High voltage line post insulators have long been used to support and insulate overhead lines. First introduced at the distribution voltage level, their use extended to transmission lines to realize the potential to reduce cost of new lines by lowering structure costs and reducing right-of-way widths.
Up until 30 years ago, porcelain line posts were the only choice available to line designers. These gave reasonably good service, except in cases where the brittle nature of porcelain led to catastrophic cascade failures. Composite line posts are more flexible and, once these became available, were soon the insulator of choice for this application. However this increased flexibility results in deflection of the composite post due to service loads and this clearly must be considered for their proper use. Combined loading tests and ultimate strength tests in particular are difficult and costly to perform because loads that can cause large deflections must be maintained perpendicular with the axial plane of symmetry of the post throughout the test. The complexity of the required pulling test rigs and the great variety of possible load combinations therefore encourage use of analytic techniques instead to provide guidance on combined loading of composite line posts. In addition, users are interested in ‘application curves’ that show combinations of loads that can be accommodated in service.
Recommendations with respect to individual load combinations are usually provided by insulator manufacturers in the form of combined loading curves to prevent unintentional overloading. Unfortunately, lack of uniformity among different manufacturers in constructing these curves has caused some confusion among users. Since developing a common recommendation for composite line posts is problematic, the IEEE WG on Insulators formed a Task Force (TF) to address the issue. The recent status of their work is summarized in this contribution to INMR by Tony Baker of K-Line Insulators USA.
Combined Loading Analysis
The combined loading problem is described in Fig. 4. Here, the fiberglass rod is shown as a horizontally mounted beam deflected through a vertical distance δ as the result of simultaneously applied loads; V (vertical load), L (longitudinal), and C or T (transverse loads).
Vertical loads include conductor weight and its associated hardware along with any ice cover. Longitudinal loads are the result of any imbalance in horizontal tensions in the adjoining span and transverse loads result from either wind loads or line angle loads, or both. The beam, if made of an effectively homogeneous material with uniform cross-section and a flexural rigidity EI, if not deflected too much (δ<10% of ι , has a deflection curve given by:
Solving (1) for deflection as a function of applied loads allows determination of total bending moment applied at the base of the beam. To satisfy the deflection limit as well as users’ desire for application curves, this moment is limited by the Maximum Design Cantilever Load (MDCL) rating for the post insulator as well as its effective length,
Effective length is the distance from the conductor support point to the top of the rigid base fitting (Fig. 4). Composite line posts are commonly installed at an inclined angle θ (Fig. 5) and the most general expression for maximum allowed bending moments in terms of applied loads are:
For the case where the transverse load is a wind load WC in a direction toward the support structure:
For the case where the transverse load is a wind load WT in a direction away from the support structure:
The TF selected a common model composite line post as defined in Table I and asked each manufacturer represented to provide a combined loading curve for it.
Each manufacturer used their preferred method of making combined loading curves. Some used the analysis outlined here, some an FEM approach and some an undisclosed proprietary method. As was found during a previous CIGRE sponsored project, all results submitted were in close agreement, validating the analysis presented here. For the θ = 0° case, (3a) and 3(b) reduce to:
and the combined loading curves for the composite line post defined in Table I according to (4a) and(4b) are given in Fig.6.
It is interesting to note that, according to Fig. 6, the vertical component of a transverse load in a direction away from the support structure counteracts the vertical weight load. As such, it appears there is significant increase in the vertical load carrying capacity of the line post. However, if the transverse load is a wind load, this increased load carrying capacity cannot be realized and Fig. 6 should be corrected, as shown in Fig. 7.
As previously noted, composite line posts are commonly installed at an inclined angle to horizontal and typical angles are in the range 12° to 15°. The combined loading curves for the common model line post defined in Table I are given in Fig. 8.
Figs. 6 and 8 show the significant effect on combined loading curves for a post installed at an inclined angle (Fig. 5) compared to one installed horizontally (Fig. 3).
In addition to the installation angle θ, line angles can also have significant effect on the combined loading curves for a particular application. The impact can be positive or negative depending on whether the angles are inside-angles or outside-angles. This concern has not yet been addressed by the TF. Certainly, the question is application sensitive and may not lend itself to inclusion in any general recommendation. Typical composite line post end fittings are ridged base and have either a horizontal clamp top (Fig. 3) or an eye type (Fig. 5) line-end fitting. The strength of these also has an impact on combined loading curves for any particular post insulator. Line end fittings have a lower strength than a base fitting but it is usual to assign a single value for the tension/compression strength of a post. As such, it is the strength of line end fittings that governs. The typical clamp top fitting has an ultimate strength rating of ±5000 lbs. and the eye type fitting ± 15,000 lbs. Each has maximum allowed working strength of 50% of ultimate. If the common model post insulator is considered to have a horizontal clamp top line end fitting, a reasonable modification for Fig. 7 to show the limitation imposed on the combined loading curve, accounting for the allowed working strength of the end fitting, is shown in Fig. 9.
The effect of end fitting strength on the combined loading curves for particular composite line posts also has yet to be addressed by the TF.
Combined Load Specifications
The TF effort to date indicates that a general recommendation for providing combined loading curves for composite line posts insulators should be feasible, but some work remains. In the meantime, the following tentative guidelines may be helpful to users in specifying composite line posts for particular applications:
• General combined loading curves for composite line post insulators should provide for maximum allowed combinations of service loads;
• It is helpful if typical service loads can be directly entered into the equations to determine combined loading limitations for any specific line post;
• Ultimate strength tests under combined loading conditions require expensive tests with respect to both facilities and time;
• Application combined loading curves based on (1) are consistent among various suppliers and lead to maintaining a maximum service loading of post insulators at MDCL or less;
• Installation angle θ has significant effect on combined loading curves;
• Line angles have a significant impact on combined loading curves and should be treated as special cases;
• Impact of end fitting strength should be included in combined loading curves.