C I R E D 20 th International Conference on Electricity Distribution Prague, 8-11 June 2009 Paper 0891 CIRED2009 Session 5 Paper No 0891 PROTECTION SCHEME FOR AN LVDC DISTRIBUTION SYSTEM Pasi SALONEN Pasi NUUTINEN, Jarmo PARTANEN Pasi PELTONIEMI Lappeenranta University of Lappeenranta University of Lappeenranta University of Technology – Finland Technology – Finland Technology – Finland psalonen@lut.fi pasi.nuutinen@lut.fi jarmo.partanen@lut.fi pasi.peltoniemi@lut.fi ABSTRACT The low voltage DC (LVDC) distribution system is a concept of new DC based distribution system. Safety of new distribution system needs to be equal or higher than traditional AC distribution systems [1]. This paper presents protection scheme for an LVDC distribution system. The analysis approaches LVDC system as a whole - from beginning of the DC district up to the customer-end protection. The analysis consist both grounded TN and ungrounded IT grounding arrangements. INTRODUCTION The LVDC distribution system [2] is a new innovation in field of electricity distribution. The drivers for new DC based distribution system development arise from need for more reliable and energy efficiency network solution compared to traditional AC based 20/0.4 kV distribution system. Due to climate change the affects of storms to public electricity distribution has increased. At the same time demand for undisturbed electricity is growing which results challenges to distribution system. The LVDC distribution system is a concept which can respond for these challenges [2]. DC based distribution system is enabled by European Union Directive 2006/95/EC [3] which defines LVDC between 75-1500 VDC. The LVDC distribution system have several benefits compared traditional 20/0.4 kV system like partly improved power quality at customer-end. The previously made analyses have shown also the techno-economical potential of LVDC distribution system [4][5][6]. BIPOLAR ± 750 VDC DISTRIBUTION SYSTEM The LVDC distribution system is more complex than traditional 20/0.4 kV distribution system. The LVDC system can be made with many different topologies [2]. One of the studied LVDC systems is bipolar ± 750 VDC system. This paper concentrates mainly to ungrounded LVDC system but also the guidelines for grounded LVDC system protection are described. In the LVDC system AC/DC conversion is made at nearby MV main line and energy is transmitted via large LVDC district to customers. Customer connections are made between a pole when DC voltage at customer is either + 750 VDC or – 750 VDC. Each customer has its own DC/AC converter located at customer-end to produce customer’s operating AC voltage. A basic implementation of LVDC distribution system is shown in figure 1. AC/DC 3.1 km 0.3 km 0.3 km 0.2 km 0.4 km 0.2 km DC/AC Figure 1. A basic implementation of LVDC distribution system in public network. Special characteristics Compared to the traditional 20/0.4 kV distribution system the special characteristics of studied bipolar ± 750 VDC system are • ungrounded IT-system in both DC and AC networks • 1-phase customer AC network • customer operating AC voltage is produced with power electronic devices • no galvanic isolation between customer AC networks and DC network The systems special characteristics are resulting from many reasons. The most desired aspects are system energy efficiency and techno-economical solutions. Due to system characteristics the LVDC system has many benefits compared to traditional 20/0.4 kV distribution system [4]. The challenges are also introduced for example in protection and safety of the system [2]. System groundings Traditional 20/0.4 kV distribution system is normally grounded TN system. Because of the power electronic devices produce galvanic connection between separate voltage levels the customer networks can’t be grounded. The groundings create short circuits through the ground which prevents the system normal operation. [1] The LV standardization [7] enables LVDC system to be grounded. Standardization defines bipolar system to be grounded in common conductor. Earlier research results have shown that in difficult grounding conditions can introduce high earth voltages over allowed limits [1][8] as in the Finnish networks where earth resistances are typically several ohms. The LVDC distribution system can be made grounded TN