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

droop. The issue is that it is possible to create an initial condition with a current that grows in the
opposite direction from the forced solution that exists in steady state. The current must have only
one transient: growing from zero to the final steady state value.
To make a stronger point, some simulation results with a single microsource connected to the
grid will be shown. At first, the simulation will not meet condition (ii). Figure 2.7 shows the
currents flowing in the static switch while Figure 2.8 shows the voltage across the static switch
on the upper plot and the current injected by the microsource on the lower plot. Figure 2.9 shows
the active power injected by the unit on the upper plot and the frequency of the microgrid on the
lower plot. From all these plots it should be noticed:
a) the current from the grid increasing, going to zero (reversing) and then increasing again on
all three phases
b) the microsource injects even more power than it is injecting in island, to feed the grid, and
backs off immediately to the requested level.
c) the load always takes the same amount of power since its voltage is unperturbed, so the extra
power that the microsource generates transiently goes into the grid.
The microsource power command is 0.2 pu, while the load takes 0.65 pu (all provided by the
unit during island mode): transiently the source generates up to 0.9 pu, with the extra power
being injected in the grid.
Igrid_a [A]
Igrid_b [A]
Igrid_c [A]
Time [s]
Figure 2.7 Three Phase Currents of the Static Switch, with Condition (ii) not Met.
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