FR2995474A1 - Photovoltaic power station for e.g. producing high voltage electrical energy, has direct current-direct current converters for receiving input voltage, and direct current-alternating current converter receiving voltages from converters - Google Patents

Abstract

The station has a set of direct current (DC)-DC converters (10-1 to 10-4) for receiving an input voltage supplied by assemblies (4-1 to 4-4) of photovoltaic panels (1), where the station provides a power of multiple hundreds of kilowatt. A DC-alternating current (AC) converter (34') receives continuous voltages supplied from the DC-DC converters. The DC-AC converter and the DC-DC converters provide a power of multiple thousands of volts, where the DC-AC converter is designed as a part of an inverter. The photovoltaic panels provide a power of a few hundreds of volts.

Description

FIELD OF THE INVENTION The present invention relates generally to photovoltaic power plants and, more particularly, to the conversion of the electrical energy supplied by the photovoltaic panels and the photovoltaic power plant. transporting this electrical energy to a high voltage transmission network. The invention is more particularly applicable to large production plants which are intended to be connected to the distribution network on high voltage transmission lines. Description of the Prior Art Photovoltaic power plants are produced on increasingly large surfaces, that is to say with a growing number of panels. These panels 15 produce electrical energy at a relatively low voltage (of a few tens of volts) and this electrical energy must be routed to a conversion transformer to a relatively high voltage (a few tens of thousands of volts). a high voltage power transmission network.

B11933 - DD13616ST 2 In such plants, the connection to the network is made via a transformer preceded by an inverter. The larger the plant, the larger the section and the length of the cables that must be provided to convey the electricity between the photovoltaic panels and the mains connection transformer. The impact of the cost of the cables, which are generally made of copper, is far from negligible on the power plant, and therefore on the cost per kilowatt hour produced. Cost Summary An object of an embodiment of the present invention is to overcome some or all of the disadvantages of conventional solar power plants. Another object of an embodiment of the present invention is to reduce the weight of the cables used between the photovoltaic panels and a mains connection transformer. Another object of an embodiment of the present invention is to provide a solution that does not require modification of the photovoltaic panels themselves. To achieve all or part of these objects and others, there is provided a photovoltaic power plant comprising: several sets of photovoltaic panels; a plurality of first DC-DC converters each receiving at input a voltage provided by one or more of said sets; and at least one second DC-AC converter receiving the DC voltages provided by said first converters. According to one embodiment of the present invention, each first converter is placed as close as possible to the set of photovoltaic panels from which it receives the voltage.

According to one embodiment of the present invention, the voltage rise ratio of each first converter is between 5 and 10. According to one embodiment of the present invention, said at least one second converter provides a voltage of several thousand volts. According to one embodiment of the present invention, each first converter provides a voltage of several thousand volts. According to one embodiment of the present invention, each second converter is part of an inverter. According to one embodiment of the present invention, each first set of photovoltaic panels provides a voltage of a few hundred volts. According to one embodiment of the present invention, said panels provide an AC voltage, third AC-DC converters being interposed between said assemblies and the first converters. According to one embodiment of the present invention, a plurality of second converters are connected in series. According to one embodiment of the present invention, the power of the plant is several hundred kilowatts. According to an embodiment of the present invention, the respective positions of the converters are selected to minimize interconnect cable sections between said assemblies and the first converters and between the converters with each other. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, features, and advantages will be set forth in detail in the following description of particular embodiments in a non-limitative manner with reference to the accompanying drawings in which: B11933 - DD13616ST 4 FIG. 1 represents the principle of a circuit for converting the electrical energy produced by a photovoltaic panel; Figure 2 shows schematically and in block form an example of a photovoltaic power plant; FIG. 3 represents, in a simplified way and in the form of blocks, an embodiment of a conversion and energy transport architecture produced by photovoltaic panels; Figure 4 shows another embodiment of a photovoltaic power plant; FIG. 5 represents yet another embodiment of a photovoltaic power station; and Figure 6 shows yet another embodiment of a photovoltaic power plant. Detailed Description The same elements have been designated by the same references in the various figures. For the sake of clarity, only the elements useful for understanding the described embodiments have been shown and will be detailed. In particular, the constitution of the photovoltaic panels has not been detailed, the embodiments described being compatible with the usual panels. FIG. 1 very schematically shows the conversion principle between a photovoltaic panel 1 and a transformer 2 for connection to the electrical distribution network (not illustrated in FIG. 1). Typically, a photovoltaic panel 1 of the type shown in FIG. 1 consists of a set of interconnected modules 12, each module comprising a matrix array of photovoltaic cells. The panel 1 provides a DC voltage V. Generally, photovoltaic cells individually generating a voltage of a few volts are electrically connected in series so that the voltage V is a voltage (commonly referred to as low) of a few tens of volts.

B11933 - DD13616ST Several panels 1 are generally associated in series to reach a voltage of a few hundred volts. To reinject the energy produced by the photovoltaic panel into the electricity network, the DC voltage must be converted into AC voltage at the frequency of this network. For this purpose, an inverter 3 is used to convert the output voltage V of the panels 1 into an AC voltage supplied at the input of the transformer 2. Most often, an inverter comprises a DC-DC converter 32 (DC / DC) followed by a DC-AC converter (DC / AC). The DC / DC converter typically operates on the principle of a switching power supply. In the example of FIG. 1, the converter is of the voltage booster type (BOOST). Such a converter comprises between a terminal 31 for applying the positive potential of the voltage V and an output terminal 33, an inductive element L in series with a diode D, a switch K connecting the midpoint between the inductance L and the diode D at a second terminal 35 for applying the voltage V (ground). A capacitive element C can be connected at the input (between the terminals 31 and 35) to stabilize the DC voltage as well as at the output of the converter (capacitor C '). The switch K is generally a power semiconductor component (a power MOS transistor or an insulated gate bipolar transistor - IGBT) and is controlled by a circuit 36 at a frequency generally greater than the kilohertz. The operation of such a converter is perfectly usual. In particular, it serves as an impedance matching device for maximum power operation from MPPT (Maximun Power Point Tracking) algorithms.

The output terminals 33 and 35 of the converter 32 are connected to the input of the converter 34. This converter 34 has the role of making the DC voltage alternating across the capacitor C '. For example, four power switches K1, K2, K3, K4 are bridged between the terminals 33 and 35. For example, the switches K1 and K3 are connected B11933 - DD13616ST 6 in series between the terminals 33 and 35 and the switches K2 and K4 are connected in series between the terminals 32 and 35, forming two "arms" of switches between the terminals 33 and 35. The alternating voltage appears by differential association of the outputs 36 and 37 of the switch arms (midpoints of the series associations of the switches respectively K1 and K3 and K2 and K4). The switches K 1, K 2, K 3, K 4 are controlled at a switching frequency of the order of one kilohertz whose pulses are modulated by a signal at the frequency of the desired AC voltage, typically 50 or 60 Hz. The switches are controlled in pairs (in the case of bipolar modulation), the switches K1 and K4 being conductive to create the alternations, for example positive, while the switches K2 and K3 are conductive to create negative alternations, for example negative ones. Generally, link inductive elements Li and L2 are connected between the terminals 36 and 37 and the input terminals of the step-up transformer 2. These inductances make it possible to filter the high-frequency component of the current generated by the cutting effected by the converter. This filtering function is, in some cases, directly provided by the primary of a network connection transformer. Here again, the operation of such a converter is in itself known. In the example of Figure 1, the converter 34 is a single-phase converter. There are also three-phase converters that operate on the same principle. Other conversion circuits can be used but they generally respect this conversion principle (cutting of the DC voltage to the frequency of the network).

The inverter 3 is common to all the panels 1 of a plant as is explained below in connection with Figure 2. Figure 2 schematically shows an example of architecture of a photovoltaic power plant.

B11933 - DD13616ST 7 In high power plants (several hundred kW) to which the described embodiments more particularly apply, several panels 1 are electrically connected to form a set or field 4i (i being between 1 and n) photovoltaic panels. Each field 4i provides a voltage of a few hundred volts (for example from 400 to 800 volts). Fields 4i are then interconnected in parallel in groups (terminal blocks symbolized by blocks 6j, j being between 1 and m).

Then, the groups 6j are connected in parallel to reach the input terminals 31 and 35 of an inverter 3. As indicated in connection with FIG. 1, the inverter 3 comprises a DC / DC conversion part whose objective is to optimize the operating point of the plant (MPPT) and a DC / AC converter providing an AC voltage. The panels of the same field 4i are interconnected in series and / or in parallel, and Si (Si, 52, ..., 5) cables connect the fields in groups. In the example of FIG. 2, there is illustrated a single set of photovoltaic fields connected to a first terminal block 61. The m terminal blocks 6j are connected by cables 7j to the inverter 3. The AC voltage is then conveyed by a cable 8 up to the voltage booster transformer 2, connected to the network 9 high voltage transport of the electrical network.

Large power plants can have tens of thousands of modules or panels arranged in fields and subfields in series and parallel arrangements to reach the inverter 3. To limit losses (by Joule effect) in cables carrying electricity in the power plant, it is necessary to use cables of large section. In the example of FIG. 2, assuming that the voltage conveyed by the cables 5i is of a few hundred volts (typically 400 to 800 volts), for a current of the order of 10 amperes, the section cables remains acceptable B11933 - DD13616ST 8 (less than twenty square millimeters). However, after the paralleling of the cables 5i in the terminal blocks 6j, the current in the cables 7j exceeds one hundred amperes. The section can then reach several hundred millimeters square. The section may be even larger on a section 7 'which generally groups in parallel the cables 7. The same is true for the cable 8 downstream of the inverter 3. For the DC sections, the cables are generally two drivers. Downstream of the inverter 3, there are 2 or 4 conductors depending on whether it is single-phase or three-phase. Due to the large dimensions of a photovoltaic power plant, the cost of the cables is not negligible in the overall cost of the plant. Typically, in a medium power plant, the length L7 of the high section cables 7j is several hundred meters. The length L8 of the cable 8 rarely exceeds ten meters. Finally, the length of the cables 5i between the photovoltaic fields 4j and the parallel connection terminal blocks 6i is on average of the order of one kilometer. It can therefore be seen that the lengths of the cables, and therefore the quantities of copper required, are far from being negligible in the production of a high power photovoltaic plant. To reduce this mass of cables, the inventors plan to raise the DC voltage as close as possible to the photovoltaic panel fields and to transport the energy at this high DC voltage to the inverter. Figure 3 schematically shows an embodiment of a plant implementing such a solution. Fields 4i of photovoltaic panels 1 are interconnected in series and / or in parallel. In the example of FIG. 3, the number of four fields (fields 41, 42, 43 and 44) has been limited to simplify. Each field 4i provides a DC voltage V which is applied at the input of a converter 10i. The outputs of the converters 10i are associated in parallel and connected to a DC / AC converter 34 '. Each converter 10i raises the DC voltage by a factor of a few units. Typically, starting from a voltage of a few hundred volts, the voltage E supplied by each converter is a few thousand volts. With respect to the inverter 3 (Figure 2), the converter 34 'has a DC input capable of receiving a voltage of several thousand volts and outputs an alternating voltage also of several thousand volts. The output of the converter 34 'is connected to the input of the transformer 2. In a variant, the converter 34' can be replaced by an inverter (DC / DC converter followed by a DC / AC converter) receiving as input a voltage of several thousand volts.

However, this is not essential insofar as the MPPT function is provided by the converters 10. The connection links 11 of the converters 10i to the converter 34 'are the longest links since the converters 10i are placed at the 4. Typically, taking the example of Figure 2, the length L11 of these cables is several kilometers. For a given power, the current flowing in the cables L11 is therefore lower because of the increase by a factor of up to 10 of the voltage. This reduces the section of the cables used, thus saving copper. Another advantage of the solution of Figure 3 is that by placing converters in parallel closer to the photovoltaic fields, it improves the robustness of the system vis-à-vis possible damage. Indeed, if a converter fails, the others continue to provide energy, which is not the case in the embodiment of Figure 2 where, if the converter 32 is faulty, the panel is down. The converters 10 are preferably those having the function of seeking the maximum operating point.

B11933 - DD13616ST 10 The 10i converters are based on the operating principle of switching power supplies. Indeed, using transformers to raise the voltage level would reduce the beneficial effect of saving copper, these transformers being formed in part of conductive windings. Technologies based on semiconductor power switches are therefore preferred. However, this then limits the possible high voltage output of the converters based on the maximum voltage that can support these power semiconductor elements. The choice of the location of the converters 10i depends on the level of current to be conveyed by the cables. Preferably, a converter is provided from the moment when the current exceeds a few tens of amperes. This choice must be adapted to the topology of the plant and in particular to the distribution of the photovoltaic panels. FIG. 4 shows another embodiment of a power plant taking again the example of the power station of FIG. 2. In this case, several sets 6j (j being between 1 20 and m) of fields 4i (i being between 1 and n) of photovoltaic panels 1 are interconnected. The converters 10i are provided at the terminal blocks 6. Thus, the section of the cables 7j of connection to an inverter 3 'can be reduced. As in the embodiment of FIG. 3, the DC / AC converter of the inverter 3 'supplies an alternating voltage of several thousand volts to the transformer 2. As a particular embodiment, assuming that the plant of FIG. 4 is a plant of several hundred kilowatts consisting of fields 4i, each providing a power of a few kilowatts (for example between 5 and 10 kilowatts) at a voltage of a few hundred volts (for example between 600 and 800 volts), the current flowing in the Si cables is of the order of 10 to 20 amperes, which requires cables with a cross section of a few tens of square millimeters (typically less than 20 millimeters). The elevation provided by the converters 10i brings the DC voltage to several thousand volts (for example a voltage of between 3600 and 6000 volts). Assuming fifteen fields 4i per set 6, the current that would have been in the embodiment of Figure 2 of the order of 200 amps is now reduced to a few tens of amperes. Therefore, we can be satisfied with a cable section 7i of a few square millimeters. It is only at the level of the connection to the inverter 3 'that several tens of square millimeters must be provided because of the paralleling of the cables 7. This increase in section can be made between a terminal block of paralleling and the inverter 3 'or directly on the terminal block of the inverter. At the output of the inverter 3 ', the current exceeds one hundred amps but does not reach the thousand amps as in the embodiment of Figure 2. Therefore, the section of the cables used remains significantly lower than that of the case of FIG. 2. FIG. 5 represents another embodiment adapted to the cases where the fields 4i (for example, four fields 41, 42, 43 and 44) of photovoltaic panels 1 are associated with their own inverters (generally designated microinverters). In such a case, each field provides an alternative YAC voltage. Between the output of each field 4i and the converter 10i with which it is associated, an AC / DC converter or rectifier 12i is then provided. In such a variant, the microinverters equipping the panel fields have the MPPT regulation elements towards the point of maximum power.

FIG. 6 represents yet another embodiment in which several inverters 3'k (k being between 1 and p) are provided, each assigned to a set of converters 10, each converter 10 being assigned to a field 4 of photovoltaic panels . The outputs of the inverters 3'k are connected in series. This amounts to associating in series B11933 - DD13616ST 12 several inverter outputs 3 'of the type of that of FIG. 3. In such a case, the voltages V1, V2,..., Vp of the inverters 3'1, 3'2, ..., 3'p are added and we obtain a voltage VT which can reach several tens of thousands of volts. Depending on the voltage level VT obtained, that is to say according to the number p of inverters 3'k, a transformer 2 (not shown in FIG. 6) is used or the ends of the series connection are connected directly. (therefore the voltage VT) to the transport network 9 (not shown in Figure 6).

An advantage of the embodiments that have been described is that they make it possible to reduce the copper mass required for the connection cables of the photovoltaic panel fields in a power plant. Typically, this mass can be reduced by a factor of several tens (of the order of 20 to 50) by raising the voltage by a factor of 5 to 10. Another advantage of the embodiments described is that they do not need to modify the photovoltaic panels nor the organization of the power plants in fields and subfields.

Another advantage of the embodiments described is that they are compatible with the use of converters based on switching power supplies. Another advantage of the architecture described is that it is compatible with the different versions of 25 photovoltaic plants (equipped with microinverters or not). It should be noted that the nature of photovoltaic panels does not matter. In particular, concentrating photovoltaic panels (CPV or Concentrated Photovoltaic) can be used.

Various embodiments have been described, various variations and modifications will be apparent to those skilled in the art. In particular, the choice of the voltage rise ratio made by the converters 10 depends on the architecture of the photovoltaic power plant in question and in particular on the voltage supplied by the fields or subfields of photovoltaic panels B11933 - DD13616ST 13 voltaic and the desired voltage output. Typically, this ratio will be chosen from a value of between 5 and 10. In particular, the embodiments described are compatible with high voltage railway electrical networks, for example 25kV-50Hz (and 25kV-60Hz) or 15kv-16, 6 Hz. In addition, the choice of the positioning of the converters 10 depends on the configuration of the photovoltaic plant which generally adapts to geographical constraints. The notion of "near" or "as close as possible" to the placement of the converters 10 will take into account these constraints. Finally, the practical implementation of the embodiments that have been described is within the abilities of those skilled in the art from the functional indications given above and the standards usually used to determine the section of the cables according to the intensity they must convey.

Claims (11)

REVENDICATIONS1. Photovoltaic plant comprising: a plurality of sets (4) of photovoltaic panels having a plurality of first DC-DC converters (10) each receiving at input a voltage provided by one or more of said sets; and at least one second DC-AC converter (34 ', 3') receiving DC voltages provided by said first converters. 10 2. Plant according to claim 1, wherein each first converter is placed closer to the assembly (4) of photovoltaic panels which it receives the voltage. The plant according to claim 1 or 2, wherein the voltage rise ratio of each first converter (10) is between 5 and 10. 4. Plant according to any one of claims 1 to 3, wherein said at least one second converter provides a voltage of several thousand volts. 20 5. Plant according to any one of claims 1 to 4, wherein each first converter provides a voltage of several thousand volts. 6. Plant according to any one of claims 1 to 5, wherein each second converter is part of an inverter. 7. Plant according to any one of claims 1 to 6, wherein each first set of photovoltaic panels provides a voltage of a few hundred volts. 30 8. Plant according to any one of claims 1 to 7, wherein said panels provide an AC voltage, third converters (12) AC-DC being interposed between said assemblies and the first converters.B11933 - DD13616ST 15 9. Plant according to any one of claims 1 to 8, wherein a plurality of second converters (3 ') are connected in series. 10. Plant according to any one of claims 1 to 9, wherein the power of the plant is several hundred kilowatts. 11. Plant according to any one of claims 1 to 10, wherein the respective positions of the converters are chosen to minimize the interconnection cable sections 10 between said sets and the first converters and between the converters them.