Control and Design of Microgrid Components - Power Systems ...

currents from the microsources increasing their measure of output power, and as a consequence
of the droop the units will operate at a frequency slightly smaller that nominal system value.
ω o
ω 1
ω
Unit a
P ao
Unit b
P a1
P bo
P b1
P
Figure 3.10 Power – Frequency Droop Characteristic.
Figure 3.10 shows the characteristic of the power versus frequency droop. Two units, a and b are
shown here: their power setpoint when connected to the grid are respectively Pao and Pbo. Both
characteristics allow power to increase as the frequency decreases. During islanding the
frequency sags to a new value, ω1, and the microsources respectively generate Pa1 and Pb1
power. Two things need to be pointed out: the difference of the sum of generation in island
(Pa1+Pb1) and during grid connection (Pao+Pbo) is the missing quota of power from the grid.
The second important point is that since the slopes of the droops are constant, the two units pick
up power in the same amount, that is one ramps up of as much in per unit as the other does.
P o
ω o
P versus Frequency droop
+ π
P
_
+
m
Δ ω
_
+
ω
1
s
δ
v
− π
Figure 3.11 Block Diagram of the Active Power Droop.
The control block that realizes the droop is represented in Figure 3.11: there are only three inputs
to the block: the desired and measured injected power and the nominal system frequency. The
measure of the angle of the voltage at the regulated bus is not needed because that information is
already implicitly fed back through the value of the measured power injected by the microsource.
The output of this block is the desired angle of the voltage at the inverter bus: this is because the
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