TRANSMISSION AND DISTRIBUTION SYSTEMS

HIGH VOLTAGE DIRECT CURRENT

Over one hundred years ago when first alternate current (AC) system was implemented it was apparent that direct current (DC) in comparison to AC would be much less used. The biggest advantage of AC over DC is its easiness of changing the voltage level. As it was said in chapter 3.1.2.1 resistance losses depend of the voltage level, thus it is vital to transmit energy with high voltage lines. However with the development of the power electronics more and more efficient rectifiers have been constructed and that resulted with DC being used in some parts of power system again.

  • GRAPH 3: COMPARISON OF AC AND DC COSTS VS DISTANCE BASED ON [11]

    • Graph 3 shows a comparison of AC and DC high voltage lines costs depending on the distance of the line. It can be seen that on lower distances AC is less expensive. However while the distance is growing AC costs grow faster than DC ones and from about 700km DC line becomes a more profitable solution. The disadvantage of transmitting DC is the need of having the rectifiers on both ends of the line. The substations containing converters are more expensive than standard ones and they cover bigger area. Nevertheless the biggest advantage of DC over AC is lower transmission losses. Since the voltage is stable DC does not move reactive power through the wire which means the reactive current does not generate additional losses. Also using DC, wires are not affected by skin effect.

    SUPERCONDUCTING CABLES

    High temperature superconducting (HTS) cables is a technology of the future. It is still in a phase of constructing and testing, however the latest researches proved that HTS cables can have a large influence on the efficiency of transmitting the energy. The main engineers and producers of HTS estimate that the new technology will lower the primary energy consumption by 10-15% by reducing the transmitting losses [12].

    Superconductivity is a phenomenon which occurs with no resistance at all in certain materials when the temperature is below the critical temperature. The materials which can turn into superconductivity are called superconductors and there are many kinds of them such as certain elements, alloys, appropriate organic compounds etc. They all vary in term of the critical temperature, thus in the point of view of the power engineering it would be vital to choose the superconductor with the highest critical temperature possible. The most appropriate material is as of yet bismuth strontium calcium copper oxide (BSCCO) with the critical temperature equaling 140K, however the issue of inventing more proper superconductors is still open.

  • FIGURE 9: EXAMPLE SUPERCONDUCTOR CABLE TAKEN FROM [13]

    • Figure 9 shows an exemplary design of superconducting cable. As can be seen, the superconductor is in shape of a tape wrapped on a copper core. The very important element in such a cable is the coolant. HTS tape have to be cooled below the critical temperature to keep the superconductivity. The cooling facility needs refrigerators to cool liquid nitrogen which will be then squashed into the cables to cool them. The process of cooling the HTS cable requires the energy. However, according to the engineers, the energy usage in the process will be lower than the resistance losses in standard cables.

    As stated before, HTS technology is in a phase of research. There are still many problems which need to be solved. The main issue is to find the most appropriate superconductor with the highest critical temperature which will also be solid enough to face years of maintenance. Once it has been found, there needs to be a manufacturing technology to produce such a superconductor. It has to be inexpensive and solid because the ill-begotten microstructure will weaken the properties of the material. Another problem is the cooling system which will require an absolute operational reliability to avoid the cable damage or nitrogen outflow.

    CONCLUSIONS

    Transmission and distribution systems are one of the key elements in the power system. The very high importance of the T&D is based on their task. They are responsible for delivering the energy from power utilities to the end users. This is why it is essential to have the most efficient and reliable T&D systems possible.

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