Pre-commissioning insulation requirements

Choice of equipment and methods of transformer oil treatment and the extent of such treatment are defined by general requirements to the insulation of a transformer. These requirements define:

  • Dielectric strength of insulation;
  • Initial aging conditions.

Some degradation of insulation (as compared to factory specifications) is expected. Therefore, the transformer must be able to withstand loads of 85% - 90% of full test loads. On the other hand, all measures must be taken to keep the insulation at factory quality level.

Vacuum treatment of transformer core

The objective of this operation is two-fold:
1. Evacuation of air from the transformer prior to oil filling;
2. Drying of insulation surface after contact with air.

Vacuum pressure and duration depend on the desired degree of dehydration. Since moisture concentrates in the surface layers, it spreads both outside and inside insulation during vacuumizing. If vacuum duration is equal to the duration of moisture exposure at the same temperature, only about 60% of absorbed moisture can be removed.

With no special heating, the maximum concentration of moisture in surface layers may be reduced, when reaching equilibrium, to 2 – 4% at residual pressure of 1 – 5 mm. Hg.

Insulation degassing
Lengthy contact of insulation with gas and its permeation into the insulation require a long degassing process.
Vacuumizing for 16 – 20 hours at 3 – 5 mm. Hg. at 10 – 20oC cannot sufficiently degas insulation – gases are emitted into the transformer oil during operation.

Transformer heating


Two objectives are reached when heating a transformer:

  1. Heating a transformer in transportation configuration to 30 – 40 oC protects it from moisture before lid opening;
  2. Heating of assembled transformer to 60 -100 oC dehydrates insulation and improves its status estimation conditions.

Methods of heating a transformer in transportation configuration are as follows:

  1. Filling the tank with pre-dried and pre-purified oil so as to cover the coils entirely.
  2. Oil tub method: filing the oil above the top yoke and heating the oil by circulating it through an oil heater.
  3. Heating the transformer with external steam of electric heaters under the bottom of the tank
  4. Heating with losses inducted in the tank’s walls by a special temporary coil.
  5. Circulation of oil through the oil heater (for transformers stored with oil loaded).

Methods of dry heating transformers:

  1. Circulation of oil heated to 80 - 100 oC through oil heater.
  2. Spraying of hot oil: the transformer is filled with oil to the level at which the lower yoke is covered, the oil circulates through the oil heater and is sprayed to the coils in vacuum.
  3. Heating by intrinsic losses in short-circuit mode.
  4. Heating by coil dissipation from direct current.
  5. Heating by intrinsic losses in short-circuit mode while powered by an underfrequency source.
  6. Purging of empty transformer tank with hot air (above 100 oC).

Insulation overheating is possible during heating by intrinsic losses from alternating current circulating in the coils in short-circuit mode or from direct current. Average coil temperature is usually limited to +95 oC, however, it may increase significantly in some areas.
It is possible that the oil level indication is not accurate enough, while heating the transformer by currents circulating in the coils when the coils are covered with oil. Serious overheating and even damage to insulation of the upper part of the coils are possible as the result of the oil level being lower than expected.
For all methods, the primary heat carrier is the oil with heating constant of several hours. To facilitate accelerated and uniform hearing of oil, it must be constantly agitated at flow rate, depending on the oil’s volume.
During heating by spraying hot oil in vacuum, insufficient convection may lead to significant deviation of temperatures in parts of insulation which are subjected directly to the sprayed oil, to the remaining insulation.
Heating is only efficient is the transformer tank is thermally insulated.

After transformer transportation, storage and installation, as well as violation of regulations pertaining to the above, the measured insulation characteristics may indicate its unacceptable condition. In this case, insulation process treatment is required, e.g. drying.
Below is a brief list and description of drying transformers prior to commissioning.

Circulation of hot dry oil

Oil heated to 80 – 85 oC circulates through a drying system (vacuum degasser) for filtration and drying. The drying process is as follows: moisture is diffused from the outer layers into relatively dry oil and is subsequently removed from the oil in the dryer. In theory, the oil can be dried to 1 – 1.5% (depending on water solubility in specific oil), if: oil average temperature is 70 – 75 oC, the temperature is uniform throughout the oil volume (may require intensive agitation) and moisture content in the oil is below 10 ppm. The method’s efficiency is significantly reduced when temperature of oil in the tank drops.

Cold vacuum method

Transformer is heated in vacuum at residual pressure of 0.05 – 0.1 mm. Hg., insulation temperature must remain above 20 oC. The method requires using a special cold trap to freeze-capture water vapor, which increases vacuum pump efficiency and facilitates additional diffusive extraction of water vapor from the tank. Drying can be stopped when water separation in the trap reaches 3 – 5 g per hour per 1 ton of insulation. Equilibrium moisture content graphs indicated that reaching 0.5% moisture content in solid insulation, requires vacuum of at least 0.1 mm. Hg. and temperature of no more than 30 oC.

Thermovacuum diffusion

This is a technology of drying insulation of 110 – 750 kV transformers. The process is illustrated in the figure below.
Transformer is heater to 80 – 85oC for heating insulation in one of the two methods:

  • Circulation of hot oil, or
  • Circulation of current in the coils.

The then oil is drained in vacuum, and the transformer is vacuumized for up to 48 hours at residual pressure corresponding to the moisture content as per equilibrium moisture state, until separation of water in condensation tower stops. The process results are considered satisfactory with 1 mm thick samples have moisture content below 1%.

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1 — vacuum booster pump, 2 — buffet tank 0.05 m3; 3 — mechanical vacuum meter, 4 — vacuum meter, 5 — transformer, 6— oil pump, 7— oil drain tank, 8— stop valve.

Oil spraying

The transformer is heated to approximately 100oC (see figure below) at residual pressure below 5 mm. Hg. (second figure below).
One of the ways is to treat the transformer for several days to reduce emission of water in the condenser to below 50 gram per hour per 1 ton of insulation; the drying process then continues at 85oC to reach 5 g per hour per 1 ton of insulation.

Another option is to spray oil at residual pressure of 1 – 2 mm. Hg. until condensation stops entirely. The method may be applied while drying very moist insulation.
Moisture content in insulation samples must not exceed 1% following the drying.

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Transformer heating: 1 — transformer; 2 — thermal insulation; 3 — oil sprayer; 4 — collector; 5 — flexible oil hose; 6 — Dn 100 oil line; 7 — transformer oil; 8 — valves; 9 — Dn 125 connector; 10 — oil pump; 11 — oil heater; 12 — signal thermometer; 13 — filter; 14— Dn 100 valves

Transformer vacuumizing: 1 — mechanical vacuum meter; 2 — electronic vacuum meter; 3, 4— Dn 100 valves; 5, 12, 15 — Dn 100 vacuum line; 6 — transformer; 7— thermal insulation; 8, 10, 14— vacuum gate Dn 100; 9 — Cold trap; 11 — Roots blower; 13 — buffer tank 0.05 m3; 16 — vacuum booster.

Cycling transformer drying

The transformer is heated by spraying hot oil in variable vacuum and periodic hot air purges to reach insulation temperature of 80 – 90oC (depending on insulation temperature).
This heating is followed by two more stages:
1) vacuumizing at residual pressure of 0.15—0.5 mm Hg.
2) heating and rinsing by oil spray.
The vacuum system is connected to cold trap, where condensation is monitored. The process may take 3 – 8 cycles, depending on actual moisture content. This method allows drying of insulation with moisture content of 6 – 8%.

Drying the transformer with hot air

Clean dry air at 100oC passes through the transformer tank and heats the magnetic core and the coils.
Air consumption must ensure the minimal difference of temperature drop between input and output air. Recommended air flow rate is approximately 600 m3/hour per 1m2 of tank surface. This method is recommended for transformers which are not designed for complete vacuum.

Insulation drying process completion criteria

1. Reaching of set temperature in most of the moisturized insulation areas and reaching of residual pressure corresponding to the moisture content limit in equilibrium conditions.

2. No more condensate in the trap, or stabilization of condensation at the rate of 3 – 5 gram per hour.

3. Stabilization of insulation impedance on the level of dry insulation.

Oil filling and saturation

Filling the transformer with oil and saturation of insulation with oil is a complex and important process of complete extraction of air and its replacement with air in the transformer and in insulation pores.

When filling the transformer with oil in vacuum, a particularly dangerous condition is inadequate tank sealing, since water condensate may form from the air during rapid expansion in vacuum.

Vacuum tightness is normally tested by measuring residual pressure in the tank after pump stop. Sealing is considered adequate if pressure increase does not exceed 5 – 20 mm hg.

In practice, the increase of pressure in dry sealed transformer with 60 tons of oil does not exceed 1 mm hg per hour. Regardless of pressure change in the tank, it is not recommended fill the oil if it is raining.

After vacuum treatment, e.g. residual pressure of 5 mm hg, approximately 0.65% of air remains in the transformer, and this value is too high, considering that the volume filled is several dozen cubic meters.

Filling the tank with oil should be a slow process, to replace the air in gaps, connections, etc. Air remains in pockets surrounded by insulation and other materials, and can only be dissolved later on. The processes of filling narrow gaps and large capillaries in the insulation are identical: it is the viscous movement of oil due to pressure difference on the inside and outside of the capillary. However, if the pressure in the gaps is equal to the residual pressure in the tank, the pressure in insulation capillaries can be significantly higher, when insulation has not been sufficiently dried and degassed.

The lower the oil’s viscosity, the higher the saturation rate; therefore the oil’s temperature should be as high as practical during filling. Oil treatment is recommended after complete assembly of the transformer and filling it with oil, sometimes with cooling system pumps on. The oil is filtered, degassed, dried and heated to a certain temperature.

In some cases, this process is considered as a supplementary stage of insulation degassing with final dissolution of air bubbles. Process duration is usually defined by the condition of treating the total volume of the oil at least twice.

Transformer oil treatment

Any oil, even if it was perfectly purified by the manufacturer, requires drying and degassing, since it is virtually impossible to prevent water and air from entering the oil.

The oil may contain particles which enter the oil during transportation. In some cases, the oil may contain particles of sorbent which were not entirely removed in the final stages of production. Sometimes inadequate cleaning of transformer tanks may cause colloids to enter the oil, which reduce its dielectric performance, lower the flashpoint etc. The required transformer oil treatment methods are: drying, degassing, filtration through mechanical filters and, in some cases, filtration through absorption filters.

Oil drying methods

The following methods are usually applied to drying transformer oil:
1. Thermal vacuum diffusion drying and degassing. This is the most common method.
2. Filtration through dry paper filter with simultaneous drying and removal of solid contaminants. The drawback is the necessity of drying the paper (e.g. to reach moisture content of 10 ppm, the paper moisture content must not exceed 2 – 3%) and the danger of paper decomposition under the influence of moisture with subsequent contamination of the oil.
3. Absorption drying using zeolite.
Using this method, up to 1000 tons of oil can be dried before the sorbent needs to be recycled. Zeolite systems are a lot cheaper than vacuum degassers. They are used when degassing is not required. Depending on the kind of oil, it may contain 9 – 11% of air and 3.5 – 5% of water (volume). Therefore, to reach air content of 0.1% and less than 1 – 2% of water (less than 10 ppm), up to 16% of vapor-gas mixture must be removed from the oil. This reduces air content by two orders of magnitude, while moisture content only drops by 3 – 4 times. However, since the significantly lower diffusion of water vapor in the oil, its removal can be a challenging task.

Vacuum degasser works in two stages. In the first stage, the oil foams in vacuum, this facilitates intensive diffusion of gases and water vapor. During the second stage a relatively slow diffusion of gases from a thin layer of oil is ensured by vacuum created in the system.

To accelerate the diffusion in the second stage, higher vacuum and more intensive oil heating is required; process efficiency changes with temperature and flow rate of the oil.

Removal of solid contaminants from transformer oil

Ensuring adequate purity of transformer oil is no less important than deep dehydration. Harmful effects of high moisture content mostly manifest themselves in increased conductivity and impurity concentration.

Oil in a new transformer may contain particles which come either from production (cellulose fibers, dust, microscopic particles of iron and copper), or from installation (sand, sorbent particles, dust and dirt from the atmosphere etc). In several cases, filtration systems were the source of contamination, such as old oil residue with fine carbon particles etc. The biggest challenge is presented by the need to efficiently remove microscopic (smaller than 25 micron) particles from the oil. This is important for both fresh oil (when adding oil) after the oil has passed through a low quality filter, and also, and especially so, when purifying oil in transformer tank. In this case, small particles, which move relatively slow through the oil, may be carried by convective currents past the filter.

An indirect check by measuring breakdown voltage of transformer oil and estimation of its variation coefficient may be indicative of presence of large, visible particles (larger than 50 – 100 micron) or a large number of particles. Measuring particle mass after filtration through a diaphragm is problematic when particle quantity is less than 50 – 30 gram per ton.

A more informative method of quantitative particle composition analysis is particle dispersion analysis.