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.
Alumina Main Page |
DrysphereTM Product Info |
Back to Technical Info
