Photovoltaic power plant

Abstract

In large PV power plants, grounding of individual PV modules may lead to problems. The present invention overcomes such problems. The basis for the invention is a PV power plant comprising one or more PV generators, each comprising a PV string and an inverter with a DC input and an AC output. The PV string comprises at least one PV module and is electrically connected to the DC input of the inverter. The inverter comprises means for controlling the DC potential at the DC input depending on the DC potential at the AC output. The AC outputs of the inverters are coupled in parallel. The novel feature of the invention is that the PV power plant further comprises an offset voltage source, which controls the DC potential at the AC outputs. Thereby, the DC potential at the DC input will be indirectly controlled, and it is thus possible to ensure that the potentials with respect to ground at the terminals of the PV modules are all non-negative or all non-positive without grounding the PV modules. Ground loops can be avoided, and there is no need for the use of transformer-based inverters.

Claims

1. A .Iadd.photovoltaic (.Iaddend.PV.Iadd.) .Iaddend.power plant comprising a PV generator, the PV generator comprising a PV string and an inverter with a DC input and an AC output, the PV string comprising at least one PV module and being electrically connected to the DC input, wherein the PV power plant further comprises an offset voltage source, which controls DC potential at the AC output.[.s.]., the offset voltage source being connected to an AC side of the inverter between ground and a neutral terminal of the AC output.

.[.2. The PV power plant according to claim 1, in which the inverter comprises a means for controlling the DC potential at the DC input depending on the DC potential at the AC output..].

3. The .Iadd.photovoltaic (.Iaddend.PV.Iadd.) .Iaddend.power plant according to claim 1, further comprising one or more additional PV generators .Iadd.each comprising a PV string and an inverter with a DC input and an AC output.Iaddend., the AC .[.outputs.]. .Iadd.output .Iaddend.of .Iadd.each of .Iaddend.the inverters being coupled in parallel.

4. The .Iadd.photovoltaic (.Iaddend.PV.Iadd.) .Iaddend.power plant according to claim 1, in which .[.the.]. .Iadd.an .Iaddend.output voltage of the offset voltage source depends on one or more of: .[.the.]. .Iadd.a .Iaddend.solar irradiation of the PV modules, .[.the.]. .Iadd.an .Iaddend.ambient temperature of the PV modules, an external reference voltage and .[.the.]. .Iadd.a .Iaddend.measured potential .[.of one or more of the.]. .Iadd.at the DC .Iaddend..[.inputs.]. .Iadd.input of the inverter.Iaddend..

5. The .Iadd.photovoltaic (.Iaddend.PV.Iadd.) .Iaddend.power plant according to claim 1, in which .[.the.]. .Iadd.an .Iaddend.output voltage of the offset voltage source is time dependent.

6. The .Iadd.photovoltaic (.Iaddend.PV.Iadd.) .Iaddend.power plant according to claim 1, in which the offset voltage source comprises at least one offset PV module.

7. The .Iadd.photovoltaic (.Iaddend.PV.Iadd.) .Iaddend.power plant according to claim 6, in which the .Iadd.at least one .Iaddend.offset PV .[.modules.]. .Iadd.module .Iaddend.is .[.are.]. arranged so that .[.they.]. .Iadd.it .Iaddend.will be subjected to the same solar irradiation and/or the same ambient temperature as the .Iadd.at least one .Iaddend.PV .[.modules.]. .Iadd.module.Iaddend..

8. The .Iadd.photovoltaic (.Iaddend.PV.Iadd.) .Iaddend.power plant according to claim 1, in which .[.the.]. .Iadd.an .Iaddend.output voltage of the offset voltage source equals approximately half of the output voltage of the PV .[.strings.]. .Iadd.string.Iaddend., and wherein the inverter comprises an electrical equalising circuit, which causes the DC potential at its DC input to be symmetric around the average DC potential at its AC output.

9. The .Iadd.photovoltaic (.Iaddend.PV.Iadd.) .Iaddend.power plant according to claim 1, in which the power plant further comprises an isolation transformer having a primary side connected to the AC .[.outputs.]. .Iadd.output.Iaddend., a secondary side and a neutral terminal on the primary side, and that the offset voltage source is connected between ground and the neutral terminal.

10. The .Iadd.photovoltaic (.Iaddend.PV.Iadd.) .Iaddend.power plant according to claim 9, in which the AC .[.outputs.]. .Iadd.output .Iaddend.and the isolation transformer comprise one or more phases.

11. The .Iadd.photovoltaic (.Iaddend.PV.Iadd.) .Iaddend.power plant according to claim 1, in which the offset voltage source forms part of one inverter.

12. The .Iadd.photovoltaic (.Iaddend.PV.Iadd.) .Iaddend.power plant according to claim 1, in which the offset voltage source is programmable and/or can be turned off.

13. A method of controlling a PV power plant, the PV power plant comprising at least one inverter with a DC input electrically connected to a PV string, an AC output and .[.a means for controlling the DC potential at the DC input depending on the DC potential at the AC output,.]. .Iadd.an offset voltage source associated with the AC output, .Iaddend.the method comprising .[.that of.]. controlling .[.the.]. .Iadd.a .Iaddend.DC potential at the AC .[.outputs.]. .Iadd.output .Iaddend.by use of .[.an.]. .Iadd.the .Iaddend.offset voltage source connected to an AC side of the at least one inverter between ground and a neutral terminal of the AC output.

14. The method of claim 13 further comprising the step of adjusting the offset voltage of the voltage source to hold the voltage .[.of one.]. the DC .[.inputs.]. .Iadd.input .Iaddend.at a voltage offset with respect to ground.

15. The method of claim 14 in which the voltage offset is substantially zero.

16. The method according to claim 13, further comprising the step of turning the offset voltage source off.

17. The .Iadd.photovoltaic (.Iaddend.PV.Iadd.) .Iaddend.power plant of claim 1, wherein the inverter comprises a three-phase AC output.

.Iadd.18. The photovoltaic (PV) power plant of claim 1, wherein the neutral terminal comprises a central connection point of three star-coupled windings. .Iaddend.

.Iadd.19. The photovoltaic (PV) power plant of claim 1, wherein the neutral terminal is connected to the inverter. .Iaddend.

.Iadd.20. A PV power plant comprising a PV generator, the PV generator comprising a PV string and an inverter with a DC input and an AC output, the PV string comprising at least one PV module and being electrically connected to the DC input, wherein the PV power plant further comprises an offset voltage source, which controls DC potential at the AC output, the offset voltage source being connected to an AC side of the inverter between ground and a neutral terminal of circuitry connected to the AC output. .Iaddend.

.Iadd.21. A system, comprising: an inverter configured to couple to a PV string comprising at least o ne PV module at a DC input thereof, and further configured to couple to an AC grid at an AC output thereof; and an offset voltage source configured to control DC potential at the AC output, wherein the offset voltage source is configured to be connected at an AC side of the inverter between ground and a neutral terminal associated with the AC output. .Iaddend.

.Iadd.22. The system of claim 21, wherein the neutral terminal comprises a central connection point of three star-coupled windings. .Iaddend.

.Iadd.23. The system of claim 21, wherein the neutral terminal is connected to the inverter. .Iaddend.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention and its advantages will become more apparent, when looking at the following description of possible embodiments of the invention, which will be described with reference to the accompanying figures, which are showing:

(2) FIG. 1 shows a schematic diagram of a prior art PV power plant,

(3) FIG. 2 shows a graph illustrating voltages obtained in the prior art PV power plant shown in FIG. 1,

(4) FIG. 3 shows a schematic diagram of another prior art PV power plant,

(5) FIG. 4 shows a graph illustrating voltages obtained in the prior art PV power plants shown in FIGS. 3 and 5,

(6) FIG. 5 shows a schematic diagram of yet another prior art PV power plant,

(7) FIG. 6 shows a schematic diagram of a modified form of the prior art PV power plant shown in FIG. 5,

(8) FIG. 7 shows a first embodiment of a PV power plant according to the invention,

(9) FIG. 8 shows a graph illustrating voltages obtained in the embodiment of the PV power plant shown in FIG. 7,

(10) FIG. 9 shows a second embodiment of a PV power plant according to the invention,

(11) FIG. 10 shows a third embodiment of a PV power plant according to the invention, and

(12) FIG. 11 shows a schematic of an isolation transformer shown in FIGS. 5, 7, 9 and 10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(13) The PV power plant 34 of FIG. 7 comprises a PV generator 2, comprising a PV string 3 and an inverter 4. The inverter 4 has a DC input 18 and a three-phase AC output 19. The inverter 4 comprises an electrical equalising circuit, which causes the DC potential at its DC input 18 to be symmetric around the average DC potential at its AC output 19. In its simplest form, the equalisation circuit may comprise a voltage divider based on resistors, inductors and/or capacitors. The PV string 3 comprises three PV modules 5 connected in series and arranged so that they will be exposed to sunlight. Each PV module 5 comprises a number of PV cells (not shown) connected as already known in the art so that they generate a single DC power output at the terminals 6 of the PV module 5. The PV string 3 is electrically connected to the DC input 18 of the inverter 4 through a positive connection 7 and a negative connection 8. The AC outputs 19 are connected electrically in parallel to a power grid 9 through a three-phase AC connection 17 and a three-phase isolation transformer 10 having a primary side 11, a secondary side 12 and a neutral terminal 13 on the primary side. An offset voltage source 14 is electrically connected between ground 15 and the neutral terminal 13.

(14) The PV power plant 34 functions as follows. The PV modules 5 convert the radiation energy received from the sun into electrical energy and thereby generate DC voltages across their terminals 6. Due to the series connection of the PV modules 5, a PV string DC voltage appears between the positive connection 7 and the negative connection 8. In typical PV power plants, the PV string DC voltages may be as high as above 1,000 V. The inverter 4 converts the PV string DC voltage at its DC input 18 into a three-phase AC voltage at its AC output 19, from where it is led to the power grid 9 through the AC connection 17 and the isolation transformer 10. The inverter 4 is controlled by a control system (not shown) to ensure that no electrical power flows from the AC output 19 to the DC input 18. The power plant 34 thus converts solar energy into electrical energy, which is delivered to the power grid 9. The PV string DC voltages and thus the output power of the power plant 34 vary with the irradiation and the ambient temperature as is already known in the art.

(15) The offset voltage is applied to the neutral terminal 13 of the isolation transformer 10, thereby causing the average DC potential at its primary side 11 to be offset from ground potential with the offset voltage. Thus, also the average DC potential with respect to ground 15 at the AC output 19 of the inverter 4 equals the offset voltage. Due to the equalising circuit in the inverter 4, the potentials with respect to ground 15 at the positive connection 7 and the negative connection 8 will be symmetrical around the offset voltage, i.e. approximately zero at one of the connections 7, 8 and approximately twice the offset voltage at the other connection 7, 8. By selecting an appropriate electrical polarity for the offset voltage source 14, it can thus be ensured that the potentials with respect to ground for all PV modules 5 are, for example, either non-negative or non-positive and nearly always very close to ground potential.

(16) When operating, the voltages appearing at the positive input 7 and negative input 8 of the inverter 4 are represented in FIG. 8. In this figure the axis 25 represents the voltage with respect to ground, and it will be seen that the voltage at the positive input 7 (represented by the line 27) is above ground potential whilst that at the negative input 8 (represented by the line 28) is below ground potential. These voltages, as well as the potential between them—the voltage across the PV string 3 (represented by the range 26)—are controlled by the characteristics of the inverter 4, the irradiation of the PV string 3, the type of solar cells used in each PV module 5 as well as other factors. The arrows 35 and 36 illustrate that fact that the potentials at the positive 7 and negative 8 inputs of the inverter 4 can be varied by the variation of the voltage output by the offset voltage source 14.

(17) The isolation transformer 10 shown in FIG. 11 comprises three star-coupled primary windings 20 on the primary side 11 and three star-coupled secondary windings 21 on the secondary side 12. The central connection point of the primary windings 20 constitutes the neutral terminal 13 of the isolation transformer 10.

(18) In the case that the PV modules 5 are of the ‘thin film’ type, the offset voltage source can be driven so that the whole PV string 3 is held at a positive potential relative to ground. Such a configuration is suitable for avoiding the problems with thin film modules discussed above. If, alternatively, the offset voltage source 14 is driven so that the positive input 7 of the inverter 4 is kept at or near ground potential, then a configuration suitable for back contact type modules is realised. The advantages of this embodiment are clear to see: since there is no requirement for the inverter 4 to be of a transformer-based (galvanically isolated) type, cost and weight can be reduced and efficiency improved.

(19) Turning now to FIG. 9 we see a second embodiment of a PV power plant according to the invention. This PV power plant 37 is similar to the PV power plant 34 of the first embodiment above, but with the addition of two or more PV generators 39, similar in design to the PV generator 2 of the first embodiment. Again, each PV generator 39 comprises a PV string 3 (not shown) and an inverter 38. Each inverter 38 has a DC input 18 and a three-phase AC output 19. The AC outputs 19 are connected electrically in parallel to a power grid 9 through a three-phase AC connection 17 and a three-phase isolation transformer 10. As before, an offset voltage source 14 is electrically connected between ground 15 and the neutral terminal 13.

(20) FIG. 9 also illustrates a controller 40 which controls the voltage and polarity of the offset voltage source 14 though a control line 41. The signal on the control line 41 is a function of the output of a comparator 42 which compares the output of a voltage measurement 45, 46 with respect to ground 15 and a reference voltage 43. The voltage measurement can be either the voltage of the positive input 7 or the negative input 8 of the inverter 4. The choice of this voltage is made by means of a switch 47. The switch 47 may be a physical switch (for example controlled directly by service personnel), or an electronic switch.

(21) It would, of course, be possible to build the functionality of the controller 40, comparator 42 and switch 47 into the inverter 4. In this case, inverter 4 becomes a ‘controller’ inverter which supplies the DC offset to all the inverters on the isolated AC side of the isolation transformer 10.

(22) The advantages of this embodiment are similar to the advantages already given for the first embodiment discussed above. In addition, it will be seen that there is no requirement to ground the appropriate input of each inverter 4, 38 individually since the offset voltage source 14 controls the voltage relative to ground on the isolated AC side of all the inverters 4, 38 to a reference point. This reference point could be set to any desired potential between positive or negative side of the PV string and thus compensate for different problems associated with different PV cell type discussed above.

(23) The reference point could also be made programmable, that is it can be varied according to the type of PV string being used, or by some other criteria. It also could be set as a function of time and thus it would be possible to changed the settings of the offset voltage during the day if required.

(24) Since the offset voltage is being produced at a single point in the circuit, and simultaneously alters the potential to ground of all the PV modules 5, there are no voltage differences between the PV modules 5, and no related ground loops between the inverters 4, 38.

(25) Since very little current flows through the voltage source, there is very little power dissipated (often of the order of 1 Watt).

(26) FIG. 10 shows a third embodiment of the invention. This is similar to the embodiment illustrated in FIG. 7, but with the addition of a second PV generator 2, and with each PV string 3 comprising four PV modules 5 connected in series. The offset voltage source 14 comprises two offset PV modules 16 connected in series. The offset PV modules 16 are similar in construction to the PV modules 5 of the PV generators 2.

(27) The number of offset PV modules 16 equals half the number of PV modules 5 in a PV string 3, wherefore the output voltage of the offset voltage source 14—the offset voltage—equals approximately half of the PV string DC voltages. Most of the time, the offset voltage source 14 is less loaded than the PV strings, wherefore most of the time, the offset voltage will be a little higher than half of the PV string DC voltages.

(28) Instead of using an equalising circuit, the DC potential at the DC input 18 of the inverters 4 may be controlled actively by the inverter control circuits. This is for instance possible in a transformer-less inverter with an unsymmetrical boost circuit.

(29) Although various embodiments of the present invention have been described and shown, the invention is not restricted thereto, but may also be embodied in other ways within the scope of the subject-matter defined in the following claims.