Environmental Engineering Reference
In-Depth Information
It is worth noting that, after compensation, the loss in the grid transmission line is reduced by
I
L
1
cos
2
θ
×
3
√
3
K
V
cos
2
θ
1
−
=
1
−
.
I
L
1
K
V
2
2
2
×
That is, the loss is reduced by at least 50%.
9.3.3 Compensation of Harmonic Currents
The above analysis is based on the assumption that there is no harmonics in the load current.
As a matter of fact, according to (9.6), all the harmonic current components, if any, are
automatically diverted into the compensation currents
i
ra
and
i
rb
since
i
a
and
i
b
only contain
the fundamental current component. Therefore, no extra effort is needed to suppress the
harmonics. It is worth noting that the current
i
rc
(
i
c
) only contains fundamental components
even if the load current contains harmonics.
9.3.4 Regulation of the DC-bus Voltage
A stable DC-link voltage is required in order for the SPC to work properly. This can be achieved
by introducing a PI controller to maintain the DC bus voltage
V
c
at the DC-bus reference voltage
V
cre f
. The output of the DC-bus voltage controller is added on to the required RMS value of
the track-side currents so that the right amount of active power can be injected into the SPC.
Because of the double frequency ripple component in the DC-bus voltage, a low-pass filter,
such as the hold filter
e
−
Ts
/
2
Ts
1
−
H
(
s
)
=
,
/
2
where
T
is the fundamental period of the system, can be adopted to measure the DC component
of
V
c
for feedback.
9.3.5 Implementation of the Compensation Strategy
The above compensation strategy can be implemented as shown in Figure 9.3. The sinusoidal
tracking algorithm (STA) (Ziarani and Konrad 2004) (see also Chapter 22) is adopted to
calculate the phase of the fundamental component of the grid line voltage
u
ab
. The phase of
the voltage
u
ab
is
+
6
2
3
ω
t
so it can be used to generate the signal sin(
ω
t
) and sin(
ω
t
−
)
+
6
) with the
needed to form the reference compensation currents. The product of sin(
ω
t
fundamental load current is
sin
√
2
I
L
1
sin
+
6
+
6
−
θ
ω
t
×
ω
t
√
2
I
L
1
2
cos
cos
2
+
3
−
θ
=
θ
−
ω
t
,
√
2
2
2
√
3
of which the DC component
I
L
1
cos
θ
ca
n be multiplied with
to obtain the required
amplitude of the track-side currents, i.e.,
√
2
1
√
3
I
L
1
cos
×
θ
.
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