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

u
+
_
K F
ω
o
+
_
ω o
s
P
u
ω o
s
Q
u
Figure 3.5 Selective Filter Diagram.
Figure 3.6 shows the magnitude and phase response of the filter. The magnitude shows that the
frequency 60Hz is passed without any alteration, i.e. unitary gain (zero dB) and zero phase shift.
For frequencies very near to 60Hz the gain is a little lower than one and phase shift is non-zero.
That is not a problem: by choosing an appropriate value for KF, it is possible to ensure that the
gain is lowered only of a fraction of a percent for the range of frequencies expected during island
operation. The phase shift is also not a problem: since all components are shifted of the same
amount, the shift can assume any arbitrary value. The calculation of P and Q is a function of the
relative shift of voltages and currents, not their absolute value. As long as both voltages and
currents are shifted of the same amount (and they are, since the frequency of voltages and
currents are the same), then their relative phasing will remain unaltered.
Figure 3.6 Selective Filter Response.
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