Method for operating a PV system, and PV system

Abstract

The application relates to a method for operating a PV system (100) at an operating point (AP) having a DC voltage (U_DC) and a DC output. The PV system (100) has a PV generator (11) and an inverter system (20). The inverter system (20) is designed to convert the DC output (P_PV) of the PV generator (11) into a three-phase electric AC total output (P_AC) and has a primary inverter (10) and at least one secondary inverter (WR2, WR3,..., WRn). The inverters (10, WR2, WR3,..., WRn) are connected in parallel on the DC side and on the AC side. The method comprises operation of the at least one secondary inverter (WR2, WR3,..., WRn) by means of a droop control curve (12.2, 12.3,..., 12.N) stored in the inverters (10, WR2, WR3,..., WRn), by specifying a setpoint value (P2_s, P3_s ,..., Pn_s) for an individual output (P2, P3,..., Pn) of the at least one secondary inverter (WR2, WR3,..., WRn) as a function of the DC voltage (U_DC) at the operating point (AP) according to the droop control curve (12.2,..., 12.N). Furthermore, the method comprises operation of the primary inverter (10) by means of MPPT control (MPPT), by specifying a setpoint value (U_DC_s) for the DC voltage (U_DC) at the operating point (AP) and setting the setpoint value by varying the individual output (P1) of the primary inverter (10). The primary inverter (10) incrementally shifts the operating point (AP) along the DC voltage (U_DC), in order to maximize the total output of the PV generator (11) resulting from the individual output (P1) of the primary inverter (10) and the use of the stored droop control curve (12.2,..., 12.n) in the at least one secondary inverter (WR2, WR3,..., WRn). The application further relates to a PV system (100).

Description

[0001] 24-169-P-WO-1-submitted version

[0002] METHOD FOR OPERATING A PV SYSTEM AND PV SYSTEM

[0003] TECHNICAL AREA

[0004] The application concerns a method for operating a PV system and a PV system that has several inverters for electrical power conversion.

[0005] STATE OF THE ART

[0006] Common photovoltaic (PV) systems comprise PV generators, which in turn consist of a number of PV modules connected in series to form PV strings. Multiple PV strings can also be connected in parallel. A PV generator, comprising a single PV string or several PV strings in parallel, can be connected to an inverter input. The inverter uses power electronic circuitry to extract electrical power in the form of direct current (DC) from the PV generator, convert it to alternating current (AC), and feed it into an AC grid or use it for other purposes, such as powering a load. Depending on the size of the PV generators, the number of DC inputs, and the inverter's rated power, a high electrical power can be present at a single DC input of the inverter.For example, a so-called central inverter can have several dozen DC inputs and convert a nominal power of several megawatts, so that PV generators with a large number of parallel-connected PV strings and each with several dozen to several hundred kilowatts of electrical nominal power can be connected to each DC input.

[0007] A PV module has a characteristic curve that describes the current or power generated by the module as a function of the voltage applied to it under a given irradiance. This characteristic curve, also called the P(U) curve, typically has a maximum power point (MPP) at which the PV generator delivers the maximum possible power (MPP power) under the given irradiance. The peak power of a PV module is a given value determined using standardized methods and represents the MPP power under optimal irradiance conditions. The sum of the characteristic curves of the individual modules in a PV generator yields the generator's characteristic curve and thus its current MPP power.This MPP power as well as the sum of the peak powers of the PV modules of a PV generator are essential design criteria of a PV system in terms of a maximum expected MPP power of a specific 24- 169- P- WO - 2 - submitted version.

[0008] PV generator and one for the technically and economically optimal dimensioning of the inverter for the operation of a PV system.

[0009] An inverter connected to a PV generator typically features MPP tracking, where a DC voltage at the inverter's DC-side connection, and thus the voltage at the PV generator, is adjusted and modified so that the PV generator delivers the maximum possible power output under a given solar irradiance. Various methods are known in the art to find and track this operating point of maximum power, the so-called Maximum Power Point (MPP), for example, when the irradiance on the PV generator changes.For example, EP 1 995656 A1 discloses a method in which an inverter specifies a setpoint for a DC voltage to be applied to a PV generator and modifies it depending on the resulting PV power, wherein the direction of the modification of the setpoint depends on the change in power during previous modifications of the DC voltage applied to the PV voltage.

[0010] Instead of a single central inverter with a nominal power rating that essentially corresponds to the sum of the peak power ratings of the connected PV generators, several so-called string inverters, each with a fraction of the nominal power rating of a central inverter, can be used for the same nominal power rating. Provided that the nominal power rating of a string inverter is on the order of the peak power of the individual PV generators, one PV generator can, for example, be connected to each of these string inverters.

[0011] However, if multiple inverters are to be used for a given PV system with a high-peak-power PV generator, and their rated power is only a fraction of the PV generator's peak power, it can be advantageous to connect several inverters in parallel on both the DC and AC sides and connect a PV generator to this parallel connection of multiple inverters on the DC side. This allows the PV generator's power to be distributed among the inverters. In such a configuration, conventional MPP tracking methods, where the inverters modify the DC voltage at their respective DC terminals, cannot be used because the same DC voltage is present at all inverters due to the parallel DC connection. Furthermore, while an inverter can influence this voltage to some extent, it cannot adjust it independently.Conventional MPP tracking methods in parallel-connected inverters would interfere with each other, and a controlled adjustment of the PV voltage would not be possible, consequently preventing a correlation between PV voltage and PV power. Furthermore, an imbalance in the power distribution between the individual inverters can occur (24-169-P-WO-3-submitted version), which can lead to lower efficiency or, in the event of escalation, to an overload of individual inverters.

[0012] TASK

[0013] The application is based on the task of providing an improved procedure for operating a PV system as well as an improved PV system.

[0014] SOLUTION

[0015] The problem is solved by a method having the features of claim 1 and a PV system having the features of claim 10. Embodiments are mentioned in the dependent claims.

[0016] DESCRIPTION

[0017] A photovoltaic (PV) system comprises a PV generator and an inverter system. The inverter system is designed to convert the DC power of the PV generator into a total three-phase AC electrical power. The inverter system includes a primary inverter and at least one secondary inverter, with the inverters connected in parallel on the DC and AC sides.

[0018] A method for operating the PV system at an operating point with a DC voltage and a DC power exhibits:

[0019] • Operation of at least one secondary inverter by means of a droop control curve stored in the inverters, by specifying a setpoint for a single power output of the at least one secondary inverter as a function of the DC voltage at the operating point according to the droop control curve,

[0020] • Operation of the primary inverter using MPPT control, by specifying a setpoint for the DC voltage at the operating point and adjusting it by varying the individual power of the primary inverter, whereby the primary inverter, using MPPT control, incrementally shifts the operating point along the DC voltage to maximize the total power of the PV generator resulting from the individual power of the primary inverter and the application of the stored droop control curve in the at least one secondary inverter.

[0021] Due to the parallel connection of the inverters in the inverter system, the same DC voltage is present at the input of all inverters. This DC voltage corresponds to the version submitted to 24-169-P-WO-4.

[0022] The DC voltage applied to the PV generator. The individual power ratings of the respective inverters can refer in particular to the respective AC output power. The total AC power, i.e., the current total system power, can be determined from the measured individual power of the primary inverter and the estimated individual power ratings of at least one secondary inverter. The individual power ratings of at least one secondary inverter can be estimated using the stored droop control curve.

[0023] The MPPT controller varies the individual power output of the primary inverter to gradually shift the DC voltage in order to achieve the optimal operating point on the PV generator's characteristic curve. This optimal operating point also depends on external factors and can be adjusted by the MPPT controller during operation, particularly continuously.

[0024] A better operating point is characterized, for example, by a higher total AC power output of the PV system, which ideally corresponds to the MPP power of the PV generator, except for conversion losses. In the case of PV oversizing, i.e., when the power output of the connected PV generators is greater than the rated power of the inverter system, a better operating point can also be characterized by operation at the rated power of the inverter system. The rated power of the inverter system is the sum of the rated power outputs of the individual inverters within the system.

[0025] This method allows for the control of secondary inverters in a system with multiple AC and DC inverters connected in parallel, using their respective droop control curves, while the primary inverter adjusts the operating point accordingly. The operating point is set via the DC voltage. This adjustment by the primary inverter can be independent of the control systems of the secondary inverters. In some embodiments, communication between the inverters in the system is not required.

[0026] A droop control curve defines the relationship between the output power of an inverter and the DC voltage at its input. The droop control curve can be predefined for the inverter's control system. The adjustment of the output power of a secondary inverter to the setpoint defined by the droop control curve at a given DC voltage is then achieved by the inverter's control system, specifically by switching semiconductor switches in an inverter bridge. The inverter then adjusts the actual output power to the setpoint defined by the droop control curve for the current DC voltage using a power electronic bridge circuit. 24-169-P-WO-5-submitted version

[0027] Preferably, the droop control curves have a base point that lies at or above the open-circuit voltage of the PV generator, and a negative slope, meaning that the power output increases as the DC voltage decreases. Furthermore, the droop control curves have a kink point, i.e., a voltage at which the straight line extending from the base point reaches the rated power of the respective inverter.

[0028] The primary inverter performs MPPT control, flexibly adjusting the operating point of the PV system. MPPT control can include MPPT tracking. The secondary inverter(s) control their power output using their respective characteristic droop control curves, directly dependent on the common DC voltage. This can optionally occur without any further control system overriding the droop control curve. This allows for simpler and more cost-effective design and easier replacement of secondary inverters without requiring changes to the overall control system of the PV system. During MPPT tracking, the primary inverter monitors the Maximum Power Point (MPP) for the entire PV system's inverter array, primarily by varying the DC voltage in relation to the total AC power, i.e., the total system power.The total AC power can be calculated, estimated, and / or measured by the primary inverter and / or a higher-level unit.

[0029] In one embodiment of the method, during MPPT control, the primary inverter shifts the operating point in a direction that depends on the direction of any change in the total AC power compared to a previous step. This allows a better operating point to be found in both directions compared to the current operating point, i.e., at a higher or lower DC voltage than the current DC voltage.

[0030] In one embodiment of the method, the primary inverter performs iterative MPP tracking, aiming for an operating point that lies at a maximum of the PV characteristic curve. In this embodiment, the MPPT control includes MPP tracking, which involves an iterative search for the operating point that lies at the maximum of the PV characteristic curve. This allows the power output of the PV generator to be maximized.

[0031] In one embodiment of the method, the total AC power considered during operation by the primary inverter within the framework of MPPT control, particularly within the framework of MPP tracking, includes the individual power of the at least one secondary inverter. The individual power of the at least one secondary inverter is determined from the stored droop control curve of the at least one secondary inverter and the DC voltage at the operating point. This allows the MPPT-24-169-P-WO-6- version to be used.

[0032] The system takes the total AC power into account when setting the operating point. The determination of the individual power output of at least one secondary inverter from the stored droop control curve of that secondary inverter and the DC voltage at the operating point can be performed, for example, in the primary inverter and / or the higher-level control unit. The higher-level control unit could, for example, be located remotely from the PV system on a server.

[0033] In one embodiment of the method, the droop control curve of at least one secondary inverter can be modified. This allows the MPPT control on the primary inverter to, for example, adjust the operating point more accurately and / or faster, and / or to simulate the start-up of the PV system.

[0034] To modify the droop control curve, one parameter of the droop control curve can be adjustable. The droop control curve can, for example, have at least partial linear form. This line has a slope and a baseline where it intersects the zero line of the power. The slope of the line and / or the position of the baseline voltage can be adjustable. This allows, for example, the shape of the control curve to be specifically modified.

[0035] The droop control curve can be modified, for example, by the primary inverter and / or the higher-level unit. Communication between the primary inverter and at least one secondary inverter can be unidirectional or bidirectional. Bidirectional communication allows parameters of the overall system, particularly the total AC power, and / or parameters of the (variable) droop control curves to be transmitted to the primary inverter. A relatively slow communication speed may suffice for this purpose, as the primary inverter remains responsible for setting the current operating point. Alternatively or additionally, the droop control curves of the secondary inverters can be modified, particularly in response to a power imbalance between the inverters in the system.

[0036] In one embodiment of the method, the droop control curve of the at least one secondary inverter is modified depending on a secondary differential current at the output of the at least one secondary inverter. This allows the droop control curves of the secondary inverters to be modified depending on a power imbalance between the inverters of the inverter system. In particular, the circulating current flowing between the inverters can be used as a measure of such an imbalance. The respective AC differential current, individually measurable at the output of each secondary inverter, represents a contribution of the respective secondary inverter to a circulating current in the overall inverter system with the primary inverter connected in parallel with the at least one secondary inverter.

[0037] In one embodiment of the method, the primary inverter detects a primary differential current at its output and uses this primary differential current to determine the droop control curve of the at least one secondary inverter, which is modified by the complementary secondary differential current. The primary inverter then uses this modified droop control curve within the framework of MPPT control, particularly MPP tracking, to determine the individual power output of the at least one secondary inverter at a given DC voltage.

[0038] In one embodiment of the method, the inverter system comprises more than one secondary inverter. The respective droop control curve for each secondary inverter specifies the setpoint for the respective individual power output as a function of the current common DC voltage. The secondary inverters can be identical in construction and, in particular, can have the same droop control curve. In one embodiment of the method, the stored droop control curves for the secondary inverters are essentially identical during operation of the inverter system.

[0039] In one embodiment, the droop control curves of the secondary inverters can be modified, particularly depending on the respective differential current at the output of each secondary inverter. Based on the primary differential current, the primary inverter determines the complementary secondary differential currents and uses the correspondingly modified stored droop control curves of the secondary inverters within the framework of MPPT control.

[0040] A photovoltaic (PV) system can be operated at a single operating point with a DC voltage and DC power output. The PV system comprises a PV generator and an inverter system. The inverter system is configured to convert the electrical DC power of the PV generator into a total three-phase electrical AC power output. The inverter system includes a primary inverter and at least one secondary inverter. The inverters are connected in parallel on both the DC and AC sides. A droop control curve stored in the inverters specifies a target value for the individual power output of the at least one secondary inverter, depending on the DC voltage. The at least one secondary inverter can be operated using this droop control curve.The primary inverter is configured to specify a setpoint for the DC voltage at the operating point using MPPT control and to adjust this setpoint by varying the individual power output of the primary inverter. The primary 24-169-P-WO-8- submitted version.

[0041] The inverter is further configured to gradually shift the operating point along the DC voltage using MPPT control in order to maximize the total power of the PV generator resulting from the individual power of the primary inverter and the application of the stored droop control curve in the at least one secondary inverter (WR2, WR3, ... , WRn).

[0042] The operating point of the PV system can be set by the primary inverter via MPPT control, which can include MPP tracking. The secondary inverter(s) can be operated with their droop control curve and react independently to the current operating point setting. This can simplify the design of the inverter system.

[0043] In one embodiment, the PV system includes current meters that measure the AC current output by each inverter. Using this AC current, the output power of each inverter can be determined. Additionally, the PV system can include differential current meters located on the AC side of each inverter. These differential currents represent, in particular, the respective contributions to the circulating current within the PV system and can be used to modify the droop control curve of each inverter.

[0044] In one embodiment of the PV system, the primary and secondary inverters are essentially identical in construction. The identical inverters have the same nominal power output, meaning that each inverter can contribute approximately the same individual power to the total AC power.

[0045] In one embodiment of the PV system, the at least one secondary inverter has a rated power that is at least twice the rated power of the primary inverter. In such an embodiment, the power convertible by the at least one secondary inverter is at least twice that convertible by the primary inverter. In such an embodiment, the secondary inverter can be operated largely constantly at an energy-optimized operating point, while the primary inverter can use its entire rated power range for MPP tracking.

[0046] In one embodiment of the PV system, the secondary inverters each have a rated power that is at least twice the rated power of the primary inverter. In such an embodiment, the secondary inverters with symmetrical power distribution can be operated largely constantly in an energy-optimized configuration. 24-169-P-WO-9-submitted version

[0047] Operating point operation, while the primary inverter can use its entire rated power range for MPP tracking.

[0048] In one embodiment, the PV system has an ammeter which is arranged on the AC side at the connection of the parallel-connected inverters to an AC grid. Using the total AC current delivered by the inverter system measured in this way, the total AC power can be determined.

[0049] BRIEF DESCRIPTION OF THE FIGURES

[0050] The registration process is further explained and described below using the examples shown in the figures.

[0051] Fig. 1 shows a first embodiment of a PV system.

[0052] Fig. 2 shows a second embodiment of the PV system.

[0053] Fig. 3 shows a third embodiment of the PV system.

[0054] Fig. 4 shows exemplary PV characteristic curves and droop control curves.

[0055] Fig. 5 shows examples of further PV characteristic curves and droop control curves.

[0056] Fig. 6 shows examples of possible circulating currents in the PV system.

[0057] The same reference symbols are used in the figures for identical or similar elements. Representations in the figures may not be to scale.

[0058] FIGURE DESCRIPTION

[0059] Fig. 1 shows a PV system 100 with a PV generator 11 and an inverter system 20. The inverter system 20 converts a DC power P_PV from the PV generator 11 into a three-phase total AC electrical power P_AC. The total AC power P_AC is fed into a three-phase AC grid 14.

[0060] The PV generator 11 produces a DC power P_PV at a given solar irradiance and a DC voltage U_DC. The functional relationship between the produced DC power P_PV and the applied DC voltage U_DC is defined by a PV characteristic curve P_PV(U). The PV generator 11 is connected to the inverter system 20 on the DC side, so that the DC voltage U_DC at the inverter 12 essentially corresponds to the DC voltage U_DC applied to the PV generator. 24-169-P-WO-10-submitted version

[0061] The inverter system 20 converts the DC power P_PV into the total AC power P_AC. In this embodiment, the total AC power P_AC is fed into an AC grid 14. Optionally, a transformer can be provided between the inverter system 20 and the AC grid 14. The configuration shown is suitable, for example, for PV systems 100 with high power outputs in the range of several hundred kilowatts to several megawatts of electrical power. In addition to feeding power into the AC grid 14, other uses of the total AC power P_AC are possible, e.g., for operating an island grid or a dedicated load.

[0062] Inverter system 20 comprises a primary inverter 10 and a plurality of secondary inverters WR2, ... , WRn. The secondary inverters WR2, ... , WRn each have control units that detect the DC voltage U_DC and control the respective inverter WR2, ... , WRn such that it delivers a respective AC current I2_AC, ... , ln_AC and generates a respective power output P2, ... , Pn. The respective power output P2, ... , Pn is defined by a droop control curve 12.2, ... , 12.n stored in the inverters 10, WR2, ... , WRn, depending on the DC voltage U_DC. A setpoint P2_s, ... , Pn_s is determined via the respective droop control curve 12.2, ... , 12. n and used in the respective secondary inverter WR2, ... , WRn for adjusting the individual power P2, ... , Pn to the respective setpoint P2_s, ... , Pn_s e.g. by means of an inverter bridge circuit.

[0063] An MPPT controller of the primary power converter 10 generates a setpoint U_DC_s for the DC voltage U_DC. Using the setpoint U_DC_s for the DC voltage U_DC, a DC controller 18 generates a setpoint P1_s for the individual power P1 of the primary power converter 10. The primary power converter 10 is controlled by means of the setpoint P1_s for the individual power P1 of the primary power converter 10 and regulates, for example by means of an inverter bridge circuit, the AC current I1_AC and the associated individual power P1 to the setpoint P1_s.

[0064] The MPPT controller estimates the individual power outputs P2, ..., Pn of the secondary inverters 12.2, ..., 12.n using the stored droop control curves 12.2, ..., 12.n, as a function of the current DC voltage U_DC. The primary inverter's own power output P1 is measured, for example, by the primary inverter and / or calculated from the AC current I1_AC and the applied AC voltage. These individual power outputs P1, P2, ..., Pn are summed by the MPPT controller to obtain the current total AC power P_AC.

[0065] MPP tracking 16 of the MPPT controller MPPT modifies the setpoint U_DC_s of the DC voltage U_DC as a function of the previously determined total AC power P_AC and the current DC voltage U_DC. An operating point AP of the PV system 100 is shifted stepwise along the DC voltage U_DC curve (24-169-P-WO-11) to achieve a better operating point AP on a PV characteristic curve P_PV(U) of the PV generator 11. The MPP tracking 16 shifts the operating point AP in a direction that depends on the direction of any change in the total AC power P_AC compared to a previous step. The MPP tracking 16 can be iterative and aim for the operating point AP that lies at a maximum of the PV characteristic curve P_PV(U).

[0066] Fig. 2 shows a second embodiment of the PV system 100. In this embodiment, the respective slopes k of the droop control curves 12.2, ... , 12. n are adjustable. The respective slopes are determined by respective k-controllers 13.2, ... , 13. n in the respective secondary inverters WR2, ... , WRn. The determination of the respective slopes k is based on differential currents I2_di, ... , ln_di measured at the respective outputs of the secondary inverters WR2, ... , WRn.

[0067] The respective differential currents I1_di, I2_di, ... , ln_di are determined via differential current meters at the respective three-phase outputs of inverters 10, WR2, ... , WRn. The respective differential currents 11_di, I2_di, ... , ln_di are a measure of the uneven distribution of the individual powers P1, P2, ... , Pn in the three-phase inverter system 20. By including the differential currents in the control of the secondary inverters WR2, ... , WRn, the MPP can also be determined, for example, during the start-up of the PV system 100.

[0068] Fig. 3 shows a third embodiment of the PV system 100. In this embodiment, the respective slopes k of the droop control curves 12.2, ... , 12.n are also adjustable. The respective slopes are determined by respective k-controllers 13.2, ... , 13.n in the respective secondary inverters WR2, ... , WRn. The determination of the respective slopes k is based on differential currents I2_di, ... , ln_di measured at the respective outputs of the secondary inverters WR2, ... , WRn.

[0069] In this embodiment, the primary inverter 10 has an MPPT control which, in addition to the total AC power P_AC and the DC voltage U_DC, also takes into account the primary differential current 11_di at the output of the primary inverter 10 to determine the setpoint U_DC_s of the DC voltage U_DC.

[0070] The primary AC current I1_AC at the output of the primary inverter 10 is measured by an ammeter and used, for example, to determine and control the primary individual power P1 of the primary inverter 10.

[0071] The respective differential currents I1_di, I2_di, ... , ln_di are determined via differential current meters at the respective outputs of inverters 10, WR2, ... , WRn. The differential currents I1_di, I2_di, ... , ln_di are a measure of the uneven distribution of the individual 24- 169- P- WO - 12 - submitted version

[0072] Power outputs P1, P2, ... , Pn in inverter system 20. Additionally, the primary differential current 11_di can be used in the MPPT control to determine the complementary secondary differential currents I2_di, ... , ln_di and the associated change in the stored slope of the droop control curves 12.2, ... , 12.n. By including the differential currents in the MPPT control, the uneven distribution of the individual power outputs P1 , P2, ... , Pn can be reduced, and a better symmetry of the power delivered to the inverters 10, WR2, ... , WRn can be achieved.

[0073] Fig. 4 shows exemplary PV characteristic curves P_PV(U) and droop control curves 12.2, 12.3 of the PV system 100. In the example shown, the PV system 100 has two secondary inverters WR2, WR3, which are identical in construction and have the same nominal power of 100 kW. The primary inverter 10 also has a nominal power of 100 kW.

[0074] In both illustrations of Figure 4, the respective droop control curves 12.2 and 12.3 are shown individually and as the sum of both droop control curves 12.2 and 12.3. Also shown is the range within which the individual power P1 of the primary inverter 10 can be set. In the example shown, the adjustable range is from 0 to 100 kW – corresponding to the nominal power of the primary inverter 10.

[0075] The resulting system control curve 12. Sys, derived from the sum 12.2+12.3 of the individual power P1 and the droop control curves 12.2, 12.3, for this inverter system 20 at a specifically set individual power P1 (here, for example, approximately 90% of the nominal power of the primary inverter 10), intersects a PV characteristic curve P_PV(U) of the PV generator 11. On the depicted PV characteristic curves P_PV(U), the point of maximum power (MPP) is located at a DC voltage of 766 V in each case.

[0076] In the upper part of Figure 4, the droop control curves 12.2 and 12.3 of the two secondary inverters WR2 and WR3 each have a slope k of 400 W / V. The sum of the two droop control curves 12.2 and 12.3 of the two secondary inverters WR2 and WR3 thus has a slope k of 800 W / V. At an operating point AP at the maximum power point (MPP), the total AC power P_AC is 235 kW at a DC voltage of 766 V. The total AC power P_AC consists of the 75 kW per secondary inverter WR2 and WR3 controlled by the droop control curve and the 85 kW of the primary power inverter 10, which is controlled by the MPPT controller of the primary power inverter 10. The operating point AP at the point of maximum power (MPP) is maintained and tracked by the primary inverter 10 continuously varying the DC voltage U_DC as a function of the total AC power P_AC. 24-169-P-WO-13-submitted version

[0077] When the PV system 100 starts from idle at a DC voltage of approximately 960 V, the individual power P1 of the primary inverter 10 is initially zero, meaning the system control curve corresponds to the sum of the droop control curves 12.2 and 12.3. The PV generator 11 is then loaded by the MPPT controller of the primary inverter 10 by increasing the individual power P1, causing the DC voltage U_DC to decrease and the system control curve 12. Sys to shift upwards. The intersection of the system control curve 12. Sys with the PV characteristic curve P_PV(U), and thus the operating point AP, simultaneously moves to the upper left on the PV characteristic curve P_PV(U), meaning the total AC power P_AC increases as the DC voltage U_DC decreases.

[0078] However, starting the PV system 100 from the open-circuit voltage of the PV generator is not possible in the example shown in Figure 4 above. Using the MPPT control, even at maximum power P1 of the primary inverter 10, only the first (rightmost) intersection point of the PV characteristic curve P_PV(U) with the system control curve 12. Sys is achievable.

[0079] In the lower part of Figure 4, the droop control curves 12.2, 12.3 of the two secondary inverters WR2, WR3 each exhibit an increased slope k of 500 W / V. The sum of the two droop control curves 12.2, 12.3 of the two secondary inverters WR2, WR3 exhibits a slope k of 1000 W / V.

[0080] In the example shown in Figure 4 below, starting up the PV system 100 is almost achievable. However, here too, only the first (rightmost) intersection of the PV characteristic curve P_PV(U) with the system control curve 12. Sys is attainable. For starting up to the maximum power point (MPP), the system control curve 12. Sys would have to lie completely above the PV characteristic curve P_PV(U), at least at the maximum power P1 of the primary inverter 10.

[0081] Therefore, the slope of the droop control curves 12.2, 12.3 is preferably relatively large during the start-up phase and can be reduced during operation. Preferably, the inflection point of the droop control curves 12.2, 12.3 lies below the inflection point voltage at a DC voltage U_DC, so that the DC voltage U_DC lies within the linearly varying range of the droop control curves 12.2, 12.3.

[0082] Operation at a single operating point AP at the point of maximum power (MPP) is also possible in the example shown in Fig. 4. The power distribution here is slightly asymmetrical, with the individual power outputs P2 and P3 of the secondary inverters WR2 and WR3 each at 90 kW and the individual power output P1 of the primary inverter 10 at 55 kW.

[0083] Fig. 5 shows examples of further PV characteristic curves P_PV(U) and droop control curves 12.2, 12.3.

[0084] The droop control curves 12.2, 12.3 have a variable slope k. In the version shown in 24-169-P-WO-14-, submitted...

[0085] For example, the PV system 100 has two secondary inverters WR2 and WR3, which are identical in construction and have the same nominal power of 100 kW. The primary inverter 10 also has a nominal power of 100 kW.

[0086] In the exemplary system shown in Figure 5, the maximum total AC power P_AC of the inverter system 20 is therefore 300 kW. However, at the point of maximum power (MPP) on the PV characteristic curve P_PV(U), the power of the PV generator 11 is already 335 kW. The PV generator 11 is therefore oversized for the inverter system 20.

[0087] In both illustrations of Figure 5, the respective droop control curves 12.2 and 12.3 are shown individually and together as 12.2 + 12.3. Also shown is the range within which the individual power P1 of the primary inverter 10 can be set. In the example shown, the adjustable range is from 0 to 100 kW – corresponding to the nominal power of the primary inverter 10.

[0088] The upper part of Figure 5 illustrates a possible start-up process from the no-load state of the PV generator 11. At point 0 on the far right, the PV system 100 starts in no-load mode with the no-load voltage of the PV generator 11. The MPPT controller of the primary inverter 10 reduces the DC voltage U_DC by applying a load via P_AC feed-in through the generation of the individual power P1. The DC power P_PV subsequently increases according to the intersection point of the system control curve 12 (see Fig. 4) with the PV characteristic curve P_PV(U).

[0089] The DC voltage U_DC drops, and the first operating point AP1, located to the left of the no-load point 0, is reached. Here, the secondary inverters WR2 and WR3 feed in a power output P2+P3 of approximately 2 x 30 kW. The total AC power P_AC thus reaches 60 kW from the secondary inverters WR2 and WR3, plus P1 = 40 kW from the primary inverter 10. The primary inverter 10 can further reduce the DC voltage U_DC by increasing the load (i.e., by increasing P1).

[0090] The next operating point AP2 is reached further to the left. The secondary inverters WR2 and WR3 feed in their respective nominal power P2=P3=100 kW. The total AC power P_AC reaches P2+P3=200 kW plus P1=20 kW from the primary inverter P1. The primary inverter 10 can further reduce the DC voltage U_DC here by increasing the load (i.e., increasing P1).

[0091] The next operating point AP3 is reached further to the left. The inverter system 20 with the three inverters 10, WR2, WR3 reaches its rated power at P1+P2+P3 = 300 kW with a symmetrical power distribution. 24-169-P-WO-15- submitted version

[0092] The lower part of Figure 5 shows another start-up attempt from the idle state of the PV generator 11. Here, the two droop control curves 12.2 and 12.3 exhibit a shallower slope k. This is advantageous for continuous operation, e.g., at the point of maximum power, but is disadvantageous during start-up.

[0093] At point 0 on the far right, the PV system 100 starts in no-load mode with the no-load voltage of the PV generator 11. The MPPT controller of the primary inverter 10 then reduces the DC voltage U_DC by applying a load via P_AC feed-in through the generation of the individual power P1. The DC power P_PV subsequently increases according to the intersection of the system control curve 12 (see Fig. 4) with the PV characteristic curve P_PV(U).

[0094] At operating point AP1 further to the left, the primary inverter 10 reaches its rated power of 100 kW. Here, the secondary inverters WR2 and WR3 feed in P2+P3 ~2x10 kW. The total AC power P_AC reaches P2+P3=20 kW plus P1=100 kW from the primary inverter 10.

[0095] The next operating point APx further to the left cannot be reached because the primary inverter 10 is already operating at its rated power of 100 kW and cannot further increase its individual power P1, and therefore the load on the PV generator. Consequently, the DC voltage U_DC cannot be further reduced by the MPPT control system.

[0096] At operating point AP1, immediately to the left of the no-load point 0, a circulating current l_circuit flows between inverters 10, WR2, and WR3 due to the unequal power distribution: 100 kW in the primary inverter 10 and approximately 20 kW each in the secondary inverters WR2 and WR3. By detecting this circulating current l_circuit, the slope of the droop control curves 12.2 and 12.3 can be increased by the respective k-controllers 13.2 and 13.3, depending on the detected circulating current l_circuit. This allows operating point AP1 to shift further to the left, enabling the next operating point APx to be reached further to the left (see the upper part of Figure 5).

[0097] On the detection and consideration of the circular flows l_circle by the k-controller 13.2,

[0098] 13.3 A start-up process can be achieved by increasing the slope k of the droop control curves 12.2,

[0099] 12.3. The k-controller can, for example, increase the slope k of the respective droop control curve 12.2, 12.3 for larger circular currents I2_di, I3_di and decrease the slope k of the droop control curves 12.2, 12.3 for smaller circular currents I2_di, I3_di.

[0100] Fig. 6 shows an example of possible circulating currents l_circuit of the PV system 100 as a function of a power imbalance of the three-phase inverters 10, WR2, WR3. In the example shown, the two secondary inverters WR2, WR3 are assumed to be identical in construction and have the same power rating. It can be seen that the circulating current l_circuit decreases with increasing power. 24-169-P-WO-16-submitted version

[0101] The imbalance between primary inverter 10 and secondary inverters WR2 and WR3 increases significantly. This can be detected via the differential current meters and taken into account in the MPPT control.

[0102] 4-169-P-WO-17-submitted version

[0103] REFERENCE MARK LIST

[0104] 10 primary inverter 11 PV generator

[0105] 12.2, 12.3, ... , 12. n Droop control curve 12. Sys System control curve

[0106] 13.2, 13.3, ... , 13. n k controller 14 AC network

[0107] 16 MPP tracking

[0108] 18 DC voltage regulators

[0109] 20 Inverter system P_PV(U) PV characteristic curve

[0110] AP1, AP2, AP3, APx Operating point

[0111] U_DC DC voltage

[0112] P_PV DC power

[0113] P_AC AC total power

[0114] WR2, WR3, ... , WRn secondary inverter P2, P3, ... , Pn individual power of the secondary inverter

[0115] P2_s, P3_s, ... , Pn_s Setpoint for individual power of the secondary inverter MPPT MPPT control U_DC_s Setpoint for the DC voltage at the operating point P1_s Setpoint for individual power of the primary inverter P1 Individual power of the primary inverter

[0116] I1_AC, I2_AC, ... , In _AC AC current

[0117] I1_di primary differential current

[0118] I2_di, ... , ln_di secondary differential current k slope of droop control curve l_circuit circulating current

[0119] MPP point of greatest power

Claims

24-169-P-WO-18-submitted version PATENT CLAIMS 1. A method for operating a PV system (100) at an operating point (AP) with a DC voltage (U_DC) and a DC power (P_PV), wherein the PV system (100) comprises a PV generator (11) and an inverter system (20), wherein the inverter system (20) is configured to convert the DC power (P_PV) of the PV generator (11) into a three-phase total electrical AC power (P_AC), wherein the inverter system (20) comprises a primary inverter (10) and at least one secondary inverter (WR2, WR3, ... , WRn), wherein the inverters (10, WR2, WR3, ... , WRn) are connected in parallel on the DC and AC sides, wherein the method comprises: Operation of at least one secondary inverter (WR2, WR3, ... , WRn) by means of a droop control curve (12.2, 12.3, ... , 12. n) stored in the inverters (10, WR2, WR3, ... , WRn), by specifying a setpoint (P2_s, P3_s, ... , Pn_s) for a single power (P2, P3, ... , Pn) of the at least one secondary inverter (WR2, WR3, ... , WRn) as a function of the DC voltage (U_DC) at the operating point (AP) according to the droop control curve (12.2, 12.3, ... , 12. n), Operation of the primary inverter (10) by means of an MPPT controller (MPPT) by specifying a setpoint (U_DC_s) for the DC voltage (U_DC) at the operating point (AP) and adjusting it by varying the individual power (P1) of the primary inverter (10), wherein the primary inverter (10) by means of the MPPT controller shifts the operating point (AP) stepwise along the DC voltage (U_DC) in order to maximize a total power of the PV generator (11) resulting from the individual power (P1) of the primary inverter (10) and the application of the stored droop control curve (12.2, ... , 12. n) in the at least one secondary inverter (WR2, WR3, ... , WRn).

2. Method according to claim 1, wherein the primary inverter (10) shifts the operating point (AP) in a direction which depends on a direction of a change in the total AC power (P_AC) compared to a previous step.

3. Method according to claim 1 or 2, wherein the primary inverter (10) performs iterative MPP tracking (16) in which an operating point (AP) is sought which corresponds to a maximum of the PV characteristic curve (P_PV(U)).

4. Method according to one of the preceding claims, wherein a total AC power (P_AC) taken into account during operation by the primary inverter (10) within the framework of MPPT control (MPPT), in particular within the framework of MPP tracking (16), determines the individual 24-169-P-WO-19-submitted version Power (P2, P3, ... , Pn) of the at least one secondary inverter (WR2, WR3, ... , WRn) is included, wherein the individual power (P2, P3, ... , Pn) of the at least one secondary inverter (WR2, WR3, ... , WRn) is determined, in particular in the primary inverter (10), from the stored droop control curve (12.2, 12.3, ... , 12. n) of the at least one secondary inverter (WR2, WR3, ... , WRn) and the DC voltage (U_DC) at the operating point (AP).

5. Method according to any of the preceding claims, wherein the droop control curve (12.2, 12.3, ... , 12. n) of the at least one secondary inverter (12.2, 12.3, ... , 12. n) is modifiable.

6. Method according to claim 5, wherein the droop control curve (12.2, 12.3, ... , 12. n) of the at least one secondary inverter (WR2, WR3, ... , WRn) is modified as a function of a secondary differential current (I2_di, I3_di, ... , ln_di) at the output of the at least one secondary inverter (WR2, WR3, ... , WRn).

7. Method according to claim 6, wherein the primary inverter (10) detects a primary differential current (11_di) at the output of the primary inverter (10) and uses the primary differential current (11_di) to determine the modified droop control curve (12.2, 12.3, ... , 12. n) of the at least one secondary inverter (WR2, WR3, ... , WRn) and uses this modified droop control curve (12.2, 12.3, ... , 12. n) within the framework of MPPT control (MPPT), in particular within the framework of MPP tracking (16), to determine the individual power (P2, P3, ... , Pn) of the at least one secondary inverter (WR2, WR3, ... , WRn).

8. Method according to one of the preceding claims, wherein the inverter system (20) has more than one secondary inverter (WR2, WR3, ... , WRn), wherein the respective droop control curve (12.2, 12.3, ... , 12. n) for the respective secondary inverter (WR2, WR3, ... , WRn) specifies the respective setpoint (P2_s, P3_s, ... , Pn_s) for the respective individual power as a function of the DC voltage (U_DC).

9. Method according to claim 8, wherein the stored droop control curves (12.2, 12.3, ... , 12. n) for the secondary inverters (WR2, WR3, ... , WRn) of the inverter system are identical.

10. Method according to claim 9, wherein the droop control curve (12.2, 12.3, ... , 12. n) of the secondary inverters (WR2, WR3, ... , WRn) is modified depending on a respective secondary differential current (I2_di, I3_di, ... , I n_di) at the output of the respective secondary inverter (WR2, WR3, ... , WRn). 24-169-P-WO-20-submitted version 11. A PV system (100) which can be operated at an operating point (OP) with a DC voltage (U_DC) and a DC power (P_PV), wherein the PV system (100) comprises a PV generator (11) and an inverter system (20) for converting the electrical DC power (P_PV) of the PV generator (11) into a three-phase total electrical AC power (P_AC), wherein the inverter system (20) comprises a primary inverter (10) and at least one secondary inverter (WR2, WR3, ... , WRn), wherein the inverters (10, WR2, WR3, ... , WRn) are connected in parallel on the DC and AC sides, and wherein a droop control curve (12.2, 12.3, ... ) is stored in the inverters.

12. n) specifies a setpoint (P2_s, P3_s, ... , Pn_s) for a single power output (P2, P3, ... , Pn) of the at least one secondary inverter (WR2, WR3, ... , WRn) as a function of the DC voltage (U_DC), wherein the at least one secondary inverter (WR2, WR3, ... , WRn) is operable with the droop control curve (12.2, 12.3, ... , 12. n ), wherein the primary inverter (10) is configured to specify a setpoint (U_DC_s) for the DC voltage (U_DC) at the operating point (AP) by means of an MPPT controller (MPPT) and to adjust it by varying the single power output (P1) of the primary inverter (10), and wherein the primary inverter (10) is further configured to use the MPPT control (MPPT) to gradually shift the operating point (AP) along the DC voltage (U_DC) to obtain a result from the single power (P1) of the primary inverter (10) and the application of the stored droop control curve (12.2, ... , 12.n) to maximize the total power output of the PV generator (11) resulting from at least one secondary inverter (WR2, WR3, ... , WRn).

12. PV system according to claim 11, wherein the PV system (100) has current meters which measure a respective AC current (I1_AC, I2_AC, I3_AC, ... , ln_AC) output by a respective inverter (10, WR2, WR3, ... , WRn).

13. PV system according to claim 11 or 12, wherein the PV system (100) has differential current meters which are arranged on the AC side of the respective inverters (10, WR2, WR3, ... , WRn).

14. PV system according to one of claims 11 to 13, wherein the primary and the at least one secondary inverter (10, WR2, WR3, ... , WRn) are essentially identical in construction.

15. PV system according to one of claims 11 to 13, wherein the at least one secondary inverter (WR2, WR3, ... , WRn) has a rated power that is at least twice the rated power of the primary inverter (10), or wherein the 24- 169- P- WO - 21 - submitted version secondary inverters (WR2, WR3, , WRn) each have a rated power that is at least twice the rated power of the primary inverter (10).

16. PV system according to one of claims 11 to 15, wherein the PV system (100) has an ammeter which is arranged on the AC side at the connection of the parallel circuit of the inverters (10, WR2, WR3, ... , WRn) with an AC network (14).

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