C I R E D 19 th International Conference on Electricity Distribution Vienna, 21-24 May 2007 Paper 0464 CIRED2007 Session 5 Paper No 0464 Page 1 / 4 APPLICATION OF LOW VOLTAGE DC-DISTRIBUTION SYSTEM – A TECHNO- ECONOMICAL STUDY Tero KAIPIA, Pasi SALONEN, Jukka LASSILA, Jarmo PARTANEN Lappeenranta University of Technology – Finland tero.kaipia@lut.fi , pasi.salonen@lut.fi , jukka.lassila@lut.fi , jarmo.partanen@lut.fi ABSTRACT Today’s electricity distribution networks construct mainly of the three-phase AC-systems. Consumer voltage in traditional distribution system is 230/400 V and the nominal frequency in Europe is 50 Hz. The European Union (EU) low voltage directive (LVD 72/23/EEC) [1] defines the boundaries for the low voltage (LV) levels used in public distribution systems. According to the directive any electrical equipment designed to be used with a voltage rating between 50-1000 V AC and between 75-1500 V DC is a low voltage instrument. The utilization of the 1 kV AC-distribution system takes full advantage of the LVD 73/23/EEC directive’s AC-definition. However, the DC voltage rating is still unexploited in public distribution systems. As the weight of cost efficiency and reliability requirements of distribution networks are expected to increase in the future, with the utilization of distributed generation, there will be demand for novel distribution techniques. The utilisation of the LV-DC- distribution opens new possibilities for network development. In this paper, the basic concepts and the techno-economical potential of the LV-DC-systems are discussed. Also aspects of developing required power electronic equipment for LV-DC-systems are introduced. INTRODUCTION Until today the main voltage levels in Finnish distribution networks have been 20 kV and 0.4 kV. Although the 1 kV AC low voltage level is today used in distribution networks, the application possibilities of LV-AC-distribution systems are limited to quite small transmission powers and short transmission distances. Hence, the techno-economic benefit of the LV-DC-system can be achieved through better transmission capacity compared to the LV-AC-system. The recognized leading themes in the distribution network development are the cost effectiveness and system reliability. LV-DC-system enables decreasing the number and length of the medium voltage branch lines. This reduces the number of possible fault situations in medium voltage network, affecting typically hundreds of customers, and so improves the quality of distribution leading to the decrease of the outage costs. The power electronics in the DC-system also enable compensating voltage drop, dips and fluctuations originating from the network at the customer’s end and so increase the customer’s voltage quality. However, adding power electronics and number of components to distribution network increases the probability of inner system faults. Component prices of power electronics have constantly been decreasing in the last decade allowing power electronic devices to be used in greater number of applications. The increase of LV-network transmission capacity, achieved with the DC-system, enables application of smaller conductor cross sections than in LV-AC-systems. Also the number of distribution transformers decrease as there will no longer be need for the public 0.4 kV LV- network, when it is replaced with the LV-DC-network. Also remarkable investment cost savings compared to the construction of MV-lines is achieved, as LV components are inexpensive compared to MV components. Mechanical engineering Metal industry Electronics and electrotechnical industry Mechanical engineering Metal industry Electronics and electrotechnical industry Figure 1 Price development in Finnish technology industry between years 1995-2006 [2]. FUNCTIONAL PRINCIPLES OF LV-DC- DISTRIBUTION The objectives in development of the LV-DC-distribution systems are:  Improvement of the quality of distribution and supply  Decrease of the costs of electricity distribution business  Open new business opportunities for component, system and service providers. Functional purposes of the LV-DC-distribution system are to provide reliable electric energy transmission from MV network to the LV customers, to transform the voltage suitable for the LV customers and provide good quality