Method in a cascaded two-level converter, control device and computer program products
View Patent ↗A method for providing a switching order signal to a cell of a cascaded two-level converter is provided. The cell includes a capacitor parallel-connected with two series-connected semiconductor devices. The cascaded two-level converter includes two or more of the cells cascade connected and arranged in a phase, divided into two phase arms, between a first pole and a second pole of a direct voltage side. The method includes measuring voltages of the capacitor of the cell; calculating a compensated voltage reference based on a voltage reference and the measured voltages of the capacitors, wherein the voltage reference corresponds to a desired ac current to be output on an ac-side; using the compensated voltage reference to calculate a switching order signal, and providing the switching order signal to the cells.
1. A method for providing a switching order signal to a cell of a cascaded two-level converter arranged to convert between alternating current and direct current, said cell comprising a capacitor parallel-connected with two series-connected semiconductor devices, which each in turn comprises at least one respective transistor with an anti-parallel connected diode, said cascaded two-level converter comprising at least two of said cells cascade connected and arranged in a phase between a first pole and a second pole of a direct voltage side, said phase being divided into two phase arms, a first phase arm being connected between said first pole and an ac phase output, and a second phase arm being arranged between said ac phase output and said second pole, comprising the steps of:
measuring voltages of said capacitors of said at least two cells;
calculating a compensated voltage reference based on a voltage reference and said measured voltages of said capacitors, said voltage reference representing the fundamental frequency output voltage on the ac phase output and corresponding to a desired ac current to be output on an ac-side;
using said compensated voltage reference to calculate a switching order signal for said cell; and
providing said switching order signal to said cell,
wherein said step of calculating a compensated voltage reference comprises utilizing the equation:
r
=
u
v
ref
+
(
u
cp
-
u
cn
)
/
4
(
u
cp
+
u
cn
)
/
4
,
wherein r is said compensated voltage reference, u v ref is said voltage reference, u cn is the sum of the cell capacitor voltages of said first phase arm and u cn is the sum of the cell capacitor voltages of said second phase arm.
2. The method as claimed in claim 1 , wherein it is performed for each of said at least two cells said step of using said compensated voltage reference to calculate a switching order signal for the respective cell and of providing said switching order signal to the respective cell.
3. The method as claimed in claim 1 , wherein said step of calculating a compensated voltage reference comprises calculating the sum of the cell capacitor voltages of said first and second phase arms.
4. The method as claimed in claim 1 , wherein said step of calculating the switching order signal is based on pulse-width modulation using a carrier wave signal.
5. The method as claimed in claim 4 , wherein said carrier wave signal is chosen so as to reflect actual network conditions.
6. Control device for controlling switching instants of a cell of a cascaded two-level converter arranged to convert between alternating current and direct current, said cell comprising a capacitor parallel-connected with two series-connected semiconductor devices, which each in turn comprises at least one respective transistor with an anti-parallel connected diode, said cascaded two-level converter comprising at least two of said cells cascade connected and arranged in a phase between a first pole and a second pole of a direct voltage side, said phase being divided into two phase arms, a first phase arm being connected between said first pole and an ac phase output, and a second phase arm being arranged between said ac phase output and said second pole, including means for implementing the method as claimed in claim 1 .
7. The control device as claimed in claim 6 , wherein said means comprises a processing means, such as a computer or microprocessor.
8. A computer program product stored on a computer readable storage medium and loadable into the internal memory of a computer controlling a cascaded two-level converter, comprising software code portions for carrying out the method as claimed in claim 1 when said product is run on said computer.
9. The method as claimed in claim 2 , wherein said step of calculating a compensated voltage reference comprises calculating the sum of the cell capacitor voltages of said first and second phase arms.
10. The method as claimed in claim 2 , wherein said step of calculating the switching order signal is based on pulse-width modulation using a carrier wave signal.
11. The method as claimed in claim 3 , wherein said step of calculating the switching order signal is based on pulse-width modulation using a carrier wave signal.