Parallel Operation of Inverters - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 290

When δ is small,

V o

EV o
Z o δ

V o
Z o

P ≈ and Q ≈ E −

Z o ,

and, roughly,

P ∼ δ and Q ∼ E .

As a result, the conventional droop control strategy for inverters with an inductive Z o takes
the form

E ∗ −

E i = n i Q i ,

ω i = ω ∗ −

m i P i .

− − ω

This strategy, consisting of the Q E and P droop, is illustrated in Figure 19.3(b). In

this case, P P and Q

= =

Q .

19.4.3 For C-inverters
When the output impedance is capacitive, then

90 ◦ and

θ =−

V o
Z o .

EV o
Z o

EV o
Z o

P =− sin δ and Q =− cos δ +

When δ is small,

V o
Z o ,

EV o
Z o δ

V o
Z o E

P ≈− and Q ≈− +

and, roughly,

P ∼− δ and Q ∼− E .

Hence, the conventional droop control strategy for C-inverters takes the form

E ∗ +

E i = n i Q i ,

ω i = ω ∗ + m i P i ,

This is illustrated in Figure 19.3(c). In this case, P P and Q =− =− Q . Note that, in order to

make sure that the Q loop are of a negative feedback, respectively,

so that the droop controller is able to regulate the frequency and the voltage, the signs before
n i Q i and m i P i are all positive, which makes them boost terms.

− E loop and the P − ω