New semiconductors push back the frontiers
23 April 1999Radical advances in HVDC converter technology and ever more ambitious realms of application, towards ever higher power capacities in one direction and ever lower power capacities in the other, have followed hard on the heels of world-wide deregulation in the power industry.
In spite of recent setbacks to the plans to interconnect the grids of ASEAN countries and the collapse of the Bakun HVDC link project, massive new undertakings for international HVDC links and back-to-back interconnectors are now appearing with increasing frequency, and radically advanced new technology is beginning to be introduced in major commercial projects.
In China, the monumental Three Gorges scheme is spawning a whole network of HVDC systems of hitherto unheard of capacities, the equally massive Trans-European High Voltage Power Bus is approaching the implementation of the first stage, having established a source of financial investment.
In Brazil, capacitor commutated voltage source converters are being installed on a major interconnection project, and new silicon carbide switching chips are being tested in a small scale HVDC link in Sweden. Major HVDC interconnection developments are also anticipated from an important new political initiative between Russia and China.
Irkutsk-Mongolia-Beijing
During the February 1999 visit to Moscow of the Premier of the Chinese State Council Zhu Rongji progress was registered on the project to construct an extremely high capacity bulk power transmission line to transport electricity from the Irkutsk region of Siberia to China. A gradual rapprochement of the sides on prices, giving hope for compromises was reported.
ABB Power Systems spokesmen say the Chinese government now supports the project to build a 600 kV, 3000 MW HVDC transmission line to supply Beijing in China with Russian electricity from Irkutskenergo, via Mongolia.
According to speakers at an EU–China energy conference in Brussels on 4 March 1999 some $1.5 billion will be needed to finance the project.
Russia has had an increasing surplus of power generation capacity in Siberia, particularly since the financial crisis of August 1998, and environmentally concerned Beijing is unlikely to build new coal-fired power plants to serve the country's fast expanding capital city.
The new 2500 km transmission line would use proven technology which has already been successfully applied on projects as big as this. It is roughly equivalent to one bipole of the Itaipu system in Brazil/ Paraguay or one of the 3000 MWe MVA two bipole links just ordered for the Three Gorges system in China.
Studies began four years ago for the scheme, which was granted approval by former Chinese Premier Li Peng. The project was endorsed by the current Chinese Premier Zhu Rongji during his February visit to Moscow last week. The two converters, in Irkutsk and Beijing, will probably be supplied by Western European companies while transmission wires, towers and insulators would most likely be sourced from Russian and Chinese companies.
Three Gorges
The mighty Three Gorges hydropower plant complex will have an installed generating capacity of 18 200 MW and will drastically reduce the threats of flooding to the 13 million residents in the lower reaches of the Yangtze River. The huge back-dam powerhouse will be sited in the middle of Xiling Gorge, one of the Yangtze Three Gorges, with the 185 m high dam located at Sandouping, Yichang City in Hubei Province.
The power generated will be transmitted to the Central China Power grid, the East China Power Grid and the Chongquin Power Grid via the Three Gorges Transmission System consisting of 9100 km of HVDC and HVAC lines, and 2475 MVA of transformer capacity. These will include two circuits of 500 kV, 3 GW double bipole overhead HVDC lines in addition to the existing 1.2 GW Gezhouba–Nanquiao (Ge–Nan) HVDC line. The first of the two new links, the recently ordered HVDC 2 link from the right bank busbar to the Liantang converter station, is under construction and the second is at the consulting stage. These will aggregate a total of 7.2 GW of transmission lines from the Three Gorges to Eastern China.
The system will serve an area extending some 2900 km from east to west and 1500 km from north to south with an installed generating capacity in 1996 of 84 GW, representing 38.6 per cent of the whole of China's capacity. Its 7766 km of 500 kV transmission lines amounted to 57 per cent of the entire country's transmission capacity.
When the Three Gorges Transmission System is commissioned in 2010, serving 200 GW of generating capacity, it will be the largest in China. The degree of dependence on the three large HVDC lines also represents security and stability threats which might impact on a wide range of supply areas if outages should ever occur. This means that great care is to be taken with the control and configuration planning, based on the following criteria:
In the event of multiple incidents, the system should, as far as is possible, following simple and reliable measurements, prevent instability and large scale blackouts.
Further ±500 kV HVDC links to the North China Power Grid and to the South China Power Grid are expected to achieve substantial reductions in regional peak loads due to the non-coincidence of annual peak demand periods.
Expansion to the west of Three Gorges is expected with the development of the 12 GW Xiluodu and 6 GW Xiangjiaba hydropower stations in due course, possibly using HVDC links of higher than 500 kV voltage.
For the recently ordered HVDC 2 link, ABB Power Systems will supply the converter stations, Siemens will supply the Liantang converter transformer and smoothing reactors, while the overhead lines will be supplied by Chinese concerns. Commercial operation of HVDC2 is due in 2003.
Garabi back-to-back link
The first commercial application of the modular back-to-back HVDC concept, using capacitor commutated converters, is the Garabi interconnector joining the Brazilian and Argentinian grids. The Garabi link is not an Itaipu extension, but it will still benefit from the Ivaipora 765 kV, 1056 Mvar series capacitor – the largest in the world – in the Itaipu to Sao Paulo link since the transmission capacity of both lines will be increased due to the power factor correction.
The main function of the Garabi back-to-back link is to convert power from the 60 Hz of the Brazilian grid to the 50 Hz of the Argentinian grid in order to facilitate power transfers between the two countries in a manner which will support power trading across national borders.
In May 1998 the Brazilian Ministry of Mines and Energy, through Eletrobras, Furnas, and Gerasul, in agreement with the Argentine government, signed a 20 year contract to import 1000 MW of firm capacity into Brazil from MEM – the wholesale energy market in Argentina. The electricity purchased by Eletrobras will be supplied commercially via the South/Southeast/Central-West interconnected systems. ABB was awarded the contract for the first ITP (independent transmission project) between the two countries.
The link is in a 500 kV HVAC line which runs from the Rincón de Santa Maria substation at the Yacyrita 1000 MW hydropower system in Argentina some 127 km to the Garabi converter, and thence some 365 km further into Brazil to the Ita substation near Rio Grande do Sir in Santa Catarina.
The converter station is a modular, compact back-to-back system, consisting of two blocks of 550 MW each. It provides an asynchronous bi-directional connection between the 50 Hz, 500 kV Argentinean AC network and the 60 Hz, 525 kV Brazilian AC network. It is being built as a private project for CIEN (Companhia de Interconexão Energética), a group headed by Endesa of Chile and Endesa of Spain formed to import energy from Argentina into Brazil.
Modular converters
The Garabi back-to-back converter ctation involves several significant technical advantages which represent a major step forward in ABB's HVDC technology. These are:
The most striking technical difference between conventional HVDC and the technology used in the Garabi Converter Station is the introduction of commutation capacitors between the converter transformer and the valves. CCC provides several additional attractive features:
Control, protection, event logging, and fault recording are integrated in the same package, ABB's new Mach 2 system, which is a fully redundant, PC-based control system. The system can be operated from remote locations in Argentina or Brazil.
Voltage source converters
The predominant technology in today's HVDC transmission systems is phase-commutated converter (PCC) technology, based on thyristors. One of the main problems with PCC is that there must be rotating machines in the receiving network, with the attendant risk of commutation failure. With the advent of the voltage source converter (VSC) the current can be switched off as well as being triggered, and therefore there is no need for an active commutation voltage from the connected network.
This opens up the possibility of using HVDC to supply "dead" networks, ie sections in which there are no rotating machines or in which the short-circuit powers of the rotating machines are very low.
HVDC was originally developed from technologies used in industrial drive systems. There the PCC (phase commutated converter) technology which is at present being used for HVDC has now almost entirely been replaced by VSC technology.
With the appearance of high switching frequency components such as IGBTs it now becomes advantageous to use pulse width modulation (PWM) technology.
In a VSC converter the ac voltage is created by very fast switching between two fixed voltages. The desired fundamental frequency voltage is created through low pass filtering of the high frequency pulse modulated voltage.
With PWM it is possible to create any phase angle or amplitude within limits by changing the PWM pattern, which can be done almost instantaneously. Thus PWM offers the possibility to control both active and reactive power independently of each other.
This makes the pulse width modulated voltage source converter a close to ideal component in the transmission network. From a system point of view it acts as a motor or generator without mass that can control active and reactive power almost instantaneously. Furthermore, it does not contribute to the short-circuit power since the ac current can be controlled.
VSC HVDC is particularly beneficial for connecting, small dispersed electricity generators to a grid. A recent example is the 60 MW connection of a wind generator park to the main grid of the island of Gotland in Sweden, using ABB's new HVDC Light technology. Thanks to the independent control of reactive and active power afforded by the VSC scheme, the varying operating conditions of the wind power units can be accommodated while increasing the utilisation of the wind turbines by some 5 to 10 per cent, thus improving the economy of the installation.
A pilot project to use HVDC Light technology with VSC to simplify large wind farm connections to the grid, while at the same time solving the reactive power problems and eliminating any instability potential completely, has been contracted by Eltra with ABB. Eltra, the Western Denmark grid operator, will use the system to study how wind farms can best be connected into transmission networks.
Field tests are planned on a 7 MW on-shore wind farm at Tjæreborg, near the port of Esjberg on Denmark's west coast where the largest and most recently built wind turbines are located, as part of a government funded programme to encourage utilities to learn how to operate large wind farms feeding an ac distribution system.
The Hellsjön project
Since the IGBT is a metal oxide semiconductor (MOS) device, and the power requirement for the control of the component is very low, it was evident that series connection of many semiconductors with good voltage distribution, even at switching frequencies in the 1 kHz range, should be possible.
In 1994 ABB started the development of VSC converters for small-scale HVDC based on IGBTs – a technology it called HVDC Light (see MPS, May 1998). With the co-operation of the local utility, VB-Elnät, they designed the transmission for operation in a commercial network.
An existing l0 km long 50 kV back-up ac line between Hellsjön and Grängesberg in central Sweden was made available for the project. VB-Elnat also provided space in the stations and connections to the ac. networks.
The transmission capacity rating was set at 3 MW, somewhat above the hydropower generator rating at Hellsjön, with a direct voltage of ±10 kV dc. The converter stations were connected to separate parts of an existing 10 kV ac network.
During the development of the project the various characteristics and behaviours of VSC converters, PWM control, IGBT valves etc were tested in digital and analogue simulations.
The complete transmission stations were connected in a round power circuit at a laboratory in Ludvika.
By the end of 1996, and after a comprehensive synthetic testing, the equipment was moved to the field for installation and further testing. On March 10, 1997 power was transmitted between Hellsjön and Grängesberg in the first voltage source converter HVDC transmission.
Converter design main circuits
The HVDC Light converter consists of the bridge, the converter control, the converter reactor, dc capacitor, and an ac filter. The bridge is a six-pulse bridge, two-level, with series connected IGBTs in each valve. Every IGBT is provided with an antiparallel diode. Auxiliary power to the gate drive unit is generated from the voltage across the IGBT. Turn on/off of each individual IGBT is effected via an optical link from the control equipment on ground potential. The semiconductors are cooled with deionized water.
The objective for the dc capacitor is primarily to provide a low inductive path for the turn-off current; and for energy storage to permit control of the power flow. The capacitor also reduces the harmonics on the dc side.
The converter generates characteristic harmonics related to the switching frequency. The ac currents are smoothed by the converter reactor and then the remaining harmonic contents in the ac bus voltage are reduced by a high-pass filter.
Depending on the system requirements the converter may be equipped with two more components:
Control
The fundamental frequency voltage across the reactor defines the power flow between the ac and dc sides. The converter firing control calculates a voltage time area across the converter reactor which is required to change the current through the reactor from actual value to the reference value. The active power flow between the converter and the ac network is controlled by changing the phase angle between the fundamental frequency voltage generated by the converter (Ug) and the ac voltage on the ac bus. The reactive power flow is determined by the amplitude of Ug which is controlled by the width of the pulses from the converter bridge.
The current order to the controller is calculated from the set power/current order or the dc voltage control. A reference voltage, equal in phase and amplitude to the fundamental frequency component of the output voltage from the bridge Ug, is calculated. The pulse pattern is generated by the pulse width modulation (PWM) where the reference voltage is compared with the triangular carrier wave.
Potential of SiC
The commercial manufacture of silicon carbide (SiC) semiconductors with high power ratings will greatly enhance the performance and reliability of voltage source converters for HVDC transmission systems.
ABB's new "Powerchips" are claimed to be able to handle voltages up to ten times higher and working temperatures several hundreds of degrees higher than traditional chips based on silicon. In addition, energy losses will be extremely low. As the new semiconductor chips are smaller and more powerful than traditional systems, the need for costly cooling is eliminated.
ABB has already started a pilot production line manufacturing the new chips capable of handling power ratings of 3500 V and 200 A. Some of these chips will shortly be installed in association with high power IGBTs in the Hellsjön transmission link.
Following a rapid of development in silicon switching devices in recent years – moving through GTOs (gate turn-off thyristors), IGBTs (integrated gate bipolar transistors) and IGCTs (integrated gate commutated thyristors) – the use of silicon carbide as the semiconductor substrate is moving the technology ahead in large quantum leaps. The IGCT already led the way by integrating the GCT power handling device with the freewheeling diode/gate driver in one integrated package. Currently manufactured SiC devices have:
They are currently in use for motor control and military high frequency systems.
In the future, electrical drives in ships, for example, will be a quarter of their present size and there will be no need for large electric filters in HVDC transmission systems.
Prospects
In general HVDC technology is advancing towards simpler systems with improved performance and increased reliability, more compact hardware and improved power quality.
Universal deregulation is bound to spawn increasing requirements for new HVDC links with capacities reaching new frontiers in maximum and minimum directions. Such systems will be required not only for economic transmission aims, but also for weak network stabilisation, power factor correction and power quality promotion.
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