WO2018052985A1 - System and method for transformerless power conversion - Google Patents

Abstract

A system and method for bilateral transfer of energy between a single or multi-phase alternating current source and either a monopolar or bipolar direct current source. There is a capacitive column which includes a number of series-connected capacitive modules. A controller is configured to successively bypass selected capacitive modules within the capacitive column in a series of charge-exchange steps that involve repetitive connection of the column, first to one such source and then to the other.

Description

SYSTEM AND METHOD FOR TRANSFORMERLESS POWER CONVERSION

BACKGROUND

[0001] This disclosure relates to conversion of electric power between a high dc voltage and a poly-phase ac voltage without the use of magnetically-based transformers.

[0002] Technologies used in transformation of electric power between alternating current (ac) and direct current (dc) have advanced rapidly over the past decades, as has the use of dc as a means of transporting large amounts of power. DC transports power more efficiently than ac and unlike ac, is able to do so at a controlled level, e.g. at a level corresponding to the maximum thermal capability of the conductors used.

[0003] High voltage dc transmission projects have grown in distance, voltage level and power rating. In this evolution, the need for dc-to-dc transformation has been limited since, irrespective of the dc transmission voltage, transformers are included in both sending and receiving terminals so that conversion to a voltage convenient to the local system is quite straight forward. However ongoing evolution of the world's electric supply system gives rise to the need for direct dc-to-dc power transformation taking advantage of the rapidly developing field of power electronics and eliminating the traditional, if rarely used, recourse of first conversion of dc to ac, then voltage transformation by magnetic ac transformers, and finally conversion back to dc by traditional ac-to-dc bridges This new need, spurred in part by growing interest in high voltage dc "macrogrids" proposed as overlays to ac systems in Europe, North America, and other areas of high electricity usage, has led to a number of proposals for capacitor-based dc-to-dc transformers to link dc lines of different voltage levels. More germane to the innovations cited herein, it has also led to a need for new methods for converting power between dc lines and ac lines; methods which avoid the use of intermediate magnetically based ac transformers and methods which reduce the cost of converting dc to ac at moderate to low power levels.

SUMMARY

[0004] Recent innovations in dc-to-dc transformers (DCTs) are capable of coupling differing dc voltages by capacitive methods. Such methods make use of columns consisting of capacitive modules to alternately accept electric charge from one dc node and then deliver it to another. The most practical such option may be the multi-module DCT, (MMDCT), which performs within a dc system in a manner analogous to performance of magnetic transformers within an ac system. The following paragraphs begin with a brief review of MMDCT operating principles, then adapt those principles to a number of embodiments for transforming power back and forth between either monopolar or bipolar dc nodes and either single or multi-phase high voltage ac nodes.

[0005] Throughout this document power electronic switches used within various disclosures are illustrated as simple switch symbols. The reader should be aware they would, in reality, usually be thyristors or insulated gate bipolar transistors (IGBTs).

[0006] In Figure 1, illustrating as background a simple MMDCT configuration 300, switch 13 first connects a column 100 of n capacitors or capacitive modules 90 between ground 1 and a higher voltage node 3 with all capacitances 90 comprising the column 100 in electrical series. Then, as in figure 2, switch 13 opens and switch 12 closes, connecting the column 100 to a lower voltage node 2 but not before (n-m) of the capacitors in the column 100 are isolated and electrically by passed - thus presenting a lower voltage to node 2. While the lesser number of capacitors, m, remains constant during the column's connection to the lower voltage node 2, the actual capacitances 90 comprising that number m may be rotated among the capacitances 90 comprising a column 100, such that, following connection to the lower voltage node 2, all capacitances 90 in the column 100 are left with equal charge. In this and subsequent illustrations, the timing and sequence of switch opening and closing is governed by logic within a controller 1000, present in all embodiments subsequently described but omitted from their drawings for the sake of simplicity.

[0007] In figure 1 resistor 80 is inserted in the column by opening switch 10 during initial charging of the capacitive column 100. Switch 10 is then closed for ongoing operation of the MMDCT 300.

[0008] Since connection to either node 2 or 3, in figure 1 is achieved through a reactance 70, 71 the energy exchange between either node 2 or 3 and the capacitive columns 100 is in the form of a resonant half sine-wave as shown in figure 3, current being interrupted at the first current zero in all cases. Thus, assuming the reactance of inductors 70 and 71 in figure 2 are equal, the period of both input and output pulses will be equal and the profile of current exchange into or out of both nodes 2, 3 will consist of a series of pulses separated by a span of zero current equal to the period of the current pulse itself as shown in figure 3. Since the current wave-form of figure 3 is very difficult to filter, three MMDCTs configurations of the form shown in figures 1 and 2 may be connected in parallel as shown in figure 4, system 301, thus smoothing the dc output as shown in figure 5 and tripling the power rating of the MMDCT with simpler filtering.

[0009] In a number of high voltage electrical devices, a sinusoidal ac wave form of either 50 or 60 Hertz is created by electrically creating a series of sequential steps, gradually increasing in magnitude, then decreasing in magnitude to zero in a manner that describes a half sine wave, after which the same regime constructs the negative half of that ac sine wave. A portion of the positive half-sine wave structure for one full three-phase ac cycle is illustrated in figure 6. It will be apparent from the operating principles of the MMDCT that, just as the number of capacitive modules 90 bypassed in figure 1 can be selected to establish the lower of two fixed dc voltages, the number can also be changed from one operating cycle to the next, thereby providing a means to form a series of steps necessary to constitute a 50 or 60 Hz wave form. It is this principle that is used in the embodiments cited herein.

[0010] All examples and features mentioned below can be combined in any technically possible way.

[0011] In one aspect, a system for bilateral transfer of energy between a single or multiphase alternating current source and either a monopolar or bipolar direct current source, includes a plurality of capacitive columns, each column comprising a plurality of series-connected capacitive modules, and a controller configured to successively bypass selected capacitive modules within each capacitive column in a series of charge-exchange steps that involve repetitive connection of the column first to one such source and then another.

[0012] Embodiments may include one of the following features, or any combination thereof. The sequence of charge-exchange steps may be caused to closely approximate an alternating sinusoidal wave form, each step being either a recipient or source of energy from or to the capacitive column. The energy transfer between the direct current and alternating current sources may be achieved by resonant transfer of half-sine wave current pulses. A number of half-sine wave pulses of differing periods may form repetitive groups. The sum of the periods within each group may be approximately equal. Each charge exchange step may have a period and approximately corresponds to a voltage within an approximate sine wave. The charge exchange steps may be assigned to different capacitive columns such that the sum of all charge exchange step periods of each capacitive column is approximately the same over an operating cycle. The controller may cause the sum of time periods associated with charge exchange steps served by any given capacitive column over each operating cycle to be approximately constant. The system may further comprise a fixed reactance with a number of taps, wherein the sum of time periods is at least in part accomplished by the controller causing adjustments in reactance by the taps on the fixed reactance.

[0013] In another aspect, a system for bilateral transfer of energy between multiple direct current nodes at different voltages, includes a capacitive column comprising a plurality of series- connected capacitive modules, and a controller that is configured to successively bypass selected capacitive modules within the capacitive column in a series of charge-exchange steps that involve repetitive connection of the column first to one node and then the other. The capacitors may receive and store energy as an intermediary between energy transfer between the capacitive column and the nodes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Fig. 1 is a schematic diagram illustrating a fundamental multi-module DCT

(MMDCT) capacitive column exchanging charge resonantly with one of two dc nodes.

[0015] Figure 2 is a schematic diagram illustrating the MMDCT of fig. 1 with the capacitive column switched so as to exchange charge with the other dc node, some capacitors within the column having been electrically isolated and bypassed.

[0016] Figure 3 shows the pattern of single, half sine wave current pulses resulting from current exchanged both with a first and second dc nodes. [0017] Figure 4 is a schematic diagram illustrating three MMDCTs in parallel, pulse input and outputs of which are equally offset from one another in time.

[0018] Figure 5 shows the advantage of a three column MMDCT of figure 4 in smoothing input and output current waveforms.

[0019] Figure 6 illustrates the concept by which individual steps of varying magnitude can closely approximate a sine wave profile.

[0020] Figure 7 is a schematic diagram illustrating an embodiment capable of exchanging power between a monopolar dc node and three-phase ac nodes.

[0021] Figure 8 shows the current contributions to sine wave formation achievable with the embodiment of figure 7.

[0022] Figure 9 is a schematic diagram illustrating the embodiment of figure 7 configured to exchange power between a bipolar dc node and three-phase ac nodes.

[0023] Figure 10 shows the current contributions to sine wave formulation achievable with the embodiment of figure 9.

[0024] Figure 1 1 illustrates that a repeating total period of capacitive column discharges can be realized if the sum of periods for all discharges are equal.

[0025] Figure 12 is a schematic diagram illustrating an eight-column configuration capable of assigning positive pulse development to the positive pole and negative pulse development to the negative pole.

[0026] Figure 13 shows the profiles that need to be approximated, given separate assignment of one group of capacitive columns for positive half cycles and another for negative half cycles.

[0027] Figure 14 is a schematic diagram illustrating an embodiment in which a single capacitive column is capable of exchanging power between a single dc node and three ac nodes. [0028] Figure 15 is a schematic diagram illustrating an embodiment in which two capacitive columns, one for each pole, are capable of exchanging power between bipolar dc nodes and three ac nodes.

[0029] Figure 16 is a schematic diagram illustrating the embodiment of figure 15 adapted to conversion between more than two dc nodes.

DETAILED DESCRIPTION

[0030] In the first embodiment of this disclosure, illustrated in figure 7, each capacitive column 100, 101, 102 is first connected to the positive dc node 3 and then to phase a ac node 110, then again to the positive dc node 3 and immediately following to phase b ac node 111, and finally, in like manner is caused to exchange energy between the dc node 3 and the phase c ac node 112. Each of the capacitive modules within capacitive columns 100, 101, and 102 are full bridge modules or functionally equivalent, i.e. able to reverse connections such that a positive charge received from dc node 3 can be presented to nodes 110, 111, or 112 as a contribution to the building of either positive or negative portion of a power frequency ac sine wave. Three columns 100, 101, 102 in figure 7 operate in the manner described above but offset in time, thus providing nine inputs to three-phase ac sinusoidal wave construction. Since each current pulse exchanged with the ac busses 110, 111, 112 from columns 100, 101 and 102, is separated from the next by an equal period of zero current, assuming charge and discharged periods can be kept equal, the embodiment shown in figure 7 will supply pulse trains to each of the ac nodes 110, 111,112 as characterized by the profile of figure 8 wherein there is a zero-current period between exchanges of charge between capacitive columns 100, 101, and 102 and each of the ac nodes, 110, 111, and 112. This will, without a recourse cited later, result in a rather course

characterization of a pure sine wave profile.

[0031] It is apparent that if the embodiment of figure 7 is used with the positive dc node 3 as a sole point of dc energy exchange, that embodiment will represent a monopolar load on what is most commonly a bipolar dc system. That imbalance can be remedied by adding a mirror image of configuration 302 in figure 7 supplying an equal level of power from the negative dc node 33 as shown in figure 9. In this case the number of pulses available for constructing three sinusoidal ac voltages doubles and, assuming that the capacitive modules within all capacitive columns 100, 101, 102, 103, 104, and 105 are supplied with full bridges or their equivalent, will eliminate the time gap between pulse exchanges and allow the profile shown in figure 10. This allows both a finer definition of an ideal sine wave and a contiguous series of pulse delivery to each of the ac nodes 110, 111, 112.

[0032] One important challenge in the foregoing embodiments will be apparent to those versed in the art. In order to provide pulses to help construction points on an ac waveform at voltages of lower and lower value, capacitive columns 100 must bypass an increasing number of capacitive modules thus increasing the total effective capacitance of a resonant discharge circuit in which the inductance of reactor 70 remains constant. Thus, absent other measures, e.g.

providing a large number of taps on reactor 70, the frequency of each pulse will reduce as lower voltage steps are constructed and the time period of such pulses achieving energy transfer to the ac nodes 110, 111, 112 will progressively increase. This could result in progressive time- displacement of wave pulse train patterns supported by various capacitive columns 100. But to keep the aggregate time duration of the three charge-discharge sequence steps of each capacitive column 100, 101, 102, 103, 104 and 105 equal through sequential closure of switches 12a, 12b, and 12c, the time period of individual charge exchanges with ac nodes 110, 111, or 112 need not be the same providing that the sum of periods of each three successive discharges is the same for each supplying capacitive column 100, 101, 102, 103, 104 and 105 as illustrated in figure 11. Since the schedule of closures of all switches 12a, 12b, and 12c is easy to determine in advance and since any given tap provided in reactors 70 may be left unused, or used in any or all of the resonant transfers of charge with the ac busses 110, 111, or 112, the number of taps needed to equalize the sum of differing periods can be achieved with a number of taps an order of magnitude less than the number of steps comprising the transition. Furthermore, small inequalities in the sum of periods for the three charge transfers assigned to any given capacitive column 100, can be accommodated while maintaining the synchronism of a given discharge schedule by slight intentional delays in closure of switches 13 in figure 7 or figure 9; a remedy which will cause a slight decrease in the utilization efficiency of the components comprising the embodiment of figure 7 or 9 but not necessarily their electrical efficiency. [0033] It is apparent that the number of capacitive columns shown in the embodiment illustrated in figure 7 and 9 can be increased providing the number of three-switch groups such as 12a, 12b, and 12c is increased accordingly. Figure 12 for example, shows an embodiment with four capacitive columns 100 per dc node and as many groups of switches 12e, 12f, and 12g. A four-column per pole embodiment allows one group of four capacitor columns 100 to be assigned to the positive portion of each of the three-phase wave forms and a second group of four capacitor columns 100 to be assigned to the negative portion of each of the three-phase wave-forms - thus permitting the capacitive modules within each of those columns to be comprised of half bridges or their equivalent. Figure 13 shows, as an example, the positive portion of a balanced three phase profile which would be assigned to charge exchanges only with the positive pole. Since two thirds of the time two separate positive voltages must be matched, four capacitive modules per pole would be needed to provide time-contiguous charge exchange pulses with each profile.

[0034] Yet another embodiment consists, in major part, of an extension in the principle on which the MMDCT was originally based, which was a two-step sequential charge exchange first with one node and then a second. That extension consists of embodiments which first make a charge exchange with one node and then a series of charge exchanges in sequence with two or more other nodes, each of the latter charge exchanges involving the same or differing capacitive modules 90.

[0035] An example embodiment of that system is illustrated in figure 14 which allows exchange between dc and three-phase ac nodes with a single capacitive column 100 per pole. For the embodiment shown in figure 14, the cycle of charge and discharge of capacitive column 100 is divided into four equal, sequential, and repeating time steps. Only one of switches 80, 81, 82, and 71 is closed at any one time. Switch 71 is closed first, resonantly charging the capacitive column 100 from the positive node 3. The same charging/discharging resonant frequency will result from closure of each switch 80, 81, and 82 inasmuch as both the total effective capacitance and reactance is the same in each case.

[0036] The net voltage on the capacitive column 100 is then adjusted by means of selective bypassing methods previously discussed, to correspond to the ac voltage step on ac phase a (node 110) and switch 80 closes adding charge to capacitor 91 which follows the voltage profile of alternating current node 110. The frequency of the resonant exchange of energy from the capacitive column 100 to capacitor 91 is determined largely by the inductance of reactor 12 and capacitor 91. Because the inductance of reactor 12 is less than the inductance of reactor 95 and the capacitance of capacitor 91 is less than the minimum capacitance of capacitive column 100, the frequency of resonant exchange of energy is minimally effected by the large variation in capacitance of capacitive column 100. After switch 80 opens, the energy delivered to the associated capacitor 91 discharges to the alternating current bus 110 through the associated reactor 95 and capacitance 92. Capacitor 92 functions just as a transmission line's series capacitor allowing capacitive column 100 to maintain a constant polarity resulting in a dc voltage offset on the ac voltage generated across capacitor 91. Series capacitor 92 blocks the dc voltage offset from being transferred to ac phase a (nodellO). With this process completed, switch 80 is opened and switch 81 closed to achieve the same transfer to ac phase b (node 111) at its voltage level at that instant and after that, switch 82 is closed to transfer energy to phase c (node 112), thus completing the four-step cycle.

[0037] Since the foregoing contributions to steps comprising each of three ac nodes 110, 111 and 112, must change polarity as well as matching the voltage profile of each ac node, the capacitive column 100 must be capable of receiving charge from the positive dc node 3 in figure 14, then exchanging charge with ac nodes of either positive or negative polarity. If there are no series capacitors 92 and to maintain the ac voltages on the three ac nodes 110, 111 and 112 without any dc offset, capacitive column 100 must discharge positive polarity pulses to capacitors 91 through either switch 80, 81, or 82 during the positive polarity halves of the respective ac cycles. During the negative polarity halves of the respective ac phases at nodes 110, 111, or 112, connections within capacitive column 100 must reverse their polarity to deliver the necessary negative pulses to capacitors 91 through switches 80, 81 and 82, - a process which can be achieved with commonly applied "full bridges" or their functional equivalent. The capacitive column 100 continues to always receive a positive charge to capacitive column 100 from node 3 during its charging sequence. To keep component rating reasonable, the resonant frequency associated with each of the four switches 71, 80, 81, and 82 will need to be high, e.g. in the order of 2,000 to 3,000 Hz. Considering characteristics of commercially available power electronic switches and the fast recombination time needed for frequency that high, the fundamental frequency ac wave shape may have the dc bias which, if necessary, can be isolated from the three-phase ac nodes by isolating series capacitors 92 in figure 14. It will be apparent as well that any ac wave shape constructed by a series of small steps, as is the case with

embodiments cited herein, high-pass filters 97, may be needed on the ac nodes 110, 111, and 112. A dc filter 97 may also be applied to dc bus 3.

[0038] To those skilled in the art, it will be apparent that the principles set forth above for transformation of energy from dc nodes to ac nodes of one or more phases, can equally well achieve conversion from ac energy from ac nodes of one or more phases to either monopolar or bipolar dc nodes.

[0039] While the embodiment shown in figure 14 will achieve its objective of power conversion between dc and ac, it would cause an unbalance on the dc system. It is apparent however that the embodiment of figure 14 could be replicated in exactly the same fashion for the negative dc node and added to the monopolar embodiment of figure 14 as shown in figure 15.

[0040] The above embodiment may, in similar fashion, allow a single capacitive column 100, comprised of capacitive modules 90 as shown in figure 1 to transform energy between multiple dc nodes, a three-node example of which is shown in figure 16. In this embodiment, as in the prior embodiment, the capacitive column 100 is first caused to exchange charge with node 3 by closure of switch 13, then, after switch 13 is opened, exchanges charge with second node 2 by closure of switch 12, and then with third node 4 through closure of switch 12a.

[0041] The foregoing embodiments represent examples systems whereby a novel system and method, consisting of first causing a charge exchange between one or more dc nodes and one or more ac nodes, can be achieved by adjusting, between charge exchanges, the number of capacitive modules 90 within various columns that are bypassed such that the voltage profile of charge approximates one or more sinusoidal ac wave forms. It will be obvious to those versed in the art that the novel method described herein may be embodied in many other of combinations of capacitive columns 100, each switchable to exchange charge with one or more ac nodes. [0042] Elements of figures are shown and described as discrete elements in a block diagram. These may be implemented as one or more of analog circuitry or digital circuitry. Alternatively, or additionally, they may be implemented with one or more microprocessors executing software instructions. The software instructions can include digital signal processing instructions.

Operations may be performed by analog circuitry or by a microprocessor executing software that performs the equivalent of the analog operation. Signal lines may be implemented as discrete analog or digital signal lines, as a discrete digital signal line with appropriate signal processing that is able to process separate signals, and/or as elements of a wireless communication system.

[0043] When processes are represented or implied in the block diagram, the steps may be performed by one element or a plurality of elements. The steps may be performed together or at different times. The elements that perform the activities may be physically the same or proximate one another, or may be physically separate. One element may perform the actions of more than one block.

[0044] Embodiments of the systems and methods described above comprise computer components and computer-implemented steps that will be apparent to those skilled in the art. For example, it should be understood by one of skill in the art that the computer-implemented steps may be stored as computer-executable instructions on a computer-readable medium such as, for example, floppy disks, hard disks, optical disks, Flash ROMS, nonvolatile ROM, and RAM. Furthermore, it should be understood by one of skill in the art that the computer-executable instructions may be executed on a variety of processors such as, for example, microprocessors, digital signal processors, gate arrays, etc. For ease of exposition, not every step or element of the systems and methods described above is described herein as part of a computer system, but those skilled in the art will recognize that each step or element may have a corresponding computer system or software component. Such computer system and/or software components are therefore enabled by describing their corresponding steps or elements (that is, their functionality), and are within the scope of the disclosure.

[0045] A number of implementations have been described. Nevertheless, it will be understood that additional modifications may be made without departing from the scope of the inventive concepts described herein, and, accordingly, other embodiments are within the scope of the following claims.

Claims

What is claimed is:

1. A system for bilateral transfer of energy between a single or multi-phase alternating current source and either a monopolar or bipolar direct current source, comprising:

a plurality of capacitive columns, each column comprising a plurality of series-connected capacitive modules; and

a controller configured to successively bypass selected capacitive modules within each capacitive column in a series of charge-exchange steps that involve repetitive connection of the column first to one such source and then another.

2. The system of claim 1, wherein the sequence of charge exchange steps is caused to closely approximate an alternating sinusoidal wave form, each step being either a recipient or source of energy from or to the capacitive column.

3. The system of claim 1, wherein energy transfer between the direct current and alternating current sources is achieved by resonant transfer of half-sine wave current pulses.

4. The system of claim 3, wherein a number of half-sine wave pulses of differing periods form repetitive groups.

5. The system of claim 4, wherein the sum of the periods within each group is

approximately equal.

6. The system of claim 1, wherein each charge exchange step has a period and

approximately corresponds to a voltage within an approximate sine wave.

7. The system of claim 6, wherein the charge exchange steps are assigned to different capacitive columns such that the sum of all charge exchange step periods of each capacitive column is approximately the same over an operating cycle.

8. The system and method of claim 1, wherein the controller causes the sum of time periods associated with charge exchange steps served by any given capacitive column over each operating cycle to be approximately constant.

9. The system of claim 8, further comprising a fixed reactance with a number of taps, wherein the sum of time periods is at least in part accomplished by the controller causing adjustments in reactance by the taps on the fixed reactance.

10. A system for bilateral transfer of energy between multiple direct current nodes at different voltages, comprising:

a capacitive column comprising a plurality of series-connected capacitive modules; and a controller that is configured to successively bypass selected capacitive modules within the capacitive column in a series of charge-exchange steps that involve repetitive connection of the column first to one node and then the other.

11. The system of claim 10, wherein capacitors receive and store energy as an intermediary between energy transfer between the capacitive column and the nodes.