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Electrical Paper-Oil Transformers and the Requirement for High Quality Adsorbing Desiccants to Prolong Transformer Life

Dr. Mark Moskovitz and Mary Beth Dawson

An uninterrupted supply of electricity is a necessity for developed and developing countries. Simply, without electricity, the global economy comes to a grinding halt. This demand for continuous and reliable power requires the utmost in efficiency and life expectancy from all components of the power grid. The power grid is a network of electrical lines distributing electricity from power plants to individuals and businesses. The “Smart Grid,” a modern concept promoted by the U.S. Department of Energy, incorporates two-way digital technology in order to improve the efficiency, reliability, and security of our nation’s electrical grid. Utilizing the smart grid concept, fluxes in energy can be eliminated by close monitoring of power usage in a given region. The smart grid concept focuses on reducing overall energy consumption as well as increasing communication in the electricity distribution system.

What different sources of electricity generation contribute to the power grid?

The way by which all resources (other than photovoltaic energy) generate electricity is founded on Faraday’s principle of electromagnetic induction. The movement of wind, water, or steam rotates a turbine connected to the main rotor of a generator. As the rotor turns the armature, a set of copper coils turns inside a magnetic field, inducing voltage and generating electricity. Power plants deliver this electrical energy from non-renewable and renewable resources in the form of direct current (DC). Non-renewable resources include fossil fuels (coal, oil, and natural gas) and nuclear energy derived by nuclear fission of fissile elements with unstable nuclei, such as Uranium-235 and Plutonium-239. Each nuclear fission reaction releases about 200 million electron volts (MeV) of energy in addition to radioactive waste. Water heated by the release of energy from nuclear fission and the burning of fossil fuels produces steam, which in turn powers a generator to create electricity. The burning of fossil fuels releases carbon dioxide into the atmosphere, contributing to greenhouse gas emissions and global warming. Scientists are now turning to naturally replenished resources as alternative sources of energy. Renewable resources include hydroelectric, wind, biomass, geothermal, tidal, and solar energy.

Hydroelectric Energy

Hydroelectric power generation requires a height differential in a body of water. The height differential between stored water and turbines denotes potential energy. As water flows downward through a pipe called a penstock, the falling water’s kinetic energy is converted to mechanical energy as it passes through a turbine connected to a generator. In some hydropower facilities, when customer demand for electricity is low such as at night, water is pumped uphill to a storage pool in a process called pumped storage and is later used to generate electricity during times of higher demand. Hydropower facilities are capable of producing large amounts of reliable power when demand for electricity is at its highest.

Wind Energy

In a sense, wind energy is a converted form of solar energy. As sunlight warms the air and warm air rises, cooler air takes its place, creating wind. Wind’s kinetic energy is converted from mechanical to electrical energy by the use of wind turbines. Each turbine consists of a 25-75 meter tower and one to three blades connected to a horizontal hub. This hub is connected to the nacelle, which houses the electrical components of the turbine, such as the gear box and generator. Sensors monitor the wind’s speed and direction, and a Yaw mechanism changes the turbine’s direction so that the blades always face the wind. The amount of energy produced is relative to the diameter of the blades; the longer the blades, the higher the energy output. Wind speed is also directly proportional to the amount of energy produced. The amount of power generated in watts is equal to: 0.5 v3 π r2 , where = the density of dry air, or 1.225 kg/m3 ; v= wind velocity in meters/second; and r= the radius of the rotor in meters. Put simply, the amount of power generated is equal to the wind’s speed cubed. Doubling the wind speed, for example, would result in an 800% increase in electricity generated. For a turbine to be viable, wind speed must be at least 8 m/s. Fluctuations in energy output occur often, and wind speed is also highest at night when there is the lowest demand for electricity. Extra energy is either wasted or warehoused in a battery storage system, providing electricity during peak demand hours.

Biomass Energy

Biomass refers to organic material from plant and animal waste, such as wood, crops, switch grass, manure, and sewage. Biomass is burned creating steam which then powers a turbine connected to an electric generator. In a process called co-firing, biomass is combined with fossil fuels and burned to reduce carbon emissions from a power plant. Gasification systems also convert biomass into synthetic gas, or syngas, which is either burned to create electricity, converted to other fuels, or used as an alternative to natural gas in a gas turbine. The natural decomposition of biomass also releases biogas, a combination of carbon dioxide and methane gas. Biogas can be captured from places like landfills and sewage treatment plants and burned to create electricity.

Geothermal Energy

Geothermal energy is generated naturally in the depths of the earth’s crust, and is found at geothermal reservoirs along plate boundaries. Geothermal energy sources create steam, turning turbines to generate electricity. In dry steam plants, steam is brought to the surface to power turbines, condensed, and then returned to the ground. In flash steam plants, hot water is depressurized into steam powering turbines, and then returned to the ground as water. In binary cycle plants, hot water heats a second liquid such as iso-butane, creating steam to power turbines.

Tidal Energy

Tidal energy is generated by the force of moving seawater through tidal barrages, tidal fences, and tidal turbines. Tidal barrages are typically built across estuaries. As the tide changes, seawater flows from one side of the barrage to the other, turning a turbine to generate electricity. Tidal fences are a series of vertical-axis turbines commonly built across channels between two land masses. Tidal turbines are like underwater wind turbines. Currents flow over the blades of the turbine, generating electricity.

Solar Energy

Solar energy may be converted into electricity either directly, by lining photovoltaic cells into solar panels, or indirectly in solar power plants. In a solar power plant, energy is collected in solar thermal collectors, warming a liquid to create steam that rotates turbines to generate electricity.

Power plants are only one component of the power grid. To increase the use of renewable energy, transmission line capacity must be improved. There are little, if any, transmission lines connecting renewable resource power plants to many areas covered by the main grid. At power plants, electricity is distributed to transmission substations with step-up transformers which convert the electricity to extremely high voltages. Electricity leaves power plants at voltages of 138-765 kilovolts (kV). Regional power substations containing step-down transformers then convert the electricity to lower voltages for distribution to individual businesses and households. Households receive voltages ranging from 120V-240V of electricity. Regional substations also contain circuit breakers and switches, allowing individual substations to be disconnected from the main grid when necessary. If there is an overload of energy, disconnecting a specific region from the main grid prevents further power outages which could spread throughout the grid. A distribution bus at each substation also splits the power into multiple directions.

Without transformers, electricity generated in power plants could not be used by consumers. A transformer, however, is the weakest link of the electrical grid. Transformers are subjected to natural aging and other processes which easily cause transformer failure. When a transformer fails, a power outage occurs. Power outages and other interruptions in the electrical grid cost the U.S. economy at least $150 billion annually. According to the U.S. Department of Energy, that translates into an annual cost of about $500 for every man, woman, and child. The Northeast Blackout of 2003 alone caused about $7 billion in losses to the economy in just six hours of time. These astounding figures reflect just how essential that transformers be maintained regularly to function properly.

What is a Transformer?

Energy from power plants must be converted to a substantially lower voltage before domestic and commercial use. Appliances and other machinery operate at specific voltages. Inconsistent voltage delivery places extra stress on electrical components, speeding wear out and breakdown. An electrical transformer converts high voltage power from power plants to usable, lower-voltage power with no loss of energy. This process is achieved by the specialized construction of the parts of a transformer. Each transformer consists of a ferromagnetic core surrounded by a series of coils, or windings. The primary winding is connected to the input wire, while the secondary winding(s) are connected to the output wire(s). The windings are separately wrapped in Kraft or pressboard paper, ensuring that all of the current flows through the windings. The alternating current running through the primary winding creates a magnetic field which shifts in response to changes in the alternating current, which in turn creates an electric potential in the secondary winding(s). According to Faraday’s Law of electromagnetic induction, emf= - N ΔBA /Δt, where emf= electromotive force (voltage induced), N= number of turns in the coil, BA= magnetic flux, and t= time in seconds. In other words, the number of turns in the winding is directly related to the amount of voltage induced in the winding. In this way, based on the relation of the number of turns in the primary winding versus number of turns in the secondary winding, transformers are able to convert high voltage power to lower voltage power.

Complications with Transformer Performance

Dielectric fluid, or transformer oil, acts as a coolant system to keep a transformer from overheating. This fluid moves vertically from the bottom of the transformer through the windings where it is heated. As it re-circulates to the bottom, it cools by going through a series of radiators, or cooling fins, and is reused. Although transformer oil has a low affinity for water, water can accumulate in a transformer as either free standing or emulsified water in the transformer oil. Freestanding water is water that has settled out of the transformer oil. Emulsified water is suspended in transformer oil, but not yet separated from the oil. Moisture may accumulate due to leaks in gaskets and welds, improper sealing allowing for water to enter the machine, inadequate drying of transformers at time of production, poor maintenance, and natural aging of insulation materials inside the transformer. This water then weakens the dielectric strength of the oil and solid insulation, and accelerates aging of the transformer’s insulating material. For each doubling of water content, transformer life is reduced by half.

Although transformer failure may occur for many reasons, moisture in transformer oil remains one of the most common, yet preventable, causes of failure. It accelerates aging of transformers dramatically, decreases the dielectric constant, and decreases dielectric strength, the maximum voltage that the transformer can handle without breaking down. Water content should be kept at or below 10ppm. Water is the main culprit impacting shelf life of transformers.

With temperature increases, the solubility of oil increases and water migrates from the insulating paper and paperboard to the oil. Water is also naturally hydrogen-bonded to the hydrocarbon chains of the insulating paper. These water molecules break apart from the cellulose polymer chains with natural aging. Additionally, freestanding water can degrade the insulating paper further by weakening the hydrogen bonds of these polymer chains, with degraded insulation then leading to increased mechanical stress. The oxidation of transformer oil is also accelerated by water and temperature increases. During this degradation process, fatty acids settle out of the oil and rest on the parts of the insulation system as sludge, significantly decreasing the system’s ability to function. Oxidation also leads to corrosion of the solid parts of the transformer, allowing oil to leach out.

Water present in transformers also causes hydrogen-induced fractures; depletion of additives such as dispersants and demulsifying agents; restricted oil flow due to emulsions, sludge, ice crystals, and microbial contamination; and impaired film strength. Water lowers transformer oil’s interfacial tension, leading to aeration. The presence of air in turn causes oxidation and cavitations, increased heat, and weakened oil films. Oil contaminated with water can also soften and flow out of bearings, causing leaks.

Transformer Maintenance

Transformers may fail due to electrical arcing caused by overloaded electrical equipment, coronas which result in power loss, and insulation material breakdown. According to the International Association of Engineering Insurers, insulation failure is the leading cause of transformer failure, caused by inadequate or defective insulation and insulation deterioration. Transformers may be regularly monitored by dissolved gas analysis (DGA) tests as well as oil and chemical tests. DGA uses gas chromatography in determining the levels of oxygen, nitrogen, carbon dioxide, hydrogen, methane, ethane, ethylene, and acetylene gases in the oil. Based on these levels, transformer problems may be identified. Chemical tests may also reveal levels of moisture, polar contaminants and other by-products of oxidation in the transformer oil. An online filtration system can periodically run these tests to prevent accumulation of contaminants and maintain the insulating properties of the oil. Certain models of these filtration systems also have a degassing mode which removes moisture and dissolved gases from the transformer when necessary.

Online oil conditioning systems, such as Mark I and Mark II, are capable of removing polar oxidation products, moisture, acids, dissolved metals, oxygen, and other insulation degradation products from transformer oil to extend transformer life. According to a study on transformer maintenance presented in Electricity Today (2003: Issue 8), “laboratory tests have shown that a combination of desiccants, adsorbents and semi-permeable hollow fiber membranes is capable of restoring and maintaining transformer oil properties to near new conditions” (Kovacevic et al., 31). Aside from the use of desiccants and adsorbents, hollow fiber membranes (HFM) can also remove moisture and dissolved gases from transformer oil. As the oil flows through the HFM, moisture and dissolved gases permeate to the shell side until equilibrium is reached. Transformer oil may be completely degassed by the HFM if gases and other products are continuously removed by a vacuum or stripping gas so equilibrium is never reached.

Other ways to keep air and moisture out of transformers include conservator systems and oil-filled inert gas systems. In a conservator system, a conservator filled with oil is connected to the main tank. Air flows in an out of the conservator through a desiccant air dryer as the oil level in the main tank rises and falls with temperature fluctuations. In an inert gas system, a nitrogen gas cylinder is connected to the main tank. A series of valves control the amount of pressure in the nitrogen cylinder and main tank as pressure changes with temperature fluctuations.

Reactive procedures for removing excess moisture in transformers include disassembling the transformer and shipping for oven drying or vacuuming the system. Both processes require excessive time, labor, and money.

The Need for an Adsorbing Desiccant to Prolong Transformer Life and Prevent Dysfunction

Transformers should provide fifty years of continuous service and be designed to be maintenance free. Because water damage greatly decreases transformer life, removing moisture with adsorbing desiccant ceases degradation to the insulation system and increases transformer life by decades. Use of adsorbing desiccant is a time and cost efficient means to remove moisture without disrupting normal use of the transformer. The use of DAI activated alumina as a desiccant is the perfect solution for removing moisture from electrical transformers.

DrySphereTM is the only activated alumina product on the market that can adsorb up to 36% of its own weight in water. Specialized dust-free spheres of alumina have large surface areas to adsorb more moisture than any other desiccant. DrySphereTM removes water content in transformer oil to a comfortable level of less than 10ppm. Once the water content in transformer oil is decreased to this level, water from the insulating paper and paperboard diffuses into the transformer oil and the water is once again removed. DrySphereTM removes moisture from all parts of the transformer, greatly improving the efficiency of the transformer and more than doubles transformer life. Consequently, DAI’s DrySphereTM is the superior product for maintenance of paper-oil transformers.

To assure consistent and reliable energy delivery and to make successful implementation of the smart grid possible, electrical engineers must consistently control, regulate, and monitor the impact of water on transformers as they are the weakest link in our energy distribution system. DrySphereTM specially designed activated alumina provides the electrical engineering community with a proven, tested and simple engineering solution to make our lives trouble-free.

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