Photovoltaic power plant

This application isa reissue of U.S. application Ser. No. 13/127,813 that issued as U.S. Pat. No. 9,287,712, that issued on Mar. 15, 2016, and is a continuation of reissue application Ser. No. 15/921,272, that isentitled to the benefit of and incorporates by reference essential subject matter disclosed in International Patent Application No. PCT/DK2009/000231 filed on Nov. 6, 2009 and Danish Patent Application No. PA 2008 01537 filed Nov. 7, 2008; and Danish Patent Application No. 2009 00896 filed on Jul. 24, 2009.

The invention regards a photovoltaic (PV) power plant.

In larger PV power plants with DC/AC-converters (inverters), the plant is typically connected to the power grid through a dedicated isolation transformer, which connects the relatively low voltage PV generator system to the medium voltage power grid. One reason for this is that the PV modules, which convert the solar energy into electrical energy, typically must have a defined potential with respect to ground. This is typically achieved by grounding all or some of the PV modules.

Grounding is normally done in order to comply with local regulations, to facilitate the detection of isolation faults and/or to avoid corrosion and/or yield reduction of the PV modules.

Detection of isolation faults may be difficult in larger systems due to the rather high leakage currents from the PV modules, especially in wet conditions. By grounding the system, leakage currents can be monitored.

Some types of PV module, notably thin film modules incorporating a TCO (transparent conductive oxide) layer are prone to irreparable damage, and consequent substantial power losses, resulting from the reaction of glass-sodium with moisture. To avoid accelerated degradation of such PV modules, it is normally required to ground the negative terminal of the PV strings, i.e. avoid that any active part of the PV modules have a negative potential with respect to the ground potential. The degradation of the PV modules depends on the potential difference between the active parts of the module and the ground. Depending on the module construction, grounded parts may be in very close distance from the active parts—accelerating the degradation.

With some other types of PV module, notably those where the terminals are all located on one side of the module—known as ‘back contact modules’—a reduction of module efficiency has been observed during operation. This appears to be due to a build up of static charge on the surface of the cell and can be counteracted by maintaining the cell below the ground potential. Thus, some back-contact PV modules require that the positive terminal is grounded in order to avoid yield losses, i.e. their terminals must have non-positive potentials.

In larger PV systems comprising several strings of PV modules and several inverters, grounding of more than one PV module may cause currents to run through the ground (ground loops). Ground loops may cause problems with controlling the power plant, increase the risk and/or the rate of corrosion and also increase problems relating to electromagnetic interference (EMI). In order to avoid ground loops, transformer-based inverters may be used, so that the DC and the AC sides of the inverters are separated galvanically. Such inverters are however, relatively heavy and expensive, and there is a demand for PV power plants, which may utilise transformer-less inverters and still ensure defined potentials with respect to ground at the PV modules. The use of transformer-less inverters in large PV power plants does however, require that, if ground loops shall be avoided, the PV generator system be configured as a network with an earthing system where the AC side of the inverters has no connection to ground at all. This is known as an ‘IT’ earthing system and is described in, for example, IEC (International Electrotechnical Commission) International Standard 60364-1—Electrical Installation in Buildings. This means, in practice, that the AC side of the system must be floating with respect to ground and can therefore not be grounded.

1 FIG. 22 23 3 24 24 18 19 3 5 5 6 5 3 18 4 7 8 19 9 15 illustrates a typical prior art power plantand comprises a single PV generator, comprising a PV stringand a transformerless (non galvanically isolated) inverter. The inverterhas a DC inputand a three-phase AC output. The PV stringcomprises three PV modulesconnected in series and arranged so that they will be exposed to sunlight. Each PV modulecomprises 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 terminalsof the PV module. The PV stringis electrically connected to the DC inputof the inverterthrough a positive connectionand a negative connection. The AC outputsare connected electrically in parallel to a power gridcomprising three power lines and a neutral line. The neutral line is connected to ground via a ground connection.

Here the system is configured as a network with an earthing system where the AC side of the inverter has a connection to ground, and the network also includes a ground connection. This is known as a ‘TN’ earthing system and is described in, for example, IEC 60364-1.

7 8 24 25 7 27 8 28 3 26 24 3 5 18 22 3 2 FIG. When operating, the voltages appearing at the positive inputand negative inputof the inverterare represented in. In the graph the axisrepresents the voltage with respect to ground, and it will be seen that the voltage at the positive input(represented by the line) is above ground potential whilst that at the negative the negative input(represented by the line) is below ground potential. These voltages, as well as the potential between them—the voltage across the PV string(represented by the range)—are controlled by the characteristics of the inverter, the irradiation of the PV string, the type of solar cells used in each PV moduleas well as other factors. Since the grounding of either side of the inverter DC inputsis not possible in this design of power plant, such a power plantwill be subject to a decrease in efficiency, and the PV stringliable to damage, resulting from the problems discussed above.

3 FIG. 1 FIG. 4 FIG. 2 FIG. 29 22 31 30 18 19 8 30 32 18 30 3 7 30 8 illustrates another prior art power plant. Here the difference from the power plantofis that the PV generatorcomprises a transformer-based (galvanically isolated) inverter, that is to say there is galvanic isolation between the DC inputand AC outputof the inverter. This allows the grounding of the negative inputof the inverterto be made using a ground connection.illustrates the voltages appearing at the DC inputsof the inverterin a similar manner to. In can be seen that the whole PV stringis 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 positive inputof the inverterwas grounded instead of the negative input, then a configuration suitable for back contact type modules would be realised.

18 30 32 7 8 30 32 In this type of power plant, whilst it is possible to control the voltages appearing at the inputs, and so minimise the decrease in efficiency and damage resulting from the problems discussed above, this advantage comes only at the cost of using a transformer-based inverter (). Such an inverter design is more expensive to produce, heavier and is less efficient in operation and so the use of such an inverter is clearly a disadvantage. A further disadvantage of this type of power plant is the requirement that a ground connectionneeds to be physically connected to the positive inputor the negative inputof the inverter. This requires additional hardware and labour to attach. In addition, changing the type of PV modules at a later date may involve the physical disconnection and/or reconnection of a ground connection, a procedure which is labour intensive and therefore a disadvantage.

5 FIG. 1 FIG. 4 FIG. 2 FIG. 33 22 19 9 17 10 11 12 13 18 9 31 8 24 32 18 24 3 7 24 8 illustrates yet another prior art power plant. Here the difference from the power plantofis that the AC outputsare connected electrically in parallel to a power gridthrough a three-phase AC connectionand a three-phase isolation transformerhaving a primary side, a secondary sideand a neutral terminalon the primary side. Such a transformer is often used in high capacity power plants, where multiple inverters are coupled in parallel, and is described in more detail below. Such a configuration allows the potential of the inverter inputsto be independent of the potential of the networkwithout the need for using a costly transformer-based inverter. The grounding of the negative inputof the invertercan be made using a ground connection.illustrates the voltages appearing at the DC inputsof the inverterin a similar manner to. In can be seen that the whole PV stringis 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 positive inputof the inverterwas grounded instead of the negative input, then a configuration suitable for back contact type modules would be realised.

32 7 8 24 32 23 23 3 6 FIG. A disadvantage of this type of power plant is the requirement that a ground connectionneeds to be physically connected to the positive inputor the negative inputof the inverter. This requires additional hardware and labour to attach. In addition, changing the type of PV modules at a later date may involve the physical disconnection and/or reconnection of a ground connection, a procedure which is labour intensive and therefore a disadvantage. In addition, if two or more PV generatorsare connected in parallel, as illustrated in, and one input of each inverteris earthed as described above, problems will arise if the characteristics or irradiation of each PV stringare not identical. This may cause unwanted voltages and consequent ground loop currents.

In all the prior art power plants illustrated above it can be seen that whilst the potential of one or more PV strings with respect to ground is an important parameter for running a power plant in an efficient manner and one which does not cause damage to the PV strings, this often only achieved by the use of expensive hardware or the labour intensive fitting of additional hardware.

Therefore, it is an object of the invention to provide a PV power plant comprising PV strings in which the grounding regime of the PV strings hinders degradation and loss of efficiency.

It is a further object of the invention to provide a PV power plant which easy to adapt to different types of PV string.

It is an even further object of the invention to provide a PV power plant in which the grounding regime of the PV strings is programmable and thus continuously adaptable. The aim of the present invention is to overcome the above mentioned and other drawbacks of known PV power plants.

The basis for the invention in a first aspect is 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 comprises at least one PV module and is electrically connected to the DC input of the inverter. 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. Thus, ground loops can be avoided, and there is no need for the use of transformer-based inverters. Furthermore, isolation faults in the power plant may be detected by monitoring the current flowing from the offset voltage source.

Preferably, the inverter comprises means for controlling the DC potential at the DC input depending on the DC potential at the AC output. Such means may be dedicated to this function or the function may be a by-product of another function within the inverter.

The PV power plant may also preferably comprise one or more additional PV generators where the AC outputs of all the inverters of these PV generators are coupled in parallel.

Preferably, the output voltage of the offset voltage source depends on the solar irradiation on and/or the ambient temperature of the PV modules. This allows for controlling the potentials at the terminals of the PV modules so that they are as close to ground potential as possible at all times.

The output voltage of the offset voltage source may additionally or alternatively depend on an external reference voltage. This allows for controlling the potentials at the terminals of the PV modules according to any preset potential, or a potential that is set remotely and/or dynamically in order to compensate for factors not immediately accessible to the PV power plant or its components.

The output voltage of the offset voltage source may additionally or alternatively depend on the measured potential of one or more of the inputs to one or more of the inverters. This allows for controlling the potentials at the terminals of the PV modules according to the potential across them produced by irradiation or, alternatively or additionally, according to a requirement to hold one or more of the inverters within certain limits, for example within a certain potential relative to ground.

The output voltage of the offset voltage source may additionally or alternatively is time dependent. This allows for controlling the potentials at the terminals of the PV modules for example according to the time of day. This is an advantage, for example, if a reverse potential is required during the hours of darkness to repair damage caused to ‘back contact modules’ during daylight hours, or for varying the potential of the PV string throughout the day following the pattern of expected irradiation. Additionally or alternatively a time dependency might follow a yearly or weekly cycle dependent upon a preset pattern connected with ambient temperature or power usage.

In one embodiment of the invention, the offset voltage source may preferably comprise at least one offset PV module. A PV module constitutes a very reliable voltage source, is easy to incorporate into a PV power plant and eliminates the need for installing any additional power generators.

The offset PV modules may preferably be are arranged so that they will be subjected to the same solar irradiation and/or the same ambient temperature as the PV modules. This is a very simple way of achieving that the potentials at the terminals of the PV modules can be as close to ground potential as possible at all times.

Preferably, the output voltage of the offset voltage source may equal approximately half of the output voltage of the PV strings, and the inverters may comprise an electrical equalising circuit, which causes the DC potential at their DC inputs to be symmetric around the average DC potential at their AC outputs. This establishes an even simpler control of the PV module potentials.

The power plant may preferably further comprise an isolation transformer having a primary side connected to the AC outputs, a secondary side and a neutral terminal on the primary side, and the offset voltage source may be connected between ground and the neutral terminal. This is a very simple way of controlling the DC potential at the AC outputs of the inverters.

The AC outputs of the inverters and the isolation transformer may preferably comprise three phases. In this way, a very stable neutral terminal may be achieved.

Alternatively or additionally the offset voltage source may form part of one of the inverter. This would allow for a compact and efficient system. This may preferentially be realised by one inverter becoming a ‘controlling’ inverter which supplies the DC offset to all the inverters on the isolated AC side of the isolation transformer.

Alternatively or additionally the offset voltage source may be programmable. By this is meant that the offset voltage source can be programmed, by means of inbuilt computer code or other means, to respond in a simple or complex manner to inputs such as temperatures, voltage measurements, time, power usage requirements or other parameters.

Alternatively or additionally the offset voltage source may be able to be turned off. By this is meant that the offset voltage source can be taken out of circuit. This has the distinct advantage that when the offset voltage source is not required (for example during night-time hours when no power is being produced) the power that it uses may be saved resulting in a more efficient PV power plant.

The basis for the invention in a second aspect is realised by 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.

This aspect of the invention may advantageously be realised by 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.

In one embodiment of the method the DC input may be the positive connection, in another embodiment of the method the DC input may be the negative connection.

Alternatively or additionally this aspect of the invention may advantageously be realised by making the voltage offset substantially zero.

Alternatively or additionally this aspect of the invention may advantageously be realised by further the step of turning the offset voltage source off. This is advantageous for the reasons described previously.

34 2 3 4 4 18 19 4 18 19 3 5 5 6 5 3 18 4 7 8 19 9 17 10 11 12 13 14 15 13 7 FIG. The PV power plantofcomprises a PV generator, comprising a PV stringand an inverter. The inverterhas a DC inputand a three-phase AC output. The invertercomprises an electrical equalising circuit, which causes the DC potential at its DC inputto be symmetric around the average DC potential at its AC output. In its simplest form, the equalisation circuit may comprise a voltage divider based on resistors, inductors and/or capacitors. The PV stringcomprises three PV modulesconnected in series and arranged so that they will be exposed to sunlight. Each PV modulecomprises 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 terminalsof the PV module. The PV stringis electrically connected to the DC inputof the inverterthrough a positive connectionand a negative connection. The AC outputsare connected electrically in parallel to a power gridthrough a three-phase AC connectionand a three-phase isolation transformerhaving a primary side, a secondary sideand a neutral terminalon the primary side. An offset voltage sourceis electrically connected between groundand the neutral terminal.

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

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

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

10 20 11 21 12 20 13 10 11 FIG. The isolation transformershown incomprises three star-coupled primary windingson the primary sideand three star-coupled secondary windingson the secondary side. The central connection point of the primary windingsconstitutes the neutral terminalof the isolation transformer.

5 3 14 7 4 4 In the case that the PV modulesare of the ‘thin film’ type, the offset voltage source can be driven so that the whole PV stringis 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 sourceis driven so that the positive inputof the inverteris 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 inverterto be of a transformer-based (galvanically isolated) type, cost and weight can be reduced and efficiency improved.

9 FIG. 37 34 39 2 39 3 38 38 18 19 19 9 17 10 14 15 13 Turning now towe see a second embodiment of a PV power plant according to the invention. This PV power plantis similar to the PV power plantof the first embodiment above, but with the addition of two or more PV generators, similar in design to the PV generatorof the first embodiment. Again, each PV generatorcomprises a PV string(not shown) and an inverter. Each inverterhas a DC inputand a three-phase AC output. The AC outputsare connected electrically in parallel to a power gridthrough a three-phase AC connectionand a three-phase isolation transformer. As before, an offset voltage sourceis electrically connected between groundand the neutral terminal.

9 FIG. 40 14 41 41 42 45 46 15 43 7 8 4 47 47 also illustrates a controllerwhich controls the voltage and polarity of the offset voltage sourcethough a control line. The signal on the control lineis a function of the output of a comparatorwhich compares the output of a voltage measurement,with respect to groundand a reference voltage. The voltage measurement can be either the voltage of the positive inputor the negative inputof the inverter. The choice of this voltage is made by means of a switch. The switchmay be a physical switch (for example controlled directly by service personnel), or an electronic switch.

40 42 47 4 4 10 It would, of course, be possible to build the functionality of the controller, comparatorand switchinto the inverter. In this case, inverterbecomes a ‘controller’ inverter which supplies the DC offset to all the inverters on the isolated AC side of the isolation transformer.

4 38 14 4 38 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,individually since the offset voltage sourcecontrols the voltage relative to ground on the isolated AC side of all the inverters,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.

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.

5 5 4 38 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, there are no voltage differences between the PV modules, and no related ground loops between the inverters,.

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

10 FIG. 7 FIG. 2 3 5 14 16 16 5 2 shows a third embodiment of the invention. This is similar to the embodiment illustrated in, but with the addition of a second PV generator, and with each PV stringcomprising four PV modulesconnected in series. The offset voltage sourcecomprises two offset PV modulesconnected in series. The offset PV modulesare similar in construction to the PV modulesof the PV generators.

16 5 3 14 14 The number of offset PV modulesequals half the number of PV modulesin a PV string, wherefore the output voltage of the offset voltage source—the offset voltage—equals approximately half of the PV string DC voltages. Most of the time, the offset voltage sourceis 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.

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

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.