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

Figure 4.9 P and Q Plane Capability with Constant Voltage and Angle X=0.1 pu.
It must be noticed that there are several choices of the impedance that allow to deliver the P and
Q inside the region, but each choice corresponds to a different inverter voltage and power angle.
For what concerns the maximum inverter voltage, it can be noticed that with X= 0.2 pu, in Figure
4.8, there is a need to have a 1.12 pu inverter voltage to reach the coordinate P=1.0 pu, Q=0.75
pu. With X=0.1 pu the inverter voltage needs to be only 1.06 pu. It is manifest how the choice of
the impedance affects the requirements on the inverter to reach the P and Q coordinates. For
instance a lower DC bus voltage can be afforded when using X=0.1 pu rather than X=0.2 pu. The
ideal design must keep into consideration the issue of DC bus magnitude because it is directly
connected to the maximum voltage that the inverter can achieve. The inductance also affects the
value of the minimum voltage that the inverter may need to synthesize when trying to reach high
values of inductive reactive power. But this value is less critical because it can always be reached
by means of a thinner modulation of the voltage waveform: on the contrary, the maximum
voltage represents a real issue because its value determines the rating that the DC bus must have.
In summary, a smaller size of the inductor corresponds to a lower cost, a lower requirement on
the DC bus rating and a smaller range of power angle. The range of the power angle cannot be
allowed to get too small to avoid errors in the power to become unacceptable. A compromise
between angle resolution, inductor price and DC bus rating is key to obtain an ideal match
between inverter capabilities and inductor.
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