Page 65 - Hybrid-Renewable Energy Systems in Microgrids

Integrated renewable energy sources with droop control techniques-based microgrid operation 49

The equation of state of charge of battery is given by Eq. (3.11),

1 t i
SOCt() = ∫ η tI () (3.13) SOC(ti)=1Q(ti)∫∞tiηc(t)Ibat
tdt
()
i
Qt () c batt
i ∞ t(t)dt
C C
()+
Q ()=t i nominal t cos t ( + α ∆Tt β ∆T 2 () t ) (3.14) Q t i = C n o m i -
1
c
c
t
 I ()  B cap
1 + A cap   I batt   nal Ct cost1 + Acap Ibatt-
t I n o m i n a l B -
C nominal cap 1 + αc∆Tt + βc∆T t
2
I = nominal (3.15)
nominal Inominal=Cnominaln
n
where n is number of hours.
6 Concept of droop control
The droop control idea has been employed by the inverter parallel operations because
of the power system inertia, and the frequency decreases with increase in load. In the
microgrid operation RES are directly connected through the power electronic ele-
ments, therefore the system inertia has been neglected. Introducing a droop control
technique will imitate the inertia of voltage source inverter (VSI) as within the syn-
chronous type of wind turbine generator system. Fig. 3.8 shows the characteristics of
droop control. The benefits of droop control methods are given below [30]:
• Simple and easy implementation
• Reliability is high
• High flexibility
• Power ratings are different

The relationship between the active power and frequency can be illustrated as:

f = f o + K Pf (P o − P ) (3.16) f=fo+KPfPo−P

Figure 3.8 General characteristics of droop control.