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.

What is claimed is: 
     
       1. A photovoltaic (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 photovoltaic (PV) power plant according to  claim 1 , further comprising one or more additional PV generators each comprising a PV string and an inverter with a DC input and an AC output, the AC outputs output of each of the inverters being coupled in parallel. 
     
     
       4. The photovoltaic (PV) power plant according to  claim 1 , in which the an output voltage of the offset voltage source depends on one or more of: the a solar irradiation of the PV modules, the an ambient temperature of the PV modules, an external reference voltage and the a measured potential of one or more of the at the DC inputs input of the inverter. 
     
     
       5. The photovoltaic (PV) power plant according to  claim 1 , in which the an output voltage of the offset voltage source is time dependent. 
     
     
       6. The photovoltaic (PV) power plant according to  claim 1 , in which the offset voltage source comprises at least one offset PV module. 
     
     
       7. The photovoltaic (PV) power plant according to  claim 6 , in which the at least one offset PV modules module is are arranged so that they it will be subjected to the same solar irradiation and/or the same ambient temperature as the at least one PV modules module. 
     
     
       8. The photovoltaic (PV) power plant according to  claim 1 , in which the an output voltage of the offset voltage source equals approximately half of the output voltage of the PV strings string, 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 photovoltaic (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 output, 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 photovoltaic (PV) power plant according to  claim 9 , in which the AC outputs output and the isolation transformer comprise one or more phases. 
     
     
       11. The photovoltaic (PV) power plant according to  claim 1 , in which the offset voltage source forms part of one inverter. 
     
     
       12. The photovoltaic (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, an offset voltage source associated with the AC output, the method comprising that of controlling the a DC potential at the AC outputs output by use of an the 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 input 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 photovoltaic (PV) power plant of  claim 1 , wherein the inverter comprises a three-phase AC output. 
     
     
       18. The photovoltaic (PV) power plant of claim 1, wherein the neutral terminal comprises a central connection point of three star-coupled windings.  
     
     
       19. The photovoltaic (PV) power plant of claim 1, wherein the neutral terminal is connected to the inverter.  
     
     
       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.  
     
     
       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.    
     
     
       22. The system of claim 21, wherein the neutral terminal comprises a central connection point of three star-coupled windings.  
     
     
       23. The system of claim 21, wherein the neutral terminal is connected to the inverter.