HV Technology
AC to DC conversion for Transmission
At the generating station, power is produced as an alternating current. However, transmission is preferred in DC form. The fundamental process that occurs in a HVDC system is conversion AC to DC by using a rectifier, transmitting the HVDC and converting the DC into AC at the receiving end by use of an inverter (Sharma, 2008). There are three main methods of conversion:
- Natural commutated converters: These converters are widely used in most HVDC systems. Conversion is done by a thyristor – a controllable semiconductor that may carry currents of high magnitudes about 4000A. By connecting the thyristor in series with the power line, a thyristor valve can be built which is capable of transmitting very high voltage in the order of hundreds of kilovolts. The valve is operated at a frequency of 50Hz – 60Hz. By varying the control angle in the thyristor, it is possible to regulate the DC level for transmission and this way, power transmitted is efficiently and rapidly controlled.
- Capacitor Commutated Convertors: Commutation capacitors are serially connected between thyristor valves and convertor transformers. The capacitors are effective in safeguarding convertor performance against commutation failure when connected to weak networks (Grigsby, 2007).
- Forced commutation convertors: These convertors are made of semiconductors capable of being turned on and off. The semiconductor devices are Gate Turn-Off thyristor GTO and Insulate Gate Bipolar transisotor IGBT. This convertor has a lot of advantages including feeding passive networks without generation and independent control of reactive and active power components.
Transmission
Transmission of HVDC can be overhead or underground. Overhead cables are installed on pylons high above the ground. The overhead systems are most preferred because of cost of installation and ease of trouble shooting. Moreover, overhead systems are capable of transmitting higher power levels compared to underground. Their main disadvantage is interruption of telecommunication systems and may not be very convenient for very populated areas and aesthetic value of the environment. On the other hand, underground systems have advantages of safety and ‘neatness’ especially in urban areas. Cost of installation is larger in the case of underground transmission.
The main components of HVDC transmission is shown in figure 1 in the appendix (Grigsby, 2007).
The transmission medium used for most HV transmissions on land is copper cables (Roberto et al., 2008). Two conductors having different polarities are mainly used. For under water transmission, the cables are solid and filled with oil. The cables are insulated with paper tapes and filled with high viscosity oil. This design has no length limit and can be used for depths up to 1000 meters deep.
Inversion at the receiving end
At the receiving end e.g. the industry, the HVDC has to be converted back to ac. This requires inversion and is achieved by use of inverters. Typically, the inverters are a reverse operation of the convertors. Figure 2 in the appendix shows convertor and inverter similarities.
Once converted to ac, the power can be stepped up or down using transformers for relevant applications. For instance, power is stepped down for low power applications like lighting and stepped up for heavy applications like moving power plants.
Conclusion
Production of electrical power is most convenient in AC form. However, transmission is best done in DC to curb costs and transmit more power. Conversion is therefore necessary. However, regulation and control of AC is better than DC hence the need for DC inversion at the point of consumption Fasbinder et al., 2010).
Recommendations
For land transmission of High Voltages, it is recommended to use cables made of extruded polythene to minimize interference to communication lines (Stephen, 2002).
References
Grigsby, L. (2007) Electric Power Generation, Transmission, and Distribution, CRC Press
Roberto R, J. Charpentier, P. and Sharma, R. (2008) High Voltage Direct Current (HVDC) Transmission Systems Technology, World Bank Review paper.
Fassbinder, S. and Deutsches, K. (2010) Practical Applications of Electrical Conductors, [online textbook] accessed 15-Feb-2012
Stephen, C. (2002) Electric Machinery and Power System Fundamentals, London: McGraw
Appendix
Figure 1: HVDC transmission system components
Figure 2: Convertor and Inverter circuits