• 专利附录
  • 专利说明
  • 权利要求
  • 类似技术
  • 同族专利
  • 引用文献
  • 法律状态
  • 优先权
    • 专利摘要:
    A controller (101) for controlling a power converter (103) connected to a photovoltaic side of a network converter of a photovoltaic system is presented. When photovoltaic panels (105) of the photovoltaic system produce electric power, the controller controls the power converter to keep its electric power independent of direct voltage (UDC) of the Network converter to enable the network converter to maximize, with maximum power point tracking, the electric power produced by the photovoltaic panels. When the photovoltaic panels do not produce electric power, the controller controls the electric power of the power converter to be an upwards convex function of the direct voltage. Thus, the network converter is enabled to apply the maximum power point tracking also when only the power converter supplies electric power. Thus, there is no need to deactivate the maximum power point tracking when the photovoltaic panels stop producing electric power.
    公开号:FI20185215A1
    申请号:FI20185215
    申请日:2018-03-08
    公开日:2019-09-09
    发明作者:Jani Hiltunen；Miko Huomo；Esa Malinen；Pasi Nuutinen；Mikko Pääkkönen
    申请人:Greenenergy Finland Oy；
    IPC主号:
    专利说明:

    A controller and a method for controlling a power converter
    20185215 PRH 08 -03- 2018
    Field of the disclosure
    The disclosure relates generally to control of electric energy. More particularly, the 5 disclosure relates to a controller, a method, and a computer program for controlling a power converter connected to a photovoltaic side of a network converter of a photovoltaic system. Furthermore, the disclosure relates to a power converter and to a photovoltaic system.
    Background
    A photovoltaic system comprises typically one or more photovoltaic panels and a network converter for supplying electric power produced by the one or more photovoltaic panels to an alternating voltage power grid or to a direct voltage power grid. The network converter is advantageously configured to carry out maximum power point tracking “MPPT” that comprises tuning direct voltage of each photovoltaic panel or direct voltage of parallel connected photovoltaic panels so that the electric power produced by the one or more photovoltaic panels under consideration is maximized. In other words, the MPPT comprises seeking a voltagecurrent operating point which maximizes electric power produced by one or more photovoltaic panels.
    In many cases, there is a need to connect one or more electric devices to a photovoltaic side of a network converter of a photovoltaic system. In this document the “photovoltaic side of a network converter” means a direct voltage terminal of the network converter for receiving electric power from one or more photovoltaic panels. An electric device connected to the photovoltaic side of a network converter can be for example a direct voltage energy storage that may comprise e.g. a battery system and/or a capacitor bank. The direct voltage energy storage can be controlled to receive electric power from the photovoltaic panels when the photovoltaic panels operate in advantageous conditions for producing electric power. On the other hand, the direct voltage energy storage can be controlled to supply electric power to a power grid via the network converter when the operation conditions of the
    20185215 PRH 08 -03- 2018 photovoltaic panels are not advantageous. It is also possible that the electric device is an electric power source such as e.g. a fuel cell or a wind turbine driven generator. A direct voltage energy storage or another electric device is typically connected to the photovoltaic side of a network converter with a power converter configured to 5 convert voltage of the electric device to voltage suitable for the photovoltaic side of the network converter.
    A photovoltaic system of the kind mentioned above is however not free from challenges. One of the challenges is that one or more electric devices connected to the photovoltaic side of a network converter should not disturb the maximum power 10 point tracking “MPPT” when photovoltaic panels produce electric power. On the other hand, the MPPT should not disturb the operation of the network converter and neither the operation of the one or more electric devices when the photovoltaic panels do not produce electric power.
    Summary
    The following presents a simplified summary in order to provide basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a 20 prelude to a more detailed description of exemplifying embodiments of the invention.
    In accordance with the invention, there is provided a new controller for controlling a power converter connected to a photovoltaic side of a network converter of a photovoltaic system. The network converter is configured to carry out the maximum power point tracking “MPPT” for maximizing electric power produced by one or more 25 photovoltaic panels of the photovoltaic system, and the power converter is suitable for connecting a direct voltage energy storage or another electric device to the photovoltaic side of the network converter.
    A controller according to the invention comprises a processing system configured to:
    20185215 PRH 08 -03- 2018
    - control, in response to a situation in which the one or more photovoltaic panels produce electric power, the power converter to keep electric power transferred via the power converter to be substantially independent of direct voltage of the network converter so as to enable the network converter to maximize, with the maximum power point tracking, the electric power produced by the one or more photovoltaic panels, and otherwise
    - control electric power transferred via the power converter to the network converter to be an upwards convex function of the direct voltage of the network converter so that the upwards convex function has a maximum between minimum and maximum values of the direct voltage.
    The electric power transferred via the power converter does not disturb the maximum power point tracking since this electric power is substantially independent of the direct voltage of the network converter, i.e. of the direct voltage of the one or more photovoltaic panels, when the one or more photovoltaic panels produce 15 electric power. On the other hand, when the one or more photovoltaic panels do not produce electric power, the behavior of the electric power transferred via the power converter to the network converter as a function of the direct voltage is arranged to emulate behavior of electric power of photovoltaic panels. Thus, the network converter can apply the maximum power point tracking also when the one or more 20 photovoltaic panels do not produce electric power. The upwards convex function having the maximum between the minimum and maximum values of the direct voltage prevents the maximum power point tracking from driving the direct voltage to one or another of the minimum and maximum values. Therefore, there is no need to deactivate the maximum power point tracking when the photovoltaic panels stop 25 producing electric power.
    In accordance with the invention, there is provided also a new power converter that comprises:
    - a first electric terminal for connecting to a photovoltaic side of a network converter of a photovoltaic system,
    20185215 PRH 08 -03- 2018
    - a second electric terminal for connecting to an electric device such as e.g. a direct voltage energy storage,
    - a main circuit for transferring electric power between the first and second electric terminals, and
    - a controller according to the invention for controlling the electric power transferred via the main circuit.
    The power converter can be for example a direct voltage-to-direct voltage “DC-DC” converter.
    In accordance with the invention, there is provided also a new photovoltaic system 10 that comprises:
    - one or more photovoltaic panels,
    - a network converter for supplying electric power produced by the one or more photovoltaic panels to a power grid and for maximizing, with the maximum power point tracking, the electric power produced by the one or more photovoltaic panels, and
    - at least one power converter according to the invention and connected to the photovoltaic side of the network converter.
    In accordance with the invention, there is provided also a new method for controlling a power converter connected to a photovoltaic side of a network converter of a 20 photovoltaic system. A method according to the invention comprises:
    - controlling, in response to a situation in which one or more photovoltaic panels of the photovoltaic system produce electric power, the power converter to keep electric power transferred via the power converter to be substantially independent of direct voltage of the network converter so as to enable the network converter to maximize, with the maximum power point tracking, the electric power produced by the one or more photovoltaic panels, and otherwise
    20185215 PRH 08 -03- 2018
    - controlling electric power transferred via the power converter to the network converter to be an upwards convex function of the direct voltage of the network converter so that the upwards convex function has a maximum between minimum and maximum values of the direct voltage.
    In accordance with the invention, there is provided also a new computer program for controlling a power converter connected to a photovoltaic side of a network converter of a photovoltaic system. A computer program according to the invention comprises computer executable instructions for controlling a programmable processing system to:
    - control, in response to a situation in which one or more photovoltaic panels of the photovoltaic system produce electric power, the power converter to keep electric power transferred via the power converter to be substantially independent of direct voltage of the network converter so as to enable the network converter to maximize, with the maximum power point tracking, the electric power produced by the one or more photovoltaic panels, and otherwise
    - control electric power transferred via the power converter to the network converter to be an upwards convex function of the direct voltage of the network converter so that the upwards convex function has a maximum between minimum and maximum values of the direct voltage.
    In accordance with the invention, there is provided also a new computer program product. The computer program product comprises a non-volatile computer readable medium, e.g. a compact disc “CD”, encoded with a computer program according to the invention.
    Various exemplifying and non-limiting embodiments of the invention are described in accompanied dependent claims.
    Exemplifying and non-limiting embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific
    20185215 PRH 08 -03- 2018 exemplifying and non-limiting embodiments when read in conjunction with the accompanying drawings.
    The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of un-recited features. The 5 features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of “a” or “an”, i.e. a singular form, throughout this document does not exclude a plurality.
    Brief description of the figures
    Exemplifying and non-limiting embodiments of the invention and their advantages 10 are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which:
    figures 1a, 1b, and 1c illustrate a photovoltaic system comprising a power converter according to an exemplifying and non-limiting embodiment ofthe invention, and figure 2 shows a flowchart of a method according to an exemplifying and non-limiting 15 embodiment of the invention for controlling a power converter connected to a photovoltaic side of a network converter of a photovoltaic system.
    Description of exemplifying and non-limiting embodiments
    The specific examples provided in the description below should not be construed as limiting the scope and/or the applicability of the accompanied claims. Lists and 20 groups of examples provided in the description are not exhaustive unless otherwise explicitly stated.
    Figure 1a illustrates a photovoltaic system according to an exemplifying and nonlimiting embodiment of the invention. This exemplifying photovoltaic system comprises electrically interconnected photovoltaic panels 105 and a network 25 converter 104 for supplying electric power produced by the photovoltaic panels 105 to a power grid 123. In this exemplifying case, the power grid 123 is an alternating voltage power grid and thus the network converter 104 is a network inverter. The network converter 104 is configured to apply the maximum power point tracking
    20185215 PRH 08 -03- 2018 “MPPT” that comprises tuning direct voltage Udc of the photovoltaic panels 105 so that the electric power produced by the photovoltaic panels 105 is maximized. The electric power produced by the photovoltaic panels 105 is Idc.pv x Udc, where Idc.pv is output current of the photovoltaic panels 105. Thus, the MPPT comprises seeking a Udc, Idc.pv -operating point which maximizes the electric power Idc.pv x Udc produced by the photovoltaic panels 105.
    In the exemplifying case illustrated in figure 1a, the photovoltaic system comprises a direct voltage energy storage 113 that comprises a battery system. It is also possible that a direct voltage energy storage comprises a capacitor bank that may comprise e.g. electric double layer capacitors “EDLC” known as “supercapacitors”. The photovoltaic system comprises a power converter 103 according to an exemplifying and non-limiting embodiment of the invention for transferring electric power between the direct voltage energy storage 113 and a photovoltaic side of the network converter 104. The photovoltaic side is a direct voltage terminal of the network converter 104 for receiving electric power from the photovoltaic panels 105. The power converter 103 comprises a first electric terminal 107 connected to the photovoltaic side of the network converter 104 and a second electric terminal 108 connected to the direct voltage energy storage 113. The power converter 103 comprises a main circuit 109 for transferring electric power between the first and second electric terminals 107 and 108. In this exemplifying case, the power converter 103 is a bi-directional direct voltage-to-direct voltage “DC-DC” converter.
    The exemplifying photovoltaic system illustrated in figure 1a further comprises a power converter 133 for transferring electric power between a direct voltage connector 124 and the photovoltaic side of the network converter 104. In the exemplifying situation shown in figure 1a, the direct voltage connector 124 is connected to batteries 116 of a car 115 that can be a pluggable hybrid-car or a full electric car. The power converter 133 can be a bi-directional direct voltage-to-direct voltage “DC-DC” converter like the power converter 103. The power converters 103 and 133 are advantageously capable of transferring electric power from the direct voltage energy storage 113 to the batteries 116 of the car 115 as well as in the opposite direction from the batteries 116 of the car 115 to the direct voltage energy storage 113. The above-mentioned power transfers take place in the direct voltage
    20185215 PRH 08 -03- 2018 side of the system shown in figure 1a and thus power losses can be low. Furthermore, the energy stored in the batteries 116 of the car 115 can be used for energizing the AC power grid 123 and thus the car 115 can be used, if needed, as a pluggable auxiliary power source.
    The power converter 103 comprises a controller 101 according to an exemplifying and non-limiting embodiment of the invention. The controller 101 comprises a processing system 102 configured to control electric power Pi = Idci x Udc transferred via the power converter 103. When the photovoltaic panels 105 produce electric power, the processing system 102 keeps the electric power Pi = Idci x Udc 10 substantially independent of the direct voltage Udc. Thus, when the photovoltaic panels 105 produce electric power, the processing system 102 controls the current Idci so that the current Idci is substantially inversely proportional to the direct voltage Udc, i.e. Idci ~ 1/Udc. Correspondingly, the power converter 133 comprises a controller 131 configured to control electric power P2 = Idc2 x Udc transferred via the 15 power converter 133. When the photovoltaic panels 105 produce electric power, the controller 131 keeps the electric power P2 = Idc2 x Udc substantially independent of the direct voltage Udc. As the electric powers Pi and P2 are substantially independent of the direct voltage Udc, the behavior of the electric power transferred by the network converter 104 as a function of the direct voltage Udc is determined 20 by the photovoltaic panels 105. Therefore, the power converters 103 and 133 do not disturb the maximum power point tracking “MPPT” that comprises tuning the direct voltage Udc so that the electric power transferred via the network converter 104, i.e. Pi + P2 + Idc.pv x Udc, is maximized.
    When the photovoltaic panels 105 do not produce electric power i.e. the electric 25 power of the photovoltaic panels 105 is zero, the processing system 102 of the power converter 103 controls the electric power Pi = Idci x Udc to be an upwards convex function of the direct voltage Udc so that the upwards convex function has a maximum between minimum and maximum values of the direct voltage Udc i.e. inside the range of variation of the direct voltage Udc. The range of variation of the 30 direct voltage Udc has advantageously a first sub-range where the upwards convex function is increasing and, above the first subrange, a second sub-range where the
    20185215 PRH 08 -03- 2018 upwards convex function is decreasing. The above-mentioned upwards convex function of the direct voltage Udc can be for example:
    Pi(Udc) = Poi - oc(Udc - Um)2, (1) where Poi is a target value of the electric power Pi transferred via the power 5 converter 103, Um is a value of Udc at which the electric power Pi achieves the target value Poi, and oc is a positive constant. The controller 131 of the power converter 133 can be configured to control the electric power P2 = Idc2 x Udc to be an upwards convex function of the direct voltage Udc when the photovoltaic panels 105 do not produce electric power. The upwards convex function related to the 10 power converter 133 can be e.g. P2(Udc) = P02 - oc(Udc - Um)2, where P02 is a target value of the electric power P2 transferred via the power converter 133. As the MPPT maximizes the electric power transferred via the network converter 104, the MPPT drives the direct voltage Udc to Um and thereby the MPPT drives the electric power Pi to the target value P01 and correspondingly the electric power P2 to the target 15 value P02. It is also possible that only one of the power converters 103 and 133 is configured to control its electric power to be an upwards convex function of the direct voltage Udc when the photovoltaic panels 105 do not produce electric power, and the other one of the power converters is configured to keep its electric power independent of the direct voltage Udc. In this exemplifying case, the behavior of the 20 total power Pi + P2 as a function of the direct voltage Udc is determined by the one of the power converters 103 and 133 that is configured to control its electric power to be the upwards convex function of the direct voltage Udc when the photovoltaic panels 105 do not produce electric power.
    It is assumed above that the MPPT maximizes the electric power as a signed 25 quantity. Thus, the MPPT drives the direct voltage Udc to Um also in cases where one or both of Pi and P2 is/are negative. For example, when both Pi and P2 are negative and the photovoltaic panels 105 do not produce electric power, the direct voltage energy storage 113 and the batteries 116 of the car 115 are charged from the power grid 123.
    20185215 PRH 08 -03- 2018
    In the exemplifying photovoltaic system illustrated in figure 1a, each of the controllers 101 and 131 comprises a signal input interface for receiving an indicator signal that expresses whether or not the photovoltaic panels 105 produce electric power. The photovoltaic system comprises a sensor 106 for detecting the current 5 Idc.pv of the photovoltaic panels 105 and for setting the indicator signal to express that the photovoltaic panels 105 produce electric power in response to a situation in which the current Idc.pv is detected to exceed a predetermined threshold. A line 121 in figure 1b illustrates the operation of the power converter 103 when the indicator signal expresses that the photovoltaic panels 105 produce electric power, and a 10 curve 122 in figure 1 b illustrates the operation of the power converter 103 when the indicator signal expresses that the photovoltaic panels 105 do not produce electric power.
    Figure 1c shows a schematic illustration of the main circuit 109 of the power converter 103. It is however worth noting that the main circuit shown in figure 1c is 15 only a non-limiting example, and it is also possible that the main circuit of a power converter according to an embodiment of the invention has some other topology. In the exemplifying case illustrated in figure 1c, the main circuit 109 comprises a first inverter 110 whose direct voltage side is connected to the first electric terminal 107 and a second inverter 111 whose direct voltage side is connected to the second 20 electric terminal 108. The main circuit 109 comprises a transformer 112 between alternating voltage sides of the first and second inverters 110 and 111. The first inverter 110 comprises controllable switches 119a, 119b, 119c, and 119d. The second inverter 111 comprises controllable switches 120a, 120b, 120c, and 120d. In the exemplifying case presented in figure 1c, the controllable switches are 25 insulated gate bipolar transistors “IGBT”. It is also possible that the controllable switches are for example gate turn-off thyristors “GTO”, metal oxide semiconductor field-effect transistors “MOSFET”, or bipolar transistors. Furthermore, the main circuit 109 comprises inductor-capacitor “LC” filters for suppressing alternating components of the voltages of the electric terminals 107 and 108.
    The implementation of the processing system 102 of the controller 101, as well as the implementation of the processing system of the controller 131, can be based on one or more processor circuits, each of which can be a programmable processor
    20185215 PRH 08 -03- 2018 circuit provided with appropriate software, a dedicated hardware processor such as for example an application specific integrated circuit “ASIC”, or a configurable hardware processor such as for example a field programmable gate array “FPGA”. Furthermore, the controller 101 as well as the controller 131 may comprise one or more memory devices such as e.g. random-access memory “RAM” circuits.
    Figure 2 shows a flowchart of a method according to an exemplifying and nonlimiting embodiment of the invention for controlling a power converter connected to a photovoltaic side of a network converter of a photovoltaic system. The method comprises the following actions:
    - action 201: controlling, in response to a situation in which one or more photovoltaic panels of the photovoltaic system produce electric power, the power converter to keep electric power transferred via the power converter to be substantially independent of direct voltage of the network converter so as to enable the network converter to maximize, with the maximum power point tracking, the electric power produced by the one or more photovoltaic panels, and otherwise
    - action 202: controlling electric power transferred via the power converter to the network converter to be an upwards convex function of the direct voltage of the network converter so that the upwards convex function has a maximum between minimum and maximum values of the direct voltage.
    In a method according to an exemplifying and non-limiting embodiment of the invention, the electric power transferred via the power converter to the network converter is controlled to be substantially:
    Po - oc(Udc - Um)2, when the electric power of the one or more photovoltaic panels is zero. Po is a target value of the electric power transferred via the power converter, Udc is the direct voltage of the network converter, Um is a value of the direct voltage at which the electric power transferred via the power converter achieves the target value, and a is a positive constant.
    20185215 PRH 08 -03- 2018
    A method according to an exemplifying and non-limiting embodiment of the invention comprises measuring whether the one or more photovoltaic panels produce electric power.
    A method according to an exemplifying and non-limiting embodiment of the 5 invention comprises receiving an indicator signal expressing whether the one or more photovoltaic panels produce electric power.
    A computer program according to an exemplifying and non-limiting embodiment of the invention comprises computer executable instructions for controlling a programmable processing system to carry out actions related to a method according 10 to any of the above-described exemplifying and non-limiting embodiments of the invention.
    A computer program according to an exemplifying and non-limiting embodiment of the invention comprises software modules for controlling a power converter connected to a photovoltaic side of a network converter of a photovoltaic system.
    The software modules comprise computer executable instructions for controlling a programmable processing system to:
    - control, in response to a situation in which one or more photovoltaic panels of the photovoltaic system produce electric power, the power converter to keep electric power transferred via the power converter to be substantially independent of direct voltage of the network converter so as to enable the network converter to maximize, with the maximum power point tracking, the electric power produced by the one or more photovoltaic panels, and otherwise
    - control electric power transferred via the power converter to the network converter to be an upwards convex function of the direct voltage of the network converter so that the upwards convex function has a maximum between minimum and maximum values of the direct voltage.
    The software modules can be for example subroutines or functions implemented with programming tools suitable for the programmable processing system.
    A computer program product according to an exemplifying and non-limiting embodiment of the invention comprises a computer readable medium, e.g. a compact disc “CD”, encoded with a computer program according to an exemplifying embodiment of invention.
    A signal according to an exemplifying and non-limiting embodiment of the invention is encoded to carry information defining a computer program according to an exemplifying embodiment of invention.
    The specific examples provided in the description given above should not be construed as limiting the scope and/or the applicability of the appended claims. Lists 10 and groups of examples provided in the description given above are not exhaustive unless otherwise explicitly stated.
    权利要求:
    Claims (15)
    [1] What is claimed is:
    1. A controller (101) for controlling a power converter (103) connected to a photovoltaic side of a network converter (104) of a photovoltaic system, the controller comprising a processing system (102) configured to:
    5 - control, in response to a situation in which one or more photovoltaic panels (105) ofthe photovoltaic system produce electric power, the power converter to keep electric power transferred (Idci x Udc) via the power converter to be substantially independent of direct voltage (Udc) of the network converter so as to enable the network converter to maximize, with maximum power point 10 tracking, the electric power (Idc.pv x Udc) produced by the one or more photovoltaic panels, characterized in that the processing system is configured to control, in response to a situation in which the electric power of the one or more photovoltaic panels is zero, the electric power (Idci x Udc) transferred via the power converter to the network 15 converter to be an upwards convex function of the direct voltage (Udc) of the network converter so that the upwards convex function has a maximum between minimum and maximum values of the direct voltage.
    [2] 2. A controller according to claim 1, wherein the processing system is configured to control, in response to the situation in which the electric power of the one or more
    20 photovoltaic panels is zero, the electric power transferred via the power converter to the network converter to be substantially:
    Po - oc(Udc - Um)2, where Po is a target value of the electric power transferred via the power converter, Udc is the direct voltage of the network converter, Um is a value of the direct voltage 25 Udc at which the electric power transferred via the power converter achieves the target value, and a is a positive constant.
    [3] 3. A controller according to claim 1 or 2, wherein the controller comprises a signal input interface for receiving an indicator signal expressing whether the one or more photovoltaic panels produce electric power.
    20185215 PRH 08 -03- 2018
    [4] 4. A power converter (103) comprising:
    - a first electric terminal (107) for connecting to a photovoltaic side of a network converter of a photovoltaic system,
    - a second electric terminal (108),
    [5] 5 - a main circuit (109) for transferring electric power between the first and second electric terminals, and
    - a controller (101) according to any of claims 1-3 for controlling the electric power transferred via the main circuit.
    5. A power converter according to claim 4, wherein the power converter is a direct 10 voltage-to-direct voltage converter.
    [6] 6. A power converter according to claim 5, wherein the main circuit (109) comprises a first inverter (110) whose direct voltage side is connected to the first electric terminal, a second inverter (111) whose direct voltage side is connected to the second electric terminal, and a transformer (112) between alternating voltage
    15 sides of the first and second inverters.
    [7] 7. A photovoltaic system comprising:
    - one or more photovoltaic panels (105),
    - a network converter (104) for supplying electric power produced by the one or more photovoltaic panels to a power grid and for maximizing, with
    20 maximum power point tracking, electric power produced by the one or more photovoltaic panels, and
    - at least one power converter (103) according to any of claims 4-6, the first electric terminal of the power converter being connected to a photovoltaic side of the network converter.
    25
    [8] 8. A photovoltaic system according to claim 7, wherein the photovoltaic system comprises a direct voltage energy storage (113) connected to the second electric terminal of the power converter.
    20185215 PRH 08 -03- 2018
    [9] 9. A photovoltaic system according to claim 8, wherein the direct voltage energy storage (113) comprises a battery system.
    [10] 10. A method for controlling a power converter connected to a photovoltaic side of a network converter of a photovoltaic system, the method comprising:
    5 - controlling (201), in response to a situation in which one or more photovoltaic panels of the photovoltaic system produce electric power, the power converter to keep electric power transferred via the power converter to be substantially independent of direct voltage of the network converter so as to enable the network converter to maximize, with maximum power point
    10 tracking, the electric power produced by the one or more photovoltaic panels, characterized in that the method comprises controlling (202), in response to a situation in which the electric power of the one or more photovoltaic panels is zero, the electric power transferred via the power converter to the network converter to be an upwards convex function of the direct voltage of the network converter so that
    15 the upwards convex function has a maximum between minimum and maximum values of the direct voltage.
    [11] 11. A method according to claim 10, wherein the electric power transferred via the power converter to the network converter is controlled to be substantially:
    Po - o.(Udc - Um)2,
    20 in response to the situation in which the electric power of the one or more photovoltaic panels is zero, Po being a target value of the electric power transferred via the power converter, Udc being the direct voltage of the network converter, Um being a value of the direct voltage Udc at which the electric power transferred via the power converter achieves the target value, and a being a positive constant.
    25
    [12] 12. A method according to claim 10 or 11, wherein the method comprises measuring whether the one or more photovoltaic panels produce electric power.
    20185215 PRH 08 -03- 2018
    [13] 13. A controller according to claim 10 or 11, wherein the method comprises receiving an indicator signal expressing whether the one or more photovoltaic panels produce electric power.
    [14] 14. A computer program for controlling a power converter connected to a 5 photovoltaic side of a network converter of a photovoltaic system, the computer program comprising computer executable instructions for controlling a programmable processing system to:
    - control, in response to a situation in which one or more photovoltaic panels of the photovoltaic system produce electric power, the power converter to 10 keep electric power transferred via the power converter to be substantially independent of direct voltage of the network converter so as to enable the network converter to maximize, with maximum power point tracking, the electric power produced by the one or more photovoltaic panels, characterized in that the computer program further comprises computer executable 15 instructions for controlling the programmable processing system to control, in response to a situation in which the electric power of the one or more photovoltaic panels is zero, the electric power transferred via the power converter to the network converter to be an upwards convex function of the direct voltage of the network converter so that the upwards convex function has a maximum between minimum 20 and maximum values of the direct voltage.
    [15] 15. A non-transitory computer readable medium encoded with a computer program according to claim 14.
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