Power conversion apparatus and maximum power point tracking control method

Abstract

The present invention provides a power conversion apparatus and a maximum power point tracking control method, related to the field of photovoltaic power generation. Maximum power point tracking control of a photovoltaic module is implemented by means of sampling a bus output current and a bus voltage at an output end of a boosted and isolated DC / DC converter connected to the photovoltaic module, and a sampling circuit used to track a signal required by the maximum power point of the photovoltaic module is moved from a primary side of a transformer to a secondary side of the transformer. In addition, a sampling resistor having a relatively high resistance may be used, so as to improve sampling precision with a small loss. This may have the advantages of high current sampling precision and low loss, and greatly reduce design difficulty. In addition, a sampling delay in a non-isolated sampling means of the sampling resistor is relatively small, and real-time control is better.

Description

Power conversion device and maximum power point tracking control method Technical Field

[0001] The present invention relates to the field of photovoltaic power generation, in particular to a power conversion device and a maximum power point tracking control method. Background Art

[0002] At a time when conventional energy shortages are becoming increasingly serious, photovoltaic power generation, as an energy-saving and environmentally friendly green power generation method, is gradually being promoted and used both at home and abroad.

[0003] Photovoltaic modules (also commonly known as solar photovoltaic panels) convert the received sunlight into electrical energy in the form of direct current. The converter then converts the direct current into another direct current to power a DC load, or converts the direct current into alternating current for grid connection or to power an AC load.

[0004] As we all know, the output power of photovoltaic modules is affected by sunlight. In order to obtain maximum power output, maximum power tracking of photovoltaic modules has become a core technology of photovoltaic power generation. With the advancement of technology and the iteration of products, it has become an indispensable and mature technology in photovoltaic power generation products (such as photovoltaic single-phase inverters, optimizers, photovoltaic control boxes, etc.).

[0005] Referring to the schematic PV curve diagram of a conventional photovoltaic module in Figure 1, one can see that the module has only one maximum power point, which is related to the module's voltage. In the prior art, tracking the module's maximum power point is typically achieved by sampling the voltage and current of the photovoltaic module (PV), calculating the module's power, and then perturbing the module's voltage.

[0006] In traditional string-type single-phase inverters, PV panels are connected in series to generate high voltage, which is then fed into a single-phase inverter. The subsequent single-phase inverter widely uses a boost circuit as a pre-stage, which then inverts the high-voltage DC power and feeds it into the grid. Because both the boost circuit and the inverter circuit are non-isolated converters and the controller and boost circuit share a common ground, the voltage and current of the PV panel string can be directly sampled using a low-cost resistor-and-op-amp method, enabling maximum power point tracking of the PV panels.

[0007] As countries pay more attention to the safety of photovoltaic power generation, optimizers and micro single-phase inverters have quickly come into everyone's view. In particular, micro single-phase inverters are becoming increasingly popular because they can achieve low-voltage isolation and component-level maximum power tracking and monitoring.

[0008] Due to the low output voltage of a single photovoltaic module, the traditional front-stage boost plus back-stage inverter solution has increasingly obvious shortcomings due to the high boost ratio and low conversion efficiency. Therefore, for micro single-phase inverters connected to a single photovoltaic module, most of the industry has chosen high-frequency isolation solutions such as flyback, LLC, push-pull, and other circuits as the front stage, using the turns ratio of the high-frequency transformer to achieve high boost. This high-frequency isolation solution isolates the photovoltaic module and the inverter. Please refer to the typical schematic diagram of a micro single-phase inverter shown in Figure 2. The photovoltaic module 210 and the inverter 220 are isolated by the high-frequency transformer 231 in the high-frequency isolation converter 230.

[0009] Since the photovoltaic module 210 and the inverter 220 are isolated, there are two current system solutions: one using one DSP and the other using two DSPs for control.

[0010] The solution using a DSP is as follows: placing the controller 240 on the low-voltage side of the photovoltaic module 210, using a resistor plus op amp method to sample the voltage Vpv and current Ipv of the photovoltaic module 210, and using an isolated sampling method to sample the AC voltage Vac and AC current Iac; or placing the controller 240 on the high-voltage inverter 220 side, using a resistor plus op amp method to sample the AC voltage Vac and AC current Iac, while using isolated sampling to sample the voltage Vpv and current Ipv of the photovoltaic module 210, and using an isolated drive method to drive the switch tube on the low-voltage side of the photovoltaic module 210 side in the high-frequency isolation converter 230. Please refer to the typical micro single-phase inverter schematic shown in Figure 3.

[0011] Here, the voltage Vpv and current Ipv of the PV panel 210 are used for maximum power algorithm control, and the AC voltage Vac and AC current Iac on the inverter side of the inverter are used to control the inverter 220. It can be seen that regardless of whether the controller 240 is placed on the PV panel 210 side or the AC side where the inverter 220 is located, isolated sampling must be used to obtain the voltage and current on the side where the controller is not located.

[0012] Current isolated sampling methods include optical isolation sampling, capacitive isolation sampling, and magnetic isolation sampling. Compared with non-isolated sampling (or direct sampling) using resistors and op amps, these methods are much more expensive and bulky. In addition, isolated sampling introduces sampling delays, which makes control inconvenient.

[0013] The solution using two DSPs for control is as follows: one controller (DSP1) is placed on the photovoltaic panel side, and one controller (DSP2) is placed on the AC side where the inverter is located. These controllers directly sample the electrical signals required for maximum power tracking and the electrical signals required for inverter control. DSP1 and DSP2 then communicate with each other through optocoupler isolation. Although this solution avoids the use of isolated sampling, it requires two DSPs, which significantly increases the cost and the number of peripheral circuits.

[0014] In addition, the power level of a photovoltaic module is usually several hundred watts, such as the commonly used 600W. The voltage Vpv of a photovoltaic module is usually tens of volts, such as more than twenty volts or more than thirty volts, and the current Ipv of a photovoltaic module is usually tens of amperes, such as between twenty and thirty amperes. In order to achieve maximum power electrical tracking, when sampling the current Ipv of a photovoltaic module, if a non-isolated sampling method of a resistor plus an operational amplifier is used, if a sampling resistor with a larger resistance value is required to obtain accurate sampling, it will result in a large sampling loss. If a sampling resistor with a smaller resistance value is required to reduce the loss, it will result in insufficient sampling accuracy. Therefore, the existing technology also has the disadvantage that the sampling of the current Ipv of a photovoltaic module cannot achieve both accuracy and loss, resulting in great design difficulty.

[0015] Summary of the Invention

[0016] The present invention provides a power conversion device for use in a photovoltaic system, comprising: a first step-up isolated DC / DC converter including a transformer, wherein the input end of the first step-up isolated DC / DC converter is used to connect to a first photovoltaic module, and the output end of the first step-up isolated DC / DC converter is used to provide a DC bus; and a controller that receives a bus voltage of the DC bus and a bus output current output by the first step-up isolated DC / DC converter, and outputs a switch control signal for controlling the operation of the first step-up isolated DC / DC converter based on the bus voltage and the bus output current output by the first step-up isolated DC / DC converter, thereby performing maximum power point tracking control of the first photovoltaic module.

[0017] Furthermore, the controller and the secondary winding of the transformer share a common ground.

[0018] Furthermore, the switch control signal for controlling the first step-up isolated DC / DC converter is controlled so as to maximize a bus output current output by the first step-up isolated DC / DC converter.

[0019] Furthermore, the controller includes: a first multiplication module, which multiplies the bus voltage and the bus output current of the first boost isolated DC / DC converter to obtain a current output power of the first boost isolated DC / DC converter; a maximum power point tracking control module, which obtains a rated current variation coefficient of the first boost isolated DC / DC converter based on a difference between the current output power of the first boost isolated DC / DC converter and the output power at a previous moment; a second multiplication module, which multiplies the rated current variation coefficient of the first boost isolated DC / DC converter by the rated current of the first boost isolated DC / DC converter to obtain a bus output current reference value of the first boost isolated DC / DC converter; a first subtraction module, which subtracts the bus output current reference value of the first boost isolated DC / DC converter from the bus output current of the first boost isolated DC / DC converter to obtain a bus output current difference of the first boost isolated DC / DC converter; and a first operation unit, which outputs the switching control signal for controlling the first boost isolated DC / DC converter based on the bus output current difference of the first boost isolated DC / DC converter.

[0020] Furthermore, an inverter is included, wherein the DC end of the inverter is connected to the DC bus, and the AC end of the inverter is used to output AC power.

[0021] Furthermore, the controller also receives the AC current and AC voltage of the AC power output by the inverter, and outputs a switch control signal for controlling the operation of the inverter according to the bus voltage, the AC current and the AC voltage.

[0022] Furthermore, the controller also includes: a second subtraction operation module, which subtracts the bus voltage reference value from the bus voltage to obtain a bus voltage difference; a second operation module, which obtains the AC current variation coefficient based on the bus voltage difference; a phase-locked control module, which outputs a phase-locked sine wave according to the AC voltage; a third multiplication operation module, which multiplies the phase-locked sine wave, the AC current variation coefficient and the AC current peak value to obtain an AC current reference value; a third subtraction operation module, which subtracts the AC current reference value from the AC current to obtain an AC current difference; and a third operation unit, which outputs a switching control signal for controlling the inverter based on the AC current difference.

[0023] Furthermore, it also includes: a second boost isolated DC / DC converter, including a transformer, the input end of the second boost isolated DC / DC converter is used to connect a second photovoltaic module, and the output end of the second boost isolated DC / DC converter is used for the DC bus; the controller also receives the bus output current of the second boost isolated DC / DC converter, and outputs a switch control signal to control the operation of the second boost isolated DC / DC converter according to the bus voltage and the bus output current of the second boost isolated DC / DC converter, so as to perform maximum power point tracking control of the second photovoltaic module.

[0024] Furthermore, the switch control signal for controlling the second step-up isolated DC / DC converter is controlled so as to maximize the bus output current output by the second step-up isolated DC / DC converter.

[0025] Furthermore, the first photovoltaic assembly is formed by one photovoltaic cell group, or by a plurality of photovoltaic cell groups connected in series and parallel.

[0026] The present application also provides a maximum power point tracking control method for a photovoltaic assembly, comprising: a voltage detection circuit detecting a bus voltage of a DC bus connected to an output end of a first boost isolated DC / DC converter; a first current detection circuit detecting a bus output current at an output end of the first boost isolated DC / DC converter; and a controller performing maximum power point tracking control of a first photovoltaic assembly based on the bus voltage and the bus output current of the first boost isolated DC / DC converter, wherein the input end of the first boost isolated DC / DC converter is used to connect to the first photovoltaic assembly.

[0027] Furthermore, the controller maximizes the bus output current of the first boost isolated DC / DC converter according to the bus voltage and the bus output current of the first boost isolated DC / DC converter, so as to achieve maximum power point tracking control of the first photovoltaic component.

[0028] Furthermore, the controller performs the following operations: multiplying the bus voltage by the bus output current of the first boost isolated DC / DC converter to obtain a current output power of the first boost isolated DC / DC converter; obtaining a rated current variation coefficient of the first boost isolated DC / DC converter based on a difference between the current output power of the first boost isolated DC / DC converter and the output power at a previous moment; multiplying the rated current variation coefficient of the first boost isolated DC / DC converter by the rated current of the first boost isolated DC / DC converter to obtain a bus output current reference value of the first boost isolated DC / DC converter; subtracting the bus output current reference value of the first boost isolated DC / DC converter from the bus output current of the first boost isolated DC / DC converter to obtain a bus output current difference of the first boost isolated DC / DC converter; and outputting a switching control signal for controlling the first boost isolated DC / DC converter based on the bus output current difference of the first boost isolated DC / DC converter to track the maximum power point of the first photovoltaic module.

[0029] Furthermore, it also includes: an AC voltage detection circuit to detect the AC voltage on the AC output side of an inverter; an AC current detection circuit to detect the AC current on the AC output side of the inverter, wherein the DC end of the inverter is connected to the DC bus, and the controller further executes: outputting a switch control signal for controlling the operation of the inverter according to the bus voltage, the AC current and the AC voltage.

[0030] Furthermore, the controller also performs the following operations: subtracting the bus voltage reference value from the bus voltage to obtain a bus voltage difference; obtaining the AC current variation coefficient based on the bus voltage difference; outputting a phase-locked sine wave based on the AC voltage; multiplying the phase-locked sine wave, the AC current variation coefficient, and the AC current peak value to obtain an AC current reference value; subtracting the AC current reference value from the AC current to obtain an AC current difference; and outputting the switch control signal for controlling the inverter based on the AC current difference.

[0031] Furthermore, the present invention further includes: a second current detection circuit detecting a bus output current of a second boost isolated DC / DC converter; the controller also performs maximum power point tracking control of a second photovoltaic assembly based on the bus voltage and the bus output current of the second boost isolated DC / DC converter, wherein the input end of the second boost isolated DC / DC converter is used to connect to the second photovoltaic assembly.

[0032] Furthermore, the controller maximizes the bus output current of the second boost isolated DC / DC converter according to the bus voltage and the bus output current of the second boost isolated DC / DC converter, so as to achieve maximum power point tracking control of the second photovoltaic module. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is a schematic diagram of the PV curve of a conventional photovoltaic module.

[0034] Figure 2 is a schematic diagram of a typical micro single-phase inverter.

[0035] Figure 3 is a schematic diagram of a typical micro single-phase inverter.

[0036] FIG4 is a schematic diagram of a typical power conversion device.

[0037] FIG5 is a schematic diagram of a power conversion device according to an embodiment of the present invention.

[0038] FIG6 is a schematic diagram of a curve showing the output power and bus output current of a boost isolated DC / DC converter.

[0039] FIG. 7 is a schematic diagram of a controller used in the power conversion device shown in FIG. 5 according to an embodiment of the present application.

[0040] FIG8 is a schematic diagram of a power conversion device according to another embodiment of the present invention.

[0041] FIG. 9 is a schematic diagram of a controller applied to the power conversion device shown in FIG. 8 according to an embodiment of the present application.

[0042] FIG10 is a schematic diagram of a power conversion device according to another embodiment of the present application.

[0043] FIG11 is a schematic diagram of a controller applied to the power conversion device shown in FIG10 . DETAILED DESCRIPTION

[0044] The following will clearly and completely describe the technical solutions of the present invention in conjunction with the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0045] FIG4 shows a schematic diagram of a typical power conversion device, which is used in a photovoltaic inverter system. As shown in FIG4 , the power conversion device includes at least a first boost-isolated DC / DC converter 230 and an inverter 220. The first boost-isolated DC / DC converter 230 includes a transformer 231. The input of the first boost-isolated DC / DC converter 230 is connected to the first photovoltaic module 210, and the output of the first boost-isolated DC / DC converter 230 is connected to the DC bus 250. The first photovoltaic module 210 is configured to convert solar energy into electrical energy. The first boost-isolated DC / DC converter 230 is configured to boost the electrical energy generated by the first photovoltaic module 210 into DC power, which is output to the DC bus 250. The transformer 231 in the first boost-isolated DC / DC converter 230 isolates the first photovoltaic module 210 from the DC bus 250. The DC terminal of the inverter 220 is connected to the DC bus 250, converting the DC power on the DC bus 250 into AC power for grid connection or powering AC loads. This is how photovoltaic power generation is achieved.

[0046] Continuing with FIG4 , the power conversion system may further include a second boost-isolated DC / DC converter 330 . The second boost-isolated DC / DC converter 330 includes a transformer 331 . The input of the second boost-isolated DC / DC converter 330 is connected to the second photovoltaic assembly 310 , and the output of the second boost-isolated DC / DC converter 330 is connected to the DC bus 250 . Similarly, the second photovoltaic assembly 310 is configured to convert light energy into electrical energy. The second boost-isolated DC / DC converter 330 is configured to boost the electrical energy generated by the second photovoltaic assembly 310 into direct current (DC) power that is output to the DC bus 250 . The transformer 331 in the second boost-isolated DC / DC converter 330 provides isolation between the second photovoltaic assembly 310 and the DC bus 250 .

[0047] In this way, using multiple isolated boost DC / DC converters as the front-end circuit and a single inverter 220 as the back-end inverter circuit can improve the power level and efficiency of the power conversion device. Furthermore, compared to traditional front-end boost circuits, the isolated boost DC / DC converter leverages the inherent turns ratio of the high-frequency transformer to achieve a high voltage boost. Figure 4 shows a typical power conversion device currently used in photovoltaic inverter systems. The number of isolated boost DC / DC converters in the front-end can be customized to meet different power requirements.

[0048] For the power conversion device shown in Figure 4, as described in the prior art, in order to follow the maximum power point of the photovoltaic module and control the inverter, it is necessary to simultaneously sample the voltage Vpv and current Ipv of the photovoltaic module and the AC voltage Vac and AC current Iac on the inverter side of the inverter. Therefore, regardless of whether the controller is placed on the photovoltaic module side or the AC side where the inverter is located, isolated sampling must be used to obtain the voltage and current on the side where the controller is not located. This results in the problems of high cost, large size, and large signal delay or loss in photovoltaic maximum power point tracking control.

[0049] For a boost-isolated DC / DC converter, the input side receives power from the PV panel, with input power Pin = Vpv * Ipv, where Vpv is the PV panel voltage and Ipv is the PV panel current. The output side delivers bus voltage Vbus and bus output current Ibus, with output power Pout = Vbus * Ibus. Due to power losses in a boost-isolated DC / DC converter, Pout is typically calculated as k * Pin, where k is greater than 0 and less than 1.

[0050] In existing technology, the power of a photovoltaic (PV) module is typically calculated by sampling its voltage and current, and the voltage of the PV module is then perturbed to track the PV module's maximum power point (MPP). (See Figures 2 and 3 for details). This maximizes the input power Pin of the step-up isolated DC / DC converter. However, this technology suffers from many of the aforementioned problems.

[0051] However, the inventors have discovered that according to the relationship between the input power Pin and the output power Pout of the step-up isolated DC / DC converter (Pout=k*Pin), if the output power Pout can be maximized, the input power Pin can also be maximized.

[0052] Based on the concept that maximizing the output power Pout of a boost isolated DC / DC converter can maximize the input power Pin, an embodiment of the present invention provides a power conversion device that can be applied to a photovoltaic power generation system. Specifically, referring to FIG5 , a schematic diagram of a power conversion device according to an embodiment of the present invention is shown. The power conversion device includes: a first boost isolated DC / DC converter 230 including a transformer 231. The input end of the first boost isolated DC / DC converter 230 is used to connect to a first photovoltaic module 210, and the output end of the first boost isolated DC / DC converter 230 is used to connect to a DC bus 250.

[0053] The controller 240 receives the bus voltage Vbus of the DC bus 250 and the bus output current Ibus1 output by the first step-up isolated DC / DC converter 230, and outputs a switch control signal C1 for controlling the operation of the first step-up isolated DC / DC converter 230 according to the bus voltage Vbus and the bus output current Ibus1 output by the first step-up isolated DC / DC converter 230, so as to perform maximum power point tracking control of the first photovoltaic component 210.

[0054] As described above, tracking the maximum power point of a photovoltaic (PV) module by sampling its voltage and output current is converted to tracking the maximum power point of the PV module by sampling the bus voltage and bus output current on the output side of the isolated DC / DC converter. This means that the sampling circuit for the signals required to track the maximum power point of the PV module has been moved from the primary side of the transformer to the secondary side of the transformer. Currently, the bus voltage Vbus of the DC bus is typically 400V, with a trend toward 600V. For a PV module with a bus voltage Vbus of 400V and a power rating of 600W, the bus output current Ibus1 output by the first step-up isolated DC / DC converter 230 is only 1.5A, which is significantly lower than the tens of amperes of the PV module current Ipv. Therefore, a larger sampling resistor can be used to improve sampling accuracy and minimize losses. Therefore, the power conversion device provided by this application combines the advantages of high current sampling accuracy and low losses, significantly reducing design complexity. In addition, the sampling delay of the non-isolated sampling method using a sampling resistor is also small, and the real-time control is better.

[0055] In practical applications, the current sampling circuit used to sample the bus output current Ibus1 output by a step-up isolated DC / DC converter 230 is typically a resistor-and-op-amp sampling circuit. This circuit not only has the advantages of a non-isolated sampling circuit in terms of size and cost, but also possesses the aforementioned advantages of high current sampling accuracy and low loss.

[0056] Furthermore, as shown in FIG5 , the first step-up isolated DC / DC converter 230 includes a switch tube, and the switch control signal C1 is used to control the switch tube in the first step-up isolated DC / DC converter 230 to achieve maximum power point tracking control of the first photovoltaic assembly 210 .

[0057] Furthermore, the first boost isolated DC / DC converter 230 can be any converter with both boost and isolation functions, such as flyback, LLC, push-pull and other circuits. The present application does not limit the specific structure of the first boost isolated DC / DC converter 230.

[0058] Typically, the first step-up isolated DC / DC converter 230 includes a primary switch unit, a transformer, and a secondary switch unit. The primary switch unit is connected between the first photovoltaic module 210 and the primary winding of the transformer 230, and the secondary switch unit is connected between the secondary winding of the transformer 230 and the DC bus 250. The aforementioned switch control signal C1 is typically used to control the switch transistors within the primary switch unit to implement maximum power point tracking control of the first photovoltaic module 210.

[0059] In practical applications, the controller 240 and the secondary winding of the transformer 231 share a common ground, so that the sampling circuit for the signal required to track the maximum power point of the photovoltaic module can be placed on the secondary side of the transformer.

[0060] Please refer to the schematic diagram of the curve between the output power and bus output current of the boost isolated DC / DC converter shown in Figure 6. The output power Pout of the boost isolated DC / DC converter is equal to Vbus*Ibus, where the bus voltage Vbus is a substantially constant voltage value. Therefore, the output power Pout of the boost isolated DC / DC converter increases with the increase of the bus output current Ibus of the boost isolated DC / DC converter, forming a substantially linear relationship. Therefore, as long as the boost isolated DC / DC converter is controlled to maximize the bus output current Ibus of the boost isolated DC / DC converter, the output power Pout of the boost isolated DC / DC converter can be maximized, thereby achieving maximum power point tracking of the first photovoltaic assembly 210.

[0061] That is, further, the switch control signal C1 of the first step-up isolated DC / DC converter 230 is controlled to maximize the bus output current Ibus1 output by the first step-up isolated DC / DC converter 230. "Maximization" here means that the controlled object approaches its maximum value that can be achieved.

[0062] Specifically, refer to FIG. 7 , which is a schematic diagram of a controller applied to the power conversion device shown in FIG. 5 according to an embodiment of the present application. As shown in FIG. 7 , the controller 240 includes:

[0063] The first multiplication module 2411 performs a multiplication operation on the bus voltage Vbus and the bus output current Ibus1 of the first boost isolation DC / DC converter 230 to obtain the current output power Pbus1 of the first boost isolation DC / DC converter 230;

[0064] The maximum power point tracking control module (MPPT) 2412 obtains the rated current variation coefficient k1 of the first boost isolated DC / DC converter 230 according to the difference between the current output power Pbus1 of the first boost isolated DC / DC converter 230 and the output power at the previous moment;

[0065] The second multiplication module 2413 multiplies the rated current variation coefficient k1 of the first boost isolated DC / DC converter 230 by the rated current Irated of the first boost isolated DC / DC converter 230 to obtain a bus output current reference value Ibus_ref1 of the first boost isolated DC / DC converter 230 ;

[0066] The first subtraction module 2414 performs a subtraction operation on the bus output current reference value Ibus_ref1 of the first boost isolated DC / DC converter 230 and the bus output current Ibus1 of the first boost isolated DC / DC converter 230 to obtain the bus output current difference Err_Ibus1 of the first boost isolated DC / DC converter 230;

[0067] The first operation module 2415 outputs a switch control signal C1 for controlling the first boost isolation DC / DC converter 230 according to the bus output current difference Err_Ibus1 of the first boost isolation DC / DC converter 230 .

[0068] Specifically, in one embodiment, the first operation module 2415 is a PI module. However, the present application does not limit the specific type of the first operation module 2415, as long as it can output the required switch control signal according to the bus output current difference Err_Ibus1.

[0069] Specifically, in practical applications, the controller 240 is a DSP or other microcontroller. Therefore, the first multiplication module 2411, the maximum power point tracking control module (MPPT) 2412, the second multiplication module 2413, the first subtraction module 2414, and the first operation module 2415 are implemented through programming.

[0070] Specifically, according to the controller principle shown in Figure 7, the loop used for maximum power point tracking control is a current loop that controls the bus output current Ibus. By calculating the output power of the first boost isolated DC / DC converter 230, the bus output current Ibus1 of the first boost isolated DC / DC converter 230 is disturbed to maximize the bus output current Ibus1 of the first boost isolated DC / DC converter 230, so as to track the maximum output power of the first boost isolated DC / DC converter 230, thereby achieving the purpose of tracking the maximum power point operation of the photovoltaic module.

[0071] The controller that usually realizes tracking the maximum power point of the photovoltaic module is also used to control the subsequent inverter. Therefore, the sampling of the signal required for tracking the maximum power point of the photovoltaic module and the sampling for controlling the inverter are both located on the secondary side of the transformer without the need for isolated sampling. Therefore, compared with the existing technology, it has the advantages of low cost, small size and no signal delay for photovoltaic maximum power point tracking control.

[0072] Referring to Figure 8 , a schematic diagram of a power conversion device according to another embodiment of the present invention is shown. The power conversion device further includes an inverter 220. The DC terminal of inverter 220 is connected to a DC bus 250, and the AC terminal of inverter 220 is used to output AC power. In other words, inverter 220 is used to invert the DC power at the DC bus into AC power to power AC loads or connect to the grid. The power conversion device shown in Figure 8 is thus implemented as a photovoltaic inverter system.

[0073] Specifically, as shown in FIG8 , the controller 240 also receives the AC current Iac and AC voltage Vac of the AC power output by the inverter 220, and outputs a switch control signal C_IVT for controlling the operation of the inverter 220 based on the bus voltage Vbus, the AC current Iac, and the AC voltage Vac, thereby converting the DC power into AC power and stabilizing the bus voltage Vbus. Because the bus voltage Vbus is stabilized by the subsequent inverter, the bus output current Ibus1 of the first step-up isolated DC / DC converter 230 can be maximized by controlling the bus output current Ibus1, thereby achieving the maximum output power of the first step-up isolated DC / DC converter 230 and tracking the maximum power point of the photovoltaic module.

[0074] Please refer to FIG9 , which is a schematic diagram of a controller used in the power conversion device shown in FIG8 according to an embodiment of the present application. As shown in FIG9 , the controller 240 further includes the following components based on the controller shown in FIG7 :

[0075] The second subtraction module 2421 performs a subtraction operation on the bus voltage reference value Vbus_ref and the bus voltage Vbus to obtain a bus voltage difference value Err_Vbus;

[0076] The second operation module 2422 obtains the AC current variation coefficient k3 according to the bus voltage difference Err_Vbus;

[0077] The phase-locked control module 2426 outputs a phase-locked sine wave Sin according to the AC voltage Vac;

[0078] The third multiplication module 2423 performs a multiplication operation on the phase-locked sine wave Sin, the AC current variation coefficient k3, and the AC current peak value Iac_pk to obtain an AC current reference value Iac_ref;

[0079] The third subtraction module 2424 performs a subtraction operation on the AC current reference value Iac_ref and the AC current Iac to obtain an AC current difference value Err_Iac;

[0080] The third operation module 2425 outputs a switch control signal C_IVT for controlling the inverter 220 according to the AC current difference Err_Iac.

[0081] Specifically, in one embodiment, the second operation module 2422 and the third operation module 2425 are PI modules. However, the present application does not limit the specific types of the second operation module 2422 and the third operation module 2425. It is sufficient as long as the second operation module 2422 can obtain the AC current variation coefficient k3 based on the bus voltage difference Err_Vbus, and as long as the third operation module 242 can output the switching control signal C_IVT for controlling the inverter 220 based on the AC current difference Err_Iac.

[0082] Similarly, the second subtraction operation module 2421 , the second operation module 2422 , the phase-locked control module 2426 , the third multiplication operation module 2423 , the third subtraction operation module 2424 and the third operation module 2425 are implemented by programming in the controller 240 .

[0083] As shown in Figure 9, the control loop of the downstream inverter includes two loops. One is the voltage loop for the bus voltage Vbus, whose output is the AC current variation coefficient k3. The AC current variation coefficient k3 is multiplied by the phase-locked sine wave Sin, and then multiplied by the AC current peak value Iac_pk to obtain the AC current reference value Iac_ref. The difference between the sampled AC current Iac and the AC current reference value Iac_ref is then used as the PI to obtain the switching control signal C_IVT for controlling the downstream inverter. The two loops of the downstream inverter are used to stabilize the bus voltage Vbus and output the sinusoidal current.

[0084] The bus voltage reference value Vbus_ref and the AC current peak value Iac_pk here are determined when the power conversion device is designed. Therefore, the bus voltage reference value Vbus_ref and the AC current peak value Iac_pk are given values.

[0085] Here, the bus voltage Vbus, the bus output current Ibus1 of the first step-up isolated DC / DC converter 230, the AC current Iac, and the AC voltage Vac are obtained by using. As shown in Figure 8, since the controller 240 and the secondary winding of the transformer in the first step-up isolated DC / DC converter 230 share a common ground, that is, the controller 240 is placed on the inverter side, and the four sampling signals of the bus voltage Vbus, the bus output current Ibus1 of the first step-up isolated DC / DC converter 230, the AC current Iac, and the AC voltage Vac are also on the inverter side. Compared with the solution of using a single controller in the prior art, the present application does not require isolated sampling, which can reduce costs and size, and does not introduce sampling delays, bringing convenience to control. Equivalent to the solution of using two controllers in the prior art, the present application can reduce one controller, thereby reducing costs and reducing peripheral circuits accordingly.

[0086] Please refer to the schematic diagram of the power conversion device of another embodiment of the application shown in Figure 10, which, on the basis of the power conversion device shown in Figure 8, also includes: a second boost isolation DC / DC converter 330, including a transformer 331, the input end of the second boost isolation DC / DC converter 330 is used to connect a second photovoltaic component 310, and the output end of the second boost isolation DC / DC converter 330 is used for the DC bus 250 to improve the power level of the power conversion device.

[0087] The controller 240 also receives the bus output current Ibus2 of the second boost isolated DC / DC converter 330, and outputs a switch control signal control C2 for controlling the operation of the second boost isolated DC / DC converter 330 according to the bus voltage Vbus and the bus output current Ibus2 of the second boost isolated DC / DC converter, so as to perform maximum power point tracking control of the second photovoltaic component 310.

[0088] The principle of implementing the maximum power point tracking of the second photovoltaic assembly 310 is the same as the principle of implementing the maximum power point tracking of the first photovoltaic assembly 210 described above, and will not be repeated here.

[0089] Specifically, please refer to the schematic diagram of the controller applied to the power conversion device shown in Figure 10 shown in Figure 11. Based on the controller shown in Figure 9, it adds a control loop for maximizing the bus output current Ibus2 of the second step-up isolated DC / DC converter 330. Specifically, it includes a multiplication module 2431, an MPPT module 2432, a multiplication module 2433, a subtraction module 2434 and an operation module 2435. Their functions are respectively the same as the first multiplication module 2411, the maximum power point tracking control module (MPPT) 2412, the second multiplication module 2413, the first subtraction module 2414 and the first operation module 2415, and are not repeated here.

[0090] In a specific implementation, the first photovoltaic assembly 210 and the second photovoltaic assembly 310 can each be formed by a single photovoltaic cell group, or both can be formed by multiple photovoltaic cell groups connected in series and parallel. Of course, one can also be formed by a single photovoltaic cell group, and the other can be formed by multiple photovoltaic cell groups connected in series and parallel. The series-parallel connection here includes series connection, parallel connection, series connection followed by parallel connection, and parallel connection followed by series connection. Regardless of the number of photovoltaic modules in the first photovoltaic assembly 210 and the second photovoltaic assembly 310, the voltage of the first photovoltaic assembly 210 and the second photovoltaic assembly 310 is relatively low, and a step-up isolated DC / DC converter is required to raise it to a higher voltage before subsequent conversion.

[0091] In the system shown in Figure 10, there are two boost isolated DC / DC converters connected to the DC bus. However, in actual applications, the number of boost isolated DC / DC converters connected to the DC bus can be set according to the power level requirements. The corresponding controller also needs a corresponding number of loops to control the operation of the boost isolated DC / DC converters to achieve tracking of the maximum power point of the PV module connected to each boost isolated DC / DC converter.

[0092] For a system with multiple boost isolated DC / DC converters, each channel only needs to sample the bus output current at its output end through a non-isolated sampling circuit. Compared with the existing technology that uses multiple isolated sampling circuits to sample the voltage of photovoltaic modules, multiple isolated sampling circuits are saved, thereby greatly reducing the cost and size of the power conversion system.

[0093] In the aforementioned power conversion systems, the subsequent stage is an inverter, enabling its output to be connected to the grid or to power an AC load. In practical applications, the subsequent stage may also be a DC / DC converter circuit, converting DC power at the bus voltage into DC power suitable for powering a DC load. This application does not limit the specific type of the subsequent circuit.

[0094] In one embodiment of the present application, a maximum power point tracking control method for a photovoltaic module is provided, which is applied to a photovoltaic power generation system. Referring to the power conversion device shown in FIG5 , the method includes: a voltage detection circuit detecting a bus voltage Vbus of a DC bus connected to an output terminal of a first step-up isolated DC / DC converter 230;

[0095] The first current detection circuit detects the bus output current Ibus1 at the output end of the first step-up isolated DC / DC converter 230;

[0096] A controller 240 performs maximum power point tracking control of the first photovoltaic assembly 210 according to the bus voltage Vbus and the bus output current Ibus1 of the first boost isolated DC / DC converter, wherein the input end of the first boost isolated DC / DC converter 230 is used to connect to the first photovoltaic assembly 210.

[0097] Similar to the above, the sampling circuit for the signal required to track the maximum power point of the photovoltaic module is moved to the secondary side of the transformer. A larger sampling resistor can be used to improve sampling accuracy while minimizing losses. Therefore, the method provided in this application combines the advantages of high current sampling accuracy with low losses, significantly reducing design complexity. Furthermore, the non-isolated sampling method using a sampling resistor minimizes sampling delay, resulting in better real-time control.

[0098] In one embodiment, the controller 240 controls the bus output current Ibus1 of the first boost isolated DC / DC converter 230 based on the bus voltage Vbus and the bus output current Ibus1 of the first boost isolated DC / DC converter to maximize the bus output current Ibus1, thereby achieving maximum power point tracking control of the first photovoltaic assembly 210. Referring to FIG. 6 and its description, the bus voltage Vbus is a substantially constant voltage value, so the output power Pout of the boost isolated DC / DC converter increases with the increase of the bus output current Ibus of the boost isolated DC / DC converter. Therefore, as long as the boost isolated DC / DC converter is controlled to maximize the bus output current Ibus of the boost isolated DC / DC converter, the output power Pout of the boost isolated DC / DC converter can be maximized, thereby achieving maximum power point tracking of the first photovoltaic assembly 210.

[0099] In one embodiment, referring to FIG. 7 and its description, the controller 240 performs:

[0100] Multiplying the bus voltage Vbus by the bus output current Ibus1 of the first boost isolated DC / DC converter to obtain a current output power Pbus1 of the first boost isolated DC / DC converter;

[0101] Obtaining a rated current variation coefficient k1 of the first boost isolated DC / DC converter according to a difference between a current output power Pbus1 of the first boost isolated DC / DC converter and an output power at a previous moment;

[0102] Multiplying the rated current variation coefficient k1 of the first boost isolated DC / DC converter by the rated current Irated of the first boost isolated DC / DC converter to obtain a bus output current reference value Ibus_ref1 of the first boost isolated DC / DC converter;

[0103] Subtracting the bus output current reference value Ibus_ref1 of the first boost isolated DC / DC converter from the bus output current Ibus1 of the first boost isolated DC / DC converter to obtain a bus output current difference Err_Ibus1 of the first boost isolated DC / DC converter;

[0104] A switch control signal C1 for controlling the first boost isolated DC / DC converter is output according to the bus output current difference Err_Ibus1 of the first boost isolated DC / DC converter to track the maximum power point of the first photovoltaic assembly 210 .

[0105] In this way, the loop used for maximum power point tracking control is a current loop that controls the bus output current Ibus. By calculating the output power of the first boost isolated DC / DC converter 230, the bus output current Ibus1 of the first boost isolated DC / DC converter 230 is disturbed to maximize the bus output current Ibus1 of the first boost isolated DC / DC converter 230, so as to track the maximum output power of the first boost isolated DC / DC converter 230, thereby achieving the purpose of tracking the maximum power point of the photovoltaic module.

[0106] In a specific embodiment, referring to the power conversion device shown in FIG8 , the method further includes:

[0107] An AC voltage detection circuit detects the AC voltage Vac at the AC output side of the inverter 220;

[0108] An AC current detection circuit detects the AC current Iac on the AC output side of the inverter 220, wherein the DC end of the inverter 220 is connected to the DC bus 250. The controller 240 further executes: outputting a switch control signal C_IVT for controlling the operation of the inverter according to the bus voltage Vbus, the AC current Iac and the AC voltage Vac.

[0109] In this manner, controller 240 also receives the AC current Iac and AC voltage Vac of the AC power output by inverter 220, and outputs a switch control signal C_IVT to control the operation of inverter 220 based on the bus voltage Vbus, AC current Iac, and AC voltage Vac, thereby converting DC power into AC power and stabilizing the bus voltage Vbus. Because the bus voltage Vbus is stabilized by the subsequent inverter, the bus output current Ibus1 of the first step-up isolated DC / DC converter 230 can be maximized by controlling the output current, thereby achieving the maximum output power of the first step-up isolated DC / DC converter 230 and tracking the maximum power point of the photovoltaic module. Furthermore, the output AC power is suitable for grid connection or AC loads.

[0110] In a specific embodiment, referring to FIG. 9 and its description, the controller further executes:

[0111] Subtracting the bus voltage reference value Vbus_ref from the bus voltage Vbus to obtain a bus voltage difference value Err_Vbus;

[0112] Obtaining the AC current variation coefficient k3 according to the bus voltage difference Err_Vbus;

[0113] Outputting a phase-locked sine wave Sin according to the AC voltage Vac;

[0114] Performing a multiplication operation on the phase-locked sine wave Sin, the AC current variation coefficient k3, and the AC current peak value Iac_pk to obtain an AC current reference value Iac_ref;

[0115] Subtracting the AC current reference value Iac_ref from the AC current Iac to obtain an AC current difference value Err_Iac;

[0116] The switching control signal C_IVT for controlling the inverter 220 is output according to the AC current difference Err_Iac.

[0117] That is, the control of the downstream inverter includes two loops: one is the voltage loop for the bus voltage Vbus, whose output is the AC current variation coefficient k3. The AC current variation coefficient k3 is multiplied by the phase-locked sine wave Sin, and then multiplied by the AC current peak value Iac_pk to obtain the AC current reference value Iac_ref. The difference between the sampled AC current Ia and the AC current reference value Iac_ref is then used as the PI to obtain the switching control signal C_IVT for controlling the downstream inverter. The two loops of the downstream inverter are used, one for stabilizing the bus voltage Vbus and the other for outputting the sinusoidal current.

[0118] In a specific embodiment, referring to the power conversion system shown in Figure 10, the method further includes: a second current detection circuit detects the bus output current Ibus2 of the second boost isolated DC / DC converter 330; the controller 240 also performs maximum power point tracking control of a second photovoltaic component 310 based on the bus voltage Vbus and the bus output current Ibus2 of the second boost isolated DC / DC converter, wherein the input end of the second boost isolated DC / DC converter 330 is used to connect to the second photovoltaic component 310.

[0119] Furthermore, the controller 240 controls the bus output current Ibus2 of the second boost isolated DC / DC converter 330 according to the bus voltage Vbus and the bus output current Ibus2 of the second boost isolated DC / DC converter to maximize the bus output current Ibus2, so as to achieve maximum power point tracking control of the second photovoltaic component 310.

[0120] In a specific implementation, the switch control signal C1 , the switch control signal C2 , and the switch control signal C_IVT may be a duty cycle control signal PWM or a frequency control signal PFM.

[0121] The current sampling circuit and voltage sampling circuit used in this application are both non-isolated sampling circuits, such as a resistor plus op amp sampling circuit. However, this application does not limit the specific structure of the non-isolated sampling circuit; any non-isolated circuit that can sample current or voltage is applicable to this application.

[0122] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A power conversion device used in a photovoltaic system, characterized in that: include: A first step-up isolated DC / DC converter, comprising a transformer, wherein an input end of the first step-up isolated DC / DC converter is used to connect a first photovoltaic module, and an output end of the first step-up isolated DC / DC converter is used for a DC bus; A controller receives a bus voltage of the DC bus and a bus output current output by the first step-up isolated DC / DC converter, and outputs a switch control signal for controlling the operation of the first step-up isolated DC / DC converter according to the bus voltage and the bus output current output by the first step-up isolated DC / DC converter, so as to perform maximum power point tracking control of the first photovoltaic assembly.

2. The power conversion device used in a photovoltaic system according to claim 1, characterized in that: The controller and the secondary winding of the transformer share a common ground.

3. The power conversion device used in a photovoltaic system according to claim 1, characterized in that: The switch control signal controlling the first step-up isolated DC / DC converter maximizes a bus output current output by the first step-up isolated DC / DC converter.

4. The power conversion device used in a photovoltaic system according to claim 1 or 3, characterized in that: The controller comprises: a first multiplication module, performing a multiplication operation on the bus voltage and the bus output current of the first step-up isolated DC / DC converter to obtain a current output power of the first step-up isolated DC / DC converter; a maximum power point tracking control module, obtaining a rated current variation coefficient of the first boost isolated DC / DC converter according to a difference between a current output power of the first boost isolated DC / DC converter and an output power at a previous moment; a second multiplication module, performing a multiplication operation on the rated current variation coefficient of the first boost isolated DC / DC converter and the rated current of the first boost isolated DC / DC converter to obtain a bus output current reference value of the first boost isolated DC / DC converter; a first subtraction operation module, performing a subtraction operation on a bus output current reference value of the first boost isolated DC / DC converter and a bus output current of the first boost isolated DC / DC converter to obtain a bus output current difference of the first boost isolated DC / DC converter; The first operation unit outputs the switch control signal for controlling the first boost isolation DC / DC converter according to the bus output current difference of the first boost isolation DC / DC converter.

5. The power conversion device used in a photovoltaic system according to claim 1, characterized in that: It also includes an inverter, wherein the DC end of the inverter is connected to the DC bus, and the AC end of the inverter is used to output AC power.

6. The power conversion device used in a photovoltaic system according to claim 5, characterized in that: The controller also receives the AC current and AC voltage of the AC power output by the inverter, and outputs a switch control signal for controlling the operation of the inverter according to the bus voltage, the AC current and the AC voltage.

7. The power conversion device used in a photovoltaic system according to claim 6, characterized in that: The controller further comprises: A second subtraction operation module performs a subtraction operation on the bus voltage reference value and the bus voltage to obtain a bus voltage difference; A second operation module, obtaining the AC current variation coefficient according to the bus voltage difference; A phase-locked control module outputs a phase-locked sine wave according to the AC voltage; A third multiplication module performs multiplication operation on the phase-locked sine wave, the AC current variation coefficient, and the AC current peak value to obtain an AC current reference value; a third subtraction operation module, performing a subtraction operation on the AC current reference value and the AC current to obtain an AC current difference; The third operation unit outputs a switch control signal for controlling the inverter according to the AC current difference.

8. The power conversion device used in a photovoltaic system according to claim 1, characterized in that: Also includes: A second step-up isolated DC / DC converter, comprising a transformer, wherein an input end of the second step-up isolated DC / DC converter is used to connect a second photovoltaic module, and an output end of the second step-up isolated DC / DC converter is used for the DC bus; The controller also receives the bus output current of the second boost isolated DC / DC converter, and outputs a switch control signal to control the operation of the second boost isolated DC / DC converter according to the bus voltage and the bus output current of the second boost isolated DC / DC converter, so as to perform maximum power point tracking control of the second photovoltaic component.

9. The power conversion device used in a photovoltaic system according to claim 8, characterized in that: The switch control signal controlling the second step-up isolated DC / DC converter maximizes a bus output current output by the second step-up isolated DC / DC converter.

10. The power conversion device used in a photovoltaic system according to claim 1, characterized in that: The first photovoltaic assembly is formed by one photovoltaic cell group, or by a plurality of photovoltaic cell groups connected in series and in parallel.

11. A maximum power point tracking control method for a photovoltaic module, characterized in that: include: A voltage detection circuit detects a bus voltage of a DC bus connected to an output end of a first step-up isolated DC / DC converter; A first current detection circuit detects a bus output current at an output end of the first step-up isolated DC / DC converter; A controller performs maximum power point tracking control of a first photovoltaic component according to the bus voltage and the bus output current of the first step-up isolated DC / DC converter, wherein the input end of the first step-up isolated DC / DC converter is used to connect the first photovoltaic component.

12. The maximum power point tracking control method of a photovoltaic module according to claim 11, characterized in that: The controller maximizes the bus output current of the first boost isolated DC / DC converter according to the bus voltage and the bus output current of the first boost isolated DC / DC converter to achieve maximum power point tracking control of the first photovoltaic assembly.

13. The maximum power point tracking control method of a photovoltaic module according to claim 11, characterized in that: The controller executes: Performing a multiplication operation on the bus voltage and the bus output current of the first step-up isolated DC / DC converter to obtain a current output power of the first step-up isolated DC / DC converter; Obtaining a rated current variation coefficient of the first boost isolated DC / DC converter according to a difference between a current output power of the first boost isolated DC / DC converter and an output power at a previous moment; performing a multiplication operation on the rated current variation coefficient of the first boost isolated DC / DC converter and the rated current of the first boost isolated DC / DC converter to obtain a bus output current reference value of the first boost isolated DC / DC converter; Subtracting a bus output current reference value of the first boost isolated DC / DC converter from the bus output current of the first boost isolated DC / DC converter to obtain a bus output current difference of the first boost isolated DC / DC converter; A switch control signal for controlling the first step-up isolated DC / DC converter is output according to the bus output current difference of the first step-up isolated DC / DC converter to track the maximum power point of the first photovoltaic assembly.

14. The maximum power point tracking control method of a photovoltaic module according to claim 13, characterized in that: Also includes: An AC voltage detection circuit detects an AC voltage on an AC output side of an inverter; an AC current detection circuit to detect the AC current at the AC output side of the inverter, wherein the DC end of the inverter is connected to the DC bus, and the controller further executes: The line voltage, the AC current and the AC voltage are used to output a switch control signal for controlling the operation of the inverter.

15. The maximum power point tracking control method of a photovoltaic module according to claim 14, characterized in that: The controller also executes: Subtracting the bus voltage reference value from the bus voltage to obtain a bus voltage difference; Obtaining the AC current variation coefficient according to the bus voltage difference; Outputting a phase-locked sine wave according to the AC voltage; Performing a multiplication operation on the phase-locked sine wave, the AC current variation coefficient, and the AC current peak value to obtain an AC current reference value; Subtracting the AC current reference value from the AC current to obtain an AC current difference; The switch control signal for controlling the inverter is output according to the AC current difference.

16. The maximum power point tracking control method of a photovoltaic module according to claim 11 or 14, characterized in that: Also includes: A second current detection circuit detects a bus output current of a second boost isolation DC / DC converter; The controller also performs maximum power point tracking control of a second photovoltaic component according to the bus voltage and the bus output current of the second step-up isolated DC / DC converter, wherein the input end of the second step-up isolated DC / DC converter is used to connect the second photovoltaic component.

17. The maximum power point tracking control method of a photovoltaic assembly according to claim 16, characterized in that: The controller maximizes the bus output current of the second boost isolated DC / DC converter according to the bus voltage and the bus output current of the second boost isolated DC / DC converter to achieve maximum power point tracking control of the second photovoltaic assembly.

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