WO2015106814A1 - Dc series mppt pv system and control method thereof - Google Patents

Dc series mppt pv system and control method thereof Download PDF

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Publication number
WO2015106814A1
WO2015106814A1 PCT/EP2014/050819 EP2014050819W WO2015106814A1 WO 2015106814 A1 WO2015106814 A1 WO 2015106814A1 EP 2014050819 W EP2014050819 W EP 2014050819W WO 2015106814 A1 WO2015106814 A1 WO 2015106814A1
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WIPO (PCT)
Prior art keywords
power
conversion
energy storage
modules
energy
Prior art date
Application number
PCT/EP2014/050819
Other languages
French (fr)
Inventor
Panagiotis Bakas
Antonis MARINOPOULOS
Original Assignee
Abb Technology Ltd
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Publication date
Application filed by Abb Technology Ltd filed Critical Abb Technology Ltd
Priority to PCT/EP2014/050819 priority Critical patent/WO2015106814A1/en
Publication of WO2015106814A1 publication Critical patent/WO2015106814A1/en

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Classifications

    • 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
    • H02J7/345—Parallel operation in networks using both storage and other dc sources, e.g. providing buffering using capacitors as storage or buffering devices
    • 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
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381—Dispersed generators
    • 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
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • H02J2300/22—The renewable source being solar energy
    • H02J2300/24—The renewable source being solar energy of photovoltaic origin
    • H02J2300/26—The renewable source being solar energy of photovoltaic origin involving maximum power point tracking control for photovoltaic sources
    • 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
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/40—Synchronising a generator for connection to a network or to another generator
    • 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

Definitions

  • the present invention relates to a power conversion system for providing power from a plurality of solar photovoltaic modules, further denoted PV modules, to a power grid .
  • the conversion system comprises a conversion device comprising a maximum power poi nt tracki ng unit, further denoted MPPT unit, and an energy storage, a control system adapted to control the flow of energy to and from the energy storage, and an i nverter adapted to be connected to the power grid .
  • the present invention further relates to a method for controlling a power conversion system and PV power plant comprising the power conversion system.
  • a problem with power conversion systems according to the state of the art is that the generated power from the PV power plant is not constant over time. This makes integrating the generated power to the grid difficult and expensive.
  • PV power plant A further problem with such prior art PV power plant is that a mismatch in power may arise due that different PV modules are subjected to different degree of solar radiation and the PV mod- ules therefore generate different amount of electric power.
  • EP1 623495 discloses power condition circuit for integrating power from a PV module to the grid .
  • the circuit comprises an energy storage.
  • AU201 1200794 discloses a power converting system for converting power from a PV power plant.
  • the system comprises a plurality of energy storages.
  • a first object of the i nvention is to provide a more cost effective power conversion system than prior art power conversion systems.
  • a second object of the invention is to provide a power conversion system that enables correction of mismatch in power between different PV modules.
  • the system comprises a plurality of conversion devices connected in series, each MPPT unit of the conversion devices comprises means for enabling connection to one PV-module or an array of PV-modules, and a first MPPT unit and a second M PPT unit of the conversion devices are connected to the inverter, wherei n the control system is configured to control the energy storages of the conversion devices so that the maximum output power to the power grid is lower than a certai n level .
  • the system enables the i nverter and all the equipment connect- ed after it to be rated in lower power than the nomi nal power of the PV modules.
  • the system furthermore enables the inverter to be used more efficiently since it will be operated closer to its nominal power rating for longer time compared to prior art systems.
  • a further advantage is that the lower power rating enables the inverter to be designed in a simpler way.
  • M PPT unit relates to a Maximum Power Point Tracking unit which controls output power from the PV module or array of PV modules connected to it.
  • the first and the second MPPT unit is directly connected to the inverter in that a terminal of the first MPPT unit is connected to a terminal of the inverter, and a terminal of the second MPPT unit is connected to another terminal of the inverter.
  • the inverter has the function of converting electric power from one form to another.
  • the inverter is a DC/AC i nverter, which has the function of converting DC power to AC power and synchronizing the AC power to an AC power grid .
  • the inverter is a DC/AC inverter, which has the function of converting DC power to a DC power grid .
  • the inverter is di- mensioned for a power rating of said certain level .
  • the inverter By dimensioning the inverter to the power rating that the control system is configured to control the energy storages, a more simple and cost effective inverter can be used than used in prior art conversion systems.
  • the energy storages are connected between the MPPT unit and the inverter.
  • the feature provides a more simple control scheme than prior art systems i n that the MPPT unit can be controlled inde- pendently of the control of the energy storage. Furthermore, the feature enables improved flexibility in the design of a DC/DC converter of the MPPT unit.
  • the control system comprises a plurality of control devices arranged so that each conversion device comprises a control device that is configured to control the energy storage of the conversion device so that the maximum output power from the conversion devices are lower than the certain level .
  • each conversion device comprises a designated control device, the redundancy of the control system is improved .
  • the overall control of the plurality of PV modules is maintained .
  • the control system is configured to correct mismatch in PV voltage between different PV modules.
  • the control system corrects the mismatch i n PV voltage by means of either charging the energy storage of the conversion device to which the PV module or the array of PV modules that generates the higher power is connected or discharging the energy storage of the conversion device or the array of PV mod- ules to which the PV module that generates the lowest power is connected .
  • the system corrects the mismatch i n PV voltage between two adjacent PV modules or two adjacent arrays of PV modules.
  • the system corrects the mismatch in PV voltage for all the PV modules of the system.
  • control system is adapted to control the energy storages of the conversion devices so that the energy storages operate with margin to their fully discharged and fully charged state.
  • control system is adapted to control the energy storages of the conversion devices so that the energy storages operate in a range of 20 to 80 % of their storage capacity.
  • each conversion device is adapted to be arranged on the backside of the PV module or the array of PV modules to which the MPPT unit is connected .
  • each conversion device By arrangi ng each conversion device as a unit on the backside of the PV module, the system can easily be connected to the PV-modules of a PV power plant and the system is protected from the environment by the PV-modules.
  • the com- ponents of the conversion device are arranged i n a casing , thereby formi ng a junction box.
  • the energy storage is a super-capacitor.
  • the super-capacitor is suited for PV power plants and particularly for use in each PV module without significantly impacting the size and weight of the PV module.
  • the system comprises sensor means for measuring a physical quantity of the PV modules that is dependent on the generated electric power of the PV modules, and means for determining the generated power based on said physical quantity.
  • the sensor means measures the current or voltage of the PV modules.
  • the invention further relates to a method for controlling a conversion system according to claim 10. The method comprises the steps of
  • the conversion system comprises a plurality of conversion devices connected in series, and a first MPPT unit and a second MPPT unit of the conversion devices are connected to the inverter, wherein the method comprises the steps of
  • the energy stor- ages are connected between the MPPT unit and the inverter, the method further comprises the steps of.
  • the correction of mismatch in PV voltage comprises
  • the mismatch i n PV voltage is compensated by controlling the energy storage of the conversion device designated for the first PV module so that the energy storage charges the difference in generated power, or by controlling the energy storage of the conversion device designated for the second PV module so that the energy storage discharges the difference in generated power,
  • the mismatch i n PV voltage is compensated by controlling the energy storage of the conversion device designated for the first PV module so that the energy storage discharge the difference i n generated power, or by controlling the energy storage of the conversion device des- ignated for the second PV module so that the energy storage charges the difference in generated power.
  • the method of correcting mismatch in PV voltage is applied to all PV module of the system.
  • the method comprises controlling the energy storages so that the energy storages operate with margin to their fully discharged and fully charged state by alternating charging and discharging the ener- gy storage of the conversion devices designated for the first and the second PV module.
  • the energy storages are controlled so that the energy storages operate in the interval of 20-80% of the capacity of the energy storages.
  • Fig . 1 shows a PV power plant connected to a power conversion system according to an embodiment of the invention.
  • Fig . 2 shows a flow chart of a first embodiment of a method for controlling the power conversion system in fig .1 .
  • Fig . 3 shows a flow chart of a second embodiment of a method for controlling the power conversion system i n fig .1 .
  • Fig . 4a shows the power profile from a PV power plant connected to a power conversion system according to state of the art.
  • Fig . 4b shows the power profile from a PV power plant connected to a power conversion system according to the invention .
  • Fig . 1 shows a PV power plant 1 connected to a power conversion system 3 according to an embodiment of the invention .
  • the PV power plant 1 comprises a pl urality of PV modules 5 that generates DC electrical power when they are subjected to solar radiation .
  • the PV modules 5 are connected to a power grid 7 by means of the power conversion system 3.
  • the power conversion system 3 has the function of converting and synchronizing the DC power from the PV modules 5 to the power grid 7.
  • the power conversion system 3 comprises a plurality of conver- sion devices 10.
  • Each conversion device 10 comprises an M PPT unit 12 and an energy storage 14.
  • the MPPT unit (Maximum Power Point Tracking ) controls output power from the PV module 5 or the array of PV modules 5 connected to it.
  • the conversion devices 10 are connected in series by means of the MPPT units 12 of the conversion devices 10.
  • Each MPPT unit 12 is connected to one PV module 5 or an array of PV modules 5.
  • the array of the PV modules 5 comprises a plurality of PV modules 5 connected in series. Accordi ngly, the output ter- minal of one of the MPPT units 12 is connected with the input terminal of an adjacent MPPT unit 1 2, etc. I n fig .
  • the conversion system 3 comprises three conversion devices 10, and one PV module 5 is connected to each of the MPPT units 12. It shall be understood that any number of conversion devices 10 could be connected in series and that one PV module 5 or an array of the PV modules 5 could be connected to each of the conversion devices 10.
  • the power conversion system 3 further comprises a control sys- tern and an i nverter 22 connected to the power grid 7.
  • the inverter 22 is for example a DC/AC inverter and the power grid 7 is an AC power grid .
  • the DC/AC i nverter converts the electric power from DC to AC so that the power can be provided to the AC power grid .
  • the inverter 22 is a DC/DC inverter and the power grid 7 is a DC power grid .
  • the DC/DC i nverter converts the electric power from a first DC form to a second DC form that is provided to the DC power grid .
  • the power conversion system 3 is arranged so that the inverter 22 is connected to a terminal of the MPPT unit 1 2 of a first of the conversion devices 10 and to a terminal of the MPPT unit 1 2 of a second of the conversion devices 10.
  • the first and the second MPPT unit 12 are di rectly connected to the inverter 22.
  • a single inverter 22 can be used for the PV power plant 1 .
  • the energy storages 14 of the conversion devices 10 are connected to the MPPT unit 12 so that the energy storages 14 receive converted DC power from the MPPT unit 12.
  • the conversion system 3 comprises sensor means for measuring a physical quantity of the PV modules 5, which physical quantity is dependent on the generated electric power of the PV modules 5, and means for determining the generated power based on the physical quantity.
  • the sensor means measure the current or voltage of the PV modules 5.
  • the control system is adapted to receive the information on the physical quantity and determini ng the generated power of the PV module 5 or array of PV modules 5 connected to the MPPT unit 12 of conversion devices 1 0.
  • the control system is adapted to control the charging and discharging of the energy storage 14 so that the maximum output power to the power grid 7 is lower than a certain level .
  • the control system is controlled so that the power rati ng provided to the i nverter 22 is the nominal power rating of the i nverter 22.
  • the conversion system 3 has the function of removing peaks in power rating to the inverter 22. Thereby, a more cost effective inverter 22 can be used . Furthermore, the inverter 22 will have a longer operative lifetime than inverters 22 in prior art conversion systems 3.
  • each conversion device 10 comprises a control device 20 that controls the chargi ng and discharging of the en- ergy storage 14 in the same conversion device 10.
  • the redundancy conversion system 3 is improved .
  • the conversion system 3 provides a modular design that enables connection or disconnection of further conversion devices 10 to the row of serial connected conversion devices 1 0.
  • the MPPT unit 12, the energy storage 14 and the control device 20 of each conversion device 10 is arranged i n a casing that is attached on the backside of the PV module 5 or one of the PV modules 5 of an array of PV modules 5 to which the MPPT unit 12 is connected .
  • control system 20 controls the charging and discharging of the energy storages 14 so that mismatch in PV voltage between different PV modules 5 is cor- rected .
  • control system 20 is configured to control the charging and discharging of the energy storages 14 so that the energy storages 14 operates with margin to their fully dis- charged and fully charged states, such as in the range between 20 to 80 % of their storage capacity.
  • margin to their fully dis- charged and fully charged states such as in the range between 20 to 80 % of their storage capacity.
  • Fig . 2 shows a flow chart of a first embodiment of a method for controlling the power conversion system 3 in fig .1 .
  • the method comprises, in a step 100, receiving i nformation on the generated power of the PV module 5 or array of PV modules 5 connected to each conversion device 10.
  • the information on the generated power is for example a physical quantity of the PV modules 5, which physical quantity is dependent on the generated electric power of the PV modules 5, such as the current or voltage of the PV modules 5.
  • the method comprises determi ning if the generated power of each the PV module 5 or array of PV modules 5 exceeds a certain level .
  • the method comprises controlling the flow electric power to or from the energy storage 14 of each conversion system 3 on basis of the determined generated power.
  • the flow electric power is controlled so that output power to the inverter 22 and thereto connected power grid 7 becomes lower than a certain level .
  • the control unit 10 receives information on the physical quantity of the PV modules 5 and determines the gen- erated power. Preferably, the control unit 10 continuously receives information on the physical quantity, determines the generated power, and regulates the charging and discharging of the energy storages 14.
  • Fig . 3 shows a flow chart of a second embodiment of a method for controlling the conversion system 3 in fig . 1 . The method comprises steps i n fig . 2 and further steps of controlling the energy storage 14 so that mismatch i n PV voltage between different PV modules 5 is corrected . In the following described method , the mismatch i n PV voltage is compensated between two different PV modules 5.
  • the method is applicable to correction of mismatch i n PV voltage for all the PV modules 5 in the conversion system 3.
  • the method comprises receiving information on generated power of a first and a second PV module 5 that are connected to different conversion devices 10.
  • the information may be the same as in step 100 related to a physical quantity that depends on the generated power of the PV module 5.
  • the method comprises determi ning a difference in generated power between the first and the second PV module 5.
  • the method comprises in a step 1 50, compensating the mismatch in PV voltage by controlling the energy storage 14 of a conversion device 10 designated for the first PV module 5 so that the energy storage 14 charges the dif- ference in generated power, or by controlling the energy storage 14 of the conversion device 10 designated for the second PV module 5 so that the energy storage 14 discharges the difference i n generated power.
  • the method comprises in a step 160, compensating the mismatch in PV voltage by controlling the energy storage 14 of a conversion device 10 designated for the first PV module 5 so that the energy storage 14 discharges the difference in generated power, or by controlling the energy stor- PV module 5 so that the energy storage 14 charges the difference i n generated power.
  • the method comprises controlling the energy storages 14 so that the energy storages 14 operate with margin to their fully discharged and fully charged state. For example, the energy storages 14 operate i n the interval of 20-80% of their capacity.
  • the method comprises the steps of controlling the ener- gy storages 14 so that the energy storages 14 operate in the i nterval of 20-80% by alternati ng charging and discharging the energy storage 14 of the conversion device 10 designated for the first and the second PV module 5.
  • Fig . 4a shows the power profile from a PV power plant 1 connected to a power conversion system 3 accordi ng to state of the art.
  • the PV power plant 1 generates power duri ng the time of the day which the PV modules 5 are subjected to sunshine.
  • the power rati ng capacity of the inverter 22 is indicated by a doted line. As it can be seen , the output power profile is relatively steep with a peak at noon .
  • the power rating capacity of the i nverter 22 is dimensioned with margin to withstand days with exceptional high sun radiation and thereby exceptional high power rating .
  • Fig . 4b shows the power profile from a PV power plant 1 connected to a power conversion system 3 according to the invention .
  • the output power from the PV power plant 1 is controlled so that the maximum output power to the power grid 7 is lower than 0, 1 kW .
  • the power rating capacity of the inverter 22 is indicated by a doted line.
  • the output power profile from the power conversion systems 3 is flatter than the output power profile of prior art power conversion systems.
  • the power rati ng capacity of the inverter 22 is set to the power rating to which the maximum output power is controlled , in this case 0, 1 kW . Accordingly, the invention enables use of a more simple and cost effective inverter 22 than prior art power conversion systems 3.
  • the present invention is not limited to the disclosed embodiments but may be modified within the framework of the claims.
  • control system 20 may comprise a central unit that controls the energy storages 5 of all of the conversion devices 10.

Abstract

A power conversion system (3) for providing power from a plurality of PV modules (5) to a power grid (7). The conversion system comprises a conversion device (10) comprising an MPPT unit (12) and an energy storage (14), a control system and an inverter (22). The conversion system comprises a plurality of conversion devices (10) connected in series, each MPPT unit comprises means for enabling connection to a PV-module. A first MPPT unit and a second MPPT unit are connected to the inverter. The control system is configured to control the energy storages so that the maximum output power to the power grid is lower than a certain level.

Description

DC SERIES MPPT PV SYSTEM AND CONTROL METHOD THEREOF
FI ELD OF THE I NVENTI ON
The present invention relates to a power conversion system for providing power from a plurality of solar photovoltaic modules, further denoted PV modules, to a power grid . The conversion system comprises a conversion device comprising a maximum power poi nt tracki ng unit, further denoted MPPT unit, and an energy storage, a control system adapted to control the flow of energy to and from the energy storage, and an i nverter adapted to be connected to the power grid .
The present invention further relates to a method for controlling a power conversion system and PV power plant comprising the power conversion system. PRIOR ART
A problem with power conversion systems according to the state of the art is that the generated power from the PV power plant is not constant over time. This makes integrating the generated power to the grid difficult and expensive.
A further problem with such prior art PV power plant is that a mismatch in power may arise due that different PV modules are subjected to different degree of solar radiation and the PV mod- ules therefore generate different amount of electric power.
EP1 623495 discloses power condition circuit for integrating power from a PV module to the grid . The circuit comprises an energy storage. AU201 1200794 discloses a power converting system for converting power from a PV power plant. The system comprises a plurality of energy storages. OBJ ECTS AND SUMMARY OF THE I NVENTION
A first object of the i nvention is to provide a more cost effective power conversion system than prior art power conversion systems. A second object of the invention is to provide a power conversion system that enables correction of mismatch in power between different PV modules.
This object is obtained by a power conversion system according to the preamble of claim 1 . The system is characterized in that the system comprises a plurality of conversion devices connected in series, each MPPT unit of the conversion devices comprises means for enabling connection to one PV-module or an array of PV-modules, and a first MPPT unit and a second M PPT unit of the conversion devices are connected to the inverter, wherei n the control system is configured to control the energy storages of the conversion devices so that the maximum output power to the power grid is lower than a certai n level .
The system enables the i nverter and all the equipment connect- ed after it to be rated in lower power than the nomi nal power of the PV modules. The system furthermore enables the inverter to be used more efficiently since it will be operated closer to its nominal power rating for longer time compared to prior art systems. A further advantage is that the lower power rating enables the inverter to be designed in a simpler way.
Moreover, i n that each conversion device is provided with the energy storage that is controlled by the control system, the pl urality of PV modules can share the same inverter with the lower power rating . Thus, the i nvention provides a cost effective conversion system. The term "M PPT unit" relates to a Maximum Power Point Tracking unit which controls output power from the PV module or array of PV modules connected to it.
The first and the second MPPT unit is directly connected to the inverter in that a terminal of the first MPPT unit is connected to a terminal of the inverter, and a terminal of the second MPPT unit is connected to another terminal of the inverter.
The inverter has the function of converting electric power from one form to another. According to an embodiment, the inverter is a DC/AC i nverter, which has the function of converting DC power to AC power and synchronizing the AC power to an AC power grid . According to another embodiment, the inverter is a DC/AC inverter, which has the function of converting DC power to a DC power grid .
Accordi ng to an embodiment of the invention , the inverter is di- mensioned for a power rating of said certain level . By dimensioning the inverter to the power rating that the control system is configured to control the energy storages, a more simple and cost effective inverter can be used than used in prior art conversion systems.
Accordi ng to an embodiment of the invention, the energy storages are connected between the MPPT unit and the inverter. The feature provides a more simple control scheme than prior art systems i n that the MPPT unit can be controlled inde- pendently of the control of the energy storage. Furthermore, the feature enables improved flexibility in the design of a DC/DC converter of the MPPT unit.
Accordi ng to an embodiment of the invention , the control system comprises a plurality of control devices arranged so that each conversion device comprises a control device that is configured to control the energy storage of the conversion device so that the maximum output power from the conversion devices are lower than the certain level . By means of configuring the control system so that each conversion device comprises a designated control device, the redundancy of the control system is improved . In the event of a malfunction of a control device of one conversion device, the overall control of the plurality of PV modules is maintained .
Accordi ng to an embodiment of the invention , the control system is configured to correct mismatch in PV voltage between different PV modules. The control system corrects the mismatch i n PV voltage by means of either charging the energy storage of the conversion device to which the PV module or the array of PV modules that generates the higher power is connected or discharging the energy storage of the conversion device or the array of PV mod- ules to which the PV module that generates the lowest power is connected . According to an embodiment of the i nvention , the system corrects the mismatch i n PV voltage between two adjacent PV modules or two adjacent arrays of PV modules. According to an embodiment the system corrects the mismatch in PV voltage for all the PV modules of the system.
Accordi ng to an embodiment of the invention , the control system is adapted to control the energy storages of the conversion devices so that the energy storages operate with margin to their fully discharged and fully charged state.
By controlling the energy storages so that they operate with margin to their fully discharged and fully charged state, it is assured that control option of both charging and discharging of the energy systems remains available for most of the times Accordi ng to an embodiment of the invention , the control system is adapted to control the energy storages of the conversion devices so that the energy storages operate in a range of 20 to 80 % of their storage capacity.
Accordi ng to an embodiment of the invention , each conversion device is adapted to be arranged on the backside of the PV module or the array of PV modules to which the MPPT unit is connected .
By arrangi ng each conversion device as a unit on the backside of the PV module, the system can easily be connected to the PV-modules of a PV power plant and the system is protected from the environment by the PV-modules. Preferably, the com- ponents of the conversion device are arranged i n a casing , thereby formi ng a junction box.
Accordi ng to an embodiment of the invention , the energy storage is a super-capacitor. The super-capacitor is suited for PV power plants and particularly for use in each PV module without significantly impacting the size and weight of the PV module.
Accordi ng to an embodiment of the invention , the system comprises sensor means for measuring a physical quantity of the PV modules that is dependent on the generated electric power of the PV modules, and means for determining the generated power based on said physical quantity. For example, the sensor means measures the current or voltage of the PV modules. The invention further relates to a method for controlling a conversion system according to claim 10. The method comprises the steps of
- controlling the energy storage so that the maximum output power to the power grid is lower than a certai n level . Accordi ng to an embodiment of the invention , the conversion system comprises a plurality of conversion devices connected in series, and a first MPPT unit and a second MPPT unit of the conversion devices are connected to the inverter, wherein the method comprises the steps of
- -controlling each of the energy storages so that the maximum output power to the power grid is lower than the certain level .
Accordi ng to an embodiment of the invention , the energy stor- ages are connected between the MPPT unit and the inverter, the method further comprises the steps of.
- controlling the energy storages so that mismatch in PV voltage between different PV modules are corrected . Accordi ng to an embodiment of the invention , the correction of mismatch in PV voltage comprises
- receiving information on generated power of a first and a second PV module,
- determining a difference in generated power between the first and the second PV module,
- if said difference has a positive value and the absol ute val ue of said difference exceeds a certai n value, the mismatch i n PV voltage is compensated by controlling the energy storage of the conversion device designated for the first PV module so that the energy storage charges the difference in generated power, or by controlling the energy storage of the conversion device designated for the second PV module so that the energy storage discharges the difference in generated power,
- if said difference has a negative value and the absolute val ue of said difference exceeds a certai n value, the mismatch i n PV voltage is compensated by controlling the energy storage of the conversion device designated for the first PV module so that the energy storage discharge the difference i n generated power, or by controlling the energy storage of the conversion device des- ignated for the second PV module so that the energy storage charges the difference in generated power. Accordi ng to an embodiment of the invention , the method of correcting mismatch in PV voltage is applied to all PV module of the system.
Accordi ng to an embodiment of the invention, the method comprises controlling the energy storages so that the energy storages operate with margin to their fully discharged and fully charged state by alternating charging and discharging the ener- gy storage of the conversion devices designated for the first and the second PV module. According to an embodiment, the energy storages are controlled so that the energy storages operate in the interval of 20-80% of the capacity of the energy storages.
BRI EF DESCRI PTI ON OF TH E DRAWI NGS
The invention will now be explained more closely by the descri ption of different embodiments of the invention and with reference to the appended figures.
Fig . 1 shows a PV power plant connected to a power conversion system according to an embodiment of the invention. Fig . 2 shows a flow chart of a first embodiment of a method for controlling the power conversion system in fig .1 .
Fig . 3 shows a flow chart of a second embodiment of a method for controlling the power conversion system i n fig .1 .
Fig . 4a shows the power profile from a PV power plant connected to a power conversion system according to state of the art. Fig . 4b shows the power profile from a PV power plant connected to a power conversion system according to the invention . DETAI LED DESCRI PTI ON OF PREFERRED EM BOD I MENTS OF THE I NVENTION
Fig . 1 shows a PV power plant 1 connected to a power conversion system 3 according to an embodiment of the invention . The PV power plant 1 comprises a pl urality of PV modules 5 that generates DC electrical power when they are subjected to solar radiation .
The PV modules 5 are connected to a power grid 7 by means of the power conversion system 3. The power conversion system 3 has the function of converting and synchronizing the DC power from the PV modules 5 to the power grid 7.
The power conversion system 3 comprises a plurality of conver- sion devices 10. Each conversion device 10 comprises an M PPT unit 12 and an energy storage 14. The MPPT unit (Maximum Power Point Tracking ) controls output power from the PV module 5 or the array of PV modules 5 connected to it. The conversion devices 10 are connected in series by means of the MPPT units 12 of the conversion devices 10. Each MPPT unit 12 is connected to one PV module 5 or an array of PV modules 5. The array of the PV modules 5 comprises a plurality of PV modules 5 connected in series. Accordi ngly, the output ter- minal of one of the MPPT units 12 is connected with the input terminal of an adjacent MPPT unit 1 2, etc. I n fig . 1 the conversion system 3 comprises three conversion devices 10, and one PV module 5 is connected to each of the MPPT units 12. It shall be understood that any number of conversion devices 10 could be connected in series and that one PV module 5 or an array of the PV modules 5 could be connected to each of the conversion devices 10.
The power conversion system 3 further comprises a control sys- tern and an i nverter 22 connected to the power grid 7. The inverter 22 is for example a DC/AC inverter and the power grid 7 is an AC power grid . The DC/AC i nverter converts the electric power from DC to AC so that the power can be provided to the AC power grid . Alternatively, the inverter 22 is a DC/DC inverter and the power grid 7 is a DC power grid . The DC/DC i nverter converts the electric power from a first DC form to a second DC form that is provided to the DC power grid .
The power conversion system 3 is arranged so that the inverter 22 is connected to a terminal of the MPPT unit 1 2 of a first of the conversion devices 10 and to a terminal of the MPPT unit 1 2 of a second of the conversion devices 10. Accordi ngly, the first and the second MPPT unit 12 are di rectly connected to the inverter 22. Thereby, a single inverter 22 can be used for the PV power plant 1 .
The energy storages 14 of the conversion devices 10 are connected to the MPPT unit 12 so that the energy storages 14 receive converted DC power from the MPPT unit 12.
The conversion system 3 comprises sensor means for measuring a physical quantity of the PV modules 5, which physical quantity is dependent on the generated electric power of the PV modules 5, and means for determining the generated power based on the physical quantity. For example, the sensor means measure the current or voltage of the PV modules 5. The control system is adapted to receive the information on the physical quantity and determini ng the generated power of the PV module 5 or array of PV modules 5 connected to the MPPT unit 12 of conversion devices 1 0. The control system is adapted to control the charging and discharging of the energy storage 14 so that the maximum output power to the power grid 7 is lower than a certain level . Preferable, the control system is controlled so that the power rati ng provided to the i nverter 22 is the nominal power rating of the i nverter 22. Accordingly, the conversion system 3 has the function of removing peaks in power rating to the inverter 22. Thereby, a more cost effective inverter 22 can be used . Furthermore, the inverter 22 will have a longer operative lifetime than inverters 22 in prior art conversion systems 3.
In the conversion system 3 shown i n fig .1 , the control system is arranged so that each conversion device 10 comprises a control device 20 that controls the chargi ng and discharging of the en- ergy storage 14 in the same conversion device 10. Thereby, the redundancy conversion system 3 is improved . Furthermore, the conversion system 3 provides a modular design that enables connection or disconnection of further conversion devices 10 to the row of serial connected conversion devices 1 0.
Preferably, the MPPT unit 12, the energy storage 14 and the control device 20 of each conversion device 10 is arranged i n a casing that is attached on the backside of the PV module 5 or one of the PV modules 5 of an array of PV modules 5 to which the MPPT unit 12 is connected .
Accordi ng to an embodiment, the control system 20 controls the charging and discharging of the energy storages 14 so that mismatch in PV voltage between different PV modules 5 is cor- rected .
Preferably, the control system 20 is configured to control the charging and discharging of the energy storages 14 so that the energy storages 14 operates with margin to their fully dis- charged and fully charged states, such as in the range between 20 to 80 % of their storage capacity. By operati ng the energy storages 14 in this range, the output power from the PV power plant 1 to the inverter 22 can be maintai ned at the specified power rating for most of the times. Moreover, the correction of mismatch in PV voltage can be maintained without interruptions.
Fig . 2 shows a flow chart of a first embodiment of a method for controlling the power conversion system 3 in fig .1 .
The method comprises, in a step 100, receiving i nformation on the generated power of the PV module 5 or array of PV modules 5 connected to each conversion device 10. The information on the generated power is for example a physical quantity of the PV modules 5, which physical quantity is dependent on the generated electric power of the PV modules 5, such as the current or voltage of the PV modules 5.
In a step 1 10, the method comprises determi ning if the generated power of each the PV module 5 or array of PV modules 5 exceeds a certain level .
In a step 120, the method comprises controlling the flow electric power to or from the energy storage 14 of each conversion system 3 on basis of the determined generated power. The flow electric power is controlled so that output power to the inverter 22 and thereto connected power grid 7 becomes lower than a certain level .
For example, the control unit 10 receives information on the physical quantity of the PV modules 5 and determines the gen- erated power. Preferably, the control unit 10 continuously receives information on the physical quantity, determines the generated power, and regulates the charging and discharging of the energy storages 14. Fig . 3 shows a flow chart of a second embodiment of a method for controlling the conversion system 3 in fig . 1 . The method comprises steps i n fig . 2 and further steps of controlling the energy storage 14 so that mismatch i n PV voltage between different PV modules 5 is corrected . In the following described method , the mismatch i n PV voltage is compensated between two different PV modules 5. It shall be understood the method is applicable to correction of mismatch i n PV voltage for all the PV modules 5 in the conversion system 3. In a step 130, the method comprises receiving information on generated power of a first and a second PV module 5 that are connected to different conversion devices 10. The information may be the same as in step 100 related to a physical quantity that depends on the generated power of the PV module 5.
In a step 140, the method comprises determi ning a difference in generated power between the first and the second PV module 5.
If the difference between the first and the second PV module 5 has a positive value and the absolute value of the difference exceeds a certain val ue, the method comprises in a step 1 50, compensating the mismatch in PV voltage by controlling the energy storage 14 of a conversion device 10 designated for the first PV module 5 so that the energy storage 14 charges the dif- ference in generated power, or by controlling the energy storage 14 of the conversion device 10 designated for the second PV module 5 so that the energy storage 14 discharges the difference i n generated power. If the difference between the first and the second PV module 5 has a negative value and the absolute value of the difference exceeds a certain value, the method comprises in a step 160, compensating the mismatch in PV voltage by controlling the energy storage 14 of a conversion device 10 designated for the first PV module 5 so that the energy storage 14 discharges the difference in generated power, or by controlling the energy stor- PV module 5 so that the energy storage 14 charges the difference i n generated power. Preferably, the method comprises controlling the energy storages 14 so that the energy storages 14 operate with margin to their fully discharged and fully charged state. For example, the energy storages 14 operate i n the interval of 20-80% of their capacity. The method comprises the steps of controlling the ener- gy storages 14 so that the energy storages 14 operate in the i nterval of 20-80% by alternati ng charging and discharging the energy storage 14 of the conversion device 10 designated for the first and the second PV module 5. Fig . 4a shows the power profile from a PV power plant 1 connected to a power conversion system 3 accordi ng to state of the art. The PV power plant 1 generates power duri ng the time of the day which the PV modules 5 are subjected to sunshine. The power rati ng capacity of the inverter 22 is indicated by a doted line. As it can be seen , the output power profile is relatively steep with a peak at noon . The power rating capacity of the i nverter 22 is dimensioned with margin to withstand days with exceptional high sun radiation and thereby exceptional high power rating .
Fig . 4b shows the power profile from a PV power plant 1 connected to a power conversion system 3 according to the invention . The output power from the PV power plant 1 is controlled so that the maximum output power to the power grid 7 is lower than 0, 1 kW . The power rating capacity of the inverter 22 is indicated by a doted line. As can be seen , the output power profile from the power conversion systems 3 is flatter than the output power profile of prior art power conversion systems. The power rati ng capacity of the inverter 22 is set to the power rating to which the maximum output power is controlled , in this case 0, 1 kW . Accordingly, the invention enables use of a more simple and cost effective inverter 22 than prior art power conversion systems 3. The present invention is not limited to the disclosed embodiments but may be modified within the framework of the claims.
For example, the control system 20 may comprise a central unit that controls the energy storages 5 of all of the conversion devices 10.

Claims

1 . A power conversion system (3) for providing power from a plurality of PV modules (5) to a power grid (7), the conversion system (3) comprises
- a conversion device (10) comprising an M PPT unit (12) and an energy storage (14),
- a control system adapted to control the flow of energy to and from the energy storage (14), and
- an inverter (22) adapted to be connected to the power grid (7), characterized in that
the conversion system (3) comprises a plurality of conversion devices (10) connected in series, each MPPT unit (12) of the conversion devices (10) comprises means for enabling connec- tion to one PV-module (5) or an array of PV-modules (5), and a first MPPT unit (12) and a second MPPT unit (12) of the conversion devices (10) are connected to the inverter (22), wherein the control system is configured to control the energy storages (14) of the conversion devices (10) so that the maximum output pow- er to the power grid (7) is lower than a certai n level .
2. The power conversion system (3) according to claim 1 , wherei n the inverter (22) is dimensioned for a power rating of said certain level .
3. The power conversion system (3) accordi ng to any of claim 1 or 2, wherei n the energy storages (14) are connected between the MPPT unit (12) and the inverter (22).
4. The power conversion system (3) accordi ng to any of the previous claims, wherein the control system comprises a plurality of control devices (20) arranged so that each conversion device (10) comprises a control device (20) that is configured to control the energy storage (14) of the conversion device (10) so that the maximum output power from the conversion devices (10) are lower than the certain level .
5. The power conversion system (3) accordi ng to any of the previous claims, wherein the control system is configured to correct mismatch in PV voltage between different PV modules (5).
6. The power conversion system (3) accordi ng to any of the previous claims, wherei n the control system is adapted to control the energy storages (14) of the conversion devices (10) so that the energy storages (14) operate with margin to their fully dis- charged and fully charged state.
7. The power conversion system (3) accordi ng to any of the previous claims, wherein each conversion device (10) is adapted to be arranged on the backside of the PV module (5) or the array of PV modules (5) to which the MPPT unit (12) is connected .
8. The power conversion system (3) accordi ng to any of the previous claims, wherei n the energy storage (14) is a super- capacitor.
9. The power conversion system (3) accordi ng to any of the previous claims, wherein power conversion system (3) comprises sensor means for measuring a physical quantity of the PV modules (5) that is dependent on the generated electric power of the PV modules (5), and means for determining the generated power based on said physical quantity.
10. A method for controlling a power conversion system (3) for providing power from a plurality of PV modules (5) to a power grid (7), the conversion system (3) comprises a conversion device (10) comprising an MPPT unit (12) and an energy storage (14), a control system controlling the flow of energy to and from the energy storage (14), and a inverter (22) connected to the power grid (7), wherein the method comprises the steps of
- controlling the energy storage (14) so that the maximum output power to the power grid (7) is lower than a certai n level .
1 1 . The method according to claim 10, wherei n the conversion system comprises a pl urality of conversion devices (10) connected i n series, and a first MPPT unit (12) and a second MPPT unit (12) of the conversion devices (10) are connected to the inverter (22), wherein the method comprises the steps of
- -controlling each of the energy storages (14) so that the maximum output power to the power grid (7) is lower than the certai n level .
12. The method according to any of claim 10 or 1 1 , wherein the energy storages (14) are connected between the MPPT unit (1 2) and the inverter (22), the method further comprises the steps of.
- controlling the energy storage (14) so that mismatch i n PV voltage between different PV modules (5) are corrected .
13. The method according to any of claim 10-12, wherein the correction of mismatch in PV voltage comprises
- receiving information on generated power of a first and a sec- ond PV module (5),
- determining a difference in generated power between the first and the second PV module (5),
- if said difference has a positive value and the absol ute val ue of said difference exceeds a certai n value, the mismatch i n PV voltage is compensated by controlling the energy storage (14) of the conversion device (10) designated for the first PV module (5) so that the energy storage (14) charges the difference i n generated power, or by controlling the energy storage (14) of the conversion device (1 0) designated for the second PV module (5) so that the energy storage (14) discharges the difference in generated power,
- if said difference has a negative value and the absolute val ue of said difference exceeds a certai n value, the mismatch i n PV voltage is compensated by controlling the energy storage (14) of the conversion device (10) designated for the first PV module (5) so that the energy storage (14) discharges the difference in generated power, or by controlling the energy storage (14) of the conversion device (10) designated for the second PV module (5) so that the energy storage (14) charges the difference i n generated power.
14. The method according to claim 1 3, wherein the correction of mismatch in PV voltage is controlled by storing or dischargi ng of energy of the energy storage (14), the method comprises
- controlling the energy storages (14) so that the energy storag- es (14) operate with margi n to their fully discharged and fully charged state by alternating charging and discharging the energy storage (14) of the conversion devices (10) designated for the first and the second PV module (5).
15. A PV power plant comprising a plurality of PV modules (5) and a power conversion system (3) according to any of claim 1 - 9.
PCT/EP2014/050819 2014-01-16 2014-01-16 Dc series mppt pv system and control method thereof WO2015106814A1 (en)

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