Design-Glass Shell


Dielectric shell material

The ability of SEDIVER Suspension Insulators to indefinitely withstand the effects of time and the elements is directly related to the structural purity of their glass dielectric shells, and to the permanent surface pre-stresses imparted by the SEDIVER toughening process. No other ceramic dielectric shell material possesses this ideal combination of desirable characteristics.


The importance of homogeneity

In the case of cap & pin dielectric materials - both porcelain and glass - their long term mechanical performance depends on their ability to resist the initiation and propagation of surface and/or internal cracks due to stress in service.
The electrical performance of these materials is similarly dependant on their structural consistency as internal discontinuity is the starting point of failure due to electrical stress concentration.


Structural purity of SEDIVER glass

Because the first step of the glass manufacturing process is the total fusion of raw materials, the homogeneity and purity of glass structure is without comparison with other materials.

However, in order to maintain this purity level throughout the glass manufacturing process, it is primordial to master a wide range of parameters to eliminate inclusions in the glass shells.

Sediver, with more than fifty years of specialised experience in insulation, has achieved this goal by:

  • Constantly monitoring all raw materials for correct chemical composition,
  • Continuous metering of the raw material mixture,
  • State of the art glass furnace design and operation, including specific refractory wall materials and automated control of internal atmosphere and temperature,
  • Multiple thermal shocks testing of every glass dielectric shell after molding and toughening.

 

Design-Toughening Process

Effects of the toughening process

Micro cracks on the surface of ceramic dielectric shell materials - both glass and porcelain - have an important effect on the long term mechanical performance of suspension insulators when submitted to "normal stress and effect of fatigue", see below. This is due to the intense concentration of stresses which occurs at critical surface cracks and which causes propagation leading to eventual failure. Since surface micro-cracks are inherent to all brittle ceramic materials, some form of compensation or control over this condition is necessary.

In SEDIVER suspension insulators, the effect of critical surface cracks is overcome by the permanent compressive pre-stresses imparted into the surface region of their glass dielectric shells by the controlled cooling process know as toughening. Because these surface compressive pre-stresses are so substantial (in the order of 250 MPa - 35,000 psi), the formation and propagation of surface micro-cracks is very strongly inhibited.

The relative effectiveness of  toughening (view explanation in video) in controlling micro-crack growth, as compared with methods employed for porcelain, is shown below.

Control of microcrack growth in porcelain and toughened glass
Load
condition		 Porcelain Toughened glass
UNSTRESSED
NORMAL
STRESS
EFFECT OF
FATIGUE

Design-Glass Shell Profile

Over the years SEDIVER engineers have developed and optimised different types of toughened glass dielectric shells, each having the special combination of characteristics described and illustrated bellow.

All shell types have been designed in accordance with the insulator profile parameters stated in IEC 60815, used as a guide for the selection of insulators in respect of polluted conditions.



Profiles
Standard Profile
(IEC 60305 &
ANSI C29.2)

Because of shallow, well-spaced under-ribs and a leakage distance in excess of standard duty requirements, this design performs well in areas of mild contamination.
Anti fog profile
(IEC 60305)

A design with a longer leakage distance than the standard profile obtained by two or three ribs of greater depth. The profile and wide spacing of the ribs promote an effective self-cleaning action by wind or rain.Their wider spacing also prevents arcing between adjacent ribs under severe contamination.
Open profile

The absence of deep under-ribs on this shell type greatly reduces pollutant accumulation on the lower surface because air flow is smooth and uninterrupted.
This design is particularly effective in desert areas where natural washing by rain is infrequent. It is also effective for tension (dead-end) strings in extreme industrial pollution and can solve ice-bridging problems when alternated with other profiles in a string.
Spherical profile

This spherical shape permits a leakage distance equivalent to that of standard profile type. The absence of under-ribs reduces pollution build-up, notably in dust-laden environments. Manual cleaning, if required is easy and efficient with the spherical profile.
External shad profile

A design with a leakage distance equivalent to that of anti fog profile type.
Elimination of under-ribs reduces pollution build-up, promotes self cleaning and facilitates manual cleaning when necessary.

 

 

Design-Metal Parts


Insulator cap design

The properties of malleable cast iron or GS cast iron used for the cap SEDIVER Insulators are listed in SEDIVER internal purchasing requirements and have been determined for their excellent resistance to fatigue, combined with high resilience at low temperature - important qualities for insulators of this type because of the cyclic nature of loads and weather conditions encountered in service. In addition, malleable cast iron and GS cast iron readily permits high thickness and excellent adherence of the hot dip galvanised coating necessary for effective protection against corrosion.


Insulator pin design

SEDIVER insulator pins are forgings made of medium carbon steel, suitably heat treated to provide an optimal combination of strength and ductility, and hot dip galvanized for protection against corrosion.

Typical Mechanical Properties
Malleable
cast iron		 Medium steel
Grade A
Tensile strength daN/mm² 35 59/71
Elastic limit daN/mm² 23 35
Elongation % 10 19
Resilience daJ/cm² 1 to 1.6 4
Coefficient of termal expansion K-1 12.10­ 11.10­
Brinell hardness HB <150 <190
Minimum thickness of galvanisation µm 85 85

Insulators with specific protection against corrosion are also available

 

Endurance

 

Today the world's longest high voltage transmission lines, operating under the most challenging conditions and requiring the highest degree of reliability, are equipped with Sediver Toughened Glass Suspension Insulators.

Two basic benefits explain this world-wide preference:

  • Toughened Glass Insulators indefenitely withstand the effects of time and the elements
  • Toughened Glass Insulators have a superior ability to endure mechanical and electrical overload conditions.

For more information about the significance of dielectric material structure and other aspects of insulator design, download the brochure.

 

No ageing

Thanks to the amorphous structure of glass and the unique mechanical properties of the toughening process, toughened glass insulators do not age. The surface compressive stresses induced by the toughening process mean that surface cracks simply cannot develop. It is common practice among utilities to quote the life time of toughened glass insulators as being far above the limit of poles, towers and conductors. In fact, it is the metallic components such as fittings and hardware that dictate this evaluation.

Numerous studies on very old toughened glass insulators have shown that the mechanical and electrical values of toughened glass insulators manufactured by SEDIVER do not decrease with time, and stay above original ratings even after 40 years in service. Examples are given as follows:

 

No ageing

Thanks to the amorphous structure of glass and the unique mechanical properties of the toughening process, toughened glass insulators do not age. The surface compressive stresses induced by the toughening process mean that surface cracks simply cannot develop. It is common practice among utilities to quote the life time of toughened glass insulators as being far above the limit of poles, towers and conductors. In fact, it is the metallic components such as fittings and hardware that dictate this evaluation.

Numerous studies on very old toughened glass insulators have shown that the mechanical and electrical values of toughened glass insulators manufactured by SEDIVER do not decrease with time, and stay above original ratings even after 40 years in service. Examples are given as follows:

 

At Glance Inspection

 

 

 

 

 

As power supply reliability becomes of more concern each year, utilities are carrying out diagnostics of their lines and insulation in order to avoid unforeseen failures. Inspecting insulators has been recognized to be very difficult to do with porcelain (buzzing each unit...), and is sometimes impossible with composite insulators. For both these types of insulator a visit to each pylon by a ground crew is necessary, using specialised equipment. On the other hand with toughened glass insulators, which cannot be punctured nor be internally cracked, a simple glance at the string gives a complete and reliable assessment of the condition of the insulation. This means that a crew in a helicopter can inspect 100 kilometres of line, or more, per hour.

As cost is also being scrutinized sharply year after year, utilities are considering with great interest the possible savings in the cost of maintenance of their power lines. The first step is the inspection cost. In fact, it is a double folded question. The real question would be: how to ensure the best level of evaluation of the lines at the cheapest cost. The periodicity or frequency of inspection, the level of confidence in the inspection work itself, are important factors to add to the overall cost of inspection of a line.

Several studies have been performed worldwide. Each time, toughened glass insulators were proven to provide the cheapest and most dependable inspection results.

A study published by Stattnet* showed the following relative global inspection costs per km:

 
Glass Porcelain Composite
10 US$ 152 US$ 240 US$
 

* "Recent experience and future plans for the newest generation of Insulators and arresters on the Statnett AC and DC Networks" from Kjell Halsan Diarmid Loudon - Statnett, Norway.
Published in Insulator 2000 – World Congress on Insulator Technologies for the Years 2000 and beyond, Barcelona November 14-17 1999.

 

Safe live line working
 

While more and more utilities are faced with the economical challenge of keeping their lines energized « whatever happens », it clearly appears that live line work has become a necessity. Live line maintenance requires specialized crews, equipment and procedures - at a higher cost than traditional dead line maintenance operations. However, the financial impacts compared to shutting down a line are negligible.

Before going on a hot line, maintenance crews have to make an assessment of the condition of the insulator strings. This has been recognized to be very difficult to do with porcelain (buzzing each unit...), and is sometimes impossible with composite insulators.

Who would dare take a risk when sending maintenance crews onto their towers not knowing if it is safe? Thanks to the unique properties of toughened glass, which cannot be punctured, nor be internally cracked, maintenance crews can work on live lines equipped with toughened glass insulators with no risk of “misbehaviour” of any insulator. A simple glance at the string gives a complete and reliable assessment of the condition of the insulation.

Utilities throughout the world are using SEDIVER toughened glass insulators for this reason.

 

There is only one possible aspect for a damaged toughened glass insulator. A simple glance is enough to know.

No internal crack or puncture is possible with toughened glass insulators.

Storage

Recommendations

Sediver insulators are delivered packed in wooden crates on pallets. This packing is designed for long-term storage; however, since wood is sensitive to humidity, if the insulators are to be stored for a long period of time it is recommended to store the crates in a ventilated area.

The wood may darken with time, but will keep its mechanical strength.


White traces on the metal parts

If for some reason the insulators have been stored for a long period of time in a confined & humid atmosphere, white traces can appear on the metal fittings. This deposit - which is simply zinc oxide - can be removed by simple brushing and/or washing of the metal parts. This deposit is totally harmless and does not affect the insulators' behaviour.


Clicking pins

When the insulators are stored in a dry atmosphere, it may become possible to rotate the pin by a few degrees - with a clicking noise. This is due to a slight shrinkage of the cement which has no consequence on the mechanical characteristics of the insulators. This phenomenon dissapears once the insulators are under load.

At time of istallation

Recommendations

The insulators must not be dragged along the ground.
Once the crate has been opened, the strings should be handled by the cap of the top insulator of the string.


No climbing on strings

Insulators and grading rings shall not be used as steps by the crew. Climbing the string may damage the insulator shells or the rings.

 

Verification of the locking device

SEDIVER cap and pin insulators are delivered in short strings. It should be noted that this pre-assembly is for ease of transportation. Hence, the position of the locking device must be checked at time of installation.
Use a wooden mallet for the operation of the locking device.

 
Split pin in coupling position Split pin in locking position
Protection against corrosion

In certain environments, the insulator metal parts deteriorate with time due to electrolytic corrosion. This is particularly true in coastal areas, tropical climates and many polluted areas. In order to extend the service life of the insulators installed in such environments, Sediver offers several solutions to improve protection against corrosion:


Galvanising

All Sediver ferrous metal fittings are hot-dip galvanised.
IEC 60383-1 requires a zinc coating mass of 600 g/m² - or 85 µm. In severe conditions, where this standard protection is known to be insufficient, Sediver offers enhanced protection of the cap and the pin by increasing the thickness of zinc from 85 µm to 110 µm.

 

Pin - zinc sleeve

In severely corrosive marine and industrial atmospheres, and in all DC applications, the galvanised coating on insulator pins may deteriorate over time and be followed by corrosion of the pin itself. To prevent this form of pin damage, SEDIVER supplies - when needed - pins with a corrosion retarding sleeve made of high-purity zinc.
(The catalogue designation which identifies the presence of a zinc sleeve is "YC" or "ZC").

The sleeve is cast directly onto the pin and is partially covered by the cement. Its position and dimensions are designed so that the sleeve acts as a sacrificial anode and thereby protects the pin against galvanic action
   

Standard pin   Pin with corrosion

retardation sleeve

Cap - zinc collar

Sediver has developed and patented a cap which is protected by a cast-on collar made of high purity zinc. Sediver recommends its use for all HVDC applications and for HVAC applications in very corrosive marine and industrial atmospheres.
(The catalogue designation which identifies the presence of a zinc collar is "ZD" )

 

Electropic insulators

Sediver has developed a special range of insulators dedicated to perform in tropical environment.
The figure represents typical toughened glass optimised insulators used in tropical areas subjected to heavy contamination:

  • to optimise the electrical stresses on the metal parts (cap and pin),
  • to have a more uniform electrical field distribution on the insulator
  • to limit the effects of discharges around the metal parts during "wetted" periods.
Pollution mechanism

More than ten million Toughened Glass Suspension insulators are now in service throughout the world in polluted areas which involve almost all possible combinations of contaminant source and weather pattern. Regardless of the contaminated condition encountered, these Sediver Insulators have successfully prevented power system disturbances due to contamination flashover, in a great cases for time periods longer than 25 years.