WO2011101030A1

WO2011101030A1 - An electric power plant and a method for control thereof

Info

Publication number: WO2011101030A1
Application number: PCT/EP2010/051994
Authority: WO
Inventors: Konstantinos Papastergiou
Original assignee: Abb Research Ltd
Priority date: 2010-02-17
Filing date: 2010-02-17
Publication date: 2011-08-25

Abstract

An electric power plant comprises a solar energy park (10), a DC line (12) connecting the park to a DC/AC converter (13) and an alternating voltage network (15) connected to the latter. An energy storing assembly (16) is connected to the DC line (12) in parallel with the solar energy park (10). An arrangement (18) collects information about operation parameters of the plant and a control unit (19) controls flow of electric power within the plant based on information from this arrangement.

Description

An electric power plant and a method for control thereof

TECHN ICAL FI ELD OF THE I NVENTION AN D BACKGROUND ART

The present invention relates to the field of electric power plants comprising a solar energy park having a plurality of solar energy panels configured to generate electric power, a DC line connecting said park to a direct voltage side of a DC/AC converter, a said DC/AC converter configured to convert direct voltage into alternating voltage and conversely as well as an alternating voltage network connected to an alternating voltage side of said DC/AC converter, as well as a method for controlling the power flow within such an electric power plant.

The invention is not directed to a particular size of such an electric power plant with respect to the maximum power which may be generated by said solar energy park or the level of the voltage on said DC line. Thus, "solar energy park" is to be inter- preted to cover an assembly of solar energy panels of one single dwelling house as well as larger solar energy parks producing a power of several MW . "Alternating voltage network" is here to be interpreted to cover the range from a local connection to a few consumers of AC power to electric power networks for distribution or transmission of high voltage AC power, in which case a transformer normally has to be arranged for connecting the alternating voltage side of said converter to the alternating voltage network. An electric power plant of the type mentioned above is schematically illustrated in appended Fig 1 showi ng a solar energy park 1 here through a DC/DC converter 2 connected to a DC line 3 connecti ng to a DC/AC converter 4 , which in its turn is through a transformer 5 connected to an alternating voltage network 6 in the form of an electric power distribution grid .

For a large solar park the infrastructure investment normally accounts for almost 50% of the total capital costs, and the size of this infrastructure investment with respect to the total capital costs may be even higher for smaller energy parks. However, the infrastructure in the form of cables, DC/AC converter, transformers and the like of such an electric plant generating electric power through solar energy panels is not at all utilized during night when the solar energy panels deliver no power and only to a small extent during cloudy days.

This problem may be at least partially addressed by providing a plant of this type with an energy storing assembly connected to said DC line in parallel with said solar energy park, an arrangement configured to collect information about operation parame- ters of the plant and a control unit configured to control flow of electric power within the plant based on information from said arrangement. Such a plant is known through WO 2008/139267 A1 , in which it is disclosed how the energy storing assembly may be charged with electric power from the solar energy park when this produces excess power, such as during sunny days, and how this electric power stored may be fed to the DC/AC converter and by that to said alternating voltage network during night and during cloudy days when the yield of the solar energy panels is zero or very low. This means a better utilization of the existing infrastructure and by that an increased return of the infrastructure investment of an electric power plant of this type.

However, it is of course an ongoing attempt to improve such plants with respect to for instance efficiency degree of utilization of the equipment thereof etc. SUMMARY OF THE I NVENTION

The object of the present invention is to provide an electric power plant of the type defined above being in at least some as- pect improved with respect to such plants already known .

This object is according to the present invention obtained by providing such a plant with the additional feature that said control unit is configured to carry out said control so as to regulate the total flow of electric power on said DC line to said DC/AC converter and by that to said alternating voltage network by adjusting the proportion of said total power fed from said solar energy park and the proportion of said total power fed from said energy storing assembly based upon information from said ar- rangement.

It may in this way be possible by this type of regulation of the total flow of electric power to the alternating voltage network based upon information about operation parameters of the plant obtain this total flow of electric power in the most appropriate way considering said information , which may relate to the charging state of the energy storing assembly, the weather, time of the day and spot price on the electric power market etc. Accordi ng to an embodiment of the invention said control unit is configured to carry out said control so as to obtain a flow of electric power from the solar energy park as well as from said energy storing assembly upon receipt of information from said arrangement that the power flow presently deliverable by said solar energy park is lower than a determined requested total flow of electric power to said DC/AC converter. This means that a determined requested total flow of electric power to said DC/AC converter may be obtained , or it may come closer to this requested total flow than should only electric power be delivered from said solar energy park to the DC/AC converter, so that the efficiency of the DC/AC converter may be increased . According to a further development of this embodiment said control unit is configured to carry out said control so as to regulate said total flow of electric power for obtai ning a said determined requested total flow of electric power constituting a maximum electric power deliverable to said DC/AC converter for obtaining a maximum of efficiency of the operation of the DC/AC converter. This means that the loss of electric power in said DC/AC converter will be kept at a minimum also during cloudy days when the electric power delivered by the solar energy park would as the only electric power delivered to the DC/AC converter result in a considerably higher loss of electric power i n said converter. Furthermore, during sunny days, especially in summer months, the electric power deliverable by the solar energy park may be high enough for obtaining a maximum efficiency of the DC/AC converter and then the proportion of electric power fed from the energy storing assembly may be zero.

Accordi ng to another embodiment of the invention said energy storing assembly comprises energy storing members in the form of electric batteries. No long connection cables are required between such electric batteries and the DC/AC converter, but they may in fact be co-located , which reduces cable transmission losses. The solar energy panels of a solar energy park and electric batteries typically produce voltages of the same order of magnitude making it suitable to use the same DC/AC converter for both these sources.

Accordi ng to another embodiment of the invention said energy storing assembly comprises at least one series connection of a plurality of electric batteries, and the number of such batteries in the series connection may be chosen to obtain a voltage level desired in the particular electric power plant according to the invention . Accordi ng to another embodiment of the invention the plant comprises at least one four quadrant DC/DC converter connect- ing said energy storing assembly to said DC line, and the control unit is configured to control the operation of said DC/DC converter for controlling flow of electric power to and from said assembly. The combination of the use of electric batteries as energy storing members and the use of such a four quadrant DC/DC converter for connecting them to the DC line opens up many possibilities to control the electric power flow within said plant and by that obtain exactly the power flow most appropriate with respect to the actual operation parameters of the plant. This means that the direction of flow of electric power to or from the energy storing assembly as well as the magnitude of this flow may be determined by correspondingly controlling said DC/DC converter. Accordi ng to a further embodiment of the invention said control unit is configured to carry out said control of said at least one DC/DC converter to feed electric power from said solar energy park to said energy storing assembly based upon corresponding information received from said arrangement. Such a control is preferably carried out during sunny days when the solar energy park may possibly generate more electric power than presently needed on the alternating voltage network or when the spot price on the electric power market is low. Accordi ng to another embodiment of the invention said control unit is configured to carry out said control of said at least one DC/DC converter so as to adj ust the level of the voltage of the electric power arriving from said solar energy park to the voltage level of the electric batteries of the energy storing assembly. The voltage level of the electric power delivered by the solar energy park is dependent upon the strength of the sun radiation and may accordingly vary considerably, but a voltage level suitable for charging the electric batteries may by this still be obtained through the control of the DC/DC converter. Accordi ng to another embodiment of the invention said control unit is configured to control said at least one DC/DC converter so as to adj ust the proportion of electric power fed from said solar energy park to said energy storing assembly with respect to the proportion of electric power fed from said solar energy park to said DC/AC converter and by that to said alternating voltage network.

Accordi ng to a further embodiment of the invention said control unit is configured to control said at least one DC/DC converter so as to adj ust the proportion of electric power fed from said energy storing assembly to said DC/AC converter with respect to the electric power fed from said solar energy park to the DC/AC converter based on information from said arrangement. The control may by this for instance be carried out so as to obtain a maximum efficiency of the operation of the DC/AC converter or for obtaining an optimum consideration of the present spot price on the electric power market. Accordi ng to another embodiment of the invention said control unit is configured to carry out control of said DC/AC converter so as to feed electric power from the alternating voltage network to said energy storing assembly based upon information from said arrangement. Such information may be a very low spot price on the electric power market, mostly during night, so that energy may be stored to a low cost for later being delivered to the alternating voltage network when the price thereof is much higher and/or there is a larger need of feeding electric power to the alternating voltage network.

Accordi ng to another embodiment of the invention said solar energy park comprises a plurality of solar energy panels connected in series so as to obtain a voltage level of electric power generated thereby requested on the DC line arriving to said DC/AC converter. This means that it will not be absolutely necessary to have a separate DC/DC converter connecting the solar energy park to said DC line.

Accordi ng to another embodiment of the invention the solar energy park has a rated power of at least 50 kW or at least 500 kW , such as > 1 MW , and the DC line may according to another embodiment of the invention be configured to apply a voltage of > 500 V or > 2 kV, such as a voltage of 500 V - 5 kV, to said DC/AC converter. These are typical suitable levels of power and voltage for a plant according to the invention .

Accordi ng to another embodiment of the invention said arrangement is configured to collect information about operation parameters in the form of the present

a) charging state of said energy storing assembly, and/or b) weather, such as sunny or cloudy, and/or

c) time of the day, such as in broad daylight or at night, and/or d) spot price on the electric power market,

and the control unit is configured to carry out said control based upon this information .

The invention also relates to a method for controlling flow of electric power within an electric power plant according to the appended independent method claim. The advantages and ad- vantageous features thereof and of embodiments of the invention defi ned in the appended dependent method claims appear clearly from the discussion above of embodiments of the electric power plant according to the invention . The method according to the invention may be carried out through a computer program loadable directly into the internal memory of a computer, such as through the Internet, and the present invention also relates to such a computer program and a corresponding computer program product. Further advantages as well as advantageous features of the invention will appear from the following description .

BRI EF DESCRI PTION OF THE DRAWI NGS

With reference to the appended drawings, below follows a specific description of embodiments of the invention cited as examples. In the drawi ngs:

Fig 1 is a very simplified view showing the general construction of a known electric power plant of the type to which the present invention relates,

Fig 2 is a very simplified view illustrating an electric power plant according to a first embodiment of the invention ,

Fig 3 is a view similar to Fig 2 of an electric power plant ac- cording to a second embodiment of the invention , and

Figs 4-6 are views of the electric power plant according to Fig 3 illustrating different modes of operation thereof. DETAI LED DESCRI PTI ON OF EMBODI MENTS OF THE I NVENTION

Explained herein are embodiments of the invention , describing an electric power plant of the invention and a method of control- ling flow of electric power within such a plant. The invention may, however, be embodied in many different forms and should not be construed as being limited to the exemplary embodiments set forth herein ; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully con- vey the concept of the invention to those skilled in the art. An electric power plant according to a first embodiment of the present invention is very schematically illustrated in Fig 2. This plant has a solar energy park 10 having a plurality of strings of series connected photovoltaic solar energy panels 1 1 . The number thereof may in the practice be much higher than shown in the Figure, and such a plant may typically have a rated power in the order of 1 MW . The plant further comprises a DC line 1 2 connecting the park to a direct voltage side of a DC/AC converter 13 of any conceivable type by means of DC breakers 14. The alternating voltage side of the DC/AC converter is connected to an alternating voltage network 15, possibly through a transformer not shown .

The invention also comprises an energy storing assembly 16 in the form of strings of electric batteries 1 7 connected in series. These electric batteries may be located close to the DC/AC converter reducing the electric power transmission losses in the connection cable between this assembly 16 and said converter 13, even if the solar energy park 10 is located at a considerable distance, such as perhaps in the order of 1 km or more , to said converter. The DC breakers 14 may be controlled to either connect the solar energy park or the energy storing assembly to the DC/AC converter depending upon operation parameters of the plant.

The plant further comprises an arrangement 1 8 configured to collect information about operation parameters of the plant, such as the strength of the sun radiation at the site of the solar energy park 1 0, the present spot price on the electric power market and the charging state of the electric batteries in the energy storing assembly 16. The arrangement may for this sake have equipment communicating with different sensors in a manner known per se. The plant further comprises a control unit 19 configured to control flow of electric power within the plant based on information from said arrangement 18. Thus, the control unit 19 will control the state of the DC breakers 14 and the operation of the DC/AC converter 13 as indicated by the control means 20 therefor. The operation of an electric power plant shown in Fig 2 may be as follows. During the day when the sun shines the solar energy park 10 may be connected to the DC line by havi ng the DC breakers 14 in the state shown in Fig 2 and by that supply electric power to the alternating voltage network 15 through the DC/AC converter 13. During night or when one or more of the solar energy panels 1 1 are shaded or in a fault condition one or more battery strings of the energy storing assembly 16 may be connected to the converter 1 3 for providing the electric power requested to the alternating voltage network together with one or more of the solar energy panel strings or alone. The electric batteries 1 7 may be charged from the alternating voltage network 1 5 by appropriate control of the DC/AC converter 13 when there is excess energy on the alternating voltage network or the spot price on the electric power market for any other reason is low.

Fig 3 illustrates an electric power plant according to a second embodiment of the invention . The parts thereof corresponding to parts of the embodiment shown in Fig 2 are provided with the same reference numerals as those used in Fig 2. This embodiment differs from the one in Fig 2 by having each battery stri ng of the energy storing assembly 16 connected to the DC line through a four quadrant DC/DC converter 21 . The control unit 19 is for that sake configured to also through control of said DC/DC converters 21 control flow of electric power within the plant as indicated by further control means 22. The arrangement of such four quadrant DC/DC converters enables controlling of flow of electric power to and from the energy storing assembly 16 by control of the operation of these converters. Possible ways of operating an electric power plant according to the embodiment shown in Fig 3 will now be explained while making reference to Figs 4-6. Fig 4 shows how the energy storing assembly 16 may through control of the DC/DC converter 21 be controlled to deliver electric power to the DC/AC converter 13 and by that to the alternating voltage network 15 when the solar energy park 1 0 is in operation but not producing enough energy to keep the DC/AC converter 13 running at maximum power, hence highest efficiency. This could happen when one or more solar energy panels in a string become faulty or when the sky is cloudy or when one or more panels/strings are shaded by nearby buildings/other strings.

Fig 5 shows how the DC breakers 14' may be closed and the DC/DC converter may be controlled for charging the electric batteries in the energy storing assembly 16 directly from the solar energy park 10. This may for instance be appropriate when the sun is shini ng in broad daylight and all the solar energy panel strings are in full operation , so that the solar energy park will possibly generate more power than needed to be fed to the alternating voltage network. Fig 6 illustrates through the arrow A how the control unit may through control of the DC/DC converters 21 control the energy storing assembly 16 to deliver electric power to the alternating voltage network 15 at nights, when no electric power may be delivered from the solar energy park. It is also shown through the dashed line B how the electric batteries may instead be charged from the alternating voltage network 15 by appropriate control of the DC/AC converter 13 and the DC/DC converters 21 , which may be a preferably option when the spot price on the electric power market is low and there is no need to provide electric power to the alternating voltage network 15 through the DC/AC converter 13. It is pointed out that the different DC breakers may be in different states, although it is shown in the Figures how these are always in the same state, so that it will be possible to for instance in the embodiment shown in Fig 4 only feed electric power from some of the solar energy panel strings. It may correspondingly be possible to control the different DC/DC converters connecting the different electric battery strings to the DC line differently, so that the latters are differently charged with electric power should that be desired .

The i nvention is of course not in any way restricted to the embodiments descri bed above, but many possi bilities to modifications thereof will be apparent to a person with skill in the art without departing from the scope of the invention as defined in the appended claims.

The energy storing assembly may have other energy stori ng members than electric batteries, such as pressure vessels, or a combination thereof.

Claims

CLAI MS

1 . An electric power plant comprising

• a solar energy park (10) havi ng a plurality of solar energy panels (1 1 ) configured to generate electric power,

• a DC line (12) connecting said park to a direct voltage side of a DC/AC converter (1 3),

• a said DC/AC converter configured to convert direct voltage into alternating voltage and conversely,

· an alternating voltage network (15) connected to an alternating voltage side of said DC/AC converter (1 3),

• an energy storing assembly (16) connected to said DC line in parallel with said solar energy park,

• an arrangement (18) configured to collect information about operation parameters of the plant, and

• a control unit (19) configured to control flow of electric power within the plant based on information from said arrangement, characterized i n that said control unit (19) is configured to carry out said control so as to regulate the total flow of electric power on said DC line (12) to said DC/AC converter (1 3) and by that to said alternating voltage network (15) by adj usti ng the proportion of said total power fed from said solar energy park (1 0) and the proportion of said total power fed from said energy storing assembly (16) based upon information from said arrangement.

2. A plant according to claim 1 , characterized i n that said control unit (19) is configured to carry out said control so as to obtain a flow of electric power from the solar energy park (1 0) as well as from said energy storing assembly (16) upon receipt of information from said arrangement (18) that the power flow presently deliverable by said solar energy park is lower than a determi ned requested total flow of electric power to said DC/AC converter (1 3).

3. A plant according to claim 2 , characterized i n that said control unit (19) is configured to carry out said control so as to regulate said total flow of electric power for obtaining a said determi ned requested total flow of electric power constituting a maximum electric power deliverable to said DC/AC converter (13) for obtaining a maximum of efficiency of the operation of the DC/AC converter.

4. A plant according to any of the preceding claims, characterized in that said energy storing assembly (16) comprises energy storing members in the form of electric batteries (1 7).

5. A plant according to claim 4 , characterized i n that said energy storing assembly (16) comprises at least one series connection of a plurality of electric batteries (1 7).

6. A plant according to claim 4 or 5, characterized in that it comprises at least one four quadrant DC/DC converter (21 ) connecting said energy stori ng assembly (16) to said DC line (12), and that the control unit (19) is configured to control the operation of said DC/DC converter (21 ) for controlling flow of electric power to and from said assembly.

7. A plant according to claim 6, characterized i n that said control unit (1 9) is configured to carry out said control of said at least one DC/DC converter (21 ) to feed electric power from said solar energy park (10) to said energy storing assembly (1 6) based upon correspondi ng i nformation received from said arrangement (1 8).

8. A plant according to claim 7, characterized i n that said con- trol unit (19) is configured to carry out said control of said at least one DC/DC converter (21 ) so as to adjust the level of the voltage of the electric power arriving from said solar energy park (10) to the voltage level of the electric batteries (1 7) of the energy storing assembly (16).

9. A plant according to claim 7 or 8, characterized in that said control unit (19) is configured to control said at least one DC/DC converter (21 ) so as to adjust the proportion of electric power fed from said solar energy park (10) to said energy stori ng as- sembly (16) with respect to the proportion of electric power fed from said solar energy park to said DC/AC converter (13) and by that to said alternati ng voltage network (15).

10. A plant according to claim 6, characterized in that said control unit (19) is configured to control said at least one DC/DC converter (21 ) so as to adjust the proportion of electric power fed from said energy storing assembly (16) to said DC/AC converter (13) with respect to the electric power fed from said solar energy park (1 0) to the DC/AC converter (1 3) based on informa- tion from said arrangement (18).

1 1 . A plant according to any of the preceding claims, characterized i n that said control unit (19) is configured to carry out control of said DC/AC converter (13) so as to feed electric power from the alternating voltage network (15) to said energy storing assembly (16) based upon information from said arrangement (1 8).

12. A plant according to any of the preceding claims, charac- terized in that said solar energy park (10) comprises a plurality of solar energy panels (1 1 ) connected in series so as to obtain a voltage level of electric power generated thereby requested on the DC line (12) arriving to said DC/AC converter (13).

13. A plant according to any of the preceding claims, characterized i n that said solar energy park (10) has a rated power of at least 50 kW or at least 500 kW , such as > 1 MW .

14. A plant according to any of the preceding claims, charac- terized in that said DC line (12) is configured to apply a voltage of > 500 V or > 2 kV, such as a voltage of 500 V - 5 kV, to said DC/AC converter.

15. A plant according to any of the preceding claims, charac- terized in that said arrangement (1 8) is configured to collect information about operation parameters in the form of the present a) charging state of said energy storing assembly and/or b) weather, such as sunny or cloudy, and/or

and that the control unit (19) is configured to carry out said control based upon this information .

16. A method of controlling flow of electric power within an electric power plant comprising a solar energy park (1 0) having a plurality of solar energy panels (1 1 ) configured to generate electric power, a DC line (12) connecting said park to a direct voltage side of a DC/AC converter (1 3), a said DC/AC converter configured to convert direct voltage into alternating voltage and conversely, an alternating voltage network (15) connected to an alternating voltage side of said DC/AC converter, and an energy storing assembly (16) connected to said DC line in parallel with said solar energy park,

in which information about operation parameters of the plant is collected and flow of electric power within the plant is controlled based upon said information collected ,

characterized in that said control is carried out so as to regulate the total flow of electric power on said DC line (12) to said DC/AC converter (1 3) and by that to said alternating voltage network (15) by adj usti ng the proportion of the total power fed from said solar energy park (10) and the proportion of said total power fed from said energy storing assembly (16) based upon said information collected .

1 7. A method according to claim 16, characterized in that said control is carried out so as to obtain a flow of electric power from the solar energy park (10) as well as from the energy storing assembly (16) when said information collected reveals that the power flow presently deliverable by said solar energy park is lower than a determined requested total flow of electric power to said DC/AC converter (13).

18. A method according to claim 1 7, characterized in that said control is carried out so as to regulate said total flow of electric power for obtaining a said determined requested total flow of electric power constituting a maximum electric power deliverable to said DC/AC converter (13) for obtaining a maximum of efficiency of the operation of the DC/AC converter.

19. A method according to claim 1 7 or 18, characterized in that said control is carried out so as to obtain a flow of electric power from the solar energy park (10) as well as from said energy storing assembly (16) to said DC/AC converter (13) when said information collected reveals that it is broad daylight but so cloudy that the power flow presently deliverable by said solar energy park is lower than said determined requested total flow of electric power to said DC/AC converter.

20. A computer program loadable directly into the internal memory of a computer, which computer program comprises computer program code for causi ng the computer to carry out a control of the steps in any of the method claims 16-19.

21 . A computer program product comprising a data storage medium readable by an electronic control unit, a computer program accordi ng to claim 20 being stored on said data storage medium.