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Basslink Basslink the high voltage DC power link between Victoria & Tasmania – the longest undersea power link in the world by Michael Goebel Back in February 2000 a tender was granted for building a high voltage DC power link between Victoria and Tasmania. It was finally commissioned in April 2006 and now feeds power in both directions between Victoria and Tasmania, depending on demand in those states. siliconchip.com.au September 2008  13 V ictoria and Tasmania have different power supply problems. Victoria has Australia’s most inefficient thermal power stations, burning brown coal – Loy Yang power station and mine is shown above. So it makes sense to use Tasmania’s hydo-electricity when it is available, especially during peak power periods. On the other hand, when Tasmania suffers droughts they often do not have enough hydro-electric power capacity to meet their own demand. When that happens, Tasmania can draw power from Victoria’s grid. But there is a problem – the 300km-wide stretch of often-wild water between the two called Bass Strait. And connecting them is not quite as easy as erecting a couple of giant pylons and stringing some cable over the water. Enter Basslink Basslink is a 400kV DC bi-directional, undersea electricity interconnector, rated to transmit 500MW continuously and 630MW peak from Tasmania. The link can operate at up to 600MW for up to 10 hours, providing that it is ‘precooled’ (six hours at no more than 300MW). In the other direction, up to 480MW can be sent to the relatively small Tasmanian grid. Basslink consists of: • a 290km-long submarine power cable, the longest of its type in the world, from McGaurans Beach near Giffard in Victoria’s Gippsland, to Four Mile Bluff, above George Town on Tasmania’s north coast. • a 60.8km overhead power line to the Victorian coast. 14  Silicon Chip • a 6.6km underground cable in Victoria. • an 11km overhead line section to the Tasmanian coast. • a 1.7km underground cable in Tasmania. The high voltage cable used for the Basslink is 15cm thick. The undersea cable alone weighs 17,400 tonnes. From McGauran Beach it runs for a few kilometres as an underground cable and finally emerges as an above- ground line running 70km to Loy Yang power station. There, the high voltage DC from Tasmania is converted into AC with the help of thyristor “converter valves” so it can be fed into the 3-phase power system. These valves have nothing to do with thermionics or vacuum tubes but are entirely solid state. The thyristors are made of pure mono crystalline silicon and are effectively used as switches. When Victorian power is being fed to Tasmania, a similar station in Tasmania transforms the DC into AC. So power transmission can be made in both directions, with similar conversion equipment for AC to DC and DC to AC at each end. Why DC? In an AC distribution system, voltage conversion is simple – just use a transformer. However, above certain power levels and over long distances, for submarine cables, high voltage AC transmission links have significant disadvantages compared to DC, despite DC having to be converted twice. For a long transmission path, the smaller losses and siliconchip.com.au reduced construction cost of a DC line can offset the additional cost of converter stations at each end of the line. Also, at high AC voltages significant amounts of energy are lost due to corona discharge, the capacitance between phases or in the case of buried cables, between phases and the soil or water in which the cable is buried. Long undersea cables have a high capacitance. While this has minimal effect for DC transmission, the current required to charge and discharge the capacitance of the cable causes additional I2R power losses when the cable is carrying AC. In addition, some AC power is lost in the dielectric. HVDC can carry more power per conductor, because for a given power rating the constant voltage in a DC line is lower than the peak voltage in an AC line. This voltage determines the insulation thickness and conductor spacing. Therefore DC transmission line corridors can be used to carry more power into an area of high power consumption, which can again lower costs. Another advantage of HVDC systems is that they require only two cables as opposed to the three needed for three phase current transmission. As a result, an HVDC overhead line also requires less space. So the key advantages of a DC link over an AC link are: • It allows power transmission between AC networks with different frequencies, or networks which cannot be synchronised. A good example of this is Japan which has 50Hz and 60Hz power grids. • Inductive and capacitive parameters do not limit the transmission capacity or the maximum length of a DC overhead line or cable. • In addition, the full conductor cross section is utilised because there is no “skin effect” at DC. Thyristor “valves” The outdoor valves for earlier systems were designed with oil-immersed thyristors with parallel/series connection of thyristors and an electromagnetic firing system. Further development went via air-insulated, air-cooled valves to the air-insulated, water cooled design, which is the state-of-the-art in HVDC valve design. The development of thyristors with higher current and voltage ratings has eliminated the need for parallel connection and reduced the number of series-connected thyristors per valve. Light triggered thyristors It has long been known that injecting photons into the gate instead of electrons can turn on thyristors. See Fig 5. The route of the predominantlyundersea HVDC interconnect between the Loy Yang power station in Victoria’s Gippsland and the Tasmanian hydro grid connection at George Town, near Launceston in northern Tasmania. This is known as LTT (Light triggered thyristor) technology and reduces the number of components in the thyristor control by up to 80%. This simplification results in increased reliability and availability of the transmission system. With LTT technology, the gating light pulse is transmitted via fibre-optic cable (itself providing high isolation), through the thyristor housing directly to the thyristor wafer. Therefore no elaborate electronics, including auxiliary supplies, are needed to control the high potential. Innovations in almost every other area of HVDC have been constantly adding to the reliability of this technology with economic benefits for users throughout the world. Light Pipe Cu Si Cu Mo A graphical diagram of one of the Siemens laser-controlled LTT thyristors which make up the “valve” used in the AC/ DC and DC/AC conversion in Basslink, with an exploded photo of the device at right. siliconchip.com.au September 2008  15 The high-performance thyristors installed in HVDC plants today typically have silicon wafer diameters of up to 125mm (6 inches), blocking voltages up to 8kV and current carrying capacity up to 4kA DC. Thus no parallel thyristors need to be installed, however, series connection is necessary to handle the HVDC voltages. The required optical gate power is just 10mW. The forward overvoltage protection is integrated into the wafer. Further benefits of direct light triggering are the unlimited black start capability and operation during system undervoltage or system faults without traditional limitations. In the case of convential (electrically triggered) thyristors (ETT), this is only possible if sufficient firing energy is stored long enough in the thyristor electronics. A simple voltage divider circuit made from standard off-the-shelf resistors and capacitors allows monitoring of the thyristors performance. Monitoring signals are transmitted at very-much-safer ground potential through another dedicated set of fibre optic cables as for the LTT. All electronic circuits needed for the evaluation of performance are now located at ground potential in a protected environment, further simplifying the system. The extent of monitoring is the same as for the ETT. It is expected that this technology will become the industry standard in HVDC thyristor valves of the 21st century, paving the way towards maintenance-free thyristor valves. Laser control The thyristors in the Basslink project are controlled by 10mW laser flashes, transmitted via glass fibres. These thyristors, which have a diameter of 100mm, were produced by Infineon and are made of silicon, molybdenum and copper. To achieve a DC voltage of 400kV, several dozen thyristors per converter valve are connected in series. All of these thyristors must trigger within 1µs in order to ensure that none are overloaded or damaged. Valve Design The modular concept of the Siemens thyristor valves permits different mechanical setups to best suit each application: single, double, quadruple valves or complete six-pulse bridges, either free standing or suspended from the building structure. The standard Siemens valves for long distance transmission are suspended from the ceiling of the valve hall to allow them to withstand earthquakes, especially important in areas prone to seismic activity such as Japan and New Zealand. The suspension insulators are designed to carry the weight and additional loads originating for example from The old and the new: at left is a bank of six mercury-arc rectifiers for a 100kV, 140A high-voltage DC supply from around 1942. At right are the HVDC valve towers in the 3000MW. ±500kV, 1000km HVDC transmission link between Guizhou and Guangdong in China. Basslink is a smaller distance but is predominantly under water, which brings in a whole new set of problems to overcome (photo courtesy Siemens). 16  Silicon Chip siliconchip.com.au an unbalanced weight distribution due to insulator failure, an earthquake or during maintenance. Connections between modules (piping of cooling circuit, fibre optic ducts, buswork and suspension insulator fixtures) are flexible in order to allow stress-free deflections of the modules inside an MVU (multiple valve unit) structure. Each valve is made up of three modules. Four arresters, each related to one valve, are located on one side of the valve tower. Ease of access for maintenance purposes, if required, is another benefit of the Siemens valve design. By varying the number of thyristors per module and the number of modules per valve, the same design can be used for all transmission voltages that may be required. Thyristor cooling The thyristors are stacked in the module with a heatsink on either side. The water connection to the heatsinks can be designed in parallel or series. The parallel cooling circuit provides all thyristors with the same water temperature. This allows a better utilisation of the thyristor capability and offers the additional advantage that electrolytic currents through the heatsinks – the cause for electrolytic corrosion – can be avoided by placing grading electrodes at strategic locations in the water circuit. The parallel wafer cooling principle has been in use for more than 25 years, with no corrosion problems ever encountered. Water cooling also does not require any deoxygenising equipment. Fire! With such enormous power involved, arcing and fire is a constant risk. This has been minimised through many steps: • All oil has been eliminated from the valve and its components. Snubber capacitors and grading capacitors use SF6 as a replacement for impregnating oil. • Only flame-retardant and self-extinguishing plastic materials are used. • A wide separation between the modular units ensures that any local overheating will not affect neighbouring units. • Careful design of the electrical connections avoids loose contacts. The past has shown that Siemens HVDC installations have never been exposed to a hazardous fire risk. The tests performed on actual components and samples in the actual configuration as used in the valve indicate that the improved design indeed is flame retardant and the risk of a major fire following a fault is extremely low or even non existent. REPLACE One end of BassLink – the 400kV DC valve hall near the Loy Yang power station in Victoria’s Gippsland. The valves hang from the ceiling, predominantly to allow them to move, protecting them from damage in case of earthquake. siliconchip.com.au September 2008  17 High Voltage DC: how it works As noted elsewhere in this article, the basic element in all HVDC conversion is the light triggered thyristor (LTT) or SCR (silicon controlled rectifier). These are connected in series banks of a hundred or more SCRs, to give a total rating which may be 500kV DC or more and hundreds of Amps. When connected in such a way, they are referred to as “valves” and each valve can be regarded as a single device. In reality, to function in this way, all the SCRs in a valve must be triggered on within one microsecond of each other. This is achieved by a laser light pulse fed to the every SCR in the valve. Once an SCR is triggered, it breaks into conduction and it continues to conduct until the load current falls to a very low value (below the “holding” current) or the voltage across it is reversed in polarity. So in effect, once an SCR or valve is triggered into conduction, it behaves just like a conventional diode, albeit a very large diode! So how are these banks of LTTs, or valves, connected to perform the conversion from AC to DC or DC to AC? And how does the same setup perform either AC/DC or DC/AC without altering the connections? To keep it simple, let’s first consider the conversion of AC to DC. Consider that all power grids are 3-phase systems, with the difference between respective AC phases being 120°. Fig.1 shows a 3-phase transformer connected to a 6-SCR bridge rectifier. Incidentally, this is exactly the same connection as used in a standard car alternator (it has six large diodes pressed into its casing.) Each diode (or LTT) conducts when + +DC THREE-PHASE TRANSFORMER (STAR/STAR) φ1 RED R φ3 φ2 BLUE W WHITE 0V PHASE 2 18  Silicon Chip D3 B it has a forward voltage across it and so each diode conducts for a maximum of 60°. The result is a DC voltage with a superimposed 300Hz AC ripple (for a 50Hz grid), as shown in Fig.2. At any one time, only two diodes will be conducting, for example, D1 & D5 or D6, D2 & D4 or D6 and so on. The result is that there are six diode-conduction periods providing the AC to DC conversion which is more familiarly known as rectification. Exactly the same setup can be used to convert the DC back to AC except that the power flow is in the other direction. In DC/AC conversion, only two SCRs conduct at any one time (as in rectification) but they need to be triggered into conduction at the right time to energise the particular transformer windings. However, the resultant waveforms on the output side of 3-phase transformers are anything but a clean sine wave. Hence quite heavy filtering is required in order to prevent large harmonics in the distribution grid. PHASE 3 T – D2 Fig.1: AC to DC conversion in an HVDC system is D4 D6 D5 essentially the same as the rectification process 0V in a 3-phase power system, whether it is the alternator of a car or in a country’s electric power grid. Six diodes (or SCRs) are required and two diodes will be conducting at any one time. Note that while we show a star-connected transformer in this case, it could just as easily be a delta-connected system. DC OUTPUT PHASE 1 D1 Fig.2: This set of waveforms shows how a 3-phase system is converted to DC using the schematic of Fig.1. The three voltage phases are separated by a phase difference of 120° and the resultant DC from the rectifiers has a superimposed ripple at six times the mains frequency, ie, 300Hz for a 50Hz grid. Because there are six periods (or combinations) of SCR conduction in this DC/AC conversion process, this is referred to as a 6-pulse converter; six trigger pulses are required for one cycle of AC which will be 50Hz or 60Hz, depending on the country where it is used. In reality, the setup outlined in Fi.g.1 is not used now. Instead, we have a more complicated system, shown in Fig.3. This uses a 3-phase transformer with star and delta-connected secondaries feeding a set of 12 SCRs or valves. This makes use of the fact that while the three phases are separated by 120°, when the outputs of a star and delta system are connected together, you effectively have six phases separated by 60°. When used for AC/DC conversion (ie, rectification), the resultant DC has 600Hz ripple (for a 50Hz grid) and there are 12 combinations of SCR conduction. To consider just one 30° conduction period, when D1 conducts, so will D5, D8 & D11 or D6, D9 & D12. Since there are 12 conduction periods, this is referred to as a 12-pulse converter. The six voltage waveforms from the star and delta windings are depicted in Fig.4. Note that we have shown the Star and Delta winding outputs with their normal amplitude relationship, whereby the phase-to-phase (delta) voltage is √3 times the phase to neutral (star) voltage, to highlight how the 6-phase system comes about. siliconchip.com.au +DC STAR SECONDARY φ1 PRIMARY WINDING THREEPHASE φ2 AC φ3 D1 D2 D3 D4 D5 D6 D7 DELTA SECONDARY D10 D8 D9 D11 D12 HIGHVOLTAGE DC 0V Fig.3: the schematic of a 12-pulse AC to DC converter. While this circuit is ostensibly an AC to DC converter, it can just as easily be run in the opposite direction, converting DC to 3-phase AC by triggering the SCRs at appropriate times to energise the relevant transformer windings. In reality, we assume that their amplitudes will be the same. For DC/AC conversion, the process is exactly the same as for a 6-pulse system except that we now have 12 SCRs (or valves) which are triggered at 12 points in the waveform to energise the star and delta windings. Because the harmonics in a 12-pulse converter are twice as high Fig.4: the phase relationship in a + 3-phase system using star and delta waveforms together, to drive the converter circuit of Fig.3. The resultant phase waveforms are separated by only 60° and the 0V resultant DC from rectification has ripple at 12 times the mains frequency, ie, 600Hz for a 50Hz system. Note that the harmonics produced in conversion can run as high as the 30th or higher and – require elaborate filtering. in frequency and reducing in amplitude by the same amount, it is much easier to filter them out and prevent them being fed into the distribution grid. Note that the schematic of Fig.3 is much simplified compared to reality and does not show all the filtering, protection and ancillary components. Note also that the DC/AC conversion process assumes that there is already 3-phase power available on the AC grid, to provide the essential triggering and synchonisation of the SCR valves. In a complete HVDC system there is DELTA φ1 STAR φ4 DELTA φ2 STAR φ5 DELTA φ3 STAR φ6 T an AC/DC converter at each end of the transmission line and in the case of the Basslink system and others throughout the world, they can transfer power in both directions. See Fig.5. So was Edison right all along? Anyone familiar with the history of electrical energy will know that Thomas Edison was a very strong proponent of DC and many early systems in towns and cities were based on DC. Ultimately though, the sheer advantages of the AC system, as promoted by Westinghouse and largely invented by Nicola Tesla, won out and now AC generation and transmission of power are universal, usually at 50Hz or 60Hz. So why is high voltage DC now being promoted? In fact, the advantages of AC over DC are still manifest. However, for power transmission over very long distances, say 800km or more or for distances of 50km or more undersea, DC has advantages in that usually only one cable instead of three, in the case of 3-phase AC transmission, is involved. By transmitting electrical power at very high voltages, eg, 400kV or higher, the resistance losses are reduced, as are the weight of the cable, its supporting structure and so on. Furthermore, DC transmission makes it possible to transfer power between two electrical grids that operate at different frequencies (50Hz & 60Hz), as happens in Japan. So Edison did not get it right. High voltage DC power transmission is the solution to problems that Edison is unlikely to ever have foreseen. SC +DC φ1 AC GRID1 φ2 φ1 AC–DC CONVERSION HIGH-VOLTAGE DC TRANSMISSION φ3 AC–DC CONVERSION φ2 AC GRID2 φ3 0V Fig.5: a complete HVDC system in schematic form. In very long distance systems, as used in China or Brazil, power transmission is usually in one direction only but a complete AC to DC and DC to AC converter is required at the start and finish of the transmission line. siliconchip.com.au September 2008  19