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
Search WWH ::




Custom Search