TECHNICAL FIELD

The present disclosure relates to a photovoltaic (PV) power system, and to a corresponding method of operation. The PV power system of the present disclosure is able to perform a maximum power point tracking (MPPT) based on a DC average voltage that is generated from DC outputs of multiple PV modules.

BACKGROUND

A PV power system typically comprises a plurality of PV strings, wherein each PV string typically comprises a plurality of PV modules (also referred to as solar panels). Each PV module produces a DC voltage output when illuminated. The DC voltage outputs of the PV modules may be converted to an AC voltage.

Most common PV power systems are based on a single box solution called “string inverter”, which is used to convert the DC voltages of the PV modules to an AC voltage. Such an inverter is arranged close to the PV strings, and contains both a DC/DC converter for performing MPPT processing, as well as an inverter stage to generate a three-phase AC power of up to 1 kV AC or more.

Another possible solution for a PV power system splits the above-described single box into a box called “string combiner”, which comprises the DC/DC power converter performing the MPPT processing, and another box for the DC/ AC inverter stage for generating up to 1 kV AC or more power. The string combiner is located next to the PV strings, from where DC power cables route a voltage of up to 1500 V or more DC to the DC/ AC inverter box.

Both solutions commonly use convection cooling, which requires a large surface area that leads to big size heat sinks, which are heavy and expensive. Moreover, big size and heavy weight PV power conversion equipment is also needed, due to the internal power loss of the solutions, which again determines the cooling requirement.

Thus, there is a need for an improved PV power system. SUMMARY

The present disclosure has accordingly the objective to provide an improved PV power system. An objective is thereby to overcome the above mentioned drawbacks. For example, an objective is to reduce a power loss of the PV power system. Another objective is to reduce the size and weight of the PV power system. Another objective is to improve the cooling of the PV power system.

These and other objectives are achieved by the solution provided in the enclosed independent claims. Advantageous implementations are further defined in the dependent claims.

According to a first aspect, the disclosure relates to a photovoltaic, PV, power system comprising: a plurality of PV strings, wherein each PV string comprises a plurality of PV modules configured to provide a DC voltage output, a first power conversion equipment connected to the plurality of PV strings, wherein the first power conversion equipment is configured to generate a DC average voltage based on the DC voltage outputs of the plurality of PV modules of the plurality of PV strings, wherein the DC voltage outputs are weighted by a sum of output currents of the plurality of PV modules, and a second power conversion equipment configured to perform MPPT processing based on the DC average voltage.

For example, the PV power system of the first aspect has the advantage that its cooling need is significantly reduced. This may be caused by a lower power loss of the PV power system of the first aspect compared to conventional solutions. Moreover, the dimensions of the PV power system of the first aspect may be reduced compared to conventional solutions.

In an implementation form of the first aspect, the first power conversion equipment is configured to perform partial power conversion (PPC) for each of the PV strings, to generate the DC average voltage of the plurality of PV modules.

The PPC can be implemented by means of low voltage semiconductors, which are small, efficient, and cost much less compared to conventional solutions. Moreover, due to the high efficiency of the PPC, the cooling need of the PV power system of the first aspect is largely reduced compared to conventional solutions. In an implementation form of the first aspect, the first power conversion equipment comprises a plurality of partial power string optimizers (PPSO) configured to perform the PPC, each PPSO being associated with one or more of the plurality of PV strings.

In an implementation form of the first aspect, the first power conversion equipment comprises one or more low voltage semiconductor devices configured to perform the PPC.

In an implementation form of the first aspect, the first power conversion equipment is configured to route the DC average voltage to the second conversion equipment via one or more cables.

In an implementation form of the first aspect, the one or more cables are configured for a voltage of up to 1500 V or more.

In an implementation form of the first aspect, the PV system is further configured to apply water-cooling to the second power conversion equipment.

In an implementation form of the first aspect, the first power conversion equipment comprises a smart string controller comprising power processing equipment connected to the plurality of PV strings.

In an implementation form of the first aspect, the second power conversion equipment comprises a DC/DC conversion stage and a DC/AC inverter stage, wherein the DC/DC conversion stage is configured to perform the MPPT processing.

In an implementation form of the first aspect, the first power conversion equipment and the second power conversion equipment are configured to exchange data via a communication link.

In an implementation form of the first aspect, the first power conversion equipment and the second power conversion equipment are configured to use the communication link to perform MPPT optimization. In an implementation form of the first aspect, the PV system further comprises a medium voltage (MV) transformer connected to the second power conversion equipment and a MV grid connected to the MV transformer.

In an implementation form of the first aspect, the MV transformer comprises low voltage windings, and wherein the DC/AC inverter stage is configured to generate a 3 -phase AC voltage, and to feed the AC voltage to the low voltage windings.

In an implementation form of the first aspect, the MV transformer is configured to deliver power to the MV grid.

In an implementation form of the first aspect, the DC/AC inverter stage is integrated into the MV transformer.

In an implementation form of the first aspect, the second power conversion equipment comprises a reverse powering module.

According to a second aspect, the disclosure relates to a method of operating a PV power system comprising a plurality of PV strings, each PV string comprising a plurality of PV modules providing a DC voltage output, wherein the method comprises: generating, by a first power conversion equipment, a DC average voltage based on the DC voltage outputs of the plurality of PV modules of the plurality of PV strings, wherein the DC voltage outputs are weighted by a sum of output currents of the plurality of PV modules; and performing, by a second power conversion equipment, MPPT processing based on the DC average voltage.

The method of the second aspect may have implementations forms, in which the method of the second aspect operates PV power systems according to respective implementation forms of the first aspect. The method of the second aspect and its implementation forms, by operating the PV power system of the first aspect or its respective implementation forms, can achieve the same advantages as mentioned above.

According to a third aspect, the disclosure relates to a computer program comprising instructions which, when the program is executed by a computer, cause the computer to control the first power conversion equipment and the second power conversion equipment, so as to perform the method according to the second aspect.

In summary of the above aspects and implementation forms, this disclosure is based on splitting a DC/DC part and a DC/ AC part of a PV power system into two different entities, namely, providing the PV power system with the first power conversion equipment and the second power conversion equipment. The DC average voltage is generated, for instance, by performing PPC based on weighted by input currents. The MPPT processing can be moved to the second power conversion equipment, and is advantageously done only based on the DC average voltage. Further, water cooling may be applied to the second power conversion equipment, for example, to several co-located inverter stages next to a MV transformer.

It has to be noted that all devices, elements, units and means described in the present application could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities.

Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof.

BRIEF DESCRIPTION OF DRAWINGS

The above described aspects and implementation forms of the present disclosure will be explained in the following description of embodiments in relation to the enclosed drawings, in which:

FIG. 1 shows a PV power system according to this disclosure.

FIG. 2 shows a PV power system according to this disclosure.

FIG. 3 illustrates PPC in a PV power system according to this disclosure.

FIG. 4 illustrates the generating of a DC average voltage in a PV power system according to this disclosure. FIG. 5 shows a method for operating a PV power system according to this disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a PV power system 100 according to an embodiment of this disclosure. The PV power system 100 comprises a plurality of PV strings 102, a first power conversion equipment

104, and a second power conversion equipment 105, which are connected. The PV power system may be connectable with the second power conversion equipment to a MV transformer and/or a power grid.

Each PV string 102 of the plurality PV strings 102 comprises a plurality of PV modules 103. The PV modules 103 of each PV string 102 are configured to provide a respective DC voltage output 101. For example, each PV module 103 of the PV string 102 may provide an individual DC voltage output, and the PV modules 103 of the PV string 102 may be connected in series to generate the DC voltage output 101.

The first power conversion equipment 104 is connected to the plurality of PV strings 102. The first power conversion equipment 104 is configured to generate a DC average voltage 106 based on the DC voltage outputs 101 output by the plurality of PV modules 103 of the plurality of PV strings 102 (an optional block for generating the DC average voltage 106 is indicated by a dashed box). These DC voltage outputs 101 are weighted by the first power conversion equipment 104 by a sum of output currents of the plurality of PV modules 103, when generating the DC average voltage 106. The first power conversion equipment 104 may be further configured to route the generated DC average voltage 106 to the second conversion equipment

105, for example, via one or more cables or the like. The one or more cables may be configured for a voltage of up to 1500 V or more. Accordingly, the DC output voltage 106 may also be up to 1500 V or more. For instance, the DC output voltage 106 may be in a range of 500-1500 V or more.

The second power conversion equipment 105 is configured to perform MPPT processing based on the DC average voltage 106 generated by the first power conversion equipment 104 (an MPPT block is indicated by a dashed box). The first power conversion equipment 104 and the second power conversion equipment 105 may be configured to work together, in order to optimize the MPPT processing. For instance, the two parts of power conversion equipment 104, 105 may use a communication link to exchange data, and to perform the MPPT processing optimization based on the data. The data may include information relevant to the MPPT processing.

FIG. 2 shows a PV power system 100 according to an embodiment of this disclosure, which builds on the embodiment shown in FIG. 1. The PV power system 100 of FIG. 2 also comprises the plurality of PV strings 102, the first power conversion equipment 104, and the second power conversion equipment 105. Same elements are labelled with the same reference signs in this disclosure.

The PV power system 100 can be designed for a large scale, industrial power plant application, for example, a so-called utility scale PV power system. The power conversion functions of the PV power system 100 are divided into the first power conversion equipment 104, which has a DC/DC conversion function and is connected to the PV power source, i.e., to the plurality of PV strings 102, and into the second power conversion equipment 105, which has a DC/AC conversion function.

As shown in FIG. 2, the second power conversion equipment 105 comprises a DC/DC conversion stage 204 and a DC/AC inverter stage 205. The DC/DC conversion stage 204 is configured to perform the MPPT processing. The DC/AC inverter stage 205 generates a 3 -phase AC voltage. In the second power conversion equipment 105, the DC/DC conversion stage 204 may be collocated with the DC/AC inverter stage 205. The second power conversion equipment 105 may also comprise more than one such DC/DC conversion stage 204 and DC/AC inverter stage 205, for instance, connected in parallel.

Accordingly, the MPPT processing is moved in this disclosure (back) to the second power conversion equipment 105, and the MPPT processing can be done more efficiently at the second power conversion equipment 105, since there is just a single DC input - namely the DC average voltage 106 - compared to more than one DC input as in conventional solutions.

In a typical installation of the PV power system 100, for example, in a utility scale power plant, the second power conversion equipment 105 may be located close to a MV transformer station 201. The MV transformer station 201 may also be a component of the PV power system 100, but can also be an external component. The DC/AC inverter stage 205 may be configured to generate the 3 -phase AC voltage, and to feed the AC voltage to lower voltage windings of the MV transformer 201. The MV transformer 201 may be connected to the second power conversion equipment 105 on one side, and may be connected to a MV grid 102 on the other side. Notably, it may also be possible to integrate the DC/ AC inverter stage 205 into the MV transformer 201 or vice versa.

The DC/AC inverter stage 205 may comprise a PV inverter. A water cooling can be applied to the second power conversion equipment 105. In case of more than one DC/AC inverter stage 205, a common water cooling can be used. Water cooling does not need the same large mechanical structures as air cooling requires. Hence, the enclosure of the second power conversion equipment 105 can be downsized significantly to smaller dimensions and less weight. Typically, half of the size or less can be achieved compared to conventional solutions.

The first power conversion equipment 104 of the PV power system 100 may comprise a smart string controller. The smart string controller can optionally comprise string fuses, disconnection switches (DSw), residual current measurements (RCM), and isolation resistance measurements (Riso), as indicated in FIG. 2. The smart string controller may be connected to the plurality of PV strings 102. The smart string controller can have inputs for many PV strings 102, each made out of several PV modules 103 in series, respectively. Although all connected PV strings 102 may have a similar orientation (e.g., to optimize their illumination), there may still occur some difference in local illumination and local temperature of the individual PV modules 103, which may lead to some difference in the produced electric voltage and current of different PV modules 103. In order to obtain the maximum power from each PV string 102, a MPPT processing optimization is beneficial. However, since the differences, especially differences in the voltages of the PV string 102, are typically small, a power conversion with limited voltage processing capability can be used.

As shown in FIG. 2, the first power conversion equipment 104 may comprise a plurality of PPSOs 203, which are configured to perform PPC for each of the PV strings 102. For instance, each PPSO 203 may be associated with one of the plurality of PV strings 102. Alternatively or additionally, the first power conversion equipment 104 may comprise one or more low voltage semiconductor devices configured to perform or participate in performing the PPC. The PPC for each of the PV strings 102 may generate the DC average voltage 106. For example, a set of PPSOs 203 can be used to convert the PV outputs 101 from the PV strings 102 into the DC average voltage 106. The DC average voltage 106 may be controlled to be the average of all the DC outputs 102, which may be further weighted by the respective output currents (see also examples of the formulas shown in FIG. 4) according to: lout ~ / Istring_n wherein V _out is the DC average voltage 106 of the plurality of PV strings 102, n denotes the n ^th PV string 102 of the plurality of PV strings 102, V _str ing n is the DC (voltage) output of all series-connected PV modules 103 of the n ^th PV string 102, I _str ing_n is the output current of the n ^th PV string 102, I _out is the total output current of all the PV strings 102, indicates a summation over all the plurality of PV strings 102.

Since a full MPPT processing (conventional solution) has been replaced by the PPSO function in the first power conversion equipment 104, the power loss may go down significantly, and the cooling need may be significantly reduced as well. This leads to a smaller, much lighter mechanical design compared to conventional solutions, at largely reduced costs of the entire smart string controller.

The PPSO function is not only much more efficient, but also much cheaper compared to conventional solutions, because it may be based on mainstream low voltage power semiconductors and other lower voltage passive components.

FIG. 3 and FIG. 4 illustrate an example of PPC in a PV power system 100 according to this disclosure. FIG. 3 shows specifically a part of the first power conversion equipment 104, which may be configured to perform the PPC for each PV string 102. The PPC may be performed, for instance, for each of n PV strings 102, to generate the DC average voltage 106. In this case, n PCC modules may be used to perform the PPC, as shown in FIG. 3. As mentioned above, the PPC for each PV string 102 may be performed by a PPSO 203, which is associated with the PV string 102. The first power conversion equipment 104 can accordingly comprise n PPSOs 203, which are configured to perform PPC.

The n PPSOs may receive the DC outputs 101 of the n PV strings 102 as inputs, and may provide a common output, namely, the DC average voltage 106. For example, all negative lines of the DC outputs 101 of the PV strings 102 may be connected to a negative common output terminal of the first power conversion equipment 105, and all positive lines of the DC outputs 101 of the PV strings 102 may be connected to the respective PPSOs 203, as shown in FIG. 2. The n paralleled PPSOs 203 may be configured to power a local PPSO voltage bus. The DC average voltage 106 output voltage may be controlled to be the current weighted average of all input voltages, for example per the formula shown in FIG. 4. The output current may be the sum of all input currents.

As shown in FIG. 4, each PPSO 203 may comprise two or more power electronic switches 401, e.g., MOSFETs, with a bidirectional power flow. Further, the PPSOs 203 may comprise a single output bridge 403. Alternatively, instead of the output bridge 403, N output bridges interleaving could be used. Moreover, the PPSOs comprise input bridges 402, which may work with shifted carrier depending on the operating point (pseudo interleaving). Moreover, one or more input/output bridges 402/403 can be encapsulated for electromagnetic compatibility (EMC) optimization. Using GaN half-bridges, e.g. 650V, may widen the operation ranges of the MPPT processing. Moreover, there is the possibility to increase the switching frequency by an enhancement of the soft switching depending on the switching frequency and the targeted efficiency.

FIG. 5 shows a schematic representation of a method 500 for operating the PV power system 100 according to an embodiment of this disclosure.

The PV power system 100 comprises a plurality of PV strings 102, each PV string 102 comprising a plurality of PV modules 103 providing a DC voltage output 101. The method 500 comprises the following steps: a step 501 of generating, by a first power conversion equipment 104, a DC average voltage 106 based on the DC voltage outputs 101 of the plurality of PV modules 103 of the plurality of PV strings 102, wherein the DC voltage outputs 101 are weighted by a sum of output currents of the plurality of PV modules 103; and a step 502 of performing, by a second power conversion equipment 105, MPPT processing based on the DC average voltage 106.

The present disclosure has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed matter, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.