Insulating media in high voltage transformers consists of paper wrapped around the conductors in the transformer coils plus mineral oil and pressboard to insulate the coils from ground. From the moment a transformer is placed in service, both the solid and liquid insulation begin a slow but irreversible process of degradation.
1) SOLID INSULATION: PAPER
i) COMPOSITION OF PAPER: CELLULOSE
Paper and press board are composed primarily of cellulose, which is a naturally occurring polymer of plant origin. From a chemical perspective, cellulose is a naturally occurring polymer. Each cellulose molecule is initially composed of approximately 1000 repeating units of a monomer that is very similar to glucose. As the cellulose molecule degrades, the polymer chain ruptures and the average number of repeating units in each cellulose molecule decreases. With this reduction in the degree of polymerization of cellulose, there is a decrease in the mechanical strength of the cellulose as well as a change in brittleness and color. As a consequence of this degradation, cellulose will reach a point at which it will no longer properly function as an insulator separating conductors. When cellulose will reach its end of life as an insulator depends greatly on the rate at which it degrades.ii) PARAMETERS THAT AFFECT DEGRADATION OF CELLULOSE
a) HEATSeveral chemical reactions contribute to the degradation of cellulose. Oxidation and hydrolysis are the most significant reactions that occur in oil-filled electrical equipment. These reactions are dependent on the amounts of oxygen, water, and acids that are in contact with the cellulose. In general, the greater the level of these components, the faster are the degradation reactions. Also, the rates of the degradation reactions are greatly dependent on temperature. As the temperature rises, the rates of chemical reactions increase.
For every 10° (Celsius) rise in temperature, reaction rates double.
Consequently, the useful life of cellulose and oil is markedly reduced at higher temperatures. Paper and oil subjected to an increased temperature of 10°C will have their lives reduced by a factor of 50%. Elevations in temperature can result from voluntary events such as increased loading, or they can result from a large number of involuntary events, such as the occurrence of fault processes (partial discharge and arcing).
b) OXYGEN
The cellulose that is present in paper, press board, and wood oxidizes readily and directly to carbon oxides. The carbon oxides (carbon dioxide and carbon monoxide) that are found in oil-filled electrical equipment result primarily from the cellulose material. This has very important consequences, since the useful life of major electrical devices such as power transformers is generally limited by the integrity of the solid insulation, the paper. It is now possible to determine more closely the extent and the rate of degradation of the cellulose by observing the levels of the carbon oxides in the oil as a function of time.
As cellulose reacts with oxygen, carbon dioxide, water, and possibly carbon monoxide are produced. Carbon monoxide is produced if there is an insufficient supply of oxygen to meet the demands of the oxidation reaction. The levels of these products in the oil continue to increase as oxidation continues. However, they never exceed concentrations in the oil that are referred to as their solubility limits, which are temperature and pressure dependent. After the solubility limit of each has been reached, further production cannot increase their concentration in the oil. If carbon monoxide and carbon dioxide were to ever exceed their solubility limits, they would form bubbles that would be lost to the atmosphere or to a gas blanket; this rarely happens. Any water that forms will fall to the bottom of the tank or be adsorbed into the solid insulation (the cellulose).
c) MOISTURE
Cellulose has a great affinity for holding water (notice how well paper towels work). Water that is held in the paper can migrate into the oil as the temperature of the system increases, or the reverse can happen as the temperature of the system decreases. In a typical large power transformer, the quantity of cellulose in the solid insulation can be several thousand pounds.
For new transformers, the moisture content of the cellulose is generally recommended to be no more than 0.5%. Water distributes between the oil and the paper in a constant ratio, depending on the temperature of the system. As the temperature increases, water moves from the paper into the oil until the distribution ratio for the new temperature is achieved. Likewise, as the temperature decreases, water moves in the opposite direction.
In addition to the water that is in the paper and the oil at the time a transformer is put into service, there is also water introduced into the system because of the ongoing oxidation of the cellulose. Water is a product of the oxidation of cellulose, and it is therefore always increasing in concentration with time. Even if the transformer were perfectly sealed, the moisture concentration of the paper would continue to increase. The rate of generation of water is determined primarily by the oxygen content of the oil and the temperature of the system. An increase in either of these factors increases the rate of water generation.
d) ACID
Cellulose can degrade by a chemical process referred to as hydrolysis. During hydrolysis, water is consumed in the breaking of the polymeric chains in the cellulose molecules. The process is catalyzed by acids. Acids are present in the oil that is in contact with the cellulose. Carboxylic acids are produced from the oil as a result of oxidation. The acid content of the oil increases as the oil oxidizes. With an increase in acidity, the degradation of the cellulose increases.
2) LIQUID INSULATION: OIL
The insulating fluid that has the greatest use in electrical equipment is mineral oil. There are insulating materials that may be superior to mineral oil with respect to both dielectric and thermal properties; however, to date, none has achieved the requisite combination of equal or better performance at an equal or better price. Consequently, mineral oil continues to serve as the major type of liquid insulation used in electrical equipment.i) COMPOSITION OF OIL
a) TYPES OF HYDROCARBONS AND PROPERTIES OF EACHMineral oil can vary greatly in its composition. All mineral oils are mixtures of hydrocarbon compounds with about 25 carbon atoms per molecule. The blend of compounds that is present in a particular oil is dependent on several factors, such as the source of the crude oil and the refining process. Crude oils from different geographical areas will have different chemical structures (arrangement of the carbon atoms within the molecules). Crude oils from some sources are higher in paraffinic compounds, whereas others are higher in naphthenic compounds. Crude oils also contain significant amounts of aromatic and polyaromatic compounds. Some of the polyaromatic compounds are termed “heterocyclics” because, besides carbon and hydrogen, they contain other atoms such as nitrogen, sulfur, and oxygen. Some heterocyclics are beneficial (e.g., oxidation inhibitors), but most are detrimental (e.g., oxidation initiators, electrical charge carriers).
The refining of crude oil for the production of dielectric fluids reduces the aromatic and polyaromatic content to enhance the dielectric properties and stability of the oil.
The terms paraffinic and naphthenic refer to the arrangement of carbon atoms in the oil molecule. Carbon atoms that are arranged in straight or branched chains, i.e., carbon atoms bonded to one another in straight or branched lines, are referred to as being paraffinic. Carbon atoms that are bonded to one another to form rings of generally five, six, or seven carbons are referred to as being naphthenic. Carbon atoms that are bonded as rings of benzene are referred to as being aromatic. Carbon atoms that are contained in “fused” benzene rings are referred to as being polyaromatic. These forms of bonded carbon atoms are depicted in Figure 1. The straight lines represent the chemical bonds between carbon atoms that are present (but not depicted) at the ends and vertices of the straight lines.
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| FIGURE1 Carbon configurations in oil molecules. |
The differences in the chemical composition will result in differences in physical properties and in the chemical behavior of the oils after they are put in service. The chemical composition has profound effects on the physical characteristics of the oil.
For electrical equipment, the main concerns are:
• Paraffinic oils tend to form waxes (solid compounds) at low temperature.
• Paraffinic oils have a lower thermal stability than that of naphthenic and aromatic oils.
• Paraffinic oils have a higher viscosity at low temperature than that of naphthenic and aromatic oils.
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| FIGURE 2 Typical oil molecule. |
b) OXIDATION INHIBITORS
Oxidation inhibitors, such as DBPC (di-tertiary butyl paracresol) and DBP (di-tertiary butylphenol), are often added to oil to retard the oxidation process. These compounds work by attracting oxygen molecules to themselves rather than allowing oxygen to bind with oil molecules. With time, the inhibitor gets consumed because of its preferential reaction with oxygen. As a result, the oil will then oxidize at a more rapid rate. The remedy is to add inhibitor to oil that has lost its antioxidant capabilities.
ii) FUNCTIONS OF OIL
a) ELECTRICAL INSULATIONThe primary function of insulating oil is to provide a dielectric medium that acts as insulation surrounding various energized conductors. Another function of the insulating oil is to provide a protective coating to the metal surfaces within the device. This coating protects against chemical reactions, such as oxidation, that can influence the integrity of connections, affect the formation of rust, and contribute to the consequent contamination of the system.
Insulating oil, however, is not a good lubricant. Despite this fact, it is widely used in load tap changers, circuit breakers, and transformers. Therefore, Its use in these devices presents a challenge to the mechanical design of the system.
b) HEAT DISSIPATION
A secondary function of the insulating fluid is to serve as a dissipater of heat. This is of particular importance in transformers where localized heating of the windings and core can be severe. The oil aids in the removal of heat from these areas and distributes the thermal energy over a generally large mass of oil and the tank of the device. Heat from the oil can then be transferred by means of conduction, convection, and radiation to the surrounding environment.
All mineral oils are comparable in their ability to conduct and dissipate heat. To ensure that a given oil will perform satisfactorily with respect to heat dissipation, several specifications are placed on the oil.
These specifications are based upon certain factors that influence the oil’s ability to dissipate heat over a wide range of possible operating conditions. These factors include such properties as viscosity, pour point, and flash point.
c) DIAGNOSTIC PURPOSES
The third function of the insulating fluid is to serve as an indicator of the operational condition of the liquid-filled equipment. The condition (both chemical and electrical) of the insulating fluid reflects the operational condition of the electrical device. In a sense, the fluid can provide diagnostic information about the electrical device much like blood can provide diagnostic information about the human body.
The condition of the blood is important as it relates to its primary function of transporting oxygen and other chemical substances to the various parts of the body. Indeed the condition of the blood is symptomatic of the overall health of the body. For example, the analysis of the blood can be used to diagnose a wide variety of health problems related to abnormal organ function.
In much the same way, insulating fluid can be viewed as serving its primary functions as an insulator and heat dissipater. It can also be viewed as serving another (and perhaps equally important) function as a diagnostic indicator of the operational health of liquid-filled equipment. This is possible because when faults develop in liquid-filled equipment, they cause energy to be dissipated through the liquid.
This energy can cause a chemical degradation of the liquid. An analysis for these degradation products can provide information about the type of fault that is present.
iii) PARAMETERS THAT AFFECT OIL DEGRADATION
a) HEATJust as temperature influences the rate of degradation of the solid insulation, so does it affect the rate of oil degradation. Although the rates of both processes are different, both are influenced by temperature in the same way. As the temperature rises, the rates of degradation reactions increase.
For every 10° (Celsius) rise in temperature, reaction rates double!
b) OXYGEN
Hydrocarbon-based insulating oil, like all products of nature, is subject to the ongoing, relentless process of oxidation. Oxidation is often referred to as aging. The abundance of oxygen in the atmosphere provides the reactant for this most common degradation reaction. The ultimate products of oxidation of hydrocarbon materials are carbon dioxide and water. However, the process of oxidation can involve the production of other compounds that are formed by intermediate reactions, such as alcohols, aldehydes, ketones, peroxides, and acids.
c) PARTIAL DISCHARGE AND THERMAL FAULTING
Of all the oil degradation processes, hydrogen gas requires the lowest amount of energy to be produced.
Hydrogen gas results from the breaking of carbon–hydrogen bonds in the oil molecules. All of the three fault processes (partial discharge, thermal faulting, and arcing) will produce hydrogen, but it is only with partial discharge or corona that hydrogen will be the only gas produced in significant quantity. In the presence of thermal faults, along with hydrogen will be the production of methane together with ethane and ethylene. The ratio of ethylene to ethane increases as the temperature of the fault increases.
d) ARCING
With arcing, acetylene is produced along with the other fault gases. Acetylene is characteristic of arcing. Because arcing can generally lead to failure over a much shorter time interval than faults of other types, even trace levels of acetylene (a few parts per million) must be taken seriously as a cause for concern.
e) ACID
High levels of acid (generally acid levels greater than 0.6 mg KOH/g of oil) cause sludge formation in the oil. Sludge is a solid product of complex chemical composition that can deposit throughout the transformer. The deposition of sludge can seriously and adversely affect heat dissipation and ultimately result in equipment failure.
3 SOURCES OF CONTAMINATION
i) EXTERNAL
External sources of contamination can generally be minimized by maintaining a sealed system, but on some types of equipment (e.g., free-breathing devices) this is not possible. Examples of external sources of contamination are moisture, oxygen, and solid debris introduced during maintenance of the equipment or during oil processing.ii) INTERNAL
Internal sources of contamination can be controlled only to a limited extent because these sources of contamination are generally chemical reactions (like the oxidation of cellulose and the oxidation of oil) that are constantly ongoing. They cannot be stopped, but their rates are determined by factors that are well understood and often controllable. Examples of these factors are temperature and the oxygen content of the system.Internal sources of contamination are:
• Nonmetallic particles such as cellulose particles from the paper and press board
• Metal particles from mechanical or electrical wear
• Moisture from the chemical degradation of cellulose (paper insulation and press board)
• Chemical degradation products of the oil that result from its oxidation (e.g., acids, aldehydes, ketones)