Greg Steeves, P.E.
Baron USA, LLC
Transformer life depends heavily on the condition of the liquid and solid insulation. Effective drying, degasification and filtration practices, in manufacturing and during installation, ensure low initial contaminant levels. However, transformers experience rising contaminant levels over time. These can be detected by testing. The liquid insulation can be maintained and restored with purification and reclamation (sometimes referred to as ‘regeneration’). But, once in the field, the solid insulation is difficult to maintain directly.
Substation transformers often experience rising moisture and gas levels. The rate of accumulation often increases as the transformer ages. Wind farm transformers are particularly subject to gassing at accelerated rates.
Various technologies and techniques are available to maintain the transformer solid and liquid insulation (both directly & indirectly); even if the transformer cannot be taken out of service. Appropriate selection and application of transformer insulation treatment technologies and techniques can extend transformer life, reduce downtime and save money. These technologies, their application in manufacturing and the field, as well as the purpose and functions of cold traps will be discussed. An overview of the Vapor Phase process and why it is not normally used in the field will also be discussed.
Transformer insulation is broadly grouped into two categories: solid and liquid. Hundreds of pounds to several tons of solid insulation is present in the form of cellulosic paper and pressboard; tape, tubes and spacers. Cellulosic solid insulation has excellent dielectric properties when its moisture content is very low and it is thoroughly impregnated with liquid insulation.
Care must be taken to reduce the moisture content throughout solid insulation during manufacturing. Low moisture content must also be maintained once in operation because the material will absorb water from the environment including moisture in the air and leaks in gaskets. Insulation also produces water as a byproduct of decomposition over time. Solid insulation can absorb water to five percent of its own weight, or more.
Liquid insulation is available in various forms, each with particular application depending on where the transformer will be used. These include paraffinic and naphthenic mineral oil; natural esters, synthetic esters and other synthetics such as silicone, R-Temp and askarels/PCB’s (no longer used). Such liquid insulation, like the solid insulation, has an affinity for moisture and gasses.
Insulating fluids will absorb moisture and gas from the environment. These dissolve into the fluid until it is saturated. Saturation level varies with temperature, and from one fluid to another. In the presence of heat, such as localized heating or arcing, the oil will produce gases known as fault gases. Other byproducts may be produced in the presence of heat and oxygen such as sludge and acids. All such contaminants contribute to the lowering of the dielectric strength and the shortening of the transformer’s life.
It is well established that the rate of solid insulation aging increases dramatically with its moisture content. Therefore, it is important that it be thoroughly dried during manufacturing; typically to less than 0.5% moisture (and sometimes <0.1% – depending on voltage class). Since it will gain moisture once in use, re-drying may be required throughout its life to maintain reliability and extend life. It is typically recommended that moisture levels be kept below 1% in the solid insulation.
A wide variety of technologies and techniques involving filtration, heat, vacuum and adsorption are available and are applied in various combinations depending on size, voltage class, transformer location and other circumstances.
Removal of Contaminants – General
Particulate, water, dissolved gases and polar contaminants lower the dielectric strength of both the solid and liquid insulation and shorten their life.
Particulate and polar contaminants are the simplest to remove. Solid particulate can be removed from the liquid insulation by passing it through porous filter media. The liquid insulation can also be used to transport particles from the solid insulation for removal. Polar contaminants can likewise be removed by passing the liquid insulation through adsorptive media such as fullers earth or activated alumina where polar molecules cling to the adsorptive media and are removed.
Removal of water and gases are often related. Though absorptive media such as molecular sieve or polymer filters can absorb some water, they cannot remove gases. However, both water and gases can be removed using vacuum processing.
Significant water removal also usually requires the addition of heat. There are a variety of methods of heat and vacuum application available. They are applied differently depending on the circumstance. Various techniques and their application are discussed below.
Water Detection and Location
The presence of water and gases in an in-service transformer is usually detected by testing the liquid insulation (oil). Oil sampling and testing allows a number of tests for assessing the health of both the solid and liquid insulation. Dissolved Gas Analysis (DGA) can provide a great deal of information about transformer health. Most of the gases generated in a transformer will be found in the oil.
Karl Fisher titration is used to determine the water content of the oil. However, unlike gases, most of the water in a transformer is in the solid insulation. The water-in-oil measurement is valuable because it can be also used to estimate the moisture content of the solid insulation. To achieve this, it is important to know the temperature of the oil in the transformer at the time the sample is taken (see Figure 1).
Depending on temperature, more than 99% of the water in a transformer may be in the solid insulation. This is important to recognize when choosing equipment and applying drying methods. A quick treatment to remove the water from the oil will not have an effective drying impact on the solid insulation.
For example: an in-service transformer with 10,000 gallons of oil and 25,000 lbs. of paper that is tested to reveal 10 ppm water in oil and 3% moisture in the paper means that there is < 0.1 gallons of water in the oil. However, it also means that there are almost 90 gallons of water in the paper. A quick dry of the oil will have little effect on the total water content of the total solid/liquid insulation system. In short order, moisture will migrate from the wet paper to the dry oil such that the oil tests wet again.
The Importance of Heat and Vacuum for Drying
To efficiently remove water dissolved in oil and imbedded in paper insulation, it must be converted from a liquid to a vapor. Liquid water can be converted to a water vapor (gas) by increasing its temperature (x-axis) and/or lowering its pressure (y-axis) (see Figure 2). Usually, both will be used except for cases where water removal quantities are small and ambient temperatures are not low.
The conversion of liquid water into vapor requires > 970 btu/lb (enthalpy / latent heat of vaporization). As this heat energy is consumed and removed from the system with the water vapor, the temperature in the insulation will drop unless additional heat is added to replace it.
Therefore, both low pressure (vacuum) and heat addition play important roles in moisture removal.
The Role of Vacuum
- Lowers the temperature that would be required to vaporize water for removal.
- Vacuum stream provides a transport mechanism for the water vapor to be removed from the transformer.
- Reduces the amount of oxygen present which increases the safety of the drying operation and reduces oxidation (thermal damage) to the insulation.
- Removes dissolved gasses from the oil (no heat required) and from the gas space without eliminating the gas ratios important to DGA interpretation.
The Role of Heat
- Increases the diffusion rate of water through the solid insulation to a surface from which it can be extracted.
- Increases the water vapor pressure so it can be extracted as vapor.
- Replaces the latent heat of water vaporization. This reduces temperature drop which, in turn, reduces slowdown of the water removal rate and the risk of freezing.
- Dramatically increases the oil’s ability to absorb water if it is being used to transport water out of the system.
- Helps dislodge sludge into a soluble form that can be transported by the oil for removal.
- Increases the effectiveness of reclamation with, and reduces the pressure drop through, fullers earth or other bed percolation processes; when used for polar contaminant removal.
Techniques of Heat and Vacuum Application
There are a number of things to consider when choosing heat and vacuum drying tools and techniques. Generally, the most efficient method is desired for a given circumstance with the economics of the needed equipment and the impact of potential power outages in view.
- Water removal from solid insulation is most efficiently done prior to impregnation with oil. Therefore, in a manufacturing situation, the process can be designed to perform thorough and efficient drying prior to impregnation so that the drying step is not the bottleneck in production. For transformers in service, solid insulation is already impregnated with oil.
- Transformers in a factory setting allow for more options for heat and vacuum application than in the field. The environment can be more closely controlled. Un-tanked active parts may also provide additional process options not available after tanking.
Heat Addition Methods
All three modes of heat transfer to the active part shall be considered. Each has an application and limitations:
I. Convection Heat Transfer: movement of heat from one place to another through the circulation of fluids such as:
a. Hot Air Circulation: uses fans and air heaters to transfer heat into the active part with air as the transport mechanism.
i. Benefits: can be used on tanked and un-tanked active parts.
ii. Limitations: cannot be done simultaneous to vacuum (air removal); presence of oxygen at elevated temperature increases risk of thermal damage to paper; difficult to apply in the field due to limits on accessibility to the active part – lack of suitable connection points on the transformer tank.
b. Hot Oil Spray: uses pump and heaters to circulate transformer oil over or around the active part using oil as the heat transport mechanism.
i. Benefits: can be used on tanked and un-tanked active parts; can be usedon an impregnated winding; can be applied to add heat while also pulling vacuum.
ii. Limitations: for initial drying of an unimpregnated assembly, oil will soak the outer paper layers slowing or impeding removal of deeply imbedded moisture with potential reduction in drying efficiency and lengthening of the shorter potential drying time.
c. Vapor Phase: hot liquid or vaporized solvent such as a derivative of kerosene (eg: Isopar) is fogged or sprayed against the active part while pulling vacuum.
i. Benefits: provides efficient heat transfer simultaneous to vacuum application without impregnating paper; the temperature and pressure can be closely controlled for effective drying in a low oxygen environment; very low final moisture levels can be achieved in the shortest possible time – both important to manufacturing processes.
ii. Limitations: may not be effective on impregnated assemblies; difficult to apply to tanked assemblies due to the limitation of available access ports for addition and extraction of the solvent; can be difficult or impossible to achieve total removal of the solvent after drying; the size of equipment needed and potential safety issues of hot solvent usage in a substation make vapor phase drying impractical in the field.
II. Conduction Heat Transfer: movement of heat within a solid, and from one solid to another in contact such as:
a. Low Frequency Heating: works by applying a current to the windings at low frequency (mHz) and low voltage.
i. Benefits: can provide efficient heating of the winding which, in turn, heats the insulation.
ii. Limitations: requires specialized equipment and careful control/monitoring; may produce uneven heating in some areas of solid insulation.
III. Radiation Heat Transfer: movement of heat through one place to another by electromagnetic waves:
a. Heating / Insulating Vacuum Chamber Walls: works by making the vacuum chamber walls hotter than the un-tanked active part.
i. Benefits: can provide heating through space and voids including vacuum.
ii. Limitations: not efficient as primary heat source; may impact other processes (such as vapor phase) by inducing solvent to evaporate into the vacuum stream.
b. Heating / Insulating Transformer Tank: works by minimizing heat loss to the environment, one step of which is radiation from the active part to the transformer tank (when no oil present). Providing heat through the transformer tank is generally not practical, so this is limited to preventing heat loss rather than adding it.
Situational Application of Heat and Vacuum from Cradle to Grave
Complete treatment of the solid and liquid insulation will involve some combination of particulate filtration, vacuum processing (gas & water removal), heat, adsorptive media reclamation (polar contaminants) and absorptive media (limited moisture removal). The combination of these will be selected to best suit the circumstances: phase of transformer life, degree of contamination, location of transformer (including environment considerations such as ambient temperature; equipment access etc.), and the ability to take an outage if the transformer is in service.
For transformers with wet solid insulation, the efficiency of the drying process, and therefore the amount of time required, will be dependent on:
- Speed and efficiency of heat addition to the insulation
- Minimization of heat loss to the surroundings.
- Pumping speed efficiency of the vacuum system
- Ability to choose techniques that overlap heat addition and pressure reduction as much as possible.
New, unimpregnated, un-tanked solid insulation with > 3-5% moisture – especially in large quantities – can be efficiently and effectively dried in a vapor phase chamber. The process is the fastest available, has a high degree of control and produces excellent repeatability. Heat can be added simultaneous to controlled pressure reduction because the transformer tank is not present as a barrier.
A chamber designed for vacuum, with solvent injection and vacuum systems, is needed with ability to collect the water and solvent from the drying process. Most chambers are designed to be flooded with liquid insulation for thorough impregnation of the dried paper with oil after drying is complete.
Oil systems are designed to degasify, filter and dry the liquid insulation. They should be selected to dry the liquid to very low levels and should treat the oil to vacuum levels as low as the vapor phase endpoint pressure. This is necessary to minimize foaming and bubble formation inside the vapor phase chamber as oil is introduced. As discussed above, this method does not easily lend itself to field application.
Hot Air Circulation
This method circulates hot air around the active part, either in its tank, or in a chamber/oven. Air circulation and vacuum application cannot occur simultaneously so this process is typically used on oil-filled devices that have smaller quantities of solid insulation.
Some processing efficiency can be gained by performing the air heating and vacuum drying in the same chamber, one after the other, without having to move the parts between steps. It may be necessary to dry the air or allow for air exchanges lest the air saturate with moisture and create condensation.
As discussed above, this method does not easily lend itself to field application.
Hot Oil Spray/Recirculation
This method, since slower than vapor phase drying, is often reserved for final drying, post impregnation and assembly. Vapor phase drying is performed for initial drying. Then, hot oil circulation/spray is used to remove surface moisture that accumulates during assembly/tanking and prior to shipment.
Field: Installation and Maintenance
Once the transformer is in the field, some additional considerations are necessary. Though it arrives dry from manufacturing, depending on the size and voltage class, the transformer may arrive without oil. The active part may be briefly exposed to moisture in air during assembly of radiators, bushings and other peripherals. If exposure time is limited, this moisture should remain near the paper surface where it can be removed prior to filling with oil.
A transformer that becomes wet while energized and in service has the additional challenge of the active part being submerged in oil.
Several purification techniques that address particulate, polar contaminants, gasses and moisture can be utilized depending on a number of factors. Considerations include:
- What contaminants are present? Most purifiers/equipment will address particulate and some moisture. Not all will address gasses. Most will not address polar contaminants, if present. If polar contaminants are affecting power factor, interfacial tension, acid number or color, a fuller earth module will need to be incorporated into the overall decontamination scheme.
- Can the transformer be taken offline for the drying process? If not, a properly designed system that addresses gasses (if present) and the water in the paper (not just in the oil) might be a consideration. If an outage can be taken, additional techniques and options are available. Offline processes are faster when they can be used.
- The voltage class; weather conditions; accessibility limits near the transformer; and a variety of other factors may also be considerations.
Online Energized Processing
If an outage cannot be scheduled but gas or moisture levels dictate that action be taken, online energized processing may be a viable option. Wind farm transformers prone to gas generation, as well as GSU or other transformers that are generating fault gases can be processed while energized to reduce combustible gasses to safe levels.
Transformer drying can also be done online. However, since > 99% of the water is in the solid insulation; and the solid insulation is submerged in oil; it becomes necessary to dry the paper using the oil as the moisture transport mechanism. This is a slow process and requires that the oil moisture levels be suppressed to very low levels to induce moisture movement from the paper to the oil.
Various styles of online energized processors are available but generally fall into two categories. The first group utilizes absorbent media such as molecular sieve or polymer absorption elements to remove water from oil in a loop outside the transformer. The limitation of such systems is two-fold. First, though the media when dry boast excellent water removal capability, this ability decreases as the media accumulate moisture. This can limit performance very quickly and may involve regeneration cycles or expensive and time-consuming media changes for any meaningful progress, if there is any at all. Secondly, these types of systems typically do not remove gasses, so one contaminant is not addressed at all. Moisture movement between the paper and oil is somewhat dependent on the operating temperature of the transformer. This can further limit the effectiveness of this type of system.
It can be useful to know the calculated number of gallons of water in the liquid/solid insulation system and the operating temperature of the transformer. The sorptive capacity of molecular sieve and polymer media varies with oil temperature. A transformer with gallons of water to remove may require numerous media changes and/or regeneration cycles which can be costly and time consuming.
The second category of online processor is vacuum dehydrators/degasifiers/purifiers. Though vacuum should never be pulled on an energized transformer, vacuum can still be used to dry and degasify oil in a loop exterior to the transformer.
Since vacuum purifiers do not absorb or saturate with water, they will consistently reduce moisture levels in the oil to low enough levels to attract moisture out of the solid insulation so that it can be transported out of the transformer in the slipstream loop for removal. Since water removal by vacuum is minimally affected by oil temperature and water is exhausted to atmosphere, drying is consistently effective. Degasification is incidental with vacuum purification and this further increases the dielectric strength of the oil as gas levels are suppressed.
Online, energized systems employ additional safeties to monitor oil flow exterior to the transformer and any alarms in the processor.
Offline Deenergized Processing
If an outage can be taken, or in the case of a new install prior to oil-fill, several additional methods are available.
High Vacuum with No Heat Addition
If the transformer has no oil in it, and moisture levels are low (such as new install), and ambient temperatures are not low, it may be possible to effectively dry the active part by connection of a vacuum hose and application of high vacuum. Moisture will evaporate under the low pressure and be transported out of the transformer by the vacuum pumps.
The ambient temperature must be above the minimum guideline for the voltage class. Very good vacuum levels must be achieved and held because the vaporization process slows down as temperatures cool inside the active part.
A cold trap may or may not be used to capture removed water for measurement.
Hot Oil Circulation without Vacuum
This technique is similar to online energized processing except that when the transformer is deenergized, much higher oil flow rates can be used. The processor can also be used to elevate the oil temperature to increase its water carrying capacity.
This method is a viable option if the transformer cannot be emptied of its oil or if the transformer tank is not vacuum rated.
This process is the slowest of the offline drying methods because the water is removed indirectly through the oil rather than directly (which would be possible with the oil removed).
Hot Oil Circulation Followed by Vacuum
This method is faster than the previous because the final stage involves moving the oil away from the active part so that moisture can be removed directly rather than through the oil.
The oil volume is circulated through the deenergized transformer at high rate and high temperature until the temperature of the active part is above a desired target. Then, the oil is rapidly removed while vacuum is applied to the transformer tank. As the solid insulation is exposed to vacuum, water evaporates directly into the air space and is removed by the vacuum pumps. Residual heat in the core steel and winding will be present to replace that removed by the evaporating water from the paper.
As drying occurs, the insulation temperature will drop, but not sufficiently to slow the drying process or approach freezing. This technique is faster than the previous, because it employs heat addition and pressure reduction with some overlap of the two. A cold trap may or may not be utilized.
Hot Oil Circulation Under Vacuum (Hot Oil Spray)
This method further reduces processing time by more completely overlapping the addition of heat and reduction of pressure.
The oil level in the transformer is reduced to the bottom of the winding so that all of the solid insulation is exposed. While reducing pressure through pulling vacuum, the remaining oil volume is circulated to the oil processor where it is dried, filtered, degassed and returned to the top of the transformer hot. Entering the transformer, it washes over the active part transferring heat into the solid insulation. The insulation, since exposed to low pressure and heat will give up its moisture to the vacuum stream. The steady influx of heat from the circulating oil minimizes cooling as moisture leaves through the vacuum line.
A cold trap may or not be utilized in this process.
Additional Considerations for Online Energized and Wind Farm Processors
Online energized systems must be low flow rate, sized to the transformer oil volume. This is necessary to minimize agitation inside the transformer tank. Extra precautions and safety measures are necessary when drying online. Gas blanketed transformers must be monitored and the head space gas replenished as it will be diminished by the degasified oil. Vacuum purification-style purifiers can be used online to remove water, gasses and particulate so can address wet transformers and gassing transformers.
Wind farm processors typically have an onboard generator to power the processor so as not to be dependent on shore power. Often the trailer will have sufficient oil storage onboard to temporarily remove the oil if the transformer will be deenergized for inspection or repair. The processing rate is sized according to the transformers that will be processed, but processors are also typically smaller than those used on other larger power and distribution transformers.
Cold traps are sometimes used to intercept water removed from the transformer specifically through the vacuum system. This has at least two benefits. First, water intercepted before the vacuum pumps does not pass through the pumps. This increases their pumping capacity. For oil-sealed vane or piston pumps, the additional benefit is that the water does not accumulate in the pump’s sealing oil. For modern dry screw vacuum pumps, this is not a concern. The second major benefit is that the moisture can be measured as a means of determining drying rate, endpoint and progress of transformer drying.
Since the boiling/vaporization temperature of liquid water is very low in a vacuum environment, the water must be collected by freezing it. Furthermore, captured ice must be kept below the sublimation temperature. This is the temperature at which ice will turn directly from solid into gas in high vacuum.
Two major types of cold traps are available to achieve the necessary temperatures. The first and simplest is a liquid nitrogen (or dry ice/acetone) trap. These types of traps use a cold medium such as liquid nitrogen to cool a thimble across which the vacuum stream flows. Moisture in the vacuum stream freezes to the thimble. The thimble can be removed periodically to thaw and measure the water to determine both removal rate and total water removed.
The second type is a cascade refrigerated trap. This style of trap uses several cascaded refrigeration loops to achieve the necessary temperatures for freezing and retaining the water. It has higher initial cost, but has the benefit of more rapid thawing and the elimination of liquid nitrogen (or dry ice/acetone).
Measurement of water removed by the vacuum system can also be achieved through instrumentation in parallel or instead of physical measurement by cold trap. These moisture monitors measure dewpoint of the vacuum stream, as well as temperature, pressure and pumping speed to provide a moisture removal rate and total moisture removed.