Photovoltaic flexible power point control method and device

Abstract

The application discloses a photovoltaic flexible power point control method and device, and the method comprises the following steps: judging whether the illumination irradiance changes at the current moment; if the illumination irradiance changes, calculating the duty ratio of the intersection point D of the load line and the photovoltaic characteristic curve after the illumination changes according to the current working mode, the load line is the connecting line of the intersection point of the power limit curve and the photovoltaic characteristic curve after the solar irradiance changes and the original point, and jumping from the current working point to the working point of the point D based on the duty ratio of the point D; if the illumination irradiance does not change, calculating the perturbation step and the perturbation direction according to the state of the current working point, making the perturbation step adaptively decrease with the cycle number, reaching the next working point from the current working point according to the calculated perturbation step and the perturbation direction, and cyclically executing until the target power point is reached. The application can track fast in the transient state and reduce the power oscillation in the steady state.

Description

Technical Field

[0001] The present invention relates to photovoltaic technology, and in particular to a photovoltaic flexible power point control method and device. Background Art

[0002] With the depletion of fossil fuels, renewable energy sources such as photovoltaics (PV) have received increasing attention in recent years. For the past few decades, photovoltaic power plants (PVPPs) have typically operated at their maximum power point (MPP) to maximize energy collection efficiency. The increasing penetration of PV power generation has brought new challenges to the power grid, such as overvoltage and overload. To address these issues, stricter regulations have been adopted to transform PV power plants. Consequently, flexible power point control strategies have been promoted. The constant power generation (CPG) algorithm, as one of the flexible power control strategies, has been applied to address these issues.

[0003] The traditional CPG algorithm works as follows when irradiation increases and decreases: (1) When irradiation increases, there is a transition between two different operating modes. Figure 1 The figure shows the operation process of the traditional CPG control algorithm when the solar irradiance increases. Point A is the intersection of the load line and the photovoltaic characteristic curve under the current irradiation, point B is the intersection of the load line and the photovoltaic characteristic curve after the solar irradiation changes, and point C is the intersection of the power limit curve and the photovoltaic characteristic curve after the solar irradiation changes. Figure 1 As shown in (a), once it is detected that the power generation at point B exceeds the limit, the PVPP switches from MPPT mode to CPG mode. In this case, the operating point will move away from the maximum power point in a fixed step size, actively reducing the photovoltaic power output. In another case, the PVPP initially operates in CPG operation mode. When the solar irradiance increases, point A moves along the load line to point B. When the power generation at point B exceeds the limit, it will continue to execute CPG mode and move towards point C to reduce power output. (2) A similar operation mode switching process can also exist in the scenario of reduced irradiance. Figure 2 The figure shows the operation process of the traditional CPG control algorithm when the solar irradiance decreases. Figure 2As shown in (a), once the PVPP detects that the power generation at point B has dropped to a limit, it will perform a rightward perturbation operation. In this case, the operating point will move toward point C with a fixed step size. Near point C, there will be a point P equal to the target power point. Therefore, it no longer perturbs to the right, but oscillates around point C. In another case, it is found that it is already at the maximum point near point C, and it no longer satisfies the point P greater than the target power point, so it oscillates around the MPP point (point C). Based on the above analysis, it can be concluded that the traditional CPG method with a fixed step size has two main disadvantages: one is that the tracking speed is relatively slow during the irradiance transition (transient), and the other is that the power oscillation in the stable stage (steady state) will cause significant power loss. Summary of the Invention

[0004] Purpose of the invention: To address the problems existing in the prior art, the present invention provides a photovoltaic flexible power point control method and device with fast transient tracking speed and low steady-state oscillation.

[0005] The photovoltaic flexible power point control method of the present invention includes:

[0006] Step S1: Determine whether the light irradiance at the current moment has changed; if the light irradiance has changed, execute step S2; otherwise, execute step S4;

[0007] Step S2: Calculating the duty cycle of the intersection D of the load line and the photovoltaic characteristic curve after the change in solar irradiance according to the current working mode, wherein the load line is a line connecting the intersection of the power limit curve and the photovoltaic characteristic curve after the change in solar irradiance and the origin;

[0008] Step S3: Jump from the current operating point to the operating point at point D in one step based on the duty cycle at point D, and execute step S4;

[0009] Step S4: Determine whether the target power point has been reached. If so, run at the target power point; otherwise, execute step S5.

[0010] Step S5: Calculate the disturbance step size and disturbance direction according to the state of the current working point, so that the disturbance step size is adaptively reduced with the number of cycles, and reach the next working point from the current working point according to the calculated disturbance step size and disturbance direction, and return to execute step S4.

[0011] Furthermore, the method for determining whether the light irradiance at the current moment has changed specifically includes:

[0012] Measure the voltage and current of the photovoltaic system at the current moment;

[0013] Calculate the current power based on the measured voltage and current;

[0014] Calculate the power difference dP between the current moment and the previous moment;

[0015] If the absolute value of the power difference dP is greater than or equal to the first preset threshold, it is determined that the light irradiance has changed.

[0016] Furthermore, the first preset thresholds in the CPG mode and the MPPT mode are different.

[0017] Furthermore, the calculation of the duty cycle of the intersection D of the first load line and the photovoltaic characteristic curve after the illumination change according to the current working mode specifically includes:

[0018] If the current working mode is CPG mode, the duty cycle of the intersection point D is calculated according to the following formula:

[0019]

[0020] If the current working mode is MPPT mode, the duty cycle of the intersection point D is calculated according to the following formula:

[0021]

[0022] Where d represents the duty cycle, V old Indicates the voltage at the previous moment, V oc represents the open circuit voltage, I represents the current at the current moment, V represents the voltage at the current moment, R load Represents the load resistance, P limit Indicates the power limit value.

[0023] Furthermore, at the beginning of each moment, the photovoltaic system is switched to the non-operating mode state, and after jumping to the operating point D, the operating mode is switched to the non-operating mode state.

[0024] Furthermore, the method for calculating the disturbance step size and disturbance direction according to the state of the current working point specifically includes:

[0025] Get the number of times the operating point crosses the power limit line Flag;

[0026] If the power difference dP* between the current power and the target power is greater than 0, the left side is used as the perturbation direction, and the perturbation step size is calculated according to the following formula:

[0027] Step = 2 2-Flag

[0028] If dP* is less than 0, the perturbation direction and perturbation step size are calculated according to the perturbation-observation method.

[0029] Furthermore, the calculation method of the marking number Flag is:

[0030] At the beginning of each moment, set the number of times Flag = 1;

[0031] If the following conditions are met each time, the number of flags is increased by 1:

[0032] (P limit >P)&(P old >P limit ) or (P>P limit )&and(P limit >P old )

[0033] Get the final number of markings Flag.

[0034] Furthermore, if the power difference dP* is greater than 0 and |dP / dV| <e th , it is determined that the target power point has been reached, and the working mode is switched to MPPT mode, where dP and dV are the power difference and voltage difference between the current moment and the previous moment, e th is the second preset threshold.

[0035] Furthermore, the method for determining whether the target power point has been reached is as follows: if the absolute value of the power difference dP* between the current power and the target power is less than a third preset threshold, it is determined that the target power point has been reached. After reaching the target power point, the operating mode is switched to CPG mode, and the disturbance step size is set to 0.

[0036] The photovoltaic flexible power point control device of the present invention includes a processor and a computer program stored in a memory and executable on the processor. The processor implements the above method when executing the program.

[0037] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: the present invention can achieve rapid transient tracking and reduce steady-state power oscillations. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 Mode transition diagram of traditional CPG control when irradiation is enhanced: (a) from MPPT mode to CPG mode; (b) from CPG mode to CPG mode;

[0039] Figure 2 Figure 2 shows the mode transition of traditional CPG control when irradiance decreases: (a) from CPG mode to CPG mode; (b) from CPG mode to MPPT mode.

[0040] Figure 3 A schematic flow chart of an embodiment of a photovoltaic flexible power point control method provided by the present invention;

[0041] Figure 4 A diagram showing the specific implementation steps of the photovoltaic flexible power point control method provided by the present invention;

[0042] Figure 5 Mode transition diagrams of the improved CPG control under enhanced irradiation: (a) from MPPT mode to CPG mode; (b) from CPG mode to CPG mode;

[0043] Figure 6 Mode transition diagram of the improved CPG control under reduced irradiation: (a) from CPG mode to CPG mode; (b) from CPG mode to MPT mode;

[0044] Figure 7 is the judgment diagram of the disturbance direction;

[0045] Figure 8 Schematic diagram of the proposed zero oscillation method;

[0046] Figure 9 The simulation results in a short period of time are as follows: (a) power waveform comparison under 7000W power limit; (b) power waveform comparison under 8000W power limit;

[0047] Figure 10 Long-term solar irradiance variation diagram: (a) stable solar irradiance; (b) unstable solar irradiance;

[0048] Figure 11 The simulation results of slowly changing solar irradiance over a long period of time: (a) voltage waveform comparison under 7000W power limit; (b) voltage waveform comparison under 8000W power limit; (c) power waveform comparison under 7000W power limit; (d) power waveform comparison under 8000W power limit;

[0049] Figure 12 The simulation results of a rapid change in solar irradiance over a long period of time: (a) voltage waveform comparison under 7000W power limit; (b) voltage waveform comparison under 8000W power limit; (c) power waveform comparison under 7000W power limit; (d) power waveform comparison under 8000W power limit;

[0050] Figure 13 Experimental results for improving the control strategy in a short period of time when irradiation changes sharply: (a) conventional CPG algorithm; (b) the algorithm of the present invention;

[0051] Figure 14 Experimental results for improving control strategies under long timescale solar irradiation. DETAILED DESCRIPTION

[0052] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0053] Example 1

[0054] This embodiment provides a photovoltaic flexible power point control method, such as Figure 3 and Figure 4 Shown, including:

[0055] Step S1: Determine whether the current light irradiance has changed. If so, proceed to step S2; otherwise, proceed to step S4.

[0056] The method for determining whether the light irradiance has changed at the current moment is as follows: measuring the voltage V and current I of the photovoltaic system at the current moment; calculating the power at the current moment based on the measured voltage and current; calculating the power difference dP between the current moment and the previous moment; if the absolute value of the power difference dP is greater than or equal to a first preset threshold, it is determined that the light irradiance has changed. The first preset threshold dP in the CPG mode is th1 and the first preset threshold dP in MPPT mode th2 Different, and in order to better judge whether the light changes in CPG mode, the power difference dP* between the current power and the target power can also be added, that is, when |dP|≥dP th1 and |dP*|≥dP* th1 , determine whether the light irradiance changes.

[0057] Step S2: Calculate the duty cycle of the intersection D of the load line and the photovoltaic characteristic curve after the light changes according to the current working mode, where the load line is the connecting line between the intersection C of the power limit curve and the photovoltaic characteristic curve after the solar irradiation changes and the origin.

[0058] The calculation method of the duty cycle of the intersection point D is analyzed below:

[0059] 1) When the light irradiance increases: from Figure 5 (a) As can be seen, the principle of similar triangles is used to consider the need to jump as close to the target power point as possible in one step. Similarly, when the irradiance increases, point A moves towards point B on the load line. Then, by applying the principle of similar triangles, point C can be derived as:

[0060] V C ≈V A =V old (1)

[0061]

[0062] V old Indicates the voltage at the previous moment, V A 、V B 、V C Represents the voltages at points A, B, and C, respectively. I A , I B , I represent the currents at points A, B, and C respectively, Voc Indicates the open circuit voltage.

[0063] Recombining formula (2), we get the following formula (3):

[0064]

[0065] The required point C is then determined by:

[0066]

[0067] R pv,D 、R pv,C Represents the resistance of point D and point C respectively, R load Indicates the load resistance.

[0068] The formula for the duty cycle of point D in the Boost circuit is:

[0069]

[0070] In addition, there will be another transition mode from CPG to CPG under increased irradiance. When the irradiance increases, point A moves toward point B on the load line. Figure 5 (b) It can be seen that in order to reduce the tracking time, point B needs to jump to point D, which is close to point E. Consider that the current at point B is approximately equal to the current at point E.

[0071] I C =I B =I≈I E (6)

[0072]

[0073] P limit Indicates the power limit value.

[0074] The formula for the duty cycle of point D in the Boost circuit is:

[0075]

[0076] 2) When the light irradiance decreases:

[0077] Similar to the case of irradiance enhancement, we briefly analyze the mode switching state under irradiance reduction. The key is to jump to the vicinity of the target point E in one step to reduce the tracking time. Figure 6 (a) and Figure 6 (b) It can be seen that point D can be constructed with the help of the inverse proportional curve and point B. The specific idea is as follows:

[0078] I C =I B =I (10)

[0079]

[0080] The formula for the duty cycle of point D in the Boost circuit is:

[0081]

[0082] In summary, it can be concluded that if the current working mode is the CPG mode, the duty cycle is calculated according to formula (5); if the current working mode is the MPPT mode, the duty cycle of the intersection point D is calculated according to formula (9).

[0083] In specific implementations, the parameter K represents the PV system's operating mode. When K = 1, the system is in CPG mode; when K = 0, the system is in MPPT mode; and when K = 2, the system is neither in CPG load shedding mode nor in MPPT maximum power output mode. This parameter is used for initialization and reset after changes in irradiance. At the beginning of each moment, the PV system is switched to a non-operating mode, i.e., K = 2.

[0084] Step S3: Jump from the current working point to the working point D in one step based on the duty cycle of point D, and execute step S4. After jumping to the working point D, the working mode is switched to the non-working mode state, that is, K=2.

[0085] Step S4: Determine whether the target power point has been reached. If so, run at the target power point; otherwise, execute step S5.

[0086] The method for determining whether the target power point has been reached is as follows: if the absolute value of the power difference dP* between the current power and the target power is less than the third preset threshold dP* th2 , it is determined that the target power point has been reached and steady state has been achieved. After reaching the target power point, the working mode is switched to CPG mode, that is, K = 1, and the perturbation step size step is set to 0 to achieve zero oscillation. In specific implementation, when the current power P is within plus or minus 200W around Plimit, it is considered to have reached steady state. In addition, if the power difference dP* is greater than 0 and |dP / dV| <e th , it is determined that the target power point has been reached, and the working mode is switched to MPPT mode, where dP and dV are the power difference and voltage difference between the current moment and the previous moment, e th is the second preset threshold.

[0087] Step S5: Calculate the disturbance step size and disturbance direction according to the state of the current working point, so that the disturbance step size is adaptively reduced with the number of cycles, and reach the next working point from the current working point according to the calculated disturbance step size and disturbance direction, and return to execute step S4.

[0088] The method for calculating the perturbation step size and the perturbation direction specifically includes:

[0089] A. Obtain the number of times the operating point crosses the power limit line Flag;

[0090] The counting method of Flag is as follows: at the beginning of each moment, set the number of times Flag to 1; if the following conditions are met once, the number of times Flag is increased by 1: (P limit >P)&(P old >P limit ) or (P>P limit )&and(P limit >P old ); if (P limit >P)&(P old >P limit ) It is considered that a certain working point P has crossed the Plimit point from above; similarly, when (P>P limit )&and(P limit >P old ), it is considered that point P has passed through point Plimit from below. Get the current mark count Flag.

[0091] B. If the power difference dP* between the current power and the target power is greater than 0, the left side is used as the disturbance direction, such as Figure 4 The perturbation step size is calculated as follows:

[0092] Step = 2 2-Flag #(14)

[0093] like Figure 7 As shown, the operating point will move to the left at a fixed step size. In this case, both point P2 and point B move to point A.

[0094] C. If dP* is less than 0, the perturbation direction and perturbation step size are calculated according to the perturbation-observation method.

[0095] according to Figure 4 In Algorithm 2, if the power generation exceeds the limit, the operating points on both sides will climb to Figure 7 (a) The constant power point (the intersection of the Plimit line and the photovoltaic characteristic curve in the figure). Otherwise, the photovoltaic power station will continue to maximize energy output ( Figure 7 (b)).

[0096] The perturbation and observation method is as follows: when crossing the power line from top to bottom, (P limit >P)&and(P old >P limit ), take positive; when crossing the power line from bottom to top, that is (P>P limit )&and(Plimit >P old ), take negative. Determine whether a stable state is reached. When this condition is met, the system reaches a steady state and stops the binary step length. The step length calculation formula is: Step = ±2 2-Flag , if positive, it goes to the left; if negative, it goes to the right.

[0097] In this embodiment, when the working point is close to point A, Figure 8 As shown in , the step length will spontaneously shorten as the perturbation direction changes. Figure 8 In (a), point P is greater than point Plimit (target power point). The variable step size formula adopted by the present invention is (14). Each time point P crosses point Plimit, the step size can be reduced by half. As point P repeatedly perturbs near point Plimit, Flag continues to increase and the step size continues to decrease. Therefore, the final operating point is getting closer and closer to point Plimit. Until the target power point is reached, the step size is selected as 0, Flag is reset to 1, and the variable K is set to 1 as a state description parameter, indicating that it is currently in a stable state in CPG mode.

[0098] Example 2

[0099] This embodiment provides a photovoltaic flexible power point control device, including a processor and a computer program stored in a memory and executable on the processor. When the processor executes the program, the method in Embodiment 1 is implemented.

[0100] Simulation Verification

[0101] In order to verify the superiority of the present invention, different illumination change conditions and different target power values ​​are selected below and demonstrated in Matlab simulation and HIL hardware-in-the-loop experiment respectively.

[0102] 1) Matlab simulation

[0103] A. Short-time scale simulation: Short-time scale results are as follows: Figure 9 shown. Figure 9 (a) and Figure 9 (b) depicts the results of the proposed algorithm under limited power conditions of 7000W and 8000W, respectively. Clearly, the shortcomings of both conventional algorithms are significantly improved under these conditions. First, steady-state power oscillations are significantly reduced. Second, the proposed algorithm successfully accelerates the tracking process at all times when solar irradiance changes. As shown in the figure above, when the irradiance changes suddenly at 8s, the tracking speed is significantly improved, meaning that a steady state can be reached more quickly.

[0104] B. Long-term simulation: as follows Figure 10 As shown in Figure 2, the long-term simulation can be divided into steady-state rise and drastic changes. Steady-state rise in solar irradiance: Figure 11 The simulation results of one day of meteorological data are shown. In this case, the solar irradiance sampling time is 400S, the size interval is 550W / m2 to 900W / m2, and the sunshine changes once per second. Figure 11 It is obvious that the PVPP operates in MPPT mode from 0 to 50 seconds and from 350 to 400 seconds, and switches to CPG mode from 50 to 350 seconds. When the PVPP operates in CPG mode, the proposed improved algorithm significantly reduces the power oscillation. Figure 12 As shown in Figure 3, it can be seen that the proposed improved algorithm is robust enough to successfully adjust to the target power point under extreme weather conditions. Compared with the traditional method, it has a steady-state performance with smaller power oscillations.

[0105] 2) HIL hardware-in-the-loop experiment

[0106] Figure 13 The proposed scheme was compared with a conventional scheme in a rapidly changing, short-term environment. The results showed that the power oscillations caused by the conventional approach became increasingly pronounced as irradiance increased. In contrast, the proposed improved scheme consistently achieved the desired results, regardless of whether the target power point was 7000W or 8000W. Experimental results demonstrate that the improved scheme exhibits superior dynamic characteristics under transient tracking and eliminates power oscillations in the steady state. Figure 14 (a) to (h) show the experimental results at a long period scale. It is clear that the improved method can achieve fast tracking and zero oscillation under any illumination changes and any target power changes.

[0107] The above disclosure is only a preferred embodiment of the present invention and cannot be used to limit the scope of the present invention. Therefore, equivalent changes made according to the claims of the present invention are still within the scope of the present invention.

Claims

1. A photovoltaic flexible power point control method, characterized in that include: Step S1: Determine whether the light irradiance at the current moment has changed; If the light irradiance changes, execute step S2, otherwise execute step S4; Step S2: Calculate the duty cycle of the intersection point D of the load line and the photovoltaic characteristic curve after the change in light according to the current working mode. The load line is the line connecting the intersection point of the power limit curve and the photovoltaic characteristic curve after the change in solar irradiance and the origin. If the current working mode is CPG mode, the duty cycle of the intersection point D is calculated according to the following formula: If the current working mode is MPPT mode, the duty cycle of the intersection point D is calculated according to the following formula: Where d represents the duty cycle, V old Indicates the voltage at the previous moment, V oc represents the open circuit voltage, I represents the current at the current moment, V represents the voltage at the current moment, R load Represents the load resistance, P limit Indicates the power limit value; Step S3: Jump from the current operating point to the operating point at point D in one step based on the duty cycle at point D, and execute step S4; Step S4: Determine whether the target power point has been reached. If so, run at the target power point; otherwise, execute step S5. Step S5: Calculate the disturbance step size and disturbance direction according to the state of the current working point, so that the disturbance step size is adaptively reduced with the number of cycles, and reach the next working point from the current working point according to the calculated disturbance step size and disturbance direction, and return to execute step S4.

2. The photovoltaic flexible power point control method according to claim 1, characterized in that: The method for determining whether the light irradiance at the current moment has changed specifically includes: Measure the voltage and current of the photovoltaic system at the current moment; Calculate the current power based on the measured voltage and current; Calculate the power difference dP between the current moment and the previous moment; If the absolute value of the power difference dP is greater than or equal to the first preset threshold, it is determined that the light irradiance has changed.

3. The photovoltaic flexible power point control method according to claim 2, characterized in that: The first preset threshold value in the CPG mode is different from that in the MPPT mode.

4. The photovoltaic flexible power point control method according to claim 1, characterized in that: At the beginning of each moment, the photovoltaic system is switched to the non-operating mode state, and after jumping to the operating point of point D, the operating mode is switched to the non-operating mode state.

5. The photovoltaic flexible power point control method according to claim 1, characterized in that: The method for calculating the disturbance step size and disturbance direction according to the state of the current working point specifically includes: Get the number of times the operating point crosses the power limit line Flag; If the power difference dP* between the current power and the target power is greater than 0, the left side is used as the perturbation direction, and the perturbation step size is calculated according to the following formula: Step=2 2-Flag If dP* is less than 0, the perturbation direction and perturbation step size are calculated according to the perturbation-observation method.

6. The photovoltaic flexible power point control method according to claim 5, characterized in that: The calculation method of the marking number Flag is: At the beginning of each moment, set the number of times Flag = 1; If the following conditions are met each time, the number of flags is increased by 1: (P limit >P)&(P old >P limit ) or (P>P limit )&and(P limit >P old ) Get the final number of markings Flag.

7. The photovoltaic flexible power point control method according to claim 5, characterized in that: If the power difference dP* is greater than 0 and |dP / dV| <e th , it is determined that the target power point has been reached, and the working mode is switched to MPPT mode, where dP and dV are the power difference and voltage difference between the current moment and the previous moment, e th is the second preset threshold.

8. The photovoltaic flexible power point control method according to claim 1, characterized in that: The method to determine whether the target power point has been reached is: If the absolute value of the power difference dP* between the current power and the target power is less than the third preset threshold, it is determined that the target power point has been reached. After reaching the target power point, the working mode is switched to CPG mode and the perturbation step size is set to 0.

9. A photovoltaic flexible power point control device comprising a processor and a computer program stored in a memory and executable on the processor, characterized in that: When the processor executes the program, the method according to any one of claims 1 to 8 is implemented.

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