Transformers originally filled with conventional transformer oil can be retrofilled with a fire-resistant oil to increase the fire safety margin of these units. Electrical service and repair companies have discovered this to be a valuable service to offer their customers.
Why Are Transformers Being Retrofilled?
Transformer owners are choosing to retrofill their units for a variety of reasons. The common
denominator between them is the need to increase the fire safety of the transformer. Because of
changing circumstances, building owners and utilities are often being advised by their insurance
companies or attorneys to protect their buildings or to reduce their exposure to potential liability
of explosion or fire.
Some of the most common reasons are:
1. Expanding a building: When a building is expanded or remodelled, a transformer that was once a safe distance from exterior walls may now be too close to use conventional transformer oil.
2. Changing regulations or fire codes: As building codes and insurance regulations change transformers may be reclassified or be required to meet more stringent fire protection guidelines.
3. Liability exposure: Transformers located near public roads or walkways may present an exposure to a potential liability that the owner would like to reduce.
Many times, a building owner will be faced with the requirement of constructing a barrier or enclosure around a padmounted transformer. Changing the dielectric fluid from conventional mineral oil to a fire resistant fluid is often a far less expensive option that may be acceptable to the regulatory parties involved. Retrofilling the transformer with a fire resistant fluid is an easy way to increase the fire safety margin of the unit, lowering the risk of fire or explosion.
Retrofill Fluids
The fire resistant oils discussed here a fire point of at least 300 degrees C. This is significantly higher than the typical 160 degrees C fire point of conventional transformer oil.
Look for the following characteristics when choosing which fire resistant fluid to use:
1. Choose a hydrocarbon fluid. There are several hydrocarbon fire-resistant fluids from which you can choose. Silicone-based fluids have not traditionally been used in oil retrofill jobs because of problems that may arise as residual oil leaches out of the core and coil of the transformer and mixes with the silicone fluid. Hydrocarbon fluids mix easily with this residual oil without any foaming or dielectric problems. In addition, hydrocarbon fluids are biodegradable.
2. Choose a fluid with lower viscosity. When comparing fluids, pay particular attention to the viscosity of the retrofill fluid. As the transformer was designed to be cooled with conventional transformer oil, it will run warmer with a thicker fire resistant fluid. Choosing a fluid with the lowest viscosity possible will minimize this problem.
There are two principal types of hydrocarbon-based fire resistant fluids: petroleum oils and synthetic oils. Petroleum based fluids are refined from crude oils. They are purified lubricant oils that have special additives. Synthetic oils are new developments of the past few years. They are manufactured from oils called PAOs (poly-alpha-olefins). Although they are compatible with normal transformer oils and petroleum fire resistant oils, they offer several advantages. They have much lower viscosities than petroleum oils, while offering the same degree of fire safety. They are clear fluids, which makes them easier to work with. In addition, they have much greater resistance to oxidation than do petroleum products.
Transformer Cooling
Transformers that were designed for use with conventional transformer oil will run warmer when
filled with a fire resistant oil. This is because of the higher viscosity of the high firepoint fluids.
Table One compares the characteristics of two fire resistant oils and conventional transformer oil.
Choosing a retrofill fluid with the lowest possible viscosity will minimize the temperature rise
associated with the retrofill. Typically, a transformer designed for conventional oil will run 2 - 6 degrees C
warmer after being retrofilled with a fire resistant fluid.
Residual Transformer Oil
A successful retrofill job depends on removing as much of the original fill transformer oil as
possible. A small amount of transformer oil will remain in the unit, saturated in the porous paper
and wood components. The majority of this residual oil will be replaced by the fire resistant fluid
within six months after the unit is retrofilled.
Mixtures of residual transformer oil and hydrocarbon based fire resistant oils will have good electrical characteristics. Because transformer oil is more flammable than the fire resistant oil, the mixture will have a lower fire point than the fire resistant fluid would by itself. If a 300 C firepoint is required, a second full or partial retrofill should be considered when the equilibrium between the two fluids has been established (approximately six months). Approximately 50 per cent of the units retrofilled will require a second drain and fill procedure because of the lowered firepoint of the mixture.
Retrofill Procedure
The procedure to retrofill conventional mineral oil with a fire resistant oil is relatively simple and
straightforward.
These are some of the key points to be used in retrofilling electrical equipment originally filled with PCB fluids or conventional transformer oil. This list should be used as a guideline; it is not intended to be a complete list of all procedures that may be needed. Of course, all work should be done in accordance with applicable regulations and good engineering practice.
Key Steps In Retrofilling
1. Access the unit in accordance with owner's regulations. Make sure that the unit is de-energized.
2. Ground all equipment (transformer, pump, tanks, etc.) to control static discharge while you are working.
3. Perform transformer insulation tests (at minimum, a "megger" test @ 2,000 volts d.c.)
4. Discharge the transformers high voltage windings and cables.
5. Reground the transformer windings.
6. Drain the existing oil.
7. Allow a minimum time of one half hour for transformer oil to drain out of the core and coil.
8. Using a small pump and hoses, manually flush the interior of the unit with warm retrofill fluid (5 per cent of the unit's oil volume is recommended). The procedure will be easier if the retrofill fluid is warmed to at least 100 degrees F. Be sure to flush down the core and coil if possible. Try to wash as much of the original fluid as possible out of the unit. Discard this flush fluid and replace the manhole as soon as possible.
9. Allow the unit to drip for 30 minutes; then vacuum or pump the remaining fluid from the bottom of the tank.
10. Replace gaskets if needed (high firepoint hydrocarbon fluids are compatible with gaskets used with conventional transformer oil).
11. If the transformer is rated for full vacuum, apply a vacuum of 30 mm Hg on the unit.
12. Begin the retrofill, with warmed fluid, if possible.
13. Filter the retrofill fluid through 5 micron filters as it is being pumped into the unit.
14. Wait before performing the next insulation tests. This gives air bubbles an opportunity to rise to the top of the fluid. The wait time is dependent on the fluid's temperature. Four hours wait time at a fluid temperature of 50-80 C is recommended.
15. Perform another set of insulation tests, as in step 3. If the test value has decreased, investigate to determine the cause.
16. Wait again before energizing the unit. This gives the retrofill fluid time to saturate any porous materials that may have become dry during the process. The wait time is dependent on the temperature of the retrofill fluid. Twenty four hours wait time is recommended.
17. Observe the unit for leaks during this wait time.
18. Energize the unit without load.
19. Wait three hours minimum after energizing, before adding the load.
20. Apply the load.
21. On the following day, check the unit's temperature and pressure, observe it again for leaks and perform other standard observations and check.
22. After the retrofill, follow standard maintenance intervals and procedures. Pay close attention to possible leaks from any old gaskets that were not replaced.
Conclusion
Retrofilling a transformer from conventional mineral oil to a fire resistant fluid can significantly
increase the fire safety of electrical equipment. Transformer service companies can solve problems
for their customers by performing this procedure, thus providing a valuable service.
Transformers that were designed to use conventional transformer oil will run slightly warmer with fire resistant fluids. Choosing a hydrocarbon based fluid with low viscosity will ensure that this temperature rise is kept to a minimum.
When performing a retrofill, remove as much of the residual transformer oil as possible. Transformer oil that leaches from the paper and wood in the unit will mix with the retrofill fluid, possibly lowering the fire point. The fluid should be tested in six months to determine if additional work is needed.
Dr. David Sundin is with Dielectric Systems, Inc. located in Milwaukee, WI.