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30 Hybrid-Renewable Energy Systems in Microgrids

4 Microgrid control and operation

An industrial solution that accommodates different renewable energy sources into a
microgrid structure is desired to operate in “Plug and Play” mode. All possible situ-
ations that may arrive during operation should be considered well in advance, and a
suitable control algorithm should be available to handle the specific situation. This
property is called Plug and Play. A number of attempts have been made by researchers
to develop a suitable control strategy for different microgrid architectures. Main func-
tion of any control scheme is to share the load among different micro sources, main-
tain the power quality, and energy management among microgrid and main grid in
case of grid-tied mode. Hierarchical control and droop-based control are the two main
control schemes applied for microgrid control in different mode of operations [11].

4.1 Hierarchical control of microgrid
Hierarchical control initiates at inner level and spreads up to higher level, performing
the task of load share, energy management, and optimal economic operation. Fig. 2.5
presents different levels and related operation through a diagram. Different levels are
described as follows [12]:
1. Field level control: This is the innermost control level in hierarchical control scheme.
Control of individual converters associated with each microsource is performed at this
level. When operating in grid-tied mode, all the sources attached to microgrid bus are
regulated to synchronize with main grid. A reference is generated for voltage and fre-
quency synchronization, and control dedicated for individual converters utilize the ref-
erence signal to bring different sources in synchronism with main grid. Some of the
sources such as solar and wind can also be regulated to generate maximum power at
some specific operating point. This is called maximum power point, and can be obtained
using inner-level control. When microgrid operates in autonomous mode, inner control
has to bear the responsibility of balancing active and reactive power generation between
source and load. This is critical for stabilization of autonomous microgrid, as circulating
current may damage the microsources under unequal voltage generation condition. When

Figure 2.5 Schematic diagram of hierarchical control.