2012 San Francisco Colloquium http : //www.cigre.org Advances in voltage source converter (VSC) technologies B4-6 500 kv VSC Transmission System for lines and cables B. JACOBSON, B. WESTMAN, M. P. BAHRMAN* ABB Sweden, USA* bjorn.jacobson@se.abb.com In many cases an elevated transmission voltage is needed to reach an optimum balance between power capacity, investment cost and losses. High voltage also implies large power blocks. The maximum allowable block size is determined by total system response to loss of links or converters. In weaker networks classic current source converters will require special measures to operate well. Here the by now traditional advantages of voltage source converters (VSC) can be used to facilitate addition of transmission capacity even to inject power into points with very low available short circuit power. Other aspects of the VSC technology like relatively small foot print and the possibility to connect isolated wind farms can be other factors leading to the selection of this particular converter type. By dividing the power of the VSC converters in two separate poles, a high utilization of converter rating and optimal lines can be obtained without surpassing the reliability criteria for the network. Two blocks of opposite polarity is preferred because they can balance out any ground current in normal operation without the need for a third conductor or cable. A key component in this scheme is the asymmetrical voltage source converter. The paper will cover the status of the HVDC converter technology, particularily with respect to two basic functional topologies namely asymmetric monopoles and the bipole configuration. Examples of two operational asymmetric links and one asymmetric link under design and construction will be covered detailing the specific requirements and operational duties that led to the selection of asymmetric links. Aspects of cable connection as well as overhead lines will be covered. Finally some future possible applications of this technology will be discussed.
1 Status of the technology VSC transmission has changed from a niche technology to being a large portion of the total HVDC market in the past five year period. Some of the aspects bringing about this shift are increased demand for advanced transmission, the build up of a large and varied pool of user experience, the increase of power and voltage ratings, the reduction of losses and last but not least the availability of more than a single supplier with now three vendors having delivered or been awarded large international VSC transmission system contracts. Examples of advanced transmission are interconnections between weak networks, power from shore, multi infeed and wind power infeed, especially from offshore generation. The pool of user experience needs not be listed here but can be found on vendor websites, e.g. [1]. Voltage ratings have gradually progressed. The highest pole voltage in operation for VSC is 350 kv (Caprivi Link, Namibia) and the highest voltage in an awarded contract is 500 kv (Skagerrak 4, Denmark Norway). The voltage level is not in principle limited by the converters, so it would be feasible to build VSC converters to the 800 kv level, this being the highest direct voltage in operation, albeit on a LCC (Line Commutated Converter or Classic ) HVDC system. The experience from the HVDC Classic insulation design could be adapted to VSC converters. Cable voltages available is cited at 500 550 kv from different vendors. Typical cable currents could be 1400 2000 A depending on thermal conditions and choice of conductor material, where copper would give higher current capacity but also higher material cost and total weight. Increased transmission capacity could be created by parallelling cables, especially for short bypass sections of an otherwise overheadline transmission, for instance when passing a river or an urban area. 3,5% 3,0% 2,5% 2,0% 1,5% 1,0% 0,5% Gen. 1 Gen. 2 HVDC Light Gen. 3 Gen. 4 HVDC Classic 0,0% 1995 2000 2005 2010 2015 Figure 1 Evolution of losses for HVDC Light (VSC Transmission). HVDC Classic typical losses shown in green for comparison. In successive generations of VSC technology, the converter station losses have been reduced step by step. The now more or less prevailing multi level technology has led to conversion losses of about 1% per station, which is a huge step from the early days of VSC when 3% was a typical value. To come down to 1% or even less, advances in topologies (multilevel converters) and switching strategies have to be paired with careful semiconductor selection and even optimization of the
semiconductors and their driving for the specific application in HVDC. The semiconductor of choice for VSC is the Insulated Gate Bipolar Transistor by virtue of low driving power, predictable behaviour during transient events, large experience bank, comparatively low losses and good availability in suitable ratings on the market. In the different types of possible multi level technologies, there is the choice to use series connection of semiconductors. The main reason for using series connection is reliability. When series connecting several semiconductors in a converter cell, single device failures are acceptable and operation of the converter can continue without need to bypass the cell with a faulty IGBT. Without series connection, the affected cell needs to be bypassed with some type of device, often a mechanical switch, otherwise operation of the converter has to be stopped because of the unpredictable status of a converter cell with no means of control left. In order to take full advantage of series connection specially designed IGBTs that fail to a stable short circuit are used. 2 Projects with asymmetric converters To date three asymmetric VSC transmission systems are in operation/construction but the reasons to select this design and the operation modes used vary: Valhall (78 MW, 150 kv) was designed as an asymmetrical monopole to make optimum use of the coaxial HVDC cable design. Caprivi (250 MW, 300 kv) was designed as an asymmetrical monopole but is prepared for a future additional converter and bipolar operation. Skagerrak 4 (715 MW, 450 kv) is designed as a monopole and will be operated in bipolar mode with Skagerrak 3 (which uses LCC technology). 2.1 Valhall The Valhall platform complex has been in operation since 1982 but the facilities needed to be upgraded and it was decided that the platforms should be supplied with power from the Norwegian AC grid and to remove the existing gas turbines installed on of the platforms. The link is 78 MW HVDC link and use asymmetrical 150 kv VSC converters and a 292 km long 150 kv HVDC cable. The power coming from shore is converted in the offshore HVDC station to AC and then distributed on 11 kv level to the hotel and production platforms. The majority of the load on the platform complex consists of compressors and pumps. Figure 2 The Valhall platform complex is located roughly halfway between UK and Norway. Figure 3 Lista onshore HVDC converter station.
The VSC converter concept offers important advantages in offshore installations as space and weight are key issues and cost drivers for the platform. The filters used in VSC design are all small and can be made compact and light weight compared to other solutions. The transmission link went into operation 2011. 2.2 Caprivi Link The Caprivi Link Interconnector is rated 300 MW at 350 kv and the transmission distance is 950 km. The link is interconnecting the AC networks of Namibia and Zambia and is important in the Southern African Pool. This is the first project that uses VSC technology together with overhead lines. The link has been design to allow a future bipolar extension and the DC overhead lines are built with two poles from the start. The scheme can in the first phase be operated in three alternative operational modes: 1. Metallic return using the second pole as return conductor 2. Earth return using one DC line conductor 3. Earth return using parallelled DC line pole conductors. The losses are particularly low in operation mode 3. The long overhead line is of course subject to lightning strikes but also to disturbances emanating from fires in the brush vegetation in the arid parts of the transmission path. The need to come back quickly to stable transmission even during adverse electrical network conditions has been solved, partly by employing conventional high voltage dc breakers and partly by developing controls and sequences that enable an adapted response to a wide range of contingencies. The transmission link went into operation 2011. Figure 4 Zambezi converter station located in the Caprivi region. The design allows integration of a future second HVDC pole. 2.3 Skagerrak 4 In February 2011, an order for a 715 MW 500 kv VSC asymmetric system was awarded for an additional parallel fourth interconnection between Norway and Denmark, the Skagerrak HVDC interconnections. The three existing HVDC links in the system all use LCC technology and have a
capacity of 1050 MW. The two first links SK1 and SK2 have been in operation since mid 1970 s. The new link SK4 will operate in bipolar operation with SK3 (in operation since the early 1990 s). The new link s HVDC voltage of 500 kv is to date the highest pole voltage for a VSC system. Particular challenges in this project, apart from the record voltage, include parallel operation to three other LCC HVDC systems and a requirement that the VSC system should be able to balance the resulting current when one of the other poles go out of operation or when other conditions in operation so demand. This has called for the first VSC system design that is required to reverse polarity. Also the project requires the full use of the reactive power capability inherent in the VSC to enable transmission even during degraded or abnormal network conditions. 3 Potential North American Applications for ±500 kv VSC HVDC North America has abundant, high quality wind resources across the Great Plains on the centre of the continent. Many of the major load centres, however, are located over 1000 km to the east or west of this region. Transmission in this prime wind region is relatively sparse reflecting its comparatively low population density. The networks across this region are separated by asynchronous boundaries between the eastern and western interconnects and with the Electric Reliability Council of Texas (ERCOT). Substantial transmission additions are required to integrate these wind resources. With its increased converter capacity and improved efficiency, the unique characteristics of VSC based HVDC transmission make it an even more attractive for integration and delivery of renewable energy from non traditional generation. Circuit capacity is now roughly equivalent to that of a 500 kv ac line but with lower line cost and lower line losses. In a traditional bipolar configuration, transmission capacity is similar to that of a double circuit 500 kv ac line but at about half the line cost. Higher transmission capacities are possible with parallel converters or alternative circuit configurations, e.g. multiple symmetrical monopoles. Unlike with an ac line, however, there is no variable reactive power demand or stability limitation due to distance effects. If the transmission path crosses sensitive or congested areas, where permits and right of way acquisition are difficult to obtain on a timely basis, underground transmission with solid dielectric, extruded dc cables can be used for all or a portion of the route without distance limitation and with due consideration of shared rights of way. Figure 5 shows the configuration for a bipolar VSC HVDC transmission link with hybrid overhead underground transmission. The link is equipped with provision for metallic return switching. The controllability of HVDC transmission mitigates inadvertent flow on parallel paths and bypasses congestion thereby reserving established transmission capacity for its intended purpose. Controllability tends to firm up HVDC transmission capacity since congestion on parallel paths is less likely to cause curtailment during periods of high wind production. Power flow on the dc transmission link can automatically track variable aggregated wind plant generation keeping it off parallel paths.
Underground Figure 5 ± 500 kv VSC HVDC link with hybrid OVHD U/G transmission and metallic return switching provision. Conventional HVDC transmission requires reactive power compensation. Large concentrations of shunt compensation lower voltage stability, increase load rejection over voltages and lower the system resonance frequency for low order harmonics. Consequently conventional HVDC must have relatively strong points for network interconnection. Wind or PV solar generation do not appreciably strengthen the ac network. These issues are mitigated with VSC HVDC since there is no need for reactive power compensation. Therefore much weaker network conditions can exist at the points of interconnection without being supplemented with conventional generation or other synchronous machines. Even radial outlet transmission from wind plants is possible with VSC HVDC as evidenced by various offshore wind projects. Furthermore, the dynamic voltage support capability of the converters and use of choppers can improve recovery of the wind farm from ac system disturbances to better fulfill grid code requirements. With conventional HVDC, temporary dc line faults are cleared quickly by converter phase control with no need for circuit breaker action. Rapid clearing of dc line faults with VSC requires dc circuit breakers. Figure 6 illustrates some of the large HVDC projects for accessing remote wind resources on the US Great Plains which are currently under active development. More lines would be required if EHV ac transmission were used. The ac system characteristics at the sending end for many of these projects require additional investment to strengthen the system interconnection. This is especially true since local conventional generation may not be dispatched when the wind is blowing strongly. Use of VSC HVDC would substantially reduce the amount of additional investment. Furthermore, the variable O&M expenses for following the wind generation would be less since there would be no need for commensurate reactive power compensation.
Zephyr TransWest Express Centennial West Rock Island Grain Belt Plains & Eastern Southern Cross Figure 6 Proposed ± 500 kv HVDC links for wind plants. Annual average wind speed at 80 m per AWS Truewind, NREL. 4 Alternatives for Congested Rights of Way It is often challenging to terminate high capacity transmission lines at key substations within a major load centre due to land use constraints, congested rights of way and reliability concerns. With HVDC transmission, possibilities exist to work around some of these constraints. One option is to transition to underground cable transmission along a shared utility corridor, beside roadways or along transmission lines. A second option is to transfer loads from lower voltage circuits to higher voltage circuits and use the freed up right of way for an HVDC line. A third option is to convert existing EHV transmission lines to dc to increase their power transfer capacity. Conversion of an ac line to a dc line to increase transfer capacity has been analyzed extensively over the years but has never been implemented. [2] Some lines have been designed with provision for eventual conversion to HVDC. Analysis must take into account corona and field effects, insulation properties, clearances and conductor current rating in order to determine the possible increase in capacity. In the United States, the National Electric Safety Code provides clearance to ground criteria consisting of a defined reference height plus an electrical component of clearance. The reference height depends on the land use of the underlying terrain traversed. The electrical component of clearance depends on the maximum ac or dc operating voltage and an overvoltage factor. For a long bipolar HVDC line the overvoltage factor is about 1.75 times the operating voltage. This factor is lower than that for an EHV ac line. Composite long rod insulators have superior pollution performance and can have longer creepage distance for a given connecting length. Insulator replacement can allow raising the transmission voltage and thereby the power capacity. Replacing the outer crossarms with horizontal vee composite insulators raises the conductor anchor point and eliminates conductor swing at the tower to obtain a higher operating voltage for a given conductor sag. By making these insulation changes to convert a
single circuit 345 kv ac line with two conductor bundles to a bipolar HVDC line for example, one could reasonably expect to increase the transfer capability by about 80% above the ac line thermal rating. 5 Conclusions VSC is an innovative but mature technology that enables new uses in transmission. Ratings are already today sufficient both in terms of current and voltage to match the near future needs of large scale transmission, in North America as well as in the rest of the world. For example, a 500 kv, 700 MW monopolar cable system has already been ordered in North Europe. The asymmetric converter in a bipolar arrangement can give an attractive combination of high power, reasonable block size and total system reliability. Both overhead lines and cables are available giving the planner larger freedom to select an optimum transmission path. Possible uses in North America could be new wind power infeed and relief of congestion in the transmission system [1] www.abb.com/hvdc Internet website [2] K. Halson, F. Loudon, I. Gutman, J. Lundquist; Feasibility of upgrading 300 kv AC lines to DC for increased power transfer capability, B2 110 CIGRE 2008