21, rue d Artois, F-75008 PARIS B4-118 CIGRE 2008 http : //www.cigre.org THE NEPTUNE REGIONAL TRANSMISSION SYSTEM 500 kv HVDC PROJECT E. STERN, J. NASH - NEPTUNE RTS, LLC (U.S.A.) C. SCHOENIGER, C. BARTZSCH - SIEMENS (GERMANY) G. ACQUAOTTA, M. BACCHINI, A. ORINI (*) - PRYSMIAN, ITALY SUMMARY The Neptune Regional Transmission System consists of a 105 km (65 mile) HVDC interconnector between Sayreville, New Jersey and Nassau County on Long Island via undersea and underground cables. Neptune RTS utilizes a monopolar HVDC metallic return scheme, with a rated terminal voltage of 500 kv DC and a bidirectional, continuous power transfer capability of 660 MW measured at the point of delivery. This paper highlights some key aspects of this project, starting from design considerations, for all vital components including direct-light-triggered thyristor (LTT) valves, Active AC filters and a state-ofthe-art control and protection system. The link is the first installed using paper-insulated, mass-impregnated cables at this high voltage level (500 kv) and size (2100 mm²). The majority of the approx.82 km HVDC cable route is underwater. It was simultaneously laid and buried on the sea bottom in close bundle with an XLPE insulated-metallic return cable and a single fibre optic cable. The land portion on Long Island is 22,5 km long and entirely installed in the shoulder and pathways alongside a major four-lane highway. Directionally drilled conduits were utilized for 6,5 km (almost 30%) of the land route to pass under bridge abutments and highway entryways and exits, thereby significantly reducing inconvenience to the public. KEYWORDS HVDC link, DC cable, submarine cable installation, HVAC cables, directional drilling, converter station, direct-light-triggered thyristor (LTT) valves, Active AC filters (*) ambrogio.orini@prysmian.com 1
1. INTRODUCTION The Neptune Regional Transmission System 500 kv HVDC Project (Neptune RTS) is the first HVDC Intertie between the electric transmission grids individually coordinated by PJM Interconnection (PJM) and New York Independent System Operator (NYISO). Project developer Neptune RTS, LLC constructed the 105 km (65 mile) HVDC interconnector between Sayreville, New Jersey and Nassau County on Long Island via undersea and underground cables. Neptune RTS s aim was to deliver an interconnector that is environmentally acceptable, technically feasible and economically viable. Aspects such as rising demand for electricity, enhanced security of supply of both AC networks as well as providing its customer Long Island Power Authority (LIPA) access to one of the most diverse and competitively-priced power markets in the United States are also addressed by Neptune RTS. Within 24 month the Neptune RTS was constructed and successfully commissioned by a consortium of Siemens and Prysmian Cables and Systems. Starting at the end of June 2007, 660 Megawatts, enough energy for 600,000 homes, can flow via the HVDC transmission system. 2. SYSTEM DESCRIPTION For various reasons, including Long Island's rapidly growing electricity consumption outpacing its ability to build new, economical "on-island" generation, the Neptune RTS was seen as an environmentally friendly, low-loss and cost-effective solution for meeting Long Island s future power needs. Neptune RTS utilizes a monopolar HVDC metallic return scheme, with a rated terminal voltage of 500 kv DC and a bidirectional, continuous power transfer capability of 660 MW measured at the point of delivery. Neptune RTS has a 4-hour overload capability of 750 MW to meet Long Island peak demands, starting from a base load of 600 MW. The general cable route is shown in Fig.1. The Sayreville HVDC Converter Station is located in Sayreville, New Jersey to interconnect the PJM system at First Energy s Raritan River Substation via a 0.8 km, underground single circuit 230 kv, 2500 mm2 conductor size, XLPE insulated cable system. The Duffy Avenue HVDC Converter Station presides in Hicksville, New York and interconnects to the LIPA Newbridge Road Substation via a 3.6 km underground single circuit 345 kv, 1600 mm2 XLPE insulated cable system. Fig.1 The 500 kv DC cables is the first installed with paper-insulated, mass-impregnated cables at this high voltage level and size (2100 mm²). The majority of the route, approx.82 km, is underwater; in this portion the HVDC cable was simultaneously laid and buried on the sea bottom in close bundle, together with an XLPE insulated-metallic return cable and a single fibre optic cable. Two installation campaigns were necessary, due to the long shallow water portion on New Jersey side, which required the use of a shallow draft barge and due to the high weight of the cable bundle. The land portion on Long Island is 22.5 km long and entirely installed in the shoulder and pathways alongside a major four-lane highway. Directionally drilled conduits were utilized for 6,5 km (almost 30%) of the land route to pass under bridge abutments and highway entryways and exits, thereby significantly reducing inconvenience to the public. 2
3. CONVERTER STATION DESIGN ASPECTS The Neptune RTS design reflects many innovations to provide a high degree of energy availability for a densely populated service territory. Fault-tolerant control systems, component redundancy at all levels of operation, extensive spare parts and well-planned quality assurance ensure high component and system reliability. To provide the highest level of reliability and availability and hence quality of the HVDC control and protection system intensive on- and off-site tests, including functional and dynamic performance test, have been carried out. Furthermore, this design goal has been achieved by installing all DC side equipment indoors. The installation of DC components in the DC halls reduces the pollution level (polluting winds from the sea carrying salt and sand, conductive dust and industrial smoke) and protects the insulation against light rain, snow, dew or fog (Fig. 2). Implementation of the Neptune RTS design characteristics reflects an employment of proven designs by the system supplier and a cooperative development of system design specifications including the customer and the interconnecting transmission owners/operators at each end of the line. This process began with the development of technical specifications for link and continued through the design, installation and testing phases of the project. Low loss design was of central importance for technical and economical optimisations. Fig 2: Duffy Avenue Converter Station aerial view This resulted in converter station designs with maximum losses of less than 1.7 percent for both stations and less than 0. 7 percent for the DC and AC cables at 660 MW power transfer. At continuous power transfer of 660 MW total system losses in the range of 11888-13932 kw have been recorded. The performance requirements for audible noise, PLC interference, radio and TV interference have been considered in the design of the converter stations. An important feature of the Neptune HVDC system is to provide the capability that enhances the stability of the AC system in case of disturbances / contingencies. During extreme disturbances the HVDC controller will automatically curtail power transfer to prevent frequency deviation beyond certain limits. For contingency conditions that may trip a number of AC lines and / or generators, DC power limitations (run-back scheme) are implemented causing the Neptune RTS to limit and regulate power flow on a constrained network. The most up to date HVDC technology and Siemens s advanced SIMATIC TDC control system is used to control, monitor and protect the Neptune RTS. The pole / converter as well as AC switchyard / station related control functions are carried out by the redundantly configured (active / hot-stand-by) pole and station control systems. A tailor-made redundant protection system for HVDC applications properly co-ordinated with pole control and the AC system protection is responsible for selective fault clearing. Diagnosis of faulty conditions is assisted by the Sequence of Events Recording (SER) and Transient Fault Recorder (TFR), time synchronised via a GPS controlled Masterclock. The thyristor valve assemblies at both Duffy and Sayreville converter stations comprise 78 thyristor levels (three redundant) and 24 non-linear reactors distributed to three thyristor modules. The three 3
modules are arranged in twin tower design to minimize component space without neglecting protection and monitoring aspects for high reliability and low maintenance requirements (Fig 3). The valves are equipped with directlight-triggered thyristors (LTT) with integrated overvoltages protection to eliminate external energy supplies and auxiliary electronic logic circuits for protection. Valve snubber circuits consist of a seriesconnected single capacitor and resistor with wire-in-water technology for the most efficient cooling possible. In the unlikely event of an outage, swift replacement of faulty individual components can be performed. Thyristors can be replaced without the opening of any water connections. Fig 3: Valves arranged in the twin tower Three in service and one spare single-phase three-winding converter transformers rated 242/121/121 MVA have been installed at each station (Duffy at 345 kv AC; Sayreville at 230 kv The 300 mh smoothing reactors installed in the DC halls are of air-core dry type design and thus maintenance free. 4. CABLE SYSTEM The submarine transmission line consists of a bundle of three cables - a 500 kv HVDC cable, a medium voltage metallic return cable and a fibre optic cable. (fig. XX). CONSTRUCTIONAL CONSTRUCTIONAL DETAILS OF 500 DETAILS kv HVDC CABLE 1 - Copper Conductor: mm 2 2100 Diameter of circular conductor: mm 51.9 2 - Conductor Screen 3 - Insulation: mass impregnated, paper. Thickness: 4 - Insulation Screen 5 - Lead Alloy Sheath: 6 - PE Sheath 7 - Reinforcement: Galvanised steel tapes 8 - Bedding: Polyester Tapes 9 - Armour: Galvanised steel wires 10 - Serving: Polypropylene strings Overall diameter of the cable Weight of In air cable In water mm mm kg/m 19.5 131 53.5 44 The HVDC cable and relevant accessories (terminations, factory and repair joints, earthing connections) were subjected to a full series of Type Testing in accordance to CIGRE (Electra No. 171 and 189) Recommendation. In particular, the cable and joints underwent mechanical (tensile/bending) tests followed by daily thermal cycles at 1.8 Uo (900 kv DC) and polarity reversals at 1.4 Uo (700 kv), followed by impulse testing at 1000 kv superposed to 500 kv DC at opposite polarity. The tests were positively concluded before starting manufacturing of the commercial cable, in December 2004. 4
The HVDC land cable has the same construction as the submarine one up to the steel tape reinforcement. A further thick polyethylene sheath is then applied on it. The land cable was supplied in long lengths, up to 1000 m in large drums, typically 4.2 m flange, weighting more than 50 tons each. The medium voltage metallic return cable consists of a 2000 mm2 copper conductor, XLPE insulation, copper screen, polyethylene sheath and galvanised steel armouring. Overall cable diameter is approx. 100 mm and weight is respectively 30.4 kg/m in air and 23.2 kg/m in water. For the land portion on Long Island, the metallic return cable was split into two cables in parallel, laid on the two sides of the HVDC cable. This configuration was chosen in order to minimise the magnetic field due to the DC current flowing in the cables to a value less than 200 mg. The fibre optic cable is a standard single-wire armoured submarine cable; it is equipped with 24, low attenuation, single mode fibres. 5. SUBMARINE CABLE INSTALLATION Cable installation was planned in two phases for several reasons: the heavy weight of the cables exceeded the abilities of a single campaign; significant length of cables to be installed by a barge in the shallow water environment of New Jersey; and different in-water installation windows allowed by the permitting authorities of New Jersey and New York State. The first section of cables was transported via the special cable ship Giulio Verne and transferred to a newly constructed cable installation barge. The first installation campaign started at the Interstate border New Jersey-New York and was performed towards the New Jersey landfall. The second campaign was entirely performed by using the Giulio Verne, in the deeper waters of the Atlantic Ocean route portion, from Interstate border New Jersey-New York to Jones Beach landfall, on Long Island (Fig.4). Joint Sayreville LP Joint W.D.<10mW.D.>10m Fig.4. Submarine cable installation layout The three cables were laid and buried simultaneously 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 5
three different tanks. The tank for the storage of the 500kV cable was a rotating platform in order to wind and unwind the cable without inducing any torsion effect. The cable system was buried for the entire route length to a target cover depth of 6 ft. The cable burial was executed by jetting /plough system (Prysmian s Hydroplow), operated from the barge for the first phase and from the C/S G. Verne for the second one. Where the cable route crossed the federal navigation channels, the permits required burial depths greater than 6ft. A pre-excavation of corridors in the channels was therefore carried out by dredging where the authorized cable levels could not be achieved by Hydroplow. At shore end the cable was pulled inside HDPE conduits previously installed by HDD technology, The conduits in New Jersey were approx. 380m long and in Long Island approx. 600m long. Infield online joints (one per cable type) were planned and performed in two route locations: in the first campaign to cross the Rail Road bridge, where the barge could not pass under the bridge along the design installation route, the cables had to be cut and re-jointed; and at the beginning of the second campaign on the C/S G. Verne a second set of infield online joints was performed in order to connect the cables laid in the two different laying campaigns. 6. LAND CABLE INSTALLATION The Neptune project involved the installation of 22,5 km of Land route, mainly on Long Island. The HVDC and the MVDC cables were installed in sections lengths from a minimum of 500m to a maximum of 1000m into polyethylene conduits. The conduits, one for each cable, have been preinstalled by means of cut and cover trench excavation techniques. The ducts were installed at 1300mm depth in a concrete surround with the appropriate spacing. The remainder of the trench were backfilled with excavated material. (Fig.5) Fig 5. HVDC trench sections in Long Island and inside HDD Horizontal directional drilling (HDD) technique was used to put conduit in place where trenching was not feasible, especially where the cable crossed the major transportation routes such as highways and train tracks. A total of 19 drills were executed along the HVDC cable route and 4 drills were executed along the 345kV HVAC cable route in Long Island. The drill length was between a minimum of approx. 130m to a maximum of approx. 600m. The cables have been installed through the conduits by means of pulling winches with the aid of a low friction water resistant silicone based compound applied on the cable during the operation; this 6
guaranteed a low friction between cable and conduit (less than 0,2) and no abrasion on the cable external sheath. 7. REACTIVE POWER AND AC VOLTAGE CONTROL FEATURES The Neptune HVDC conversion process at Duffy Avenue Converter Station draws a maximum of 468 MVAr and the two 345/138 kv autotransformers require approximately 100 MVAr at power import levels of 750 MW. To address these needs and comply with specified AC voltage change limits during switching a total of 525 MVAr shunt capacitance has been installed, subdivided into five banks of 105 MVAr To. A 115 MVAr shunt reactor is also installed at Duffy Avenue Converter Station to counteract capacitive MVAr transfer into the AC system during low transfer levels. The HVDC reactive power control (RPC) system provides several control features. The RPC is capable to automatically control VAr transfer into the 138 kv AC network by switching of the reactive banks based upon a MVAr set point (Q mode). System operation in voltage control mode (V mode) allows the RPC to maintain AC system voltage within set point high and low limits at the 138 kv bus, while keeping the voltage of the 345 kv system within the specified limits. The RPC system of the Sayreville Converter Station manages the reactive power or AC voltage control in a similar manner. Five AC filter banks of 107 MVAr rating each and a shunt reactor 80 MVAr have been installed to fulfil the specified limits including the requirement to maintain a power factor between 0.95 lagging and 0.95 leading when the Sayreville Converter Station is operating at any level within its operating range. The high degree of compensation combined with high AC system impedances introduces the potential for considerable temporary overvoltages (TOV) after load rejection. A control limitation strategy minimizes the impact of TOV by restarting the DC system and restoring power transfer to predisturbance levels as fast as possible (Vac 1.3 pu@100 msec and Vac 1.2 pu@500 msec). During AC system faults, the HVDC system will be kept in operation to continue power transfer / DC current flow or to facilitate fast recovery. An AC filter tripping logic is implemented in the DC station control system as a backup limitation strategy. Moreover, best engineering practices have been applied in the design of both converter stations and in particular of the AC filters to avoid low-order harmonic resonances that substantially increase temporary overvoltage crest values above the fundamentalfrequency magnitude. 8. HARMONIC AC FILTERING The operation of an HVDC converter inherently generates harmonic currents, which if left unaddressed may propagate through the interconnected transmission system, perhaps even amplified due to system resonances. Besides power quality impacts, one potential consequence of harmonic currents injected into a transmission system is interference with telephone circuits due to mutual coupling with transmission lines. Transmission rights-of-way on Long Island and in New Jersey are typically congested, and often have adjacent phone circuits. Accordingly, the harmonic current (ITlevel) design target specified for the Neptune HVDC Project is very low, less than typically specified for HVDC systems. To mitigate these harmonic effects, four triple-tuned AC filters plus one low order (Sayreville: doubletuned) harmonic filter are applied to the AC bus of each converter station. These passive AC filters are tuned to the characteristic harmonics 11 th / 13 th, 23 rd / 25 th, 35 th / 37 th and 47 th / 49 th. Considering the presence of substantial harmonic emissions caused by existing loads as well as low-order resonance conditions, additional filtering is provided for the 3 rd and 5 th harmonic (Fig 6). Active AC filters have been provided at each converter station to substantially reduce the harmonic current injection and consequently the likelihood of telephone interference. 7
The connection to the 345 / 230 kv AC system is achieved by means of a passive triple-tuned filter, forming a so-called hybrid filter (Duffy Avenue: TT 5/12/24, Sayreville: TT 12/24/36). The passive part can be used as a conventional passive filter if the active filter device is by-passed for maintenance purposes. These active filters are designed to improve the effectiveness of passive tuned AC filters and to mitigate harmonic currents flowing in the AC network at frequencies not or insufficiently covered by the installed passive filters. Fig 6: AC filters in operation at Duffy Avenue The active filter does not appear to the AC network like a passive impedance. Therefore, it is free of resonance with other network elements. The active filter system measures the harmonic currents flowing into 230 kv Raritan River Substation / harmonic voltages at the Duffy Avenue 345 kv bus and produces a voltage which is able to counteract the harmonic current flow into the AC network. In order to ensure the high availability, reliability targets two Active AC filters have been installed at each converter station. Each phase of the active AC filter comprises a voltage-sourced IGBT converter, fast acting switches to bypass and protect the converter in case of overvoltages or overcurrent, filter equipment, a surge arrester and means to bypass the active filter device and isolate it from the passive branch. A highspeed control and protection system processes the currents and/or voltages measured using appropriate sensors, and produces the control pulses for the IGBT s. The active filter equipment is assembled in a container-type house with separate rooms for the IGBT converter equipment, the electronic and control equipment and converter cooling equipment. Each active AC filter is designed to mitigate simultaneously at sixteen harmonics. The active filter can act on each harmonic individually, either characteristic harmonics or non-characteristic ones. One harmonic controller is dedicated to each harmonic selected for elimination by the action of the active filter. The sum of all harmonic controller outputs gives the waveform required by the active filter. Settings can be changed easily and consequently, the filter performance can be adjusted according to identified problems without changes in the hardware. During commissioning, comprehensive measurements carried out in the whole range of operation prove the AC filter design. In spite of considerable pre-existing harmonics, the measurements confirm the excellent harmonic performance of the installed filters. Generally, the THD level is lower during HVDC operation and thus the harmonic contribution of the converter stations is negligible: Sayreville: without HVDC: THD 0.75 %; with HVDC in operation: THD 0.48 0.67 % Duffy Ave: without HVDC: THD 1.80 %; with HVDC in operation: THD 0.69 1.23 % At actively filtered frequencies (except 7 th and 9 th ), the residual harmonic currents flowing into the 230 kv PJM AC system are in the range between 0.1 A and 0.3 A. For the 7 th and 9 th harmonic a residual current of approximately 1.0 A has been achieved. Records on 69 kv and 138 kv overhead lines emanating from Newbridge Road Substation show an considerably low increase of IT-levels in the range 1000 2000 A due to the operation of the Duffy Avenue Converter Station. 8