1) INTRODUCTION
Low voltage bushings are an important component of
distribution transformers, whether they are pole mounted
or pad mounted, and are often relegated to low priority.
These bushings, however, are responsible for the withdrawal of operation of many transformers in distribution
utilities, primarily because of leaks. These leaks generally
occur due to deterioration of the gaskets caused by the
exposure to sunlight, excessive tightening, or heating of
the cables’ connection terminals to higher than nominal
temperatures.
These substitutions add unnecessary costs to the distribution system, which could simply be avoided with the
application of thermoplastic bushings supplied by The H-J
Family of Companies, which include technologies that
prevent accelerated deterioration of the gaskets.
H-J's philosophy is that the bushings have a design that
allows their life span, including their component parts, to
be the same as the transformer’s, avoiding unnecessary
removal of the transformer before it completes its life cycle.
2) THERMOPLASTIC H-J BUSHINGS
2.1) Electrical and Dielectric Characteristics
The electrical and dielectric characteristics of low voltage
H-J thermoplastic bushings are as follows:
- Rated voltage (kV)
- Rated current (A)
- 160, 250, 400, 630, 800, 1000, 1250 e 1600
- Power frequency withstand voltage dry and wet, 1 min (kV)
- Rated lighting impulse withstand voltage (kV)
2.2) Constructive characteristics
- Insulating body
- BT melting temperature (°C)
- UV stable
- In accordance with ASTM G 154, 1500h
- Terminals
- Olhal ou Spade (2, 4 or more holes)
- Terminal type
- Gaskets
2.3) Constructive details
Figures 1 and 2 show details of the H-J thermoplastic
bushings with an eye and a spade type terminal, which can
be two, four or more NEMA holes for any rated current.
We, also, highlight the following:
- In H-J bushings only two gaskets are used in the
sealing system. One installed between the terminal and the
insulator and another installed between the insulator and
the tank;
- The gaskets are totally protected from sunlight, avoiding their deterioration for this reason;
- The H-J thermoplastic bushing uses the philosophy of
a tightening nut for each gasket. In this way, the lower nut, installed in the conductor,
tightens the gasket between the terminal and the insulation body. In turn, the thermoplastic inner nut tightens the
gasket between the insulation body and the tank.
Thus, uniform tightening of the gaskets is achieved,
helping to prevent leakage;
- It should be noted that for the gasket installed near the
tank, there is a compression limit (between 25 and 30% of
its width) given by the contact between the insulating body
and the tank, avoiding unnecessary compression;
On the other hand, the gasket between the terminal and
the insulator is installed by means of an automatic process,
without human interference, with torque control, which
guarantees an adequate tightening;
- H-J thermoplastic bushings withstand higher bending
loads when compared to porcelain bushings, supporting
strains that may be caused by the weight of installed
cables, thus reducing the possibility of leaks under these
conditions.
Figure 1
Figure 2
3) BUSHING ASSEMBLY
In addition to the aforementioned, it is important to note
that thermoplastic bushings are assembled at the HJ
factory in the United States, by a fully automated,
human-free system, with control of all steps of the process,
assuring total quality to the product.
The use of HJ thermoplastic bushings reduces the operating costs (Opex) of distribution utilities or users of transformers in commercial and industrial installations, as it
minimizes the need to withdraw from operation transformers that are in good operating conditions, significantly
reducing leaks in the secondary bushings.
This is due to the better protection from exposure to
sunlight and to proper compression control of the gaskets.
In addition, because the thermoplastic bushings withstand
much higher tensile stresses than porcelain bushings, the
stress on the gaskets is reduced, also minimizing the possibility of leaks.
Additionally, from the point of view of the manufacturer,
the bushing has the advantage of being supplied
completely assembled, tested and with all components.
This avoids the need to separately purchase parts and
components and minimizes assembly time on the transformer.
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