Hybrid microgrid system for wind power electrical generation
View Patent ↗A system and a method for controlling a hybrid microgrid system (HMS) is disclosed. The HMS includes a WTG, an RSC, a GSC, a DC-link connecting the RSC and the GSC, a PV system that outputs a DC current to the DC-link, a rechargeable battery, a bidirectional BBC connected between the DC-link and the rechargeable battery, and a controller. The method for controlling the HMS includes: preparing a definition set including a characteristic element c i and equations defining desired value c i *, a fractional order sliding mode surface ζ i , and a control law element u i cnt ; monitoring c i (t) and the HMS status; calculating the equations based on monitored information; and controlling the HMS based on the u i cnt (t) calculated and in accordance with a global sliding mode control with fractional order terms. The ζ i comprises a fractional time integral and fractional time derivative of e i (t), where e i (t)=c i (t)−c i *(t). The u i cnt (t) satisfies ζ i ( t ) d ζ i ( t ) dt < 0 , when ζ i (t)≠0.
1. A hybrid microgrid system (HMS) for wind power electrical generation comprising:
a wind turbine (WT) and an electric generator;
a grid side converter (GSC) configured to output a power to a point of common coupling (PCC);
a DC-link configured to receive a power from the WT and the electric generator and to supply a power to the GSC;
a rechargeable battery configured to exchange a power between the DC-link;
a load configured to receive a power via the PCC;
a utility grid configured to exchange power via the PCC; and
a controller comprising: a processor; a memory; a bus-line; and I/O port,
wherein, the controller is configured to control the HMS by executing a program installed in the memory and in accordance with a global sliding mode control with fractional order terms (GSMCFO) method, and wherein, the program comprises a definition set customized for the HMS and to be referred in applying the GSMCFO method to the HMS,
wherein the definition set comprises: a characteristic element c i to be measured; and equations defining a desired value c i * of the characteristic element c i , a fractional order sliding mode (FOSM) surface (i of the characteristic element c i , and a control law element u i cnt of the characteristic element c i , and
wherein the controller is further configured to
monitor the characteristic element c i (t) and a related status of the HMS,
calculate at least one of the equations defined in the definition set based on the characteristic element c i (t) monitored and the status of the HMS monitored, and
control the HMS based on the control law element u i cnt (t) calculated,
wherein the equation defining the FOSM surface ζ i (t) for the characteristic element c i (t) comprises a fractional time integral of a tracking error e i (t) and a fractional time derivative of the tracking error e i (t), wherein the tracking error e i (t) for the characteristic element c i (t) is defined as,
e i ( t )= c i ( t )− c i *( t ),
wherein the equation defining the control law element u i cnt (t) is configured to satisfy a condition
ζ
i
(
t
)
d
ζ
i
(
t
)
dt
<
0
,
so far as ζ i (t) is not zero.
2. The hybrid microgrid system of claim 1 , wherein the definition set further comprises: a minimum value SOC min in and a maximum value SOC max of the state of charge (SOC) of the rechargeable battery; and equations defining a power imbalance ΔP and a power balance condition of the HMS, given respectively as,
Δ P=P re +P ug −P dem −P b
Δ P= 0,
wherein P re represents a total power generated by the WT and the electric generator, P ug , a grid power exchanged between the utility grid and the PCC, P dem , a load demand, P b , a battery power exchanged between the rechargeable battery and the DC-link,
wherein the controller is further configured to
monitor elements required to calculate a power imbalance ΔP and a SOC,
calculate a power imbalance ΔP with the equation given in the definition set, and
control the battery power P b and the grid power P ug to satisfy and maintain the power balance condition, under a restriction that the SOC of the rechargeable battery satisfies a condition,
SOC min ≤SOC≤SOC max .
3. The hybrid microgrid system of claim 2 , wherein the WT comprises:
a wind turbine generator and the electric generator comprises a rotor and a stator;
a rotor side converter (RSC) configured to receive an AC power from the electric generator and output an RSC output DC current to the DC-link;
a solar photovoltaic (PV) system configured to output a PV output DC current to the DC-link; and
a bidirectional buck-boost converter (BBBC) connected between the DC-link and the rechargeable battery and configured to control a power exchanged between the rechargeable battery and the DC-link,
and wherein, the definition set further comprises an equation defining an equivalent control law element u i eqv for the characteristic element c i , wherein the equivalent control law element u i eqv comprises a maximum disturbance term R t 1−μ δ i , representing a possible maximum value of a lumped external disturbances and parametric perturbations to the tracking error e i (t), wherein, R t 1−μ represents a Riemann-Liouville fractional integration, and δ i , a positive function,
wherein, the equation defining the control law element u i cnt (t) comprises a function SG (ζ i (t)) given by a signum function sgn(ζ i (t)) or one of its smooth approximations including
tan
h
(
ζ
i
(
t
)
θ
)
,
wherein, θ(>0).
4. The hybrid microgrid system of claim 3 , wherein the characteristic element c 1 is an angular frequency ω r of the WT, and the characteristic element c 2 is a d-axis stator current I ds , wherein the desired value ω r * for the angular frequency ω r of the WT is defined as,
c
1
*
(
t
)
=
ω
r
*
=
λ
*
V
w
R
,
wherein λ* denotes a desired tip speed ratio, giving a maximum power coefficient for the turbine with a blade radius R, at a wind speed V w ,
wherein the desired value I ds * for the d-axis stator current I ds of the rotor is given as,
c 2 *( t )= I ds =0
wherein the equations defining the FOSM surfaces ζ i (t) for the characteristic elements c 1 (i=1, 2) are given as,
ζ 1 ( t )= k 1 R t μ e 1 ( t )+σ 1 R t 1-μ e 1 ( t )+ R t 2-μ e 1 ( t ),
ζ 2 ( t )= k 2 R t μ e 2 ( t )+ R t 1-μ e 2 ( t )
wherein, 0<μ<1, k 1 , k 2 , and σ 1 are positive constants, R t μ denotes a Riemann-Liouville fractional integration, R t 1-μ and R t 2-μ denote Riemann-Liouville fractional derivations,
wherein the equivalent control law element u 1 eqv (t) and the control law element u i cnt (t) of the angular frequency ω r are given by q-axis stator voltages V qs eqv (t) and V qs cnt (t), and defined respectively as,
u
1
eqv
(
t
)
=
V
qs
eqv
(
t
)
=
-
1
a
(
k
1
e
1
(
t
)
+
σ
1
e
.
1
(
t
)
+
R
t
1
-
μ
δ
1
-
ω
¨
r
*
+
a
[
L
q
ω
r
I
ds
+
R
s
I
qs
+
λ
r
ω
r
]
)
,
u
1
cnt
(
t
)
=
V
qs
cnt
(
t
)
=
V
qs
eqv
(
t
)
-
1
a
[
ϱ
1
R
t
1
-
μ
SG
(
ζ
1
(
t
)
)
❘
"\[LeftBracketingBar]"
ζ
1
(
t
)
❘
"\[RightBracketingBar]"
α
+
γ
1
R
t
1
-
μ
ζ
1
(
t
)
]
,
wherein the equivalent control law element u 2 eqv (t) and the control law element u 2 cnt (t) of the d-axis stator current I ds of the rotor are given by d-axis stator voltages V ds eqv (t) and V ds cnt (t), and defined respectively as
u
2
eqv
(
t
)
=
V
ds
eqv
(
t
)
=
-
L
d
(
k
2
e
2
(
t
)
+
[
L
q
ω
r
I
qs
-
R
s
I
ds
]
L
d
+
R
t
1
-
μ
δ
2
-
I
.
ds
*
)
,
u
2
cnt
(
t
)
=
V
ds
cnt
(
t
)
=
V
ds
eqv
(
t
)
-
L
d
[
ϱ
1
R
t
1
-
μ
SG
(
ζ
2
(
t
)
)
❘
"\[LeftBracketingBar]"
ζ
2
(
t
)
❘
"\[RightBracketingBar]"
α
+
γ
2
R
t
1
-
μ
ζ
2
(
t
)
]
,
wherein α is given by
a
=
3
P
2
Ar
2
JL
q
,
wherein P denotes a number of pole pairs of the rotor, Λ r , a rotor flux, J, an inertia of mechanical shaft of the wind turbine generator, L q , a q-axis self-inductance of the stator, Rs, a stator resistance, I qs , a q-axis stator current, Λ r , a rotor flux, L d , a d-axis self-inductance of the stator, R t 1-μ δ 1 and R t 1-μ δ 2 represent the maximum disturbance terms, α ∈ (0, 1), i and γ i (i=1, 2) are positive constants.
5. The hybrid microgrid system of claim 3 , wherein the DC-link further comprises a DC-bus and a DC-link capacitor, and wherein the characteristic element c 3 is a DC-link voltage V dc ,
the desired value V dc * defined for the DC-link voltage V dc is given as
c
3
*
(
t
)
=
V
dc
*
=
{
V
pv
MPPT
;
when
V
dc
min
≤
V
pv
MPPT
≤
V
dc
max
,
V
dc
nom
;
when
V
pv
MPPT
<
V
dc
min
,
or
V
pv
MPPT
>
V
dc
max
,
wherein V pv MPPT represents an output voltage of the PV system under a maximum power point tracking (MPPT) operation, V dc min , V dc max and V dc nom represent a minimum allowable value, a maximum allowable value, and a nominal value of the DC-link voltages, each predetermined respectively,
the equation defining the FOSM surfaces ζ 3 (t) of the DC-link voltage is given as,
ζ 3 ( t )= k 3 R t μ e 3 ( t )+ R t 1-μ e 3 ( t )
wherein, μ ∈ (0, 1) and k 3 are positive constants, R t 1-μ denotes a Riemann-Liouville fractional integration, R t 1-μ denotes a Riemann-Liouville fractional derivation,
wherein the equations defining the equivalent control law element u 3 eqv (t) and the control law element u 3 cnt (t) of the DC-link voltage V dc are given by d-axis AC output currents I d eqv (t) and I d cnt (t) from the GSC, and defined respectively as,
u
3
eqv
(
t
)
=
I
d
eqv
(
t
)
=
2
3
C
dc
V
dc
V
d
(
k
3
e
3
(
t
)
+
P
w
C
dc
V
dc
+
I
pv
C
dc
+
(
1
-
D
)
C
dc
I
b
+
R
t
1
-
μ
δ
3
-
V
.
dc
*
(
t
)
)
,
u
3
cnt
(
t
)
=
I
d
cnt
(
t
)
=
I
d
eqv
(
t
)
+
2
3
[
ϱ
3
R
t
1
-
μ
SG
(
ζ
3
(
t
)
)
❘
"\[LeftBracketingBar]"
ζ
3
(
t
)
❘
"\[RightBracketingBar]"
α
+
γ
3
R
t
1
-
μ
ζ
3
(
t
)
]
,
wherein C dc denotes a capacitance of the DC-link capacitor, V d , a voltage of the AC output from the GSC, P w , an output power from the RSC, I pv , an output current from the PV system, D, a duty cycle ratio of the BBBC, I b , a battery output current, R t 1-μ δ 3 is the maximum disturbance term, α ∈ (0, 1), 3 and γ 3 are positive constants.
6. The hybrid microgrid system of claim 3 , wherein the GSC is further configured to output an AC output to a point of common coupling (PCC) via a grid side filter,
wherein the characteristic elements c i (t) (i=4, 5) are a d-axis AC current I d (i=4) of the AC output from the GSC, and a q-axis AC current I q (i=5) of the AC output from the GSC for, respectively,
the desired values defined for the d-axis AC current and the q-axis AC current are given respectively as
c
4
*
(
t
)
=
I
d
*
=
I
d
=
2
3
P
dem
-
P
ug
V
d
,
c 5 *( t )= I q *=0,
wherein P dem and P ug represent a power demand at the load and a power exchanged between the PCC and the utility grid, respectively, wherein P ug >0, when provided from the utility grid to the PCC, P ug <0, when provided from the GSC to the utility grid, V d , a d-axis AC voltage of the AC output from the GSC,
the equations defining the FOSM surfaces ζ i (t) for the d-axis AC current I d (i=4) and the q-axis AC current I q (i=5) are given respectively as,
ζ 4 =k 4 R t μ e 4 ( t )+ R t 1-μ e 4 ( t ),
ζ 5 ( t )= k 5 R t μ e 4 ( t )+ R t 1-μ e 5 ( t ),
wherein μ ∈ (0, 1), k 4 , and k 5 are positive constants, R t μ and R t 1-μ each denotes a Riemann-Liouville fractional integration and a Riemann-Liouville fractional derivation, respectively,
wherein the equations defining the equivalent control law element u 4 eqv (t) and the control law element u 4 cnt (t) of the d-axis AC current I d are given by d-axis AC voltages V d eqv (t) and V d cnt (t) of the AC output from the GSC, and defined respectively as,
u
4
eqv
(
t
)
=
V
d
eqv
(
t
)
=
-
L
f
(
k
4
e
4
(
t
)
-
I
.
d
*
-
1
L
f
[
R
f
I
d
+
U
d
-
L
f
ω
g
I
q
-
R
t
1
-
μ
δ
4
]
)
,
u
4
cnt
(
t
)
=
V
d
cnt
(
t
)
=
V
d
eqv
(
t
)
-
L
f
[
ϱ
4
R
t
1
-
μ
SW
(
ζ
4
(
t
)
)
❘
"\[LeftBracketingBar]"
ζ
4
(
t
)
❘
"\[RightBracketingBar]"
α
+
γ
4
R
t
1
-
μ
ζ
4
(
t
)
]
,
wherein the equations defining the equivalent control law element u 5 eqv (t) and the control law element u 5 cnt (t) of the q-axis AC current I q are given by q-axis AC voltages V q eqv (t) and V q cnt (t) of the AC output from the GSC, and defined respectively as,
u
5
eqv
(
t
)
=
V
d
eqv
(
t
)
=
-
L
f
(
k
5
e
5
(
t
)
-
I
.
d
*
-
1
L
f
[
R
f
I
d
+
U
d
-
L
f
ω
g
I
q
-
R
t
1
-
μ
δ
5
]
)
,
u
5
cnt
(
t
)
=
V
d
cnt
(
t
)
=
V
d
eqv
(
t
)
-
L
f
[
ϱ
5
R
t
1
-
μ
SW
(
ζ
5
(
t
)
)
❘
"\[LeftBracketingBar]"
ζ
5
(
t
)
❘
"\[RightBracketingBar]"
α
+
γ
5
R
t
1
-
μ
ζ
5
(
t
)
]
,
wherein L f and R f denote a grid side filter inductance and a grid side filter resistance, respectively, U d and U q , a d-axis and a q-axis voltages at a point of common coupling (PCC), respectively, ωg, an electrical angular frequency of the AC output from the GSC, R t 1-μ δ i ,(i=4, 5) are the maximum disturbance terms, α ∈ (0, 1), i and γ i (i=4, 5) are positive constants.
7. The HMS of claim 3 , wherein the BBBC is configured to facilitate charging of the rechargeable battery while operating as a buck converter, and to facilitate discharging to the DC-link while operating as a boost converter,
and wherein the characteristic element c 1 (i=6) is a battery current I b ,
the desired value I b * defined for the battery current I b is given as,
c
6
*
(
t
)
=
I
d
*
=
I
d
=
2
3
P
dem
-
P
ug
V
d
,
wherein P re represents a sum of powers generated by the WT, P ug a power supplied by the utility grid, P dem , a load demand, and V b , a battery voltage,
the equation defining the FOSM surfaces ζ 6 (t) of the battery current is given as,
ζ 6 ( t )= k 6 R t 1-μ e 6 + R t 1-μ e 6 ,
wherein, μ ∈ (0, 1) and k 6 are positive constants, R t 1-μ denotes a Riemann-Liouville fractional integration, R t 1-μ denotes a Riemann-Liouville fractional derivation,
wherein the equations defining the equivalent control law element u 6 eqv (t) and the control law element u 6 cnt (t) of the battery current I b are given by duty cycles D eqv (t) and D cnt (t) of the BBBC, and defined respectively as,
u
6
eqv
(
t
)
=
D
eqv
(
t
)
=
L
b
V
dc
(
k
6
e
6
(
t
)
+
V
b
L
b
-
I
b
R
b
L
b
+
R
t
1
-
μ
δ
6
-
I
b
*
)
,
u
6
cnt
(
t
)
=
D
cnt
(
t
)
=
D
eqv
(
t
)
-
L
b
[
ϱ
6
R
t
1
-
μ
SG
(
ζ
6
(
t
)
)
❘
"\[LeftBracketingBar]"
ζ
6
(
t
)
❘
"\[RightBracketingBar]"
α
+
γ
6
R
t
1
-
μ
ζ
6
(
t
)
]
,
wherein L b denotes a battery inductance, V b , a battery voltage, R b , a battery resistance, V dc , a DC-link voltage, R t 1-μ δ 6 represents the maximum disturbance term, α ∈ (0, 1), 6 and γ 6 are positive constants.