21, rue d’Artois, F-75008 PARIS B1_105_2010 CIGRE 2010 http : //www.cigre.org

200 kV DC extruded cables crossing the Bay

M. BACCHINI T. WESTERWELLER N. KELLEY R. GRAMPA (*) Siemens, Prysmian Power Cables M. MARELLI Germany and Systems, Prysmian Powerlink, Italy U.S.A.

SUMMARY

In the Bay of San Francisco, in California, there is the first case in the world of commercial transmission of electrical energy in direct current at 200 kV with Multilevel VSC technology and cables with extruded insulation: it is the “Trans Bay Cable” (TBC) project.

The innovative submarine HVDC link consists of two cables in bipolar configuration and has the capability to transmit 400 MW over a length of 55 miles i.e. 88 km. The connection between converter station in Potrero - in San Francisco - and Pittsburg - in the East Bay - allows the closure of the loop of the already existing “Greater Bay Area” transmission system, thus reinforcing the power flow directed to the center of San Francisco. This will decrease transmission grid congestion in the East Bay and will also avoid the need of building additional power plants in the City of San Francisco.

Cables are manufactured by Prysmian in a factory close to Naples, in Italy, which is fully dedicated to submarine cables. Design for this specific project includes a copper conductor with a cross section of 1100 mm2, extruded insulation, lead sheath and armour of steel wires. The two HVDC cables (+/- 200 kV) are laid in bundle and buried at a depth of 6 ft (1.8 m), in a safe corridor separate from any existing AC cables. Laying operations are carried out by two different vessels: the “Giulio Verne” (one of the largest cableships in the world) for most of the route and a suitably equipped barge for the very shallow waters in the East Bay. The use of extruded insulation cables in the HVDC links showed a large increase during last years, due to some advantages that polymeric insulation can offer in comparison to traditional laminated mass impregnated cables and since the large technological progress happened, which involved materials, manufacturing process, system integration.

KEYWORDS

HVDC – VSC – Submarine cable – Extruded Cable – Multilevel converter – Interconnection.

(*) Corresponding author Renato Grampa [[email protected]]

PROJECT BACKGROUND

The Trans Bay Cable Project (TBC Project) is a 400 MW, ±200 kV, submarine-based, point-to-point, DC transmission system which transmits power from the generation resource-rich area of Pittsburg, California to the City and County of San Francisco load-pocket (as shown in Figure X).

The Project was developed by Trans Bay Cable, LLC and it will be owned by the Pittsburg Power Company upon Commercial Operation. Extruded DC and AC cable systems were provided by Prysmian and voltage-source DC converter stations were provided by Siemens as consortium partners under an engineer-procure-construct (EPC) contract. Commercial Operation is expected to be achieved during the first quarter of 2010. The benefits of the Project are many. The electric capacity and energy delivered by the Project to the City and County of San Francisco load-pocket improves power supply reliability and provides for future load-growth needs. In addition, overall electric system losses are substantially reduced.

This focus of this paper is the 200 kV extruded DC submarine cable technology and installation provided by Prysmian.

PROJECT DESCRIPTION AND INTERFACES

The Project is integrated into the California ISO Control Area and interconnects with the Pacific Gas & Electric (PG&E) Pittsburg 230 kV Substation in Pittsburg, California and the PG&E Potrero 115 kV Substation in San. Francisco, California. A basic scheme of the link is reported in Figure 1.

a.c. d.c. a.c. transmission transmission transmission 115 kV 200 kV 230 kV

Fig. 1 Basic layout of the whole connection

Specifically, the Project components include the:  Pittsburg 230 kV Extruded AC Cable System which interconnects the Pittsburg Converter Station with the PG&E System at the PG&E Pittsburg Substation;  Pittsburg Converter Station based upon Siemens’ Modular Multi-level Converter HVDC Plus Converter Technology;  Land and Submarine ±200 kV Extruded DC Cable Technology which interconnects the Pittsburg Converter Station with the Potrero Converter Station;  Potrero Converter Station based upon Siemens’ Modular Multi-level Converter HVDC Plus Converter Technology; and  Potrero 115 kV Extruded AC Cable System which interconnects the Potrero Converter Station with the PG&E System at the PG&E Potrero Substation.

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HVDC SYSTEM AND PLUS TECHNOLOGY

A new modular multilevel converter (MMC) topology has been introduced by Siemens into HVDC applications during recent years. The concept of this topology is based on the converter arms acting as controllable voltage sources with a high number of possible discrete voltage steps. This allows for forming an approximate sine wave in terms of adjustable magnitude of the voltage to the AC terminal. This principle is shown in Figure 2.

Each of these variable voltage sources is designed with a number of identical but individually controllable sub-modules. Each sub-module is a two-terminal component which can be switched

Fig. 2 - Converter in Modular Multilevel topology and control principle between a state with full module voltage and a state with zero module voltage in both current directions. Dependent on the current direction, the capacitor can be charged or discharged. Besides auxiliary components and electronics, each sub-module consists of an IGBT half bridge and a capacitor unit.

By serially connecting many modules an +U /2 d elegant multilevel topology can be constructed. It is possible to individually and selectively control each of the individual sub-modules in a converter arm.

The total voltage of the two converter arms in one phase unit equals the DC voltage, and by adjusting the ratio of the converter arm voltages in one phase

-Ud/2 module, the desired sinusoidal voltage at

the AC terminal can be achieved. See Fig. 3 - AC and DC Voltages controlled by Converter Module Voltages Figure 3.

DEVELOPMENT OF EXTRUDED CABLES FOR HVDC TRANSMISSION

The TBC Project makes use of a new generation of HVDC cables, which are insulated with an extruded synthetic compound. If compared to traditional mass impregnated or oil filled cables, extruded cables generally provide several advantages. For DC application, they comprise use of simpler manufacturing technologies, a lower production cost, and possibility to operate cables at higher temperature than mass impregnated. These facts have been the main drivers which led to a large increase of the use of extruded insulation cables in the HVDC links during last years.

Prysmian’s first stages of research was done at the end of years ‘90s on several polymeric insulating materials and included life test under increasing electrical stress, definition of coefficient of life (N, defined by using the very well known Weibull distribution law), investigation of space charge 2 distribution by applying PEA technology (Pulsed Electroacustic Analysis) to flat models of different polymeric insulating materials. Electrical conductivity of polymeric materials was investigated as well, by using flat models at different temperatures and electrical stresses.

The second part of the study was done on model cables, insulated with most promising materials; the extensive test program consisted of: - d.c. dielectric strength, cold and hot conditions (from 20 to 90°C); - L.I. test (cold and hot) and L.I. superimposed onto HVDC of opposite polarity (cold and hot) - HVDC life test with step voltage increasing up to breakdown, with thermal cycles at 70°C; - HVDC life test as above, with daily polarity reversals.

Eventually, the third part of the development project considered the best combination of semi- conductive and insulating materials. A full-size 200 kV 630 mm² cable and relevant accessories (flexible factory and repair joint, rigid repair joint, terminations) was submitted to a long-term test, following recommendation included in Cigré Technical Brochure 219 for DC VSC cable systems.

The knowledge acquired during previous years and the very positive outcome of all development tests suggested to continue with development of HVDC extruded cables and a further pre-qualification started at increased voltage. A test protocol was prepared and implemented to allow a full qualification with and without polarity reversal, thus allowing use of DC extruded cables for both LCC and VSC converter technologies. Full system, including cable and accessories, has been subject to the following voltages, with thermal cycles in accordance to mentioned Cigré TB 219: - VSC System, U0 = 300 kV: o Voltage Cycle test: 435 kV, 70°C o Up2,S = 640 kV o Up2,O = 360 kV - LCC System, U0 = 250 kV: o Voltage Cycle test: 365 kV, 70°C o Polarity reversal: 315 kV, 70°C o Up1 = 500 kV (impulse superposed to DC at opposite polarity)

Full test was successfully completed thus confirming that the applied materials and technology are fit for use for HVDC transmission: Additionally it further proved the importance of extensive development programs to move towards to industrial application with reasonable confidence.

CABLE MANUFACTURING AND TESTING

The most significant cable part of the TBC Project is the ±200 kV DC submarine cable. The power rating of the connection is 400 MW, which is considered as increased by 5% at sending point thus giving a nominal rated current of 1050 A.

Specific environmental and installation conditions led to design a copper conductor size of 1100 mm2, which is of a compacted circular design, constructed from annealed plain copper wires and filled with water blocking compound to limit water propagation in case of cable severance.

The insulation system consists of an inner semi-conducting screen layer, the insulation compound and an outer semi-conducting insulation screen, extruded simultaneously. The insulation is composed of a cross-linked polyethylene suitable for HVDC application. Fig. 4 - The submarine 200 kV DC cable for TBC Project A semi-conducting water swelling tape is then applied between the

3 outer semi-conducting screen and the metallic sheath in order to limit water propagation along the cable core in case of cable damage. The metallic sheath is made of lead alloy, over which it is extruded a layer of polyethylene compound. The “armouring” includes bedding, armour and serving, applied in one common process. Armour is made of one layer of galvanised steel wires. Serving is made of polypropylene strings that provide a degree of abrasion protection and reduce cable friction during laying. In order to distinguish the two poles during laying; the polypropylene serving of the cables is black and yellow with two different patterns. Cable is shown in Figure 4, its manufacturing flow is visually described in the diagram of Figure 5.

CONDUCTOR STRANDING

TURNTABLE

DE-GASSING TANK EXTRUSION LINE (CCV)

LEAD EXTRUDER

TURNTABLES JOINTING AREA

PE SHEATH EXTRUDER ARMOURING MACHINE

TURNTABLE

Fig. 5 - Manufacturing flow diagram for submarine extruded cables

Factory tests on submarine cable were made in three steps: firstly individual extrusion runs were tested under AC voltage, then flexible joints were locally tested with AC voltage and verified with X-Rays analysis, and finally each whole cable - 83 km long, including factory joints - was DC tested, in accordance with Cigrè Technical Brochure No.219.

A type test on the specific cable design and relevant accessories was requested. A miniature circuit was installed as shown in Figure 6, and test performed according to Cigré TB 219 for VSC Systems with U0 = 200 kV:

o Voltage test: 290 kV, 70°C o Up2,S = 430 kV o Up2,O = 250 kV o L.I. withstand test, Up1 = 400 kV

Type test has been successfully completed, including mechanical tests required for

submarine cables as recommended in Fig. 6 - Type Test loop for DC cable Electra No. 171.

Cable for the land portion of the DC connection has the same core construction of the submarine cable, finished to the PE sheath. AC cables connecting the converter stations to Pittsburg and Potrero networks are respectively 1200 mm2 Cu 230 kV XLPE according to IEC 62067 and 1200 mm2 Cu 115 kV XLPE according to IEC 60840.

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INSTALLATION DESIGN AND WORKS

Once manufacture was completed, submarine and land cables were shipped from Europe to the US, to be installed.

Submarine cable installation

The marine cables transport, laying and protection were performed using various marine spreads. Due to relatively shallow water in some sections of the route the cable lay was undertaken by two separate laying vessels. A barge was used for the shallow water section of the route and the cable ship Giulio Verne was used for the deeper water sections. The two spreads have operated simultaneously:  The cable laying barge has carry out the installation from the George Miller Jr Memorial Bridge crossing (official name of Benicia-Martinez Bridge), at approx. 23 km distance from the landing point in Pittsburg to the Pittsburg landing location. This route section, characterized by shallow water depth, was localized mainly in the (see Figure 7, blue route)  The cable installation from west side of George Miller Jr Memorial Bridge crossing to San Francisco landing location for a total distance of approx. 62km was performed by the cable ship Giulio Verne. (see Figure 7, yellow route)

Fig 7 -. Submarine cable installation plan

In order to meet the challenging completion date of the cable installation program, a decision was made to transport the submarine cables by a dedicated vessel from the Italian factory to US through the Panama Canal. In San Francisco the cable lengths were transshipped from the transport vessel onto the Giulio Verne and then sailed to the initial installation location at George Miller Jr Memorial Bridge crossing where a length of approx 23 Km of each of the three cables was transferred onto the laying barge.

Once transfer of cables was completed the laying barge and Giulio Verne started the cables installation sailing in opposite direction along the planned cable route.

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The three cables (two power and one optical cables) were laid and buried simultaneously in a bundle configuration. This methodology provided several advantages: such as reduced installation time; reduced costs for protection; and avoid leaving the cable unprotected on the seabed. The three cables were stored on board the lay vessels in three different tanks.

The cable system was buried for the entire route length to a target cover depth of 6 ft. The cable burial was executed by means of Prysmian’s Hydroplow system (jetting/plough action), operated from the lay barge along the shallow water section and from the C/S G. Verne for the deeper water section. Some short sections of cables were laid first and then were protected by post lay burial (PLB) or other approved methods such as concrete mattresses, during a subsequent separate operation, typically:  Areas of landing points between HDD conduits exit at sea and start/end position of simultaneous lay and burial operations.  Section of cable at interface between barge installation and Giulio Verne installation.  Crossing with other services.

At shore end the cables were pulled inside HDPE conduits previously installed by HDD technology, The conduits in San Francisco were approx. 290m long and in Pittsburg approx. 270m long.

Particular effort was put in the Project to successfully fulfil the strict environmental requirements by the local regulatory Agencies, including work controls and installation windows limitations.

At landing points the submarine cables were connected to the land cables by means of sea/land joints. Sea/land joints were assembled in suitable underground concrete chambers. At sea/land joint location sacrificial anodes were placed in the surrounding wet ground and connected to the sea cable armouring by means of disconnectable link box. The main scope is to provide a reference electrode for periodic checks of the corrosion status of the cable armouring and provide local corrosion protection to the armour wires.

Land cable installation

HVDC Section

The Trans Bay Cable project involved the installation of 1 km of land route in Pittsburg and 0,4 km of land route in San Francisco. The HVDC cables were installed in single length in San Francisco while in Pittsburg the cables were installed in two sections and a joint was inserted approximately at mid route.

The cables were installed in conduits, one for each cable. The conduits were pre- installed by means of “cut and cover trench excavation” techniques. The ducts were installed in a concrete surround with the appropriate spacing. The remainder of the trench was backfilled with material having suitable thermal characteristics. Horizontal directional drilling (HDD) technique was used to put conduit in place in single location approx. 200m long in Pittsburg where open cut excavation was not feasible due to environmental Fig.8 - HVDC trench section constraints.

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After cable installation the conduits were filled with grout of controlled thermal characteristics in order to improve thermal dissipation and to maintain the required cable rating.

HVAC Section

The project included also two HVAC cable sections to connect the converter stations to local power Authority substations. In Pittsburg side one 230kV cable circuit 1km long was installed. At San Francisco side two 115kV circuits 0,3km long each were installed. AC cables have been installed in conduits. Conduits have not been filled with thermal grout after cable installation excepted where horizontal directional drilling (HDD) technique was used to put conduit in place in single location - approx. 200m long - in Pittsburg, as open cut excavation was not feasible due to environmental constraints.

COMPLETION AND COMMISSIONING

At the end of the installation, all AC and DC cables were tested in accordance with relevant standards The 200kV DC cables were tested to 290 kV (reversal polarity) for a duration of 15 min. AC cables were tested by using a resonant test set, as shown in Figure 9.

Figure 9: Photo of the Test Setup

The 230kV circuit cables in Pittsburg were tested at a voltage of 228 kV, i.e 1.7 times Uo, for one hour; in the same way the 115kV cable circuits in Potrero were tested to 128 kV for one hour. During the High Voltage AC tests, accessories have been PD monitored.

Jacket integrity testing was also carried out on all land cables at 10 kV for 1 minute in accordance to IEC Standards.

CONCLUSION

The successful realization of the TBC Project confirmed that available HVDC cable technologies, enhanced installation techniques and overall project management approach, allow to optimize the choice of the interconnection system form a technical and economical perspective, based on the power ratings and environmental and practical conditions.

The described project is the first in the world which utilize ±200 kV DC extruded cables and can be considered as a milestone for further improvement of cable technology, system integration and optimum use of combined resources, thus giving confidence for a future increase of this type of HVDC connections.

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BIBLIOGRAPHY

[1] CIGRE TB 219 – Recommendations for testing DC extruded cable systems for power transmission at a rated voltage up to 250 kV [2] CIGRE TB 269 – VSC Transmission [3] HVDC Workshop, Rome, Nov 2008 – HVDC cable connections. State of the art and future perspectives – A. Orini, et al [4] HVDC Workshop, Rome, Nov 2008 – Benefits of HVDC for System Interconnection – D. Retzmann et al. [5] Cigré SC B4 Colloquium, Bergen, June 2009 – Challenges and Achievements for new HVDC Cable Connections – M. Marelli et al.

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