WO2021000253A1

WO2021000253A1 - Photovoltaic system and control method thereof

Info

Publication number: WO2021000253A1
Application number: PCT/CN2019/094364
Authority: WO
Inventors: Bing Huang; Yun Wu; Dario BIGNOLI; Li Zeng
Original assignee: Marich Holdings The Netherlands B.V.
Priority date: 2019-07-02
Filing date: 2019-07-02
Publication date: 2021-01-07

Abstract

A photovoltaic system (100) and a control method for the photovoltaic system (100). The photovoltaic system (100) comprises: a photovoltaic array (110); an inverter stage (130) comprising a DC-DC converter (131) and a DC-AC converter (132) that are cascaded with each other via a bus (138) and coupled between the photovoltaic array (110) and an AC power grid (200); a plurality of electrical switches (121, 122); and a controller (150) configured to: in response to detecting a fault in the photovoltaic system (100) or the AC power grid (200), send a disconnecting signal to the plurality of electrical switches (121, 122) to disconnect the inverter stage (130) from the AC power grid (200) and the photovoltaic array (110), respectively; and control the DC-DC converter (131) to convert a DC voltage on a DC input side (139) of the inverter stage (130) into a DC voltage on the bus (138). In the photovoltaic system (100), the DC side voltage of the photovoltaic system (100) can be decreased below the safe voltage within a predefined time, and the efficiency of the inverter is effectively improved.

Description

PHOTOVOLTAIC SYSTEM AND CONTROL METHOD THEREOF

FIELD

Embodiments of present disclosure generally relate to photovoltaic power generation, and more particularly, to a photovoltaic system and a control method of the photovoltaic system.

BACKGROUND

In the photovoltaic system, the photovoltaic (PV) DC line can generally reach a high voltage of 600-1100V. Due to poor contact of the line or damage of the insulation, the DC arc phenomenon is easily caused, causing a fire. When a fire is generated, if there is light radiating onto the PV cells, the PV cells will continue to generate electricity, and the high voltage will always exist. This makes the fire fighting work very dangerous.

In order to improve the safety, the US National Electrical Code requires that the PV system should provide rapid shutdown (RSD) function. Moreover, in the US National Electrical Code NEC 690.12, after the RSD device is started, the voltage of the PV system at a distance from the PV cell array should be reduced to below 30V within 30 second. This requires the DC input port voltage of the PV inverter to be reduced to below 30V within 30 second.

In the conventional PV system, for the purpose of reducing the DC input port voltage of the PV inverter as quickly as possible during the rapid shutdown, a method of adding a discharge circuit to the DC input side of the PV inverter is employed. However, this discharge circuit has a risk of failure and reduced efficiency of the inverter.

SUMMARY

Embodiments of the present disclosure provide a photovoltaic system and a control method for a photovoltaic system.

In a first aspect, a photovoltaic system is provided. The photovoltaic system comprises: a photovoltaic array; an inverter stage comprising a DC-DC converter and a DC-AC converter that are cascaded with each other via a bus and coupled between the photovoltaic array and an AC power grid; a plurality of electrical switches; and a controller configured to: in response to detecting a fault in the photovoltaic system or the AC power grid, send a disconnecting signal to the plurality of electrical switches to disconnect the inverter stage from the AC power grid and the photovoltaic array, respectively; and control the DC-DC converter to convert a DC voltage on a DC input side of the inverter stage into a DC voltage on the bus.

In some embodiments, the controller is configured to control the DC-DC converter by: determining a DC voltage drop in the inverter stage over a predetermined period of time; and in response to the DC voltage drop exceeding a threshold, controlling the DC-DC converter to convert the DC voltage on the DC input side of the inverter stage into the DC voltage on the bus.

In some embodiments, the DC voltage drop in the inverter stage over the predetermined period of time is determined based on at least one of the following: a voltage drop on the DC input side over the predetermined period of time, or a voltage drop on the bus over the predetermined period of time.

In some embodiments, the controller is configured to control the DC-DC converter to convert the DC voltage on the DC input side of the inverter stage into the DC voltage on the bus in response to at least one of the following: the voltage drop on the DC input side exceeding a first threshold, or the voltage drop on the bus exceeding a second threshold.

In some embodiments, the controller (150) is further configured to: in response to the conversion of the DC voltage on the DC input side of the inverter stage into the DC voltage on the bus, obtain the DC voltage on the DC input side, and in response to the DC voltage on the DC input side falling below a first predetermined voltage, control the DC-DC converter to cease converting the DC voltage on the DC input side into the DC voltage on the bus.

In some embodiments, the controller is further configured to: in response to the conversion of the DC voltage on the DC input side of the inverter stage into the DC voltage on the bus, obtain the DC voltage on the bus, and in response to the DC voltage on the bus exceeding a second predetermined voltage, control the inverter stage to perform normal operations corresponding to operations before the fault was detected.

In some embodiments, the bus is configured to provide power to auxiliary devices of the photovoltaic system.

In some embodiments, the plurality of electrical switches comprises: a first plurality of switches configured to connect or disconnect the photovoltaic array with the inverter stage; and a second plurality of switches configured to connect or disconnect the inverter stage with the AC power grid.

In a second aspect, a control method for the photovoltaic system is provided. The control method comprises: in response to detecting a fault in a photovoltaic system or a AC power grid, sending a disconnecting signal to a plurality of electrical switches to disconnect an inverter stage of the photovoltaic system from the AC power grid and a photovoltaic array, respectively, wherein the inverter stage comprises a DC-DC converter and a DC-AC converter that are cascaded with each other via a bus and is coupled between the photovoltaic array and the AC power grid; and controlling the DC-DC converter to convert a DC voltage on a DC input side of the inverter stage into a DC voltage on the bus.

In some embodiments, controlling the DC-DC converter to convert the DC voltage on the DC input side of the inverter stage into the DC voltage on the bus comprise: determining a DC voltage drop in the inverter stage over a predetermined period of time; and in response to the DC voltage drop exceeding a threshold, controlling the DC-DC converter to convert the DC voltage on the DC input side of the inverter stage into the DC voltage on the bus.

In some embodiments, determining the DC voltage drop of the inverter stage over the predetermined period of time comprises at least one of: determining a voltage drop on the DC input side over the predetermined period of time, or determining a voltage drop on the bus over the predetermined period of time.

In some embodiments, controlling the DC-DC converter to convert the DC voltage on the DC input side of the inverter stage into the DC voltage on the bus in response to at least one of the following: the voltage drop on the DC input side exceeding a first threshold, or the voltage drop on the bus exceeding a second threshold.

In some embodiments, the control method further comprises: in response to the conversion of the DC voltage on the DC input side of the inverter stage into the DC voltage on the bus, obtaining the DC voltage on the DC input side, and in response to the DC voltage of the DC input side falling below a first predetermined voltage, controlling the DC-DC converter to cease converting the DC voltage on the DC input side into the DC voltage on the bus.

In some embodiments, the control method further comprises: in response to the conversion of the DC voltage on the DC input side of the inverter stage into the DC voltage on the bus, obtaining the DC voltage on the bus, and in response to the DC voltage on the bus exceeding a second predetermined voltage, controlling the inverter stage to perform normal operations corresponding to operations before the fault was detected.

DESCRIPTION OF DRAWINGS

Drawings described herein are provided to further explain the present disclosure and constitute a part of the present disclosure. The example embodiments of the disclosure and the explanation thereof are used to explain the present disclosure, rather than to limit the present disclosure improperly.

FIG. 1 illustrates a schematic diagram of a PV system 100 coupled to an AC power grid 200 in accordance with an embodiment of the present disclosure.

FIG. 2 illustrates details of PV system 100 (a PV array 110 is not shown) in accordance with an embodiment of the present disclosure.

FIG. 3 illustrates a flowchart of a control method 300 for the PV system 100, in accordance with an embodiment of the present disclosure.

FIG. 4 illustrates a flowchart of a further control method 400 for the method 300, in accordance with some embodiments of the present disclosure.

FIG. 5 illustrates a flowchart of a further control method 500 for the method 300, in accordance with some embodiments of the present disclosure.

Throughout the drawings, the same or similar reference symbols are used to indicate the same or similar elements.

DETAILED DESCRIPTION OF EMBODIEMTNS

Principles of the present disclosure will now be described with reference to several example embodiments shown in the drawings. Though example embodiments of the present disclosure are illustrated in the drawings, it is to be understood that the embodiments are described only to facilitate those skilled in the art in better understanding and thereby achieving the present disclosure, rather than to limit the scope of the disclosure in any manner.

The term “comprises” or “includes” and its variants are to be read as open terms that mean “includes, but is not limited to. ” The term “or” is to be read as “and/or” unless the context clearly indicates otherwise. The term “based on” is to be read as “based at least in part on. ” The term “being operable to” is to mean a function, an action, a motion or a state can be achieved by an operation induced by a user or an external mechanism. The term “one embodiment” and “an embodiment” are to be read as “at least one embodiment. ” The term “another embodiment” is to be read as “at least one other embodiment. ” The terms “first, ” “second, ” and the like may refer to different or same objects. Other definitions, explicit and implicit, may be included below. A definition of a term is consistent throughout the description unless the context clearly indicates otherwise.

Unless specified or limited otherwise, the terms “mounted, ” “connected, ” “supported, ” and “coupled” and variations thereof are used broadly and encompass direct and indirect mountings, connections, supports, and couplings. Furthermore, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. In the description below, like reference numerals and labels are used to describe the same, similar or corresponding parts in the figures. Other definitions, explicit and implicit, may be included below.

As discussed above, in a conventional PV system, a discharge circuit is usually provided on the DC input side in order to reduce the DC voltage on the input side of the PV inverter stage as quickly as possible to meet the requirements of NEC 690.12 during the rapid shutdown. However, the discharge circuit will reduce the efficiency of the PV inverter stage and the reliability of the system.

According to embodiments of the present disclosure, a DC-DC circuit in the PV inverter stage is utilized to convert a DC voltage on the input side of the PV inverter stage to a DC voltage on the bus between the DC-DC circuit and the DC-AC circuit of the PV inverter stage. In this way, the voltage on the DC input side close to the PV cell array may fall below the required voltage value within a short period of time. Moreover, since the power on the bus of the PV inverter stage is provided to the auxiliary devices (e.g. control circuits for the PV inverter stage) of the PV system, the power transferred to the bus will be quickly consumed, which in turn promotes a rapid drop in the DC voltage.

FIG. 1 illustrates a schematic diagram of a PV system 100 coupled to an AC power grid 200 in accordance with an embodiment of the present disclosure. Examples of the PV system 100 includes, but are not limited to, power generation systems that converts solar energy into electrical energy. Such systems can generate electrical power independently and can be integrated into the power grid. In some embodiments, the PV system 100 may be a centralized system for medium or large PV power generation. In alterative embodiments, the PV system 100 may be implemented as a distributed system for small PV power generation.

The PV system 100 comprises a PV array 110. In the example shown in FIG. 1, the PV array 110 comprises PV cells 111-1, 111-2, 111-3, 111-4, 111-5, 111-6, 111-7 and 111-8 (collectively referred to as “PV cell (s) ” 111) . It is to be understood that the number of the PC cells 111 as shown herein is merely for illustration, without suggesting any limitation as to the scope of the present disclosure. Those PV cells 111 are operable to convert the energy of sunlight directly into electricity by the photovoltaic effect. To increase the output voltage, in some embodiments, the PV cells 111 in the PV array can be arranged in a plurality of rows or strings. Other suitable arrangements of the PV cells 111 are possible as well.

The PV system 100 further comprises an inverter stage 130. The inverter stage 130 is coupled between the PV array 110 and an AC power grid 200. In operation, the inverter stage 130 can convert the DC power generated by the PV array 110 into AC power, so as to provide power for the AC grid 200.

FIG. 2 illustrates details of PV system 100 (the PV array 110 is not shown) in accordance with an embodiment of the present disclosure. As shown, the inverter stage 130 comprises a DC-DC converter 131 and a DC-AC converter 132. These

converters

131 and 132 are cascaded with each other via a bus 138. The DC-DC converter 131 may be implemented by a BOOST circuit and can provide an adjustment to the voltage output by the PV array. For example, the DC-DC converter 131 may achieve the function of Maximum Power Point Tracking (MPPT) by adjusting the DC voltage. The DC-AC converter 132 can be configured to convert the DC voltage on the bus 138 into AC voltage. The DC-DC convert 131 and the DC-AC convert 132 may be implemented with power semiconductor devices such as metal-oxide-semiconductor field effect transistor (MOSFET) , insulated-gate bipolar transistors (IGBTs) , integrated gate-commutated thyristors (IGCTs) , gate turn-off thyristors (GTOs) , MOS-controlled thyristors (MCTs) , and like.

The PV system 100 further comprises electrical switches 121. In some embodiments, the electrical switches 121 may be DC switches configured to connect or disconnect the inverter stage 136 (in particular, the DC-DC converter 131) with the PV array 110. In performing the rapid shutdown (RSD) operation, the switches 121 are capable of breaking the electrical connection of the PV array 110 to the inverter stage 130, thereby disconnecting the PV array 110 form the AC power grid 200 in an efficient manner. For example, in some embodiments, at least some of the electrical switches 121 may be implemented by circuit breakers with high current breaking capability. In this way, the electrical switches 121 can break the output current of the PV array under a high current. Of course, other types of switches are also possible.

The electrical switches 122 are operable to connect or disconnect the PV inverter stage 130 (in particular, the DC-AC converter 132) to or from the AC power grid 200. In performing the rapid shutdown operation, the switches 122 are capable of disconnecting the electrical connection of the AC power grid 200 to the entire PV system 100. The electrical switches 122 may be implemented by relays and/or any other suitable types of devices.

As shown in FIG. 1, the PV system 100 also comprises a controller 150 which is configured to control the operations/functionalities of the PV system 100. For example, in operation, the controller 150 can be configured to receive and process the sensed voltage and current signals and to control operations of the DC-DC converter 131, the DC-AC converter 132 and the

electrical switches

121 and 122.

The controller 150 may detect the fault in the photovoltaic system 100 and/or the AC power grid 200. To this end, in some embodiments, the controller 150 may receive measurements of the voltage and/or current. Example of such measurements include, but are not limited to, the DC voltage or current readings of a DC input side 139 of the inverter stage 130, the bus voltage readings of the bus 138, the voltage or current readings of the AC output side of the inverter stage 130, or the grid voltage readings. Based on the received measurements, the controller 150 determines whether the PV system 100 and the power grid 200 are faulty. Alternatively, or in addition, in some embodiments, the controller 150 may also receive a fault signal from other devices or receive a fault signal from a remote device.

If a fault is detected in the photovoltaic system 100 or the AC power grid 200, the PV system 100 will perform the RSD operation to electrically separate the PV cells with from the power grid. To this end, the controller 150 sends a disconnecting signal to the

electrical switches

121, 122 to disconnect the inverter stage 130 from the AC power grid 200 and the photovoltaic array 110, respectively. The disconnecting signal can be sent in a wired and/or wireless manner, such that the PV array 110 is disconnected from the inverter stage 130 and the inverter stage 130 is disconnected from the AC power grid 200.

In response to the detected fault, the controller 150 is further configured to control the DC-DC converter 131 to convert a DC voltage on a DC input side 139 of the inverter stage 130 into a DC voltage on the bus 138. For example, during the RSD operation, the inverter stage 130 may enter the rapid shutdown (RSD) mode. In this RSD mode, the DC-DC converter 131 of the inverter stage 130 is operated to convert the DC voltage on the DC input side 139 of the inverter stage 130 to the DC voltage on the bus between the DC-DC converter 131 and the DC-AC converter 132. For example, the DC-DC converter 131 can perform DC voltage conversion in the pulse width modulation (PWM) manner.

In this way, the remaining power on the DC input side 139 of the inverter stage 130 will be quickly transferred to the bus 138, rather than being dissipated by the conventional discharge circuit or dissipated by the electrical elements on the DC input side 139. Due to the transfer of the energy, the DC voltage on the DC input side 139 can be much more rapidly decreased.

In some embodiments, the bus 138 is configured to provide power the auxiliary devices of the PV system 100. For example, there may be several auxiliary devices or circuits in the PV system 100, such as control circuits for the PV system 100, which require DC power supply to power them. In this situation, the bus 138 may be electrically connected with the auxiliary devices in the PV system 100 to feed the power on the bus 138 to these auxiliary devices or circuits. In this way, the power transferred from the DC input side 139 to the bus 138 during the RSD can be consumed more quickly, which further speeds up the voltage drop on the DC input side 139. Moreover, such an approach also allows the remaining electrical power on the DC input side 139 to be effectively utilized without being dissipated in vain.

In some embodiments, the controller 150 may be configured to control the DC-DC converter 131 in the following way. First, the controller 150 may determine a DC voltage drop in the inverter stage 130 over a predetermined period of time. Only by way of example, the period of time may be of a length of 3 seconds. Any other suitable time length is possible as well.

In particular, when the PV system 100 is in rapid shutdown, the DC voltage on the DC input side 139 and the bus 138 will drop significantly. Therefore, the voltage on the DC input side 139 or the bus 138 can be used as a basis for determining whether the PV system 100 is in rapid shutdown. In some embodiments, a voltage drop on the DC input side 139 over the predetermined period of time can be detected as the DC voltage drop in the inverter stage 130. Alternatively, or in addition, the controller 150 may sense the voltage drop on the bus 138 over the predetermined period of time as the DC voltage drop in the inverter stage 130.

However, since the output voltage on the PV array 110 is affected by the solar radiation, the output voltage generally has large fluctuations, which in turn affects the DC voltage on the DC input side 139 of the inverter stage 130. Moreover, the DC voltage on the bus 138 is relatively stable, and sometimes it is not possible to respond to the RSD in time. Therefore, if it is judged that the PV system 100 is in the rapid shutdown only based on the DC voltage of the DC input side 139 or the bus 138, an erroneous judgment or a missing judgment may occur. In some embodiments, both of the DC voltage on the DC input side 139 and the DC voltage on the bus 138 may be used as a basis for determining the RSD state of PV system 100, and a more accurate judgment can be obtained.

Then the controller 150 may compare the drop of DC voltage in this period of time with a predetermined threshold. If the DC voltage drop exceeds the threshold, it can be determined that the PV system 100 has entered the RSD state. In some embodiments, the controller 150 may compare the DC voltage drop on the DC input side 139 with a first threshold. Alternatively, the controller 150 may compare the DC voltage drop on the bus 138 with a second threshold. In addition, the controller may simultaneously perform a comparison of the DC voltage on the DC input side 139 and a comparison of the DC voltage on the bus 138. The first threshold and the second threshold may be the same, or may be different from each other.

Accordingly, the controller 150 may control the DC-DC converter 131 to convert the DC voltage on the DC input side 139 of the inverter stage 130 into the DC voltage on the bus 138. That is, the inverter stage 130 is caused to enter the RSD mode. By controlling the inverter stage 130 based on the detected DC voltage drop thereof, the controller 150 can control entry into the RSD mode upon a more accurate basis.

In some embodiments, after the inverter stage 130 enters the RSD mode, the controller 150 may perform further operations to determine whether the task of decreasing the DC voltage is completed. For example, the controller 150 obtains or monitors the DC voltage on the DC input side 139, and compares the DC voltage on the DC input side 139 with a safe voltage. If the DC voltage on the DC input side 139 falls below a safe voltage, the controller 150 controls the DC-DC converter 131 to cease converting the DC voltage on the DC input side 139 into the DC voltage on the bus 138. That is, in the case that the voltage on the DC input side 139 has fallen below the safe voltage, the inverter stage 130 may exit the RSD mode, and then, for example, both of the DC-DC and DC-

AC converters

131 and 132 of the inverter stage 130 stops operating.

In some embodiments, after the inverter stage 130 enters the RSD mode, the controller 150 may perform further operations to determine whether the fault is eliminated and whether the PV system is still in the RSD state. For example, the controller 150 obtains or monitors the DC voltage on the bus 138, and compares the DC voltage on the bus 138 with a setting voltage. If the DC voltage on the bus 138 comes back the setting voltage, the controller control the inverter stage 130 to perform normal operations corresponding to operations before the fault was detected. That is, in the case that the DC voltage on the bus 138 rises back and reach a certain setting voltage, it indicates that the fault has been eliminated and the PV array 110 and AC power grid 200 have been reconnected. Then, the inverter stage 130 may exit the RSD mode, and may resume the normal operations (for example, both the DC-DC converter 131 and the DC-AC converter 132 is operated to convert the DC power generated by the PV array 110 to the AC power provided to the AC power grid 200) , which are the operations performed before the failure.

Furthermore, as shown in FIG. 2, the controller 150 comprises

control circuits

151 and 152. For example, the control circuit 151 may include a DC-DC DSP for the DC-DC converter 131 and a DC-AC DSP for the DC-AC converter 132, and the control circuit 152 may include a microprocessor and a communication platform, wherein the communication platform may communicates with the remote device in a wired and/or wireless manner to transmit or receive status information or instructions, and the microprocessor may further process the information from the control circuit 151 and the remote device.

As shown, the inverter stage 130 further comprises bulk capacitors 133. The bulk capacitors 133 are connected between the positive and negative lines of the DC bus 138. The bulk capacitors 133 can regulate the flow of energy between the DC-DC circuit 131 and the DC-AC circuit 132, and stabilize the DC voltage on the bus 138. The electrical power stored in the bulk capacitors 133 may be provided to auxiliary devices of the PV system 100, for example,

control circuits

151 and 152.

Furthermore, the PV system 100 may comprise a grid leakage current detector 135, current/voltage sensing devices (not illustrated in the figure) and a line filter 134. The voltage/current sensing devices can sense the voltage and current required to control the

converters

131 and 132 and the

electrical switches

121 and 122. For example, the voltage/current sensing device may provide the DC voltage or current readings of the DC input side 139 of the inverter stage 130, the bus voltage readings of the bus 138, the voltage or current readings of the AC output side of the inverter stage 130, and the grid voltage readings. Based on this, a fault in the PV system and the AC power grid can be monitored, and the RSD operation can be performed in a timely manner in the event of the fault. In some embodiments, the electrical switches 122 described above, the grid leakage current detector 135, current/voltage sensing devices and a line filter 134 may be integrated with the DC-DC converter 131 and DC-AC converter 132 to form a single inverter apparatus. In alterative embodiments, the electrical switches 122, the grid leakage current detector 135, current/voltage sensing devices and a line filter 134 may be formed separately from the DC-DC converter 131 and DC-AC converter 132.

Embodiments of the present disclosure are capable of achieving multiple advantages over conventional approaches.

There are some circuits including capacitors, such as a DC EMI filter 140 (as shown in FIG. 2) , on the DC input side of the inverter stage 130. Due to the capacitors on the DC input side 139 of the inverter stage 130, when the rapid shutdown (RSD) operation is performed, even if the DC switches 121 disconnects the electrical connection of the PV array 110 to the input side 139 of the inverter stage 130, it is also often difficult to quickly drop the voltage of the input side of the inverter stage 130 to a safe voltage.

The conventional approach is to dispose a discharge circuit on the DC input side of the inverter stage. The discharge circuit generally includes resistors, through which the remaining energy in the capacitors is dissipated, thereby causing the voltage on the DC input side to drop to a safe value. Moreover, in the conventional PV system, the DC-DC and DC-AC converters of the inverter stage will stop working once the system is performing RSD operation. That is, the electrical energy flow from the DC input side to the AC output side of the invert stage will be interrupted during the RSD.

In contrast, according the embodiments of the present disclosure, when the PV system 100 is in the rapid shutdown, the DC-DC converter 131 of the inverter stage 130 performs a voltage conversion operation, which causes the power on the DC input side 139 to be transferred to the bus 138, thereby rapidly decreasing the voltage on the DC input side 139. In addition, the power on the bus 138 is not wasted, but is provided to the auxiliary devices or circuits in the system as the power supply to assist the auxiliary devices or circuits. In such way, the DC side voltage of the PV system can be decreased below the safe voltage within a predefined time to comply with NEC 690.12, and the efficiency of the inverter is effectively improved.

FIG. 3 illustrates a flowchart of a control method 300 for the PV system 100. The method 300 may be implemented by the controller 150 as described above.

At block 301, the controller 150 detects the fault in the photovoltaic system 100 or the AC power grid 200.

At block 302, in response to the fault in the photovoltaic system 100 or the AC power grid 200 is detected, the controller 150 sends a disconnecting signal to a plurality of

electrical switches

121 and 122 to disconnect the inverter stage 130 of the photovoltaic system 100 from the AC power grid 200 and the photovoltaic array 110, respectively, wherein the inverter stage 130 comprises the DC-DC converter 131 and the DC-AC converter 132 that are cascaded with each other via the bus 138 and is coupled between the photovoltaic array 110 and the AC power grid 200.

At block 303, the controller 150 controls the DC-DC converter 131 to convert the DC voltage on the DC input side 139 of the inverter stage 130 into the DC voltage on the bus 138.

In some embodiments, controlling the DC-DC converter 131 to convert the DC voltage on the DC input side 139 of the inverter stage 130 into the DC voltage on the bus 138 comprises: determining a DC voltage drop in the inverter stage 130 over a predetermined period of time, and in response to the DC voltage drop exceeding a threshold, controlling the DC-DC converter 131 to convert the DC voltage on the DC input side (139) of the inverter stage 130 into the DC voltage on the bus 138.

In some embodiments, determining the DC voltage drop of the inverter stage 130 over the predetermined period of time comprises at least one of: determining the voltage drop on the DC input side 139 over the predetermined period of time or determining the voltage drop on the bus 138 over the predetermined period of time.

In some embodiments, controlling the DC-DC converter 131 to convert the DC voltage on the DC input side 139 of the inverter stage 130 into the DC voltage on the bus 138 in response to at least one of: the voltage drop on the DC input side 139 exceeding a first threshold, or the voltage drop on the bus 138 exceeding a second threshold.

FIG. 4 illustrates a flowchart of a further control method 400 for the method 300 for the PV system 100.

At 401, in response to the conversion of the DC voltage on the DC input side 139 of the inverter stage 130 into the DC voltage on the bus 138, the controller 150 obtains the DC voltage on the DC input side 139.

At 402, the controller 150 determines whether the DC voltage of the DC input side 139 falling below the safe voltage.

At 403, in response to the DC voltage of the DC input side 139 falling below the safe voltage, the controller 150 controls the DC-DC converter 131 to cease converting the DC voltage on the DC input side 139 into the DC voltage on the bus 138.

FIG. 5 illustrates a flowchart of a further control method 500 for the method 300 for the PV system 100.

At 501, in response to the conversion of the DC voltage on the DC input side 139 of the inverter stage 130 into the DC voltage on the bus 138, the controller 150 obtains the DC voltage on the bus 138.

At 502, the controller 150 determines whether the DC voltage on the bus 138 exceeding a setting voltage.

At 503, in response to the DC voltage on the bus 138 exceeding the setting voltage, the controller 150 controls the inverter stage 130 to perform normal operations corresponding to operations before the fault was detected.

While several inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the inventive embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure.

Claims

A photovoltaic system (100) comprising:

a photovoltaic array (110) ;

an inverter stage (130) comprising a DC-DC converter (131) and a DC-AC converter (132) that are cascaded with each other via a bus (138) and coupled between the photovoltaic array (110) and an AC power grid (200) ;

a plurality of electrical switches (121, 122) ; and

a controller (150) configured to:

in response to detecting a fault in the photovoltaic system (100) or the AC power grid (200) , send a disconnecting signal to the plurality of electrical switches (121, 122) to disconnect the inverter stage (130) from the AC power grid (200) and the photovoltaic array (110) , respectively; and

control the DC-DC converter (131) to convert a DC voltage on a DC input side (139) of the inverter stage (130) into a DC voltage on the bus (138) .
The photovoltaic system (100) of claim 1, wherein the controller (150) is configured to control the DC-DC converter (131) by:

determining a DC voltage drop in the inverter stage (130) over a predetermined period of time; and

in response to the DC voltage drop exceeding a threshold, controlling the DC-DC converter (131) to convert the DC voltage on the DC input side (139) of the inverter stage (130) into the DC voltage on the bus (138) .
The photovoltaic system (100) of claim 2, wherein the DC voltage drop in the inverter stage (130) over the predetermined period of time is determined based on at least one of the following:

a voltage drop on the DC input side (139) over the predetermined period of time, or

a voltage drop on the bus (138) over the predetermined period of time.
The photovoltaic system (100) of claim 3, wherein the controller (150) is configured to control the DC-DC converter (131) to convert the DC voltage on the DC input side (139) of the inverter stage (130) into the DC voltage on the bus (138) in response to at least one of the following:

the voltage drop on the DC input side (139) exceeding a first threshold, or

the voltage drop on the bus (138) exceeding a second threshold.
The photovoltaic system (100) of any of claims 1-4, wherein the controller (150) is further configured to:

in response to the conversion of the DC voltage on the DC input side (139) of the inverter stage (130) into the DC voltage on the bus (138) , obtain the DC voltage on the DC input side (139) , and

in response to the DC voltage on the DC input side (139) falling below a first predetermined voltage, control the DC-DC converter (131) to cease converting the DC voltage on the DC input side (139) into the DC voltage on the bus (138) .
The photovoltaic system (100) of any of claims 1-4, wherein the controller (150) is further configured to:

in response to the conversion of the DC voltage on the DC input side (139) of the inverter stage (130) into the DC voltage on the bus (138) , obtain the DC voltage on the bus (138) , and

in response to the DC voltage on the bus (138) exceeding a second predetermined voltage, control the inverter stage (130) to perform normal operations corresponding to operations before the fault was detected.
The photovoltaic system (100) of any of claims 1-4, wherein the bus is configured to provide power to auxiliary devices of the photovoltaic system (100) .
The photovoltaic system of any of claims 1-4, wherein the plurality of electrical switches (121, 122) comprises:

a first plurality of switches (121) configured to connect or disconnect the photovoltaic array (110) with the inverter stage (130) ; and

a second plurality of switches (122) configured to connect or disconnect the inverter stage (130) with the AC power grid (200) .
A control method for a photovoltaic system (100) , comprising:

in response to detecting a fault in a photovoltaic system (100) or a AC power grid (200) , sending a disconnecting signal to a plurality of electrical switches (121, 122) to disconnect an inverter stage (130) of the photovoltaic system (100) from the AC power grid (200) and a photovoltaic array (110) , respectively, wherein the inverter stage (130) comprises a DC-DC converter (131) and a DC-AC converter (132) that are cascaded with each other via a bus (138) and is coupled between the photovoltaic array (110) and the AC power grid (200) ; and

controlling the DC-DC converter (131) to convert a DC voltage on a DC input side (139) of the inverter stage (130) into a DC voltage on the bus (138) .
The control method of claim 9, wherein controlling the DC-DC converter (131) to convert the DC voltage on the DC input side (139) of the inverter stage (130) into the DC voltage on the bus (138) comprise:

determining a DC voltage drop in the inverter stage (130) over a predetermined period of time; and

in response to the DC voltage drop exceeding a threshold, controlling the DC-DC converter (131) to convert the DC voltage on the DC input side (139) of the inverter stage (130) into the DC voltage on the bus (138) .
The control method of claim 10, wherein determining the DC voltage drop of the inverter stage (130) over the predetermined period of time comprises at least one of:

determining a voltage drop on the DC input side (139) over the predetermined period of time, or

determining a voltage drop on the bus (138) over the predetermined period of time.
The control method of claim 11, wherein controlling the DC-DC converter (131) to convert the DC voltage on the DC input side (139) of the inverter stage (130) into the DC voltage on the bus (138) in response to at least one of the following:

the voltage drop on the DC input side (139) exceeding a first threshold, or

the voltage drop on the bus (138) exceeding a second threshold.
The control method of any of claims 9-12, further comprising:

in response to the conversion of the DC voltage on the DC input side (139) of the inverter stage (130) into the DC voltage on the bus (138) , obtaining the DC voltage on the DC input side (139) , and

in response to the DC voltage of the DC input side (139) falling below a first predetermined voltage, controlling the DC-DC converter (131) to cease converting the DC voltage on the DC input side (139) into the DC voltage on the bus (138) .
The control method of any of claims 9-12, further comprising:

in response to the conversion of the DC voltage on the DC input side (139) of the inverter stage (130) into the DC voltage on the bus (138) , obtaining the DC voltage on the bus (138) , and

in response to the DC voltage on the bus (138) exceeding a second predetermined voltage, controlling the inverter stage (130) to perform normal operations corresponding to operations before the fault was detected.