Wood Pole Overhead Lines
This book concentrates on the mechanical aspects of distribution wood pole lines, including live line working, environmental influences, climate change and international standards. Other topics include statutory requirements, safety, profiling, traditional and probabilistic design, weather loads, bare and covered conductors, different types of overhead systems, conductor choice, construction and maintenance.
Inspec keywords: overhead line conductors; wood; power overhead lines; inspection; overhead line mechanical characteristics; foundations; condition monitoring; lightning protection; poles and towers; failure analysis
Other keywords: overhead line failure; conductor sag; component susceptibility; wood pole decay mechanism; covered conductor; condition assessment; wood pole overhead line; lightning protection; live line working; weather load; tension; cross-arm design; foundation design; insulated conductor; bare conductor; inspection technique; probabilistic design standard; line susceptibility; line construction; pole design
Subjects: Overhead power lines; Inspection and quality control; Power line supports, insulators and connectors; Geotechnical structures; Reliability; Power system protection; Engineering mechanics; Inspection and quality control
In this chapter, the problem of supplying power economically is covered from the various aspects of choosing underground or overhead, the route, the line voltage, wayleave and the various relevant electricity regulations. Although looking initially at new lines, much of this also applies to refurbishment or change of use of lines. By the end of this chapter, the different functions served by undergrounding or overhead construction will be established. Sometimes, ease of establishing an underground network can mean it is just as easy for someone to dig it up. Wayleave and environmental problems are major negatives for overhead lines, but faults are generally easier to find and quicker to repair compared with underground cables. However, this chapter will not soke this perennial argument it is not intended to but it may help an enlightened choice to be made.
The routing and construction of overhead lines is governed by a number of statutes and regulations. The first part of this chapter gives a broad outline of the relevant legislation relating to rights to erect a line and general safety and environmental considerations. The second part lists the key points of the main acts and regulations. The four key acts of parliament and regulations that apply to overhead lines will be covered in detail. This chapter aims to outline the statutory process required to obtain consent for overhead line changes and construction, and identify safety issues relating to overhead lines. In addition, the statutory obligations that have to be followed in the construction and maintenance of overhead lines are described in relation to: wayleave, safety, mitigation of environmental effects.
This chapter looks at surveying and profiling essential first requirements for any new line construction or for refurbishment. Traditional ground-based techniques are covered first, followed by a description of the increasing trend to use helicopters and laser profiling. The chapter is split into three main areas: 1. traditional surveying /profiling for a proposed new green field route; 2. refurbishing an existing line; and 3. laser profiling using helicopters.
In essence, the electricity industry supported ENALR 111 and presented it to the Department of Energy representatives (who had been members of the Baldock Enquiry) as a half-way house between deterministic and probabilistic principles for future wood pole designs. Load-factored design has been used in the introduction of the site-specific design methodology that was established to meet the requirements of the 1988 electricity regulations for the design of wood pole overhead lines. This approach was then endorsed with the 'deemed to comply' epithet such that the industry contended that lines designed in this way met the 'fitness for purpose' clause of the 1988 regulations. The new ESQCR 2002 regulations still leave the onus on the distributor to ensure that lines are safe, reliable and in all ways 'fit for purpose' as far as 'reasonably practical'. The recent CENELEC standards BS EN 50341 and BS EN 50423 have brought in deterministic and probabilistic designs for new lines under the UK NNA BS EN 50423-3-9, although in respect of wood pole lines only the deterministic approach is declared.
Throughout the twentieth century electricity became arguably the most important commodity provided to the home and industry and now interacts with almost every aspect of our lifestyle. Distribution overhead line designs supplying electricity up to 33 kV, unlike electrical goods, have not necessarily changed all that much, yet there have been significant developments in design criteria that should be noted. Overhead line reliability and safety have been heavily criticised in recent years and so there is a need to address the issues of future overhead line design. This chapter looks at some of the fundamental steps that are adopted in developing a new overhead line wood pole design for the UK environment together with some historical references and the effects of new European legislation.
Today, there are many software design packages that will allow the OHL design engineer to do all the necessary calculations at the touch of a few buttons. However, it is necessary to appreciate the engineering behind any software, as it is essential to understand what is happening and to feed in the engineering knowledge. This chapter deals with the mechanical aspects of pole and cross-arm strengths and foundation capabilities. The next chapter will deal with the mechanical side of conductors the tensions in them that cause stresses on the poles and the sag that is relevant for maintaining line clearances in most weather conditions.
The next two chapters look at conductors the workhorse of the overhead line. Chapter 8 looks at the electrical choice for conductors and how to calculate what is required. This chapter, however, continues with the mechanical theme of chapter 6. The conductor that goes up must maintain its statutory clearance to ground for the next 40 or 50 years, whether cold or up to its maximum allowable operating temperature (normally 50 °C but now often higher). Mechanical and metallurgical creep cause a conductor to stretch and so slacken off between the poles and this must be allowed for Conductors also take wind and ice loads and. within reason, should not break or be strained beyond their elastic limit under severe weather conditions. There is. therefore, a need to make further allowances e.g. use factors of safety. Finally, not all lines are on level ground. Hills are always present and sag/tension calculations must take account of the fact that an intermediate pole may be on top of a hill or deep in a gully. This chapter therefore seeks to cover the most general aspects of conductor sag and tension calculations, including the weather loads and uneven ground and compares manual methods of line design with computer software design packages.
This chapter considers the geometry of conductors and the various materials used. This is followed by a look at the electrical requirements, how to calculate them and a look at the current ratings and greasing (corrosion) requirements of bare and covered conductors. The covering can be simply a thin sheath or full insulation. In the former, the conductor is covered to reduce clashing problems or for safety or space reasons but it is not insulated. It must be remembered, however, that the best choice electrically may not be the best choice mechanically or even environmentally.
This chapter looks briefly at the various types of conductor on the market with particular reference to covered and insulated conductors. Aspects such as current carrying capacity, weather loads and line design have been covered in other chapters. A brief introduction to novel conductors that may be used in the future on wood pole lines is given at the end of the chapter.
Wood pole overhead line construction has changed significantly over the past few years. Most of the changes have been brought about by the need to mechanise a lot of the work previously achieved by manual techniques. Due to the downsizing of the workforce, previous methods are no longer practical Line teams now need all the latest equipment in order to do the same amount of work previously undertaken by large teams of linesmen. The following chapter describes the construction of overhead lines in the order of work undertaken.
The Electricity Supply Regulations 1988 (Part V, Paragraph 24) state: 'The supplier shall take all reasonably practicable steps to inspect his installations to ensure compliance with these Regulations'. The electricity utilities (or DNOs as they are now known in the UK) therefore have a responsibility to inspect their networks and, in the context of this book in particular, their overhead lines. This is not only essential to maintain supplies in a costeffective maimer and to be certain that regulatory clearances etc. are complied with, but also to provide a legal defence in any litigation where third parties may try to blame the DNO for incidents involving electricity supply lines. Unfortunately, the regulations give no guidance on methods or frequency of inspections and so it is up to the DNO to decide upon a regime that will deliver the required network performance in terms of supply and safely.
Overhead line asset owners have a statutory obligation under the Electricity Supply Regulations to ensure that the network equipment is sufficient for the purposes for and the circumstances in which they are used and so constructed, installed (both electrically and mechanically), used and maintained as to prevent danger and interruption of supply, so far as is reasonably practicable. In order to achieve this, inspection regimes have historically been introduced to consider the onsite condition of these assets and therefore determine what should be done to avoid both interruption of supply and danger to the public. Chapter 11 explained the techniques and philosophy of inspection; this chapter considers line component inspection in detail. When inspecting overhead lines thought should be given to the continued stability of the line and its safe operation. The inspection should consider each component inspected to be fit for purpose for a duration not less than 15 years from the date of inspection.
Recent years have seen the derivation and population of health indices to rank assets on the basis of condition (in relation to proximity to end of life) as a means of assisting asset managers in formulating and justifying replacement or refurbishment plans. This chapter looks at the concept of health indices, which are based on utilising existing information, and summarises the experience gained over the past few years with developing and implementing health indices. This enables initial health indices to be derived and populated quickly, without the need for large-scale information collection. This gives a knowledge of present asset condition that can be immediately used to provide a very structured, well defined assessment of future condition assessment activities that can be used to focus and direct future asset management activities.
Line failures can occur through gradual and predictable deterioration of line components. They can also occur through unseen vandalism or storm damage. Frequent inspection coupled with a probabilistic analysis of the benefits from maintenance can extend line life economically. However, there will always be a number of factors that no amount of economic inspection or maintenance can stop from leading to premature line failure. Some of these can be avoided not by inspection/maintenance but by policy changes. Examples of this could be improved lightning protection in susceptible areas, the use of covered conductors in forest, wildlife and leisure areas and a re-think on line design to avoid buildmg-m future ferroresonance problems.
This chapter looks at the decay processes in wood poles used for electricity distribution networks. It covers the commonly used rot detection techniques as well as the various remedial methods employed. The use of these data to evaluate pole strength and remnant life is then discussed. The need to link wood poles to the load they are required to carry is emphasised and a simple evaluation technique is demonstrated.
This chapter has highlighted many areas in the UK where overhead lines are under environmental attack. An acceptable failure limit has to be decided so that the extent of mitigation or condition assessment techniques can be kept to a costeffective level. This chapter considered various components of an overhead line including conductors, insulators, poles supports, fittings, foundations, pole-mounted transformers, pole-mounted switchgear, lightning protection, and line design. The weather effects considered in this chapter are lightning, snow, ice, wind, pollution, temperature and others.
This chapter looks at the historical development of working on live lines and illustrates the basics of hot-glove (live line) working on distribution wood pole lines in the UK.
This chapter briefly explains what lightning is, how to decide risk levels and how to protect against it.
This chapter looks at the future trends in overhead line design, construction and maintenance and also at draft regulations and how these may affect the current situation. The future is something that can be anticipated and precautions taken current data can be extrapolated and the present situation gauged in terms of overhead line supply quality and economic performance. For the future, the challenges of climate change and new regulations and practices require preparations now. The future trend for overhead lines has to be based on two areas what sort of environment the overhead line is likely to face for the next 50 years and how supply quality can be maintained costeffectively.
