Photovoltaic power plant
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.
1 . 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 outputs, 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 PV power plant according to claim 1 , further comprising one or more additional PV generators, the AC outputs of the inverters being coupled in parallel.
4 . The PV power plant according to claim 1 , in which the output voltage of the offset voltage source depends on one or more of:
the solar irradiation of the PV modules,
the ambient temperature of the PV modules,
an external reference voltage and
the measured potential of one or more of the inputs.
5 . The PV power plant according to claim 1 , in which the output voltage of the offset voltage source is time dependent.
6 . The PV power plant according to claim 1 , in which the offset voltage source comprises at least one offset PV module.
7 . The PV power plant according to claim 6 , in which the offset PV modules are arranged so that they will be subjected to the same solar irradiation and/or the same ambient temperature as the PV modules.
8 . The PV power plant according to claim 1 , in which the output voltage of the offset voltage source equals approximately half of the output voltage of the PV strings, 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 PV 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, 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 PV power plant according to claim 9 , in which the AC outputs and the isolation transformer comprise one or more phases.
11 . The PV power plant according to claim 1 , in which the offset voltage source forms part of one inverter.
12 . The PV 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, the method comprising that of controlling the DC potential at the AC outputs by use of an 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 voltage of the voltage source to hold the voltage of one of the DC inputs 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 PV power plant of claim 1 , wherein the inverter comprises a three-phase AC output.
18. A photovoltaic (PV) power plant comprising a PV generator with an inverter with a DC input and a three-phase AC output, wherein the PV power plant further comprises an offset voltage source, which controls DC potential at the three-phase 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 three-phase AC output, wherein the neutral terminal is a central connection point of three star-coupled windings connected to the three-phase AC output.
19. The PV power plant according to claim 18 , further comprising one or more additional PV generators each comprising an inverter with a DC input and an AC output, the AC output of each of the inverters being coupled in parallel.
20. The PV power plant according to claim 18 , in which an output voltage of the offset voltage source depends on one or more of: a solar irradiation of PV modules when connected to the inverter, an ambient temperature of the PV modules, an external reference voltage and a measured potential at an input of the inverter.
21. The PV power plant according to claim 18 , wherein the offset voltage source comprises at least one offset PV module.
22. The PV power plant according to claim 21 , wherein the at least one offset PV module is arranged so that it will be subjected to the same solar irradiation and/or the same ambient temperature as the at least one PV module.
23. The PV power plant according to claim 18 , wherein an output voltage of the offset voltage source equals approximately half of the output voltage of a PV string when connected to the inverter, and wherein the inverter comprises an electrical equalising circuit, which causes the DC potential at its DC input to be symmetric around an average DC potential at its AC output.
24. The PV power plant according to claim 18 , wherein the power plant further comprises an isolation transformer having a primary side connected to the three-phase AC output and a secondary side for connection to an AC grid.
25. The PV power plant according to claim 18 , wherein the offset voltage source forms part of one inverter.
26. The PV power plant of claim 18 , wherein the central connection point is connected to the inverter.
27. 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, a three-phase AC output and an offset voltage source associated with the three-phase AC output, the method comprising controlling a DC potential at the three-phase AC output by use of the offset voltage source connected to an AC side of the at least one inverter between ground and a neutral terminal of circuitry connected to the three-phase AC output, wherein the neutral terminal is a central connection point of three star-coupled windings connected to the three-phase AC output.
28. The method of claim 27 , further comprising adjusting an offset voltage of the voltage source to hold a voltage at the DC input at a voltage offset with respect to ground.
29. The method according to claim 27 , further comprising turning the offset voltage source off.
30. A system, comprising:
an inverter configured to couple to a PV string comprising at least one PV module at a DC input thereof, and further configured to couple to an AC grid at a three-phase AC output thereof;
a transformer having a primary side and a secondary side, wherein the primary side is coupled to the three-phase AC output; and
an offset voltage source configured to control DC potential at the three-phase 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 of circuitry connected to the three-phase AC output, wherein the neutral terminal is a central connection point of three star-coupled windings connected to the three-phase AC output.