Environmental Engineering Reference
In-Depth Information
40
20
0
−20
−40
0.2
0.22
0.24
0.26
0.28
0.3
Time [s]
Figure 22.9
A noisy distorted square wave
22.7.2 With a Noisy Distorted Square Wave
In this simulation, the input signal
v
, as
s
hown in Figure 22.9, is a square wave with the
period of 19 ms and the amplitude of 20
√
2 V, added with the harmonic signal
v
h
(
t
) given in
(22.8). Simulations were carried out with the same parameters used in the previous subsection
for the SOGI-PLL and the STA to extract the fundamental component of
v
. The results
are shown in the left column of Figure 22.10 for the SOGI-PLL and in the right column
of Figure 22.10 for the STA. The SOGI-PLL caused noticeable frequency variations and
the STA was not able to track the frequency. The STA caused noticeable variations in the
amplitude detected and the SOGI-PLL was not able to track the amplitude. The recovered
signals
e
have a significant amount of harmonics. The STA had difficulties in tracking the
phase as well.
22.8 Experimental Results with SOGI-PLL and STA
Various experiments were carried out with a TI renewable energy kit, which is equipped with
a floating point DSP TMS320F28335, with a sampling frequency of 10 kHz. The voltage
signal
was properly conditioned and read by the DSP via the on-chip Analog to Digital
Converter (ADC) module. Three experiments were carried out with the same parameters used
in simulations. One experiment was carried out with an input signal taken from the utility grid
and the other two were carried out with the voltage signals adopted in the simulations, which
were generated by the same DSP through a Digital to Analog (DAC) module.
v
22.8.1 With a Voltage Taken from the Grid
The grid voltage was scaled down with a single-phase transformer and then shifted and
conditioned by op-amps to form a signal that could be read by the ADC module. The same
experiment was carried out for the SOGI-PLL and the STA, respectively, and the results are
shown in the left column of Figure 22.11 for the SOGI-PLL and in the right column of Figure
22.11 for the STA, respectively. The phase was tracked well by both methods. The SOGI-PLL
took about one cycle to produce the correct amplitude and a half cycle to produce the correct
frequency while the STA took about two cycles to produce the correct amplitude and one cycle
to produce the correct frequency, all with noticeable overshoots.
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