HVDC Inter-Island Link Upgrade Project
Pole 1 Replacement Options
Report No:
Revision:
Date:
598-05-4000-1
2
1 August 2005
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HVDC Inter-Island Link Upgrade Project - 598-05-4000-1
Pole 1 Replacement Options
Disclaimer
This report was prepared under the supervision of Teshmont Consultants LP. (“Teshmont”),
whose responsibility is limited to the scope of work as shown herein. Teshmont disclaims
responsibility for the work of others incorporated or referenced herein. This report has been
prepared exclusively for Transpower NZ Ltd. and the project identified herein and must not be
reused or modified without the prior written authorization of Teshmont.
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Pole 1 Replacement Options
Table of Contents
Page
1. Introduction ..................................................................................................................1
2. Recent HVDC Refurbishment Projects .......................................................................1
2.1
2.2
2.3
Nelson River Bipole 1- Pole 1 .............................................................................................1
Nelson River Bipole 1 Pole 2 ..............................................................................................2
Pacific Intertie Mercury-Arc Valve Replacement ................................................................4
3. Costs of Replacement of Benmore-Haywards Pole 1 on Piece by Piece Basis.........8
3.1
3.2
3.3
Option 1 - Piece by Piece Equipment Replacement ..........................................................8
Option 2: Single Pole with Equivalent Rating and Voltage of the Existing Pole1 ..............9
Option 3: Replace with Single Pole with Equivalent Rating to Pole 2..............................10
4. Discussion..................................................................................................................10
5. Conclusions ...............................................................................................................11
6. References.................................................................................................................12
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Pole 1 Replacement Options
1. Introduction
This document describes an assessment of two possibilities for replacement of Pole 1 of the
Benmore-Haywards HVDC link; i) complete replacement and ii) replacement of the pole by
replacement of individual pieces of equipment. The assessment was carried out at the request of
Transpower.
Renewal of HVDC systems by other owners in recent years has followed both of the
philosophies noted above. The reasons for following a particular philosophy of replacement are
primarily economic but may not be exclusively so. This memo discusses in a qualitative way
some of the factors considered in some of the recent HVDC refurbishments.
The systems that are most relevant to this discussion are the replacement by Manitoba Hydro of
the mercury-arc valves on the Nelson River Bipole 1 system and the recent replacement of the
mercury-arc valves by Bonneville Power Authority (BPA) and Los Angeles Department of Water
and Power (LADWP) on the Pacific Intertie system.
The memo concludes, based on cost and other factors, that complete replacement of the
equipment with new equipment is the preferable replacement option for NZ HVDC Pole 1.
2. Recent HVDC Refurbishment Projects
There are two projects that have recently been refurbished in conjunction with phase out of the
mercury-arc valves at these stations; the Nelson River Bipole 1 Valve replacement and the MAV
replacement of the Pacific Intertie. It is useful to consider the circumstances that lead to the
upgrade paths selected in each case as they may be directly relevant to the upgrade of the
HVDC Hybrid Link in New Zealand.
2.1
Nelson River Bipole 1- Pole 1
The Nelson River Bipole 1 HVDC system consists of three mercury-arc valve groups in each
pole. The MAV’s were replaced in two steps. The first Pole consisting of three mercury-arc valve
groups was replaced in the early 1990’s while the second pole was replaced in 2004. The
replacement of the first pole was justified based on the shortage of spare parts, especially
porcelains, for the mercury arc valves.
When the first pole was replaced, the six individual valve groups in each 6-pulse valve group
were replaced by three double-valves, which could be accommodated within the original valve
halls as shown in Figure 2-1. This required the replacement of all of the equipment in the valve
halls except the bypass switch, which needed to be retained due to the series configuration of
valve groups within each pole. Other equipment that was replaced included the valve cooling
equipment and the valve group controls. Oil filled reactors in the ac and dc filters were also
replaced to reduce maintenance effort and reduce a source of unreliability.
Valve damping components that were no longer needed following removal of the mercury-arc
(MA) valves were removed.
The converter buildings, converter transformers, ac filters and synchronous condensers were not
replaced. The Bipole level controls were also retained. These controls are of an analogue design
and are not duplicated. Spares are not a problem as Manitoba Hydro has its own in-house
designs of the control cards and has the ability to manufacture replacements locally.
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Nelson River Pole 1 - Rearrangement of Equipment within Each 6-Pulse Valve Group
During Valve Replacement
Figure 2-1
2.2
Nelson River Bipole 1 Pole 2
By 2003, it had become clear that the second pole of mercury arc valves should also be
replaced. Reasons for the replacement included
•
•
•
Deteriorating reliability of the MAV's and associated equipment.
Continuing high maintenance costs for the valves combined with a decline in the number
of skilled personnel available for maintenance.
Concern for the converter transformers, which are stressed by arc-backs and CAB’s.
The condition of the converter transformers was known to be deteriorating and Manitoba
Hydro had already embarked on a programme of providing spares for each type of
transformer.
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•
Favourable market conditions. Due to the lull in HVDC market at the time, and because
the timing was just after the replacement of the MAV’s at Celilo it was felt that the prices
would be favourable.
Other items of note include:
•
It was decided not to replace the controls (with the exception of the valve firing controls)
as these controls were still functioning reliably and Manitoba Hydro personnel were fully
capable of maintaining the controls.
•
The buildings at both converter stations were in good shape and no significant changes
would be needed to accommodate the new valves.
•
The insulators for the valve stands had recently been replaced.
•
Manitoba Hydro did not want to replace the converter transformers as they had already
initiated a program of obtaining converter transformer spares.
•
Manitoba Hydro wanted to perform much of the construction work using its own
construction forces.
•
Much of the development work for the roll-on valve had already been completed by
suppliers for the Celilo valve replacement and thus the supplier was in a good position to
offer a favourable price. Another supplier also offered a roll-on valve of a new design at a
very competitive price. A third valve design similar to the earlier Pole 1 replacement
carried out earlier was not competitive with the two roll-on designs.
All of the above factors made it extremely favourable for use of a roll-on replacement valve
rather than a full replacement of the MAV equipment. The final configuration selected was a onefor one replacement of the six MAV’s in the valve hall with thyristor valves. As with the Pole 1
replacement, it was necessary to retain the bypass switch because of the series valve groups in
the pole.
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Nelson River Bipole 1 Pole 2 – Showing One-for-One Replacement of Each MAV
In One Valve Group
Figure 2-2
2.3
Pacific Intertie Mercury-Arc Valve Replacement
The Pacific Intertie project in the western USA has also undergone a recent upgrade in which
the MAV’s have been replaced by thyristor valves. The replacement method used was different
at the two ends primarily due to condition of equipment.
The configuration of the Pacific Intertie prior to and after the upgrade is shown conceptually in
Figure 2-3 and Figure 2-4. The initial project included three 133 kV valve groups per pole. Due to
conservative line design, it was possible to add a fourth series 100 kV group at a later time. This
was implemented using thyristors rather than mercury-arc valves. Later it was decided to add
parallel valve groups to fully use the current capability of the line conductors. The parallel valve
groups consisted of a single 500 kV - 550 MW valve group in each pole.
At the Celilo Station, the Converter transformers and other HVDC equipment were in relatively
good condition. As the Celilo area has lower seismic requirements and has had fewer
earthquakes during the lifetime of the plant than the Los Angeles region there was less concern
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with the condition of the building or equipment. The primary concern was over the condition of
the mercury-arc valves, which could easily be replaced on a one-for-one basis. These factors
made the selection of roll-on valves to replace the mercury arc valves at Celilo extremely
favourable.
At Sylmar, the situation was quite different compared to Celilo:
•
The converter transformers were not in good condition due to the effects of earthquakes.
•
The original converter building was not in good condition due again to the effects of an
earthquake that had resulted in damage to the MAVs. There was also a safety concern
due to the spillage of mercury in earlier earthquakes.
•
Even the converter transformers of the 1100 MW expansion were considered suspect as
some damage had occurred in the Northridge earthquake. The bushings of the spare
transformers were also damaged. Thus it was felt that transformers with higher
earthquake withstand capability were needed.
•
The building of the 1100 MW expansion valve groups was designed to higher seismic
standards and was large enough to accommodate a single 500 kV 12-pulse valve group
per pole with sufficient rating to replace all five valve groups in the pole. A new building
was not needed.
•
Replacing the five existing valve groups with a single converter would significantly
reduce maintenance and would solve an ac system restriction that existed between the
two buses at the Sylmar station.
Taken together these factors clearly pointed to a different upgrade path than was chosen at
Celilo. It was decided to replace all of the HVDC equipment at Sylmar with two new ±500 kV
valve groups with a total rating of approximately 3200 MW.
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Sylmar Station
12P-THY
12P-THY
6P-THY
6P-MA
6P-MA
6P-MA
6P-MA
6P-MA
6P-MA
6P-THY
6P-MA
6P-MA
6P-MA
6P-MA
6P-MA
6P-MA
12P-THY
6P-THY
6P-THY
12P-THY
Celilo Station
Valve Group Configuration of the Pacific Intertie prior to MAV Replacement
Figure 2-3
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Valve Group Configuration of the Pacific Intertie after MAV Replacement
Figure 2-4
The equipment shown in red in Figure 2-4 indicates the main equipment changed in the upgrade
of the Pacific Intertie project.
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3. Costs of Replacement of Benmore-Haywards Pole 1 on Piece by Piece Basis
The cost of replacing the HVDC equipment in Pole 1 at Benmore and Haywards has been
estimated to determine whether there is any advantage to taking a piece-by-piece replacement
approach or a complete replacement approach. The cost of equipment and construction only
was considered without factoring in such factors as costs due to outage time. The costs were
determined using Teshmont’s in-house HVDC estimating sheet, which is based on prices for
recent projects, and pricing information requested from suppliers for other projects. Due to the
short time frame of this work, it was not possible to obtain new costs for this project.
Items not included in the cost estimates any of the Options are:
•
•
•
•
•
•
3.1
AC breakers for the converter valve groups. It is assumed that the existing breakers
would be used in the one-for-one replacement option
Synchronous condensers. - In any new configuration including the piece-by piece
replacement, it should be reviewed whether the condensers should be installed at the
tertiary of the converter transformers. From an operational point of view it would be
preferable if the condensers were connected at the 110 kV or 220 kV buses using unit
transformers. However this would not be easy to achieve in the piece-by-piece
replacement scenario due to space constraints at the site. The Haywards synchronous
condensers are an important part of the system voltage control in the Wellington region
and are needed to provide adequate short circuit capacity for reliable HVDC recovery
following disturbances. The inertia of the condensers also helps to reduce the frequency
drops during transient disturbances. For these reasons it is expected that the routine
maintenance synchronous condensers would continue regardless of whether the HVDC
is replaced on a piecemeal or complete basis. The cost of new unit transformers has
been added for the complete HVDC replacement options. For the piece-by-piece
replacement, the cost of the converter transformers includes an allowance for tertiary
windings to accommodate the condensers.
Taxes and duties.
Interest during construction.
Cost of losses is not considered but by inspection the losses in the piece-by piece
replacement option where all four valve groups are retained as in the present
configuration, would be significantly higher than if pole 1 were replaced by a single 12pulse valve group at each station.
Cost of outages needed when replacing equipment on a piece-by-piece basis.
Option 1 - Piece by Piece Equipment Replacement
The estimated costs shown in Table 3-1 below were determined based on replacing the
equipment at each converter station with the four valve groups retained in the same
configuration as the existing Pole 1 configuration.
The cost includes a contingency amount for additional civil works to strengthen the building or
improve foundations/retaining walls for other equipment as needed. The amount cannot be
established with confidence at the moment but, as some subsidence in both the converter
buildings and synchronous condenser foundations has been noted, it is considered prudent to
add an allowance to permit the necessary remedial work.
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Table 3-1
Estimated Cost Breakdown (per station) For a Replacement of Pole 1 Valve Groups
On a Piece by Piece Basis
Unit Cost
Total Cost USD
4 Valve Groups
$4,700,000
$18,600,000
4 Control Sets
$1,500,000
$6,000,000
1 DC Filter (Reuse existing)
$0
$0
4 Smoothing Reactor Coils(2 per Pole)
$1,150,000
$4,600,000
DC Valve Group Switching for 4 groups
$475,000
$1,900,000
(bypass switch and disconnects)
4 Valve Group Cooling
14 Converter Transformers (2 spare)
DC Line Switching
AC Filters
Electrode Line Switching
(disconnects plus commutating switches/etc)
$575,000
$2,250,0003
$2,300,000
$31,500,000
$240,000
$950,000
$480,000
$1,900,000
$800,000
Miscellaneous (Arresters, Auxiliary power,
Instrument Transformers)
$2,320,000
Installation of Converter Equipment
Civil Work
Converter Building
Contingency for additional civil work, seismic
strengthening, correction of settlement1
$16,300,000
$1,900,000
$1,900,000
$3,000,000
Valve Hall Air Conditioning/Air handling (four halls)
Overhead2
TOTAL
$500,000
$2,000,000
$19,440,000
$114,940,000
Notes:
1. Civil work contingency is very preliminary. The work would be defined following detailed site
investigations. A substantial value is needed due to indications of settlement of the converter
building and synchronous condenser foundations at Haywards.
2. Overhead includes engineering, project management, owner's administration, and 10% overall
contingency.
3. Includes cost of tertiary winding for the condensers at Haywards. At Benmore the transformers
would be slightly cheaper but it would be necessary to add a new interconnecting transformer
between the 16 kV and 220 kV bus.
3.2
Option 2: Single Pole with Equivalent Rating and Voltage of the Existing Pole1
Costs for 600 MW, 270 kV, 1 valve group per pole (per station): USD$63,800,000
This assumes a new converter valve hall adjacent to the pole 2 building, new ac filters, new dc
side switches, new step-up transformers for the synchronous condensers at Haywards and a
third 220/16 kV interconnecting bank at Benmore. New smoothing reactors are also assumed at
each station. Cost of dismantling and disposal of the existing Pole 1 equipment is not included.
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3.3
Option 3: Replace with Single Pole with Equivalent Rating to Pole 2
Costs for 560 MW, 350 kV, 1 valve group per pole (per station): ........................USD$65,800,000
The assumptions are the same as for Option 2.
4. Discussion
Many of the technical issues that provide support for replacing or refurbishing Pole 1 have been
documented in the condition assessment of the Pole 1 equipment by Transpower[1] and the 17
July report prepared by Mike O’Brien [2]. These factors include many of the same factors that
have lead to the replacement of mercury arc valves at other locations.
•
High cost of maintaining mercury arc valves
•
Declining number of skilled personnel to maintain valves both within New Zealand and
worldwide.
•
Health concerns with mercury
•
Earthquake risks ( the MAV’s, damping components and pole 1 converter building at
Haywards would not be capable of withstanding the design basis earthquake)
•
Concern over converter transformer condition due to the cumulative effects of arc-backs
and CAB’s.
•
Age of equipment especially valves, bushings, valve-damping equipment. Spares of this
equipment are no longer available.
•
The ac filter capacitors are also questionable due to seismic concerns and corrosion of
the capacitor can mounts. While spares can be obtained, the cost of individual spare
capacitor cans is significantly higher than cost of replacing the complete filters with new
components.
•
Fire risk
•
Concern over control system reliability
•
Audible noise concerns with older equipment
•
Synchronous condenser unreliability (the synchronous condensers on the HVDC
transformer tertiaries are required for voltage control of the HVDC system)
As these reports indicate that most of the major equipment in pole 1 is in need of major
refurbishment or replacement, the above issues provide a relatively strong case for complete
replacement rather than piece-by-piece replacement.
Piece by piece replacement would be advantageous only if there were some major equipment
that was in good condition with sufficient remaining life to make it worth maintaining. The
equipment would also need to have a low risk of sudden deterioration due to end of life effects.
In the case of Pole 1 there is very little equipment that has significant useful life remaining. The
MAV’s, converter transformers, ac filters and dc controls are all reaching end of life. The
converter buildings especially at Haywards may not have adequate seismic capability. The only
major equipment that appears worth retaining is the synchronous condensers (C7-C10) which
can likely be refurbished for much less than replacement cost. The condensers are needed to
provide short circuit strength inertia and voltage control in the Haywards area, regardless of
which upgrade option is selected, and therefore planned maintenance expenditures on the
condensers should continue.
Comparing the cost of these replacement options further strengthens the case for full
replacement rather than piece-by-piece replacement. The cost estimates indicate that installation
of a new single 12-pulse valve group to replace the four 6-pulse valve groups now making up
Pole 1 would be about 55% of the cost of a one-for-one replacement of the existing equipment.
As there would be significantly less equipment in the single 12-pulse valve group configuration,
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the control complexity would be less, the amount of maintenance needed would be less and the
reliability would be expected to be higher than the existing arrangement. The control system
would be a standard solution common for both poles.
The configuration at Benmore would seem to be more likely to be suitable for a piece-by-piece
replacement as there are some limitations in interconnecting bank capacity between the 16 kV
busses and the 220 kV. However, the above cost estimates indicate that the price differential
between a completely new pole and piece-by-piece replacement is high enough to allow the
purchase of an additional interconnection bank between the 16 kV bus and the 220 kV bus. This
would make it possible to flexibly dispatch the Benmore generation for use on the South Island
or for transmission to the North using either pole of the HVDC.
The piece-by-piece replacement solution would also involve very extensive outages of existing
capacity, and significant difficulties (especially at Haywards) in working around and close to other
operational equipment. This could add huge additional costs not factored into the above cost
estimates. These technical difficulties and associated costs could well completely rule out the
piece-by-piece replacement option.
Installation of a completely new 12-pulse converter at the 220 kV buses at both Benmore and
Haywards would result in a symmetrical HVDC circuit configuration and would remove some
operating limitations of the existing system. The new pole could be completely constructed while
the existing pole 1 equipment remains in service. This would avoid the inconvenience and costs
associated with long outages and allow commissioning to begin at any suitable time. It would
also be possible to switch back to the existing Pole 1 equipment when not actively testing during
commissioning.
Consolidation of the four existing 6-pulse valve groups in Pole 1 into a single new 12-pulse
converter would provide many advantages including reduced maintenance, reduced footprint,
reduced audible noise and simplified operation.
5. Conclusions
The condition assessment indicates that there is very little major equipment or infrastructure
(buildings) in Pole 1 at Benmore or Haywards that can be maintained in service without
substantial effort and cost for refurbishment. It is also significantly cheaper (by a ratio of about
1.8 to 1) to install a single new 12-pulse converter per pole rather than refurbishing the 4 existing
6-pulse valve groups at each station. A single new 12-pulse converter would have lower losses
and simplified main circuit configuration. This would improve reliability and make operation and
maintenance easier. As there is very little existing equipment in Pole 1 that would have
substantial remaining life with little risk of sudden deterioration or failure, there are no apparent
technical or financial benefits to upgrade Pole 1 of the HVDC link on a piece by-piece basis.
Construction of a completely new 12-pulse converter would also permit the existing Pole 1
equipment to remain in service until the new Pole 1 equipment can be commissioned thus
avoiding inconvenience and costs associated with long outages during construction and
commissioning.
Based on these considerations, the most suitable way to refurbish the existing Pole 1 equipment
would be complete replacement as a single 12-pulse converter at each station.
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6. References
1.
“HVDC Link Pole 1 Condition And Risk Assessment” dated June 2005 prepared by Transpower
2.
“HVDC Transmission Pole Life Extension Options” dated 17th July 2005
prepared by Michael O’Brien
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