Arresters |
Reprinted from
Vol 16, 2012
issue of
Hubbell
TIPS&NEWS
www.hubbellpowersystems.com
Case History : Leaving the Past Behind:
Upgrading Surge Protection
Leaving the
Past Behind:
Upgrading Surge Protection
By Denny Lenk
Principal Engineer
Hubbell Power Systems
Based on the article "Surge Arresters: Utility Surge Protection Upgrade Considerations," Published in NEMA
electroindustry, July 2011, page 14.
U
tilities typically begin large scale improvements to their
surge arrester ‘fleet’ in one of two ways: When they rebuild
their system following a wide-scale disaster and after they
realize that older arresters can significantly reduce system
reliability and efficiency.
For example, the tsunami in Japan and the recent severe tornado
activity in the United States caused devastating damage to life and
property. These violent, natural events also severely damaged the
electrical grids in these areas.
In Japan, the damage sustained by the three nuclear plants received
major news coverage because of the environmental issues associated
with the potential of nuclear meltdown. What did not get primetime news coverage was the massive amount of work needed to
repair and rebuild the damaged electrical infrastructure.
345 kV System Gapped SiC Arrester
The electrical grid consists of several important segments, including:
• Voltage Generation at low voltage (LV) level (fossil fuel/nuclear
generating plant, hydro dam, wind farm, or solar panels)
• Transforming from LV generation to high voltage (HV) transmission
voltages
• Transmission of HV from generation to load (customer location)
• Voltage transformation back to LV distribution (for residential or
commercial use)
Mobile Sub w/ gapped SiC Arrester
2
In each of these segments, proper performance
of installed electrical power equipment is
critical to the reliable and efficient delivery of
electricity to the end user. Surge arresters help
insure this performance.
RELIABILITY
Under normal operating conditions, the
reliability of the electrical grid is enhanced
by the installation of surge arresters adjacent
to each piece of power equipment. The sole
purpose of the surge arrester is to protect the
electrical insulation of the adjacent equipment
from potentially damaging over voltage surges,
by diverting the over voltage surge away from
the equipment and through the adjacent surge
arrester. If not diverted, the over voltage surge
could damage the equipment. A lightning
strike, on or near a power line, is a typical type
of over voltage surge.
Surge protection can be achieved in a number
of ways. In the late 1800s, surge arresters took
the form of a simple rod gap, installed across
the power equipment. While these simple
devices adequately protected equipment on
the early low voltage (LV) distribution systems,
they could not reliably protect equipment
installed on higher voltage (HV) systems, which
evolved as the electrical grid grew.
The transition of the grid to higher system
voltages was critical to efficiently transmit
power from the often remotely located
generating plants to the end users. As system
voltages increased from LV toward ultra high
voltage (UHV) 800-kV, arrester manufacturers
have continuously improved arrester designs to
assure that the expensive HV equipment (like
transformers) was properly protected.
(SiC block). These critical components were
assembled inside a sealed porcelain housing, to
insure electrical integrity in all environmental
conditions.
In this design, the gap performed the gapspark-over function, while the non-linear
resistance SiC block limited the magnitude
of the the arrester current, allowing the
seriesconnected gap to reseal. Unlike a fuse
which, by design, fails open when it operates
properly (necessitating replacement), the surge
arrester is designed to perform its protective
function repeatedly without failure. The
implementation of gapped, SiC surge arresters
was critical to assuring that the equipment
installed on the new HV systems had the
best possible protection against potentially
damaging overvoltage surges.
The mid-1970s marked the introduction of
the metal oxide varistor (MOV), which has a
much higher exponent of non-linearity when
compared to the silicon-carbide blocks. Because
of the excellent non-linearity of the MOV, this
next generation surge arrester was designed
without internal gaps. At system operating
voltage, the MOV gapless surge arrester
appears as a high resistance to ground. When
exposed to an overvoltage surge (lightning
strike, for example), the MOV discs become
highly conductive (turns on).
Continued>>
In the United States, the post WWII period
(from the late 1940s through the mid 1970s),
marked the start of the ‘modern era’ of
surge arrester design. Arrester manufacturers
introduced the first gapped silicon-carbide (SiC)
surge arresters, which used internal spark gaps
with a precisely controlled, spark-over response
characteristic. Connected in series with each gap
assembly was a non-linear resistance element
230 kV System Gapped SiC Arrester
3
It redirects the surge to ground and, in doing so,
limits the exposure of the equipment’s insulation to
acceptable voltage levels.
MOVS: IMPROVED PROTECTIVE MARGIN AND
HIGHER ENERGY CAPABILITY
While the gapped SiC arresters provided state-of-the
art protection when manufactured, recent testing
has confirmed that the MOV gapless arresters,
manufactured since the mid-1970s, actually provide
improved performance characteristics.
The most important improvement provided by
gapless MOV surge arresters is their inherently
lower protective levels, critical to extending service
life of aging, possibly degraded, electrical insulation
REDUCE SYSTEM LOSSES
It should also be noted that all HV surge arresters,
by design, dissipate power from the grid when
operating at normal system voltage levels. For the
gapped SiC arrester, the continuous power loss is
a result of high resistive current flowing through
the arresters grading resistors. MOV gapless surge
arresters do not require this resistive grading
structure.
Comparison tests have confirmed that MOV arresters
consume less continuous watts from the grid than
comparably rated gapped-SiC arresters. This energy
saving feature is consistent with the government’s
mandate for utilities to reduce energy losses on the
grid. As an example, replacement of a single early
1960s vintage 120-kV rated gapped SiC arrester
. . . replacement of a single early 1960s vintage 120-kV rated
gapped SiC arrester with a similarly rated MOV gapless arrester
would result in an annual energy savings of more than 1000-kWh.
of power equipment that has been in service for
many years.
Replacement of gapped SiC arresters with gapless
MOV surge arresters is a simple, cost effective way
of extending the service life of expensive, aging
equipment and, at the same time, minimizing
unplanned
power
outages.
For
example,
replacement of a gapped SiC by a new gapless MOV
surge arrester on an older 69-kV transformer would
be less than 2% of the cost of transformer
replacement. Similarly, for an older 345-kV station,
arrester replacement would be less than 1% of the
cost of replacing the transformer.
Gapless MOV surge arresters also have higher
energy absorbing capability, minimizing the chance
of failure, when discharging an over voltage surge.
Laboratory testing indicates that HV MOV gapless
arresters have an energy discharge capability
approaching twice that of comparably rated
gapped, SiC arresters.
with a similarly rated MOV gapless arrester would
result in an annual energy savings of more than
1000-kWh.
It is estimated that a large quantity of gapped SiC
high voltage surge arresters may still be installed on
utility power systems. Some utilities understand the
benefits of gapless MOV arresters versus old style
gapped SiC arresters and have initiated replacement
programs. Others have little or no information on
these 30-50 year old surge arresters.
To address this concern, the NEMA 8LA Surge
Arrester Section has developed a web site http://
www.nemaarresters.com/, targeted to provide
information on gapped SiC arresters, including an
identification guide and detailed discussions of
arrester replacement considerations.
Hubbell has a policy of continuous product improvement. We reserve the right to change design and specifications without notice.
Copyright 2012 Hubbell Incorporated
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