Environmental Engineering Reference
In-Depth Information
inductor is desired because a smaller inductor is more cost-effective and it allows a higher
neutral current with higher frequency (when
V
DC
is the same). However, too small an inductor
results in very large surge of current flowing through the switches. Hence, the inductor
L
N
should be designed with more consideration on this point, i.e.
1
2
V
DC
L
N
≥
δ
m
,
di
dt
where
1 mH is quite enough
(Rashid 1993). An extreme condition is that the current flowing through the inductor is a
triangle waveform with a frequency equal to the switching frequency
f
s
. Then the following
condition should be met:
δ
m
is the maximum allowable
of the switches. Usually, 0
.
V
DC
8
I
m
f
s
,
L
N
≥
(11.16)
where
I
m
is the maximum allowable current of the switch. Usually, this condition is stronger
than the previous one and the inductor is chosen according to (11.15) and (11.16), i.e.
V
DC
8
I
m
f
s
≤
kV
DC
4
L
N
≤
fI
N
.
π
Another factor which should be taken into account is that too small a
u
N
means a weak
control action. Hence, if the neutral current is normally large, then a small inductor is desired;
if the neutral current is normally small, then a larger inductor is required.
11.5 Simulation Results
In the following simulations, a single-phase buck converter is used to simulate the neutral
current
i
N
of a three-phase inverter. The fundamental frequency of this buck converter can
be set at different frequencies to simulate different harmonic components in a neutral line.
The parameters of the neutral leg used in these simulations are
L
N
=
10 mH,
C
N
=
4000
μ
F,
V
DC
=
900 V and
f
s
=
10 kHz. The controller is designed as
K
i
=
100 and
K
v
=
4000, i.e.
with
10000 rad/s.
In order to see the signals clearly, the signals
i
c
and
u
N
arefilteredbyaholdfilter
ω
i
=
ω
o
=
1
f
s
s
f
s
1
e
−
−
F
(
s
)
=
.
s
This is a notch filter, of which the notching frequencies are the switching frequency
f
s
and its
integer multiples. Hence, the switching effects are eliminated.
11.5.1 With i
N
=
0
This is an ideal situation. The simulation results are shown in Figure 11.9. The peak value of
the shift is less than 0
.
01 V; the duty cycle of the firing pulse
q
is about 50% and
u
N
is about
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