NAME : Urs Krüsi
COUNTRY : Switzerland
REGISTRATION NUMBER : 496
GROUP REF. : A2
PREF. SUBJECT : PS2
QUESTION N° : 2-3
Resin-Impregnated-Synthetics bushings – oil-free during whole life cycle
Contamination of the natural environment by transformer oil, as well as by other oils and
hydrocarbons has to be avoided. Thus, risks of any oil spill have to be minimized. The
utilities develop also contingency plans and procedures, including training personnel and
purchasing special equipment, to handle potential accidental spills and leaks of transformer
fluid. Application of oil, in case of an accidental failure of the power equipment, is also
related to a risk of fire.
Minimizing the extent of application of oil in power transmission and distribution equipment
is one of the remedies to the above problems. On the other hand oil- and oil-paper electrical
insulation have a number of advantages, such as being partial-discharge (PD) free, having low
loss factor (tanδ), and giving a possibility of effective heat evacuation by oil circulation. Dry,
oil-free solutions can only be applied if these advantages of oil insulation are not
compromised.
One example is the resin-impregnated-paper (RIP) bushing technology with dry insulator
filling, a low tanδ and PD free technology but not using any liquid insulating material.
In RIP insulation, as well as in the oil-paper, the low value of tanδ is achieved by the
application of a thoroughly dried paper in the insulating material. If the moisture reenters the
paper again, tanδ can increase to large values. The RIP transformer bushings in operation are
well protected from humidity penetration. On the air side of the bushing the porcelain or
glass-fiber-composite tube with a silicone insulator constitute a perfect humidity barrier. The
transformer side of the bushing, at which the paper insulation is exposed to the surface, is
immersed in dry transformer oil.
Apart of the installed bushings, utilities have a need of storing transformer bushings as spare
parts for years. For such long-term storage, the transformer side of the bushings require
special protection to prevent moisture contamination. This can be accomplished by storing a
bushing in a tank or tub filled with oil. Another solution is to use a special moisture-proof
wrapping with a moisture absorbing agent but this constitutes a risk of an accidental break of
in the wrapping material. The moisture absorbing materials themselves require regular checkups and may have to be replaced. Very often special oil-filled tanks are used to protect the
transformer side of the bushings during storage, as shown in Figure 1.
Figure 1: MICAFIL RIP bushings stored as spare parts for long time at one of the utilities in Switzerland
Paper A2 211 2012, New Paper-Free Insulation Technology for Dry High-Voltage Condenser
Bushings, describes the novel resin-impregnated-synthetics (RIS) technology in which the
dense paper is substituted by a fabric made of synthetic polymer fibers. Such fabric is
impregnated by particle-filled epoxy resin which is then hardened to form a solid dry
condenser core. The silicone rubber insulation is directly molded on the air side of the core.
The RIS bushings contain no cellulose nor any other material with high moisture absorption
ability. This fact, together with the application of the inorganic filler, impermeable to
humidity, frees the RIS core material from any sensitivity to a humid storage environment.
This has been proven in a test in a controlled environment of a climatic chamber at 40°C and
a relative humidity of 95% running for 205 days. This extremely high humidity conditions
correspond to almost 50g/m3 absolute water content in air. The result of the measurements is
shown in Figure 2. The tanδ has been stable, with no noticeable change, during the whole test
period. The initial room temperature value of tanδ is very low, less than a half of the
maximum of 0.7% allowed by the corresponding IEC standard. The value of tanδ measured
later on, during the test, is even lower as the loss factor of the applied RIS material decreases
with increasing temperature till its minimum occurring at around 40°C – 60°C.
This result shows that the RIS bushings can be long-time stored without a need of any
humidity protection. The RIS technology is thus oil-free not only in operation but also during
storage.
Dissipation factor as function of storage time (40°C, 95% RH)
Dissipation factor tanδ [%]
0.8
Maximum allowed by IEC 60137 (2008)
for a new bushing
0.6
RIS 123kV oil-air bushing at room
temperature prior to climatic test
at 40°C during climatic test
0.4
0.2
0
0
50
100
150
200
250
Storage time [days]
Figure 2: Time dependence of tanδ of a 123kV oil-air RIS bushing in extremely high humidity. Test procedure
and climatic chamber showed alongside.
The environmental impact of the new RIS bushings, calculated as the CO2 emission equivalent, has
also been estimated and a comparison was carried out with the conventional RIP technology. An
example of a 123 kV oil-air transformer bushing operating at a current of average 875A was taken.
The result of the analysis of the impact of the materials used in the bushing is given in Figure 3
showing the advantage of the new RIS technology with respect to RIP. This is mainly due to the lower
amount of materials of significant impact, such as aluminum and epoxy resin, applied in the RIS
bushing.
When the total lifetime impact is taken, as shown in Figure 4, then the operational electric loss related
impact dominates the total value and the difference between the two technologies is small. When
further compared to the losses of the transformer, the losses of the bushings constitute a very small
fraction and one can say that both the RIP and the RIS technologies are characterized by the very low
environmental impact with a slight advantage in favor of the RIS.
Total
Steel
Silicone rubber
Paper
Polyurethane
Polybutadiene
Synthetic fibers
Inorganic fillers
and glass fibers
Epoxy resin
Copper
Aluminum
RIP
RIS
Figure 3: CO2 emission equivalent impact from the materials used in 123kV RIP and RIS bushings.
RIP
RIS
Figure 4: CO2 emission equivalent of the impact from materials and losses over the lifetime of 25 years of
123kV RIP and RIS bushings.