WO2017019966A1

WO2017019966A1 - System and method for smoothing multiple pulse-based generator outputs

Info

Publication number: WO2017019966A1
Application number: PCT/US2016/044737
Authority: WO
Inventors: Lionel O. Barthold; Dennis A. Woodford; Maryam SALIMI
Original assignee: Barthold Lionel O; Woodford Dennis A
Priority date: 2015-07-29
Filing date: 2016-07-29
Publication date: 2017-02-02

Abstract

Systems and methods by which multiple dc generation sources can each use single-column capacitive dc-to-dc transformers to aggregate the respective signals at a collector bus, but wherein the sum of the resulting single-pulse wave trains are caused to be equally offset in time from one another so as aggregate to a near constant dc current.

Description

SYSTEM AND METHOD FOR SMOOTHING MULTIPLE PULSE-BASED

GENERATOR OUTPUTS

FIELD

The system and method cited below pertains to the interconnection of multiple electrical generators, the output from each of which is a series of current pulses separated by a period of zero current, in a manner that produces an aggregate current that is a relatively constant dc current. "Pulses," as described and referred to herein will, for convenience of explanation, be considered as ideal symmetrical unipolar wave forms. However the embodiments and methods constituting this invention will apply equally well to unipolar but irregularly shaped wave forms. The system and method is particularly germane, but not limited to, wind-farms made up of multiple generation platforms.

BACKGROUND

Direct current is becoming an increasingly important contributor to electric power generation, delivery, and utilization systems. Its use in wind-generation systems is attractive for two primary reasons.

1. Wind generators, operating at variable speed generate a variable frequency alternating current which is impossible to combine with current from other generators also operating at different speeds. It is becoming conventional that the variable frequency ac current from each such generator is first converted to dc then back to ac at a common frequency so all generator current outputs can be aggregated in synchronism. Combined outputs of all generators are eventually stepped up to a voltage high enough to economically transport the output of the total windfarm to a point on the high voltage ac network either through ac or dc transmission.

2. Within the past several years researchers have focused considerable attention to development of an efficient dc-to-dc transformer - one that performs within a dc system just as a magnetic transformer performs within an ac system.

It has become apparent that since asynchronous ac wind energy is first converted to dc, a dc-to-dc transfomier will allow it to remain dc and but stepped up as dc, to a voltage suitable for transmission to the ac network. For off-shore wind farms that electrical architecture has two major advantages. First, it substitutes a relatively light and compact capacitor-based dc-dc transformer for a traditional ac magnetic transformer. Second it permits use of dc cable which is much less expensive than ac cable.

DESCRIPTION OF PRIOR ART

A number of principles have been proposed for dc-to-dc transformation, the simplest and most promising being a repetitive two-step system 60 illustrated in figures 1A and IB wherein a column of capacitors 10 is first connected between a primary bus 1 and ground 3 through a switch 4 with all the capacitors connected in series (step 1, figure 1A), then, by opening the first switch 4 and closing the second switch 5, comiected between a secondary lower voltage bus 2 and ground while some of the capacitors within the column 10 are electrically bypassed and removed from the circuit (step 2, figure IB). Control systems operate in such a manner that charge among active and bypassed capacitors is equalized. In that way energy can be transformed between two very different dc voltages. Depending on the charge transfer principle used, reactors 6 may be required at some point in the circuit.

The bypassing action cited above can be achieved by placing each capacitor in the series capacitor column 10 within a conventional "half-bridge" 7 shown in figure 2, or in some cases with a "full-bridge" 8 illustrated in figure 3. The half-bridge 7 is capable of either inserting the capacitor into the series chain, or isolating it and bypassing its position in the chain. The full- bridge 8 does the same thing but is also capable of reversing the polarity of particular capacitors.

Alternatives methods have been proposed for the above-described two-step bus-to-bus energy transfer. System 60, figures 1A and IB, the simplest, relies on a resonant exchange of energy between each bus in turn, through a reactor 6 and the capacitor chain 10 that results in a resonant current zero after the first half-cycle of current flow as shown in figure 4. When output current is flowing, input current is zero and vice versa.

Figures 5 A and 5B show a configuration of a system 61, similar to system 60, figures 1A and IB, but capable of supporting a non-resonant method of charge exchange between busses 1 and 2. In this case a capacitive column 12, again comprised of either half modules 7 or full modules 8, can, by special bypass logic, be made to transfer charge between busses 1, 2 in a non- resonant manner. This type of system has been shown capable of producing a more constant current wave-form on both input and output, and example of which is shown in fig. 6, but requires a higher voltage rating for its reactor 9 and, more importantly, in the need for a power control signal to assure proper operation.

Both resonant and non-resonant systems produce pulsed input and output wave forms not directly useful to a dc system. Consequently each is presumed to require at least three parallel columns 13 (which can have a half bridge or full bridge configuration), configured as shown in system 62, figure 7 for the resonant transfer option, outputs of which are offset in time so as to produce, in the aggregate, a moderately smooth wave form as shown in figure 8. In figure 7 switches 4, 14, and 24 close sequentially - each equally offset from one another in timing. Each switch 4, 14, and 24, opens at the first current zero of the resonant energy transfer through inductance 6 and the capacitance of the associated column 13. In like manner, switches 5, IS, or 25 close immediately after the opening of the corresponding switches 4, 14, or 24 to resonantly transfer energy to the secondary bus 2, the frequency of that transfer determined by reactance 6 and the capacitance of column 13. Thus the sequential operation of switch pairs 4 and 5, 14, and 15, and 24 and 25, each equally offset from one another, result in a composite input and output waveform such as A, B, and C in figure 8, also equally offset from one another. At no time is there a galvanic connection between busses 1 and 2 during that sequence.

SUMMARY

The present disclosure avoids the need for multiple columns of capacitors such as depicted in figure 7 in simple charge-transfer dc-to-dc transformers discussed above in cases where current from multiple dc sources is aggregated at a common point. Several embodiments described below enable the use of single-column dc-to-dc transformers while still producing a levelized wave-form; doing so by causing the single-pulse output wave trains from a number of individual dc-to-dc transformers to be equally off-set in time from one another - thus aggregating to a maximally levelized composite current flow at the point of collection. This results in reductions of weight, volume and cost of the total system.

All examples and features mentioned below can be combined in any technically possible way. In one aspect, a system for aggregating a plurality of separate electrical current pulse trains generated by a plurality of dc-to-dc transformers includes a collector (e.g., a bus) that is configured to receive the pulse trains from the dc-to-dc transformers, and a control system that is configured to control a relative timing of the pulse trains generated by the plurality of dc-to-dc transformers, before the pulse trains are delivered to the bus.

Embodiments may include one of the following features, or any combination thereof. The pulses of the pulse trains generated by the plurality of dc-to-dc transformers may be equally offset in time from one another. The control system may comprise a time-keeping system that is used to control a relative timing of the pulse trains generated by the plurality of dc-to-dc transformers such that the pulses of the pulse trains generated by the plurality of dc-to-dc transformers are equally offset in time from one another. The control system may comprise a controller associated with each transformer, where a controller is operably coupled with each transformer. The control system may further comprise a communications system that operably connects the controllers together such that the controllers can coordinate phasing of the pulse trains generated by the plurality of dc-to-dc transformers. The controllers may be configured to adjust a relative time offset of the pulse trains generated by the plurality of dc-to-dc transformers, such that the pulses of the pulse trains generated by the plurality of dc-to-dc transformers remain equally offset in time from one another.

Embodiments may include one of the following features, or any combination thereof. The dc-to-dc transformers may comprise single-column capacitive dc-to-dc transformers. The control system may comprise a control and communication link that connects each of the single-column capacitive dc-to-dc transformers to a point at which current wave trains from each transformer are aggregated. The control system, based on the time of occurrence of a high point or a low point in the aggregated current wave form, may cause one or more of the transformers to advance or delay a relative time offset of its pulse train such that the peak of its pulses occur at a time of minimum current of the aggregated current wave form.

In another aspect, a method of aggregating a plurality of separate electrical current pulse trains generated by a plurality of dc-to-dc transformers, includes aggregating the pulse trains from the dc-to-dc transformers and controlling a relative timing of the pulse trains generated by the plurality of dc-to-dc transformers, before the pulse trains are aggregating. Embodiments may include one of the following features, or any combination thereof. The pulses of the pulse trains generated by the plurality of dc-to-dc transformers may be equally offset in time from one another. The controlling step may comprise controlling a relative timing of the pulse trains generated by the plurality of dc-to-dc transformers such that the pulses of the pulse trains generated by the plurality of dc-to-dc transformers are equally offset in time from one another. The controlling step may comprise providing a controller associated with each transformer, where a controller is operably coupled with each transformer. The controlling step may further comprise operably connecting the controllers together and then using the controllers to coordinate phasing of the pulse trains generated by the plurality of dc-to-dc transformers.

The controllers may be configured to adjust a relative time offset of the pulse trains generated by the plurality of dc-to-dc transformers, such that the pulses of the pulse trains generated by the plurality of dc-to-dc transformers remain equally offset in time from one another.

Embodiments may include one of the following features, or any combination thereof. The dc-to-dc transformers may comprise single-column capacitive dc-to-dc transformers. The controlling step may comprise connecting each of the single-column capacitive dc-to-dc transformers to a point at which current wave trains from each transformer are aggregated. The controlling step, based on the time of occurrence of a high point or a low point in the aggregated current wave form, may cause one or more of the transformers to advance or delay a relative time offset of its pulse train such that the peak of its pulses occur at a time of minimum current of the aggregated current wave form.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1A and I B illustrate a prior art single column resonant transfer capacitive dc-to- dc transformer in which the column is alternately connected first to a primary bus and then to a secondary bus.

Figure 2 illustrates a prior art example of a half-bridge capable of either including the associated capacitor in the column of figures 1 A and IB or isolating and bypassing it. Figure 3 illustrates a prior art example full-bridge, capable of either including the associated capacitor in the column of figures 1A and IB, doing so while reversing the polarity of its connection, or isolating and bypassing it.

Figure 4 illustrates the prior art wave forms and relative timing of input and output pulses from a resonant-transfer example of a capacitive dc-to-dc transformer.

Figures 5A and 5B illustrate a prior art single column controlled current capacitive dc-to- dc transformer in which the column is alternately connected first to a primary bus and then to a secondary bus.

Figure 6 illustrates example wave forms and relative timing of input and output pulses from a prior art controlled-current example of a capacitive dc-to-dc transformer.

Figure 7 illustrates the parallel aggregation of three resonant transfer dc-to-dc transformers, each equally offset in wave-train timing from the other, in a prior art system.

Figure 8 illustrates an example offset in pulse train timing between three prior art paralleled resonant-transfer dc-to-dc transformers.

Figure 9 illustrates a system wherein an aggregation of wind turbine-generators in which asynchronous ac output voltage current is first converted to dc voltage, then stepped up to a higher dc voltage by a three-column dc-to-dc transformer and ultimately to a still higher dc transmission voltage, both dc-to-dc transformations using, as examples, the resonant transfer transformer illustrated in figure 7.

Figure 10 illustrates a system wherein for a group of wind turbine generators each is controlled to produce an equal offset in wave-train timing from the other by a high-accuracy clock.

Figure 1 1 illustrates control functionality for clock-controlled wave -train timing within a representative wind-turbine based dc-to-dc transformer.

Figures 12A, B and C illustrate an example resonant transfer current waveform, and the leveling benefit of summing first six, then sixteen resonant transfer current waveforms, respectively. Figure 13 illustrates a system wherein communication and control links can enable equal offset in the timing of resonant pulse output wave trains from multiple capacitive dc-to-dc transformers.

Figure 14 illustrates control functionality for planned sequence controlled wave-train timing within a representative wind-turbine based dc-to-dc transformer.

Figure 15 illustrates a system with the addition of a control and communication link to the point at which current wave trains from all capacitive dc-to-dc transformers are aggregated, thus allowing adjustment of the timing of specified transformers in a way to smooth the aggregated wave-shape.

Figure 16 illustrates control functionality for composite wave form controlled wave-train timing within a representative wind-turbine based dc-to-dc transformer.

DETAILED DESCRIPTION OF EMBODIMENTS

In the present disclosure, a system that aggregates a number of separate electrical current pulse trains generated by a plurality of dc-to-dc transformers, includes a collector (e.g., a bus) that is configured to receive the pulse trains from the dc-to-dc transformers, and a control system that is configured to control a relative timing of the pulse trains generated by the plurality of dc- to-dc transformers, before the pulse trains are delivered to the bus. The pulses of the pulse trains are preferably equally offset in time from one another. The disclosure also includes methods that are implemented by the disclosed systems.

The control system may use a time-keeping system to control the relative timing of the pulse trains so that the pulses are equally offset in time from one another. There is a controller operably coupled with each transformer. There is also a communications system that connects the controllers together such that the controllers can coordinate phasing of the pulse trains. The controllers can adjust the relative time offset of the pulse trains so that the pulses remain equally offset in time from one another as the system changes (e.g., as one or more generators are dropped from or added to the system).

The dc-to-dc transformers are single-column capacitive transformers. These transformers can all be connected to a point at which current wave trains from all of the transformers are aggregated. The control system, based on the time of occurrence of a high point or a low point in the aggregated current wave form, can control one or more of the transformers to advance or delay a relative time offset of its pulse train such that the peak of its pulses occur at a time of minimum current of the aggregated current wave form.

Figure 9 shows a schematic of a multiple wind generator system 63 in which current output of each of a number of asynchronous wind-driven generators 20 is first converted to dc by an ac-to-dc converter 21 but remaining as dc, is then stepped up to produce, for the resonant transfer example, a current pulse train of the nature shown in figure 4 by a dc-to-dc transformer such as shown in figures 1A and IB. By use of a three, single column dc-to-dc transformers 23 for each generator 20, arrayed as shown in figure 7, in which the phasing of currents wave trains from each is offset by 120 electrical degrees from the others as shown in figure 8, a relatively smooth aggregate current output is produced. The collected current flowing into the collector bus 30 in figure 9 can then be stepped up to a transmission-level voltage by another three- column dc-to-dc transformer 26 and sent to a remote system.

The preferred embodiments of this invention, illustrated in figures 10, 13, and 15, eliminate the need for multiple dc-to-dc transformer columns within each wind-turbine unit, allowing each to generate a single pulse chain as shown in figure 4, but controlling the relative timing of pulse trains from each generator in a manner so as to allow each to deliver to a collector bus 30 a pulse series equally offset in time from all other pulse trains reaching that bus.

Suppose, for example, a group of n wind turbine-generators constitute a control group, recognizing that the wind farm may be comprised of any number of such control groups. In the first preferred embodiment of this invention, the dc-to-dc transformer associated with each wind turbine is equipped with a controller 40 of system 64, figure 10. Controller 40, an example of which is detailed in figure 1 1 , establishes the timing of both charge and discharge steps within dc-to-dc transformer 23 based on absolute time established by an on-board high-accuracy clock 44, the functionality of which is shown in figure 1 1. Clock 44, allows a phase control logic system 45 via transformer control line 41 to control the exact switching time of all switches within the dc-to-dc transformer 23 within individual wind turbine systems (each wind turbine system including a generator 20, an ac-to-dc converter 21, and a dc-to-dc transformer 23) so as initiate the relative phasing of its output current pulse train at a time resulting in each such pulse train being displaced from the others by 2P/n where P is the period of one charge-discharge cycle and n is the number of generators in a control group. For example, if the charge/discharge period of a dc-to-dc transformer is 2 milliseconds, and there are ten wind turbines in a group such as that shown in figure 10, then each controller 40 would establish a charge/discharge pattern displaced in time by 0.2 milliseconds from that of the prior wind turbine resulting in a relative smooth aggregate dc current wave form with a ripple period of 0.2 milliseconds. Figure 12A illustrates the current input or output wave-form of a single dc-to-dc transformer. Figure 12B illustrates the current input or output wave-form of the composite current when six such current wave-forms are superimposed . Figure 12C illustrates the current input or output wave-form of a composite current when sixteen such current wave-forms are superimposed.

Another multiple generator system 65, illustrated in figure 13, is enabled by a communication link 42 between individual wind turbines controllers 40a (controllers 40a shown in figure 14). In this embodiment, the optimum offset in phasing of individual turbine-generator output current wave trains, rather than being determined by absolute time, is determined by time displacement from the phasing of a reference turbine-generator within the system 65, where the reference generator can be any generator of system 65. In this case the phase control logic system 45a within each wind turbine-generator 20 causes its output wave train to be delayed by an interval specific to that wind turbine-generator such that the aggregation of wave trains from all wind turbine generators is maximally smoothed as was illustrated by figures 12B and 12C. Also, in this embodiment, as well as system 64 in figure 10, control logic may be enabled to adjust the phasing of individual wind turbines 20 such that, in the event one or more wind turbines 20 become inoperative, or if the number of generators changes for any other reason, the phasing of those wind turbines remaining operative are equally offset in timing, thereby continuing to produce a maximally smooth aggregated current output.

Another multiple generator system 66, illustrated in figure 15, eliminates the need for time-based coordination among pulse-trains emanating from various wind turbine-generators 20 by adding a communication link 43 between associated controllers 49, individual dc-to-dc transformers 23, and the collector bus 30 where either voltage or current profile can be monitored at the point where current inputs from all turbines 20 are aggregated before transformation to a higher voltage dc-to-dc transformers 26. In this embodiment, the functionality of which is further illustrated in fig. 16, the phase control logic system 45b associated with each wind turbine-generator 20 is aware of the both the voltage profile on bus 30 and aggregated current profile being fed from bus 30 to the step-up dc-to-dc transformers 26. Each wind turbine-generator within system 66 can then, through logic controller 45b, select a phasing of its own output current wave train to "smooth" the voltage on the collector bus 30 or the aggregate current flowing from that bus by timing its current pulse at the low point of that aggregated current.

While figures 9, 10, 13, and 15 are representative of how wind turbines may be connected to a collector system, those versed in the art will recognize the adaptability of the embodiments illustrated above to other collector system arrangements as well and, in fact, to any array of generating sources producing a pooled output, one non-limiting example of which is the output of multiple solar-voltaic arrays.

in all the above embodiments, it is recognized that the change in energy withdrawn in electrical form from a given wind turbine-generator due to adjustments in output pulse-train phasing will be at a frequency much too high to materially affect the speed or momentum of the turbine's mechanical system. Also, the pulse frequencies considered for dc-to-dc capacitor-based transformers are at least an order of magnitude higher than any mechanical resonance frequency associated with the wind-turbine drive train. The impact of pulse frequencies on the wind-turbine drive train may be minimized further by filter or capacitor application across the terminals of the dc side output of the rectifier that receives its ac power from the generator.

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 aggregating a plurality of separate electrical current pulse trains generated by a plurality of dc-to-dc transformers, comprising:

a collector bus that is configured to receive the pulse trains from the dc-to-dc

transformers; and

a control system that is configured to control a relative timing of the pulse trains generated by the plurality of dc-to-dc transformers, before the pulse trains are delivered to the bus.

2. The system of claim 1 , wherein the pulses of the pulse trains generated by the plurality of dc-to-dc transformers are equally offset in time from one another.

3. The system of claim 2, wherein the control system comprises a time-keeping system that is used to control a relative timing of the pulse trains generated by the plurality of dc-to-dc transformers such that the pulses of the pulse trains generated by the plurality of dc-to-dc transformers are equally offset in time from one another.

4. The system of claim 1 , wherein the control system comprises a controller associated with each transformer, where a controller is operably coupled with each transformer.

5. The system of claim 4, wherein the control system further comprises a communications system that operably connects the controllers together such that the controllers can coordinate phasing of the pulse trains generated by the plurality of dc-to-dc transformers.

6. The system of claim 5, wherein the controllers are configured to adjust a relative time offset of the pulse trains generated by the plurality of dc-to-dc transformers, such that the pulses of the pulse trains generated by the plurality of dc-to-dc transformers remain equally offset in time from one another.

7. The system of claim 1, wherein the dc-to-dc transformers comprise single-column capacitive dc-to-dc transformers.

8. The system of claim 7, wherein the control system comprises a control and

communication link that connects each of the single-column capacitive dc-to-dc transformers to a point at which current wave trains from each transformer are aggregated.

9. The system of claim 8, wherein the control system, based on the time of occurrence of a high point or a low point in the aggregated current wave form, causes one or more of the transformers to advance or delay a relative time offset of its pulse train such that the peak of its pulses occur at a time of minimum current of the aggregated current wave form.

10. A method of aggregating a plurality of separate electrical current pulse trains generated by a plurality of dc-to-dc transformers, comprising:

collecting the pulse trains from the dc-to-dc transformers; and

controlling a relative timing of the pulse trains generated by the plurality of dc-to-dc transformers, before the pulse trains are collected.

1 1 . The method of claim 10, wherein the pulses of the pulse trains generated by the plurality of dc-to-dc transformers are equally offset in time from one another.

12. The method of claim 1 1 , wherein the controlling step comprises controlling a relative timing of the pulse trains generated by the plurality of dc-to-dc transformers such that the pulses of the pulse trains generated by the plurality of dc-to-dc transformers are equally offset in time from one another.

13. The method of claim 10, wherein the controlling step comprises providing a controller associated with each transformer, where a controller is operably coupled with each transformer.

14. The method of claim 13, wherein the controlling step further comprises operably connecting the controllers together such that the controllers can coordinate phasing of the pulse trains generated by the plurality of dc-to-dc transformers.

15. The method of claim 14, wherein the controllers are configured to adjust a relative time offset of the pulse trains generated by the plurality of dc-to-dc transformers, such that the pulses of the pulse trains generated by the plurality of dc-to-dc transformers remain equally offset in time from one another.

16. The method of claim 10, wherein the dc-to-dc transformers comprise single-column capacitive dc-to-dc transformers. Title: System and Method for Smoothing Multiple Pulse-Based Generator Outputs

First-named inventor: Lionel Barthold

Attorney docket number: 18500-00029

17. The method of claim 16, wherein the controlling step comprises connecting each of the single-column capacitive dc-to-dc transformers to a point at which current wave trains from each transformer are aggregated.

18. The method of claim 17, wherein the controlling step, based on the time of occurrence of a high point or a low point in the aggregated current wave form, causes one or more of the transformers to advance or delay a relative time offset of its pulse train such that the peak of its pulses occur at a time of minimum current of the aggregated current wave form.