2012 Paris Session
B4-309
http : //www.cigre.org
Tres Amigas : A flexible gateway for renewable energy exchange between the
three asynchronous AC networks in the USA
N M KIRBY1, N M MACLEOD2, D STIDHAM3, M REYNOLDS4
1
Alstom Grid, USA
2
Alstom Grid, UK
3
Tres Amigas LLC
4
Power Engineers, USA
SUMMARY
Tres Amigas is an ambitious and innovative project to provide a three way 5000MW interconnection
between the three independent networks in the USA. The electric grid in the USA is operated as three
separate asynchronous 60Hz systems, covering the western states (WECC), eastern states (Eastern
Interconnected System) and the state of Texas (ERCOT). The only connections between these
systems presently, are through a series of small and now ageing, HVDC back-to-back stations.
Located near the border of New Mexico and Texas, the Tres Amigas Super-station will consist
ultimately of six HVDC back-to-back converter stations, which can be configured to connect to each
of the three AC systems. The scheme will transport power across the links to exploit the differences in
energy process on the three regions. More significantly, because of the much higher power rating than
is presently available through the existing links, it will allow a much broader reach for exchange and
trading of the high levels of renewable energy which are expected to be realized in the near future
across the region.
The initial three converter stations will be built using VSC HVDC technology to provide up to
2250MW of transmission capacity, since VSC has the benefit of being capable of operating reliably,
despite the relatively weak 345kV systems in the proposed site area. To improve the AC infrastructure
in the region and allow the station to export renewable energy, new 500kV and possibly 765kV lines
will be built over time, which will strengthen the networks, and it is envisaged that these stronger AC
systems will allow the remaining three converter stations to be built using LCC HVDC technology.
The LCC stations can be designed with high levels of continuous and short-time overload capability to
maximize the power transfer capability of the scheme during a scheduled or forced outage of a single
pole.
Although the first stage of the installation is being designed as a single link, the system is planned with
operational flexibility and future expansion in mind, and facilities are being incorporated into the
design which will allow for this, including an accessible DC busbar in the back to back circuit. There
is the potential to incorporate other novel features in the future including large-scale battery energy
storage and eventually superconducting cables. The first stage and each of the future 5 stages will be
capable of independent, individual control, and they may also be placed under the control of an overall
coordinating controller in the form of an operational and market management system.
When the six HVDC links are fully installed this will potentially provide energy gateways for up to
5000MW of power to be exchanged between the three regional AC networks in effectively a multiterminal configuration.
[email protected]
A key feature of the Tres Amigas development and operating principle is maximum use of renewable
energy, not only in the HVDC power transmitted but also in the operation of the superstation itself.
The station will both transmit and make use of wind, solar and other available forms of renewable
energy throughout the regional AC networks.
Construction is anticipated to start in mid-2012, with the individual poles being commissioned from
2014 onwards. When completed this will be the largest back-to-back HVDC installation in the world,
and a unique facility to enhance the transmission network in the USA.
KEYWORDS
HVDC, VSC, IGBT, Tres Amigas, Renewable, New Mexico
1. INTRODUCTION
The application of large scale Voltage Source Back to Back HVDC Converters to the three North
American (NERC-Regional) Grids of WECC, SPP and ERCOT presents unique challenges and
significant power and transmission scheduling opportunities. The Tres Amigas (TASS) Project is a
unique staged and planned multi-step development of the physical facilities and will have profound
marketing impact on energy and transmission markets in these three regions. The TASS project is
expected to support the significant growth of renewable energy projects by providing transmission
access for newly developing wind generation, and time of day diversity in an area of the United States.
While VSC technologies have been provided in other HVDC arrangements, this project uniquely
applies up to six back to back HVDC converters in an arrangement using proven GIS/GIL technology,
leading edge IGBT and conventional thyristor-based converters in an arrangement offering maximum
flexibility to expand and properly interconnect up to 5000 MW of transmission capacity, in a flexible
real time controlled environment. As shown in Figure 5 this station will be located in Central East
New Mexico allowing access to WECC, SPP and ERCOT networks while continuing isolated
interconnection between all three regions, in the middle of a large concentration of proposed
renewable energy areas of the United States desert south west.
2. VOLTAGE SOURCE CONVERTER TECHNOLOGY
A number of different VSC HVDC technologies have been used for power transmission, however, in
the last few years there has been a consolidation from all manufacturers onto a single solution, the
Modular Multi-level Converter (MMC). The basic topology of a MMC converter is a 6–pulse bridge
arrangement, as shown in Figure 1.
Figure 1 - Modular Multi-level Converter topology
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Each “chain link” or sub- module contains a distributed DC capacitor, which acts as the source of
energy, and an arrangement of power electronic switches. These switches use Insulated Gate Bipolar
Transistors (IGBT), which are bi-directional semi-conductor devices which can be turned on and
turned off by a local gate electronic control board. For the Tres Amigas project which uses a Back–
to–back converter solution, the arrangement of one module is shown in Figure 2, together with its
output voltage waveform.
Module Output voltage
U
Figure 2 - Power electronic sub-module and output voltage waveform
Each DC capacitor is normally operated at a DC voltage of 1.8kV, hence for the operational voltage of
the Tres Amigas converter station, of ±326kV, there are many hundreds of series connected submodules. When switched into and out of service in the correct sequence the MMC topology is able to
generate an output voltage waveform which is virtually sinusoidal, as illustrated in Figure 3,
The presence of the connection transformer, between the converter and the 345kV AC network further
improves the quality of the voltage sine wave exported on to the AC system. No harmonic filters are
required on the AC side of the VSC converter to control harmonic distortion, which provides a
significant reduction on the site area requirements, compared with a classic Line Commutated
Converter (LCC) technology. As the Tres Amigas converter operates as a symmetrical monopole, the
connection transformer is connected at a point in the 6–pulse converter which has no continuous DC
voltage stress. This means that the transformers for the Tres Amigas VSC converters can be of a
conventional two winding design, without the cost and complexity of design and testing normally
associated with an HVDC converter transformer.
AC Supply Voltage
Converter Voltage
Voltage (kV)
Current (kA)
AC Phase Current
Time (ms)
Detail of
Converter
Voltage
Figure 3 - Converter output voltage waveform
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The VSC converters are Alstom Grid Maxsine systems, and in the configuration for this project the
IGBTs used are commercially available 3.3kV devices, which can switch up to 1500A. However a
number of protective measures need to be implemented, to ensure that these devices are not damaged
under adverse operating scenarios. In the event of a short circuit on the DC bus, the fault current in
the in-built free-wheeling diode of the IGBT pack construction, may be beyond the short term current
capacity of the diodes. To protect against this, a protective thyristor is includes in the sub-module
design, as shown in Figure 4, which is gated into conduction if the fault current exceeds a specified
design threshold.
Capacitor +ve
Test Terminal
IGBT1
D1
Main
Terminal
1
R1
C1
IGBT2
SW1
T1
D2
Capacitor
-ve
Terminal
Figure 4 - Protective features of the sub-module
Unlike power thyristors which are constructed from a single silicon wafer, IGBTs contain multiple
parallel-connected wafers, each with a wire bond to the terminals of the pack. An open circuit on one
of these connections could lead to a disruptive failure of the IGBT device. To protect against this
eventuality, a fast acting (<4ms) mechanical by-pass switch is installed, which will short out the
complete sub-module upon detection of an excessive voltage across the device. This by-pass switch is
powered and controlled by the gate electronic unit installed in each sub-module. Connected in parallel
with the DC capacitor is a resistor, which will discharge the stored energy in the capacitor to make it
safe for maintenance handling.
For further information the reader is referred to the documents listed in the Bibliography.
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PROJECT DESCRIPTION
3.1
Project Arrangement
The initial project concept called for the development of a three way HVDC Bus with the potential use
of High Temperature Superconductor Cable (HTSC) and HVDC Power Circuit Breakers (HVDCPCB) systems. After significant research, market and reliability investigations, it was determined that
this concept could not be executed with the current technology offerings in HVDC and without a
significant technological risk for these items of hardware and the proper protection of the IGBT based
HVDC valves. A new concept was formed in late 2010 and it was determined that technology on the
HVAC side of the station would initially be configured to quickly change configuration (in as short as
1 minute or between existing 15 minute power & transmission scheduling timing windows). Figure 6,
shows the proposed layout of the fully expanded Tres Amigas Project. The reader should note that
both VSC and conventional Line Commutated Converters (LCC) are proposed in the full system build
out. The TASS concept will start with the development of a single 750 MW VSC converter, to allow
the connection of the converter to one area of relatively limited Short Circuit Strength (SC) in New
Mexico and allow a simplified HVAC filtering and reactive deployment strategies. Further project
details are available in the documents listed in the Bibliography.
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Figure 5 - Project Location in the United States
The concept will be expanded to the full 5000 MW conversion potential by adding 3 x 750 MW VSC
and 3 x 920 MW LCC converters. Each of the converters will be configured in a new concept call
“folded Back to Back” in order that the HVDC bus (~ 300 kVdc) is exposed to allow the future
possibility of HVDC-PCB switching and the utilization of future HTSC cables to extend the TASS
system to further extend the node concept. The use of other technologies is also made possible by the
arrangement of the HVDC station bus in an “outdoor configuration”.
The center of complexity was moved to the 345 kV HVAC side of the station and will be implemented
using presently available current 345kV GIS and GIL Technology from Alstom Grid. The 345 kV bus
is to be configured to allow flexible additions of new bay positions, while being able to maintain a
high capacity (up to 10,000 Ampere rated main bus). Each line position, PCB position, and expansion
points will be configured with a “dead front - termination” installation concept to allow the addition
and expansion of the system without outage of the main bus. These arrangements in GIS will allow
compact and modular design, and implement the use of fast motor operated disconnect switches within
the GIS to make rapid configuration changes to accommodate; change of direction, regional
assignment, and maintenance switching; in a fully automated and interlocked system to assure
isolation of each of the NERC Regional networks is always assured.
The first VSC
750 MW
Converter
Figure 6 - Fully Expanded 5000 MW TASS Site Overview
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3.2
Network Issues & Challenges
At the present time the TASS Management team is preparing appropriate system interconnection
studies, under the management of Burns and McDonald working through the first two interconnected
utilities Xcel (SPP) and PNM (WECC). New 345 kV Transmission lines are planned to Tolk
Substation in Texas and Blackwater Substation near Clovis New Mexico. Although these studies have
not yet been fully completed, there are “weak network” challenges on the PNM side of the TASS
Project which favor the application of new VSC technology, capable of operating to lower Equivalent
Short Circuit Ratio Levels (ESCR). Line routing and environmental permitting and related work is in
progress for these first two interconnections. Current systems studies are reviewing a wide range of
reactive support options on the Western side of the station. The use of VSC converter technology
allows real-time control of reactive balance, with a smooth venire control, and with the possibility of
more complex control of and coordination of external devices such as SVCs and Synchronous
condensers/STATCOMs, as the dynamic reactive requirements change over a wide range of ac
network contingencies. Early results of studies continue to show significant reactive reinforcement on
the Western side of TASS will be required, and moderate to no additional reactive support on the
Eastern side of the TASS station appear needed (subject to continued studies and changing network
conditions).
3.3
Project Details and staging
The first stage of the project will install the first VSC converter and the base GIS/GIL system. The
second stage will add a second VSC converter, and additional 345 kV GIS/GIL bays and transmission
line positions, as well as extending the significant (14.4 -24.9Y kV) medium voltage (MV) station
service distribution system. Transmission expansion is expected to SPP at this level, as well as
transmission improvements to WECC. The 3rd through the 6th stages will be fully market driven and
be subject to interconnection studies, transmission line siting processes, and local renewables
development near the TASS Station, and availability of project funding.
Battery Energy Storage (BESS) systems will augment the medium voltage station service distribution
system, and be initially sized at 100 MWhr (2 x 50 MWhr). Other wind, solar and biomass projects
may be developed locally and can be interconnected at 345, 115, or 14.4-24.9Y kV levels.
The Figure 8 shows a simulated view of the first 750 MW VSC Converter, showing the folded design
with the HVDC Air Insulated (AIS)switchyard and the bottom of the figure and the HVAC lines from
the GIS/GIL system entering from the top of Figure 7.
Figure 7 - Folded BtB VSC Converter External View
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3.4
Station Single Line
The TASS Station is to be built in stages, with each stage taking advantage of substation infrastructure
developed in earlier stages. The first Stage of development is shown in Figure 8, and shows the
incoming 345 kV transmission from Xcel’s Tolk Substation (and Coal fired Thermal Generation
Station) in Western Texas (SPP-Grid) and the Western connection to Blackwater Station of Public
Service of New Mexico (PNM) 345 kV Transmission line. The PNM which extends to the
Albuquerque metropolitan region, along with significant Wind Generation Station Developments
connected to this line. The Battery Energy Storage Systems (BESS) are connected via the 14.4-24.9Y
to 345 kV Transformation, as well as the possible addition of synchronous condensers. The
Blackwater Substation (PNM) is located approximately 20 miles south of the TASS site.
Future expansion of the TASS facility will be achieved by repeating the Stage 1 circuit for each phase.
Figure 8 - 1st Stage Development One Line
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4. CONCLUSION
The commercial environment for development of the US power networks has emerged such that it is
open to both traditional utilities and private sector organizations as standalone business opportunities.
The Tres Amigas project is a private sector business concept, which has been developed to give a
staged, expandable system, using available state-of-the-art technology, taking advantage of the unique
geographical combination of access to all 3 power networks, and a large combination of renewable
energy sources. The paper has shown the detailed plan for development of the Tres Amigas station,
which will provide the largest incremental rise for many years in energy exchange capacity between
the 3 electric networks in the US.
BIBLIOGRAPHY
“VSC HVDC Converter Design with Fault Blocking Capability for OHL Applications” – NM
MacLeod, CD Barker, RS Whitehouse, W Liang - EPRI HVDC & FACTS Conference, Palo Alto August 2011
“The Tres Amigas Superstation - Uniting The Nation’s Electric Power Grid” – D. Stidham - EPRI
HVDC & FACTS Conference, Palo Alto - August 2011
“Reactive Power Loading of Components within a Multi-Module HVDC VSC Converter” – CD
Barker, NM Kirby - IEEE PES EPEC Conference, Winnipeg - September 2011
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