US 2021/0203247 A1 · MODULAR MULTILEVEL CONVERTER WITH MULTI-PORT DC POWER FLOW CONTROL AND CONTROL METHOD THEREOF

Abstract

A modular multilevel converter with multi-port DC power flow control includes a modular multilevel converter topology and a DC power flow controller, wherein an input terminal of the DC power flow controller is connected in series to uppermost submodules of upper arms of the modular multilevel converter topology, and with power transfer and energy interaction between the modular multilevel converter topology and an AC power grid, a converter station or a solid state transformer based on the modular multilevel converter topology has a function of direct current power flow control. The output terminal of the DC power flow controller adopts DC/DC converters to regulate the output voltage, and a plurality of DC/DC converters share a common DC bus with the ports connected with different DC outlet lines, thereby providing a plurality of flexible and controllable DC ports.

A method for modulating a modular multilevel converter with multi-port DC power flow control, wherein the modular multilevel converter comprises a modular multilevel converter topology and a DC power flow controller, the modular multilevel converter topology composed of a plurality of upper and lower arms of identical configuration, each arm being formed by a plurality of cascaded submodules;

an input terminal of the DC power flow controller is connected in series to uppermost submodules of the upper arms of the modular multilevel converter topology, and with power transfer and energy interaction between the modular multilevel converter topology and an AC power grid, a converter station or a solid state transformer based on the modular multilevel converter topology has a function of DC power flow control;
a carrier phase shift modulation strategy is adopted for the DC power flow controller and the modular multilevel converter topology, and the input terminal of the DC power flow controller and the submodules of the modular multilevel topology take individual modulation strategies;
ignoring modification of the DC power flow controller and circulating current control of the arms of the modular multilevel converter topology and other balance control to a voltage reference of the arms, the following formulas are obtained:
{
u
ap


0
=
x
1

u
d
2
-
y
1

u
aref
u
ap

=
x
2

u
d
2
-
y
2

u
aref
u
an
=
u
d
2
+
u
aref
where x1, y1 are a DC component coefficient and a fundamental frequency AC component coefficient of the single-phase half bridge voltage reference of the DC power flow controller, respectively, x2, y2 are a DC component coefficient and a fundamental frequency AC component coefficient of the upper arms voltage reference of the modular multilevel converter topology, respectively, uap0 is an AC side output voltage reference of the single-phase half bridge, uap, uan are output voltage references of the upper and lower arms of the modular multilevel converter topology, respectively, uaref is an AC side output voltage reference of the modular multilevel converter topology, uaref=U sin(ωt), U is the amplitude of phase voltage of AC grid, ω signifies grid voltage angular frequency, t signifies time and Ud is an DC bus voltage of the DC power flow controller;
the reference coefficients satisfy:
{
x
1
+
x
2
=
1
y
1
+
y
2
=
1
x
1
,
x
2
>
0
y
1
,
y
2
>
0
where Uc0 is the DC component of the DC power flow controller capacitor voltage, Uc1 is the DC component of the submodule capacitor voltage, and n is the count of the submodules of each arm of the modular multilevel converter topology, so that average switching functions of the single-phase half bridge of the DC power flow controller and the submodules of the upper and lower arms of the modular multilevel converter topology are obtained as follows:
{
F
a

p

0

(
t
)
=
x
1

U
d
2

U
c

0

(
1
-
m
1

sin

(
ω

t
)
)
F
a

p

(
t
)
=
x
2

U
d
2

n

U
c

1

(
1
-
m
2

sin

(
ω

t
)
)
F
a

n

(
t
)
=
U
d
2

n

U
c

1

(
1
+
m

sin

(
ω

t
)
)
where m1, m2, m are voltage modulation coefficients of the single-phase half bridge of the DC power flow controller and the submodules of the upper and lower arms of the modular multilevel converter topology, respectively:
m
1
=
y
1

U
x
1

U
a
2


m
2
=
y
1

U
x
1

U
d
2


m
=
U
U
d
2
where U is the amplitude of an AC side phase voltage;
for a DC power flow controller with an input terminal of three-phase half bridge structure, the sum of input side currents at upper and lower terminals of the capacitor is the sum of DC components of the three-phase arms current of the modular multilevel converter topology, and assuming that the sum of the DC components of three-phase arms current of the modular multilevel converter topology connected to the DC power flow controller is Id, then
Id=iSMp0−+iSMp0+
where iSMp0− is the input side current at the lower terminal of the capacitor of the DC power flow controller;
isMp0+ is the input side current at the upper terminal of the capacitor of the DC power flow controller;
then, the currents of the upper and lower arms of the modular multilevel converter topology are as follows:
iap(t)=−⅓Id−½ia(t)=−⅓Id−½I sin(ωt−φ)
ian(t)=−⅓Id+½ia(t)=−⅓Id+½I sin(ωt−φ)
where ia is an A-phase AC current;
φ is a power factor angle;
ω is an alternating voltage frequency;
t is time;
I denotes the amplitude of phase current of AC grid;
iap(t) and ian(t) are A-phase upper and lower arms current of the modular multilevel converter topology, respectively;
the input side currents of the capacitor of the DC power flow controller are determined by multiplying the arm current of each phase by the switching function of the single-phase half bridge and then merging the three phases:
i
SMp


0
-

(
t
)
=

∑
k
=
a
,
b
,
c

F
kp


0

(
t
)
·
(
-
i
kp

(
t
)
)
=

-
x
1

3

U
d

I
8

U
c


0

m
1


cos


ϕ
+
x
1

U
d

I
d
2

U
c


0
i
SMp


0
+

(
t
)
=

∑
k
=
a
,
b
,
c

(
1
-
F
kp


0

(
t
)
)

(
-
i
kp

(
t
)
)
=

I
d
+
x
1

3

U
d

I
8

U
c


0

m
1

cos


ϕ
-
x
1

U
d

I
d
2

U
c


0
where Fkp0(t) is the average switching function of the single-phase half bridge of the DC power flow controller;
ikp(t) is single-phase upper and lower arms current of the modular multilevel converter topology;
and a, b and c are the three phases;
for a DC power flow controller with an input terminal of single-phase half bridge structure, the input side currents of the capacitor include a DC component as well as a low-frequency AC component, and assuming that the DC component of the single-phase arm current of the modular multilevel converter topology embedded with the DC power flow controller is Id, then an upper arm current and a lower arm current of the modular multilevel converter are set as:
iap(t)=−Id−½ia(t)=−Id−½I sin(ωt−φ)
ian(t)=−Id+½ia(t)=−Id+½I sin(ωt−φ)
the input side currents of the capacitor of the DC power flow controller are determined by multiplying the arm current of the phase and the switching function of the single-phase half bridge:
i
SMp


0
-

(
t
)
=

F
ap


0

(
t
)
·
(
-
i
ap

(
t
)
)
=

x
1

U
d

I
d
2

U
c


0

(
1
-
m
1

sin

(
ω


t
)
)
+

x
1

U
d

I
4

U
c


0

(
sin

(
ω


t
-
ϕ
)
-
1
2

m
1

(
cos


ϕ
-
cos

(
2


ω


t
-
ϕ
)
)
)
i
SMp


0
+

(
t
)
=

(
1
-
F
ap


0

(
t
)
)

(
-
i
ap

(
t
)
)
=

I
d
+
1
2

I


sin

(
ω


t
-
ϕ
)
-
x
1

U
d

I
d
2

U
c


0

(
1
-
m
1

sin

(
ω


t
)
)
-

x
1

U
d

I
4

U
c


0

(
sin

(
ω


t
-
ϕ
)
-
1
2

m
1

(
cos


ϕ
-
cos

(
2


ω


t
-
ϕ
)
)
)
an input and output power balance equation of the DC power flow controller is as follows:
PPFC=UdiSMp0++(Ud−Uc0)iSMp0−=PMVDC
where PPEC is an input active power of the DC power flow controller;
PMVDC is an output active power of a medium-voltage DC side;
and a, b and c are the three phases;
based on calculation and analysis, with appropriately selected DC component coefficients and fundamental frequency AC component coefficients of voltage reference, the voltage modulation coefficients of the single-phase half bridge of the DC power flow controller and the upper and lower arms of the modular multilevel converter topology are enabled to meet the energy balance of the DC power flow controller.

686 more elements

Modular multilevel converter with multi-port dc power flow control and control method thereof

Life of this patent

Modular multilevel converter with multi-port dc power flow control and control method thereof

Life of this patent

Abstract

Claims12