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

Q versus E Droop
E req
Q
m Q
_
+
E o
Figure 3.8 Q versus Load Voltage, E, Droop Block.
The inputs of this block are the desired voltage at the regulated bus and the current injection of
reactive power from the microsource. The output is the new value of voltage request that
replaces the one commanded from outside. This new value is obtained from the linear
characteristic of the droop.
Figure 3.8 expands the block labeled as “Q versus E droop” in Figure 3.2 to allow to see the
details. This block is responsible for modifying the value of the reference voltage that is
commanded from outside. When two units are located electrically near each other and given two
voltage setpoints, they will try to achieve those requested voltages by injecting reactive power. If
the two setpoints are somewhat different from each other, then one machine will inject a large
amount of capacitive power, while the other will inject inductive power.
This situation comes as a consequence that the units will have to create the requested difference
of voltage by injecting a large current over the small impedance that there is between the units.
In this scenario reactive current will flow from one unit to the other, creating the problem of the
circulating currents. These currents flow in the machines, reducing the amount of ratings
available to face new load requests.
To mitigate this problem, a reactive power versus voltage droop is adopted. This characteristic is
designed to convert the external command of the voltage E req into the value E o . The larger is
the amount of capacitive power that is injected, the lower this value is allowed to sag compared
to the external request. Conversely, E o is allowed to swell as inductive current is injected. In
this way, if two neighboring units have voltage setpoints E req that are too far apart, then the
actual commands E o of the units will result nearer to each other. This correction successfully
limits the circulating reactive currents because it limits the reactive power injections to achieve
the adjusted voltages. The characteristic is represented in Figure 3.9 where it is possible to see
how the block corrects the reference voltage according to the sign of the injected reactive power.
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