IEEE Power & Energy Magazine - March/April 2016 - 77

Refurbishments of aging electrical infrastructure
bring their own set of challenges with regards to planning,
design, and execution.
cooling). To account for conditions where the dc line insulators cannot withstand full voltage, pole 3 is equipped with
a reduced dc voltage operation mode at 250 kV (0.71 p.u.).
The Haywards substation is located in a region that experiences high seismicity due to its proximity to an active geological fault requiring the converter station design to be able to
handle these conditions. The Benmore converter station does
not experience as high activity but is designed to the same
standards. A strategy devised by Transpower specified how
to achieve a high level of seismic resilience. This included
structural diversity between poles 3 and 2 to avoid common
mode failures; seismic isolation of the converter valve hall,
converter transformers, and control system; and additional
spare equipment and qualification of outdoor equipment to
meet the standards set by IEEE 693, 2005 high-performance
level including shake table testing of plant items. The Haywards site was also assessed for seismic performance based
on the 2,500-year return period spectra. This qualifies the site
as having exceeded the IEEE 693 high-performance level.
The purpose of this additional assessment was to determine
the need for additional spares or other mitigation strategies.
The buildings, however, were designed to the local building code requirements using the site-specific earthquake
spectra. The converter building was decoupled from any horizontal ground movement by installing lead rubber bearings
that have freedom of motion for ±600 mm. This provides
isolation for the thyristor valves, converter transformers, pole
control equipment, and auxiliary plant inside the building.
A number of factors posed a challenge to the site and
switchyard layouts. These include safety considerations, land
constraints, audible noise limits, and keeping pole 1 in service
until three months before pole 3 was commissioned. These factors affected design choices during planning and construction.
Audible noise limits were applicable to both the Haywards
and Benmore locations. Due to residential areas in close
proximity to the Haywards site, very stringent weighted
noise limits were required. The noise limits at the Benmore
site are due to a recreational area on the southern boundary,
located across a body of water. It has little activity during the
winter but is a popular attraction in the summer for camping.
Maintenance access was required to be wider to allow
movement of both scissor-lift and boom-type mobile elevated
work platforms. This stemmed from an increased focus on
safety as well as changes that occurred in occupational safety
and health regulations. The increased width of the maintenance access areas affected the ac switchyards, dc switchyards, ac filter compounds, and cable terminal stations.
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ieee power & energy magazine

Haywards and Benmore are operating substations, and
the converter facilities must be built within a constrained
area. The site at Haywards substation is bounded on two
sides by state highways; a regional park is to the north, and
a residential area is at the southern boundary. Land around
the substation available for development was not level and
required civil work to become usable. The Benmore substation is in the same location as the Benmore dam (and is constrained by the surrounding terrain).
One of the primary design aspects was the design coordination between the original pole 2 and new pole 3. The
energy capabilities and ratings of the pole 2 arresters could
not be exceeded. The new "A"-arresters were rated to withstand the energy duties caused by a bipolar load rejection.
This means that the increased transmission system rating
and the extended reactive compensation scheme of the bipolar system were taken into account. In a similar fashion, the
new arresters installed at the dc neutral bus coordinate with
the existing "E"-arresters in a way to ensure lower switching
surge/low frequency overvoltages for pole 2 equipment.
A unique reactive power control (RPC) system was developed for the HVdc link. The objective of the Haywards station is
to control the 220- and 110-kV ac bus voltages. This is accomplished through the coordinated control of the reactive power
supply and on-load tap changers of the substation transformers.
The RPC maintains the 220- and 110-kV bus voltages while
minimizing loading and balancing the output of the synchronous condensers as well as keeping the number of tap-changer
operations to a minimum. The objective of the RPC at the Benmore station is to either control the voltage at the 220-kV bus or
control reactive power exchange with the ac system. It accomplishes this by a similar strategy used at the Haywards station.
A new controller serves to minimize the absolute frequency difference of both ac systems connected to the HVdc
link and is called a frequency-keeping controller (FKC).
If the control system determines that both ac frequencies
are the same, system reserves can be sourced on either ac
island system as if the HVdc link was an ac link. The FKC
was developed to supplement two existing controllers, the
frequency stabilization control (FSC) and spinning reserve
sharing (SRS). These two controllers were already in place
prior to the refurbishment. If a sudden frequency deviation
occurs on either island, the FSC provides frequency support
from the opposite island's ac system. The SRS controller
functions to share any frequency deviation outside of the
normal operating frequency bands between the two islands.
It enables generators in both power systems to provide power
march/april 2016

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