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

needs of power. This solution would solve this problem and the grid would see a constant power
demand from the microgrid.
The basic requirement for the load dispatch configuration is the need for a measure of the power
flowing in those branches so that the control can compare it against the desired reference
amount. Figure 3.21 shows this configuration, where it is possible to see that the power from the
line and the power from the unit always add up to the load request. When a load increases the
unit increases its power output to maintain a constant branch power flow. This configuration
implies feedback of the line current and the voltage at the point where the unit is installed to
calculate the power flow in the feeder.
This section shows the steady state characteristics on the F-ω plane that allow to enforce limits
on frequency and unit power output, P. In this case the units are configured to regulate the flow
of power in the feeder that connects to the utility, F. Figure 3.21 shows the setup: when
connected to the grid, every load change is matched by a different power injection from the unit
since the control holds the flow of power coming from the grid, F_line, to a constant value.
During island mode all the units participate in matching the power demand as loads change.
I
Feeder
E
Loads
P
source
X
F
line
E , I Feeder
Rest of the
System
Micro-Source
Figure 3.21 Diagram of a Unit Regulating Feeder Power Flow.
The case when the grid fails while operating in the load dispatch mode can be handled by
changing the sign of the power-frequency droop characteristic. During island mode there is less
need to import power from the feeders so that the characteristic is designed to reduce power as
the frequency reduces.
Figure 3.22 shows the load tracking configuration for the whole microgrid. This mode of
operation earned this name because once the required level of power requests are set, any change
in load within the microgrid will be matched by a corresponding amount of power injected by the
units to keep the flow in the feeder constant. This configuration has the advantage that the
microgrid looks exactly like a dispatchable load from the utility point of view. New agreements
in electric contracts can be signed with the utility: for instance the microgrid could agree to
behave like a constant load during a determinate period of time of the day since it can
automatically track all its internal changes of load with the local generation.
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