Low-Voltage Direct Current and Power Electronics Reduce Power Cuts and Create a Smart Grid

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The world's first low-voltage direct current (LVDC) micro grid used continuously for electricity distribution was a collaborative creation of Lappeenranta University of Technology (LUT) and the Suur-Savon Sähkö electricity company. Direct current in the electrical network combined with power electronics reduce power cuts and enable the intelligent use of the electrical network.

The dissertation by Pasi Nuutinen, M.Sc. (Tech.), deals with a direct current distribution network for research purposes. It is the first ±750 volt low-voltage direct current micro network continuously distributing electricity to actual users in a sparsely populated area. The system developed by LUT's LVDC research group for the distribution network of Suur-Savon Sähkö includes a nearly two-kilometre LVDC distribution network and a rectifier, a battery-based energy reserve and three customer-end inverters. The inverters convert the direct current for use as alternating current in four detached houses.

Compatibility with the end user is one of the most important features of the customer-end inverter. Compared with the current electrical network, a low-voltage direct current distribution system contains power electronics that does not cause disturbances for the customer and the environment. In the research work, disturbances were controlled with correctly executed filtering and the structure of the inverter.

The present customer network is protected by fuses and line protection switches. To ensure their operation, short-circuit current must be fed into the inverter, and the current capacity of the inverter must significantly exceed that needed in normal use. By using alternative protection methods developed in the research, such as the inverter's intelligent protection system, oversized inverters that increase costs and complexity are no longer needed.

In low-voltage direct current distribution, the existing alternating current network is partly or entirely replaced by a direct current network. One of the advantages of direct current distribution compared to alternating current distribution is the possibility to transfer power multifold through a low-voltage cable. Medium-voltage overhead lines, which are susceptible to disturbances, can thus be replaced with with less costly low-voltage cables. This increases the reliability of electricity distribution especially in sparsely populated areas.

The dissertation of Pasi Nuutisen, M.Sc. (Tech.), in the field of electrical engineering is entitled Power Electronic Converters in Low-Voltage Direct Current Distribution – Analysis and Implementation. It will undergo a public examination on 18 December 2015 at 12.00 noon at Lappeenranta University of Technology, auditorium 1383. The opponent is Professor Kimmo Kauhaniemi from the University of Vaasa. The custos is Professor Pertti Silventoinen from Lappeenranta University of Technology.