Photovoltaic surplus power utilization method and photovoltaic surplus power utilization system

Abstract

The present invention relates to a photovoltaic surplus power utilization method and a photovoltaic surplus power utilization system, which are applied to a photovoltaic system having at least two inverters. The photovoltaic system operates in a grid-connected mode or a grid-disconnected mode. An output end of each inverter is connected to an alternating-current coupling point. A controller measures the power or voltage at the alternating-current coupling point and determines, on the basis of the power or the voltage, whether there is surplus power at the alternating-current coupling point. When it is detected that there is surplus power, the power at the alternating-current coupling point is distributed to an inverter which has no surplus power. When all the inverters reach a maximum-power operation state, grid-connected delivery processing or grid-disconnected step-down processing is performed on the power at the alternating-current coupling point. The present application can achieve the efficient utilization of photovoltaic surplus power, and also can avoid the impact of communication latency on photovoltaic power supply control, thereby improving the timeliness and accuracy of photovoltaic power control, such that photovoltaic energy can be utilized to the maximum extent.

Description

FIELD

[0001] The present disclosure relates to the field of photovoltaic control technologies, and more particularly, to a photovoltaic surplus power utilization method and a photovoltaic surplus power utilization system. BACKGROUND

[0002] In a scenario of multi-machine parallel cooperative power supply for renewable new energy power generation devices, it is necessary to abstract a central controller to manage the devices uniformly. In a management process, two aspects of requirements need to be basically satisfied: on the one hand, realizing a response to a basic requirement such as a power grid dispatching requirement and a local load power supply requirement; on the other hand, realizing maximized utilization of renewable energy, and improving an economic efficiency and environmental friendliness of a product.

[0003] Currently, most power supply control methods for a multi-machine parallel scenario are to select one device as a master machine and the rest of the devices as slave machines. The master machine comprehensively determines whether a current unit can increase or decrease an output based on photovoltaic control status, energy storage operation status, and meter power feedback reported by the slave machines, and outputs a current-sharing power command to realize unified dispatching of a device thereof and other units. However, due to intermittency, volatility, and unpredictability of photovoltaic power generation, and a communication delay existing in all links of real-time interaction of key information, the master machine is difficult to accurately evaluate a green power output capacity of a current unit, which may lead to problems such as impaired performance and photovoltaic curtailment during control.

[0004] In addition, for full application of photovoltaic power, current common control strategies are regulated based on the premise that a maximum photovoltaic power is known. However, a maximum photovoltaic power can be more accurately known only when an MPPT control reaches a maximum power point. Therefore, it is difficult to obtain an accurate maximum photovoltaic power from both the physical level and the algorithm level, which may lead to a non-negligible error in a control strategy using the accurate maximum photovoltaic power as a key parameter. On the other hand, a multi-machine parallel control strategy in an off-grid mode usually takes voltage and phase consistency among the slave machines as a control target to uniformly dispatch the power. However, transfer of the surplus power of the renewable energy among the devices is ignored, making it difficult to realize efficient utilization of the photovoltaic power. SUMMARY

[0005] A main objective of the present disclosure is to overcome at least one of the aforementioned drawbacks and to provide a photovoltaic surplus power utilization method and a photovoltaic surplus power utilization system, which is capable of achieving high-efficiency utilization of photovoltaic surplus power. Also, an impact of a communication delay on photovoltaic power supply control can be avoided, and real-time performance and an accuracy of the photovoltaic power control can be improved. In this way, photovoltaic energy can be maximized.

[0006] To achieve the above objectives, the technical solution adopted by the present disclosure is as follows.

[0007] The present disclosure provides a photovoltaic surplus power utilization method, applied to a photovoltaic system including at least two inverters. The photovoltaic system operates in a grid-connected mode. An output terminal of each of the at least two inverters is connected to an alternating current coupling point. A controller is configured to: detect power at the alternating current coupling point; determine whether a power surplus exists at the alternating current coupling point based on the detected power; allocate, when the power surplus is detected, the power at the alternating current coupling point to an inverter without surplus power; and perform, when each of the at least two inverters reaches a maximum power operating state, grid-connected export on the power at the alternating current coupling point.

[0008] According to an embodiment of the present disclosure, each of the at least two inverters is configured to calculate power thereof and transmit a power parameter to the controller. The controller is configured to: determine, based on the power parameter fed back by each of the at least two inverters, whether each of the at least two inverters has the surplus power or reaches the maximum power operating state; and adjust and control the operation of the alternating current coupling point and each of the at least two inverters in combination with a surplus power condition at the alternating current coupling point.

[0009] According to an embodiment of the present disclosure, the controller is configured to: determine, based on the power parameter fed back by each of the at least two inverters, whether each of the at least two inverters is an inverter generating the surplus power or an inverter generating no surplus power, where the inverter generating the surplus power is designated as a first inverter, and the inverter generating no surplus power is designated as a second inverter; and allocate the power at the alternating current coupling point to the second inverter when the surplus power is detected at the alternating current coupling point.

[0010] According to an embodiment of the present disclosure, said allocating the power at the alternating current coupling point to the second inverter includes: adjusting the power delivered from the alternating current coupling point to the second inverter, and increasing the absorbed power of the second inverter.

[0011] According to an embodiment of the present disclosure, the controller is configured to: when each of the at least two inverters reaches the maximum power operating state, compare the power at the alternating current coupling point with a grid connection threshold PBREF; and when the power at the alternating current coupling point is not smaller than the grid connection threshold PBREF, control the alternating current coupling point to be grid-connected, and regulate grid-connected output power of the alternating current coupling point.

[0012] According to an embodiment of the present disclosure, the grid connection threshold PBREF is not smaller than a maximum grid connection power limit value configured for the photovoltaic system, and the grid-connected output power of the alternating current coupling point is equal to the maximum grid connection power limit value.

[0013] According to an embodiment of the present disclosure, when the maximum grid connection power limit value Plimit configured for the photovoltaic system is smaller than k% of a total capacity of the at least two inverters in the photovoltaic system, during initialization of the method, the grid connection threshold PBREF is set to k% of the total capacity of the at least two inverters, and a power monitoring threshold PAREF for determining whether the surplus power exists at the alternating current coupling point is set to zero, where k is a coefficient ranging from 1 to 100. When the power at the alternating current coupling point is not smaller than the grid connection threshold PBREF, the grid connection threshold is lowered to P’BREF, where P’BREF=Plimit, and the power monitoring threshold is simultaneously lowered to P’AREF, where P’AREF=Plimit-PBREF.

[0014] According to an embodiment of the present disclosure, when the maximum grid connection power limit value Plimit configured for the photovoltaic system is smaller than k% of a total capacity of the at least two inverters in the photovoltaic system, during initialization of the method, the grid connection threshold PBREF is set to k% of the total capacity of the at least two inverters, and a power monitoring threshold PAREF for determining whether the surplus power exists at the alternating current coupling point is set to zero, where k is a coefficient ranging from 0 to 100. When the power at the alternating current coupling point is not smaller than the grid connection threshold PBREF, the grid connection threshold is lowered to P’BREF, where P’BREF=Plimit, and the power monitoring threshold is simultaneously lowered to P’AREF, where P’AREF=Plimit-PBREF.

[0015] According to an embodiment of the present disclosure, said each of the at least two inverters reaching the maximum power operating state includes: first, each of the at least two inverters being in a state of generating the surplus power; and second, a part of the at least two inverters being in the state of generating the surplus power, and the remaining inverters of the at least two inverters being in an energy absorption saturation state.

[0016] According to an embodiment of the present disclosure, the controller is separately arranged or integrated into a control module of one of the at least two inverters.

[0017] According to an embodiment of the present disclosure, the controller is separately arranged in a gateway or an electrical box.

[0018] In an exemplary embodiment, the present disclosure further provides a photovoltaic surplus power utilization method, applied to a photovoltaic system including at least two inverters. The photovoltaic system operates in an off-grid mode. An output terminal of each of the at least two inverters is connected to an alternating current coupling point. A controller is configured to: detect a voltage of the alternating current coupling point; determine whether regulation is required based on a magnitude of the voltage of the alternating current coupling point; process, when regulation is performed, power at the alternating current coupling point to an inverter without surplus power; and perform, when each of the at least two inverters reaches a maximum power operating state, voltage reduction or limiting processing on the voltage at the alternating current coupling point.

[0019] According to an embodiment of the present disclosure, each of the at least two inverters is configured to calculate power thereof and transmit a power parameter to the controller. The controller is configured to: determine, based on the power parameter fed back by each of the at least two inverters, whether each of the at least two inverters has the surplus power or reaches the maximum power operating state; and adjust and control the operation of the alternating current coupling point and each of the at least two inverters in combination with a surplus power condition at the alternating current coupling point.

[0020] According to an embodiment of the present disclosure, the controller is configured to: determine, based on the power parameter fed back by each of the at least two inverters, whether each of the at least two inverters is an inverter generating the surplus power or an inverter generating no surplus power, wherein the inverter generating the surplus power is designated as a first inverter, and the inverter generating no surplus power is designated as a second inverter; and adjust, when the detected voltage of the alternating current coupling point is greater than a predetermined voltage threshold, an output voltage of the first inverter and an output voltage of the second inverter to adapt to a voltage change of the alternating current coupling point.

[0021] According to an embodiment of the present disclosure, when each of the at least two inverters reaches the maximum power operating state, voltage reduction or limiting processing is performed on the voltage at the alternating current coupling point, to enable the voltage at the alternating current coupling point to be within an allowable range.

[0022] According to an embodiment of the present disclosure, the method is provided with a first voltage threshold VAREF and a second voltage threshold VBREF, the first voltage threshold VAREF being smaller than the second voltage threshold VBREF. When the voltage of the alternating current coupling point is not smaller than the first voltage threshold VAREF, the power at the alternating current coupling point is allocated to the inverter without the surplus power by processing a voltage circulating current among the at least two inverters. When the voltage of the alternating current coupling point is not smaller than the second voltage threshold VBREF, the power at the alternating current coupling point is processed and allocated to the inverter without the surplus power, and the voltage reduction or limiting processing is simultaneously performed on the voltage at the alternating current coupling point.

[0023] According to an embodiment of the present disclosure, the each of the at least two inverters reaching the maximum power operating state includes: first, each of the at least two inverters being in a state of generating the surplus power; and second, a part of the at least two inverters being in the state of generating the surplus power, and the remaining inverters of the at least two inverters being in an energy absorption saturation state.

[0024] According to an embodiment of the present disclosure, the controller is separately arranged or integrated into a control module of one of the at least two inverters.

[0025] According to an embodiment of the present disclosure, the controller is separately arranged in a gateway or an electrical box.

[0026] In an exemplary embodiment, the present disclosure provides a photovoltaic surplus power utilization system, applied to a photovoltaic system including at least two inverters. The photovoltaic system operates in a grid-connected mode. An output terminal of each of the at least two inverters is connected to an alternating current coupling point. The photovoltaic surplus power utilization system includes: a power detection unit configured to detect power at the alternating current coupling point and determine whether surplus power exists at the alternating current coupling point based on the detected power; an inverter state detection unit configured to determine whether each of the at least two inverters has the surplus power or reaches a maximum power operating state based on a power parameter fed back by each of the at least two inverters; a power allocation unit configured to allocate the power at the alternating current coupling point to an inverter without the surplus power when the surplus power is detected; and a grid-connected export unit configured to perform, when each of the at least two inverters reaches the maximum power operating state, grid-connected export of the power at the alternating current coupling point.

[0027] In an exemplary embodiment, the present disclosure provides a photovoltaic surplus power utilization system, applied to a photovoltaic system including at least two inverters. The photovoltaic system operates in an off-grid mode. An output terminal of each of the at least two inverters is connected to an alternating current coupling point. The photovoltaic surplus power utilization system includes: a voltage detection unit configured to detect a voltage of the alternating current coupling point and determine whether the voltage of the alternating current coupling point exceeds a predetermined voltage threshold based on the detected voltage; an inverter state detection unit configured to determine whether each of the at least two inverters has surplus power or reaches a maximum power operating state based on a power parameter fed back by each of the at least two inverters; an average current processing unit configured to process power at the alternating current coupling point to an inverter without the surplus power when the voltage of the alternating current coupling point exceeds the voltage threshold; and a voltage regulation unit configured to perform, when each of the at least two inverters reaches the maximum power operating state, voltage reduction processing on the voltage at the alternating current coupling point.

[0028] Compared with the related art, advantages and advantageous effects of the photovoltaic surplus power utilization method of the present disclosure are as follows.

[0029] With the photovoltaic surplus power utilization method and the photovoltaic surplus power utilization system of the present disclosure, the photovoltaic surplus power is detected by having each inverter evaluate an electricity quantity the inverter generates. The controller is configured to detect the power or the voltage at the alternating current coupling point, and perform power or voltage regulation based on a detected result. Throughout the surplus power utilization control, the controller only needs to monitor and control the alternating current coupling point, and regulation of the inverter needs to be adjusted only according to an instruction of the controller. In this way, overall control can be simpler, and energy transfer of the photovoltaic surplus power among the inverters can be realized, maximizing utilization of renewable energy. Further, in the grid-connected mode, zero-power grid connection or grid connection at the maximum power limit value can be realized, ensuring that the grid connection power at which a power grid operates is adjustable after grid connection, and guaranteeing compliance and controllability of grid-connected power output.

[0030] Further, by providing a control module in each inverter, detection of the photovoltaic surplus power is “localized”, realizing timely reporting of the inverter’s own photovoltaic power generation status. Unlike traditional photovoltaic surplus power estimation performed in the controller, this approach can avoid an adverse effect caused by the intermittency, volatility, and unpredictability of photovoltaic power generation. Also, the reporting is real-time, which can avoid an adverse effect of a communication delay during real-time interaction of key information. Therefore, the controller of the present disclosure can accurately assess a current green power output capacity of each inverter, improving real-time performance and an accuracy of photovoltaic power control, and further maximizing utilization of photovoltaic energy. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Some specific embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings in an exemplary and non-limiting manner. The same reference numerals in the accompanying drawings denote the same or similar components or parts. It should be understood by those skilled in the art that these accompanying drawings are not necessarily drawn to scale. In the accompanying drawings:

[0032] FIG. 1 is a schematic diagram of a control flow of a photovoltaic surplus power utilization method according to an embodiment of the present disclosure, where a photovoltaic system is in a grid-connected mode;

[0033] FIG. 2 is a schematic diagram of a control logic of a grid connection threshold according to an embodiment of the present disclosure;

[0034] FIG. 3 is a schematic diagram of a control flow of a photovoltaic surplus power utilization method according to another embodiment of the present disclosure, where the photovoltaic system is in an off-grid mode;

[0035] FIG. 4 is a schematic structural diagram of a photovoltaic surplus power utilization system according to an embodiment of the present disclosure, where the photovoltaic system is in a grid-connected mode; and

[0036] FIG. 5 is a schematic structural diagram of a photovoltaic surplus power utilization system according to another embodiment of the present disclosure, where the photovoltaic system is in an off-grid mode. DETAILED DESCRIPTION

[0037] Technical solutions in the present disclosure will be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are a part of the embodiments of the present disclosure, but not all the embodiments. Based on the embodiments in the present disclosure, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present disclosure.

[0038] In addition, the technical features involved in the different embodiments of the present disclosure described below can be combined with each other as long as the technical features do not conflict.

[0039] First Embodiment

[0040] This embodiment describes a photovoltaic surplus power utilization method, applied to a photovoltaic system including at least two inverters. In this embodiment, the photovoltaic system operates in a grid-connected mode. Here, the grid-connected mode refers to a power grid and an inverter performing grid-connected operation, rather than grid-connected power output. Whether a surplus electric quantity generated by photovoltaics is delivered to the power grid (i.e., grid-connected power output) is determined according to the final output of all inverters.

[0041] An output terminal of each inverter is connected to an alternating current coupling point. The controller is configured to: detect power at the alternating current coupling point; determine whether a power surplus exists at the alternating current coupling point based on the detected power; allocate, when the power surplus is detected, the power at the alternating current coupling point to an inverter without surplus power; and perform grid-connected export on the power at the alternating current coupling point through the surplus power utilization method according to the embodiments of the present disclosure.

[0042] First, a concept of the surplus power is explained. The surplus power refers to a power surplus remaining after the device meets a demand of a local load and a charging demand of an energy storage battery. For example, the surplus power at the inverter means a photovoltaic power surplus still remaining for an inverter serving as a photovoltaic-energy storage device after the inverter meets a local load demand thereof and the charging demand of the energy storage battery connected to the inverter.

[0043] It should be understood that whether the power surplus exists in the photovoltaic system can be detected through detection on the alternating current coupling point in the present disclosure. If the power surplus exists, it indicates that there is an inverter outputting the surplus power.

[0044] Detection of the photovoltaic surplus power of each inverter is performed separately in each inverter. Each inverter is configured to calculate power thereof and transmit a power parameter to the controller. The controller is configured to: determine, based on the power parameter fed back by each of the at least two inverters, an operating state of each inverter; and adjust and control the operation of the alternating current coupling point and each of the at least two inverters in combination with a surplus power condition at the alternating current coupling point. That is, photovoltaic surplus power estimation means a “localized” operation in each inverter, so that each inverter can timely report photovoltaic power generation status thereof, unlike a conventional mode where a surplus photovoltaic power is estimated by the controller. In this way, not only an adverse effect caused by intermittency, volatility, and unpredictability of photovoltaic power generation can be avoided, but also real-time reporting can be ensured, to avoid an adverse effect brought by a communication delay and lag of estimation calculation during real-time interaction of key information. Therefore, the controller of the present disclosure can accurately evaluate the current green power output capacity of each inverter, improving real-time performance and an accuracy of photovoltaic power control, and further maximizing utilization of photovoltaic energy.

[0045] A control module is arranged in each inverter. The control module is configured to adjust the power thereof according to an output instruction of the controller. Arranging the control module in each inverter can realize “localized” detection of the photovoltaic surplus power and timely reporting of photovoltaic power generation conditions thereof. Different from the traditional photovoltaic surplus power estimation implemented in the controller, the localized detection can avoid the adverse effect resulting from the intermittency, volatility, and unpredictability of photovoltaic power generation. Also, real-time reporting is realized to avoid the adverse effect caused by the communication delay during real-time interaction of key information. Therefore, the controller of the present disclosure can accurately assess the current green power output capacity of each inverter, improving the real-time performance and accuracy of photovoltaic power control, and thereby maximizing the utilization of photovoltaic energy.

[0046] In the process of adjusting the power thereof according to an output instruction of the controller, the procedure is shown in the flowchart of FIG. 1.

[0047] When the power surplus exists at the alternating current coupling point, the controller is configured to: determine, based on the power parameter fed back by each inverter, whether each inverter is an inverter generating the surplus power or an inverter generating no surplus power, where the inverter generating the surplus power is designated as a first inverter, and the inverter generating no surplus power is designated as a second inverter; and transfer the power at the alternating current coupling point to the second inverter. Definitions of the first inverter and the second inverter are determined according to an operating condition of each inverter and are dynamically variable. For example, with the continuous change in the operating state of the inverter, an inverter that originally generates no surplus power may turn into the inverter generating the surplus power when photovoltaic output power thereof increases, i.e., the inverter changes from “the second inverter” to “the first inverter”. Similarly, as the operating state of the inverter varies, an inverter that originally generates the surplus power may become the inverter generating no surplus power when the photovoltaic output power thereof reduces, i.e., the inverter changes from “the first inverter” to “the second inverter”. This method takes a real-time operating state of each inverter as the criterion when performing surplus power utilization control.

[0048] As illustrated in FIG. 1, when the photovoltaic system operates in the grid-connected mode, the power at the alternating current coupling point is transferred to the second inverter by adjusting the power delivered from the alternating current coupling point to the second inverter, and increasing the absorbed power of the second inverter.

[0049] When the photovoltaic system is in the grid-connected mode, the power at the alternating current coupling point may increase along with an increase of photovoltaic power generation of the photovoltaic system only when all inverters reach a maximum power operating state. Otherwise, as long as one inverter fails to reach the maximum power operating state, the controller may transfer the photovoltaic surplus power to a corresponding inverter for energy absorption. That is, the power of the alternating current coupling point may be controlled to zero. When all inverters reach the maximum power operating state, it means that each inverter reaches a maximum absorption capacity. If the surplus power still exists in this case, the power at the alternating current coupling point cannot be controlled to zero, and strict regulation on the grid connection power needs to be performed in the grid-connected mode.

[0050] The “each of the at least two inverters reaching the maximum power operating state” mentioned above includes: first, each of the at least two inverters being in a state of generating the surplus power; and second, a part of the at least two inverters being in the state of generating the surplus power, and the remaining inverters of the at least two inverters being in an energy absorption saturation state.

[0051] In the grid-connected mode, to guarantee compliance and controllability of grid-connected power output, a grid connection threshold PBREF is set in the controller. With the setting of the grid connection threshold PBREF, that the grid connection power delivered from the alternating current coupling point to the power grid is controllable can be ensured, to avoid an adverse impact on the power grid caused by fluctuations of the grid connection power. The controller is configured to: compare the power at the alternating current coupling point with a grid connection threshold PBREF; and when the power at the alternating current coupling point is not smaller than the grid connection threshold PBREF, perform grid-connected power output at the alternating current coupling point. In this case, the second inverter still maintains power absorption, and the grid connection power for grid-connected power output is kept at a fixed value below the grid connection threshold PBREF. In this way, the grid-connected power output from the alternating current coupling point to the power grid can be stabilized within a normal range, so that the grid-connected output power is compliant and controllable.

[0052] In an embodiment, the grid connection threshold PBREF is not smaller than a maximum grid connection power limit value configured for the photovoltaic system. The grid- connected output is performed at the alternating current coupling point, and the grid-connected output power may be maintained within a range of the maximum grid connection power limit.

[0053] However, due to grid connection condition restrictions imposed by some countries or regions, grid connection needs to involve a specific TOU mode. That is, a permission mechanism for grid connection varies in different time periods. For example, grid connection is prohibited during some off-peak electricity consumption periods, which means the maximum grid connection power limit value is set to zero or a small value. Undoubtedly, a contradiction may occur: when the power of the alternating current coupling point is detected to be greater than zero (indicating existence of the photovoltaic surplus power), power current sharing control needs to be performed to transfer the photovoltaic surplus power to the second inverters. However, the grid connection conditions are also satisfied simultaneously, which may in turn restrict transfer of the photovoltaic surplus power among the respective second inverters. In view of this, conditional optimization is performed for a scenario where the maximum grid connection power limit value of the power grid is relatively small, as shown in FIG. 2.

[0054] When the maximum grid connection power limit value Plimit configured for the photovoltaic system is smaller than k% ( k is a coefficient ranging from 1 to 100, or k is a coefficient ranging from 0 to 100) of a total capacity of all inverters in the photovoltaic system, during initialization of the method, the grid connection threshold PBREF is set to k% of the total capacity of all inverters, and a power monitoring threshold PAREF for determining whether the surplus power exists at the alternating current coupling point is set to zero. In this way, when the surplus power exists at the alternating current coupling point and the power is smaller than the grid connection threshold PBREF, the power at the alternating current coupling point can be continuously absorbed by the second inverter.

[0055] When the power at the alternating current coupling point is not smaller than the grid connection threshold PBREF, the grid connection condition is triggered. However, the maximum grid connection power limit value Plimit is obviously smaller than the grid connection threshold PBREF at this moment. Therefore, when the power at the alternating current coupling point is not smaller than the grid connection threshold PBREF, the grid connection threshold is lowered to P’BREF, where P’BREF=Plimit. In addition, the power monitoring threshold is simultaneously lowered to P’AREF, where P’AREF=Plimit-PBREF. After the grid connection threshold and a power detection threshold are lowered, if the power of the alternating current coupling point is greater than the grid connection threshold and the power detection threshold, the first inverter performs grid-connected power output of the power at the alternating current coupling point, and the second inverter absorbs the power from the alternating current coupling point. The two inverters perform coordinated control on the grid connection power to limit the power to a value not greater than P’BREF.

[0056] In addition, if the maximum grid connection power limit value Plimit is zero, after the above-mentioned grid connection threshold and power detection threshold are lowered, the power at the alternating current coupling point is greater than the grid connection threshold and the power detection threshold. Therefore, the second inverter can still absorb the power from the alternating current coupling point. Also, PI regulation can be performed on the power at the alternating current coupling point to reduce the power at the alternating current coupling point.

[0057] The above-mentioned controller may be separately arranged in a gateway or an electrical box. Each inverter is communicatively connected to the controller. Each inverter is configured to calculate the power thereof and transmit the power parameter to the controller. The controller is configured to determine the operating state of each inverter according to the power parameter fed back by each inverter, and adjust and control the operation of the alternating current coupling point and each inverter in combination with a power surplus condition at the alternating current coupling point.

[0058] The above-mentioned controller may directly adopt a control module of one inverter as a master. A main control function is integrally arranged in the control module of one inverter. All inverters are communicatively connected to one another. The remaining inverters other than the master serve as slaves, to form master-slave control.

[0059] The above two control modes shall fall within the protection scope of the present disclosure. A core focus of the present disclosure lies in realizing transfer and absorption of the photovoltaic surplus power among various inverters, enabling each local inverter to maximize utilization of the photovoltaic power. Also, reliable power output in the grid-connected mode can be realized, and stable operation of the power grid can be ensured. The protection scope shall not be limited by a specific implementation mode adopted for the main control function.

[0060] Each inverter evaluates an electric quantity generated by itself to output a measured actual result of the photovoltaic surplus power, while the controller detects the power at the alternating current coupling point and performs power feedback control. In this way, overall control can be simpler, energy transfer of the photovoltaic surplus power among the inverters can be realized, and renewable energy can be utilized to a maximum extent. In addition, in the grid-connected mode, zero-power grid connection or grid connection according to the maximum grid connection power limit value can be realized, to guarantee compliance of power grid operation after grid connection and controllability of the grid-connected power.

[0061] Second Embodiment

[0062] This embodiment describes a photovoltaic surplus power utilization method, applied to a photovoltaic system including at least two inverters. The photovoltaic system operates in an off-grid mode. In this case, a flow chart of the photovoltaic surplus power utilization method is shown in FIG. 3. An output terminal of each of the at least two inverters is connected to an alternating current coupling point. A controller is configured to: detect a voltage of the alternating current coupling point; determine whether regulation is required based on a magnitude of the voltage of the alternating current coupling point; process and allocate, when regulation is performed, power at the alternating current coupling point to an inverter without surplus power. In this processing procedure, a port voltage circulating current of the first inverter is processed to the second inverter, to realize processing and allocation of the power between the first inverter and the second inverter. However, when all inverters reach a maximum power operating state, voltage reduction or limiting processing is performed on the voltage at the alternating current coupling point.

[0063] The method according to this embodiment is provided with a first voltage threshold VAREF and a second voltage threshold VBREF. The first voltage threshold VAREF is smaller than the second voltage threshold VBREF. When the voltage of the alternating current coupling point is not smaller than the first voltage threshold VAREF, the power at the alternating current coupling point is processed and allocated to the inverter without the surplus power by processing a voltage circulating current among the inverters. When the voltage of the alternating current coupling point is not smaller than the second voltage threshold VBREF, the power at the alternating current coupling point is processed and allocated to the inverter without the surplus power, and the voltage reduction or limiting processing is simultaneously performed on the voltage at the alternating current coupling point.

[0064] Each of the at least two inverters is configured to calculate power thereof and transmit a power parameter to the controller. The controller is configured to: determine, based on the power parameter fed back by each of the at least two inverters, whether each of the at least two inverters has the surplus power or reaches the maximum power operating state; and adjust and control the operation of the alternating current coupling point and each of the at least two inverters in combination with a surplus power condition at the alternating current coupling point. The controller is configured to: determine, based on the power parameter fed back by each of the at least two inverters, whether each of the at least two inverters is an inverter generating the surplus power or an inverter generating no surplus power, where the inverter generating the surplus power is designated as a first inverter, and the inverter generating no surplus power is designated as a second inverter; and adjust, when the detected voltage of the alternating current coupling point is greater than a predetermined voltage threshold VAREF, an output voltage of the first inverter and an output voltage of the second inverter to adapt to a voltage change of the alternating current coupling point. The first inverter boosts an output voltage thereof and transmits the boosted voltage to the alternating current coupling point, so as to further raise the voltage of the alternating current coupling point. However, the second inverter reduces a voltage thereof to lower the voltage of the alternating current coupling point and pull back the voltage of the alternating current coupling point that is boosted by the first inverter. In this way, power transfer from the first inverter to the second inverter can be realized by means of voltage circulating current between the inverters.

[0065] When all inverters reach the maximum power operating state, the voltage of the alternating current coupling point starts to be greater than the predetermined voltage threshold VAREF and gradually approaches the second voltage threshold VBREF. When the voltage of the alternating current coupling point is not smaller than the predetermined voltage threshold VAREF, the power of the alternating current coupling point is processed and allocated to the inverter generating no surplus power, and the voltage of the alternating current coupling point is regulated. The regulation method may adopt existing technologies such as PI regulation, enabling the voltage at the alternating current coupling point to be within an allowable range. That is, the voltage is controlled to be below the second voltage threshold VBREF. Such an allowable range may be determined according to a configuration of the photovoltaic system, and generally refers to an overvoltage and undervoltage range of the photovoltaic system.

[0066] The each of the at least two inverters reaching the maximum power operating state includes: first, each of the at least two inverters being in a state of generating the surplus power; and second, a part of the at least two inverters being in the state of generating the surplus power, and the remaining inverters of the at least two inverters being in an energy absorption saturation state.

[0067] The above-mentioned controller may be separately arranged in the gateway or the electrical box. Each inverter is in communication connection with the controller. Each inverter is configured to calculate the power thereof and transmit the power parameter to the controller. The controller is configured to determine the operating state of each inverter according to the power parameter fed back by each inverter, and adjust and control the operation of the alternating current coupling point and each inverter in combination with the power surplus condition at the alternating current coupling point.

[0068] The above-mentioned controller may directly adopt the control module of one inverter as the master. The main control function is integrally arranged in the control module of one inverter. All inverters are communicatively connected to one another. The remaining inverters other than the master serve as slaves, to form master-slave control.

[0069] The above two control modes shall fall within the protection scope of the present disclosure. The core focus of the present disclosure lies in realizing transfer and absorption of the photovoltaic surplus power among various inverters, enabling each local inverter to maximize utilization of the photovoltaic power. Also, reliable operation under the grid-connected mode can be realized, and effective operation of the photovoltaic system can be ensured. The protection scope shall not be limited by a specific implementation mode adopted for the main control function.

[0070] In summary, with the photovoltaic surplus power utilization method according to the present disclosure, detection of the photovoltaic surplus power can be realized through each inverter evaluating an electric quantity generated by the inverter. However, the controller is configured to detect the power or the voltage at the alternating current coupling point, and perform power regulation or voltage regulation based on a detection result. In an entire control process of surplus power utilization, the controller only needs to monitor and control the alternating current coupling point, while regulation of the inverter only needs to be adjusted according to an instruction of the controller. Therefore, overall control can be simplified, energy transfer of the photovoltaic surplus power among the inverters can be realized, and renewable energy can be utilized to the maximum extent. In addition, under the grid-connected mode, zero-power grid connection or grid connection performed by the maximum grid connection power limit value can be realized, to guarantee the compliance of power grid operation after grid connection and the controllability of the grid-connected power.

[0071] Regardless of whether the photovoltaic system operates under the grid-connected mode or the off-grid mode, surplus power generation status of each inverter in the photovoltaic system may be divided into three categories: none of the inverters generate the surplus power, all inverters generate the surplus power, and partial inverters generate the surplus power. Comprehensively, six operating conditions may be determined for energy flow control among the inverters and power control at the alternating current coupling point, namely: all inverters in the grid-connected mode generating no surplus power, all inverters in the grid-connected mode generating the surplus power, partial inverters in the grid-connected mode generating the surplus power, all inverters in the off-grid mode generating no surplus power, all inverters in the off-grid mode generating the surplus power, and partial inverters in the off-grid mode generating the surplus power.

[0072] The above six operating conditions are described in detail as follows. A control flow of the photovoltaic surplus power utilization method in the grid-connected mode is shown in FIG. 2. A control flow of the photovoltaic surplus power utilization method in the off-grid mode is shown in FIG. 3.

[0073] When the photovoltaic system operates in the grid-connected mode and partial inverters generate the surplus power, namely the photovoltaic system is provided with some first inverters and some second inverters, output power of the surplus-power-generating first inverter increases with the maximum power point tracking of the photovoltaic power. Also, when the controller detects that the power at the alternating current coupling point is greater than zero, the controller controls the other second inverters without the surplus power to reduce power output or increase power absorption, to lower the power at the alternating current coupling point, and dynamically realize transfer of the photovoltaic surplus power among the second inverters, thereby alleviating photovoltaic curtailment. When the surplus power still exists after the power is transferred among the second inverters, which means all the second inverters reach a maximum absorption capacity, in this case the power at the alternating current coupling point may be continuously raised. When the power rises to the grid connection threshold PBREF, the controller issues an instruction to control grid connection of the photovoltaic system, and controls the grid feed-in power of the unit surplus power of the photovoltaic system at the maximum grid connection power limit value configured for the system or another fixed value not greater than the grid connection threshold PBREF.

[0074] When the photovoltaic system operates in the grid-connected mode, and none of the inverters generate the surplus power, that is, all inverters are the second inverters, the controller skips the execution of photovoltaic surplus power utilization control, to avoid interference with normal operation of other conventional control links.

[0075] When the photovoltaic system operates in the grid-connected mode and all inverters generate the surplus power, that is, all inverters are the first inverters, each inverter feeds the surplus power thereof to the alternating current coupling point. When the power at the alternating current coupling point reaches the grid connection threshold PBREF, the controller controls the photovoltaic system to perform grid connection, and controls the grid feed-in power of the unit surplus power of the photovoltaic system at the maximum grid connection power limit value configured for the system or the other fixed value not greater than the grid connection threshold PBREF.

[0076] When the photovoltaic system operates in the off-grid mode, and partial inverters generate the surplus power, that is, the photovoltaic system includes some first inverters and some second inverters, the output power of the surplus-power-generating first inverters increases with the maximum power point tracking of the photovoltaic power. The output voltage of the first inverter is transiently regulated to boost the voltage of the first inverter and raise the voltage at the alternating current coupling point. Also, when the controller detects that the voltage at the alternating current coupling point is greater than the predetermined voltage threshold, the controller controls other second inverters without the surplus power to perform corresponding voltage adjustment and reduce output voltages thereof. In this way, the photovoltaic surplus power can be “transferred” from a port of the first inverter to the second inverter by means of a circulating current, to maximize utilization of photovoltaic energy. When the surplus power still exists after the power is transferred among the second inverters, that is, each inverter reaches the maximum absorption capacity, in this case, the controller cannot control the voltage at the alternating current coupling point to be close to the first voltage threshold VAREF only through the second inverters. Therefore, the controller can perform PI regulation and control on the voltage at the alternating current coupling point to ensure that the voltage at the alternating current coupling point is controlled within the allowable range (not greater than the second voltage threshold VBREF).

[0077] When the photovoltaic system operates in the off-grid mode, and none of the inverters generate the surplus power, that is, all inverters are the second inverters, the controller skips the execution of photovoltaic surplus power utilization control, to avoid interference with normal operation of other conventional control links.

[0078] When the photovoltaic system operates in the off-grid mode and all inverters generate the surplus power, that is, all inverters are the first inverters, each inverter feeds the surplus power thereof to the alternating current coupling point. The controller performs PI regulation and control on the voltage at the alternating current coupling point, to ensure that the voltage at the alternating current coupling point is controlled within the allowable range.

[0079] Third embodiment

[0080] This embodiment describes a photovoltaic surplus power utilization system, applied to a photovoltaic system including at least two inverters. The photovoltaic system operates in a grid-connected mode. An output terminal of each of the at least two inverters is connected to an alternating current coupling point. As illustrated in FIG. 4, the photovoltaic surplus power utilization system includes: a power detection unit configured to detect power at the alternating current coupling point and determine whether the surplus power exists at the alternating current coupling point based on the detected power; an inverter state detection unit configured to determine whether each of the at least two inverters has the surplus power or reaches a maximum power operating state based on a power parameter fed back by each of the at least two inverters; a power allocation unit configured to allocate the power at the alternating current coupling point to an inverter without the surplus power when the surplus power is detected; and a grid-connected export unit configured to perform, when each of the at least two inverters reaches the maximum power operating state, grid-connected export of the power at the alternating current coupling point.

[0081] Fourth embodiment

[0082] This embodiment describes a photovoltaic surplus power utilization system, applied to a photovoltaic system including at least two inverters. The photovoltaic system operates under an off-grid mode. An output terminal of each of the at least two inverters is connected to an alternating current coupling point. As illustrated in FIG. 5, the photovoltaic surplus power utilization system includes: a voltage detection unit configured to detect a voltage of the alternating current coupling point and determine whether the voltage of the alternating current coupling point exceeds a predetermined voltage threshold based on the detected voltage; an inverter state detection unit configured to determine whether each of the at least two inverters has surplus power or reaches a maximum power operating state based on a power parameter fed back by each of the at least two inverters; an average current processing unit configured to process the power at the alternating current coupling point to an inverter without the surplus power when the voltage of the alternating current coupling point exceeds the voltage threshold; and a voltage regulation unit configured to perform, when each of the at least two inverters reaches the maximum power operating state, voltage reduction or limiting processing on the voltage at the alternating current coupling point.

[0083] The above embodiments are only intended to illustrate the technical concept and features of the present disclosure, so that those skilled in the art can understand the content of the present disclosure and implement the technical solution accordingly, and shall not be used to limit the protection scope of the present disclosure. All equivalent alterations and modifications made based on the essential spirit of the present disclosure shall fall within the protection scope of the present disclosure.

Claims

1. A photovoltaic surplus power utilization method, applied to a photovoltaic system comprising at least two inverters, wherein the photovoltaic system operates in a grid-connected mode, and wherein an output terminal of each of the at least two inverters is connected to an alternating current coupling point,wherein a controller is configured to:detect power at the alternating current coupling point;determine whether a power surplus exists at the alternating current coupling point based on the detected power;allocate, when the power surplus is detected, the power at the alternating current coupling point to an inverter without surplus power; andperform, when each of the at least two inverters reaches a maximum power operating state, grid-connected export on the power at the alternating current coupling point.

2. The photovoltaic surplus power utilization method according to claim 1, wherein:each of the at least two inverters is configured to calculate power thereof and transmit a power parameter to the controller,the controller is configured to:determine, based on the power parameter fed back by each of the at least two inverters, whether each of the at least two inverters has the surplus power or reaches the maximum power operating state; andadjust and control the operation of the alternating current coupling point and each of the at least two inverters in combination with a surplus power condition at the alternating current coupling point.

3. The photovoltaic surplus power utilization method according to claim 1, wherein the controller is configured to:determine, based on the power parameter fed back by each of the at least two inverters, whether each of the at least two inverters is an inverter generating the surplus power or aninverter generating no surplus power, wherein the inverter generating the surplus power is designated as a first inverter, and the inverter generating no surplus power is designated as a second inverter; andallocate the power at the alternating current coupling point to the second inverter when the surplus power is detected at the alternating current coupling point.

4. The photovoltaic surplus power utilization method according to claim 3, wherein said allocating the power at the alternating current coupling point to the second inverter comprises:adjusting the power delivered from the alternating current coupling point to the second inverter, and increasing the absorbed power of the second inverter.

5. The photovoltaic surplus power utilization method according to any one of claims 1 to 4, wherein the controller is configured to:when each of the at least two inverters reaches the maximum power operating state, compare the power at the alternating current coupling point with a grid connection threshold PBREF; andwhen the power at the alternating current coupling point is not smaller than the grid connection threshold PBREF, control the alternating current coupling point to be grid-connected, and regulate grid-connected output power of the alternating current coupling point.

6. The photovoltaic surplus power utilization method according to claim 5, wherein the grid connection threshold PBREF is not smaller than a maximum grid connection power limit value configured for the photovoltaic system, and the grid-connected output power of the alternating current coupling point is equal to the maximum grid connection power limit value.

7. The photovoltaic surplus power utilization method according to claim 5, wherein when the maximum grid connection power limit value Plimit configured for the photovoltaic system is smaller than k% of a total capacity of the at least two inverters in the photovoltaic system, during initialization of the method, the grid connection threshold PBREF is set to k% of the total capacity of the at least two inverters, and a power monitoring threshold PAREF for determining whether the surplus power exists at the alternating current coupling point is set to zero, where k is a coefficient ranging from 1 to 100, wherein:when the power at the alternating current coupling point is not smaller than the gridconnection threshold PBREF, the grid connection threshold is lowered to P’BREF, where P’BREF=Plimit, and the power monitoring threshold is simultaneously lowered to P’AREF, where P’AREF=Plimit-PBREF.

8. The photovoltaic surplus power utilization method according to claim 5, wherein when the maximum grid connection power limit value Plimit configured for the photovoltaic system is smaller than k% of a total capacity of the at least two inverters in the photovoltaic system, during initialization of the method, the grid connection threshold PBREF is set to k% of the total capacity of the at least two inverters, and a power monitoring threshold PAREF for determining whether the surplus power exists at the alternating current coupling point is set to zero, where k is a coefficient ranging from 0 to 100, wherein:when the power at the alternating current coupling point is not smaller than the grid connection threshold PBREF, the grid connection threshold is lowered to P’BREF, where P’BREF=Plimit, and the power monitoring threshold is simultaneously lowered to P’AREF, where P’AREF=Plimit-PBREF.

9. The photovoltaic surplus power utilization method according to claim 1, wherein said each of the at least two inverters reaching the maximum power operating state comprises:first, each of the at least two inverters being in a state of generating the surplus power; and second, a part of the at least two inverters being in the state of generating the surplus power, and the remaining inverters of the at least two inverters being in an energy absorption saturation state.

10. The photovoltaic surplus power utilization method according to claim 1, wherein the controller is separately arranged or integrated into a control module of one of the at least two inverters.

11. The photovoltaic surplus power utilization method according to claim 10, wherein the controller is separately arranged in a gateway or an electrical box.

12. A photovoltaic surplus power utilization method, applied to a photovoltaic system comprising at least two inverters, wherein the photovoltaic system operates in an off-grid mode, and wherein an output terminal of each of the at least two inverters is connected to an alternatingcurrent coupling point,wherein a controller is configured to:detect a voltage of the alternating current coupling point;determine whether regulation is required based on a magnitude of the voltage of the alternating current coupling point;process, when regulation is performed, power at the alternating current coupling point to an inverter without surplus power; andperform, when each of the at least two inverters reaches a maximum power operating state, voltage reduction or limiting processing on the voltage at the alternating current coupling point.

13. The photovoltaic surplus power utilization method according to claim 12, wherein:each of the at least two inverters is configured to calculate power thereof and transmit a power parameter to the controller, wherein:the controller is configured to:determine, based on the power parameter fed back by each of the at least two inverters, whether each of the at least two inverters has the surplus power or reaches the maximum power operating state; andadjust and control the operation of the alternating current coupling point and each of the at least two inverters in combination with a surplus power condition at the alternating current coupling point.

14. The photovoltaic surplus power utilization method according to claim 12, wherein the controller is configured to:determine, based on the power parameter fed back by each of the at least two inverters, whether each of the at least two inverters is an inverter generating the surplus power or an inverter generating no surplus power, wherein the inverter generating the surplus power is designated as a first inverter, and the inverter generating no surplus power is designated as a second inverter; andadjust, when the detected voltage of the alternating current coupling point is greater than a predetermined voltage threshold, an output voltage of the first inverter and an output voltage of the second inverter to adapt to a voltage change of the alternating current coupling point.

15. The photovoltaic surplus power utilization method according to any one of claims 12to 14, wherein when each of the at least two inverters reaches the maximum power operating state, voltage reduction or limiting processing is performed on the voltage at the alternating current coupling point, to enable the voltage at the alternating current coupling point to be within an allowable range.

16. The photovoltaic surplus power utilization method according to claim 15, wherein the method is provided with a first voltage threshold VAREF and a second voltage threshold VBREF, the first voltage threshold VAREF being smaller than the second voltage threshold VBREF, wherein:when the voltage of the alternating current coupling point is not smaller than the first voltage threshold VAREF, the power at the alternating current coupling point is allocated to the inverter without the surplus power by processing a voltage circulating current among the at least two inverters; andwhen the voltage of the alternating current coupling point is not smaller than the second voltage threshold VBREF, the power at the alternating current coupling point is processed and allocated to the inverter without the surplus power, and the voltage reduction or limiting processing is simultaneously performed on the voltage at the alternating current coupling point.

17. The photovoltaic surplus power utilization method according to claim 15, wherein said each of the at least two inverters reaching the maximum power operating state comprises:first, each of the at least two inverters being in a state of generating the surplus power; and second, a part of the at least two inverters being in the state of generating the surplus power, and the remaining inverters of the at least two inverters being in an energy absorption saturation state.

18. The photovoltaic surplus power utilization method according to claim 12, wherein the controller is separately arranged or integrated into a control module of one of the at least two inverters.

19. The photovoltaic surplus power utilization method according to claim 18, wherein the controller is separately arranged in a gateway or an electrical box.

20. A photovoltaic surplus power utilization system, applied to a photovoltaic system comprising at least two inverters, wherein the photovoltaic system operates in a grid-connected mode, and an output terminal of each of the at least two inverters is connected to an alternating current coupling point, wherein the photovoltaic surplus power utilization system comprises:a power detection unit configured to detect power at the alternating current coupling point and determine whether surplus power exists at the alternating current coupling point based on the detected power;an inverter state detection unit configured to determine whether each of the at least two inverters has the surplus power or reaches a maximum power operating state based on a power parameter fed back by each of the at least two inverters;a power allocation unit configured to allocate the power at the alternating current coupling point to an inverter without the surplus power when the surplus power is detected; anda grid-connected export unit configured to perform, when each of the at least two inverters reaches the maximum power operating state, grid-connected export of the power at the alternating current coupling point.

21. A photovoltaic surplus power utilization system, applied to a photovoltaic system comprising at least two inverters, wherein the photovoltaic system operates in an off-grid mode, and an output terminal of each of the at least two inverters is connected to an alternating current coupling point, wherein the photovoltaic surplus power utilization system comprises:a voltage detection unit configured to detect a voltage of the alternating current coupling point and determine whether the voltage of the alternating current coupling point exceeds a predetermined voltage threshold based on the detected voltage;an inverter state detection unit configured to determine whether each of the at least two inverters has surplus power or reaches a maximum power operating state based on a power parameter fed back by each of the at least two inverters;an average current processing unit configured to process power at the alternating current coupling point to an inverter without the surplus power when the voltage of the alternating current coupling point exceeds the voltage threshold; anda voltage regulation unit configured to perform, when each of the at least two inverters reaches the maximum power operating state, voltage reduction processing on the voltage at the alternating current coupling point.

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