Page 42 - Hybrid-Renewable Energy Systems in Microgrids

26 Hybrid-Renewable Energy Systems in Microgrids

V
2 PV +Q inon miq
inon mid
Inond=23PinonVmid+QinonV I nond = 3 V 2 +V 2 (2.8)
miqVmid2+Vmiq2 mid miq
Constant power-type load equations,

2 PV miq −QV mid
i
i
Iiq=23PiVmiq−QiVmidVmid2+Vmiq2 I iq = 2 2 (2.9)
3 V mid +V miq
2 PV +QV
I = i mid i miq (2.10)
id 2 2
3 V mid +V miq

On the basis of the dynamic model of a parallel converter microgrid system, both
transient and steady-state operation analyses can be performed. However, a microgrid
operating in autonomous mode will only operate when voltage and frequency stabili-
zation condition is met. To achieve the required control, a droop control or hierarchical
control is employed. Subsequent sections discuss different architectures of microgrid
Iid=23PiVmid+QiVmiqVmid2+Vmiq2 and relevant control strategies.

3.1 AC microgrid
In AC microgrid structure, an AC bus is created, and all microsources with variable
frequency and variable voltage AC output are connected to AC bus through AC/AC
power electronics converter. Sources with DC output are connected to AC bus through
DC/AC converter. Depending upon the mode of operation, an autonomous microgrid
is connected to AC loads through AC bus. A microgrid operating in grid-tied mode is
connected to main grid through AC bus where local AC loads are also connected [7].
Fig. 2.2 presents the schematic diagram of AC microgrid structure.
Due to the intermittent nature of different microsources, the availability of elec-
trical power from all sources does not happen at the same time. In addition, the mag-
nitude of power availability from different sources varies in different time intervals.
Therefore one of the main challenges is to share the load among different sources.
In grid-tied mode, the main grid takes care of the frequency stability issues. The
microgrid has to manage the control of individual power electronic converters asso-
ciated with individual microsources. In addition, a central control unit is required
for energy management between microgrid and main grid. When sufficient power
is available from microsources, all local loads are fed by microgrid itself. Main grid
provides support only for frequency stabilization. When a condition of insufficient
power from microgrid arises, main grid supplies power to microgrid. In case of
surplus power availability from microgrid, a control provision for power flow from
microgrid to main grid is required. All these controls are provided through central
control unit.