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

instantaneous calculation of the radius of the ellipse (the magnitude of the voltage) will yield a
quantity pulsating at twice the nominal system frequency. If the unbalance is less than 3 percent,
the magnitude of the pulsation can be very small. The active power measure will have the same
issue, showing a small amplitude ripple at twice the system frequency superimposed over the
traditional value.
Filtering the signals of voltage magnitude and power is the solution that can eliminate the
presence of the small ripples. In the hardware realization measures are brought in by sensing
equipment and translated to digital values. These values are plagued by noise coming from the
sensing devices and from the connecting cables that carry those measurements. The digital
values inside the DSP environment need to be filtered to minimize the effect of these noises. It
turns out that those filtering levels may also be enough to eliminate the 120Hz ripples in the
measures due to low levels of unbalance present in the system. In the hardware laboratory setup
the voltage supply is rather well balanced, showing a 0.2 percent level of unbalance at the 480V
point of common coupling.
The values of P,Q and regulated voltage magnitude, V are filtered before being introduced in the
controller. The filter is a low pass, single time constant transfer function:
1
H ( s)
=
1+τ
s
The time constant τ is worth 30ms, for P, Q and V. Figure 5.1 shows the frequency response of
this filter.
Figure 5.1 Filter Gain as a Function of Frequency.
The test system is one microsource in island connected to an unbalanced load. The load is
unbalanced by removing the connection at one of the three phases. As a consequence, the current
in that particular phase will be zero. The resulting unbalance in the voltage is 5.12%.
66