Environmental Engineering Reference
In-Depth Information
Fig. 3.5
Diagram of VOC scheme
Feedback linearization techniques [
22
,
23
], which show an enhanced performance
in simulation, however, are sensitive to model uncertainties. Hence sliding-mode
control will be applied in the chapter.
The grid-side PWM converter model is shown in Fig.
3.4
, where L is the
inductor of the grid side filter, R the resistance, C the DC-link capacitor. The input
line voltages and currents are expressed by the notations v
k
and i
k
,k = a, b, c.
In this chapter, the voltage-oriented control (VOC) scheme is used. It is a dual-
loop structure including DC-link voltage outer loop and dq-axes current inner
loop. A reference frame is chosen to rotate synchronously with the grid voltage
space vector and the d-axis is made to orient to the grid voltage space vector. The
block diagram of VOC is shown in Fig.
3.5
. For converter, VOC can achieve
decoupling between active and reactive currents as the d-axis of the synchronously
rotating d-q frame is aligned with the grid voltage vector. The mathematical model
in d-q synchronous rotating reference frame can be expressed by [
16
]
8
<
Li
d
¼
Ri
d
þ
xLi
q
þ
e
d
s
d
u
dc
Li
q
¼
Ri
q
xLi
d
þ
e
q
s
q
u
dc
C u
dc
¼ð
s
d
i
d
þ
s
q
i
q
Þ
i
L
ð
3
:
12
Þ
:
where i
d
,i
q
are the d- and q-axis current components of the converter; s
d
, s
q
the
d- and q-axis switching control signals in d-q reference frame; e
d
and e
q
the d- and
q-axis voltage component of the three-phase supply; x the angular frequency of
the power source.
It can be seen from Eq. (
3.12
) that it is difficult to design the regulators due to
the multiplication of the state variables by the control inputs. To design the
controllers, the dynamical Eq. (
3.12
) should be simplified based on the power
balance between AC and DC sides of the system. The active power of AC and DC
sides are expressed by the notations P
ac
and P
dc
respectively. Neglecting the