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

A
16
DG
B
22
DG
Static
Switch
C
8
D
11
DG
DG
Grid
Sensitive
Loads
PCC
Non Sensitive
Loads
Figure 3.14 Unit Power Configuration.
Figure 3.15 shows the steady state P-ω characteristics for two units when using the constant
minimum slope:
Δω
m = −
Eq. 3.3
P max
This slope allows power to change between P=0 and P=Pmax as frequency changes of Δω,
shown in Figure 3.15 with the thick dashed line. All the other characteristics are simply parallel
to this one. If the system is importing from the grid before islanding, then the resulting
frequency, ω_imp is smaller than the system frequency ωo, as already seen. It is possible that
one of the units reaches maximum power in island mode, as shown by unit 2 at frequency
ω_imp. The steady state characteristic slope switches to vertical as soon as the maximum power
limit has been reached and the operating point moves downward vertically as shown by the
arrows in Figure 3.15 as load increases. Opposite considerations take place when unit is
exporting and new frequency ω_exp is larger than nominal. It is possible that if the load is very
small that one of the units has reached the limit P=0. At that point, the slope of the characteristic
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