Observer-based control system for grid-connected DC/AC converters with LCL-filter
Systems related to controlling a DC/AC converter. A control system uses a nonlinear adaptive observer to estimate the state variables inverter current and converter voltage using a sensed grid current and a bus voltage as inputs. For non-observable points (such as when the duty cycle=0.5), the required information can be found from the DC bus voltage.
1. A system for use in controlling a DC/AC converter, the system comprising:
a non-linear observer subsystem for estimating an inverter current from said converter and for estimating a converter voltage,
wherein
outputs of said subsystem are used to control transistors of said converter; and
said subsystem receives, as input,
a grid current from said converter, and
a bus voltage of an input to said converter.
2. A system according to claim 1 wherein said system further comprises a proportional resonant controller and a linear state feedback controller.
3. A system according to claim 1 further comprising a sinusoidal pulse width modulation modulator, said modulator being used to send control signals to said transistors of said converter, said modulator creating said control signals from outputs of said subsystem.
4. A system according to claim 1 wherein said subsystem further receives as input a value derived from a duty ratio of a full bridge inverter, said full bridge inverter comprising part of said DC/AC converter.
5. A system according to claim 1 wherein said subsystem has dynamics which is given by:
x
^
.
1
=
1
L
g
x
^
3
-
1
L
g
v
g
+
ψ
11
(
X
,
u
)
x
~
1
+
ψ
14
(
X
,
u
)
x
~
4
x
^
.
2
=
1
L
inv
x
^
4
u
-
1
L
inv
x
^
3
+
ψ
21
(
X
,
u
)
x
~
1
+
ψ
24
(
X
,
u
)
x
~
4
x
^
.
3
=
1
C
x
^
2
-
1
C
x
1
+
ψ
31
(
X
,
u
)
x
~
1
+
ψ
34
(
X
,
u
)
x
~
4
x
^
.
4
=
η
^
-
1
C
BUS
x
^
2
u
+
ψ
41
(
X
,
u
)
x
~
1
+
ψ
44
(
X
,
u
)
x
~
4
η
^
.
=
γ
x
~
4
where {tilde over (x)} i =x i −{circumflex over (x)} i for i=1, . . . , 4 and {tilde over (η)}=η−{circumflex over (η)} and
where
x 1 =i g is a grid current;
x 2 =i inv is an inverter current;
x 3 =v C is a converter voltage; and
x 4 =v BUS is an input bus voltage for said system.
6. A system according to claim 1 wherein an inverter current in said subsystem resets when an operating point coincides with a singular point detected from a DC-bus voltage of said converter.
7. A system according to claim 1 wherein said DC/AC converter comprises an LCL-filter.