Material Selection Study
Electrical Components: Transmission & Distribution
A-96/239
®
Zytel HTN
®
high temperature nylon resin
Cooper Picks a Winner for Transformer Bushing Well
Cooper Power Systems has again proven its
leadership in distribution transformer components
with a new high-voltage bushing well made of
DuPont Zytel® HTN high temperature nylon resin.
Zytel® HTN is a newly developed thermoplastic
polyamide polymer. Introduced in 1994, it is the
latest advance in nylon polymers from DuPont, the
people who invented nylon nearly 60 years ago.
Cooper’s selection of Zytel® HTN came after
exhaustive studies of the performance of a range
of thermoset and thermoplastic materials. Cooper
and DuPont Engineering Polymers worked as joint
development partners in this effort.
Why not a thermoset?
Cooper had been making bushing wells from
thermoset epoxy resin since the late 1960’s. So
why change? To continue meeting the needs of
utility and other customers for reliable components
at competitive cost, Cooper management realized
that they needed a material that would perform
better than thermosets in several ways. These
included:
■ Improved resistance to cracking and chipping
during handling.
■ More uniform mechanical properties.
■ Toughness and consistent properties to allow
lighter, less bulky designs.
■ Recyclability to permit efficient reuse of
molding scrap.
■ More economical production.
For the past decade, more and more manufacturers
of motors, relays, solenoids, transformers and
various other electrical products have been replacing thermosets with engineering thermoplastics.
Start
with
DuPont
H-66308
New bushing well made of Zytel® HTN meets
needs for reliable performance in hot oil and humid
high-temperature environments.
More recently, Cooper Power Systems has taken
the lead in adopting engineering thermoplastics for
components used in power transmission and
distribution equipment. For fuse assemblies, tap
changer switches and tri-clamp bushings, Cooper
relies on such DuPont polymers as Rynite® PET
thermoplastic polyester resin, Zytel® nylon resin,
and Zytel® HTN high temperature nylon resin.
Evaluating candidate materials
For its new bushing well, Cooper required a
material that could maintain strength, toughness
and dielectric properties under the severe conditions encountered in service. Bushing wells are
exposed to oil and air at temperatures in excess of
100°C. Relative humidity can reach 100% when
the transformer enclosure is flooded.
Cooper’s evaluation of material engineering data
and its own testing resulted in the selection of
Zytel® HTN because it met all requirements with a
wider margin of safety than alternative polymers.
(continued on next page)
Superior high-temperature properties
Figure 1: Temperature Effect
On Dynamic Modulus
Storage Modulus (GPa)
10
■ Zytel® HTN
● Nylon 66
▲ PBT polyester
8
6
Figure 2: HDT vs. High-Temperature Tensile
Heat deflection temperature @ 1.8 MPa (264Psi) °C
Figure 1 compares the effect of temperature on the
dynamic modulus of several glass-reinforced engineering thermoplastic materials, including Zytel® HTN,
nylon 66 and PBT thermoplastic polyester. Amounts of
glass used are typical of electrical component and other
applications. Dynamic modulus is a measurement of a
material’s stiffness. Good stiffness at elevated temperatures is important if bushing wells are to resist deformation at abnormally high temperatures that occur during
electrical overloads.
270
®
Zytel HTN
260
250
Nylon 66
240
230
PET polyester
220
Thermoset epoxy
210
PBT polyester
200
0
50
100
150
200
Tensile strength @ 100°C, MPa
Data shown are for glass-reinforced resins with glass contents as follows:
Zytel® HTN, 35%; Nylon 66, 33%; PET polyester, 30%; Thermoset
epoxy,30%; and PBT polyester, 30%.
4
their dimensions well when temperature changes. Its
coefficient of linear thermal expansion is less than
0.5 x 10-4/°C.
2
0
0
40
80
120
160
200
240
Temperature (°C)
Method: DMA. Condition: DAM. Note: Data shown are for glass-reinforced
resins with glass contents as follows: Zytel® HTN, 35%; Nylon 66, 33%; and
PBT polyester, 30%.
The dynamic modulus of Zytel® HTN is the highest of
the materials over the full temperature range, and it is
nearly constant until the temperature reaches 120°C.
These data suggest that Zytel® HTN can provide a much
greater safety factor than the other materials for parts
subjected to mechanical load at high temperatures.
Zytel® HTN is virtually unaffected by mineral oil, an
important requirement for bushing wells. After 1,000
hours of exposure to mineral oil at 100°C, tensile
strength was 97% of the original value, elongation was
100% and flexural modulus was 99%.
Good high-temperature dielectrics
The dielectric strength of Zytel® HTN is relatively constant over the service temperature range of bushing
wells so that these components can provide good insulation even at overload temperatures.
The bottom line
Figure 2 shows that Zytel® HTN has a higher heat
deflection temperature and greater high-temperature
tensile strength than other candidate polymers often used
in electrical equipment. Like dynamic modulus, both
properties are important for good bushing well performance under load at elevated temperatures. Heat deflection temperature, a measurement of flexural strength, is
determined under a load of 1.8 MPa (264 psi).
Adding it all up, Zytel® HTN makes a bushing well
that’s quality engineered for long service life in distribution transformers. Superior toughness, strength, dielectrics, oil resistance, stability in a hot, humid service
environment plus attractive molding economics. Zytel®
HTN has it all.
Moisture, oil have little effect
Cooper Power Systems, Inc., Pewaukee, WI
Call: (414) 691-0070. Fax: (414) 691-1148.
®
Zytel HTN is much less affected by moisture than
conventional nylon and typical thermoplastic polyesters.
The relatively small changes due to exposure to moisture
won’t compromise bushing well performance. The
material’s tensile strength decreases by only 2% with
exposure at 50% relative humidity.
For more information on transformer components…
For more information on materials…
DuPont Engineering Polymers, Wilmington, DE
Call: (800) 441-0575.
Dimensional stability. After two years of exposure at
50% relative humidity at 23°C, 3.2mm (0.125in.) thick
specimens of Zytel® HTN picked up only about 0.7%
moisture. In addition, components of Zytel® HTN hold
DuPont Engineering Polymers