HOLLOW COMPOSITE INSULATORS

In generation, transmission and distribution of electric energy, for technical reasons, it’s necessary to vary the voltage level of the electricity and to measure and monitor it. This is done by using specific electrical apparatus, static or rotating, to pass from medium voltage of the generation to the high voltage of transmission and back to the medium and low voltage for distribution but also for measuring, metering and for protection and interruption of electric networks.

Insulators are used in all these electrical apparatuses: their aim is to insulate two metal parts with different voltage levels (usually one is grounded) in order to avoid short-circuits which cause the breakdown of the network or damages and destruction of electrical apparatus.

SAVER is able to supply hollow composite insulators as well as assembled bushings which use SF 6 as insulating media.
Bushings are used to carry conductors, at live voltage, inside all types of electrical apparatus such as transformer, switchgears or a bus-duct and through building walls.
Bushings are grouped into non condenser and condenser and among the latter are divided considering their application into air-to-air bushings, air-to-SF6, air-to-oil, through-wall (etc…) and their insulating media (gas insulated, RIP, OIP). For the composite insulator it’s important to know the insulating media and the specific application as well as the pollution level, altitude installation and so on.

In high and very high voltage applications, SF6 gas is often used as insulating media since it is non-ageing, non-poisonous, non-flammable, has good dielectric, arc-quenching and thermal capabilities and it’s possible to control the dielectric strength through its density (by a pressure compensated device).
Insulators are divided into porcelain or composite insulators, with silicon housing for outdoor applications or without housing for indoor applications or inside the apparatus (usually vacuum-impregnated).

Composite insulators manufacturing complexity is increasing exponentially as the rated voltage increases; for this reason, electrical and mechanical design must be very accurate. Also their manufacturing must be implemented with the highest accuracy and precision possible to avoid any minor problem or defect which could cause catastrophic effects (flashovers and/or apparatus fault)

Most insulators currently in use are porcelain ceramic insulators, but over the last decades composite insulators – that were developed 30 years ago - have increased their market share, which is constantly increasing for their important advantages:

  • safety for workers and apparatus. Because of their design, materials and technology of manufacturing, hollow composite insulators can’t explode like porcelain insulators in case of fault. Porcelain is a very rigid and brittle material and in case of a manufacturing defect (not always possible to detect) or for a high thermal stress, can break. For gas insulated apparatus in normal service conditions (usually service pressure is 8-9 MPa) or under internal overpressures (above 1,2 MPa) , the fault of a porcelain could mean a real explosion with “porcelain bullets” thrown at high distances. Fault of composite insulators, if any, is a yielding of fibers which lead to the release of the internal pressure or overpressure. This is for sure one of the main advantages of hollow composite insulators in comparison with porcelain insulators, considering the usual presence of people working in the installations but also nearby private houses and offices;
  • high hydrophobicity of the silicon which means no need to wash their housing from time to time (less maintenance costs);
  • very short lead time of composite insulators manufactured in Saver HV Division in comparison to average lead time of high voltage porcelain insulator manufacturers;
  • Lighter weight compared to porcelain insulators and this is a double advantage in terms of cheaper transportation costs and of possibility to design leaner (that means less expensive) electrical apparatus;
  • composite insulators are less fragile compared to porcelain insulators and this make their handling much easier and faster during the assembly of apparatus or during their installation in service, moreover, in case of damage of the silicon shed, very often it can be repaired.

For many years SAVER has been manufacturing and selling thousands of filament wound tubes to European producers of hollow composite insulators . Therefore, we have vast experience in the production of tubes for this application and their gluing to end fittings. In 2007 SAVER decided to start producing not only the filament wound tube glued to metal flanges, which is the ”core competence” of the composite insulator, but also the finished insulator with silicon housing.
In fact, hollow composite insulator is composed of a tube which provide its mechanical stiffness (a FRP tube), assembled with metallic flanges and covered by a silicon housing that grants the required electrical insulation. In particular, silicon housing bestows the creepage distance necessary to avoid any flashover and it is protecting filament wound tubes from UV rays.
The electrical and mechanical dimensioning, the installation of insulators in high polluted areas or at high altitudes, corona effect problems are all aspects to be considered in a correct technical dimensioning, design and fabrication process of these products.

Examples of simulation of electrical field for a correct design of and insulators and its shield:

SAVER made a huge effort in the investments for external tests, machines, human capital, tools, control and testing equipments. SAVER’s strategy was to constantly preserve and increase its position in the electric transmission and distribution, especially in the High Voltage field. Today SAVER is among the best equipped worldwide producers of hollow composite insulators delivering worldwide to all the main and well-known producers of HV OEM.

As for all other product lines, considering the highly specific and challenging application of high voltage field, Saver has selected only the best, tested and certified European raw materials in order to guarantee the highest possible quality and reliability. High voltage OEM leaders can homologate hollow composite insulators by checking their performances through challenging mechanical and electrical tests and simulating their behavior and ageing in the long term. Saver has passed all tests prescribed by international IEC 61462 Standard and by Customer Specific more severe tests, required by the most important manufacturers of electrical equipment.

Design test Acc to IEC 61462

  • Tests on interfaces and connections of end fittings (IEC 61462 § 7.2)
  • Tests on shed and housing material (IEC 61462 § 7.3) which includes:
    • Hardness test (IEC 61462 § 7.3.1)
    • Accelerated weathering test (IEC 61462 § 7.3.2)
    • Tracking and erosion test (IEC 61462 § 7.3.3)
    • Flammability test (IEC 61462 § 7.3.4)
  • Tests on the tube material (IEC 61462 § 7.4)
    • Dye penetration (IEC 61462 § 7.4.1)
    • Water diffusion test (IEC 61462 § 7.4.2)

Other customer-specific tests

  • Thermal ageing under internal pressure (2000 hours at Tmax under MSP)
  • Thermal stability (10 cycles of 24 h between Tmin and Tmax under MSP)

Type test:

Type tests are performed to measure the correspondence between design calculations and manufacturing realization. All tests are performed whenever there is a change in dimensions or design. All type tests are performed internally in Saver by using strain gauges.

  • Internal pressure test (IEC 61462 § 8.4.1)
    • Stage 1: test at 2,0 × maximum service pressure (MSP)
    • Stage 2: test at 4,0 × maximum service pressure
    • Stage 3: test at specified internal pressure (SIP) level (if SIP > 4 x MSP)
    • Stage 4: test up to burst-pressure
  • Bending test (IEC 61462 § 8.4.2)
    • Stage 1: test at maximum mechanical load (MML)
    • Stage 2: test at 1,5 × maximum mechanical load
    • Stage 3: test at 2,5 × maximum mechanical load
    • Stage 4: test up to failure

Routine tests:

Routine tests are performed to check the quality of standard production. All routine tests are performed internally in Saver, upon agreement between manufacturer and purchaser.

  • Bending test
  • Pressure test
  • Tightness test (in helium with mass spectrometer device)
  • 3D coordinate measuring machine
  • Visual inspection

Saver has already successfully fulfilled the homologation for here after listed applications:

  • 36 through 170kV Live Tank Circuit Breakers
  • 245 through 800kV Live Tank Circuit Breakers
  • 72 through 362 kV SF6 to air bushings for Dead Tank Circuit Breakers
  • 72 through 800kV instrument transformers
  • Cable terminators
  • Surge Arresters

In order to increasingly competitive in the manufacturing of hollow composite insulators, Saver has decided to dedicate one plant to High voltage applications with highly automated industrial production capacity of about fifty-thousands hollow composite insulators per year.

Many thousands of our hollow composite insulators have already been installed and in service for many years in many different worldwide geographical areas and severe service conditions.

ELECTRICAL FIELD ANALYSIS:

  • Optimization of insulator dimensions;
  • Design of internal / external shield;
  • Definition of composite insulator housing;
  • control of electric stress along the materials

Finite Element Method Analysis