Page 66 - Hybrid-Renewable Energy Systems in Microgrids

50 Hybrid-Renewable Energy Systems in Microgrids

Also, the reactive power and voltage equation can be given by:

V=Vo+KQVQo−Q V = V o + K QV (Q o −Q ) (3.17)

where,
f o —rated values of frequency of the DG unit
V o —rated values of voltage of the DG unit
P o , Q o —real and reactive power set points of the inverter.
K Pf , K QV —droop coefficients
Then the droop coefficients of the inverter can be expressed as given below:

∆f
KP=∆fPmax K P = (3.18)
P
max
∆V
KQ=∆VQ K Q = (3.19)
Q
where,
P max , Q max —deliver maximum active and reactive power of the inverter
∆f—maximum frequency
∆V—maximum voltage deviation
When more number of DG units are connected to the parallel inverters, the power
sharing of load depends on the droop characteristics. The concept of the droop charac-
teristics is when the load increases, then the reference frequency could be decreased.

6.1 Droop control techniques in microgrid
6.1.1 Virtual impedance droop control

The relationship of this droop control technique is that the inner current and voltage
loops. The general block diagram of virtual impedance droop control is shown in
Fig. 3.9. The output voltage of the virtual impedance droop control can be given by,

vo*=vref−ZDs.io v * o = v ref − Z D () s i. o (3.20)

The park's transformation of the impedance angle Θ can be illustrated by given
below,

)
)
()
−
*
*
w=w*−GpsP−P*sinθ−Q− Q*cosθ ω = ω − Gs  ( −PP sin θ ( −QQ cos θ   (3.21)
*
p
+
θ
()PP cos
E=E*−GQsP−P*cosθ+Q−Q*sinθ] E = E * − Gs  − *   θ ( −QQ* )sin] (3.22)
Q
The output impedance of the inverter has become a new control variable by using
this droop control. According to Eqs. (3.21) and (3.22), we can control the phase angle