2012 Paris Session B4-309
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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.
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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 (
