IEEE Power & Energy Magazine - March/April 2016 - 31

The experience level of personnel, both in engineering
and maintenance, is a critical factor in maintaining
the operability of an older station.

figure 7. Aging harmonic filter equipment increases maintenance. (Photo courtesy of Gene Wolf.)

changers, fans, pumps, gaskets, and oil replacement. the
scope of the work depends on what the operational and
maintenance experience has been with the units.

Smoothing Reactors
In the early days of hVdc converters, the first smoothing
reactors were oil filled, which are closely related to the converter transformers. air-core reactors were introduced as the
technology matured. each type of reactor has its own particular set of aging difficulties and defined service life. the
oil-filled smoothing reactors have many of the same issues
as found with transformers, and like the transformers, refurbishment can be focused on component replacement or the
total replacement of the reactor. the key to the degree of
work is based on good maintenance records, a necessity for
performing a conditional assessment for life extension.
air-core reactors are simpler in design than oil-filled reactors. they are considerably less expensive to replace but offer
similar challenges performing the conditional life assessment.
refurbishment can be accomplished, but in most cases it is
more advantageous to replace them outright. when replacing
these apparatuses, the engineer needs to take into consideration the support insulators, space available, foundations, and
the extensive magnetic fields these devices produce.

AC and dc Yard Equipment
hVdc and Facts installations are tightly interconnected
with the associated ac and dc yard equipment. the ac and
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ieee power & energy magazine

dc yard equipment as well as "balance of plant" facilities
need attention in any refurbishment. a life-assessment study
should include both the ac and dc yard equipment.
ac yard equipment includes the interconnecting facilities
such as power circuit breakers, circuit switchers, disconnecting switches, and instrument transformers. these equipment
types are similar to what electric utilities are accustomed
to repairing and replacing over time. however, because
the impact of failure of this equipment will often mean an
extended outage of the hVdc or Facts facility, this equipment should be included in the life-extension strategy.
dc yard equipment is more specialized and includes
dc switches, instrument transformers, and dc filters. It is
likely to be in the extra hV range of voltage, and replacements can have long lead times. other equipment would
include what is often considered balance of plant. critical
systems include yard surfacing, grounding, fencing, foundations, and steel structures. these systems do need to be
periodically assessed as the general condition of the hVdc
or Facts facilities is dependent upon good condition of
these systems.
the building that houses the thyristor valves and control
systems also needs to be a part of the life assessment. roof
leaks; heating, ventilation, and air-conditioning (hVac)
failures; building electrical system problems; door hardware;
and security systems all need to be in good working order to
assure the long-term operation of the hVdc or Facts facility. these items present some challenges to utilities that normally focus on electrical equipment, and not buildings or
their roofs.
grounding systems are critical in these stations and are
often neglected since they are generally hidden. the system conditions can change over time, resulting in higher
fault duty than when the station was commissioned. the
condition of the ground grid can deteriorate over time.
a poor grounding system can result in poor equipment
operation as well as become a safety hazard. because the
ground-system condition is difficult to determine with a
visual inspection, periodically testing the ground grid is a
good idea. the testing may also include digging up a few
connections to determine the condition. hVdc and Facts
stations can create circulating currents that can result in
fencing and grounding issues. Fence posts should be periodically checked to ensure integrity.
harmonic filters require special attention (see Figure 7).
they are switched in and out frequently so the switching
march/april 2016

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2016