Utility Reliability Problems
To
gain a broader understanding of power system reliability, it is
necessary to understand the root causes of system faults and system
failures. A description of major failure modes is provided below.
- Underground Cable
- Transformer Failures
- Lightning
- Tree Contact
- Birds
- Squirrels
- Snakes
- Insects
- Bears, Bison, and Cattle
- Mice, Rats, and Gophers
- Vandalism
1. Underground Cable
A major reliability concern pertaining to underground cables is
electrochemical treeing.
Treeing occurs when moisture penetration in the presence of an electric
field reduces the dielectric strength of cable insulation. When the
dielectric strength is degraded sufficiently, transients caused by
lightning or switching can result in dielectric breakdown.
Electrochemical treeing usually affects extruded dielectric cable such as
cross-linked polyethylene (XLPE) and ethylene-propylene rubber (EPR), and is largely attributed to insulation impurities and bad manufacturing.
To
reduce failures related to electrochemical treeing, a utility can
install surge protection on riser poles (transitions from overhead to
underground), can purchase tree-retardant cable, and can test cable
reels before accepting them from the manufacturer.
Existing cable can be tested and replaced if problems are found. One way to do this is to apply a DC voltage withstand test
(approximately 3 times nominal RMS voltage). Since cables will either
pass or not pass this test, information about the state of cable
deterioration cannot be determined.
Another popular method for cable testing is
to inject a small signal into one end
and check for reflections that will occur at partial discharge points.
Other methods are measuring the power factor over a range of frequencies
(dielectric spectroscopy), analyzing physical insulation samples in a
lab for polymeric breakdown (degree of polymerization), and using cable
indentors to test the hardness of the insulation.
Tracing an Underground Cable Fault (VIDEO)
Cant see this video? Click here to watch it on Youtube.
Not all underground cable system failures are due to cable insulation.
A substantial percentage occurs at splices, terminations, and joints.
Major causes are due to water ingress and poor workmanship. Heat shrink
covers can be used to waterproof these junctions and improve
reliability.
The last major reliability concern for underground cable is
dig-ins.
This is when excavation equipment cuts through one or more cables. To
prevent dig-ins, utilities should encourage the public to have cable
routes identified before initiating site excavation. In extreme cases
where high reliability is required, utilities can place cable in
concrete-encased duct banks.
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2. Transformer Failures
Transformers
are critical links in power systems, and can take a long time to
replace if they fail. Through faults cause extreme physical stress on
transformer windings, and are the major cause of transformer failures.
Overloads rarely result in transformer failures, but do cause thermal aging of winding insulation.
When
a transformer becomes hot, the insulation on the windings slowly breaks
down and becomes brittle over time. The rate of thermal breakdown
approximately doubles for every
10°C. 10°C is referred to as the
“Montsinger Factor” and is a rule of thumb describing the Arrhenius theory of electrolytic dissociation.
Because of this exponential relationship, transformer overloads can result in
rapid transformer aging.
When thermal aging has caused insulation to become sufficiently
brittle, the next fault current that passes through the transformer will
mechanically shake the windings, a crack will form in the insulation,
and an internal transformer fault will result.
Extreme hot-spot temperatures in liquid-filled transformers can also result in failure.
This
is because the hot spot can cause free bubbles that reduce the
dielectric strength of the liquid. Even if free bubbles are not formed,
high temperatures will increase internal tank pressure and may result in
overflow or tank rupture.
Failure of 230/69kV Autotransformer (VIDEO)
Cant see this video? Click here to watch it on Youtube.
Many transformers are fitted with
load tap changers (LTCs) for voltage regulation. These mechanically moving devices have historically been prone
to failure and can substantially reduce the reliability of a transformer.
Manufacturers have addressed this problem and new LTC models using vacuum technology have succeeded in reducing failure rates.
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3. Lightning
A
lightning strike occurs when the voltage generated between a cloud and
the ground exceeds the dielectric strength of the air. This results in a
massive current stroke that
usually exceeds 30,000 amps.
To make matters worse, most strokes consist of multiple discharges
within a fraction of a second. Lightning is the major reliability
concern for utilities located in high keraunic areas.
An isokeraunic map for the world is shown in Figure 1 below.
Lightning can affect power systems through
direct strikes (the stroke contacts the power system)
or through indirect strikes (the stroke contacts something in close
proximity and induces a traveling voltage wave on the power system).
Lightning can be protected against by:
- Having a high system BIL,
- Using shield wires,
- Using surge arrestors to clamp voltages across equipment, and
- Having a low impedance ground.
Direct strikes are
virtually impossible to protect against on a distribution system.
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4. Tree Contact
Trees continuously grow,
can fall over onto conductors, can drop branches onto conductors, can
push conductors together, and can serve as gateway for animals. This is
why many utilities spend more on tree trimming than on any other
preventative maintenance activity.
When a tree branch bridges two conductors, a fault does not occur immediately.
This is because a moist tree branch
has a substantial resistance.
A small current begins to flow and starts to dry out the wood fibers.
After several minutes, the cellulose will carbonize, resistance will be
greatly reduced, and a short circuit will occur. Branches brushing
against a single phase conductor typically do not result in system
faults.
Faults due to tree contact can be reduced by using tree wire.
This is overhead wire with an insulated jacket similar to cable. Tree
wire can be effective, but faults tend to result in conductor burndown
since they will not motor (move themselves along the conductor) like
faults on bare conductor.
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5. Birds
Birds are the
most common cause of animal faults
on both transmission systems and air insulated substations. Different
types of birds cause different types of problems, but they can generally
be classified as nesting birds, roosting birds, raptors, and
woodpeckers.
Nesting birds commonly build their homes on transmission towers and in substations. Nesting materials can cause faults, and bird excrement can contaminate insulators.
Nesting
birds also attract predators such as raccoons, snakes, and cats. These
predators can be a worse reliability problem than the birds themselves.
Roosting birds
use electrical equipment to rest on or to search for prey. They can be
electrocuted by bridging conductors with their wings, and their
excrement can contaminate insulators. To prevent birds from roosting,
anti-roosting devices can be placed on attractive sites. For locations
that cater to thousands of roosting birds, more extreme deterrent
methods such as pyrotechnics can be used.
Raptors
are birds of prey such as eagles, hawks, ospreys, owls, and vultures.
Reliability problems are similar to other roosting and nesting birds,
but special consideration may be required since most raptors are
protected by the federal government.
Woodpeckers
peck holes in wood with their beaks as they search for insects. This
does not harm trees (the bark regenerates), but can cause devastating
damage to utility poles. This can be prevented by using steel poles, by
using repellent, or by tricking a woodpecker into believing that there
is already a resident woodpecker (woodpeckers are quite territorial).
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6. Squirrels
Squirrels are a reliability concern for all overhead distribution systems
near wooded areas. Squirrels will not typically climb utility poles, but will leap onto them from nearby trees.
They cause faults by bridging grounded equipment with phase conductors. Squirrel problems can be mitigated by cutting down nearby access trees or by installing animal guards on insulators.
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7. Snakes
Snakes are major reliability concerns in
both substations and underground systems. They can squeeze through very small openings, can climb almost anything, and have the length to easily span phase conductors.
Snakes are
usually searching for food (birds in substations and mice in underground
systems), and removing the food supply can often remove the snake
problem.
Special “snake fences” are also available.
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8. Insects
It is becoming more common for
fire ants to build nests in pad mounted equipment.
Their nesting materials can cause
short circuits, the ants can eat away at conductor insulation, and they make equipment maintenance a challenge.
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9. Bears, Bison, and Cattle
These large animals do not typically cause short circuits,
but degrade the structural integrity of poles
by rubbing on guy wires. Bears can also destroy wooden poles by using
them as scratching posts, and black bears can climb wooden utility
poles. These problems can be addressed by placing fences around poles
and guy wire anchors.
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10. Mice, Rats, and Gophers
These rodents cause faults
by gnawing through the insulation of underground cable.
They are the most common cause of animal-related outages on underground
equipment. To make matters worse, they will attract snakes (also a
reliability problem).
Equipment cabinets should be tightly sealed to prevent these small animals from entering. Ultrasonic devices can also be used to keep rodents away (ultrasonic devices will not keep snakes away).
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11. Vandalism
Vandalism
can take many different forms, from people shooting insulators with
rifles to professional thieves stealing conductor wire for scrap metal.
Addressing these reliability problems will vary greatly from situation
to situation.
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Reference: The electric power engineering handbook – L.L. Grigsby (Purchase hardcover book from Amazon