AC technology may have won the historical 'war of currents', but DC is on the rise.
The famous "war of currents" between Thomas Edison, champion of direct current systems, and alternating current advocate Nikola Tesla was won by Tesla. Until a few decades ago, practically all transmission systems were AC. Technological advances, however, have caused DC to gain favour and, although most systems are still AC, Edison might still have the last laugh.
The development of high-power electronics, as in mercury arc valves and semiconductor devices such as thyristors, has led DC to become the preferred option for transmitting large amounts of power over long distances.
"We strongly believe that high voltage direct current (HVDC) will play an increasing role in the future of electric power transmission," says Abhay Kumar, lead engineer and project manager, HVDC, with ABB in Sweden.
There are now many HVDC transmission systems around the world, the largest of which transmits 6,400MW of power at 800kV a distance of more than 2,000km. Those systems use today's AC generators, generating voltages up to about 25kV. For DC transmission the voltages produced by AC generators must be scaled up with transformers, as usual, but then rectified and smoothed out with semiconductor device systems for transmission. On the user end, the process is reversed at an inverter station, where the DC current is transformed back to AC by a semiconductor device installation, and the voltage scaled down with transformers.
ABB has already developed high-voltage AC generators, however. It has manufactured a high-voltage AC generator, called the Powerformer, capable of generating up to 400kV. If generating at sufficiently high voltages, these would allow dispensing with a transformer at the generating end, while rectification in an AC to DC converter would still be necessary. This has already been done in the Troll A Precomp-ression Electrical Drive System Project, of Statoil, in Norway. The project combines ABB's HVDC Light technology with the company's VHV Motor (Very High Voltage Motor).
An alternative, using DC generators, may be possible, but voltage limitations mean that there is currently no development towards HVDC generators.
A spokesman for Siemens AG, Andrea Haas, said: "Using DC generators in HVDC systems is a complex technology. The first grids in the world used DC generators. Later, everybody switched to AC generators as they had higher efficiency."
Superconductivity is another area that might affect HVDC electricity transmission, as superconducting transmission lines could be used in future HVDC systems. "This is possible," Kumar says, "but, due to the complication of cooling the cable, it is something for the future as HVDC is typically an application for long cables and superconduction is advantageous for short distances."
Traditional AC generators will continue to be used in the foreseeable future, and technological advances regarding HVDC systems have therefore focused on the semiconductor devices used in the rectifier and inverter stations. The two main ways of achieving conversion are "Natural commutated converters" and "Forced commutated converters".
The first are the most used in today's HVDC systems. Their crucial component is the thyristor, which acts as a switch. Thyristors are mainly used with high currents and voltages, to control alternating currents, where the change of polarity of the current causes it to switch off automatically.
The thyristor is made of four layers of alternating N and P-type material (while diodes are made of two layers, and transistors are made of three layers). The main terminals are across all four layers and are called anode and cathode. The control terminal is called the gate and is attached to P-type material near the cathode. In a conventional thyristor, once it has been switched on it cannot be switched off until the anode current falls below a value called the holding current.
A thyristor can carry currents up to 4000A and can block voltages up to 10kV. By connecting thyristors in series it is possible to set up a thyristor valve which can operate at voltages of several hundred kV. The valve is operated at the grid frequency and by means of a control angle it is possible to change the DC voltage level.
Thyristors are used as six-inch devices for ultra high voltage applications such as the 800kV, 6,400MW of the Xiangjiaba installation in China. The rating will be further increased to 7,000MW and beyond.
The low-voltage control circuits used to switch thyristors on and off have to be isolated from the high voltages of transmission lines. This is usually done optically, where the control electronics sends light pulses along optical fibres to the high-side control electronics. In a system called direct light triggering, light pulses from the control electronics are used to switch light-triggered thyristors (LTTs).
Forced commutated converters, also known as voltage source converters (VSC), offer several advantages. Two main types of semiconductors are used in the voltage source converters: the GTO and the IGBT. The operation of the converter is based on Pulse width modulation (PWM). PWM allows creating, nearly instantaneously, almost any phase angle and/or amplitude – by changing the PWM pattern.
The IGBT is a three-terminal device which can be both, switched on and off. Siemens HVDC Plus system, for example, is a new generation of converters using voltage-sourced converter technology and IGBTs. Because an IGBT has both turn-on and turn-off capability the commutation processes in the converter are independent of the AC system voltage. The IBGTs can turn on and off very rapidly, and are often used to synthesise complex waveforms.
ABB's HVDC Light converter is a two-level three-phase bridge with six valves, each consisting of series-connected IGBTs.
The HVDC Plus and HVDC Light systems offer several advantages including less space requirements, supply of very weak systems and black-start capability allowing to restart a collapsed network.
Systems such as HVDC Plus and HVDC Light are suitable for DC links in the low to medium power range application up to 1,000MW and relatively shorter distances (hundreds of kilometres). Currently various companies are developing IGBT devices for higher power applications.
Siemens' Haas says typical losses of thyristor HVDC are 1.4 to 1.5 per cent for both converter stations, including transformers, with the equivalent figure for Siemens' VSC2.8 per cent. He said in future silicium carbide semiconductors could be used, as one of the options for reducing losses.
ABB, Siemens and other companies are conducting research on all kinds of power electronic devices to increase power and reduce losses.
The cost comparison between high voltage AC and HVDC systems can be tricky. It has to be based on the technology used in the converter stations at both ends of the system, i.e. mainly whether the HVDC system is thyristor-based or VSC (Voltage source converter)-based.
In a recent paper, Dr BR Andersen, of Andersen Power Electronic Solutions Ltd, of the UK, said he strongly believes that the growth in environmental opposition and the need for energy diversity will result in a dramatic growth in the application of HVDC schemes, as a solution to future power transmission challenges. He emphasised the need to educate the public, training by planers and advisors to investors, and R&D, primarily by HVDC equipment manufacturers.