Page 45 - Hybrid-Renewable Energy Systems in Microgrids

Microgrid architecture, control, and operation 29

countries. Another example is a DC microgrid of New Zealand, which also feeds DC
power to data center but operates at 220-V LVDC level compared to MVDC level of
previous examples. DC microgrid at Sweden operates at 380 V, whereas the microgrid
at Japan works at 400 V.

3.3 AC–DC hybrid microgrid

Be it AC microgrid structure or DC microgrid structure, a number of semi-con-
ductor devices based on power electronic converters are required essentially for
interface of different microsources. An AC–DC hybrid microgrid structure has
been proposed in literature, with the aim of reducing number of converters [10].
In hybrid microgrid structure, both AC and DC buses are created. All the sources
with AC output are connected to AC bus. Sources with DC output are connected to
DC bus through DC/DC converters. Both AC and DC buses are connected to each
other through a bidirectional converter, which allows flow of power between both
buses in both directions. A schematic diagram of AC–DC hybrid microgrid is pre-
sented in Fig. 2.4. A hybrid microgrid structure accommodates both AC and DC
loads. Because AC and DC buses can interact with each other and with main grid
through bidirectional converter, therefore control scheme becomes more complex.
A suitable coordination scheme is required for facilitating power transfer between
AC and DC networks.

Figure 2.4 AC–DC hybrid microgrid structure.