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Compartison between electric performances of LV cables in AC and DC distribution systems

pubblicazioni - Articolo

Compartison between electric performances of LV cables in AC and DC distribution systems

The need of a high quality of the electricity supply has induced many customers to equip their plants with appliances (UPS) that have inside a DC section in which it is inserted a storage system. Moreover the technological progress has recently made available on the market various small power generators (in the order of the tens of kW) that often generate electricity in direct current. Therefore it can be thought to integrate the various utilizer and generating devices by means of a local DC distribution system interfaced with the public distribution network by means of one or more inverters. This substantial modification will involve new strategies conceming the control and protection philosophies for the new distribution systems and also for load and distributed generators. In this paper it will be considered the use of a DC low voltage local network; in particular, under the hypothesis to employ the existing LV AC cables, a comparison concerning losses and transmissible power between AC and DC distribution scenario is performed. This paper was developed within the activity "Ricerca di Sistema" (DM 28/0212003). 1. Introduction Starting from the last years it is possible to see the continuous growing of the distributed generation (DG) resources, like fuel cells, solar PV systerns, microturbines, etc, which are connected to the low voltage (LV) electrical network by DC/AC electronic converters. The same interface technology is also used for storage systems like accumulator batteries, flywheels etc, that are part of the devices devoted to improve the power quality (upS, Custom Power Systerns, etc). Further, there are many loads that are supplied by interposing proper devices which perform at least one ACIDC conversion and that require a very good power quality. In tbis scenario of increasing loads there could be serious problerns in managing the network, mainly when the penetration level of the DG becomes very high. In fact, other than critical conditions due to the voltage profile along the lines or due to the behavior of the line protection devices, there could be nndesired islanded operations of the power system (the DG supplies its power to a part ofthe distribution system that has been separated from the utility’s power system). Therefore it seerns to be useful to investigate a possible DC distribution solutions (taking into account the increasingly widespread use of DC current both in the generators and in the electrical equipments), to detine the voltage level, to evaluate the risks for people, components and devices, to investigate the circuital solutions that allow to connect both loads and generators directly to the DC, and to use the existing components (mainly the LV AC cables that can be used also in DC) evidencing the corresponding performances when integrated in the new system. This paper will analyze in particular tbis latter aspect in the hypothesis of a balanced load located at the end of the line. This hypothesis is conservative as the voltage drop with the load at the end of the line is greater than when the load is distributed along the line. 2. Evolution or the distribution network In presence of sensitive loads or DC loads supplied by ACIDC converters and distributed generators, the DC distribution system in Fig. I(a) makes it possible to gain some advantage respecting of the AC system in Fig. 1.1 (b). ne AC (a) Fig. 1.1: Comparison between the distribution in direct current (a) and in altemating current (b).

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