System-level low voltage ride-through control for the renewable power plant connecting to weak grid
According to an aspect of the present disclosure, a system-level low voltage ride-through (LVRT) control system for a renewable power plant (RPP) connecting to a weak grid is provided. The system includes a voltage boundary vector trajectory model that captures geometric relationships of system-side responses to determine voltage controllable boundaries. The system includes a discrete and clustering matrix-based calculation algorithm that computes optimal current commands within the constructed boundaries.
1 . A method for system-level low voltage ride-through control of a renewable power plant connected to a weak grid and including a plurality of units, the method comprising:
determining voltage controllable boundaries by capturing geometric relationships of system-side responses using a voltage boundary vector trajectory model that accounts for transformer connection transformations, wherein voltage controllable boundaries are determined using:
V
lim
2
=
(
X
∑
I
Q
+
+
A
U
)
2
+
(
-
X
∑
I
Q
-
+
V
g
-
)
2
+
2
(
X
∑
I
Q
+
+
A
U
)
(
-
X
∑
I
Q
-
+
V
g
-
)
cos
(
γ
P
C
C
+
Δγ
-
k
π
3
)
A
U
=
V
g
+
2
-
(
X
∑
I
P
+
)
2
,
Δγ
=
X
∑
(
I
P
+
-
I
P
.
❘
"\[LeftBracketingBar]"
0
❘
"\[RightBracketingBar]"
+
)
/
V
g
+
k
=
0
,
1
,
2
,
3
,
4
,
5
wherein the X Σ , V g + and V g − is measured by an online impedance estimator;
discretizing positive sequence reactive current I Q + to an m-dimensional vector where m denotes discrete resolution, and wherein:
I
Q
.
des
+
=
[
0
:
I
lim
2
-
I
P
+
2
/
m
:
I
lim
2
-
I
P
+
2
]
(
16
)
where m denotes discrete resolution;
substituting the discretized I Q.des + into voltage boundary equations to solve for vectors about negative sequence reactive current I Q − ;
grouping the solved vectors into two matrices B up and B down according to norm of each solution vector, wherein:
{
B
up
=
[
I
Q
.
des
1.1
.1
-
L
I
Q
.
des
1.1
.
k
-
M
O
M
I
Q
.
des
1.
m
.1
-
L
I
Q
.
des
1.
m
.
k
-
]
B
down
=
[
I
Q
.
des
2.1
.1
-
L
I
Q
.
des
2.1
.
k
-
M
O
M
I
Q
.
des
2.
m
.1
-
L
I
Q
.
des
2.
m
.
k
-
]
where:
I
Q
-
.
des
1.
m
.
k
and
I
Q
-
.
des
2.
m
.
k
denote the two solutions obtained from the k-th equation at the m-th discrete point; and
M, L, and O are placeholders, indicating values of
I
Q
=
.
des
1.
m
.
k
and
I
Q
=
.
des
2.
m
.
k
across rows and columns;
calculating a maximum value of each row of the B up and a minimum value of each row of the B down and combine them into I up and I down respectively, such that:
I up.i ≤I down.i
computing optimal current commands within the voltage controllable boundaries using the grouped matrices;
transmitting the optimal current commands to all units within the renewable power plant, wherein the units include wind turbines, photovoltaics, and static var generators;
at a grid-connected inverter for each unit, mathematically transforming an optimal current command issued to the unit to obtain a switching signal of a thyristor; and
controlling the output three-phase current of the unit using the switching signal calculated from the optimal current command issued to the unit.