Reliability and vulnerability of critical infrastructure : electricity distribution

In recent years the failure frequency of transforme rs increased. In particular fires and explosions of main oilfilled transformers are considered as critical. The refore, international experiences of transformer fa ilures at nuclear and non-nuclear power plants and at substat ions have been investigated in more detail. Consequ ences of transformer failures with respect to a reliable e ctric power supply and distribution are addresse d and proactive measures to enhance the reliability of cr itical infrastructure and to avoid blackouts are de scribed. Berg Heinz-Peter, Fritze Nicole Reliability and vulnerability of critical infrastru cture – electricity distribution 10 step-up transformer, i.e. 75 kV, 400 kV, 220 kV or 110 kV as necessary. Figure 1. Electricity transmission and distribution system Supported by tall metal towers, the lines transporting these voltages can run into hundreds o f kilometres. The grid voltage has then to be reduced to a sub-transmission voltage, typically 26 kV, 33 kV or 69 kV, in terminal stations (also known as power substations). Sub-transmission lines supply power from terminal stations to large industrial customers and other lower voltage terminal stations, where the voltage is stepped down to 11 kV for load points through a distribution network lines. Finally, the transmission voltage is reduced to the level adapted for household use, i.e. 415 V (3-phase) or 240 V (1-phase) at distribution substations adjacent to the residential, commercial and small to medium industrial customers in the US, in Europe the transmission voltage is reduced to 400 V or 230 V. Figure 2 shows a typical electrical network system, in which power is transformed to the voltages most suitable for the different parts of the system [16] . Figure 2. Typical electrical power network At every point where there is a change in voltage, a transformer is needed that steps the voltage either up or down. There are essentially five levels of voltages in the US (United States Department of Energy 2006) used for transmitting and distributing alternating current (AC) power ( Table 1): UltraHigh Voltage (UHV, 1100 kV), Extra-High Voltage (EHV, 345 to 765 kV), High Voltage (HV, 115 to 230 kV), medium (or sub-transmission) voltage (MV, 34.5 to 115 kV), and distribution voltage (2.5 to 35 kV). The UHV, EHV, HV, and MV equipment is mainly located at power plants or at substations in the electric grid representing high voltage electric systems facilities used to switch generators, equipment and circuits or lines of the system on an d out. , while distribution-level transformers are located in the distribution network on poles, in buildings, in service vaults, or on outdoor pads. Substations can be large with several transformers and dozens of switches. As electricity transport is most efficient at high voltage, transformers at generating stations step u p low-voltage power from generation plants and use thousands of kilometers (e. g. in the U.S. about 340,000 km) of high-voltage transmission lines to move power over substantial distances to distribution systems, where transformers step down the voltage for customer use. Table 1. Alternative current voltage classes Transmission Voltages Distribution Voltages