Page 47 - Hybrid-Renewable Energy Systems in Microgrids

Microgrid architecture, control, and operation 31

a microgrid moves from autonomous mode of operation to grid-tied mode, or vice versa,
the inner control performs the islanding detection and smooth change of mode. A desired
microgrid inner control is one that can handle both planned and unplanned islanding of
microgrid [13].
2. Management level control: A Microgrid Central Controller performs at management level
and establishes a synchronism between microgrid and main grid. As an algorithm, vari-
ous techniques such zero crossing method, grid voltage filtering method, or phase locked
loop methods are used for obtaining point of synchronism. In addition to that, a decision
regarding continuing with a load or shedding a load is also taken at management level. An
economic operation of microgrid requires optimal generation from different microsources.
This task is also performed at management level control [14].
3. Grid level control: This is the outermost control layer in hierarchical control scheme, in
which several microgrids operating in parallel are managed and coordinated. Coordination
between several Microgrid Central Controllers is achieved at grid-level control [15].

5 Mathematical model of hierarchical control

A hierarchical control based operation of grid-interacted microgrid structure is
often found to be very big in size. A mathematical model for such a big system
requires mathematical tools that simplify the analysis. Normally such big systems
are presented in linearized state space model for steady-state operation around an
operating point. Dynamic model of a dedicated individual microgrid structure is
presented as follows:

d
dt xt() = Ax t() + But () (2.11) ddtx(t)=Ax(t)+Bu(t)y(t)=Cx(t)
yt () = Cx t () + Dut ()
+Du(t)

A dynamic model of all such dedicated microgrid units is obtained separately. Size
of the overall system consisting a number of individual microgrids becomes signifi-
cantly large. To simplify the analysis, model-order reduction technique has been sug-
gested in literature [22]. A linearized, reduced-order model of the complete microgrid
system is presented as follows:

  A 0   B 1 
1
[][]
 AB   0 A   B 
  [][]   =  2   2   (2.12) ABCD=A 1 00A 2 B 1 B 2 C 1 C 2 D

CD
D
 [  CC 2 ][]  
1
where A 1 and A 2 contain r dominant and (n − r) nondominant modes of A. As we
know that most of the renewable energy sources are indeterministic in nature, the