WO2012066143A2

WO2012066143A2 - Power electronic converter stage

Info

Publication number: WO2012066143A2
Application number: PCT/EP2011/070585
Authority: WO
Inventors: Jens Friebe; Klaus Rigbers
Original assignee: Sma Solar Technology Ag
Priority date: 2010-11-19
Filing date: 2011-11-21
Publication date: 2012-05-24
Also published as: WO2012066143A3; DE102010060687A1

Abstract

A power electronic converter stage comprises a plurality of converters (5 to 8, 18) of different maximum converter power values connected in parallel, wherein the maximum converter power values of at least n converters (5 to 8) being a subset of the plurality of converters (5 to 8, 18), n being an integer higher than or equal to 3, comprise a maximum power value ratio of 2⁰ to 2¹ to 2² to 2^(n-1). The plurality of converters includes one further converter (18) in addition to the n converters (5 to 8), and it provides a cumulative maximum stage power. A present stage power which is lower than the cumulative maximum stage power is provided by a suitable selection of converters from the plurality of converters (5 to 8, 18), wherein the further converter (18) is part of each selection of converters.

Description

POWER ELECTRONIC CONVERTER STAGE

FIELD

The invention relates to a power electronic converter stage comprising a plurality of converters of differing converter maximum powers connected in parallel.

Particularly, the power electronic converter stage is a DC/DC converter stage or a DC/AC converter stage of an inverter for feeding electric energy provided by a photovoltaic generator into an AC power grid.

The electric power provided by a photovoltaic generator is subject to strong fluctuations. It depends on the position of the sun, clouds, the temperature and other factors. Thus, it is a challenge to feed the electric power provided by the photovoltaic generator, independently of its absolute magnitude, at a high efficiency into the AC power grid. An electric converter, like for example a DC/DC converter or a DC/AC converter, typically displays its optimum efficiency close to its maximum converter power value, i.e. close to the maximum power forwarded by it, only, whereas it just has an efficiency which is lower by several percent points at a half of its maximum converter power value, for example. The same problem of efficiency due to fluctuating electric power like in photovoltaic generators also arises in other electric generators, like for example wind power generators.

BACKGROUND

In a power electronic converter stage of an inverter for feeding electric energy provided by a photovoltaic generator into an AC power grid, it is known to provide a plurality of identical converters which are principally connected in parallel. Of these converters, however, only so many are operated as they are required for feeding the present power of the photovoltaic generator into the AC power grid. In this way, the presently active converters are operated at a power which is closer to their maximum converter power values as compared to a case in which all converters of the converter stage are simultaneously active. Correspondingly, the efficiency of the converter stage is increased. This concept results in a maximum increase of the efficiency, when the present power of the photovoltaic generator is transferred by using such a number of converters of the converter stage which can no longer been reduced without being unable to transfer the complete present power of the photovoltaic generator. When the known power electronic converter stage only transfers very low electric powers, however, situations may occur in which even the few converters used only have a low efficiency at the power presently transferred by them. Such cases may also occur with higher powers transferred, if the number of converters of the converter stage is comparatively low, and if the sum of the converter maximum powers may thus only be adjusted to the present power of the photovoltaic generator in coarse steps.

From Zumel, P.; Fernandez, C; de Castro, A.; Garcia, O.: "Efficiency improvement in multiphase converter by changing dynamically the number of phases" in Power Electronics Specialists Conference, 2006, PESC Ό6. 37th IEEE a further converter stage comprising a plurality of identical converters connected in parallel is known. For increasing the efficiency, the number of the converters operated in an interleaving mode is dynamically varied as a function of a connected load to reduce the unavoidable losses with small loads. For increasing the efficiency of a power electronic power stage which is a buck converter, it is known from Jia Wei: "High Frequency High-Efficiency Voltage Regulators for Future Microprocessors" Dissertation, Virginia State University, Blacksburg, Virginia, USA to connect one converter of a smaller dimension in parallel to a plurality of converters of an equal, large dimension which are also connected in parallel with regard to each other. By means of this additional, so-called "Baby-Buck Channel", small powers are transferred at a high efficiency at which even only a single one of the large dimension converters connected in parallel would only have a very low efficiency. In this way, however, only a small part of the efficiency problem occurring with strongly fluctuating electric powers to be transferred by a power electric converter stage is solved. DE 198 05 926 A1 discloses a converter stage comprising a plurality of converters connected in parallel. At each point in time, only one of these converters is operated as an active voltage controller whereas the other operating converters are operating at full power. When the converter which is presently operated as the voltage controller reaches its maximum power, a further converter is being operated which then overtakes the voltage control. Vice versa, if the converter which is presently operated as the voltage controller gets close to zero power, it is switched off and the voltage control is transferred to one of the other converters previously operated at full power. In this way, different converters of the DC/DC converter stage are operated for controlling the voltage at different powers transferred by the power converter stage. The efficiency of this known power stage varies with the efficiency of the converter which is presently operated as the voltage controller. This efficiency may be quite low, if this converter only transfers a comparatively low power.

DE 102 16 252 A1 discloses a DC/DC converter stage comprising two converters connected in parallel. One of these converters is operated at a higher frequency than the other and it is preferably of a smaller dimension with regard to the maximum power transferred. The smaller converter has a higher efficiency at small powers which decreases towards higher powers, whereas the efficiency of the bigger converter increases with increasing power transferred. Thus, a controller of the known converter stage only operates the bigger converter with higher powers to be transferred. On the other hand, the smaller converter is switched off with higher powers to be transferred. In a transient area between the operation of only the small converter and only the bigger converter, both converters are operated. DE 102 23 771 A1 discloses a power electric converter stage comprising a plurality of converters connected in parallel. In one embodiment of this known converter stage, the individual converters have a same dimension, i.e. a same maximum converter power value. In another embodiment the maximum converter power values are increasing from one converter to the next bigger one by a factor of 2. This allows to adjust a high number of different powers to be transferred as a sum of maximum converter power values of different groups selected from the total number of converters. This allows for always operating the known power electronic converter stage at a total of the maximum converter power values of the converters presently operated which is only a little higher than the power which is presently to be transferred. Nevertheless, with a limited number of converters, the steps at which the totals of the maximum converter power values of the converters presently operated may be provided will be quite large. It is known that the efficiency of converters may be increased by using special electric or electronic components. However, this strongly increases the cost of the converters, particularly if the maximum converter power value is to be high.

Thus, there still is a need for a power electric converter stage which has a high efficiency close to the efficiency provided by a converter operated close to its maximum converter power value over a large range of powers to be transferred.

SUMMARY OF THE INVENTION

The invention provides a power electronic converter stage comprising at plurality of converters of different maximum converter power values connected in parallel, wherein the plurality of converters provides for a cumulative maximum stage power, wherein a present stage power which is lower than the maximum stage power is provided with a suitable selection of converters from the plurality of converters connected in parallel, wherein the maximum converter power values of at least n of the converters, n being an integer higher than or equal to 3, display a maximum power value ratio of 2° to 2¹ to 2² to 2^(n"1), wherein the plurality of converters connected in parallel includes one further converter in addition the n converters, and wherein this further converter belongs to each selection of converters by which the present stage power is provided.

Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and the detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined by the claims.

SHORT DESCRIPTION OF THE DRAWINGS

The invention will be better understood with reference to the accompanying drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. I n the drawings, like reference numerals designate corresponding parts throughout the several views. illustrates a prior art power electric converter stage comprising four converters connected in parallel whose converter maximum powers display a ration of 1 : 2 : 4 : 8. is a plot of the efficiency of the converter stage according to Fig. 1 over the stage power during activation of its three smallest converters (depicted with a dotted line) in comparison to the efficiency of a power electronic converter stage which activates three converters of a same maximum converter power value over the same power range (depicted with a full line). illustrates a power electronic converter stage according to the present invention comprising four converters connected in parallel whose maximum converter powers values display a ratio of 1 : 2 : 4 : 8 and one further converter connected in parallel comprising the smallest maximum converter power value of the four other converters. is a plot of the efficiency of the converter stage according to Fig. 3 over the stage power during activation of the three smallest converters of the four converters with continuous operation of the one further converter (depicted with a dotted line) in comparison to the efficiency of a power electronic converter stage which provides the same stage power with four converters of a same maximum converter power value connected in parallel (depicted with a full line). is a more detailed embodiment of the converter stage according to Fig. 3 designed as a boost converter between an input side DC voltage link and an output side DC voltage link. illustrates a further more detailed embodiment of the converter stage according to Fig. 3 designed as a buck converter between an input side DC voltage link and an output side DC voltage link; and

Fig. 7 illustrates a further more detailed embodiment of the converter stage according to Fig. 3 designed as an inverter between an input side DC voltage link and an AC output. DETAILED DESCRIPTION

I n the power electronic converter stage according to the present invention, the converter maximum powers of at least three, preferably at least four and even more preferably five or more converters connected in parallel display a maximum power value ratio of 2° to 2¹ to 2² to 2^(n"1). This means that the next bigger converter comprises twice the maximum converter power value than the biggest previous converter. This ratio of the maximum converter power values of these n converters allows for providing sums of their maximum converter power values in close steps with a comparatively low number of converters. At these sums of the maximum converter power values, i.e. at the corresponding selections from the n converters, the converter stage displays its maximum efficiency. Particularly, the steps between these sums of the maximum converter power values at which a maximum efficiency is obtained is only as big as the maximum converter power value of the smallest converter of the n converters. Thus, this smallest maximum converter power value defines the steps at which the stage power of the converter stage can be provided at maximum efficiency. Even with only three converters these steps are much smaller than the third part of the overall sum of the maximum converter power values, i.e. less than half of this part. With four converters the steps are smaller than a third the fourth part of the overall sum of the maximum converter power values. Already due to this, the average efficiency of the power electronic converter stage according to the present invention is quite high and only fluctuates a little from small powers up to the overall sum of all maximum converter power values.

In addition to that, the power electronic converter stage according to the present invention comprises one further converter whose maximum converter power value is as big as the smallest maximum converter power value of the n converters whose maximum converter power values display the ratio of 2° to 2¹ to 2² to 2^(n"1). The function of this further converter is to cover the steps or distances between the possible sums of the converter maximum powers of the n converters. Thus, the n converters may always be operated at their maximum converter power value, if they are operated at all, to provide a power at a maximum efficiency which is slightly lower than the present stage power and which is supplemented by the further converter to provide the full present stage power. As a result, only the further converter is operated at varying power, whereas the n converters are either operated at their maximum converter power value or not at all. This makes operating the n converters very easy. Further, it allows for designing only the further converter in such a way that is has a high efficiency also at small converter powers below its maximum converter power value to provide for a high efficiency of the entire converter stage at all present stage powers. The effort to be taken for this single one further converter comprising a high efficiency beginning with a converter power close to zero up to its maximum converter power value is particularly low in the power electronic converter stage according to the present invention as this single further converter only has the smallest maximum converter power value of the n converters, i. e. of all converters connected in parallel. This means that even with a very expensive construction of the one further converter the overall costs of the power electronic converter stage are not strongly increased. Further, the total power of the power electronic converter stage may be easily doubled by adding a simple converter which comprises twice the maximum converter power value of the previously highest maximum converter power value of the n converters but which displays a high efficiency at its maximum converter power value only. Thus, increasing the maximum power of the converter stage by a factor of 2 is possible at comparatively low cost.

For the purpose of also providing a high efficiency at small converter powers clearly below its maximum converter power value, the further converter preferably comprises another design or higher value components than the n converters or both. Further, it has to be cared for that the further converter displays a longer lifetime than all other converters of the power electronic converter stage of the present invention as it belongs to each selection of converters by which a present stage power of the power electronic converter stage is provided. The further converter is permanently operated and it is supplemented by a suitable selection of converters from the n converters which are all operated at a power of their or close to their maximum converter power value, which provides for a optimum overall efficiency.

On the other hand, the n converters are typically of a same kind and may only differ in their maximum converter power value and the respective dimension of their parts or components.

The ratio of the maximum converter power values of the n converters of 2° to 2¹ to 2² to 2^(n"1) needs not to be kept exactly. Often, this is even not possible due to unavoidable tolerances. It proves to be sufficient if the maximum converter power values of any two of the n converters which have consecutive maximum converter power value display a ratio of their maximum converter power values in a range from 1 to 1.5 to 1 to 3.0. However, it is preferred if the ratio is only from 1 to 1.5 to 1 to 2.0. The power electronic converter stage according to the present invention may be particularly be designed as a DC/DC converter stage or a DC/AC converter stage. In principle, however, it may also be an AC/DC converter stage. For example, it may be a boost converter, a buck converter or an inverter, particularly a rectifier. Due to the high range of powers over which the power electronic converter stage provides for stage powers at which it displays a very high efficiency, it is particularly suited as a DC/DC converter stage or a DC/AC converter stage in an inverter for feeding electric energy provided by a photovoltaic generator into an AC power grid.

Referring now in greater detail to the drawings, Fig. 1 illustrates a power electronic converter stage 1 in a one-line diagram. The power electronic converter stage 1 transfers electric power between an input 2 and an output 3 by means of four converters 5, 6, 7 and 8 connected in parallel. A controller 9 always activates that selection of converters from the converters 5 to 8, which is defined by the sum of the maximum converter power values of the selected converters being closest to the actual stage power from above as compared to all other possible selections from the converters 5 to 8. I.e. the selected converters are all operated as close as possible to their maximum converter power value to provide the present stage power. By means of a ratio of the maximum converter power values of the converters 5 to 8 of 1 : 2 : 4 : 8, the maximum difference between the present stage power and the sum of the maximum converter power values of the selected converters is limited to the maximum converter power value of the smallest converter 5. This step in power is clearly less than a third of the average maximum converter power value of the four converters 5 to 8. This average maximum converter power value is exactly 3.75 times the maximum converter power value of the smallest converter 5.

Fig. 2 is a plot of the efficiency of the power electronic converter stage of Fig. 1 during activation of its three smallest converters 5 to 7 connected in parallel whose maximum converter power values are assumed to be 300 W. 600 W and 1200 W here. The efficiency is plotted over the present stage power with a dotted line. For comparison, a full line depicts the efficiency of a power electronic converter stage which provides a same maximum stage power of 2100 W with three converters of a same maximum converter power value of 700 W connected in parallel. The plot only shows the range of the efficiency from 96 to 99 % over the stage power up to the maximum stage power of 2100 W. Due to the ratio of the maximum converter power values of 1 : 2 : 4, the range of the stage power up to the maximum stage power of the converter stage of 2100 W may be divided in seven equal ranges of the present stage power in which - except of close to a stage power of zero - always a comparatively high efficiency is obtained, because the converters selected for providing the present stage power are all operated comparatively close to their maximum converter power value. In contrast, the three converters of equal maximum converter power value only allow for a division of the present stage power in three equal ranges, the efficiency of the converter stage displaying very low values at the beginning of each of these ranges. Only at two points of the stage power, the efficiency of the converter stage with three converters of equal size exceeds the efficiency of the converter stage having the converters with the ratio of their maximum converter power values of 1 : 2 : 4. Everywhere else the efficiency of this converter stage according to Fig. 1 is much higher.

Fig. 3 illustrates a power electronic converter stage 1 according to the present invention which, in addition to the n converters of the converter stage 1 according to Fig. 1 , comprises one further converter 18 connected in parallel. The maximum converter power value of this further converter 18 is as high as the maximum converter power value of the smallest converter 5 of the converters 5 to 8. However, the further converter 18 is of another construction than the converters 5 to 8, and due to this other construction it is able to provide for a very high efficiency in transferring power between the input 2 and the output 3, even if it transfers a power much lower than its maximum converter power value. Fig. 4 depicts the course of the efficiency over the present stage power of the power electronic converter stage according to Fig. 3 with a dotted line. However, the range of the stage power depicted does not include the operation of the biggest fourth converter 8 of the converters 5 to 8. The three converters 3 to 7 connected in parallel are assumed to have maximum converter power values of 375 W, 750 W and 1500 W here, and the one further converter 18 of the other kind thus also has the smallest maximum converter power value of 375 W. This further converter comprises its maximum efficiency already at a power much lower than its maximum converter power value and a particularly high efficiency over a large range of its converter power. Due to the additional converter, there is a further 8^th division of the maximum stage power in which the actual stage power is provided by another selection of converters. Additionally, the average efficiency of the converter stage is very high particularly as compared to the average efficiency of the comparison example of a converter stage comprising four converters of a same maximum converter power value of 750 W each, whose efficiency is plotted with a full line here. Further, the relative average efficiency, i. e. the average efficiency as compared to the absolute maximum of the efficiency, is also significantly higher than in case of the converter stage whose efficiency is depicted in Fig. 2 and which only has n converters of a same kind displaying. The maximum efficiencies of t h e converters depend on their construction and components. In Fig. 4, it is assumed that the maximum efficiency of the further converter is slightly higher than the maximum efficiencies of all other converters which are assumed as being equal. That the absolute maximum of the efficiency in Fig. 4 is lower than that one in Fig. 2 has no meaning and only reflects free assumptions on which these plots are based. Fig. 5 illustrates an embodiment of the power electronic converter stage 1 according to the present invention as a boost converter 10 between an input side DC voltage link 11 , which is indicated by means of a buffer capacitor 12 and an output side DC voltage link 13 which is also indicated by means of a buffer capacitor 14. The converters 5 to 8 with one by one doubled maximum converter power values do not differ in their construction and their parts despite the dimensions of these parts or electronic components. They each comprise the usual circuitry of a boost converter including a inductor 15, a switch 16 and a diode 17. The switches of the individual converters may in a usual way be operated in an interleaving mode to have an as uniform power flow through the converter stage 1 as possible. The further converter 18 is also of the typical boost converter kind but comprises higher value components, particularly another switch than the switch 16 of the converter 5 of equal maximum converter power value. This higher value components result in a higher efficiency of the further converter 18.

Fig. 6 illustrates another embodiment of the power electronic converter stage 1 according to the present invention as a buck converter 19 between an input side DC voltage link 11 indicated by means of a buffer capacitor 12 and an output side DC voltage link 13 also indicated by means of a buffer capacitor 14. The converters 5 to 8 each comprise the inductor 15, the switch 16 and the diode 17 here in a circuitry typical for a buck converter. The same applies to the further converter 18 which again consists of higher value components. The switches of the individual converters may be operated in a known interleaving mode to have an as uniform power flow through the converter stage 1 as possible. Fig. 7 illustrates a further embodiment of the power electronic converter stage 1 according to the present invention as an inverter 20 between an input side DC voltage link 11 indicated by means of a buffer capacitor 12 and an AC output 21. Each of the converters 5 to 8 displaying different maximum converter power values comprises a half bridge 22 of switches 23 and 24 and an inductor 25 connecting the central point of the half bridge 22 to the AC output. The operation of the switches 23 and 24 of each half bridge 22 takes place in a usual way for an inverter, whereas the operation of the active converters 5 to 8 is typically coordinated according to an interleaving mode. The further converter 18 which is arranged in parallel to the converters 5 to 8 also comprises a half bridge with switches 23 and 24 and an inductor 25 connected to its centre but it is made of higher value components to provide for a better efficiency at present converter powers below its maximum converter power value.

Many variations and modifications may be made to the preferred embodiments of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of the present invention, as defined by the following claims.

LIST OF REFERENCE NUMERALS power electronic converter stage

input

output

converter

controller

boost converter

DC voltage link

buffer capacitor

DC voltage link

buffer capacitor

inductor

switch

diode

further converter

buck converter

inverter

AC output

half bridge

switch

inductor

Claims

1. A power electronic converter stage comprising

- a plurality of converters (5 to 8, 18) of different maximum converter power values connected in parallel,

- wherein the plurality of converters (5 to 8, 18) provides a cumulative maximum stage power,

- wherein a present stage power that is lower than the cumulative maximum stage power is provided by a selection of converters from the plurality of converters (5 to 8, 18), and

- wherein the maximum converter power values of at least n converters (5 to 8) being a subset of the plurality of converters (5 to 8, 18), n being an integer higher than or equal to 3, comprise a maximum power value ratio of 2° to 2¹ to 2² to 2^(n"1),

characterized in

- that the plurality of converters includes one further converter (18) in addition to the n converters (5 to 8), and

- that this further converter (18) is part of each selection of converters.

2. The power electronic converter stage (1 ) of claim 1 , wherein the maximum converter power value of the further converter (18) is as high as the smallest maximum converter power value of the n converters (5 to 8).

3. The power electronic converter stage (1) of claim 1 or 2, wherein the further converter (18), at a converter power that is lower than its maximum converter power value comprises a higher efficiency than the n converters (5 to 8).

4. The power electronic converter stage (1) of claim 3, wherein the further converter (18) is of another design than the n converters (5 to 8) .

5. The power electronic converter stage (1 ) of claim 3 or 4, wherein the further converter (18) comprises other components than the n converters (5 to 8).

6. The power electronic converter stage (1 ) of any of the claims 1 to 5, wherein the n converters (5 to 8) are of a same design.

7. The power electronic converter stage (1) of claim 6, wherein the n converters (5 to 8) only differ with regard to their maximum converter power value.

8. The power electronic converter stage (1) of any of the claims 1 to 7, wherein the present stage power is provided by that selection of converters in which the sum of the maximum converter power values of the selected converters (5 to 8, 18) comes closest to the present stage power from above.

9. The power electronic converter stage (1) of claim 8, wherein, for providing each present stage power, the n converters (5 to 8), so far as belonging to the selection of converters, are operated at their maximum converter power value, and that the further converter (18) is operated at variable power to provide for the remainder of the stage power.

10. The power electronic converter stage (1) of any of the claims 1 to 9, wherein n is equal to or higher than 4.

1 1. The power electronic converter stage (1) of any of the preceding claims, and being a converter stage of an inverter for feeding electric energy provided by a photovoltaic generator into an AC power grid.

12. The power electronic converter stage (1) of any of the preceding claims, and being a DC/DC converter stage.

13. The power electronic converter stage (1) of claim 12, and being a boost converter (10).

14. The power electronic converter stage (1) of claim 12, and being a buck converter (19).

15. The power electronic converter stage (1 ) of any of the preceding claims 1 to 11 , and being a DC/AC converter stage.