2.1    INTRODUCTION

This section of the EIR Addendum describes the technical refinements and improvements that are planned to be made to the Trans Bay Cable Project (Project) associated with the decision to adopt Siemens HVDC PLUS technology and design components into the proposed Project design for the converter stations in San Francisco and Pittsburg. The Siemens HVDC PLUS technology only recently became commercially available and was not available for consideration in the Draft and Final EIRs for this Project.

The technology associated with the Siemens HVDC PLUS system is proven, reliable, and capable of meeting all of the Project objectives and goals consistent with the California Independent System Operator's (CAISO) approval of the Project on September 8, 2005. The Siemens PLUS technology/converter station design is also compatible with the necessary interconnections to the PG&E substations in Pittsburg and San Francisco. The physical advantages of the Siemens HVDC PLUS design (e.g., smaller footprint, lower building heights, less operational noise, etc.) relative to the Siemens "conventional" HVDC design evaluated in the Draft and Final EIRs were summarized previously in Section 1.0 of this EIR Addendum. Section 3.0 of this EIR Addendum provides additional analysis and documentation showing that the potentially significant environmental impacts of the now adopted Siemens HVDC PLUS technology/design are in all cases equal to or less than those identified in the Draft and Final EIRs.

The balance of this section is organized as follows:

• 2.2 – Project Objectives


• 2.3 – HVDC PLUS Technology/Converter Stations


• 2.4 – Submarine Cable Design and Installation


• 2.5 – San Francisco Converter Station


• 2.6 – Pittsburg Converter Station

2.2    PROJECT OBJECTIVES

The purpose and need for the proposed Project, including the Project objectives, are as described in Section 2.3 (Purpose and Need for Project) of the Draft EIR and Section 3.2 (Project Objectives) of the Final EIR.

2.3    HVDC PLUS TECHNOLOGY/CONVERTER STATIONS

2.3.1    Overview

The Siemens HVDC PLUS (IGBT Technology) is an innovative application in the field of HVDC transmission systems. As opposed to the conventional HVDC, its design provides technical as well as economical advantages. The HVDC PLUS terminology is used for a HVDC transmission system which is based on Voltage Sourced Converter (VSC) Insulated Gate Bipolar Transistor (IGBT) technology. The extension "PLUS" stands for Power Link Universal Systems. Some important attributes are:

• Ability to feed AC systems with low short circuit power as well as passive networks with no local power generation.


• Continuously adjustable reactive power support to the area's AC transmission system. This system capability supports control of the AC bus voltage and improved system stability. Active and reactive power exchange can be controlled independently from each other within the total power rating of a station.

These features make HVDC PLUS a desirable alternative to conventional thyristor-based HVDC systems such as that evaluated in the Draft and Final EIRs. A conventional HVDC can only operate in AC systems with appropriate short-circuit power and additional measures are needed to achieve reliable operation. An HVDC PLUS converter is equipped with IGBT semiconductor devices that can be turned on and off in a controlled manner.

Thyristor converters as used in conventional HVDC systems always require reactive power, which reduces the amount of useable electrical power available. The reactive power demand varies according to the useable power transferred. Additional power components such as switched capacitor banks or Static Var Compensators (SVC) have been used to supply the reactive power demand of the converter station. In a HVDC PLUS system, each of the stations can control useable and reactive power flow independently from each other within the total MVA power ratings. Thus, the HVDC PLUS technology/design not only can transmit more useable power from one AC network to another one but also offers the possibility of controlling the AC bus voltage and improving the AC system stability.

2.3.2    Converter Station Components

The Trans Bay Cable Project, with the adoption of HVDC PLUS technology, has a power rating of 400 MW, which is the same power rating as the conventional HVDC design evaluated in the Draft and Final EIRs. This power transfer is achieved with DC voltages of ±200 kV and a DC current of 1,000 Amps (A) respectively. The transmission system consists of two stations-the San Francisco Converter Station and the Pittsburg Converter Station, connected by a submarine/onshore HVDC cable system as previously proposed and assessed in the Draft and Final EIRs.

The planned Trans Bay Cable HVDC PLUS converter stations consist of the following primary components (common for both converter stations):

• AC Feeder


• Disconnect Switches, Measuring Equipment, etc.)


• Single-phase transformer with tertiary winding for Station Auxiliary Power Supply


• Voltage Sourced Converter with ±200 kV Insulated Gate Bipolar Transistor valve groups and air-core Phase Reactors


• High voltage DC circuit


• Auxiliary Systems


• Cooling Systems (closed circuit)


• Control and Protection Systems

The core of the HVDC PLUS converter stations is the IGBT based converter where the conversion from AC to DC (i.e., Pittsburg Converter Station) and vice-versa (i.e., San Francisco Converter Station) takes place. Power Modules are made up of Insulated Gate Bipolar Transistors and form part of the Voltage Sourced Converter. Two DC Outputs are produced-one with +200 kV and one at -200 kV. Both outputs feed the High Voltage DC circuit. The IGBT equipment is enclosed in a conventional steel frame building, providing shielding, noise reduction, and other protection schemes.

With the proposed HVDC PLUS converter station design, the converter transformers are located in the AC Yard and do not penetrate the converter building with bushings. The transformers adjust the voltage at the AC busbar of the converter station to the required entry voltage of the converter. In addition, the transformers are equipped with a tertiary winding supplying the station auxiliary power for all systems.

The Control and Protection System contains all control and protection components, including measuring equipment, monitoring, as well as interface systems for communication. The main functions of the HVDC control system are to ensure operational safety and reliable energy transmission which operates in a highly efficient manner and flexible energy flow that responds to sudden changes in demand thus contributing to network stability.

With the proposed HVDC PLUS converter station design, all control and protection systems that contribute to the availability of energy are configured redundantly. This covers any potential single fault in the control and protection equipment without loss of power.

Refer to Sections 2.5 (San Francisco Converter Station) and 2.6 (Pittsburg Converter Station) for more information regarding the planned converter station layouts, components, and physical appearance.

2.4    SUBMARINE CABLE DESIGN AND INSTALLATION

In order to accommodate the HVDC PLUS converter station design addressed in this EIR Addendum, the design of the submarine HVDC cable system bundle between Pittsburg and San Francisco has been modified. The carrying capacity of the submarine cable is still 400 MW as evaluated in the Draft and Final EIRs, however, the planned cable design now consists of two, 200 kV cables (one positive, one negative) and a fiber optic communication cable (refer to Figure 2.4-1). The HVDC submarine cable system design addressed in the Draft and Final EIRs consisted of one 400 kV cable, one 12 kV return (ground) line, and a fiber optic cable. The HVDC submarine cable system would still be approximately 10 inches in diameter and would be bundled prior to burial at a target depth of 3 to 6 feet below the bottom of the Bay using a hydroplow deployed from a cable laying vessel as described in the Draft and Final EIRs. The submarine cable route and construction methods are as evaluated in the Draft and Final EIRs. The operational characteristics (e.g., electric and magnetic fields and heat) would be essentially the same as those described in the Draft and Final EIRs.

2.5    SAN FRANCISCO CONVERTER STATION

The planned converter station location in San Francisco is the San Francisco HWC (Mitigated) Converter Station site is a subset of the San Francisco HWC (Mitigated) site as evaluated in the Draft and Final EIRs and Section 1.2.2 of this EIR Addendum. The HVDC PLUS converter station design allows for a smaller footprint, fewer components, and lower building/structure heights than the conventional HVDC converter station design evaluated in the Draft and Final EIRs.

The San Francisco HWC (Mitigated) HVDC PLUS site and layout are shown on Figures
2.5-1 and 2.5-2, respectively. Figure 2.5-1 also shows the onshore DC and AC cable routings which are essentially the same as those evaluated in the Draft and Final EIRs. An elevation view of the HVDC PLUS layout at this site is shown on Figure 2.5-3 and photosimulations are shown on Figures 2.5-4 through 2.5-7.

2.6    PITTSBURG CONVERTER STATION

The planned converter station location in Pittsburg is the Pittsburg West Tenth Street Alternative 1 site evaluated in the Draft and Final EIRs, as modified. The smaller HVDC PLUS converter station footprint allows the Pittsburg West Tenth Street Alternative 1 converter station layout evaluated in the Draft and Final EIRs to be shifted onto the eastern portion of the previously evaluated converter station site. This change also allows for an approximate 255-foot buffer (part of the previously evaluated Pittsburg West Tenth Street Alternative 2 site) between the new screening/security wall located along the southern converter station boundary line and the north side of West Tenth Street, which may be landscaped consistent with design review requirements to help visually screen the facility. The converter station operational access road would be constructed along the western boundary of the buffer area.

The Pittsburg West Tenth Street Alternative HVDC PLUS site and layout are shown on Figures 2.6-1 and 2.6-2, respectively. A temporary construction access road (30 feet wide) would be constructed along the northerly portion of the previously assessed Pittsburg West Tenth Street Alternative 1 site (refer to Figures 2.6-1 and 2.6-2). Figure 2.6-1 also shows the onshore DC and AC cable routings which are essentially the same as those evaluated in the Draft and Final EIRs. An elevation view of the HVDC PLUS layout at this site is shown on Figure 2.6-3. A photosimulation with landscaping in the buffer area is shown on Figure
2.6-4, and a photosimulation without landscaping is shown on Figure 2.6-5.

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