Modular multilevel DC/DC converter for HVDC applications
View Patent ↗A DC converter is suitable for continuous operation for connecting high-voltage DC networks having different voltages. The DC converter has a first partial converter and a second partial converter, which are connected in series with each other, forming a converter series circuit. The converter series circuit extends between the DC terminals of a DC connection. The second partial converter extends between the DC terminals of a second DC connection. The first partial converter and the second partial converter are connected to each other via a power exchange device, such that the exchange of electrical power between the first partial converter and the second partial converter is made possible via the power exchange device.
1. A DC-to-DC voltage converter for connecting HVDC grids having different voltages carried on first DC voltage terminals and on second DC voltage terminals, respectively, the DC-to-DC voltage converter comprising:
a first partial converter and a second partial converter connected in series with one another to form a converter series circuit;
said converter series circuit extending between the first DC voltage terminals carrying a first DC voltage and said second partial converter extending between the second DC voltage terminals carrying a second DC voltage; and
a power exchange device connecting said first partial converter and said second partial converter to one another for enabling an exchange of electric power between said first partial converter and said second partial converter via said power exchange device;
wherein at least one of said first and second partial converters is a self-commutated converter having a power semiconductor switch to be selectively switched on and off;
and wherein at least one of said first partial converter and said second partial converter are in each case modular multi-level converters, which have converter arms having a series circuit composed of two-pole sub modules, wherein each sub module is equipped with at least one energy storage device and power semiconductor switches; and
wherein each said sub module is at least one of a full-bridge circuit, a double-module circuit, or a half-bridge circuit.
2. The DC-to-DC voltage converter according to claim 1 , wherein each of said first and second partial converters forms an AC voltage connection and said power exchange device connects said AC voltage connection of said first partial converter to said AC voltage connection of said second partial converter.
3. The DC-to-DC voltage converter according to claim 2 , wherein said power exchange device is a transformer.
4. The DC-to-DC voltage converter according to claim 3 , wherein said transformer has a primary winding connected to said AC voltage connection of said first partial converter and a secondary winding connected to said AC voltage connection of said second partial converter.
5. The DC-to-DC voltage converter according to claim 1 , wherein each one of said first and second partial converters has at least one phase module that forms two series-connected converters, and a potential point between said converter arms of a respective said phase module forms an AC voltage phase of an AC voltage connection.
6. The DC-to-DC voltage converter according to claim 5 , wherein each one of said first and second partial converters has three phase modules and AC voltage connections of said phase modules are coupled to one another via a three-phase transformer.
7. The DC-to-DC voltage converter according to claim 5 , wherein at least one of said first and second partial converters forms two phase modules and the AC voltage connection of said phase modules is coupled to the AC voltage connection of a respectively other said partial converter via a single-phase transformer.
8. The DC-to-DC voltage converter according to claim 5 , wherein each said partial converter or at least one of said first and second partial converters has only one phase module with the AC voltage phase assigned to a capacitor auxiliary phase, the capacitor auxiliary phase is connected to a DC voltage terminal of the first or second DC voltage connection via a capacitor, and wherein the AC voltage phase and the capacitor auxiliary phase assigned thereto are connected to one another via a winding of a single-phase transformer.
9. The DC-to-DC voltage converter according to claim 1 , wherein each converter arm of said first partial converter includes bidirectionally blocking sub modules and sub modules with half-bridge circuits.
10. The DC-to-DC voltage converter according to claim 1 , wherein said second partial converter exclusively has sub modules with half-bridge circuits, wherein a translation ratio between the first DC voltage connection and the second DC voltage connection is greater than or equal to 2.
11. The DC-to-DC voltage converter according to claim 1 , wherein a number (n vB ) of bidirectionally blocking sub modules is given by
n
VB
=
U
DC
2
2
Um
,
wherein U DC2 is a nominal voltage of the second DC voltage grid and Um is a voltage dropping across the energy storage device of the bidirectionally blocking sub module.
12. The DC-to-DC voltage converter according to claim 11 , wherein a number (n HB ) of sub modules with half-bridge circuits in each converter arm of the first partial converter is given according to the equation
n
HB
=
U
DC
1
U
m
(
2
u
¨
-
3
2
u
¨
)
,
wherein U DC1 is the nominal voltage of the DC voltage grid present at the first DC voltage connection and üis the translation ratio of the DC-to-DC voltage converter.
13. The DC-to-DC voltage converter according to claim 1 , wherein said power exchange device has an autotransformer connected to said first and second partial converters via devices for isolating DC voltage potentials.
14. The DC-to-DC voltage converter according to claim 1 , wherein a dielectric strength of said converter series circuit formed of said first and second partial converters is geater than a dielectric strength of a single partial converter of said converter series circuit.
15. A DC-to-DC voltage converter for connecting HVDC grids having different voltages, the DC-to-DC voltage converter comprising:
a first partial converter and a second partial converter connected in series with one another to form a converter series circuit;
said converter series circuit extending between DC voltage terminals of a first DC voltage connection and said second partial converter extending between DC voltage terminals of a second DC voltage connection; and
a power exchange device connecting said first partial converter and said second partial converter to one another for enabling an exchange of electric power between said first partial converter and said second partial converter via said power exchange device;
wherein at least one of said first and second partial converters is a self-commutated converter having a power semiconductor switch to be selectively switched on and off;
wherein at least one of said first partial converter and said second partial converter are in each case modular multi-level converters, which have converter arms having a series circuit composed of two-pole sub modules;
wherein each sub module is equipped with at least one energy storage device and power semiconductor switches, and each said sub module is a full-bridge circuit, a double-module circuit, or a half-bridge circuit; and
wherein a number (n vB ) of bidirectionally blocking sub modules is given by
n
VB
=
U
DC
2
2
Um
,
wherein U DC2 is a nominal voltage of the second DC voltage grid and Um is a voltage dropping across the energy storage device of the bidirectionally blocking sub module.
16. The DC-to-DC voltage converter according to claim 15 , wherein a number (n HB ) of sub modules with half-bridge circuits in each converter arm of said first partial converter is given according to the equation
n
HB
=
U
DC
1
U
m
(
2
u
¨
-
3
2
u
¨
)
,
wherein U DC1 is the nominal voltage of the DC voltage grid present at the first DC voltage connection and U is the translation ratio of the DC-to-DC voltage converter.