IEEE Power & Energy Magazine - March/April 2016 - 59

describes the various projects which have been completed
or are presently undergoing some form of refurbishment by
Alston Grid (now GE) and others.

Nelson River Bipole 1 Refurbishment
The Nelson River HVdc system, in Manitoba, Canada, currently consists of two HVdc bipoles (bipole 1 of ±463.5 kV
and bipole 2 of ±500 kV) constructed on the Interlake corridor with an overhead transmission line of about 900 km. As
the backbone of the Manitoba Hydro transmission system,
the Nelson River HVdc system transmits nearly 70% of the
generated power in Manitoba to the southern major load centers, and its reliable operation is critical for the load serving
in Manitoba and power exports.
Nelson River bipole 1 was originally supplied in the form
of mercury-arc converters, as shown in Figure 1. The HVdc
link was commissioned in 1971, with a rating of 1,620 MW,
transmitted on the 890 km of HVdc overhead lines from the
northern area of the province, to Winnipeg. The dc voltage
rating of 463.5 kV, created from three six-pulse bridges in

figure 1. The Nelson River mercury-arc valve group.
(Photo courtesy of GE.)

figure 2. The Nelson River pole 1 thyristor valve group 13.
(Photo courtesy of GE.)
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series per pole, remains the highest dc voltage ever achieved
by mercury-arc valves. The dc configuration was in the form
of a bipole, with 3 × 154.5 kV series valve groups per pole
(one of the six-valve groups is shown in Figure 1). Due to
the relatively weak southern ac system, six 160-Mvar synchronous condensers were installed to provide support on
voltage, reactive power, and system inertia.
The mercury-arc valves were maintained by Manitoba
Hydro, and even though the maintenance regime was laborintensive, they were able to achieve relatively high levels of
availability. However, when the second bipole of the Nelson
River was installed in 1978 using thyristors, it was clear that
the use of mercury-arc technology was in decline. This resulted
from much higher maintenance requirements and health, safety,
and environmental concerns associated with mercury.
The Nelson River mercury-arc valves shown in Figure 1
were replaced with new thyristor valves beginning in the early
1990s. The high-level pole control equipment was retained
because it was still functional, and sufficient spares and
expertise existed within Manitoba Hydro to maintain this
equipment well into the future. However, the new thyristor valves required new valve-control functions and valvebase electronics to individually control each thyristor in the
bridge. In addition to the thyristor valves and part of the control systems, a new single-circuit water cooling system was
installed for each valve group.
Of the six series-valve groups that make up bipole 1,
three were replaced with thyristor valves, one valve group
at a time, by removing all valve hall equipment and building
new structures in the existing valve halls from the floor up.
Figure 2 shows the valves for valve group 13 that, as the topmost valve group, are elevated for insulation at 300 kVdc at
the bottom of the structure.
The outage duration for the replacement of each valve
group was about 20 weeks. Manitoba Hydro carried out
all of the removal, installation, and testing work themselves, relying on Alstom Grid (now GE) for the design
and supply of the equipment and supervision of the of the
work on site.
In 2004 the three valve groups of bipole 1, pole 2 were
replaced according to a different strategy using roll-off, rollon thyristor valves, rather than demolishing and removing
everything from the valve hall and building up from the
floor. In pole 2, the mercury-arc support structures and rails
were retained, and new replacement thyristor valves were
manufactured to quickly replace the old valves.
In addition to the replacement of the valve groups in
bipole 1, Manitoba Hydro saw a significant number of
converter transformer failures in bipole 2, and a replacement strategy was implemented for all of the transformers
in both bipoles 1 and 2. This strategy has recently been
adjusted to take into account the new bipole 3 which is currently under construction.
Manitoba Hydro has continued to refurbish the key components
of bipole 1 to extend its life, including the following equipment:
march/april 2016

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