Archives for May 2013

Continuing on from our preview of the first chapter of Derek Goulsbra’s new book, available here in part one, this post previews the start of chapter 2 ‘Medium Voltage Cable Accessories: A Theoretical and Practical Appraisal.

Chapter 2 – Electrical breakdown of air and solid dielectrics

In order to understand accessories and how they perform it is necessary to look at the fundamental electrical breakdown of air and solid dielectrics, which are, of necessity, mixed in dry cable accessories.

2.1: Electrical Breakdown of Air

Air at normal temperature and pressure, which means a temperature of 20 ° C and pressure of 760 mm Hg, has an intrinsic breakdown strength of 3 MV m-1 . This means that if a voltage is applied between two electrodes, which provide a uniform field, and the electrodes are spaced 10 mm apart, electrical breakdown of the air in the form of an arc will occur at 30 kV. This value is the same for a.c. (the peak value), d.c. and impulse. The electric strength is simply found by dividing the applied voltage at breakdown by the electrode separation. In the present example:

Breakdown strength E = Voltage applied / Electrode separation
= 30 kV per cm
= 3 kV per mm
= 3 MV per m

A uniform field is shown in fig.1. By definition lines of equal voltage are spaced equally across the shortest distance of the electrodes.

In practice, the intrinsic electric strength of air is of little use because conductors at high voltage are not conducive to producing a uniform field and the earth plane is often a flat sheet interspersed with bolt heads and nuts for various fixings. At the other end of the scale from a uniform field is the point-plane configuration shown in fig. 2.

Figure 1: Uniform Field

In this situation, it will be noted that the equipotential field lines are much closer together near the point electrode and spaced further apart as the earth electrode is approached. If the separation of the electrodes is again 10 mm, it will be found that the voltage required to break down the air will be as low as 10 kV, depending upon the sharpness of the point. The reason for this is that as the field lines are close together near the point, the breakdown strength of the air, i.e. 3 MV m-1 will be reached with only a few kV applied. At this point the air breaks down locally to create corona around the needle electrode. There will not be complete breakdown of the gap initially because the electric stress nearer the earth plane is very much lower. The corona produced does, however, ionise the air thus reducing its electric strength and if the voltage is increased further, total breakdown occurs at a much lower level than if the electrodes presented a uniform field.

Corona occurs in many situations without causing complete failure of the system – e.g. on overhead power lines and around insulators on these lines, especially when there is rain or fog in the atmosphere.

Whenever air is used as part of the insulation in electrical equipment, there is the possibility of corona occurring. It is not desirable for this to happen in an enclosed space and thus designers ensure that sharp profiles are eliminated and spacing is adequate to prevent corona at working voltage.

It should be borne in mind that corona in an enclosed piece of switchgear or cable box can, if the humidity is high, create various acids, which will eventually corrode the metal work of the equipment and this must be avoided. It will be shown later that this effect can also be caused by badly installed cable termination.

Derek Goulsbra has been heavily involved in product development, failure analysis and engineering and jointer training for over thirty years. His new book, published by Nexans, is a detailed look at the Medium Voltage cable accessories which will be of value to jointers and engineers alike.

The book contains theoretical and practical appraisals on the workings and failure modes of a range of cabling accessories, including separable connectors, terminations, joints and associated components, as well as a practical consideration of cable preparation using present day techniques and tools prior to installing the accessory. Derek rounds the book up with a set of examples of failures which are presented with explanations of how these could have been avoided.

We have been lucky enough to get an early copy of the book and have featured the following teaser from chapters 1 and 2.

Chapter 1 – Introduction

The cables used for electrical distribution until the 1960’s almost exclusively used impregnated paper to insulate the conductors and were usually three core. A change to polymeric insulation then gradually gathered momentum and with it a change in the technology required for cable accessories, although, ironically some of the first dry terminations were developed for paper cable.

Early so-called dry type accessories for polymeric cables were usually of the pushon variety produced from elastomers and used on single core cable. The sound basic technology employed in the early days ensured that such products continue to be supplied today.

Heat shrinkable terminations were introduced in the late 1960’s and had the distinct advantage of being range taking and capable of use on both single and three core cables. Heat shrinkable joints were developed a few years later following the success of terminations and their suitability for both paper and polymeric cables resulted in a major revolution in cable accessory technology. This technology was a complete departure from the previous techniques, which required a cast iron box or lead sheath filled with oil or compound.

Cold shrink products made an appearance a little later and are now proving popular within the industry. The debate as to which system is preferable continues. It is fair to say that the heat shrinkable system is the most versatile and well known, but on single core polymeric cables in particular cold shrink systems are seen to have certain advantages. It is not the intention of the author to express a preference for any one system.

The use of screened elbows to terminate cables is now widely accepted and it will be shown later in the book that this system is theoretically superior to other systems for certain applications.

The early development of dry cable terminations concentrated on three core paper cables in the United Kingdom and single core polymeric cables in the United States of America. The accessories for paper cable were originally developed for paper belted cable rated at 11 kV using heat shrink technology; whilst for polymeric cable elastomeric push on systems were developed principally for 8, 15 and 25 kV.

As with many totally new concepts, it was shown that extensive laboratory development and testing is no substitute for field experience and many of the early terminations failed. The failures were generally not the result of poor installation, as each one was carefully monitored at this stage. The first heat shrinkable terminations developed for paper cable were pole top mounted and therefore exposed to the elements. The materials used were not themselves to blame and the formulation has changed little since that time. Several weak links in the chain were, however, present simple things such as inspection windows in cable lugs allowing water into the termination and the sealant used did not always exclude moisture. However, the major problem was the regular occurrence of breakdown between one core and the lead sheath.

Gradually, as the problems were identified, the weaknesses were eliminated and the products became reliable. At the present time dry type accessories, if installed correctly, will give satisfactory operation for the life of the system. Many terminations installed thirty years ago are still in service today.

As the practices of utilities have change and many of the jointers and engineers have retired, the need to make the installation of accessories less skill sensitive is the main driving force for the manufacturer. However, there are still some fundamental rules which should be obeyed to ensure satisfactory installation and hence trouble free service life.

A constant problem within the industry is the shortage of fully trained jointers to install the ever increasing variety of products in the market place.

The following chapters will examine how accessories work, why they can fail and good practices required ensuring trouble free service. It is hoped that there will be information of interest to engineers and jointers alike which will enable them to better understand the workings of cable accessories and hence reduce the potential for problems at a later stage.

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