EP3973626A1 - Modular cascaded charge-pump converter - Google Patents
Modular cascaded charge-pump converterInfo
- Publication number
- EP3973626A1 EP3973626A1 EP19727319.6A EP19727319A EP3973626A1 EP 3973626 A1 EP3973626 A1 EP 3973626A1 EP 19727319 A EP19727319 A EP 19727319A EP 3973626 A1 EP3973626 A1 EP 3973626A1
- Authority
- EP
- European Patent Office
- Prior art keywords
- cell
- converter
- energy storage
- storage element
- charge
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Pending
Links
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- 238000004590 computer program Methods 0.000 claims description 14
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Classifications
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M3/00—Conversion of dc power input into dc power output
- H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
- H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
- H02M3/06—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using resistors or capacitors, e.g. potential divider
- H02M3/07—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using resistors or capacitors, e.g. potential divider using capacitors charged and discharged alternately by semiconductor devices with control electrode, e.g. charge pumps
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/0067—Converter structures employing plural converter units, other than for parallel operation of the units on a single load
- H02M1/007—Plural converter units in cascade
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/0067—Converter structures employing plural converter units, other than for parallel operation of the units on a single load
- H02M1/0077—Plural converter units whose outputs are connected in series
Definitions
- the present invention generally relates to converters. More particularly the present invention relates to a converter for converting between a first and a second voltage.
- a conventional high voltage direct current (HVDC) converter station has high-voltage transformers that steps-up/down an AC voltage to/from the voltage of a DC-bus.
- a converter valve is then used to generate a DC voltage in the transmitting end and an AC voltage on the receiving end.
- an inverter For solar power plants an inverter is used to generate an AC voltage which is then stepped up to a higher voltage using a conventional transformer.
- Transforming a low voltage to high voltage is usually done with a transformer.
- the coils of the transformer must be tightly coupled to one another via a magnetic circuit in the form of a magnetic core and use large core and coil wires. It is also not possible to convert a DC voltage using a transformer. The requirement of tight coupling gives challenging isolation problems when the transfer ratio is high (high output/input voltage).
- WO 93/20610 discloses a transformerless power conversion system that includes a plurality of series connected capacitors, a load circuit connected to a plurality of capacitors, where the load circuit charges a plurality of capacitors from a voltage source at a predetermined voltage.
- the system also includes a circuit that reverses the polarity of the accumulated charge in selected ones of the plurality of capacitors.
- the polarity reversal circuit comprises a plurality of inductance circuits that can be coupled by switching to a corresponding capacitor different from the selected capacitors, to form a resonant circuit for reversing the polarity of the charge accumulated in that capacitor.
- WO 96/16468 discloses a DC-to-DC voltage convertor that is made up of a capacitor array having plural capacitor elements and a plurality of switches which are switchable between at least two states. When the switches are switched in the first state, the capacitor elements are connected in series, and when the switches are connected in the second state, the capacitor elements are connected in parallel.
- the DC-to-DC voltage convertor may be configured as a step-down converter or a step-up converter.
- Another type of converter is the Marx DC-to-DC converter, which is for instance described by Etienne Veilleux in a thesis named”DC Power Flow Controller and Marx DC-DC Converter for Multiterminal HVDC System”, Department of Electrical & Computer Engineering, McGill University, Montreal, Canada, 2013.
- the invention addresses at least some of the above-mentioned problems.
- the present invention is directed towards solving at least some of the above-mentioned problems.
- a converter converting between a first voltage and a second voltage, the converter having a first connection port for the first voltage, a second connection port for the second voltage and a stack of cells, where each cell comprises an energy storage element.
- the cells are configured to sequentially move a charge between the energy storage elements from one end of the stack to another end of the stack, where the sequential moving of a charge comprises a cell being configured to move the charge between itself and a neighbour cell.
- a computer program product for controlling conversion between a first voltage on a first connection port and a second voltage on a second connection port of a converter comprising a stack of cells, each cell comprising an energy storage element, the computer program product comprising a data carrier with computer program code configured to cause a control unit to, when the computer program code is loaded into the control unit
- control the converter to sequentially move a charge between the energy storage elements from one end of the stack to another end of the stack comprises controlling a cell to move the charge between itself and a neighbour cell in the stack.
- Each cell may additionally comprise an inductor.
- a cell when moving the charge between itself and a neighbour cell in the stack, is configured to form an LC circuit comprising the own inductor and own energy storage element together with the energy storage element of the neighbor cell
- the moving of the charge from a cell to a neighbour cell comprises the cell forming an LC circuit comprising the own inductor and own energy storage element together with the energy storage element of the neighbour cell.
- a cell when moving the charge between the cells, is configured to connect a first charging branch comprising the own energy storage element in parallel with a second charging branch comprising the energy storage element of the neighbour cell and to remove the parallel connection when the charge has been transferred from one of the energy storage elements to the other.
- the moving of the charge from a cell to a neighbour cell comprises connecting a first charging branch comprising the own energy storage element in parallel with a second charging branch comprising the energy storage element of the neighbour cell and removing the parallel connection when the charge has been transferred.
- the cell is further controllable to connect the own energy storage element in series with the energy storage element of the neighbour cell after the charge has been moved.
- the method further comprises connecting the own energy storage element of the cell in series with the energy storage element of the neighbour cell after the charge has been moved. It is possible that the own energy storage element of the cell is connected in series with the energy storage element of the neighbour cell before the charge is transferred.
- the cell may be further controllable to remove the series connection when connecting the charging branches in parallel with each other.
- the method further comprises removing the series connection when connecting the charging branches in parallel with each other.
- the first converter connection port may be provided at one end of the stack, while the second converter connection port may be provided across the whole of the stack.
- a cell may comprise a first and a second semiconductive element, where a first end of the own energy storage element is connected to a first end of the energy storage element of the neighbour cell via a first interconnecting branch comprising the first semiconductive element and a second end of the own energy storage element is connected to a second end of the energy storage element of the neighbour cell via a second interconnecting branch comprising the second semiconductive element in series with the own inductor.
- a least one of the first and second semiconductive element is furthermore a controllable semiconductive element, such as a controllable unidirectional conducting element or a controllable bidirectional conducting element.
- the first semiconductive element may be an uncontrollable unidirectional semiconductive element and the second semiconductive element may be a controllable semiconductive element or vice versa.
- a cell may furthermore comprise a third interconnecting branch connected between the second end of the own energy storage element and the first end of the energy storage element of the neighbour cell, where the third interconnecting branch comprises a third semiconductive element, which may be a non-controllable semiconductive element such as a non- controllable unidirectional conducting element, or a controllable semiconductive element, such as controllable unidirectional conducting element or a controllable bidirectional conducting element.
- a third semiconductive element which may be a non-controllable semiconductive element such as a non- controllable unidirectional conducting element, or a controllable semiconductive element, such as controllable unidirectional conducting element or a controllable bidirectional conducting element.
- a cell may additionally comprise a fourth interconnecting branch connected between the first end of the own energy storage element and second end of the energy storage element of the neighbour cell, where the fourth interconnecting branch may comprise a fourth semiconductive element that may be a controllable semiconductive element, such as a controlled unidirectional and bidirectional conducting element.
- the energy storage elements may be capacitors.
- the capacitance of the capacitor of this cell may be k times the capacitance C of the capacitor at the cell at the top of the stack.
- the inductance of the inductor of the cell at position k may then be the inductance of the inductor of the cell at the top divided by k.
- the converter may additionally comprise a number of sections, where the cells in the stack are provided in different sections and the sections have a different number of parallel cells.
- the number of parallel cells in the sections may for instance vary by one cell from one section to the next from one end in the stack to the other, such as from the top of the stack towards the bottom.
- the converter may further comprise a control unit controlling the cells.
- the invention has a number of advantages. Conversion is achieved in one step without the use of a transformer. The converter is also power and cost efficient and the voltage stress on the output is low.
- fig. l schematically shows a first realization of a converter having a first and a second converter connection port and a stack of cells of a first type
- fig. 2 schematically shows a second realization of a converter having a first and a second converter connection port and a stack of cells of a second type
- fig. 3 schematically shows a third realization of a converter having a first and a second converter connection port and a stack of cells of a third type
- fig. 4 schematically shows the structure of the cell according to the first type comprising an energy storage element, an inductor as well as a first, second and third semiconductive element
- fig. 5 schematically shows an LC circuit created through the cell of fig. 4 and an energy storage element of a neighbour cell
- fig. 6 schematically shows a current and voltage of the cell in fig. 4 as well as a charge being transferred between this cell and a neighbour cell
- fig. 7 shows a method step of operating the converter according to the first realization
- fig. 8 schematically shows a number of steps for moving a charge between a controlled cell and its neighbour
- fig. 9a schematically shows the charging of an energy storage element of a first cell in the stack of the first converter realization
- fig. 9b schematically shows moving of a charge from the first to the second cell in the first converter realization
- fig. 10a schematically shows currents circulating in the stack when moving the charge together with the output current from the converter of the first converter realization
- fig. 10b schematically shows a relationship between the values of the inductors and energy storage elements in the cells of the first type of converter realization
- fig. 11 schematically shows a variation of the first converter realization for providing negative output voltages and where the first and second converter connection ports are placed at the opposite ends of the stack compared with fig. 1,
- fig. 12 shows a fourth type of converter realization comprising cells of a fourth type
- fig. 13 schematically shows a computer program product in the form of a CD ROM disc with a computer program code performing the functionality of the control units controlling the converter.
- Fig. 1 shows a first realization of a converter 10 for converting between a first voltage Vi and a second voltage V2.
- Each cell comprises an energy storage element Cl, C2, C3, C4.
- Each cell may additionally comprise an inductor Li, L2, L3 and L4.
- the cells are connected in cascade.
- the converter 10 comprises a first converter connection port CPi having a first and a second converter connection terminal Ti and T2 and a second converter connection port CP2 having a third and a fourth converter connection terminal T3 and T4.
- the first converter connection port CPi has a first voltage Vi between the first and second converter connection terminals Ti and T2 while the second converter connection port CP2 has a second voltage V2 between the third and fourth converter connection terminals T3 and T4.
- the voltages may with advantage be direct current (DC) voltages.
- the first converter connection port CPi may additionally be provided at one end of the stack, while the second converter connection port CP may be provided across the whole of the stack.
- first connection port inductor LCPi and first connection port capacitor CCPi where the first connection port inductor LCPi may be a smoothing inductor and the first connection port capacitor CCPi is also a first connection port energy storage element, i.e. an energy storage element associated with and connected to the first converter connection port CPi.
- a first positive end of the first converter connection port capacitor CCPi is connected to the first converter connection terminal Ti via the first converter connection port inductor LCPi, while a second negative end of the first converter connection port capacitor CCPi is directly connected to the second converter terminal T2 of the first converter connection port CPi.
- the first converter connection port capacitor CCPi is connected in series with the first converter connection port inductor LCPi between the first and second converter connection terminals Ti and T2 of the first converter connection port CPi.
- the fourth converter connection terminal T4 of the second converter connection port CP2 is in this converter realization joined with the second converter connection terminal T2 of the first converter connection port CP2.
- the second end of the first converter connection port capacitor CCTi is also connected to the fourth converter connection terminal T4.
- the converter 10 comprises a stack of cascaded cells, where each cell is according to a first type, which is generally depicted in fig. 4.
- a cell Cn which may be an nth cell in the stack may comprise a first cell connection port CEPi and a second cell connection port CEP2, where the first cell connection port CEPi has a first cell connection terminal TCEi and a second cell connection terminal TCE2 and the second cell connection port CEP2 has a third cell connection terminal TCE3 and a fourth cell connection terminal TCE4.
- a first interconnecting branch IBi comprising a first semiconductive element.
- IBi comprising a first semiconductive element
- the second interconnecting branch IB2 connected between the second cell connection terminal TCE2 and the fourth cell connection terminal TCE4.
- the second interconnecting branch IB2 comprises a cell inductor Ln and a second semiconductive element. At least one of the first and second semiconductive element is a controllable semiconductive element.
- the first semiconductive element Dn is an uncontrollable unidirectional semiconductive element in the form of a diode and the second semiconductive element Sni is a first switch Sni that may be a controllable semiconductive element, such as a controllable unidirectional conducting element like a thyristor or a controllable bidirectional conducting element like a transistor.
- the first semiconductive element Dn is a diode having an anode connected to the first cell connection terminal TCEi and a cathode connected to the third cell connection terminal TCE3, while the first switch Sni is realized as a thyristor.
- the anode of the thyristor is in this case connected to the fourth cell connection terminal TCE4, while the cathode is connected to the second cell connection terminal TCE2.
- the connection of the first switch Sni to one of the cell connection terminals is also made via the inductor Ln and in the example given here the first switch Sni is connected to the second cell connection terminal TCE2 via the cell inductor Ln.
- an energy storage element in the cell in this case realized as a cell capacitor Cn, where a first positive end of the cell capacitor Cn is connected to the third cell connection terminal TCE3 and a second negative end of the cell capacitor Cn is connected to the fourth cell connection terminal TCE4.
- third interconnecting branch IB3 interconnecting the first and fourth cell connection terminals TCEi and TCE4, which third interconnecting branch IB3 also comprises a third semiconductive element, which may be a non-controllable unidirectional conducting element or a controllable semiconductive element, such as a controllable unidirectional conducting element like a thyristor or a controllable bidirectional conducting element like a transistor.
- a controllable semiconductor device in the form of a second switch Sn2, for instance using a transistor such as an
- IGBT Integrated-Gate Bipolar Transistor
- a similar neighbour cell may be connected with its third cell connection terminal to the first cell connection terminal TCEi of the cell
- the first end of the own energy storage element Cn of the cell is connected to a first end of the energy storage element of the neighbour cell via the first interconnecting branch IBi that comprises the first semiconductive element Dn and the second end of the own energy storage element Cn is connected to a second end of the energy storage element of the neighbour cell via the second interconnecting branch IB2 that comprises the second semiconductive element Sni in series with the own inductor Ln.
- the third interconnecting branch IB3 is connected between the second end of the own energy storage element Cn and the first end of the energy storage element of the neighbour cell.
- first cell connection terminal of a first cell CEi is connected to a junction between the first converter connection port capacitor CCPi and the first converter connection port inductor LCPi, while the second cell connection terminal of the first cell CEi is connected to the first and fourth converter connection terminals T2 and T4.
- the third cell connection terminal of the first cell CEi is further connected to the first cell connection terminal of a second cell CE2, while the fourth cell connection terminal of the first cell CEi is connected to the second cell connection terminal of the second cell CE2.
- the third and fourth cell connection terminals of the second cell CE2 are in the same way connected to the first and second cell connection terminals of a third cell CE3, which in turn has third and fourth cell connection terminals connected to first and second cell connection terminals of a fourth cell CE4.
- the fourth cell is the last cell in the stack and the third cell connection terminal of this fourth cell CE4 is connected to the third converter connection terminal T3 of the second converter connection port CP2 via a second converter connection port inductor LCP2, which may be a smoothing inductor.
- the first, second and third semiconductive elements of the first cell CEi are a diode Di and a first and second switch Sn and S12
- the first, second and third semiconductive elements of the second cell CE2 are a diode D2 and a first and second switch S21 and S22
- the first, second and third semiconductive elements of the third cell are a diode D3 and a first and second switch S31 and S32
- the first, second and third semiconductive elements of the fourth cell are a diode D4 and a first and second switch S41 and S42.
- the converter in fig. 1 may be functioning as step up converter in which the first voltage Vi is input to the first converter connection port CPi and the second voltage V2 is obtained at the second converter connection port CP2, where the second voltage V2 is higher than the first voltage Vi.
- Fig. 2 schematically shows a second converter realization with a similar structure that may be used as a unidirectional DC/DC step-down converter, where an input voltage V2 is applied at the second converter connection port CP2 and an output voltage Vi is obtained at the first converter connection port CPi.
- the second converter connection terminal T2 is used as a connection terminal of both the first and second converter connection ports.
- the first semiconductive element in the first interconnecting branch IBi in this case is realized as a switch, here in the form of a controllable unidirectional conducting element, in this case exemplified by a thyristor with the anode pointing upwards in the stack and the cathode pointing downwards in the stack towards the first converter connection port CPi.
- the second semiconductive element in the second interconnecting branch is in turn a first non-controllable unidirectional conducting element, here in the form of a diode with the cathode connected to the second end of the cell capacitor and the anode connected to the cell inductor.
- semiconductive element in the third interconnecting branch has the same realization as a second non-controllable unidirectional conducting element, here in the form of a diode, with the anode connected to the fourth cell connection terminal and the cathode connected to the first cell connection terminal.
- the first cell has a switch Si and a first and second diode Dn and Di2 as the first, second and third semiconductive element
- the second cell has a switch S2 and a first and second diode D21 and D22 and as the first, second and third semiconductive element
- the third cell has a switch S3 and a first and second diode D31 and D32 as the first, second and third semiconductive element
- the fourth cell has a switch S4 and a first and second diode D41 and D42 as the first, second and third semiconductive element
- the fifth cell has a switch S5 and a first and second diode D51 and D52 as the first, second and third semiconductive element.
- control unit 12 controlling the cells.
- the control unit 12 provides control signals to the switches Si, S2, S3, S4 and S5 via their gate units. These control signals are in the gate units adapted to a voltage level required by the switches.
- Fig. 3 schematically shows a third realization of the converter 10 of similar structure as both the two previous converter realizations.
- This third converter realization may be used both as a step up and a step-down converter, where in each case each semiconductive element is realized as a switch.
- the first and second semiconductive elements may in this case both be realized as first and second switches, each in the form of an anti parallel thyristor pair, while the third semiconductive switch may be realized as a transistor with anti-parallel diode. It is possible that also the third semiconductive element is realized as an anti-parallel thyristor pair.
- the first and second semiconductive elements may also be realized as transistors with anti-parallel diodes.
- the first cell has a first, second and third switch Sn, S12 and S13
- the second cell has a first, second and third switch S21, S22 and S23
- the third cell has a first, second and third switch S31, S32 and S33
- the fourth cell has a first, second and third switch S41, S42 and S43
- the fifth cell has a first, second and third switch S51, S52 and S53 as the first, second and third semiconductive element.
- switches were realized as single or pairs of anti-parallel thyristors or as IGBTs with or without anti-parallel diodes. It should be realized that other types of switches are possible, such as Bimode Insulated-Gate Transistors (BiGTs) and Reverse Conducting (RC)- IGBTs.
- BiGTs Bimode Insulated-Gate Transistors
- RC Reverse Conducting
- fig. 5 schematically shows an LC circuit formed by a controlled cell, i.e. a cell having its switches being operated, and the energy storage element of a neighboring cell
- fig. 6 shows the current circulating in the LC circuit and a charge being transmitted to the energy storage element of the controlled cell from the energy storage element of the neighbour cell.
- a voltage is applied on one of the converter connection ports at one end of the stack and the resulting current is used to charge the cell at this end of the stack.
- a sequential moving of a charge is made between the energy storage elements of the cells from one end of the stack to another end of the stack.
- a sequential moving of a charge from the above-mentioned end of the stack to the other end of the stack in order to provide an output voltage on the other converter connection port When moving a charge from one cell to another, a cell CEn is controlled to cause the move to be made in relation to a neighbour cell. In the sequential moving of a charge a cell is thereby configured to move the charge between itself and a neighbour cell.
- the move may more particularly involve connecting a first charging branch CBi in parallel with a second charging branch CB2, where the first charging branch CBi comprises the energy storage element or capacitor Cn of the controlled cell CEn and the second charging branch comprises the energy storage element or capacitor Cn-i of the neighbour cell.
- One of the charging branches may also comprise the inductor Ln of the controlled cell.
- the two branches may form an LC circuit.
- the capacitor Cn and diode Dn of the controlled cell CEn are placed in the first charging branch CBi, while the inductor Ln and first switch Sni of the controlled cell CEn and capacitor Cn-i of the neighbour cello are placed in the second charging branch CB2.
- the controlled cell When moving a charge between itself and a neighbour cell the controlled cell may thus be configured to form an LC circuit comprising the own inductor Ln and own energy storage element Cn together with the energy storage element Cn-i of the neighbour cell. It can additionally be seen that the controlled cell may be configured to connect a first charging branch comprising the own energy storage element in parallel with a second charging branch comprising the energy storage element Cn-i of the neighbour cell.
- the first or the second charging branch may additionally comprise the own inductor Ln.
- the capacitor delivering the charge has a voltage that is higher than the voltage of the capacitor receiving the charge. If for instance a capacitor in the cell at a lower end of the stack is to transfer a charge to the capacitor of a controlled cell immediately above the cell with the charge delivering capacitor, then the first switch Sni of the controlled cell is turned on, which causes the charge delivering capacitor Cn-i of the neighboring cell and the inductor Ln of the controlled cell CEn to be connected in parallel with the capacitor Cn of the controlled cell CEn. There is thereby also formed an LC circuit comprising the two capacitors Cn and Cn-i and the inductor Ln.
- the diode Dn in the controlled cell CEn will be conducting. A current I is thereby delivered to the charge receiving capacitor Cn and thereby the voltage Un of this capacitor Cn increases. At the same time the voltage across the charge delivering capacitor will sink. However, the rate of change of the voltage may differ based on the capacitor sizes.
- the current I stops increasing when the voltages have become equal and the inductor Ln is then charged to its maximum. The current continues to flow until it reaches zero by which the diode Dn blocks any current return. Thereby a charge Q is moved from the charge delivering capacitor Cn-i to the charge receiving capacitor Cn.
- the controlled cell CEn may additionally be configured to remove the parallel connection when the charge has been transferred from one of the capacitors to the other. After the move of the charge the cell may further be controllable, via the second switch Sn2, to connect the own energy storage element Cn in series with the energy storage element of the neighbour cell.
- the first converter connection port CPi is an input port with an applied input voltage Vi, which as an example is a DC voltage
- the second converter connection port CP2 is an output port providing an output voltage V2 that may also be a DC voltage.
- the first and second converter connection terminals Ti and T2 are input terminals of the converter 10 and the third and fourth converter connection terminals T3 and T4 are output terminals of the converter 10.
- the converter 10 is also a step-up converter.
- the first converter connection port capacitor CCPi may be larger than the capacitor Cl of the first cell CEi
- the capacitor Cl of the first cell CEi be larger than the capacitor C2 of the second cell CE2
- the capacitor C2 of the second cell CE2 be larger than the capacitor C3 of the third cell CE3
- the capacitor C3 of the third cell may be larger than the capacitor C4 of the fourth cell CE4.
- the second switch S12 of the first cell CEi may thus be
- the second switch S22 of the second cell CE2 may interconnect the second end of the second cell capacitor C2 with the first end of the first cell capacitor Cl
- the second switch S32 of the third cell CE3 may interconnect the second end of the third cell capacitor C3 with the first end of the second cell capacitor C2
- the second switch S42 of the fourth cell CE4 may interconnect the second end of the fourth cell capacitor C4 with the first end of the third cell capacitor C3.
- the first converter connection port capacitor CCPi is charged by the input voltage Vi, which input voltage can thereby be seen as being provided by a DC voltage source.
- the control of the converter involves sequentially moving a charge Q from one end of the stack to the other, step 14.
- the movement may start with charging of the capacitor Cl of the first cell CEi. This may be done through the control unit turning off the second switch S12 and turning on the first switch S11 of the first cell CEi. As can be seen in fig. 9a, this leads to the DC source forming a charging loop with the capacitor Cl and inductor Li of the first cell CEi. Another way that this could be looked at is as if a charge is moved from the first converter connection port capacitor CCPi to the first cell capacitor Cl.
- the DC source applies a voltage across the first converter connection port capacitor CCPi, which capacitor is then connected in parallel with the first cell capacitor Cl and inductor Li, thereby the first connection port capacitor CCPi forms an LC circuit together with the inductor Li and capacitor Cl of the first cell CEi used to move a charge from the first converter connection port capacitor CCPi to the first cell capacitor Cl.
- the second switch S12 of the first cell CEi is then turned on so that the capacitor Cl is connected in series with the first converter connection port capacitor CCPi.
- the sequential movement of the charge Q from one end of the stack to the other involves moving the charge between a controlled cell and its neighbor.
- the second cell CE2 is the controlled cell.
- the own capacitor C2 of the second controlled cell CE2 may be connected in series with the capacitor of the neighbour cell, i.e. with the capacitor Cl of the first cell CEi.
- the cell CE2 is further controllable to remove the series connection when connecting the capacitors in parallel with each other. A charge Q is thus moved between a cell and its neighbor, step 16-.
- this may more particularly involve the control unit removing the series connection between the capacitor C2 of the controlled second cell CE2 and the capacitor Cl of its neighbor CEi, step 16a, and forming an LC circuit through connecting the first charging branch comprising the capacitor C2 of the controlled second cell CE2 in parallel with the second charging branch comprising the capacitor Cl of the first neighbor cell CEi, step 16b.
- the branches comprises the inductor L2 there is thus formed an LC circuit comprising the two capacitors Cl and C2 and the inductor Li. Both the removing of the series connection and the forming of the LC circuit may be achieved through the control unit turning on the first switch S21 of the second cell CE2.
- step 16b The turning on of the first switch S21 will achieve the connecting of the first charging branch in parallel with the second charging branch, step 16b, and thereby a current will run from the first end of the first cell capacitor Cl towards the first end of the second cell capacitor C2. Thereby a charge Q is moved from the capacitor Cl of the first cell CEi to the capacitor C2 of the second cell CE2. Once the charge Q has been moved, the parallel connection of the two charging branches is then removed, step 16c, which may be automatically achieved by the diode
- step i6d This may then be followed by connecting the capacitor C2 of the second cell CE2 in series with the capacitor Cl of the first cell CEi, step i6d, which may be achieved through the control unit turning on the second switch S22 of the second cell CE2.
- This procedure may then be followed sequentially for each cell in the stack until the capacitor C4 of the last cell has been charged, which leads to a stepped -p output voltage V2 being delivered on the output port CP2.
- This can also be expressed in the following way. For a turned on first switching element in a current cell k, where k ranges from 1 to n-i, a charge is transferred between the capacitor of the current cell and the capacitor of the neighboring lower cell, the charge is thereby transferred to or from the capacitor of the lower cell.
- the output voltage is hence the sum of the voltage of at least some of the other cell capacitors.
- the output voltage is thus a sum of voltages of cells that are not controlled to move the charge.
- This type of operation has a number of advantages. Conversion is achieved in one step without the use of a transformer. The converter is also power and cost efficient and the voltage stress on the output is low. Through using an inductor in the cells, the switching losses and Electromagnetic interference (EMI) may also be lowered.
- EMI Electromagnetic interference
- the invention has a number of further advantages. It is possible to convert a DC voltage with very high conversion ratio and provide mega-high- voltage DC transmissions.
- the converter may also become short-circuit proof. It further eliminates the need for expensive, and difficult to design, high-voltage transformers. It is also a good fit for connecting solar power plants to future MV/HV transmission grids.
- the converter can also be used for high-ratio voltage conversion of Off-shore wind power and can thereby interconnect an off-shore wind farm with an HVDC collection grid.
- the converter further allows parallel and interleaved operation for higher, smoother and redundant output power.
- the resonant type of switching employed has a number of further advantages: It may provide a Sinusoidal current with a limited dl/dt.
- Fig. 10a and 10b schematically show the current sizes and capacitor and inductor sizes of the first converter realization.
- the average input current is n x Iout,avg were n is the number of levels in the stack and Iout,avg is the average output current.
- the average current circulating between the capacitor of a cell and the capacitor of its neighbor will therefore be very high, where the average current at the highest level, the fourth level, may be two times the average output current, the average circulating current at the second highest level, the third level, is three times the average output current, the average circulating current at the second lowest level, the second level is four times the average output current and the average circulating current at the lowest level, the first level is five times the average output current.
- the second switch in each cell is hard switched but only see the average output current Iout,avg.
- the average output current (discharge current) of the top cell capacitor C4 is equal to the converter output current.
- the third cell CE3 In order to keep the top capacitor voltage constant, the third cell CE3 must charge this capacitor C4 with an average current of the same magnitude. It will also need to supply the average converter output current since this current always flow.
- the average output current of the cell capacitor of the third cell CE3 is hence twice the converter output current. This reasoning is true also for the succeeding lower cells in the cascaded converter.
- CCPi may be larger than the capacitor Cl of the first cell CEi, the capacitor Ci of the first cell CEi be larger than the capacitor C2 of the second cell CE2, the capacitor C2 of the second cell CE2 may be larger than the capacitor C3 of the third cell CE3 and the capacitor C3 of the third cell may be larger than the capacitor C4 of the fourth cell CE4.
- the fourth cell capacitor C4 may for instance have a capacitance of C, the third cell capacitor C3 a capacitance of 2*C, the second cell capacitor C2 a capacitance of 3*C and the first cell capacitor Ci a capacitance of 4*C and the first connection port capacitor CCPi a capacitance of 5*C.
- the inductance of the fourth cell inductor L4 is L
- the inductance of the third cell inductor L3 is L/2
- the inductance of the second cell inductor L2 is L/3
- the inductance of the first cell inductor Li is L/4.
- the capacitance of the capacitor is k times the capacitance C of the capacitor in the top cell and the inductance of the inductor in the cell is the inductance of the inductor of the top cell divided by k.
- At least one of the cells in the stack, and possibly also more cells, may thereby be connected in parallel with at least one further cell for paralleling and cascading purposes.
- the converter may be seen as comprising a number of sections, where the cells in the stack are provided in different sections and the sections have a different number of parallel cells.
- the number of parallel cells in the sections may additionally vary by one cell from one section to the next from one end in the stack to the other, such as from the top of the stack towards the bottom
- the first converter realization may be a unidirectional DC/DC step up converter, where the charging current is delivered from the first end of the capacitor in a neighbour cell to the first end of the capacitor of a controlled cell above it.
- the second converter realization is a unidirectional step-down converter and in this case the charging current in a cell has the opposite direction compared with in the first converter realization. A charge is also moved from the capacitor of a controlled cell to the capacitor of the neighbour cell below it.
- the converter realization in fig. 3 is a bidirectional DC/DC step up/step down converter in which the charging current of an LC circuit may have any of the two described directions.
- Fig. 11 shows a variation of the first converter realization to be used for negative voltages.
- the first converter connection port may be placed at the top of the stack of cells, which is here to a fifth cell in the stack.
- the second converter connection port is here provided between the first connection terminal of the first cell and the fourth connection terminal of the fifth cell.
- a DC source may because of this be connected between the first and second converter port terminals, where the second converter port terminal is grounded and the first converter port terminal is placed at the first converter port inductor.
- This converter realization may be combined with the converter of the first realization in order to be connected to a symmetric HVDC link.
- Fig. 12 shows a further converter realization comprising a stack of five cells, which converter 10 may be used for step down and step up operation for both positive and negative voltages.
- each cell additionally comprises a fourth interconnecting branch, where the fourth
- interconnecting branch is connected between the third and second cell connection terminals and the third and fourth interconnecting branches comprise third and fourth semiconducting elements that in this case are transistors, while the first and second interconnecting branches comprise first and second semiconducting elements realized as anti-parallel thyristor pairs. Current can flow in both directions through the converter.
- first cell there is in this case a first thyristor switch S11 in the first interconnecting branch, a second thyristor switch S12 in the second interconnecting branch, a third transistor switch S13 in the third interconnecting branch and a fourth transistor switch S14 in the fourth interconnecting branch.
- second cell there is a first thyristor switch S21 in the first interconnecting branch, a second thyristor switch S22 in the second interconnecting branch, a third transistor switch S23 in the third interconnecting branch and a fourth transistor switch S24 in the fourth interconnecting branch.
- first thyristor switch S31 in the first interconnecting branch In the third cell there a first thyristor switch S31 in the first interconnecting branch, a second thyristor switch S32 in the second interconnecting branch, a third transistor switch S33 in the third interconnecting branch and a fourth transistor switch S34 in the fourth interconnecting branch.
- fourth cell there is a first thyristor switch S41 in the first interconnecting branch, a second thyristor switch S42 in the second interconnecting branch, a third transistor switch S43 in the third interconnecting branch and a fourth transistor switch S14 in the fourth interconnecting branch.
- first thyristor switch S51 in the first interconnecting branch a second thyristor switch S52 in the second interconnecting branch, a third transistor switch S53 in the third interconnecting branch and a fourth transistor switch S54 in the fourth interconnecting branch.
- the control unit may be realized as a processor acting on computer program code.
- This computer program code may be provided with the processor in a dedicated circuit, such as a Field-programmable Gate Array (FPGA) or an Application Specific Integrated Circuit (ASIC).
- the processor may also act on computer program instructions in a program memory.
- the computer program instructions may additionally be provided on a data carrier, which performs the functionality of the control unit when loaded into such a memory.
- Fig. 13 schematically shows such a data carrier 18 in the form a CD ROM disk with such code 20.
- Other possible types of data carriers are memory sticks.
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US5270913A (en) | 1992-04-06 | 1993-12-14 | D.C. Transformation, Inc. | Compact and efficient transformerless power conversion system |
US5581454A (en) | 1994-11-22 | 1996-12-03 | Collins; Hansel | High power switched capacitor voltage conversion and regulation apparatus |
US7554221B2 (en) * | 2004-05-04 | 2009-06-30 | Stangenes Industries, Inc. | High voltage pulsed power supply using solid state switches with droop compensation |
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