Arrangement, method and computer program product concerned with tapping of power from a DC power line to an AC power line
View Patent ↗An arrangement for tapping power from a DC power line to an AC power line includes power transfer modules between two DC potentials, each including a first branch with a string of converter cells in parallel with a second branch including a capacitor and being connected to an AC phase. There is at least one control unit that controls the arrangement considering one or more of a) distributing appropriate AC and DC voltages in converter output voltages of all series connected modules, b) maintaining/setting cell capacitor voltages in specific range and allowing boost mode operation, c) performing possible balancing of the introduced capacitor and d) employing an alternate approach of using passive filters to mitigate low order harmonics.
1. An arrangement for converting between alternating current, AC, and direct current, DC, said arrangement comprising:
a number of power transfer modules connected in series between two DC potentials, where one DC potential is the voltage of a DC power line and each power transfer module comprises a first branch comprising a string of converter cells, wherein a cell provides a voltage contribution of a cell capacitor or zero, said first branch being connected in parallel with a second branch comprising a capacitor and a secondary winding of a transformer, the primary winding of which is connected to a corresponding AC phase of an AC network;
at least one control unit comprising:
an adding unit configured to:
obtain an AC voltage reference and a DC dependent control contribution, said DC dependent control contribution being based on a DC voltage term depending on the number of power transfer modules and on the difference between the two DC potentials; and
add the DC dependent control contribution to the AC voltage reference for forming a first control expression; and
a dividing unit configured to:
obtain the first control expression;
obtain a cell voltage reference sum for use in a second control expression, where the cell voltage reference sum is a sum of cell capacitor voltage references provided for a corresponding power transfer module; and
divide the first control expression with the second control expression in order to obtain a reference voltage signal for use in PWM control of the cells of the string of a corresponding power transfer module, where the signal is compared with a triangular carrier wave.
2. The arrangement according to claim 1 , wherein said DC voltage term is used as the DC dependent control contribution.
3. The arrangement according to claim 2 , wherein a string of converter cells of a power transfer module forms a phase leg and the control unit further comprising:
a processing unit configured to obtain a phase leg current and process the phase leg current for obtaining a third control term only based on harmonic components of the phase leg current;
another further subtracting unit configured to obtain a second branch voltage and the third control term and subtract the third control term from the second branch voltage in order to obtain a zero sequence reference voltage to be used for forming said reference voltage signal.
4. The arrangement according to claim 1 , wherein the adding unit is further configured to add a zero sequence reference voltage to the DC dependent control contribution and the AC voltage reference for forming the first control expression, the control unit further comprising a further subtracting unit configured to obtain a second branch voltage without fundamental frequency component, the DC voltage term and subtract the second branch voltage without fundamental frequency component from the DC voltage term in order to obtain the DC dependent control contribution.
5. The arrangement according to claim 1 , wherein a string of converter cells of a power transfer module forms a phase leg and the control unit further comprising:
a processing unit configured to obtain a phase leg current and process the phase leg current for obtaining a third control term only based on harmonic components of the phase leg current; and
another further subtracting unit configured to obtain a second branch voltage and the third control term and subtract the third control term from the second branch voltage in order to obtain a zero sequence reference voltage to be used for forming said reference voltage signal.
6. The arrangement according to claim 5 , wherein the phase leg current is a zero sequence component of the phase leg current and the second branch voltage is a zero sequence component of the second branch voltage.
7. The arrangement according to claim 5 , wherein the processing unit comprises a low pass filter and a subtracting unit, the low pass filter being connected to a negative input of the subtracting unit, where the phase leg current is received by an input of the low pass filter and a positive input of the subtracting unit.
8. The arrangement according to claim 1 , further comprising at least one full-bridge cell connected between the DC power line and the series connected power transfer modules, the control unit further comprising a control element controlling the full-bridge cell for counteracting common mode harmonic voltages generated in the phase legs.
9. The arrangement according to claim 1 , further comprising a reactor in each power transfer module, said reactor being connected in the first branch together with the string of cells.
10. The arrangement according to claim 1 , further comprising a surge arrester connected in series with all the primary windings.
11. The arrangement according to claim 1 , wherein the cells in the string of cells comprise half-bridge converter cells.
12. Arrangement according to claim 1 , wherein the cells in the string of cells comprise full-bridge converter cells.
13. Arrangement according to claim 1 , wherein the number of modules are more than two.
14. A method of controlling a power transfer module for converting between alternating current, AC, and direct current, DC, the power transfer module being a power transfer module in a group of power transfer modules connected in series between two DC potentials, where one DC potential is the voltage of a DC power line and each power transfer module comprises a first branch comprising a string of converter cells, wherein a cell provides a voltage contribution of a cell capacitor or zero, said first branch being connected in parallel with a second branch comprising a capacitor and a secondary winding of a transformer, the primary winding of which is connected to a corresponding AC phase of an AC network, the method comprising:
obtaining an AC voltage reference;
obtaining a DC dependent control contribution, said DC dependent control contribution being based on a DC voltage term depending on the number of power transfer modules and the difference between the two DC potentials;
adding the DC dependent control contribution to the AC voltage reference for forming a first control expression;
obtaining a cell voltage reference sum for use in a second control expression, where the cell voltage reference sum is a sum of cell capacitor voltage references provided for a corresponding power transfer module;
dividing the first control expression with the second control expression for obtaining a reference voltage signal for PWM control; and
PWM controlling the cells of the string of the power transfer module using said reference voltage signal as a control signal in PWM control of these cells where the signal is compared with a triangular carrier wave.
15. The method according to claim 14 , wherein said DC voltage term is used as the DC dependent control contribution.
16. The method according to claim 14 , wherein the adding of the DC dependent control contribution to the AC voltage reference comprises adding the DC dependent control contribution to a sum of the AC voltage reference and a zero sequence reference voltage and further comprising processing the DC voltage term through subtracting a second branch voltage without fundamental frequency component from the DC voltage term in order to obtain the DC dependent control contribution.
17. The method according to claim 14 , wherein a string of converter cells of a power transfer module forms a phase leg and further comprising:
obtaining a second branch voltage;
obtaining a phase leg current;
processing the phase leg current for obtaining a third control term only based on harmonic components of the phase leg current; and
subtracting the third control term from the second branch voltage in order to obtain a zero sequence reference voltage for use in forming the reference voltage signal.
18. The method according to claim 17 , wherein the second branch voltage is a zero sequence component of the second branch voltage and the phase leg current is a zero sequence component of the phase leg current.
19. The method according to claim 14 , the arrangement further comprising at least one full-bridge cell connected between the DC power line and the series connected power transfer modules, the method further comprising controlling the full-bridge cell for counteracting common mode harmonic voltages generated in the phase legs.
20. A computer program product for controlling a power transfer module for converting between alternating current, AC, and direct current, DC, the power transfer module being a power transfer module in a group of power transfer modules connected in series between two DC potentials, where one DC potential is the voltage of a DC power line and each power transfer module comprises a first branch comprising a string of converter cells, wherein a cell provides a voltage contribution of a cell capacitor or zero, said first branch being connected in parallel with a second branch comprising a capacitor and a secondary winding of a transformer, the primary winding of which is connected to a corresponding AC phase of an AC network, the computer program product comprising a non-transitory data carrier with computer program code configured to cause a control unit to:
obtain an AC voltage reference;
obtain a DC dependent control contribution, said DC dependent control contribution being based on a DC voltage term depending on the number of power transfer modules and the difference between the two DC potentials;
add the DC dependent control contribution to the AC voltage reference for forming a first control expression;
obtain a cell voltage reference sum for use in a second control expression, where the cell voltage reference sum is a sum of cell capacitor voltage references provided for a corresponding power transfer module;
divide the first control expression with the second control expression for obtaining a reference voltage signal for PWM control; and
PWM control the cells of the string using said reference voltage signal as a control signal in PWM control of these cells where the signal is compared with a triangular carrier wave.