WO2020019103A1

WO2020019103A1 - Photovoltaic power system

Info

Publication number: WO2020019103A1
Application number: PCT/CN2018/096605
Authority: WO
Inventors: Xiaobo Yang; Wenliang Zhang; Sheng ZONG; Chunming YUAN; Guoxing FAN
Original assignee: Abb Schweiz Ag
Priority date: 2018-07-23
Filing date: 2018-07-23
Publication date: 2020-01-30

Abstract

It provides a photovoltaic power system (1), including: at least one PV panel assembly (100, 101), having at least one PV panel (140, 141), a central power converter (11) having a DC side and an AC side to be electrically coupled to an AC grid, at least one distributed power converter (120, 121) each having a first DC side electrically coupled to a corresponding one of the at least one PV panel assembly (100, 101) and a second DC side electrically coupled with the DC side of the central power converter (11), and a control system (13), for controlling operation of the central power converter (11) and the at least one distributed power converter (120, 121) so that electric power flows in a forward direction or a backward direction; wherein: in the forward direction, the electric power generated by the at least one PV panel (140, 141) of the at least one PV panel assembly (100, 101) from electromagnetic radiation is supplied to the AC grid; and in the backward direction, the electric power from the AC grid is supplied to the at least one PV panel (140, 141) of the at least one PV panel assembly (100, 101). It provides an alternative solution capable of controlling bidirectional power flow, in which either supplying snow-melting current to the PV panel (140, 141) or converting and transmitting electric power generated by the PV panel (140, 141) to the AC grid.

Description

PHOTOVOLTAIC POWER SYSTEM

Technical Field

The invention relates to photovoltaic power system, and more particularly to a photovoltaic power system having means for removing snow.

Background Art

Recently, a photovoltaic power system where photovoltaic panels and a power system are interconnected through a converter, has been widely used. Since the photovoltaic panel (PV panel) obtains the generated power by electromagnetic radiation, a quantity of the generated power is reduced or the power generation is not possible when the electromagnetic radiation is partially or completely blocked. As one of causes which block the electromagnetic radiation, there is snow that is accumulated on the photovoltaic panel.

In order to remove the snow covering the photovoltaic panels, Patent JP 2000 156 940 A discloses a method for controlling a converter in which an inverter is constructed by a bidirectional converter.

The technical solution as disclosed by the Patent JP 2000 156 940 A has a diode connected with the string of photovoltaic panels in anti-series to block current flow from the converter to the photovoltaic panel, and a controllable bypass device connected with the diode in parallel for conducting snow-melting current from the converter to the photovoltaic panel. When the snow melting current is supplied by supplying the fixed voltage to the PV panel with the bidirectional converter, a fixed voltage is applied to a string of photovoltaic panels by the converter, and the photovoltaic panels are heated and the snow melting is achieved by making a snow melting current flow from the converter to the photovoltaic panels.

Brief Summary of the Invention

It is therefore an objective of the invention to provide a photovoltaic power system, including: at least one PV panel assembly having at least one PV panel, a central power converter having a DC side and an AC side to be electrically coupled to an AC grid, at least one distributed power converter each having a first DC side electrically coupled to a corresponding one of the at least one PV panel assembly and a second DC side electrically coupled with the DC side of the central power converter, and a control system, for controlling operation of the central power converter and the at least one distributed power converter so that electric power flows in a forward direction or a backward direction; wherein: in the forward direction, the electric power generated by the at least one photovoltaic panel of the at least one PV panel assembly from electromagnetic radiation is supplied to the AC grid; and in the backward direction, the electric power from the AC grid is supplied to the at least one photovoltaic panel of the at least one PV panel assembly.

By having the photovoltaic power system according to present invention, in light of the absence of the bypass device connected with the diode in parallel for conducting snow-melting current from the converter to the photovoltaic panel, it provides an alternative solution capable of controlling bidirectional power flow, in which either supplying snow-melting current to the PV panel or converting and transmitting electric power generated by the PV panel to grid.

Preferably, the photovoltaic power system further includes a voltage sensor for measuring voltage at the DC side of the central power converter and at least one current sensor each for measuring current of a corresponding one of the at least one PV panel assembly; wherein: the control system controls the operation of the central power converter so as to regulate the voltage at its DC side based on the voltage measurement by the voltage sensor, and the control system controls the operation of the distributed power converter so as to regulate its current based on the current measurement by the current sensor. This allows the dependence between voltage and current of the DC bus to be de-coupled by separately control of them by separate devices, the central power converter and the distributed power converter. This makes it possible for flexibly supplying snow-melting current under different ambient temperature conditions.

Preferably, the distributed power converter is a bi-directional DC-DC converter, and the central power converter is a bi-directional DC-AC converter.

Preferably, the at least one photovoltaic panel of the PV panel assembly are connected in series.

Preferably, the control system controls the operation of the distributed power converter based on the current measurement and the voltage measurement so that it operates in MPPT when the electric power flows in the forward direction.

Preferably, the value of the output current of the distributed power converter in the backward direction is a function of ambient temperature of the photovoltaic power system.

Brief Description of the Drawings

The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments which are illustrated in the drawings, in which:

FIG. 1 is a configuration view of a photovoltaic power system according to an embodiment of present invention;

FIG. 2 shows a typical voltage-current characteristics of PV panel according to an embodiment of present invention;

FIG. 3 shows converter topology the distributed power converter uses according to an embodiment of present invention;

FIG. 4 shows bidirectional converter according to an embodiment of present invention; and

FIG. 5 depicts a non-isolated topology in the form of a three phase 2-level inverter according to an embodiment of present invention.

The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

Preferred Embodiments of the Invention

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Note, the headings are for organizational purposes only and are not meant to be used to limit or interpret the description or claims. Furthermore, note that the word "may" is used throughout this application in a permissive sense (i.e., having the potential to, being able to) , not a mandatory sense (i.e., must) . "The term "include" , and derivations thereof, mean "including, but not limited to" . The term "connected" means "directly or indirectly connected" , and the term "coupled" means "directly or indirectly connected" .

FIG. 1 is a configuration view of a photovoltaic power system according to an embodiment of present invention. As shown in FIG. 1, a photovoltaic power system 1 includes at least one

PV panel assembly

100, 101, a central power converter 11, at least one

distributed power converter

120, 121, and a control system 13. The skilled person should understand that they can choose any number of the PV panel assemblies and any number of distributed power converters upon the requirement of the photovoltaic power system. In this embodiment, two

PV panel assemblies

100, 101 and two

distributed power converters

120, 121 are exemplified for description of present invention. The

PV panel assembly

100, 101 includes at least one PV panel 140, 141. The central power converter 11 has a DC side and an AC side to be electrically coupled to an AC grid. The

distributed power converter

120, 121 has a first DC side electrically coupled to a corresponding one of the at least one

PV panel assembly

100, 101 and a second DC side electrically coupled with the DC side of the central power converter 11. The skilled person should understand that they can choose any number of the PV panels consisting of one of the PV panel assemblies upon the requirement of the photovoltaic power system. The photovoltaic panels of the same PV panel assembly are connected in series to form a string.

The PV panel 140, 141 can be an array of PV cells wherein the individual cells can take on any variety of constructions such as but not limited to monocrystalline, polycrystalline, and thin film. The array of PV cells are typically (but not necessarily always) assembled in close proximity to one another and in some forms can be coupled with a common carrier/substrate to form a unitary PV cell construction. Such a unitary construction can allow for ease of transportation, installation, and maintenance. The unitary construction can be mechanically and/or electrically connected with other unitary constructions to form the array of PV cells that provide power. The PV panel can include an arrangement of sufficient numbers of individual PV cells to provide any range of power, such as power above 100 W to set forth just one non-limiting example.

FIG. 2 shows a typical voltage-current characteristics of PV panel. A horizontal axis illustrates a terminal voltage of the PV panel, and a vertical axis illustrates a total value of the current which is output from the PV panel. As shown in FIG 2, the PV cell is able to operate in either of the 1 ^st quadrant and the 4 ^th quadrant of the V-I characteristics. Where the PV panel operates in the 1 ^st quadrant, it is capable of producing and/or contributing useful electric power to the grid. While in the 4 ^th quadrant, each of the PV cells of the PV panel operates as a diode under forward conduction mode conducting a current injected into it; under this operation condition, the PV panels become passive load and dissipate heat. The snow covering the PV panel will be melt when a water membrane formed between PV panel surface and the snow coating, which helps the whole snow pack drifts from the PV panel. PV panel may operate in either of two modes, producing electric power to the grid or behaving as passive load and dissipating heat.

The PV system 1 of the first embodiment, has two operation modes. One is a power generating mode that in the forward direction Df, the electric power generated by the photovoltaic panel 140, 141of the

PV panel assembly

100, 101 from electromagnetic radiation is supplied to the AC grid; the other is a snow melting mode that in the backward direction Db, the electric power from the AC grid is supplied to the photovoltaic panel 140, 141 of the

PV panel assembly

100, 101.

By the two modes, in a state of being with solar radiation, the control system 13 controls operation of the central power converter 11 and the

distributed power converter

120, 121 so that electric power flows in the forward direction Df, where the PV power system 1 operates in the power generating mode. And, at the time of snow, the control system 13 controls operation of the central power converter 11 and the

distributed power converter

120, 121 so that the electric power flows in the backward direction Db, where the PV power system 1 operates in the snow melting mode.

In this embodiment for example, in the state of being with solar radiation, the DC electric power obtained from the

PV panel assembly

100, 101 is converted by the

distributed power converter

120, 121 and output at its second DC side, which, in turn, is converted by the central power converter 11 outputting and supplying an AC electric power at its AC side to the grid. In the state of snow melting, the AC electric power obtained from the grid is converted by the central power converter 11 outputting and supplying DC electric power at its DC side, which, in turn, is converted by the

distributed power converter

120, 121 outputting and supplying DC electric power at its first DC side to the

PV panel assembly

100, 101 to increase a temperature of the PV panel.

The

distributed power converter

120, 121 can take on a number of forms and in general is structured to bi-directional DC-DC converter. As will be described in more detail below, the

distributed power converter

120, 121 can include/be integrated or coupled with other functioning components such as a Maximum Power Point Tracker (MPPT) . The additional functioning components can either be included with the

distributed power converter

120, 121 or implemented by the control system 13.

As will be appreciated, in the state of being with solar radiation, an MPPT module can be used to seek out the maximum power point of the

PV panel assembly

100, 101. In some embodiments the MPPT and distributed

power converter

120, 121 can be structured such that the input voltage to the distributed

power converter

120, 121 is regulated by the MPPT module to “float” to whatever voltage yields maximum power from the PV panel assembly 100, 101 (in turn the distributed

power converter

120, 121 can be structured such that its output is at a fixed voltage but the current allowed to “float” ) . The MPPT can be located within the physical confines of a housing of the distributed

power converter

120, 121 and/or integrated or coupled to it, but the MPPT can alternatively be located elsewhere in the

PV panel assembly

100, 101. It will be appreciated that the MPPT can be implemented via digital and/or analog techniques, and can take on a variety of forms including perturb and observe, incremental conductance, current sweep, and constant voltage, to set forth just a few nonlimiting examples.

The central power converter 11 to which one or more distributed

power converter

120, 121 are connected can take on a variety of forms. For example, the central power converter 11 can be a bidirectional DC-AC converter, non-limiting embodiments of which are shown and discussed further below. In some embodiments the central power converter 11 can include a transformer. While the distributed

power converters

120, 121 are shown as standing off from the

PV panel assemblies

100, 101, in some embodiments the distributed

power converters

120, 121 can be integrated with or in closer proximity to the

PV panel assemblies

100, 101, no matter what the form of the central power converter 11. The central power converter 11 can provide active and reactive power, AC voltage control, low voltage ride through, and randomization of timing for trip and reconnection.

The central power converter 11 can be connected to distributed

power converters

120, 121 through plug and play devices. For example, a DC bus 15 routed between the central power converter 11 and distributed

power converter

120, 121 can include on at least one end a connector (male or female) that incorporates plug and play features. As will be appreciated, plug and play connectors permit are unlike hardwired connections in that they permit rapid connection and disconnection, and may also provide some degree of environmental protection. The plug and play connectors can furthermore have features that permit the connector to be secured in place, such as through a clip, etc. In one embodiment a power outlet of the distributed

power converter

120, 121 can be a fixed base terminal (male or female) structured to receive a cable having a complementary shaped connection device. In another form the distributed

power converter

120, 121 can include a cable, for example a hardwired cable, that has a cable ending with a plug and play device (male or female) useful for connection with a complementary plug and play device associated with the central power converter 11. The plug and play connection devices can thus be considered to encompass terminal side and cable ended side connections, whether those connections are male or female. The plug and play connection devices can be installed in a separate cabinet, or together with the central inverter in the same cabinet.

The plug and play devices can also be used in other components such as: between the

PV panel assembly

100, 101 and the distributed

power converter

120, 121; in the mechanical connection between the distributed

power converter

120, 121 and the DC bus 15 (which in one form is an LVDC bus) ; and in the mechanical connection between the DC bus 15 and the central power converter 11.

Furthermore, it will be appreciated that a voltage sensor 16 and a current sensor 17 can be incorporated into one or more components of the PV power system 1, such as but not limited to the distributed

power converter

120, 121 and/or the central power converter 11. The sensors can include one or more communications devices (transmitter, receiver, transceiver, for example) can transmit to and receive data from the control system 13, and can do so using any variety of techniques. For example, the sensors can use industrial bus, cell and/or pager networks, satellite, licensed and/or unlicensed radio, and power line communication. Furthermore, the sensors can be used in a network environment such as fixed wireless, mesh network, etc. The smart meter can be structured to communicate status information such as but not limited to power, voltage, and current. In some forms the status information can be real-time while in others can additionally and/or alternatively include historic information. The status information can be compiled through measurement and/or be calculated.

In some embodiment, the voltage sensor 16 can be a voltage transformer arranged at the DC side of the central power converter 11, for measuring voltage at the DC side of the central power converter 11. Besides, the current sensor 17 can be a current transformer arranged at the

PV panel assembly

100, 101, for measuring current of a corresponding one of the

PV panel assembly

100, 101.

In some embodiment, the control system 13 may control the operation of the distributed

power converter

120, 121 based on the current measurement and the voltage measurement so that it operates in maximum power point tracker (MPPT) when the electric power flows in the forward direction.

An operation point of the PV panel 140, 141 at the time of the snow melting run will be descried by using FIG. 2.

The value of the output current of the distributed

power converter

120, 121 in the backward direction is a function of ambient temperature of the photovoltaic power system. When a heavy snow is on the panel, since the temperature of the panel is low, it is desirable to run the snow-melting following the characteristic of the string becomes V-I characteristic which is illustrated as “Heavy snow coating” . To keep the operation point on the desired curve, for example at operation point A, by taking the voltage and current count at point A as reference, the control system 13 may control the operation of the central power converter 11 so as to regulate the voltage at its DC side based on the voltage measurement by the voltage sensor 16; further, the control system 13 may control the operation of the distributed

power converter

120, 121 so as to regulate its current based on the current measurement by the current sensor 17. If the time elapses, the temperature of the

PV panel

100, 101 is further increased, and V-I characteristic is to be changed to point B on the curve indicated by “Moderate snow coating” . To keep the operation point at point B on the new curve, similarly, the control system 13 takes the voltage and current count at point B as reference and controls the operation of the central power converter 11 so as to regulate the voltage at its DC side based on the voltage measurement by the voltage sensor 16, and further controls the operation of the distributed

power converter

120, 121 so as to regulate its current based on the current measurement by the current sensor 17. In some cases, distributed

power converter

120, 121 can also regulate its current based on the current measurement by the current sensor 17.

In the embodiment of the instant application, the dependence between voltage and current of the DC bus 15 can be de-coupled by separately control of them by separate devices, the central power converter and the distributed power converter. This makes it possible for flexibly supplying snow-melting current under different ambient temperature conditions.

Still another feature of the present application provides wherein the distributed power converter is used for conversion electric power for melting the snow in the backward direction, and wherein the distributed power converter also provides function of MPPT in the forward direction.

The distributed

power converter

120, 121 can take on a variety of isolated or non-isolated forms in any of the various embodiments herein, such as buck-boost converter, as shown in FIG. 3. The distributed power converter includes two DC sides UDC1, UDC2, two capacitors C1, C2, and four controllable semiconductors each with anti-paralleled diode Q1, Q2, Q3, Q4. Some DC-DC converters are designed to move power in only one direction, from dedicated input to output, for example full-bridge converter which can be made bidirectional and able to move power in either direction by paralleling some of their diodes D5, D6, D7, D8 with controllable power semiconductors Q5, Q6, Q7, Q8 as shown in FIG. 4, while keeping the rest of elements unchanged, including four controllable semiconductors each with anti-paralleled diode Q1, Q2, Q3, Q4 and two capacitors C1, C2 and the two DC sides UDC1, UDC2.

In similar fashion, the central power converter 11 can also take on a variety of forms.

A DC-AC converter with bidirectional power flow can be realized by coupling a PWM inverter to the DC-link. The DC-link quantity is then impressed by an energy storage element that is common to both stages, which is a capacitor C for the voltage DC-link or an inductor L for the current DC-link. For example, FIG. 5 depicts a non-isolated topology in the form of a three phase 2-level inverter, including six controllable semiconductors each with anti-paralleled diode Q1 to Q6 linked in full bridge and a capacitor. Other topologies included three level converter, modular multilevel converter, Z-source inverter and so forth.

Though the present invention has been described on the basis of some preferred embodiments, those skilled in the art should appreciate that those embodiments should by no way limit the scope of the present invention. Without departing from the spirit and concept of the present invention, any variations and modifications to the embodiments should be within the apprehension of those with ordinary knowledge and skills in the art, and therefore fall in the scope of the present invention which is defined by the accompanied claims.

Claims

A photovoltaic power system, including:

at least one PV panel assembly having at least one PV panel;

a central power converter having a DC side and an AC side to be electrically coupled to an AC grid;

at least one distributed power converter each having a first DC side electrically coupled to a corresponding one of the at least one PV panel assembly and a second DC side electrically coupled with the DC side of the central power converter; and

a control system, for controlling operation of the central power converter and the at least one distributed power converter so that electric power flows in a forward direction or a backward direction;

wherein:

in the forward direction, the electric power generated by the at least one photovoltaic panel of the at least one PV panel assembly from electromagnetic radiation is supplied to the AC grid; and

in the backward direction, the electric power from the AC grid is supplied to the at least one photovoltaic panel of the at least one PV panel assembly.
The photovoltaic power system according to claim 1, further including:

a voltage sensor, for measuring voltage at the DC side of the central power converter;

wherein:

the control system controls the operation of the central power converter so as to regulate the voltage at its DC side based on the voltage measurement by the voltage sensor.
The photovoltaic power system according to claim 2, further including:

at least one current sensor each for measuring current of a corresponding one of the at least one PV panel assembly;

wherein:

the control system controls the operation of the distributed power converter so as to regulate its current based on the current measurement by the current sensor.
The photovoltaic power system according to claim 1 or 2 or 3, further including:

a DC bus, electrically coupling the second DC side of the distributed power converter and the DC side of the central power converter.
The photovoltaic power system according to claim 1 or 2 or 3, wherein:

the distributed power converter is a bi-directional DC-DC converter; and

the central power converter is a bi-directional DC-AC converter.
The photovoltaic power system according to claim 1 or 2 or 3, wherein:

the at least one photovoltaic panel of the PV panel assembly are connected in series.
The photovoltaic power system according to claim 3, wherein:

the control system controls the operation of the distributed power converter based on the current measurement and the voltage measurement so that it operates in MPPT when the electric power flows in the forward direction.
The photovoltaic power system according to claim 3, wherein:

the value of the output current of the distributed power converter in the backward direction is a function of ambient temperature of the photovoltaic power system.