IEEE Power & Energy Magazine - March/April 2016 - 29

Valve Assemblies Age Too

figure 4. A 1980s analog human-machine interface.
(Photo courtesy of Gene Wolf.)

about the entire power system and voltage control apparatuses. these requirements all rely on fast, high capacity, and
reliable communication systems. because these systems are
often located in remote areas, the lead time and expense for
communication systems needs to be identified early in the
refurbishment process.
physical and cybersecurity is becoming increasingly
important. hVdc and Facts installations play a critical role
in power system operation and need to be protected just as
other important assets. new security requirements need to
be identified and planned with life extension activities.

figure 5. A 1980s air-cooled thyristor valve module.
(Photo courtesy of Gene Wolf.)
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ieee power & energy magazine

the control system may be the brain of the hVdc converters
or Facts controllers, but the valve assembly is the heart. like
the other components, it has a finite life, which is subject to the
effects of aging and the advancement of technology. the hVdc
schemes were proven by the mercury-arc valves technology
but were quickly replaced when thyristor valve technology
became available. those early thyristor valve designs have
aged and reliability is in jeopardy; some form of replacement
is now in their future. those early thyristor valve assemblies
were generally self-supporting, floor- mounted designs and
were either water or air cooled (see Figure 5). water-cooled
suspended thyristor valve assemblies became popular in the
1980s for areas with high seismic requirements.
the suspension valve proved to be very cost-effective,
which led to its adoption throughout the industry. the suspension valve design uses insulators attached to the building's
roof trusses to support the valve as opposed to floor attachment. replacing floor-mounted configurations with suspended
valves has presented some unique challenges that must be
addressed in the refurbishment process. typically, these valve
halls were sized to provide adequate air-gap spacing for the
valve and the voltage at which it operates. putting new valves
into these existing valve halls, however, can require some
creativity. the refurbisher also needs to bear in mind that the
old valve halls may not have sufficient height clearances for
a suspended valve design. the aging buildings may not have
the load-bearing capabilities needed for a suspension design
either. the refurbisher also needs to investigate items such as
ventilation, air-filtration, and fire-protection systems.
In addition to the space and roof-support issues, the
replacement of air-cooled valves has other considerations.
they are housed in large enclosures mounted over floor gratings for the forced air cooling. these floor grates could be
a limiting factor once the enclosures are removed and the
refurbisher has to deal with them. all of these factors show
the exact importance of the detailed analysis of the valve
hall and the proposed replacement valve assemblies. one of
these issues is a show stopper, but there are challenges getting all the parameters in agreement.
grates can be incorporated into designs, and trusses can
be reinforced. roof heights may not be adjustable, but valve
heights can be. It would be great if it were simple to remove
the valves, electronics, and other components associated
with the valve assemblies, such as fiber optics, surge arresters, transducers, heat sinks, and cooling systems, and drop in
the new equipment. Unfortunately, nothing is simple when it
comes to the refurbishment of valve assemblies.
the decision to replace the valves or refurbish them
lies in the thyristor-valve performance. the owner needs
to also ask the following questions. has there been a significant increase in thyristor-valve component failure rates?
has maintenance increased to the point that the spare parts
inventory is being depleted? are spare parts readily available? Is the manufacturer still in the business?
march/april 2016

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