by Thomas F. Garrity, Siemens
Despite lingering effects of the economic recession, investment in transmission projects for grid reliability, congestion relief and the integration of new and cleaner generation sources has continued with only a modest slowdown in projects.
The evolution of smart grid technology for the U.S. power systems represents a significant opportunity to relieve congestion on the bulk power supply network. Eliminating or reducing congestion can facilitate the integration of renewable generation, such as wind and solar, and allow greater access to cleaner, lower-emission generation sources.
Siemens Energy Inc.’s U.S. energy transmission projects are rapidly progressing with construction. Two examples in various stages of construction are San Francisco’s Trans Bay Cable Project and the Static VAR Compensator Project near Chicago.
Trans Bay Cable Project
The Trans Bay Cable Project comprises a high-voltage direct current (HVDC) connection between HVDC converter stations in Pittsburg, Calif., and San Francisco (see Figure 1). The contract was awarded by Trans Bay Cable LLC, the owner, to the Trans Bay Cable consortium of Siemens Energy Inc. and Prysmian Construction Services Inc. The Trans Bay Cable link is designed to transmit 400 MW of power between Pittsburgh and San Francisco at a transmission voltage of +/- 200 kV. The transmission link is a 53-mile-long HVDC cable for the land and underwater portions of the connection, with most of the cable underwater.
Figure 1: Transmission route for HVDC submarine cable across San Francisco Bay
In addition to providing access to an additional power source for San Francisco, the smart HVDC Power Link Universal System (PLUS) technology for the transmission link will supply reactive power support for enhanced voltage control to the Pacific Gas & Electric Co. grid. The innovative voltage source converters will provide +/- 145 MVAR of reactive power at Pittsburgh and +/- 170 MVAR at the Potrero Converter Station in San Francisco. The voltage support capability of the HVDC PLUS technology is an example of the Siemens smart grid control for enhancing grid performance.
Figure 2: Air core reactors installed in the ac switchyard at Pittsburgh, Calif., with the valve hall in the background
The voltage source converters at the heart of the HVDC PLUS solution employ insulated gate bipolar transistors (IGBT) semiconductor devices configured in a modular multilevel converter (MMC), which provides many switching steps to reduce harmonics, minimize losses and reduce high-frequency noise. The multilevel converter design virtually can eliminate the need for alternating current (ac) harmonic filters in most cases, thereby contributing to significant space savings. Another advantage of the design allows the use of standard ac transformers.
|Figure 3: DC cable terminations in the Pittsburgh Converter Station; DC wall bushings installed at the valve|
Converter station construction is progressing, and the submarine cable will be laid this fall to achieve the March 2010 service date. The commissioning phase and system and transmission tests are scheduled to be complete by February 2010. Figure 2 shows progress on the Pittsburgh converter station.
The underground segment of the HVDC cable terminations have been completed at the Pittsburgh Converter Station, and the location of the cables in the direct current (dc) switchyard is shown in Figure 3. These cables will connect to the submarine cable portion of the dc link to complete the transmission path between Pittsburgh and Potrero (see Figure 4). The submarine cable is scheduled to be installed in October-November using a special cable-laying vessel designed for this purpose. The submarine cable is manufactured in one continuous length for each pole that will span the entire length of the bay.
|Figure 4: Hi pot testing of the ac cables at the Potrero Converter Station|
The Potrero Converter Station construction is on schedule for the project, and site activity is progressing. Commissioning teams to test all the station components, new voltage source converters and controls have been mobilized, and work has commenced on this new innovative design (see Figures 5 and 6).
Elmhurst Static VAR Compensation
The Elmhurst substation on the Commonwealth Edison Co. (ComEd) system is a few miles south of Chicago O’Hare International Airport. ComEd is a unit of Chicago-based Exelon Corp., one of the nation’s largest electric utilities with some 5.4 million customers. ComEd provides service to some 3.8 million customers across northern Illinois.
Two Siemens’ static VAR compensators (SVCs) are being installed at the existing Elmhurst substation to contribute to voltage support and stabilization of the northeast subzone (NESZ) within the heavily loaded ComEd service territory. ComEd’s systems analysis indicated the need for two 300-MVAR SVCs at the Elmhurst substation with nearly 100 percent availability to provide for dynamic reactive power support. The installation of the SVCs will help prevent voltage drops to unacceptable levels, possibly resulting in voltage collapse.
Figure 5: DC cable terminations at the Potrero Converter Station
Siemens Energy in a consortium partnership with Beta Engineering LLC of Pineville, La., is performing the turnkey project. The identical SVCs are rated at 0/+300 MVAR each and consist of three thyristor switched capacitor (TSC)s elements.
Figure 6: Transformers at Potrero with residences in background
During normal operation, the SVCs will control only the existing mechanically switched capacitor banks (MSCs) at the Elmhurst substation to provide steady state voltage support. In the event of a voltage drop, the TSCs will respond immediately and enhance the voltage stability of the network. This is another example of range of solutions offered by Siemens Energy that enable transmission grids to perform smarter and more efficiently. Because of the importance of switching steps, there are two star-connected TSCs each rated 75 MVAR, and one delta-connected, TSC-rated 150 MVAR.
Construction has commenced, and site work is progressing as seen in Figure 7).
Figure 7: Foundations for the major equipment and site grading started at Elmhurst
The Elmhurst SVCs will enhance the availability and quality of energy in the greater Chicago area, providing high-speed, dynamic reactor power to support voltage recovery following system faults and maintain improving steady state stability under extreme system loads or contingencies.
Figure 8: Foundation preparation at Elmhurst; nearby residences beyond fence and tree line
An important consideration for the Elmhurst site is the nearby proximity of residences (see Figure 8). Because of these residences adjacent to the site, a noise barrier has been designed to attenuate the sound levels that may be generated by the SVC, thereby shielding the residential area.
Thomas F. Garrity is vice president and senior consultant of high-voltage systems at Siemens.