A multi-point stepping method and its combination with CVT for MPPT tracking, and a photovoltaic inverter.

Abstract

This invention belongs to the field of photovoltaic power generation technology, and specifically relates to a multi-point stepping method and its combination with a CVT for MPPT tracking, as well as a photovoltaic inverter. This invention provides a novel MPPT tracking method based on a multi-point stepping method. It collects the output voltage and current signals from the solar panel bus to construct a P-U characteristic curve, samples points on the P-U characteristic curve, and determines the location of the maximum power point based on the slope of the sampled points. Then, it controls the bus to output the voltage value corresponding to the maximum power location. This method can improve the stability and efficiency of MPPT tracking.

Description

Technical Field

[0001] This invention belongs to the field of photovoltaic power generation technology, and specifically relates to an MPPT tracking method based on multi-point stepping method and its combination with CVT, as well as a photovoltaic inverter. Background Technology

[0002] In a grid-connected solar power generation system, a solar cell is a semiconductor device that directly converts light energy into electrical energy. Under a certain amount of light energy, it is of great significance to effectively increase the output power of solar cells, thereby making full use of solar energy and improving the power generation efficiency of the entire grid-connected solar power generation system.

[0003] Research on the output power characteristics of solar cells revealed that the output power P of a solar cell is related to the light intensity W and the ambient temperature T, such as... Figure 1 The figure shows the PU characteristic curves under the same T but different W, where U is the output voltage of the solar cell; Figure 2 These are the PU characteristic curves under the same W but different T conditions. From... Figure 1 It can be seen that the maximum output power of a solar cell increases with increasing light intensity, and there is a unique maximum output power point under the same lighting conditions. To the left of the maximum output power point, the output power increases approximately linearly with the increase of the solar cell output voltage; after reaching the maximum output power point, the output power begins to decrease rapidly, and the rate of decrease is much greater than the rate of increase. Figure 2 In the middle, the overall trend of solar cell output power change is similar to Figure 1 resemblance.

[0004] Therefore, the output power of solar cells is closely related to light intensity and ambient temperature. When the external environment changes, the output power of solar cells will also change significantly. Controlling the photovoltaic power generation system to make the solar cells work near their maximum output power point is an effective way to improve the efficiency of solar cells.

[0005] MPPT (Maximum Power Point Tracking) is a widely used power point control strategy for solar cells. It tracks the maximum output power point of the solar cells by real-time changes to the operating state of the grid-connected power generation system, thereby achieving maximum power output. Specifically, by changing the control input, the output voltage of the solar cell changes. The difference between the output power and the output voltage before and after the change is used to determine the current operating point on the PU characteristic curve, and then to determine the next step in changing the control input, thus gradually bringing the solar cell's operating point closer to the maximum output power point.

[0006] MPPT tracking in photovoltaic inverters includes various methods such as CVT, perturbation method, and conductance method. Currently, most existing controllers that adopt MPPT control strategies use the voltage perturbation method to track the maximum output power point of solar cells, which has the problems of low perturbation tracking rate and poor tracking stability. Summary of the Invention

[0007] To address the aforementioned problems, this invention provides an MPPT tracking method based on a multi-point stepping method and its combination with a CVT, as well as a photovoltaic inverter.

[0008] The specific technical solution of this invention is as follows:

[0009] This invention provides an MPPT tracking method based on a multi-point stepping method. The controller in the photovoltaic inverter is equipped with a maximum power point tracking module. This module acquires output voltage and current signals from the output ports of the solar panels and controls the output voltage of the bus based on the processing results of these signals. The specific processing method for the output voltage and current signals by the maximum power point tracking module is as follows:

[0010] S1: Calculate the output power based on the output voltage signal and the output current signal;

[0011] S2: Construct a PU characteristic curve with output voltage as the horizontal axis and output power as the vertical axis;

[0012] S3: Starting from the starting point on the PU characteristic curve, crawl towards the end point according to the step size s, and take the crawled points as sampling points;

[0013] S4: Within the sampling period t, the sampling point is sampled n times;

[0014] S5: Compare the slopes of n sampling points, determine the location of the maximum power point based on the slope, and send the voltage signal corresponding to the maximum power to the bus of the solar panel.

[0015] A maximum power point tracking (MPPT) method based on a multi-point stepping method combined with a CVT is disclosed. The controller of the photovoltaic inverter is equipped with a maximum power point tracking module, and the photovoltaic inverter integrates a boost circuit. The maximum power point tracking module collects output voltage and current signals from the bus of the solar panels and controls the output voltage of the bus based on the processing results of these signals. The specific processing method for the output voltage and current signals by the maximum power point tracking module is as follows:

[0016] S100: Acquire the input DC voltage signal of the boost circuit and compare the input DC voltage signal with the output voltage signal of the bus. If the DC voltage signal is greater than the output voltage signal, perform the MPPT tracking method based on the multi-point stepping method. If the DC voltage signal is less than the output voltage signal, proceed to step S200.

[0017] S200: Calculates the output power based on the output voltage signal and the output current signal;

[0018] S300: Construct a PU characteristic curve with output voltage as the horizontal axis and output power as the vertical axis;

[0019] S400: Determine the maximum power point on the PU characteristic curve and compare the output voltage value corresponding to the maximum power point with the maximum power point voltage command value. If the output voltage value is equal to the maximum power point voltage command value, send a command to the bus of the solar panel to keep the output voltage unchanged. If the output voltage value is greater than the maximum power point voltage command value, send a command to the bus of the solar panel to decrease the output voltage. If the output voltage value is less than the maximum power point voltage command value, send a command to the bus of the solar panel to increase the output voltage.

[0020] A photovoltaic inverter includes a main circuit and a control circuit. The main circuit includes a photovoltaic string, a bus, a boost converter, an inverter circuit, and a filter circuit. The control circuit includes a controller. The photovoltaic string transmits a DC voltage signal to the boost converter via the bus. The boost converter boosts the DC voltage signal input from the photovoltaic string and transmits it to the inverter circuit. The inverter circuit converts the boosted DC voltage signal into an AC voltage signal, which is then filtered by the filter circuit and connected to the power grid. The controller is equipped with a maximum power point tracking (MPPT) module, which executes any of the MPPT tracking methods described above.

[0021] The beneficial effects achieved by this invention are as follows:

[0022] This invention provides a novel MPPT tracking method based on a multi-point stepping method. It collects the output voltage and current signals of the solar panel bus to construct a PU characteristic curve, samples the points on the PU characteristic curve, and determines the location of the maximum power point based on the slope of the sampled points. Then, it controls the bus to output the voltage value corresponding to the maximum power location. This method can improve the stability and efficiency of MPPT tracking. Attached Figure Description

[0023] Figure 1 and Figure 2 This is a schematic diagram of the PU characteristic curve in the prior art;

[0024] Figure 3 This is a schematic diagram of the photovoltaic inverter in this invention;

[0025] Figure 4 This is a flowchart of the MPPT tracking method based on the multi-point stepping method in this invention;

[0026] Figure 5 and Figure 6 This is a schematic diagram of the PU characteristic curve in this invention;

[0027] Figure 7 This is a flowchart of the MPPT tracking method based on the combination of multi-point stepping method and CVT in this invention;

[0028] Figure 8 This is a structural diagram of the boost circuit in this invention. Detailed Implementation

[0029] The present invention will be further described below with reference to the accompanying drawings and embodiments. The following embodiments are only used to explain the invention and are not intended to limit the scope of protection of the present invention.

[0030] like Figure 3 The schematic diagram of the photovoltaic inverter shown includes a main circuit and a control circuit. The main circuit includes a photovoltaic string, a bus, a boost converter, an inverter circuit, and a filter circuit. The control circuit includes a controller. The photovoltaic string transmits a DC voltage signal to the boost converter via the bus. The boost converter boosts the DC voltage signal input from the photovoltaic array and transmits it to the inverter circuit. The inverter circuit converts the boosted DC voltage signal into an AC voltage signal, which is then filtered by the filter circuit before being connected to the power grid. The controller is equipped with a maximum power point tracking (MPPT) module. The MPPT module is used to collect the output voltage and current signals from the output ports of the solar panels. By tracking the output voltage and current signals, it determines the actual output voltage and sends it to the bus to control the bus to output the appropriate voltage and current. To implement this method, the present invention preferably uses a multi-point stepping method and a combination of the multi-point stepping method and a CVT. These two methods are described below.

[0031] In one embodiment, the present invention provides an MPPT tracking method based on a multi-point stepping method, such as... Figure 4 As shown, the specific processing method of the maximum power point tracking module for the output voltage signal and the output current signal is as follows:

[0032] S1: Calculate the output power based on the output voltage signal and the output current signal;

[0033] S2: Construct a PU characteristic curve with output voltage as the horizontal axis and output power as the vertical axis;

[0034] S3: Starting from the initial point on the PU characteristic curve, crawl towards the endpoint with a step size 's', and use the crawled points as sampling points, such as... Figure 5 , Figure 6 As shown;

[0035] S4: Within the sampling period t, the sampling point is sampled n times;

[0036] S5: Compare the slopes of n sampling points, determine the location of the maximum power point based on the slope, and send the voltage signal corresponding to the maximum power to the bus of the solar panel.

[0037] This invention provides a novel MPPT tracking method based on a multi-point stepping method. It collects the output voltage and current signals of the solar panel bus to construct a PU characteristic curve, samples the points on the PU characteristic curve, and determines the location of the maximum power point based on the slope of the sampled points. Then, it controls the bus to output the voltage value corresponding to the maximum power location. This method can improve the stability and efficiency of MPPT tracking.

[0038] In this embodiment, the crawling step period in step S3 is ΔT.

[0039] In this embodiment, the value of step size s in step S3 and the value of sampling period t in step S4 are determined according to the direction of the sampling point.

[0040] The method for determining the direction of sampling points in this embodiment is as follows:

[0041] Determine the slope of two adjacent sampling points. If the slope of the later sampling point is greater than that of the earlier sampling point, then crawl in the direction of the later sampling point; otherwise, crawl in the opposite direction. Figure 5 , Figure 6 As shown.

[0042] In another embodiment, the present invention provides an MPPT tracking method based on a combination of multi-point stepping and CVT, wherein the photovoltaic inverter also integrates a boost circuit, such as... Figure 7 As shown, the maximum power point tracking module processes the output voltage and output current signals in the following ways:

[0043] S100: Collect the input DC voltage signal of the boost circuit and compare the input DC voltage signal with the output voltage signal of the bus. If the DC voltage signal is greater than the output voltage signal, proceed to step S1; if the DC voltage signal is less than the output voltage signal, proceed to step S200.

[0044] S1: Calculate the output power based on the output voltage signal and the output current signal;

[0045] S2: Construct a PU characteristic curve with output voltage as the horizontal axis and output power as the vertical axis;

[0046] S3: Starting from the starting point on the PU characteristic curve, crawl towards the end point according to the step size s, and take the crawled points as sampling points;

[0047] S4: Within the sampling period t, the sampling point is sampled n times;

[0048] S5: Compare the slope of n sampling points, determine the location of the maximum power point based on the slope, and send the voltage signal corresponding to the maximum power to the bus of the solar panel;

[0049] S200: Calculates the output power based on the output voltage signal and the output current signal;

[0050] S300: Construct a PU characteristic curve with output voltage as the horizontal axis and output power as the vertical axis;

[0051] S400: Determine the maximum power point on the PU characteristic curve and compare the output voltage value corresponding to the maximum power point with the maximum power point voltage command value. If the output voltage value is equal to the maximum power point voltage command value, send a command to the bus of the solar panel to keep the output voltage unchanged. If the output voltage value is greater than the maximum power point voltage command value, send a command to the bus of the solar panel to decrease the output voltage. If the output voltage value is less than the maximum power point voltage command value, send a command to the bus of the solar panel to increase the output voltage.

[0052] In this invention, the input DC voltage signal of the boost circuit is compared with the output voltage signal of the bus to determine whether the boost circuit is in bypass mode. If it is a bypass module, a PU characteristic curve is constructed, and the voltage and current values ​​of the control bus are determined based on the maximum power point. If not, the output voltage value corresponding to the maximum power point on the PU characteristic curve and the maximum power point voltage command value are used to control the bus voltage. The combination of these two methods further improves the tracking efficiency of MPPT.

[0053] like Figure 8As shown, the boost circuit in this embodiment includes a boost1 circuit, a boost2 circuit, and a first bypass (including a diode, i.e., a diode connected in parallel with the boost1 circuit) and a second bypass (including a diode, i.e., a diode connected in parallel with the boost2 circuit) connected to the boost1 circuit and the boost2 circuit respectively. The first bypass and the second bypass receive DC voltage signals from bus1+ and bus2+ respectively. When the DC input voltage signals of the first bypass and the second bypass are greater than the output voltage signals on the bus, the boost1 circuit and the boost2 circuit are bypassed and do not work.

[0054] In this embodiment, the maximum power point voltage command value is equal to the coefficient multiplied by the open-circuit voltage of the solar panel, and the maximum power point voltage command value is set by the maximum power tracking module.

[0055] The coefficient described in this embodiment is 0.78.

[0056] In this embodiment, when the boost circuit is bypassed, the actual output voltage of the solar panel is the bus voltage; otherwise, the actual output voltage is the PV voltage.

[0057] While this specification contains numerous specific implementation details, these should not be construed as limiting the scope of any invention or the scope of the claims, but rather as descriptions of features that can embody specific embodiments of a particular invention. Specific features described in this specification within the context of an independent embodiment may also be implemented in combination with a single embodiment. Conversely, various features described within the context of a single embodiment may also be implemented independently in multiple embodiments, or in any suitable sub-combination. Furthermore, while features may be described for combination and even initially claimed in this way, one or more features from a claimed combination may be removed from that combination in some cases, and the claimed combination may be redirected to a sub-combination or a variation thereof.

[0058] Specific implementations of the subject matter have been described. Other implementations are within the scope of the following claims. For example, the activities described in the claims can be performed in a different order and still achieve the desired result. As an example, the processes described in the drawings do not necessarily require a specific order or sequence to be shown in order to achieve the desired result. In certain implementations, multitasking and parallel processing may be advantageous.

Claims

1. A maximum power point tracking (MPPT) method based on a multi-point stepping method combined with a CVT, wherein the controller of the photovoltaic inverter is equipped with a maximum power point tracking module, and the photovoltaic inverter integrates a boost circuit; the maximum power point tracking module acquires output voltage and output current signals from the bus of the solar panels, and controls the output voltage of the bus based on the processing results of the output voltage and output current signals, characterized in that... The specific processing method of the maximum power point tracking module for the output voltage signal and the output current signal is as follows: S100: Acquire the input DC voltage signal of the boost circuit and compare the input DC voltage signal with the output voltage signal of the bus. If the DC voltage signal is greater than the output voltage signal, perform the MPPT tracking method based on the multi-point stepping method. If the DC voltage signal is less than the output voltage signal, proceed to step S200. S200: Calculates the output power based on the output voltage signal and the output current signal; S300: Construct a PU characteristic curve with output voltage as the horizontal axis and output power as the vertical axis; S400: Determine the maximum power point on the PU characteristic curve and compare the output voltage value corresponding to the maximum power point with the maximum power point voltage command value. If the output voltage value is equal to the maximum power point voltage command value, send a command to the bus of the solar panel to keep the output voltage unchanged. If the output voltage value is greater than the maximum power point voltage command value, send a command to the bus of the solar panel to decrease the output voltage. If the output voltage value is less than the maximum power point voltage command value, send a command to the bus of the solar panel to increase the output voltage. The multi-point stepping MPPT tracking method includes the following steps: S1: Calculate the output power based on the output voltage signal and the output current signal; S2: Construct a PU characteristic curve with output voltage as the horizontal axis and output power as the vertical axis; S3: Starting from the starting point on the PU characteristic curve, crawl towards the end point according to the step size s, and take the crawled points as sampling points; S4: Within the sampling period t, the sampling point is sampled n times; S5: Compare the slopes of n sampling points, determine the location of the maximum power point based on the slope, and send the voltage signal corresponding to the maximum power to the bus of the solar panel.

2. The MPPT tracking method based on the combination of multi-point stepping method and CVT as described in claim 1, characterized in that, In step S3, the crawling step period is ΔT.

3. The MPPT tracking method based on the combination of multi-point stepping method and CVT as described in claim 1, characterized in that, The value of step size s in step S3 and the value of sampling period t in step S4 are determined according to the direction of the sampling point.

4. The MPPT tracking method based on the combination of multi-point stepping method and CVT as described in claim 1, characterized in that, The specific method for determining the direction of sampling points is as follows: Determine the slope of two adjacent sampling points. If the slope of the later sampling point is greater than that of the earlier sampling point, crawl in the direction of the later sampling point; otherwise, crawl in the opposite direction.

5. The MPPT tracking method based on the combination of multi-point stepping method and CVT as described in claim 1, characterized in that, The boost circuit includes a boost1 circuit, a boost2 circuit, and a first bypass and a second bypass connected to the boost1 circuit and the boost2 circuit, respectively. The first bypass and the second bypass receive DC voltage signals from bus1+ and bus2+, respectively. When the DC input voltage signals of the first bypass and the second bypass are greater than the output voltage signals on the bus, the boost1 circuit and the boost2 circuit are bypassed and do not work.

6. The MPPT tracking method based on the combination of multi-point stepping method and CVT as described in claim 1, characterized in that, The maximum power point voltage command value is equal to the coefficient multiplied by the open-circuit voltage of the solar panel, and the maximum power point voltage command value is set by the maximum power tracking module.

7. The MPPT tracking method based on the combination of multi-point stepping method and CVT as described in claim 6, characterized in that, The coefficient is 0.

78.

8. The MPPT tracking method based on the combination of multi-point stepping method and CVT as described in claim 5, characterized in that, When the boost circuit is bypassed, the actual output voltage of the solar panel is the bus voltage; otherwise, the actual output voltage is the PV voltage.

9. A photovoltaic inverter, characterized in that, The photovoltaic inverter includes a main circuit and a control circuit. The main circuit includes a photovoltaic string, a bus, a boost converter, an inverter circuit, and a filter circuit. The control circuit includes a controller. The photovoltaic string transmits a DC voltage signal to the boost converter via the bus. The boost converter boosts the DC voltage signal input from the photovoltaic string and transmits it to the inverter circuit. The inverter circuit converts the boosted DC voltage signal into an AC voltage signal, which is then filtered by the filter circuit and connected to the grid. The controller is equipped with a maximum power point tracking (MPPT) module, which executes the MPPT tracking method as described in any one of claims 1-8.

Images