unlikely due to critical steel angles going out of production. Although some people have obtained L13 drawings, it’s not clear which if any represent the finished design. The tower sizes are not consistently stated either. The drawings below are not taken from any official or final material and may not represent the production design. Once the towers are clearly photographed it will be easier to double-check the drawings.

L13 is named in [Wylfa–Pentir] and depicted with a grossly inaccurate diagram. A much cleaner depiction appears in Power Pylons of the Future (pp. 6–7) where Bystrup give the height of L13 as 56 metres tall. (In Power Pylons of the Future - 3rd edition the height was corrected to 50 m by way of the gridlines.) Bystrup’s diagram shows the bar thickness but omits a considerable amount of bracing. L13 standard and low height are depicted but not named on page 31 of [North Humber–High Marnham]. In this document, the full-height tower can be taken to be L13 as it matches Bystrup’s drawing, and the low-height tower can be taken to be L13 as it shares the same design of tower body.

The D60 drawing is manually redrawn from a portion of drawing “OVERHEAD DIVERSION ROUTE REGULATION 5(2)(o) AND 6(2) PYLON LAYOUT”, found on page 17 of document Lower Thames Crossing, 2.9 Engineering Drawings and Sections, Volume H (overhead diversion routes and pylon general arrangement), Version 2.0, December 2023. The height is given here as 51.2 metres, instead of the expected 50.952 metres.

The following diagrams are shown to scale at 12 pixels per metre:

Tower details

UK Statutory Instrument 2013 No. 3200 The National Grid (King’s Lynn B Power Station Connection) Order 2013 (SI 2013 No. 3200) lists a number of L13 tower types, from which some of the dimensions can be obtained. [Wylfa to Pentir] page 25 (PDF page 31) gives the base width of “L13 STD” (L13 D STD depicted).

Crossarm width is the total width across the widest crossarm; depending on the method of measurements this may instead be the middle conductor-to-conductor distance, which is near enough the same quantity.