Page 49 - Hybrid-Renewable Energy Systems in Microgrids

Microgrid architecture, control, and operation 33

EV
P = sin φ (2.13) P=EVXsinφ
X

EV cos φ −V 2
Q = (2.14) Q=EVcosφ−V X
2
X
Because power generators are operated at small power angle due to stability con-
cerns, an approximation can be made as follows:

≈
sin φφ
cos φ ≈1
sinφ≍φcosφ≍1

On the basis of the aforementioned approximation, a linear relation between active
power P and power angle and reactive power Q and (E − V), respectively, is estab-
lished. This linear relation can be utilized to produce droop characteristics for indi-
vidual converters as follows:

∆ ω
m pi = (2.15) mpi=∆w∆Pi
∆P
i

∆V
n = (2.16) nqi = ∆V∆Qi
qi
∆Q i
where m pi and n qi are active and reactive droop coefficients, which govern the
ratio of active and reactive power supplied from ith converter. Active power droop
coefficient is ratio of change in frequency to change in active power. Reactive power
droop coefficient is taken as ratio of change in voltage to change in reactive power.
Normally, a maximum change of 1% in nominal frequency and a maximum of 2%
change in nominal voltage are accepted under droop control scheme. A droop con-
trol has been identified as a potential solution of the requirement of Plug and Play
feature of microgrid operation. This control scheme provides a without communica-
tion control over power transfer, high flexibility, and high reliability for different-
capacity microgrid structures. However, the droop control is associated with some
drawbacks as well [20–22]. Droop control scheme completely depends upon the local
data, which has dependency on transmission line impedances. When a droop control-
ler is employed for active power share between two parallel converters, the accuracy
of power share depends upon line impedance ratio. If there is a mismatch between
line impedance, the accuracy of power share gets compromised. Dynamic response of