B4-102 CROSS SOUND CABLE PROJECT SECOND GENERATION VSC TECHNOLOGY FOR HVDC

Transcription

1 21, rue d'artois, F Paris B4-102 Session 2004 CIGRÉ CROSS SOUND CABLE PROJECT SECOND GENERATION VSC TECHNOLOGY FOR HVDC B. D. RAILING G. MOREAU L. RONSTRÖM* J. J. MILLER P. BARD J. LINDBERG P. STECKLEY TransÉnergieUS Hydro-Québec ABB Power Technologies (USA) (Canada) (Sweden) 1 INTRODUCTION The Cross Sound Cable Project (CSC) is a HVDC transmission system that interconnects the electricity market regions of New England and New York as illustrated in Figure 1. TransÉnergieUS Ltd. developed the CSC project, the first merchant transmission project in operation in the USA. CSC is a bi-directional 330 MW transmission system consisting of two voltage source AC/DC converter (VSC) stations connected by a pair of 40-kilometer DC submarine cables. Transmission capacity over the CSC project was offered to market participants through an open season process and a firm capacity purchase agreement was executed with the successful bidder. A key feature of the HVDC technology in CSC is the ability to precisely control power transfers in accordance with scheduled transactions by those who have purchased rights to its capacity. Additional benefits of the selected HVDC technology include: Figure 1 Geographical location of Cross Sound Cable Reactive power/ac voltage control at each converter to support the connected AC networks Converter stations have a compact layout with the majority of equipment housed within a typical warehouse-style building Use of DC cables with solid dielectric insulation (contains no oil) Power transfer is continuously controllable from 0 MW to 330 MW delivered in both directions without the need for equipment switching Modular design allows for comprehensive factory testing which leads to a short field-testing and commissioning program This paper describes the second-generation VSC technology employed in the CSC project, presents the main circuit one-line diagram and control system features, and discusses the commissioning and operational experience. *leif.ronstrom@se.abb.com

2 2 GENERAL SYSTEM DESCRIPTION 2.1 AC Networks As depicted in Figures 1 and 2, CSC links the AC networks of Connecticut and Long Island, New York. These AC networks are contained within two separately operated control regions in the northeast United States with independent energy and capacity markets. In Connecticut, CSC s northern converter station interconnects with the AC transmission grid via a short tap to the existing 345 kv overhead transmission line that extends from the nearby 115 kv East Shore to the 345 kv Scovill Rock substation. At East Shore a pair of autotransformers step the 115 kv voltage up to 345 kv. The Scovill Rock substation facilities include two additional 345 kv network circuits and a generator connection. This interconnection location was selected for CSC as it offers access to the 345 kv bulk transmission network in Connecticut and provides for a short HVDC cable section to the submarine cable landfall in New Haven Harbor. The CSC converter station on Long Island connects to the 138 kv bulk AC transmission system at the Shoreham substation. Four 138 kv transmission lines emanate from the Shoreham substation providing adequate transmission capacity to support rated power transfer over CSC. Again, this location was selected as it provides easy access to both the existing AC network and the HVDC submarine cable landfall. No new AC transmission beyond the short direct connection ties were required to interconnect CSC to the existing AC networks. Detail of the VSC interconnection breaker arrangement can be found in Figure 5. Power generation plants are located in close proximity to both converter stations. In Connecticut, a single generation unit rated at 511 MVA connects into the 115 kv substation at East Figure 2 Cross Sound Cable Interconnections Shore and a unit rated 461 MVA is connected into the Scovill Rock 345 kv substation. On Long Island, three 99 MVA generators are connected at the Wading River 138 kv substation, which is a short distance from Shoreham. Two GT units of 56 MVA and 17.5 MVA also supply the Shoreham 69 kv substation. 2.2 DC Transmission System Converter Equipment The first commercial HVDC transmission projects using VSC were rated up to 60 MW. They used two-level bridges and solid dielectric DC cables designed for ±80 kv DC [1,6]. For CSC, a single three-phase VSC, capable of 346 MVA was chosen. The converter is a 3-level bridge, using IGBT positions in place of diodes for the neutral-point clamping. The IGBT positions are based on a new generation IGBT and diode pack, divided into sub-modules to enable different current ratings using the same physical dimensions. The Figure 3 The Shoreham Converter Station converter DC voltage has been raised to ±150 kv, but the

3 selected PWM (Pulse Width Modulation) pattern switches the valves on and off only between + or 150 kv and 0 V, thus keeping the well-proven switching amplitude (+80 to 80 kv) and the number of IGBT positions in a valve from the previous 2-level VSC-based transmission projects. The switching frequency is 1260 Hz (21 st harmonic), which is half the frequency of the previous 2-level VSC projects. The selection of these parameters gives only half of the losses of an equally large two-level converter, and still no increase of harmonics. Each converter station can operate in any point within the PQ diagram shown in Figure 4, since active and reactive power are controlled completely independent. The IGBT valves are cooled with a glycol/water mixture and installed in modular valve housings. One phase is built up of four housings, containing valves and high-density DC capacitors. The full three-phase bridge uses twelve housings. The IGBT valve assembly has been modified so that opening of the cooling water circuit at replacement of an individual IGBT is no longer necessary. Separate housings for the control equipment and for the cooling water treatment and pumps complete the converter, entirely located inside the building seen in Figure 3. Air core, water cooled phase reactors (L4) connect the converter to the 200 kv AC filter bus, where three shunt filters, tuned to the 21 st, 41 st, and 25 th harmonics are connected via breakers, equipped with synchronous closing functionality. The power line carrier (PLC) filtering equipment (components L1, L2, L3, C1, C2 C3 and C4) is installed on the secondary side of the converter transformer. Figure 4 Steady-state PQ diagram Three standard single-phase AC transformers are used to match the AC network voltages to the converter AC voltage. A spare unit is provided at each converter station. The incoming breaker has associated pre-insertion resistors to minimize the transients at converter energisation. A 3 rd harmonic filter is installed on the DC side to remove the component due to the 3 rd harmonic modulation technique implemented in the controls (more information in the section below). 345 or 138 kv Shoreham only UDC_P1 IDC_P kv Q1 U_PCC U_AC I_VSC 3 x 120 MVA Q2 L1 L2 L3 200 kv L4 C1 C2 C3 C4 Auxiliary power rd 3 harmonic filter -150 kv Converter Building st th Mvar st Enclosure UDC_P2 IDC_P2 Figure 5 Converter Station Single-line Diagram Control and Protection The control and protection system is built up in the same way as in the DirectLink HVDC project. See the description in reference [1]. In CSC, telecommunication is added between the stations to enable fast runback for specific network disturbances. To ensure reliability, the control system is duplicated with one control system active and the other in standby. Each control system consists of pole control and protection, and valve control units (VCU).

4 The latter receives the on/off switching signals from the pole control and executes the switching orders to the valves. The main objective of the control system is to control the transferred active and reactive power independently. The ability to control reactive power may be used either to keep the reactive power exchange or the AC voltage constant at the point of common coupling (PCC), independent of the DC voltage or active power control modes. By using PWM the resulting converter voltage at the converter terminals is achieved by switching the valves according to a modulation scheme determined by several factors, such as calculated reference voltage, available DC voltage, harmonic generation and currents in the valves. The pole control consists of a state feedback controller for the converter current control. The active and reactive currents are controlled independently. The controller is synchronized to the fundamental network voltage using a phase-locked loop (PLL). The DC voltage, active power, reactive power and AC voltage controls calculate the reference currents to the converter current control. In steady state, one converter must operate in DC voltage control and the other in active power control. u dc+ X i v X t i t PCC DC Filter u f B u pcc VCU p ref q ref u ac ref u dcu dc y ref Pole Control converter ac current control APC, RPC & ACVC DCVC PLL i v u f i t u pcc Figure 6 Converter together with the pole controller; the latter consists of the converter AC current control, DC voltage control (DCVC), active power control (APC), reactive power control (RPC) and the AC voltage control (ACVC). The control is synchronized to the network fundamental voltage using a phase locked loop, PLL. The converter protections are integrated in the same duplicated main computer system, but operate independently of the control systems. All primary measuring units, cabling and computer boards are separated between control and protection, thereby ensuring that a fault in one control measurement device does not affect the protection system. Third-harmonic modulation. A zero sequence third-harmonic component is added to the sinusoidal reference voltage to reduce the peak AC converter voltage in order that, with the same DC voltage, an approximately 15 % increased AC-side fundamental-frequency voltage is available from the VSC. DC voltage balance control. The three-level converter is mid-point grounded and balancing of the DC pole voltages is needed to prevent any ground current circulation. Certain space vectors are used in the modulation to balance the bridge. Low order harmonic suppression. Minor non-linearities in the valve switching create low-order harmonics. To prevent their amplification, a special controller was designed to act on the PWM pattern in order to minimize the low order (5 th and 7 th ) harmonic currents at the PCC. The end result is that the PCC voltage levels at these harmonics are hardly affected by converter operation and there is very little contribution from these frequencies to the IT product.

5 AC voltage control. This control feature includes a slope (droop), similar to the voltage regulators on generators. The reactive power capability is dependent on the active power transfer, as shown in Figure 4. A strong network may limit the possibility of reaching the AC voltage set point due to the reactive power limit. The use of a slope mitigates this issue and has the additional advantage of avoiding hunting between the converter AC voltage control and similar controls on nearby generators. Sub-synchronous damping control. To ensure that sub-synchronous oscillations are not amplified by the converter control, a sub-synchronous damping controller (SSDC) is used. The angular frequency deviation given by a PLL is band-pass filtered to extract the sensitive frequency range. This signal is used in the SSDC and the limited output signal is added to the current orders. The SSDC is an integral part of the control system and cannot be turned off. It is active in the entire sensitive frequency range DC transmission cables, installation and rating A pair of 40 km long, DC transmission cables rated ±150 kv and 1200 ADC steady state, were designed [3] for both land and submarine installation. These cables are solid dielectric with a 1300 mm 2 copper conductor. The submarine cable uses a lead alloy metal sheath, PE protective sheath, galvanized steel armor wires, and an outer protective layer of polypropylene yarn bonded with bitumen. A specialized ship for submarine cable installation was used to transport and install the pair of DC transmission cables and a 192-core fiber optic cable. Installation of the submarine cable system was performed using a hydraulic jet plow during the last two weeks of May The submarine cable system was buried to a minimum depth of approximately 2.0 m below the seabed. Approximately 600 m of the cable system was installed in a horizontal direct drill (HDD) pipe below shellfish beds. A small water-cooling system is used to cool the cable system in the HDD. 3 COMMISSIONING Installation of the converter stations was completed during July The commissioning program was modified to achieve reliable transmission to support the Long Island and New England utilities during the remainder of the summer 2002 peak period. Additional commissioning personnel were assigned by the manufacturer and owner to complete the critical terminal and transmission testing as soon as possible. It was decided to defer a portion of the performance tests until after the summer 2002 period with the understanding that the converter stations would be staffed with commissioning personnel, and data acquisition equipment would be used to constantly monitor the critical AC and DC side parameters. The critical terminal and transmission commissioning tests were completed by August 8, Additional testing periods were scheduled as needed between Aug 2002 and Sept 2003 to complete the performance measurements. 4 STEADY STATE AND TRANSIENT PERFORMANCE 4.1 AC Side Harmonics For both converter stations, a maximum increase of 1% was allowed for individual voltage harmonics between the 2 nd and the 85 th. Total harmonic distortion (THD) was limited to 2% in New Haven and 3% in Shoreham. Finally a limit was set at 35 for the telephone interference factor (TIF) in New Haven and at 12 ka rms for the IT product in Shoreham. The limits for individual harmonic distortion and THD were easily met at both stations. As seen in the top curve of Figure 7, only low order harmonics are present in the voltage spectrum. However, they are not caused by converter operation, as they are also present when the converters are blocked. Recorded TIF stayed around 20 at New Haven with the converters in operation. The requirements for the IT product were harder to meet at Shoreham because of the presence of important voltage components at the 5 th and 7 th harmonics resulting in an IT between 14 and 20 ka rms. Low-order harmonic controller settings were subsequently optimized through numerous field tests to finally reach an IT product of 9.5 to 10 ka rms. The 5 th and 7 th harmonic currents are now kept below 2 A rms as seen in the second curve of Figure 7.

6 4.2 DC Side Harmonics With the land portion of the DC cables being very short, there is little exposure for telephone interference and therefore no strict limits were imposed on the DC side harmonics. For information purposes, they were nevertheless measured during commissioning. The HV conductor current (IDC_P1 and P2) was made available for recording via AC/DC current transducers while the total DC cable current information came from Rogowski coils installed around the cables. It can be appreciated from the top curve of Figure 8 that the converter DC side current has a major component at the 2 nd harmonic with other important odd and even triplen components up to the 21 st. The bottom curve with a log scale indicates that the components are greatly reduced due to the shielding provided by the DC cable screen. 4.3 Conducted and Radiated Noise Conducted noise is controlled by the PLC filtering equipment installed on the secondary side of the power transformer. Between 50 and 300 khz, the maximum level for the voltage on the primary side of the transformer was specified at 8.9 mv rms in a 1 khz bandwidth. The level was lowered by 6 db between 300 and 500 khz. These levels were not exceeded at either converter station. The specification asked for a maximum radiated noise level of 100 µv/m between 500 khz and 1 GHz at a distance of 460 m from any energized equipment. This level was not exceeded at either converter station. 4.4 Audible Sound The station design was done with careful attention to audible noise requirements. Both sites are located within industrial areas (70 dba limit at property line), but the audible noise impact was to be kept lower in certain directions. The stations were arranged with respect to those directions. The station building has been used as an acoustical screen for sound from the cooling fans, PLC filters and power transformers. The AC and DC CSC - Original Estim ated vs. Actual Loss Curve filters, IGBT valves and cooling 24 pumps are located inside the station Estimated Losses 20 building. Hence, the acoustical Actual Losses impact from the converter stations 16 has been minimized and the local 12 noise limits have not been exceeded. 4.5 Losses Actual CSC transmission system losses were measured during commissioning of the project. All converter station auxiliary power was supplied by the CSC power Losses (MW) Inve rt er Output Powe r (M W) Figure 9 Total Transmission System Losses, including station and cable losses

7 transformer tertiary windings during measurements. The cooling systems for the IGBT valves, phase reactors, building areas and power transformers were operated at maximum to simulate cooling load at 40ºC dry-bulb air temperature. The actual loss curve shown in Figure 9 was created using the actual MW values from the 345 kv and 138 kv utility revenue meters at the PCC at each converter. As seen in Figure 9, actual measured losses were found to be lower than the initially estimated losses, especially at high MW power transfer. The three-level bridge valve design, higher currents and lower switching frequency used in CSC offered superior power loss performance than anticipated as compared to the two-level bridge design used in earlier VSC projects such as DirectLink [1]. 4.6 Behavior during transients and faults Special care has been taken to minimize the transients for the two frequent converter operations consisting in converter energization and converter deblocking. This was achieved via pre-insertion resistors associated to the incoming breaker, AC filter circuit breakers equipped with a synchronous closing function and a staggered sequence for the converter deblocking. For more information, the reader can consult Sections 5.1 and 5.2 of reference [4], which is related to a similar project in Australia, the Murraylink project. The CSC converters are continuously operated in AC voltage control and this is quite appreciated by the neighboring utilities. CSC now takes care easily of the steady state voltage regulation at the connecting stations. It is also improving the dynamic stability as seen in Figures 10 and 11. Figure 10 corresponds to multiple AC faults during a severe thunderstorm on Long Island. The converter did not only exhibit its robustness in riding through the disturbances but also supported the voltage up to its reactive power limit of 125 Mvar while keeping the active power fairly constant. A similar behavior is observed in Figure 11 for an AC fault in Connecticut. 5 OPERATIONAL EXPERIENCE 5.1 Restoration after the August blackout Following the blackout of the northeastern portion of North America on August 14, 2003, CSC was instrumental in restoring electric power to customers on Long Island. CSC was the first transmission link to Long Island that was put into service after the blackout. The link transported 330 MW to the island, enough power to restore electric service to over 330,000 homes. Furthermore, CSC s AC voltage control feature proved valuable by stabilizing AC system voltage in Connecticut and on Long Island during and subsequent to the network restoration (refer to example presented in Figure 10). 5.2 Sub-synchronous torsional interaction Sub-synchronous torsional interaction (SSTI) may be described as interaction of the active control systems of transmission equipment such as a FACTs device or HVDC exciting the torsional modes of

8 a turbine generator shaft. There is a potential of SSTI risk only if the AC network results in a high degree of coupling between the HVDC converter and generator. The frequency range of concern for the torsional modes is about 7 Hz to 30 Hz. To prevent adverse interaction, even for situations with a high degree of unit coupling, the pole control at the HVDC converters includes a sub-synchronous damping controller (SSDC) [5]. The manufacturer s experience from delivered HVDC installations demonstrates that adverse SSTI can be avoided by proper design practices. A method of screening for units that may have a high potential for coupling or interaction with the HVDC facility is assessment of a Unit Interaction Factor (UIF) [2]. UIF screening of generation near the HVDC converters was performed for various AC network configurations to determine if any units may be considered a high SSTI risk. To ensure proper control system design, suspect units were then subject to more detailed analysis which modeled the actual HVDC control system and SSDC under various modes of HVDC converter operation and determined the small-signal electrical damping torque of the turbine rotor. Studies demonstrated that all HVDC control modes would contribute positive damping over the frequency range of concern. However, with consideration for CSC being the first application of VSC based technology in North America, some local generation owners elected to have torsional stress monitoring and relay systems installed as back-up protection. There has been no occurrence of any SSTI phenomenon in the operational experience to date. 5.3 Availability The Cross Sound Cable availability was approximately 97.5% for the period ending December 31, Punch list and final adjustment work was completed during this time period. The most serious forced outage lasted for 14.6 hours due to a fault in the IGBT valve control at the Shoreham converter station. 6 CONCLUSION New design concepts were used to expand the existing VSC technology from the 60 MW, 2-level, ±80 kv range to the 330 MW, 3-level, ±150 kv range. This was made possible by the use of enhanced control and protection systems, larger power electronic components and new arrangements of the modular converter design. DC transmission cables using solid dielectric insulation were developed for the ±150 kv range for both land and submarine cable applications. Installation and commissioning of the converter stations and DC transmission cables was completed approximately 24 months after contract signing. All performance criteria for power quality, transfer capability, audible sound and losses were met. 7 REFERENCES [1] B. D. Railing, J. J. Miller, G. Moreau, J. Wasborg, Y. Jiang-Häfner, D. Stanley, The DirectLink VSC-based HVDC Project and its Commissioning, [CIGRÉ 2002, Paper no ]. [2] High-Voltage Direct Current Handbook, First Edition, Electric Power Research Institute, TR s, [3] A. Ericsson, M. Jeroense, J. Miller, L. Palmqvist, B. Railing, P. Riffon: HVDC Light Cable Systems The Latest Projects, [Nordic Insulation Symposium, Tampere, Finland, 2003]. [4] I. Mattsson, A. Ericsson, B. D. Railing, J. J. Miller, B. Williams, G. Moreau, C. D. Clarke, Murraylink, the Longest Underground HVDC Cable in the World, [CIGRÉ Paris August 2004]. [5] Y. Jiang-Häfner, H. Duchen, K. Linden, M.Hyttinen, P. F. de Toledo, T. Tulkiewicz, A. K. Skytt, H. Björklund, Improvement of subsynchronous torsional damping using VSC HVDC, [PowerCon 2002 Conference, Kunming, China 2002]. [6] U. Axelsson, A. Holm, C. Liljegren, K. Eriksson, L. Weimers, Gotland HVDC Light Transmission Worlds First Commercial Small Scale DC Transmission, [CIRED Conference, Nice, France, 1999].