Page 43 - Hybrid-Renewable Energy Systems in Microgrids

Microgrid architecture, control, and operation 27

Figure 2.2 AC microgrid structure.

In autonomous mode of operation, the microgrid is supposed to operate and
take care of energy management and stability-related issues on its own. In such a
case, loads are to be divided into normal and critical load types. When sufficient
power is available, all loads can be fed. Under deficiency of power, critical loads
are given priority. For the purpose of frequency stabilization of microgrid, suf-
ficient battery bank storage is required. This requires determination of optimum
size of battery storage and related control scheme. In general, the initial cost of
renewable-based microsources is quiet high. A smart economic management sys-
tem helps in optimizing the economic operation of microgrid. Because most of
the loads in power structure are of AC nature, therefore AC microgrid structure is
suitable for feeding most of the loads. However, because most of the microsources
generate DC power, number of power electronic converters required for DC/AC
conversion increases. This reduces the overall efficiency. In addition, the quality
of output power becomes poor, and more harmonics is introduced due to switching
operation of converters.
Attempts have been made worldwide to develop a large-scale microgrid feed-
ing electrical loads at large scale. One such AC microgrid was developed at Greek
Island, Kythnos, which consists of solar PV and diesel-based distributed sources.
A 12-kWp energy from PV and 85-kWh energy from battery bank are made avail-
able at the microgrid. Another example of AC microgrid is a 610-kW microgrid
installed at Hachinohe, Japan. This microgrid consists of PV array, wind turbine,
and controllable digester gas engines, as well as a battery storage of lead acid
battery.