IEEE Power & Energy Magazine - March/April 2016 - 50

Benefits Realized
the main benefits of the upgrade included increased maintainability, reliability, and life expectancy, which were realized
through the use of modern equipment and design principles.
in addition, the annual maintenance required for the valves
and valve-cooling systems was significantly reduced. the
new control systems and cooling systems were designed in a
fully redundant configuration, allowing any particular piece
of equipment to be taken out of service without compromising
the ability of the station to operate at full capacity.
the original thyristor valves utilized 4,800 thyristors per
circuit (in groups of four in parallel), while the new equipment utilizes 360 with thyristors in series. this results in
fewer points of failure.
the original equipment utilized evaporative coolers,
blowers, and a glycol-based heat transport system to air cool
the valves. this equipment had high maintenance requirements and was inefficient. the new equipment is directly
cooled using a water/glycol cooling loop and dry-type fin-fan
coolers and uses less than one-sixth the energy of the original
cooling equipment. While the old cooling system was inadequate for service on hot days, the new system will not be limited by outside temperature. the new control system includes
increased functionality, allows for remote operation, and
includes an enhanced interface with the system operator.

Highgate Converter
Refurbishment Project
Assessing the Need for Refurbishment
the Highgate BtB converter station was originally commissioned in 1985 to provide energy interchange between HQ
(north) and Vermont (south) during an outage of the Vermont Yankee generating station. the rating for the station
prior to the refurbishment project in 2012 was 200 MW,
with a continuous overload capacity of 225 MW for ambient temperatures below 30 °C. in 2012, VelCo and aBB
refurbished the Highgate BtB converter station to extend the
life of the facility another 30 years and to eliminate certain
known deficiencies that were discovered over the years of
operation. the refurbishment project lasted approximately
18 months from contract signing to commissioning. During
the refurbishment, the station was completely shut down for
only 28 days to remove, install, commission, and test the
new system.
the Highgate converter station operated very reliably
over its first 20 years. over time, however, the number of
forced outages due to control and cooling system issues and
the length and frequency of scheduled maintenance outages
started to increase. even as the failures increased, the facility was becoming more important to the system with utilization increasing from about 77% to over 84%.
the continued operation of the facility was discussed
with stakeholders, vendors, and other utilities, and a general consensus was reached. if the facility was expected
march/april 2016

table 4. The project schedule.
Event

Timeline

Issue tender for design, supply,
and install of new HVdc valves,
controls, and cooling equipment

October 2011

Contract for above work signed
with ABB Canada

September 2012

Station outage to remove old
April-November
equipment, install new equipment, 2014
and commission new systems

Provisional acceptance of ABB
systems

October 2014

Return HVdc station to
commercial service

November 2014

Operational support, training,
updating plant drawings, and
project close out

November 2014
to March 2015

to run for another ten years, then minimal cost and effort
should be expended, and the existing C&p systems should
just be maintained. if the facility was expected to operate
for another 10-15 years, refurbishment would be a judgment
call by the owners. for anything over 15 years, a refurbishment of the C&p systems and the valve cooling equipment
would be necessary for reliable operation. following review
of the alternatives and system needs, it became evident that
system reliability needs could be best served by retaining the
facility in Highgate for at least another 25-30 years.
these factors prompted VelCo to take a hard look at its
expectations for the future utilization of the facility in the
short and long term and also examine contingency plans in
the event of a catastrophic failure. VelCo decided to perform an assessment to determine if refurbishment/partial
replacement would be viable or whether complete replacement would be a better option.
in 2009, VelCo carried out a technical life assessment
to determine the condition of major equipment and identify
major vulnerabilities of the system and the mitigation measures. VelCo was assisted in the work by RBJ engineering.
as a first step, the condition of the converter transformers
and thyristor valves were assessed because they comprise at
least 60% of the cost of a new facility. any doubts in these
areas would have sealed the case for complete replacement
of the entire facility.
VelCo also contracted High energy inc. to perform an
independent assessment of the condition of the balance of
other equipment. for purposes of the assessment the equipment was grouped in the following areas: C&p, cooling system, yard equipment, and ancillary equipment.

Converter Transformers
the valve side windings of the converter transformers,
including the spare, had been replaced in 1997-1998 to
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2016