Q: I’ve heard that the oxidation stability of vegetable oils used in transformers is generally lower than mineral oil. Is this a significant issue?
A: Thin films of vegetable oil, exposed to heat and air flow, do oxidize much faster than mineral oil. However, in ANSI/IEEE standard designs (not the free breathing type often used in European designs) the exposure to oxygen is insignificant. Additionally, the oxidation byproducts of mineral oil are much more harmful to the performance of a transformer than those from vegetable oil. FR3 fluid also contains a food grade preservative. Accelerated life tests, Doble Engineering PFVO tests and data from the field all show FR3 fluid very suitable for the application.
For more details, please refer to reference information entitled Oxidation Stability of Envirotemp® FR3 fluid, Cooper Power Systems Document R900-20-2.
Q: But competitors claim that their fluid can pass an ASTM Oxidation Stability test. Shouldn’t I be concerned that FR3 fluid turns to gel?
A: No, for several reasons:
- The ASTM method was developed to test mineral oils, not vegetable oils. And even for mineral oils, ASTM states that there is no claim of correlation between the test results and field performance.
- To achieve the better test results in the ASTM method, two very important key fluid properties are negatively impacted: both the pour point and viscosity are increased.
- Doble Engineering recommends their oxidation stability test (PFVO). In that test, despite competitive claims of superior oxidation stability, FR3 fluid excelled. The FR3 fluid testing, as reported by Doble Engineering (2002 Int’l Conference), produced no detectable sludge.
- No supplier of vegetable oil-based dielectric coolants is recommending use of their products in free breathing Euro-designed transformers (use is not recommended for mineral oil units either). For more details, please refer to reference information entitled Oxidation Stability of Envirotemp FR3, CPS Document R900-20-2.
Q: If FR3 fluid biodegrades so quickly, should I be concerned about the potential for this material breaking down inside the transformer during a normal product life span?
A: We have no more concern for this than we do with mineral oil. Even though mineral oil has a much slower biodegradation rate, it also biodegrades. Even with transformers lasting 40 plus years, biodegradation has not been a problem. The internal conditions for biodegradation are lacking: this includes free water, unlimited oxygen, oil-eating microbes and thin layers of fluid.
Q: I’ve heard that FR3 fluid-filled transformers typically produce higher transformer insulation power factor readings than mineral oil filled units. Is this a significant concern?
A: No. Insulation power factor readings of new mineral oil units are sometimes used as an
indicator of moisture content in the insulating paper. For operating mineral oil-filled
transformers, power factor readings are an indicator of the amount of polar contaminants
within the insulation system.
It’s important to remember that the energy loss associated with power factor is
insignificant compared to core and winding losses. The more important application is
comparing significant changes between power factor readings over time.
Because natural ester fluids have inherently higher dissipation factor values than mineral oils,
the insulation system using the esters will result in inherently higher power factor values as
well. If a power unit has power factor (p.f.) of 0.5% in mineral oil, the resulting p.f. with FR3
fluid would be approximately double that value. The difference with distribution transformers
would be smaller. The industry first experienced inherently higher power factors with askeral
transformers; p.f. values on these units would often reach into the mid-teens, even for
distribution with our performance issues class. For more details, please refer to the reference
information entitled: Power Factor Discussion, CPS Document R900-20-1.
Q: I’ve heard a lot about transformer life extension using FR3 fluid. How can it do
that? What proof do you have?
A: The consensus in the industry is that there is a direct correlation between transformer life
and paper life. Paper aging rate is primarily related to operating temperature. When paper
ages, it produces moisture and de-polymerizes. This results in reduced dielectric performance
and weaker physical strength. Once this happens, the transformer is less tolerant to voltage
impulses and through-faults. Moisture in paper increases the rate of degradation.
FR3 fluid slows down the aging process in several ways. First, FR3 fluid extracts more
moisture from the paper than mineral oil, which has a much higher moisture saturation value.
Second, FR3 fluid removes dissolved moisture by hydrolysis. Finally, the by-product of that
process actually provides additional protection to the aging paper through a process called
transesterification. FR3 fluid essentially takes the compounding effect of moisture out of the
aging equation.
IEEE C57 Standards indicate that when a new insulation system is proposed, it must pass the
accelerated life tests of C57.100. The standard includes a full scale test using multiple
transformers and a small scale test using multiple samples. The results of this testing showed
the insulation system with FR3 fluid out-lasted the mineral oil system (4 to 8 times, at the
same operating temperature). Peer-reviewed technical papers on the remarkable results of
both tests are published by IEEE and are available from Cooper Power Systems.
Q: Is it correct that FR3 fluid has limitations due to cold-temperature operation?
A: It depends on the issue. With a -21° C pour point (similar to R-Temp® fluid), FR3 fluid
has higher temperature limits on power switching than standard mineral oil. Typical limits
when the switching contacts are immersed in the coolant are -10° C.
Increased viscosity is another issue that needs to be addressed when handling FR3 fluid at
colder temperatures. Cold starting units, with continuous pumping as well as physical jarring
of de-energized transformers, should be avoided when temperatures are below -21 °C.
On the other hand, full load cold start of distribution transformers, chilled to temperatures
between -40° C and -30° C, showed no signs of thermal runaway. R-Temp fluid units have
been functioning for Arco, Alaska, near the Arctic Circle, without any reported problems. Many
FR3 fluid units have also been operating in very cold climates with no reported problems.
For operation of power transformers with LTCs in very cold weather (-10 °C), we recommend
the use of our synthetic ester fluid, Envirotemp 200® fluid in the LTC chamber. The pour point
of Envirotemp 200 fluid is below -50° C.
Q: I don’t need or want longer life from my transformers. I want to drive them harder.
Can I?
A: Probably. Our proven improved paper aging rate allows a choice between longer life,
higher overloadability, or a combination. Based on the data obtained from the C57.100 tests
described above, the temperature rise for equivalent life between the two insulation systems
is 21° C. Assuming all the other materials can withstand the higher operating temperature,
and there is no practical danger to the public regarding touch temperature, a unit designed
with FR3 fluid can run at an 86° C rise vs. a 65° C mineral oil designed unit. This represents
approximately a 20% increase in its rating. For oil retrofills, the increase is somewhat
reduced due to the higher viscosity of FR3 fluid. The increase in rating will vary from
approximately 14% to 19%, depending on the transformer size and design.
Q: Does the aging rate of insulating paper in older mineral oil-filled transformers
change after retrofilling? And, is the aging rate improvement comparable to that seen
in accelerated aging tests using new units and paper?
A: Yes, the C57.100 small scale tests were performed using paper initially aged in mineral oil
and then further aged in FR3 fluid. The aging rate changed immediately and the water
moisture content of the paper dropped to a low level within 500 hours.
Q: How much life extension can I expect after retrofilling with FR3 fluid?
A: While there can be no exact answer to the question due to many variables, we do know
that the paper aging rate will reduce by a factor between 4 to 8 times. We also know that the
moisture level of the paper will immediately start to drop, improving the dielectric performance
of the paper. After reviewing our aging data, a major IOU determined that, on average, the
remaining life will at least double.
Q: Are there other benefits in retrofilling besides insulation life extension?
A: Yes, particularly in the areas of enhanced fire safety and improved environmental
performance.
In comparison to mineral oil-filled transformers, FM Global recently reduced the separation
distances as little as one tenth. This is based on their contention that a pool-oil fire is all but
impossible with FR3 fluid. Other advantages over mineral oil include reduced tendencies for
coking, gassing and sludging.
Q: What is the recommended procedure for retrofilling?
A: The answer depends on the size, voltage, type of transformer and permissible down time.
We do recommend contracting the services of a certified power transformer retrofiller, such as
Waukesha Electric Systems Services, for medium and large units. For small power units,
distribution units, and for the initial retrofills, at least, we recommend contacting CPS Services.
Those directly involved with the retrofilling operation should become familiar with both the
Envirotemp FR3 Storage and Handling Guide, Bulletin No. 99048, and the Envirotemp
FR3 Retrofilling Guide, Bulletin No. 00046.
Q: How does CPS warrant retrofills with FR3 fluid?
A: Currently we offer our standard warranty which guarantees that our fluid will meet its
acceptance values. The condition of the transformer and the retrofilling process determines
the success of the retrofill. CPS will also work with you to structure extended warranties for a
premium. Contact the Fluids Products Group for additional information and updates.
Submit your specific question here.
Call the Ask-A-Fluids-Engineer Hotline 800-643-4335. (Monday through Friday 8 a.m. to 5 p.m. Central Time).
Q: How does FR3 fluid perform in LTCs? Are any special precautions recommended?
A: FR3 fluid works well as an LTC fluid under most conditions. It has been tested and
approved for use in the Cooper Power Systems' Quik DriveTM LTCs used in voltage regulators. Tests, under load
break operations, have shown FR3 fluid to better maintain its dielectric strength than mineral
oil.
There are two precautions: At temperatures below -10° C, the LTC may not meet its ratings.
Either a temperature sensitive lockout or the application of the synthetic ester, E-200, is
recommended. The other issue is that many LTCs, especially the modern units, tend to be
free breathing design. We strongly recommend using E-200 for such designs and making sure that a
moisture desiccant is provided and properly maintained for either fluid.
Q: If FR3 reduces paper aging, why not increase the allowable transformer hot spot
temperature rise for new units?
A: The IEEE standard for distribution transformers is 65° C rise, and in some cases 55/65° C.
Since 1999, Cooper has offered FR3 fluid-filled transformers for both ratings. In 2004, Cooper began
offering higher-rise Envirotran® products for use in substation and pole applications where there
will be no public contact potential. Even though IEEE testing with FR3 fluid indicates that
paper ages the same rate at 21° C higher temperatures than in mineral oil, other materials
need to be analyzed to insure they can also withstand the higher operating temperatures.
Q: Can we use the same equipment for handling and processing mineral oil and FR3
fluid? Are the fluids miscible? Are any special precautions recommended?
A: FR3 fluid is miscible with mineral oil and does not create the problems associated with
trace contamination of silicone in mineral oil. While it’s obviously advisable to totally avoid
blending of dielectric materials, it takes more than 7% mineral oil contamination to reduce the
fire point of FR3 fluid. It’s especially important to avoid cross contamination to maintain the
exceptional environmental properties of FR3 fluid. Flushing the mineral oil processing
equipment and hoses with FR3 fluid is recommended. FR3 fluid is also miscible with
synthetic esters, synthetic hydrocarbons (PAOs) and R-Temp fluid. Refer to Bulletin 99048,
Envirotemp FR3 Fluid Storage and Handling Guide, for additional information.
Q: Is FR3 fluid compatible with the other materials used in the transformer?
A: To date, we’ve concluded that FR3 fluid has equal or less effect on conventional
transformer materials than mineral oil. A power transformer manufacturer has reported similar
results with their materials. Aged, brittle gaskets should be replaced whenever practical. If not,
additional tightening may be required to seal the gaskets.
Q: Some of our power transformers have conservators that are free breathing design.
Can we retrofill such units with FR3 fluid?
A: Although we’ve successfully used FR3 fluid in some free-breathing transformers for over
5 years, we don’t advocate this practice until we have further data. Units with free-breathing
conservators should be retrofitted with bladders.
Q: Can FR3 fluid be used to retrofill an askarel (PCB)-filled transformer?
A: Yes, successful retrofills of askarel transformers have been reported. However, due to the
leaching of residual PCBs, the environmental attributes of pure FR3 fluid are impacted and if
enough PCBs leach from the paper, the unit may still be classified as a PCB transformer.
Also, if the unit was specifically designed to use the low-viscosity askeral material, resulting
temperature rise (at a given load) will be higher than that of a retrofilled mineral oil unit.
Q: We rely on DGA (Dissolved Gas Analysis) as a key preventive maintenance tool. Can
I use the existing DGA methods with FR3 fluid?
A: DGA has proven effective with FR3 fluid for condition diagnostics. The same fault gasses
are produced. IEEE C57.104 DGA Guide is applicable, particularly the “condition” and “key
gases” methods. Since we do not have a “reported field failures” database, or ratio methods
for determining type of failure, these diagnostic processes are not recommended at this time.
The same sampling methods and equipment are appropriate for FR3 fluid, although the
“headspace” is easier and preferred over the “direct injection” equipment. Since we do not
have any field failures to obtain data from, the ratio diagnostic methods can not be verified as
applicable for FR3 fluid.
