FI128564B - A controller and a method for controlling a power converter - Google Patents

A controller and a method for controlling a power converter Download PDF

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Publication number
FI128564B
FI128564B FI20185215A FI20185215A FI128564B FI 128564 B FI128564 B FI 128564B FI 20185215 A FI20185215 A FI 20185215A FI 20185215 A FI20185215 A FI 20185215A FI 128564 B FI128564 B FI 128564B
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FI
Finland
Prior art keywords
converter
power
electric power
photovoltaic
direct voltage
Prior art date
Application number
FI20185215A
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Finnish (fi)
Swedish (sv)
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FI20185215A1 (en
Inventor
Jani Hiltunen
Miko Huomo
Esa Malinen
Pasi Nuutinen
Mikko Pääkkönen
Original Assignee
Greenenergy Finland Oy
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Priority to FI20185215A priority Critical patent/FI128564B/en
Publication of FI20185215A1 publication Critical patent/FI20185215A1/en
Application granted granted Critical
Publication of FI128564B publication Critical patent/FI128564B/en

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of DC power input into DC power output
    • H02M3/22—Conversion of DC power input into DC power output with intermediate conversion into AC
    • H02M3/24—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters
    • H02M3/28—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate AC
    • H02M3/325—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate AC using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate AC using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J7/00—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries
    • H02J7/34—Parallel operation in networks using both storage and other DC sources, e.g. providing buffering
    • H02J7/35—Parallel operation in networks using both storage and other DC sources, e.g. providing buffering with light sensitive cells
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output
    • H02M7/42—Conversion of DC power input into AC power output without possibility of reversal
    • H02M7/44—Conversion of DC power input into AC power output without possibility of reversal by static converters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02S—GENERATION OF ELECTRIC POWER BY CONVERSION OF INFRARED RADIATION, VISIBLE LIGHT OR ULTRAVIOLET LIGHT, e.g. USING PHOTOVOLTAIC [PV] MODULES
    • H02S40/00—Components or accessories in combination with PV modules, not provided for in groups H02S10/00 - H02S30/00
    • H02S40/30—Electrical components
    • H02S40/38—Energy storage means, e.g. batteries, structurally associated with PV modules
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/56—Power conversion systems, e.g. maximum power point trackers
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E70/00—Other energy conversion or management systems reducing GHG emissions
    • Y02E70/30—Systems combining energy storage with energy generation of non-fossil origin

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Control Of Electrical Variables (AREA)

Abstract

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.

Description

A controller and a method for controlling a power converter Field of the disclosure The disclosure relates generally to control of electric energy.
More particularly, the 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 voltage- current operating point which maximizes electric power produced by one or more photovoltaic panels. o 20 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 5 the “photovoltaic side of a network converter” means a direct voltage terminal of the N network converter for receiving electric power from one or more photovoltaic panels. z An electric device connected to the photovoltaic side of a network converter can be so 25 for example a direct voltage energy storage that may comprise e.g. a battery system N 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 N 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 thephotovoltaic 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 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 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.
Publication US2017317503 describes a power storage control apparatus that — controls charging or discharging of battery without affecting the maximum power point tracking performed by a Power Conditioning System “PCS”. The maximum power extraction from the solar photovoltaic module is prevented by interfering constant voltage control or constant current control with the maximum power point tracking. The power storage control apparatus provides a power storage function to the power generation system, without affecting the maximum power point tracking performed by the PCS. O Publication CN103269117 describes a system for multi-energy convergence N coordinated control. The system comprises a direct current bus, a first alternating O current/direct current converter, a second alternating current/direct current N 25 converter, a bi-directional direct current/ direct current converter, a third alternating E current/direct current converter, a coordinating controller, a current/voltage LO detection device, a direct current/direct current converter capable of being externally io connected with a direct-current load, and a direct current/alternating current > converter capable of being externally connected with an alternating current load. The coordinating controller receives information transmitted by the current/voltage detection device and generates corresponding voltage control signals and transmitsthe signals to the corresponding first alternating current/direct current converter, the second alternating current/direct current converter, the bi-directional direct current/ direct current converter, the third alternating current/direct current converter, the direct current/direct current converter and the direct current/alternating current converter. Publication CN104716655 describes a control system that comprises an upper layer control part and a lower layer control part for controlling a photovoltaic system. The upper layer control part is used for selecting a working mode of the photovoltaic system, and the lower layer control part is used for battery charge-discharge control and for inverter control. The control system provides flexible inverter control concerning active and reactive output according to up-to-date data indicative of photovoltaic cell maximum power, load power, and mains supply power. Publication Velasco de La Fuente, D. et al.: Photovoltaic Power System with Battery Backup with Grid-Connection and Islanded Operation Capabilities, IEEE Transactions on Industrial Electronics 2013-04-01, Vol. 60, No. 4, pages 1571 - 1581, describes a photovoltaic power system with a battery backup. The photovoltaic power system has grid connection and islanded operation capabilities.
A direct current/direct current “DC/DC” converter manages the battery charge and controls a DC-link voltage to enable a maximum power point tracking reference to be followed.
Publication Saliha, A. et al: DC bus voltage balancing of multi-inverter in © photovoltaic system, 16th International Power Electronics and Motion Control N Conference and Exposition, 2014-09-21, pages 1059-1065, describes a direct O current “DC” bus voltage balancing in a multi-inverter photovoltaic system. The N 25 maximum power point tracking extracts maximum power from a photovoltaic array E connected to each DC link through a DC/DC converter.
O 3 Summary S 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 criticalelements 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 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 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: - 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 o 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 N maximum power point tracking since this electric power is substantially independent E 25 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 co electric power.
On the other hand, when the one or more photovoltaic panels do not 2 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 networkconverter can apply the maximum power point tracking also when the one or more 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 5 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 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, - 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. = 20 In accordance with the invention, there is provided also a new photovoltaic system = that comprises: <Q N . N - one or more photovoltaic panels, = > - anetwork converter for supplying electric power produced by the one or more
LO N photovoltaic panels to a power grid and for maximizing, with the maximum
LO = 25 power point tracking, the electric power produced by the one or more N 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 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 - 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: N - control, in response to a situation in which one or more photovoltaic panels > of the photovoltaic system produce electric power, the power converter to N keep electric power transferred via the power converter to be substantially E 25 independent of direct voltage of the network converter so as to enable the 2 network converter to maximize, with the maximum power point tracking, the je electric power produced by the one or more photovoltaic panels, and 2 otherwise - control electric power transferred via the power converter to the network converter to be an upwards convex function of the direct voltage of thenetwork 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 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 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 o 20 Exemplifying and non-limiting embodiments of the invention and their advantages > are explained in greater detail below in the sense of examples and with reference 5 to the accompanying drawings, in which:
N - figures 1a, 1b, and 1c illustrate a photovoltaic system comprising a power converter E according to an exemplifying and non-limiting embodiment of the invention, and
LO io 25 figure 2 shows a flowchart of a method according to an exemplifying and non-limiting > 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 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 non- limiting embodiment of the invention. This exemplifying photovoltaic system comprises electrically interconnected photovoltaic panels 105 and a network 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 "MPPT” that comprises tuning direct voltage Upc 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 Ipc pv X Upc, where Ipc pv is output current of the photovoltaic panels 105. Thus, the MPPT comprises seeking a Upc, Ipcpv -operating point which maximizes the electric power Ipcpv * Upc 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 = 25 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. E 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 o connected to the direct voltage energy storage 113. The power converter 103 N 30 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 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, asa 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 P1 = Ipc1 x Upc transferred via the power converter 103. When the photovoltaic panels 105 produce electric power, the processing system 102 keeps the electric power P1 = Ipc1 x Upc substantially independent of the direct voltage Upc.
Thus, when the photovoltaic panels 105 produce electric power, the processing system 102 controls the current o Ipc1 so that the current Inca is substantially inversely proportional to the direct voltage S Upc, i.e.
Ipc1 > 1/Upc.
Correspondingly, the power converter 133 comprises a 5 25 — controller 131 configured to control electric power P2 = Ipc2 Xx Upc transferred via the N power converter 133. When the photovoltaic panels 105 produce electric power, the E controller 131 keeps the electric power P2 = Ipc2 x Upc substantially independent of LO the direct voltage Upc.
As the electric powers P1 and P2 are substantially 3 independent of the direct voltage Upc, the behavior of the electric power transferred > 30 by the network converter 104 as a function of the direct voltage Upc is determined 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 directvoltage Upc so that the electric power transferred via the network converter 104, i.e. P4 + P2 + Ipc.pv * Upc, is maximized.
When the photovoltaic panels 105 do not produce electric power i.e. the electric power of the photovoltaic panels 105 is zero, the processing system 102 of the power converter 103 controls the electric power P1 = Ipc1 Xx Upc to be an upwards convex function of the direct voltage Upc so that the upwards convex function has a maximum between minimum and maximum values of the direct voltage Upc i.e. inside the range of variation of the direct voltage Upc. The range of variation of the direct voltage Upc has advantageously a first sub-range where the upwards convex function is increasing and, above the first subrange, a second sub-range where the upwards convex function is decreasing. The above-mentioned upwards convex function of the direct voltage Upc can be for example: P1(Upc) = Po1 — a(Upc — Um)?, (1) where Poi is a target value of the electric power P1 transferred via the power converter 103, Um is a value of Upc at which the electric power P1 achieves the target value Pm, and a is a positive constant. The controller 131 of the power converter 133 can be configured to control the electric power P2 = Ipc2 X Upc to be an upwards convex function of the direct voltage Upc when the photovoltaic panels 105 do not produce electric power. The upwards convex function related to the — power converter 133 can be e.g. P2(Upc) = Poa — a(Upc — Um)?, where Poo 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 N drives the direct voltage Upc to Um and thereby the MPPT drives the electric power ? P4 to the target value Poi and correspondingly the electric power P2 to the target N 25 value Poo. It is also possible that only one of the power converters 103 and 133 is i configured to control its electric power to be an upwards convex function of the direct = voltage Upc when the photovoltaic panels 105 do not produce electric power, and o the other one of the power converters is configured to keep its electric power | independent of the direct voltage Upc. In this exemplifying case, the behavior of the total power P1 + Pa as a function of the direct voltage Upc is determined by the one of the power converters 103 and 133 that is configured to control its electric powerto be the upwards convex function of the direct voltage Upc when the photovoltaic panels 105 do not produce electric power.
It is assumed above that the MPPT maximizes the electric power as a signed quantity. Thus, the MPPT drives the direct voltage Upc to Um also in cases where one or both of P1 and Pa is/are negative. For example, when both P1 and Pz 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.
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 Inc, 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 Ipc 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 curve 122 in figure 1b 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 N only a non-limiting example, and it is also possible that the main circuit of a power O converter according to an embodiment of the invention has some other topology. In N 25 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 LO and a second inverter 111 whose direct voltage side is connected to the second io 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 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 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 non- limiting 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 N as to enable the network converter to maximize, with the maximum power O point tracking, the electric power produced by the one or more photovoltaic N 25 panels, and otherwise
T = © - action 202: controlling electric power transferred via the power converter to N 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 N 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 — a(Upc — Um)?, 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, Upc 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.
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 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 to any of the above-described exemplifying and non-limiting embodiments of the invention.
oO 2 A computer program according to an exemplifying and non-limiting embodiment of 5 the invention comprises software modules for controlling a power converter N connected to a photovoltaic side of a network converter of a photovoltaic system.
E The software modules comprise computer executable instructions for controlling a W 25 programmable processing system to: > 2 - control, in response to a situation in which one or more photovoltaic panels N 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 thenetwork 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 and groups of examples provided in the description given above are not exhaustive unless otherwise explicitly stated.
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Claims (15)

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: - control, in response to a situation in which one or more photovoltaic panels (105) of the photovoltaic system produce electric power, the power converter to keep electric power transferred (Inc1 x Unc) via the power converter to be substantially independent of direct voltage (Upc) of the network converter so as to enable the network converter to maximize, with maximum power point tracking, the electric power (Ipcpv * Upc) 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 (Ipc1 Xx Upc) transferred via the power converter to the network converter to be an upwards convex function of the direct voltage (Upc) of the network converter so that the upwards convex function has a maximum between minimum and maximum values of the direct voltage.
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 photovoltaic panels is zero, the electric power transferred via the power converter o to the network converter to be substantially:
O = Po — a(Upc — Um)?, <Q 3 where Po is a target value of the electric power transferred via the power converter, E Upc is the direct voltage of the network converter, Um is a value of the direct voltage LO 25 Upc at which the electric power transferred via the power converter achieves the 3 target value, and a is a positive constant. N 3. Acontroller 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.
4. A power converter (103) comprising: - afirst electric terminal (107) for connecting to a photovoltaic side of a network converter of a photovoltaic system, - a second electric terminal (108), and - a main circuit (109) for transferring electric power between the first and second electric terminals, characterized in that the power converter further comprises: - a controller (101) according to any of claims 1-3 for controlling the electric power transferred via the main circuit.
5. AA power converter according to claim 4, wherein the power converter is a direct voltage-to-direct voltage converter.
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 sides of the first and second inverters.
7. A photovoltaic system comprising: - one or more photovoltaic panels (105), and oO
O N - a network converter (104) for supplying electric power produced by the one 2 20 or more photovoltaic panels to a power grid and for maximizing, with
N N maximum power point tracking, electric power produced by the one or more E photovoltaic panels,
LO N characterized in that the photovoltaic system further comprises: 00 N - 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.
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.
9. A photovoltaic system according to claim 8, wherein the direct voltage energy storage (113) comprises a battery system.
10. A method for controlling a power converter connected to a photovoltaic side of a network converter of a photovoltaic system, the method comprising: - 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 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 the upwards convex function has a maximum between minimum and maximum values of the direct voltage.
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:
N 5 Po — a(Upc — Um)?,
N I in response to the situation in which the electric power of the one or more a photovoltaic panels is zero, Po being a target value of the electric power transferred = 25 — via the power converter, Upc being the direct voltage of the network converter, Um 3 being a value of the direct voltage Upc at which the electric power transferred via N the power converter achieves the target value, and a being a positive constant.
12. A method according to claim 10 or 11, wherein the method comprises measuring whether the one or more photovoltaic panels produce electric power.
13. A method 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. A computer program for controlling a power converter connected to a 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 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 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 o converter so that the upwards convex function has a maximum between minimum > and maximum values of the direct voltage.
>
15. A non-transitory computer readable medium encoded with a computer N program according to claim 14.
j
O 3
N
FI20185215A 2018-03-08 2018-03-08 A controller and a method for controlling a power converter FI128564B (en)

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