DE102018108462B4 - Method for operating a photovoltaic inverter, inverter and installation with such an inverter - Google Patents

Abstract

A method is described for operating a photovoltaic (PV) inverter (5), to whose DC input (5.3) a PV generator (2) is connected, which comprises a series connection of PV modules (3), and whose AC Output (5.4) for feeding electrical power to a power grid (7) is connected. The procedure includes the steps:
Determining a first electrical power P ₁ at a current operating point A _{1 of} the PV generator (2) at a first generator voltage U ₁ , and a second electrical power P ₂ at a second operating point A _{2 of} the PV generator (2) at a second generator voltage U ₂ , which differs from the first generator voltage U ₁ according to U ₂ = U ₁ + ΔU by a tracking increment ΔU,
Checking whether a power plateau (LP) of a PU characteristic curve (31) of the PV generator (2) is present, comprising a comparison of the first P ₁ with the second electrical power P ₂ ,
- Selection of those operating point A ₁ , A ₂ as a new operating point A _{new of} the PV generator (2), which has a greater electrical power P ₁ , P ₂ , if according to the test no power plateau (LP) is present, and otherwise selecting that operating point A ₁ , A ₂ as a new operating point A _{new of} the PV generator (2), which has a view from a gentler point of operation of the inverter (5) and / or its components, and
- Operating the PV generator (2) in the new operating point Aneu as the current operating point A ₁ .

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates to methods for operating a photovoltaic (PV) inverter, a PV inverter designed to carry out the method according to the invention and a PV system with such a PV inverter.

STATE OF THE ART

Conventional MPP tracking methods pursue the goal of operating a PV generator at a maximum power point (MPP). However, the operating point of maximum power is not necessarily also an operating point at which the PV inverter and / or its components are operated gently. Specifically, it may be, for example, that at the MPP operating point of the PV generator, for example, an increased power dissipation of the PV inverter, a lower efficiency of the PV inverter and / or with respect to a generator voltage at the MPP operating point increased stress of semiconductor switches of the PV inverter available. The non-gentle operation of the PV inverter, its components and / or other components of the PV system can lead to an increased failure rate in the medium term. It is therefore desirable that a tracking method not only aim at maximizing the power drawn from the PV array, but also taking into account the operation of the PV inverter and / or its components and / or other components of the PV system. This is particularly relevant if a P-U characteristic of the PV generator has a power plateau in which there is no power change, but at least no significant change, over an extended range of the generator voltage. Such power plateaus can occur, for example, by using DC / DC converters (so-called power optimizers) assigned to PV modules.

The font US 2008/0 150 366 A1 discloses a PV system with a series circuit of PV modules, wherein each PV module of the series circuit is associated with a DC / DC converter as a power optimizer. The power optimizer is designed to operate the PV module assigned to it in its respective MPP operating point of maximum power. The series connection is connected to a DC input of an inverter. In this case, the inverter is set up to maintain the voltage at its DC input at a constant value, which corresponds to a permissible working voltage for the components of the inverter. Since each PV module has a power optimizer, it is always assumed that there is a power plateau in the PU characteristic of the PV generator. The ever-present power plateau is countered with a suitably selected but temporally constant generator voltage at the DC input of the inverter.

In the article RONCERO-CLEMENTE, C., et al .: Comparison of three MPPT algorithms for three-level neutral-point-clamped qz-source inverters. In: 2013 International Conference Workshop Compatibility and Power Electronics, 2013, pp. 80-85, three conventional algorithms for tracking a point of maximum performance are compared by means of simulations. The algorithms are based on a dP / dV feedback, a disturbance and subsequent observation, as well as a rising conductance. Each of the algorithms is suitable for a 3L NPC quasi-Z source inverter with a shoot-through duty cycle as a controlled variable.

In the article HOU, C.L., et al: Application of adaptive algorithm of solar cell battery charger. In: 2004 IEEE International Conference on Electric Utility Deregulation, Restructuring and Power Technologies. Proceedings, 2004, pp. 810-813, an adaptive algorithm for maximum power point tracking (MPPT) is disclosed. In the adaptive algorithm, a tracking increment is changed depending on the distance to the maximum power point (MPP). The adaptive algorithm can speed up search speed and improve tracking precision during MPPT.

The sole purpose of the MPP tracking procedures described in the articles is always to maximize the power taken from a PV generator.

OBJECT OF THE INVENTION

The invention has for its object to provide a method for operating a PV inverter, in which both a maximization of the PV generator taken power, as well as a gentle operation of the PV inverter and / or its components is taken into account. In particular, the method is intended to detect the presence of a power plateau of a P-U characteristic of a PV generator connected to the PV inverter during ongoing operation of the PV inverter and, in the case of a power plateau, to allow gentle operation of the PV inverter and / or its components. The invention is further based on the object to show a PV inverter with a method according to the invention, as well as a PV system with such a PV inverter.

SOLUTION

The object of the invention is achieved by a method for operating a PV inverter with the features of independent claim 1. The dependent claims 2 to 7 are directed to preferred embodiments of the method. Claim 8 is directed to a PV inverter for carrying out a method according to the invention. The claims 9 to 11 relate to advantageous embodiments of the PV inverter. Claim 12 is directed to a PV system with a PV inverter according to the invention. Claims 13 and 14 relate to advantageous embodiments of the PV system.

DESCRIPTION OF THE INVENTION

• - Bestimmen einer ersten elektrischen Leistung P₁ in einem aktuellen Arbeitspunkt A₁ des PV-Generators bei einer ersten Generatorspannung U₁ , und Bestimmen einer zweiten elektrischen Leistung P₂ in einem zweiten Arbeitspunkt A₂ des PV-Generators bei einer zweiten Generatorspannung U₂ , die sich von der ersten Generatorspannung U₁ gemäß U₂ = U₁+ΔU um eine Tracking-Schrittweite ΔU unterscheidet,
• - Prüfen, ob ein Leistungsplateau einer P-U-Kennlinie des PV-Generators vorliegt umfassend einen Vergleich der ersten P₁ mit der zweiten elektrischen Leistung P₂ ,
• - Wahl desjenigen Arbeitspunktes A₁ , A₂ als neuen Arbeitspunkt A_neu des PV-Generators, der eine größere elektrische Leistung P₁ , P₂ aufweist, wenn gemäß der Prüfung kein Leistungsplateau vorliegt, und andernfalls Wahl desjenigen Arbeitspunktes A₁ , A₂ als neuen Arbeitspunkt A_neu des PV-Generators, der einen unter einem Gesichtspunkt schonenderen Betrieb des Wechselrichters und/oder dessen Komponenten aufweist, und
• - Betreiben des PV - Generators in dem neuen Arbeitspunkt Aneu als aktueller Arbeitspunkt A₁ .

A method according to the invention for operating a PV inverter, to the DC input of which a PV generator is connected, which comprises a series connection of PV modules, and whose AC output is connected to a power supply network for supplying electrical power, comprises the following method steps :

• - Determining a first electrical power P ₁ in a current operating point A ₁ of the PV generator at a first generator voltage U ₁ , and determining a second electrical power P ₂ in a second working point A ₂ of the PV generator at a second generator voltage U ₂ that differ from the first generator voltage U ₁ according to U ₂ = U ₁ + ΔU differs by a tracking step size ΔU,
• Check whether there is a power plateau of a PU characteristic of the PV generator comprising a comparison of the first one P ₁ with the second electric power P ₂ .
• - Choice of that operating point A ₁ . A ₂ as a new working point A _new of the PV generator, which has greater electrical power P ₁ . P ₂ if, according to the examination, there is no performance plateau and, if so, if that work point is chosen A ₁ . A ₂ as a new working point A _new the PV generator, which has a view from a more gentle operation of the inverter and / or its components, and
• - Operating the PV generator in the new operating point Aneu as the current operating point A ₁ ,

The method according to the invention thus aims in particular at a method for tracking an operating point associated with the PV generator as a function of external conditions, for example radiation changes, which have an effect on a shape of a PU characteristic of the PV generator. The method changes depending on the shape of the PU characteristic a target direction in the tracking of the operating point. In particular, when no power plateau can be recognized in the PU characteristic, the operating point of the PV generator is changed with the aim of increasing the power taken from the PV generator. If, on the other hand, a power plateau is to be identified in the PU characteristic curve, then the operating point of the PV generator is tracked so that a more gentle operation of the PV inverter, its components or other components of the PV system is connected. During operation of the PV inverter, the method independently checks the shape of the PU characteristic curve and adapts the respective target direction when tracking the operating point to the currently prevailing basic conditions. For this purpose, the method determines a first electric power similar to a conventional MPP tracking method P ₁ at a current operating point A ₁ and a second electric power P ₂ at a second operating point A ₂ of the PV generator. It is the current working point A ₁ assigned first generator voltage U ₁ different from the second operating point A ₂ associated second generator voltage U ₂ , Unlike the conventional MPP tracking method, however, a comparison of the first P ₁ with the second performance P ₂ First checked whether there is a performance plateau of the PU characteristic. In this context, a power plateau means a region of the PU characteristic which is flattened with regard to the power, that is to say a region of the generator voltage U in which the power taken from the PV generator does not change, at least not significantly. Only if the test reveals that there is no performance plateau of the PU characteristic is that of the two operating points A ₁ . A ₂ chosen as a new working point Aneu, who has a greater performance P ₁ . P ₂ having. Only then does the method decide to select the new operating point Aneu taking into account an increase in power. On the other hand, if the test detects a performance plateau of the PU characteristic, the method no longer pursues the goal of increasing the power taken from the PV generator. An increase in performance would be due to the power plateau anyway only conditionally or slightly possible. Rather, therefore, the new operating point is decided A _new from the two working points A ₁ and A ₂ with the aim to operate the PV inverter, its components and / or other components of a PV system containing the PV inverter more gently.

Depending on the shape of the PU characteristic curve, it is therefore decided during operation of the PV inverter to set a target when selecting the new operating point Aneu. Specifically, the choice of the new operating point Aneu either with the goal, a the PV array generator, or with the aim of enabling a more gentle operation of the PV inverter. Finally, the PV generator is operated at the selected new operating point Aneu. The latter objective is pursued in particular if no appreciable increase in power is to be expected which justifies operation of the PV inverter, its components and / or other components of the PV system under higher load conditions - ie less gentle conditions. In other words, the method according to the invention only pursues the goal of increasing a power taken from the PV generator if a power increase associated with the selection of the new operating point Aneu is achieved at the new operating point A _new possibly higher load of the PV inverter whose components and / or other components justify the PV system. In summary, the method according to the invention takes into account as target (e) both a maximization of performance and at the same time a gentler operation of the PV inverter. The preferred one of the two targets is decided depending on the existing PU characteristic. Since both objectives are taken into account in principle, the method according to the invention allows a tracking of the MPP together with a generally more gentle operation and therefore a longer average life of the PV inverter, its components and / or other components of the PV system in comparison with conventional tracking methods. Investment.

In one embodiment of the method, the method steps are repeated, in particular carried out repeatedly at regular time intervals. In this case, the repetition of the method steps of the PV generator within a predetermined voltage window at a plurality of operating points A _i with i> 2 with different generator voltages U _i operated. In this way, a change in the PU characteristic due to weather changes, for example due to radiation and / or temperature changes account. When changing from the first operating point A ₁ to the second operating point A ₂ For example, a tracking direction of a previous run of the method steps can be maintained if, during the preceding pass, the second operating point A ₂ was chosen as the new working point Aneu, and vice versa. The tracking direction is characterized by a sign of the tracking increment .DELTA.U.

Keeping a previous tracking direction below the o. G. Framework conditions make sense, since such a movement of the operating point in a given direction can be continued successively.

• K₁ ) Gemäß dem ersten Kriterium wird auf die Existenz eines Leistungsplateaus geschlossen, wenn ein Betrag einer spannungsbezogenen Leistungsänderung |ΔP/ΔU|=|(P_k-P_i)/(U_k-U_i)| für die Mehrzahl von Arbeitspunkten A_i , A_k mit unterschiedlichen Generatorspannungen U_k , U_i innerhalb des vorgegebenen Spannungsfensters einen vorgegebenen Schwellwert S > 0 unterschreitet. Optional kann gefordert werden, dass der Betrag der spannungsbezogenen Leistungsänderung |ΔP/ΔU|=|(P_k-P_i)/(U_k-U_i)| für alle Arbeitspunkte A_i, A_k mit unterschiedlichen Generatorspannungen U_k, U_i innerhalb des vorgegebenen Spannungsfensters den vorgegebenen Schwellwert S unterschreitet.
• K₂ ) Gemäß dem zweiten Kriterium wird auf die Existenz eines Leistungsplateaus geschlossen, wenn die Mehrzahl der Arbeitspunkte A_i innerhalb des vorgegebenen Spannungsfensters jeweils Leistungswerte P_i (U_i ) aufweisen, die innerhalb eines vorgegebenen Leistungsfensters ΔP liegen. Optional kann verschärft gefordert werden, dass ein Leistungsplateau erst dann vorliegt, wenn alle Arbeitspunkte A_i innerhalb des vorgegebenen Spannungsfensters jeweils Leistungswerte P_i (U_i ) aufweisen, die innerhalb eines vorgegebenen Leistungsfensters ΔP liegen. Bei beiden Kriterien K₁ , K₂ wird über das vorgegebene Spannungsfenster eingestellt, welche Spannungsausdehnung ein innerhalb des Leistungsfensters ΔP liegender Bereich der P-U-Kennlinie aufweisen muss, damit die Prüfung ihn als Leistungsplateau interpretiert. Vorteilhafterweise weist das Spannungsfenster eine Breite von mindestens 20 V oder 2% einer Leerlauf-Spannung des PV-Generators, bevorzugt von mindestens 40 V oder 4% einer Leerlauf-Spannung des PV-Generators und, besonders bevorzugt von mindestens 60 V oder 6% einer Leerlauf-Spannung des PV-Generators auf. Das vorgegebene Leistungsfenster ΔP weist vorteilhafterweise eine Breite von maximal 75 W oder 1,5% einer Nennleistung des PV-Generators, bevorzugt von maximal 50 W oder 1,0% einer Nennleistung des PV-Generators und besonders bevorzugt von maximal 30 W oder 0,6% einer Nennleistung des PV-Generators auf.

In a further embodiment of the method with repeatedly performed method steps at a plurality of operating points, the test is performed on a power plateau of the PU characteristic, at least taking into account one of the following criteria K ₁ . K ₂ :

• K ₁ According to the first criterion, the existence of a power plateau is inferred when an amount of a voltage-related power change | ΔP / ΔU | = | (P _k -P _i ) / (U _k -U _i ) | for the majority of work points A _i . A _k with different generator voltages U _k . U _i falls below a predetermined threshold S> 0 within the specified voltage window. Optionally, it may be required that the amount of the voltage-related power change | ΔP / ΔU | = | (P _k -P _i ) / (U _k -U _i ) | for all operating points A _i , A _k with different generator voltages U _k , U _i falls below the predetermined threshold S within the predetermined voltage window.
• K ₂ According to the second criterion, the existence of a power plateau is concluded when the plurality of operating points A _i within the given voltage window each power values P _i ( U _i ) which are within a predetermined power window ΔP. Optionally, it may be more stringent that a performance plateau is only available if all operating points A _i within the given voltage window each power values P _i ( U _i ) which are within a predetermined power window ΔP. For both criteria K ₁ . K ₂ is set over the predetermined voltage window, which voltage extension must have a lying within the power window .DELTA.P range of the PU characteristic, so that the test interprets him as a power plateau. Advantageously, the voltage window has a width of at least 20 V or 2% of an open circuit voltage of the PV generator, preferably of at least 40 V or 4% of an open circuit voltage of the PV generator and, more preferably, of at least 60 V or 6% Open circuit voltage of the PV generator. The predetermined power window ΔP advantageously has a maximum width of 75 W or 1.5% of a nominal power of the PV generator, preferably of at most 50 W or 1.0% of a rated power of the PV generator and particularly preferably of a maximum of 30 W or 0, 6% of a rated power of the PV generator.

• - eine Abnahme einer Generatorspannung U innerhalb eines vorgegebenen Spannungsbereiches,
• - eine Abnahme eines Generatorstromes I innerhalb eines vorgegebenen Strombereiches,
• - eine Zunahme eines Wirkungsgrades des PV-Wechselrichters, dessen Komponenten und/oder anderen Komponenten der PV-Anlage,
• - eine Abnahme einer Verlustleistung des PV-Wechselrichters, dessen Komponenten und/oder anderen Komponenten der PV-Anlage, und
• - eine Abnahme der Beanspruchung beziehungsweise eine Maximierung der Lebensdauer des PV-Wechselrichters, dessen Komponenten und/oder anderen Komponenten der PV-Anlage.

Für jede der Randbedingungen lässt sich eine Generatorspannung U als Zielwert festlegen. Für den Fall, dass ein Leistungsplateau identifiziert wird, wird der neue Arbeitspunkt Aneu so gewählt, dass mit dem neuen Arbeitspunkt A_neu eine sukzessive Annäherung an den Zielwert der Generatorspannung U - gegebenenfalls sogar dessen Erreichen - verbunden ist. Dabei kann die dem Zielwert entsprechende Generatorspannung U, die dem unter einem Gesichtspunkt schonenderen Betrieb des PV-Wechselrichters zugeordnet ist, oder die einen Sollwert darstellt, bei der der Wechselrichter und/oder dessen Komponenten besonders schonend betrieben werden, fest vorgegeben sein. Alternativ dazu kann der Zielwert der Generatorspannung U jedoch auch erst im laufenden Betrieb des PV-Wechselrichters ermittelt werden.In a further embodiment of the method, the operation of the PV inverter and / or its components, which is gentler from a viewpoint, takes into account at least one of the following boundary conditions:

• - a decrease of a generator voltage U within a predetermined voltage range,
• a decrease of a generator current I within a predetermined current range,
• an increase in the efficiency of the PV inverter, its components and / or other components of the PV system,
• - A decrease in the power loss of the PV inverter, its components and / or other components of the PV system, and
• - A decrease in the load or maximize the life of the PV inverter, its components and / or other components of the PV system.

For each of the boundary conditions, a generator voltage U can be set as the target value. In the event that a performance plateau is identified, the new operating point Aneu is chosen so that with the new operating point A _new a successive approximation to the target value of the generator voltage U - possibly even reaching it - is connected. In this case, the generator voltage corresponding to the target value U, which is assigned to the gentler from a viewpoint operation of the PV inverter, or which represents a target value at which the inverter and / or its components are operated particularly gently, be fixed. Alternatively, however, the target value of the generator voltage U can also be determined only during ongoing operation of the PV inverter.

It is known that a DC / DC converter assigned to a PV module, as a power optimizer, promotes formation of a power plateau in a region of the PU characteristic. Usually, the more DC / DC converters are present as power optimizers of individual PV modules in the PV generator, the more the power plateau is formed. It is therefore desirable, in the event that a power plateau is identified within the PU characteristic, to test whether DC / DC converters are also present as power optimizers in the PV generator that generate the identified power plateau. In one embodiment of the method, if a power plateau is present according to the test, a plausibility check of the power plateau additionally takes place by means of a tracking speed which is changed relative to a preset tracking speed of the PV inverter. In particular, the plausibility check is carried out by means of a characteristic sweep over a range of the generator voltage U , A PU characteristic determined via the characteristic sweep is compared with the PU characteristic, which is based on discrete changes in the generator voltage U taking into account the (pre-set) tracking increment .DELTA.U and using the preset tracking speed. Depending on the comparison, an existence of a DC / DC converter assigned to a PV module is then concluded as a power optimizer within the PV generator. In this case, the effect is exploited that a DC / DC converter as a power optimizer always requires a certain reaction time to respond to a control deviation, such as a change in the generator voltage U. If the control deviation occurs faster than the response time of the power optimizer, a new equilibrium state of the power optimizer can not be established. As a result, one observes a tracking-dependent PU characteristic. A characteristic sweep within the meaning of the invention has a high rate of change dU / dt of the generator voltage U on and thus corresponds to a high tracking speed at which the power optimizer is not, or only conditionally to a changing generator voltage U can react. The choice of the operating point A ₁ . A ₂ The new operating point Aneu of the PV generator, which has the operation of the PV inverter and / or its components which is gentler from a viewpoint, takes place only if the existence of a DC / DC converter assigned to the PV module is concluded as a power optimizer , Often only from a power plateau LP which exists over a longer period of time, justifying the initiation of a gentler operation of the PV inverter.

In a further embodiment of the method, if, according to the test, a power plateau is present, the tracking step size ΔU is increased. The magnification may be more than 3 times, more than 6 times, in particular more than 9 times the original tracking increment .DELTA.U. With the increased tracking step size .DELTA.U ', an expansion of the voltage range assigned to the power plateau, in particular its upper Umax and / or lower voltage limit Umin, can be estimated or determined rapidly. The estimation is much faster than would be possible using the preset tracking increment .DELTA.U - with a usually rather low voltage value. In addition to the above purpose, increasing the preset tracking pitch ΔU also results in better resolution in the comparison of the first one P ₁ with the second electric power P ₂ , So can be great changes between the first P ₁ and the second electric power P ₂ a smaller tracking increment .DELTA.U 'relative to the preset tracking increment .DELTA.U, and a larger tracking increment .DELTA.U' relative to the preset tracking increment .DELTA.U for small changes. In principle, the enlarged tracking Step size .DELTA.U 'not constantly available as an enlarged tracking step size .DELTA.U'. Rather, it is sufficient if the increased tracking increment .DELTA.U 'is present only over a limited period of time, and is reset in a subsequent period back to the original or preset tracking step size .DELTA.U, possibly even reduced relative thereto.

• - einen DC-Eingang zum Anschluss eines PV-Generators, der eine Reihenschaltung von PV-Modulen umfasst,
• - einen AC-Ausgang zum Anschluss des PV-Wechselrichters an ein Energieversorgungsnetz,
• - einen DC/AC-Wandler zur Umwandlung von Gleichstrom in Wechselstrom,
• - ein Messgerät zur Bestimmung einer dem PV-Generator entnommenen Leistung P, und
• - eine Steuerung. Dabei ist die Steuerung in Verbindung mit dem Messgerät zur Durchführung des erfindungsgemäßen Verfahrens ausgelegt. Es ergeben sich die bereits im Zusammenhang mit dem erfindungsgemäßen Verfahren erläuterten Vorteile.

A PV inverter according to the invention which carries out the method according to the invention comprises

• a DC input for connection of a PV generator comprising a series connection of PV modules,
• an AC output for connection of the PV inverter to a power supply network,
• a DC / AC converter for converting direct current into alternating current,
• a measuring device for determining a power P, taken from the PV generator, and
• - a controller. The control is designed in conjunction with the measuring device for carrying out the method according to the invention. This results in the advantages already explained in connection with the method according to the invention.

In one embodiment of the PV inverter, the PV inverter is designed as a single-stage PV inverter and is designed to generate a generator voltage U of the PV generator, e.g. via a corresponding control of the DC / AC converter, to vary. In an alternative embodiment, the PV inverter is designed as a multi-stage, in particular as a two-stage PV inverter with a DC / DC converter upstream of the DC / AC converter. In this case, the two-stage PV inverter is designed to vary a generator voltage U of the PV generator via a corresponding control of the DC / DC converter. Regardless of whether it is a single-stage or multi-stage PV inverter, the PV inverter can be designed as a single-phase or multi-phase, in particular three-phase PV inverter.

• - einen PV-Generator mit einer Reihenschaltung von PV-Modulen, und
• - einen eingangsseitig mit dem PV-Generator und ausgangsseitig mit einem Energieversorgungsnetz verbundenen erfindungsgemäßen PV - Wechselrichter. Bei dem PV-Generator kann es sich insbesondere um einen PV-String mit einer Reihenschaltung von PV-Modulen handeln. Alternativ dazu kann der PV-Generator jedoch auch eine Parallelschaltung mehrerer PV-Strings aufweisen, wobei jeder PV-String eine Reihenschaltung von PV-Modulen umfasst. Auch hier ergeben sich die bereits im Zusammenhang mit dem erfindungsgemäßen Verfahren erläuterten Vorteile.

A PV system according to the invention comprises

• - a PV generator with a series connection of PV modules, and
• - An input side of the PV generator and the output side connected to a power grid according to the invention PV inverter. In particular, the PV generator can be a PV string with a series connection of PV modules. Alternatively, however, the PV generator may also comprise a parallel connection of multiple PV strings, each PV string comprising a series connection of PV modules. Here too, the advantages already explained in connection with the method according to the invention result.

According to one embodiment of the PV system according to the invention, the PV generator has at least one PV module with a DC / DC converter assigned to the PV module as a power optimizer. In a further embodiment of the PV system, a plurality of PV modules, optionally all PV modules of the PV generator, have a DC / DC converter assigned to the respective PV module as a power optimizer.

list of figures

• 1 zeigt eine Ausführungsform einer erfindungsgemäßen PV-Anlage;
• 2 zeigt ein Flussdiagramm einer Ausführungsform des erfindungsgemäßen Verfahrens, das mit der PV - Anlage aus 1 durchgeführt werden kann;
• 3a zeigt eine schematische Darstellung des erfindungsgemäßen Verfahrens am Beispiel einer P-U-Kennlinie ohne Leistungsplateau; und
• 3b zeigt eine schematische Darstellung des erfindungsgemäßen Verfahrens am Beispiel einer P-U-Kennlinie mit einem Leistungsplateau

In the following the invention will be further explained and described with reference to preferred embodiments shown in the figures.

• 1 shows an embodiment of a PV system according to the invention;
• 2 FIG. 2 shows a flowchart of an embodiment of the method according to the invention that is compatible with the PV system 1 can be carried out;
• 3a shows a schematic representation of the inventive method using the example of a PU characteristic without power plateau; and
• 3b shows a schematic representation of the method according to the invention using the example of a PU characteristic with a power plateau

DESCRIPTION OF THE FIGURES

1 shows an embodiment of the PV system according to the invention 1 with a PV generator 2 and a PV inverter 5 whose DC input 5.3 with the PV generator 2 connected is. The PV generator 2 includes a PV string with a series connection of PV modules 3 , One of the PV modules 3 has a PV module 3 assigned DC / DC converter 4 as a power optimizer, with its input 4.1 with the corresponding PV module 3 and with his exit 4.2 with the remaining PV modules 3 the series circuit is connected. The PV inverter 5 is an example as a two-stage PV inverter with a DC / DC converter 5.2 and a connected DC / AC converter 5.1 educated. An AC output 5.4 of the PV inverter 5 is for feeding electrical power to a power grid 7 connected. The PV inverter 5 includes a meter 6 for determining electrical parameters of the PV generator 2 . for example, a generator voltage U , a generator current I and / or a power P of the PV generator 2 , The PV inverter 5 is via a control unit 8th , which is designed for carrying out the method according to the invention, controlled. The control unit 8th is for receiving measurement data with the measuring device 6 connected. The control unit 8th can as a separate control unit 8th the PV system 1 available. Alternatively, however, it may also be in the PV inverter 5 be arranged, and there optionally as part of a anyway in the PV inverter 5 be formed existing controller. The control 8th may for control and / or communication purposes with the operating as a power optimizer DC / DC converter 4 be connected, which is indicated by the dashed line in 1 is shown. A corresponding connection between the control unit 8th and the DC / DC converter 4 as a power optimizer is not mandatory.

The PV inverter 5 is in 1 through the three illustrated cross lines in the connecting line between the AC output 5.4 and the power grid 7 exemplary as a three-phase PV inverter 5 shown. Alternatively, however, it is also possible that the PV inverter 5 an AC output 5.4 having a different number of phases. For example, the PV inverter 5 be formed single phase. For carrying out the method according to the invention and for the PV system according to the invention 1 It is basically irrelevant which cause a Leistungsplateau LP within the PU characteristic. Therefore, it is also not mandatory that the PV system 1 even a PV module 3 with a the PV module 3 assigned DC / DC converter 4 as a power optimizer. Alternatively, instead of the in 1 illustrated single PV module 3 also several, optionally all PV modules 3 of the PV generator 2 a DC / DC converter 4 as a power optimizer.

In 2 a flow chart of an embodiment of the method according to the invention is shown. Starting point is a situation where the PV generator 2 the PV system 1 in a current operating point A ₁ with a first generator voltage U ₁ and a first generator current I ₁ is operated. In a first step S1 becomes a PV generator 2 in the current operating point A ₁ taken first performance P ₁ certainly. In a subsequent second step S2 the PV generator changes 2 from the current operating point A ₁ in a second operating point A ₂ by a second generator voltage U ₂ and a second generator current I ₂ is marked. In a third step S3 becomes a second working point A ₂ associated second service P ₂ certainly. In a subsequent fourth step S4 is by comparing the first P ₁ with the second performance P ₂ , If appropriate, taking into account further previously determined services P _i (with i> 2) from previous operating points Ai (with i> 2), checks whether from a performance plateau LP within the PU characteristic of the PV generator 2 is to go out. In the event that according to the examination no Leistungsplateau LP If the PU characteristic is present, the method changes to a fifth step S5 in which the new operating point Aneu is selected. The new operating point corresponds to this A _new the working point A ₁ . A ₂ its performance P ₁ . P ₂ has a greater value relative to the other. In a sixth step S6 becomes the PV generator 2 with the new operating point Aneu as the current operating point A ₁ operated and the process branches back to the first step S1 in which again in the current operating point A ₁ the this working point A ₁ assigned first performance P ₁ is determined.

In the event that according to the test in the fourth step S4 from a performance plateau of the PU characteristic, the process branches to an optional seventh step S7 , in which a plausibility check of the power plateau takes place. The plausibility check can check if the PV generator 2 PV modules 3 comprising a DC / DC converter 4 is assigned as a power optimizer. As explained above, the plausibility check can take place via a characteristic sweep of at least one region of the PU characteristic curve with an increased rate of change dU / dt of the generator voltage U relative to the preset tracking speed. In an optional eighth step S8 It checks whether the plausibility check carried out is an existence of DC / DC converters 4 as a power optimizer within the PV generator 2 approved. If this is not the case, it can be assumed that in the fourth step S4 identified performance plateau LP attributable to a radiation change made during the process and is not real, or at least not present over a longer period of time. In this case, it makes sense to continue with a selection of the new operating point Aneu aimed at maximizing the performance, and the method branches off to the fifth step S5 , On the other hand, it is in the eighth step due to the plausibility check S8 from an existence of DC / DC converters 4 as a power optimizer within the PV generator 2 to go out, then closes a ninth step S9 at which the selection of the new operating point A _new from the current one A ₁ and the second operating point A ₂ based on a more gentle operation of the PV inverter 5 whose components and / or other components of the PV system are selected. The goal here is the generator voltage U to approximate a predetermined or determined during the process target value, the gentler operation of the PV inverter 5 features. For example, this can reduce the generator voltage U be connected to bridge switch of the PV inverter 5 less demanding. Alternatively, the new operating point Aneu can be chosen so that with the selection of a decrease in the generator current I For example, a current load of components of the PV inverter 5 to reduce. Furthermore, the selection of the new operating point Aneu, taking into account a reduced power loss P _loss or an increase in the lifetime T _live of the PV inverter 5 and / or its components are selected. In a subsequent tenth step S10 becomes the PV generator 2 in the new working point Aneu as current working point A ₁ operated and the process branches back to the first step S1 in which repeatedly the first performance P ₁ it is determined that the PV generator 2 in the (now changed) current operating point A ₁ is removed.

The plausibility check in the seventh step S7 and their review in the eighth step S8 are optional steps that do not necessarily have to be done. This is in 1 by a dashed representation of the process steps S7 . S8 illustrated diagram elements illustrated. In an embodiment of the method in which a plausibility check does not occur, the fourth step branches S4 either to the fifth step S5 or directly to the ninth step S9 , Alternatively or cumulatively to the plausibility check in the seventh S7 and eighth step S8 can be between the fourth S4 and the ninth step S9 an extension of the power plateau LP be ascertained by an increase in the preset tracking increment .DELTA.U (in 2 not shown). The first working point A ₁ can then be at one end of the power plateau LP is placed. It is also possible that both working points A ₁ and A ₂ then at the ends of the power plateau LP are placed.

3a shows a schematic representation of the method according to the invention using the example of a PU characteristic 31, the no Leistungsplateau LP but instead an MPP operating point A _MPP maximum power P _MPP at a generator voltage U _MPP having. Such a PU characteristic curve 31 is present, for example, when the PV generator 2 the PV system 1 homogeneous radiation through each of its PV modules 3 learns, and only a maximum of one or a few PV modules 3 a DC / DC converter 4 as a power optimizer. The starting point is a current working point A ₁ of the PV generator 2 at a first generator voltage U ₁ and a first performance P ₁ , After determination of the first performance P ₁ in the current operating point A ₁ the PV generator changes 2 in a second operating point A ₂ , in which now one the second operating point A ₂ associated second service P ₂ is determined. The method according to the invention checks by means of a comparison of the first P ₁ and the second performance P ₂ - If appropriate, taking into account further power values P _i previous operating points A _i - if the PU characteristic 31 is a Leistungsplateau LP having. The test comes to the result that no performance plateau in the PU characteristic 31 is identified. Consequently, the selection of the new operating point Aneu with the aim of enlarging the PV generator 2 taken from electrical power P and it becomes the second operating point A ₂ selected as the new operating point Aneu. The PV generator 2 is thus operated in the new operating point Aneu, the future operating point A ₁ of the PV generator represents. The process steps are now with the (just changed) current operating point A ₁ go through repeatedly. In this case, a tracking direction of the previous pass is maintained, as in the last pass the second operating point A ₂ as a new workspot A _new was selected. Correspondingly, in the repeated pass, a (new) second operating point with further reduced generator voltage U < U ₂ , Even in the repeated pass, the method recognizes that the PU characteristic 31 has no power plateau LP and will make the selection of the new operating point taking into account a power maximization. As a result, the operating point A _{i of} the PV generator becomes 2 successively in the direction of the MPP operating point A _MPP with maximum power P _MPP approach, as is the case with conventional MPP tracking methods.

3b shows the schematic sequence of the inventive method using the example of a PU characteristic 32, in contrast to the PU characteristic 31 from 3a but now a performance plateau LP having. Such a performance plateau LP can occur during (partial) shading of the PV generator 2 the PV system 1 out 1 due to the DC / DC converters operating as power optimizers 4 on the PV modules 3 mint. The performance plateau is in 3b particularly pronounced, eg because every PV module 3 of the PV generator 2 a corresponding DC / DC converter 4 assigned. Also a use of different types of PV modules 3 within the PV generator 2 However, this can be an expression of a performance plateau LP within the PU characteristic 32 or an existing performance plateau LP strengthen. The performance plateau LP the PU characteristic 32 extends between a minimum Umin and a maximum voltage limit Umax of the generator voltage U arranged.

Starting point of the description of the 3b is again a current working point A ₁ of the PV generator 2 at a first generator voltage U ₁ , According to the method according to the invention becomes the current operating point A ₁ assigned first performance P ₁ determined before the PV generator 2 in the second working point A ₂ is offset, in turn, the second operating point A ₂ associated second service P ₂ is determined. Im in 3b the case shown represents the second power P ₂ in about the first performance P ₁ ie P ₂ ( U ₂ ) ≈ P ₁ ( U ₁ ). Therefore comes a comparison of the first P ₁ with the second performance P ₂ - if necessary, taking into account further performance values P _i past working points A _i -now to the result that the PU characteristic 32 a performance plateau LP having. Accordingly, a new operating point, Aneu, is chosen not for the purpose of maximizing the power, but for the target, the PV inverter 5 , its components and / or other components of the PV system 1 operate more gently under at least one aspect. As an example, the point of view, semiconductor switch of the inverter 5 less high voltages suspend and thus less stress. The aim of the procedure is therefore in 3b to minimize the generator voltage U and approximate a predefined target value. In the present example, the target value can be a generator voltage U be given by almost 0V, since it is indeed a question of minimizing the voltage stress of the semiconductor switches as possible. Consequently, as a new working point A _new the second working point A ₂ selected because of its generator voltage U ₂ closer to the target value of the generator voltage U = 0V. The PV generator 2 is operated in the new operating point Aneu, which henceforth (at least for the next run of the method steps) the current operating point A ₁ represents.

In the repeated passage of the method steps becomes a tracking direction of the previous pass due to the choice of the new operating point A _new maintained in the last pass. Consequently, a second operating point A ₂ in the repeated passage of the method steps one relative to the second generator voltage U ₂ the previous pass further reduced generator voltage U on. Also in the repeated run the comparison of the achievements delivers that the Leistungsplateau LP still exists. As a result, in the course of the process, the generator voltage U gradually approaches the target value of the generator voltage U = 0V until the generator voltage U reaches the minimum voltage limit Umin of the power plateau LP reached. Another reduction in generator voltage U would now have a reduction in the power taken from the generator P result. The method detects the power change when falling below the minimum voltage limit Umin the power plateau LP , which is now outside the predetermined power window .DELTA.P. Accordingly, the generator voltage is avoided U continue to decrease and the next tracking step will be in opposite tracking direction (here: increase the generator voltage U ) carried out. As a result, the generator voltage moves U also in the subsequent steps by means of a successive change between an enlargement and a reduction of the generator voltage U in the immediate vicinity of the minimum voltage limit Umin of the power plateau LP ,

Now falls the performance plateau LP For example, due to a change in the irradiation conditions away, the PU characteristic 32 changes and decreases one to the PU characteristic 31 from 3a similar form. The inventive method now recognizes automatically the elimination of the power plateau LP within the PU characteristic 32 and responds with a conventional MPP tracking method, which is the new operating point A _new based on an enlargement of the PV generator 2 taken power P selects. This is not an intervention of an operator of the PV system 1 required. Rather, the method reacts to the currently present form of the PU characteristic 32 and fits a target direction when selecting the new operating point A _new - The performance-oriented or more gentle operation of the inverter 5 and / or its components - automatically.

LIST OF REFERENCE NUMBERS

1

PV system

2

PV generator

3

PV module

4

DC / DC converter

4.1

entrance

4.2

output

5

PV inverters

5.1

DC / AC converter

5.2

DC / DC converter

5.3

DC input

5.4

AC output

6

gauge

7

Energy supply network (EVN)

8th

control

31

P-U characteristic

32

P-U characteristic

S1-S10

step

U ₁ , U ₂

generator voltage

U _min , U _max

voltage limit

A ₁ , A ₂

working

A _goal

working

A _MPP

MPP operating point

P _MPP

MPP performance

U _MPP

MPP voltage

LP

performance plateau

Claims (14)

Method for operating a photovoltaic (PV) inverter (5), to whose DC input (5.3) a PV generator (2) is connected, which comprises a series connection of PV modules (3), and whose AC output ( 5.4) for feeding electrical power to a power grid (7) is connected to the method steps: - determining a first electrical power P ₁ in a current operating point A _{1 of} the PV generator (2) at a first generator voltage U ₁ , and determining a second electrical power P ₂ in a second operating point A _{2 of} the PV generator (2) at a second generator voltage U ₂ , which differs from the first generator voltage U ₁ according to U ₂ = U ₁ + ΔU by a tracking step size ΔU, Check whether a power plateau (LP) of a PU characteristic curve (31) of the PV generator (2) is present, comprising a comparison of the first P ₁ with the second electric power P ₂ , - selection of that operating point A ₁ , A ₂ as a new working point t A _{new of} the PV generator (2) having a greater electrical power P ₁ , P ₂ , when according to the test no power plateau (LP) is present, and otherwise selecting that operating point A ₁ , A ₂ as the new operating point A _new of PV generator (2), which has a view of a gentler operation of the inverter (5) and / or its components, and - operating the PV generator (2) in the new operating point Aneu as the current operating point A ₁ . Method according to Claim 1 , characterized in that the method steps are repeatedly, in particular at regular intervals repeatedly performed, wherein by means of the repetition of the process steps of the PV generator (2) within a predetermined voltage window at a plurality of operating points A _i with i> 2 with different generator voltages U _i is operated. Method according to Claim 2 , characterized in that the check on a power plateau (LP) of the PU characteristic comprises at least one of the following criteria: - an amount of the voltage-related power change | ΔP / ΔU | = | (P _k -P _i ) / (U _k -U _i ) | falls below a predetermined threshold S> 0 for the plurality of operating points A _i , A _k with different generator voltages U _k , U _i within the predetermined voltage window, - the plurality of operating points A _i within the predetermined voltage window, have power values P _i (U _i ) within a given power window ΔP. Method according to one of the preceding claims, characterized in that the operation of the PV inverter (5) and / or its components, which is gentler from a viewpoint, takes into account at least one of the following boundary conditions: - a decrease of a generator voltage U within a predefined voltage range - a decrease in a Generator current I within a predetermined current range - an increase in efficiency of the PV inverter (5), its components and / or other components of the PV system (1), - a decrease in power loss P _{loss of} the PV inverter (5), whose Components and / or other components of the PV system (1), - a decrease in the load or a maximization of the service life T _{leb of} the PV inverter (5), its components and / or other components of the PV system (1). Method according to one of the preceding claims, characterized in that when, according to the test, a power plateau (LP) is present, additionally a plausibility check by means of a relative to a preset tracking speed of the PV inverter (5) changed tracking speed, in particular by means of a characteristic curve sweep over a range of the generator voltage U takes place, - wherein a determined over the characteristic sweep PU characteristic with the PU characteristic (31, 32) is compared, based on discrete changes in the generator voltage U, taking into account the tracking Step ΔU, and - depending on the comparison, an existence of a DC / DC converter (4) associated with a PV module (3) as a power optimizer within the PV generator (2) is concluded, and - wherein the choice of that operating point A ₁ , A ₂ as a new operating point A _{new of} the PV generator (2), the scho from one point of view Nenderen operation of the PV inverter (5) and / or its components, only takes place when the existence of a the PV module (3) associated DC / DC converter (4) is closed as a power optimizer. Method according to one of the preceding claims, characterized in that when, according to the examination, a power plateau (LP) is present, an increase of the tracking increment .DELTA.U by more than 3 times the original tracking increment .DELTA.U takes place. Method according to one of the preceding Claims 2 to 6 , characterized in that in a change from the first A ₁ to the second operating point A _2, a tracking direction of a previous pass of the method steps is maintained if the second operating point A _{2 was selected} as the new operating point Aneu in the previous pass, and otherwise vice versa. PV inverter (5), which performs the procedure according to one of Claims 1 to 8th comprising, - a DC input (5.3) for connecting a PV generator (2) comprising a series connection of PV modules (3), - an AC output (5.4) for connecting the PV inverter (5) to a power supply network (7), - a DC / AC converter (5.1) for converting direct current into alternating current, - a measuring device (6) for determining a power P taken from the PV generator (2), and - a controller (8 ) used in conjunction with the measuring device (6) for carrying out the method according to one of Claims 1 to 8th is designed. PV inverter (5) after Claim 8 characterized in that the PV inverter (5) is designed as a single-stage PV inverter (5), and is designed to vary a generator voltage U of the PV generator (2). PV inverter (5) after Claim 8 characterized in that the PV inverter (5) as a two-stage PV inverter (5) with a DC / AC converter (5.1) upstream DC / DC converter (5.2) is formed, wherein the PV inverter (5) is designed to vary a generator voltage U of the PV generator (2) via a corresponding control of the DC / DC converter (5.2). PV inverter (5) after one of the Claims 8 to 10 characterized in that the PV inverter (5) is designed as a single-phase or three-phase PV inverter (5). PV system (1) comprising - a PV generator (2) with a series connection of PV modules (3), and - a PV inverter connected on the input side to the PV generator (2) and to a power supply network (7) on the output side (5) after one of Claims 9 to 12 , PV system (1) according to Claim 12 , Characterized in that the photovoltaic generator (2) comprises at least one PV module (3) with a PV module (3) associated with the DC / DC converter (4) as a performance optimizer. PV system (1) according to Claim 13 , characterized in that a plurality of PV modules (3) of the PV generator (2) have a respective PV module (3) associated DC / DC converter (4) as a power optimizer.

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