Photovoltaic inverter low-voltage ride-through control method and system based on improved MPPT (Maximum Power Point Tracking)

Abstract

The invention discloses a photovoltaic inverter low-voltage ride-through control method and system based on improved MPPT (maximum power point tracking), relates to the technical field of power distribution and power electronics, and solves the problem of low-voltage ride-through failure caused by rapid rise of direct-current side voltage and dramatic increase of power grid current due to unbalanced system power when the voltage of a power grid drops. On the basis of traditional photovoltaic MPPT control, improved photovoltaic MPPT control is provided, an adjusting coefficient is added on the basis of the traditional MPPT control, the product of the adjusting coefficient and the direct current bus voltage is used as a new reference value, and on the other hand, in order to eliminate the influence of the PCC voltage on the grid side current, a full feed-forward path of the PCC voltage is introduced. The photovoltaic power is automatically converged to the active power according to the proposed control scheme, so that the overvoltage of the direct current bus is limited.

Description

Technical Field

[0001] The present invention belongs to the field of power distribution and power electronics technology in power systems, and in particular relates to a photovoltaic inverter low voltage ride through control method and system based on improved MPPT. Background Art

[0002] With the increase in the installed capacity of photovoltaic power stations in my country, more and more photovoltaic power is connected to the grid. The connection of large-scale photovoltaic power stations to the grid will have a huge impact on the safe operation of the entire power system. Once a distribution line fails, it will cause an instantaneous peak in the grid current, which will have a huge impact on the converter. If a voltage drop or other fault occurs on the grid side, the DC side overvoltage protection or inverter overcurrent protection will be triggered, causing the photovoltaic grid-connected system to be directly disconnected from the grid, which will cause serious problems such as power oscillation and frequency over-limit in the grid. However, when the grid voltage drops, the photovoltaic grid-connected power generation system is at risk of DC side overvoltage and grid side overcurrent, which will trigger system protection, causing the photovoltaic power generation system to be disconnected, and have an adverse effect on the stability of the grid. Therefore, the grid-connected inverter must have low voltage ride-through capability.

[0003] When the grid voltage drops, only a part of the active power generated by the photovoltaic panels can be transmitted to the grid, resulting in power imbalance, which will lead to a rapid increase in the DC side voltage and a sharp increase in the grid current, which may trigger overvoltage or overcurrent protection and cause low voltage ride-through failure. For the traditional photovoltaic grid-connected power generation system, the main goal of the boost converter is to extract the maximum power of the photovoltaic panel, rather than to adjust the output voltage, so MPPT control must be adopted. The inverter grid-connected control mode usually includes two types: directional vector control and direct power control. At present, the research on directional vector control is relatively extensive, mainly focusing on independent control of active and reactive components through coordinate transformation and decoupling. According to the requirements of photovoltaic grid-connected low voltage ride-through, the control needs to be further improved. Once a voltage drop occurs, the MPPT is first controlled to further reduce the power output of the pv array to balance the input and output power of the inverter. However, there are problems such as slow control convergence, weak anti-interference ability, and low robustness in the photovoltaic LVRT under the existing control mode. Summary of the invention

[0004] In view of the above-mentioned existing problems, the present invention proposes a photovoltaic inverter low voltage ride-through control strategy based on improved MPPT control, which aims to ensure the stable operation of the photovoltaic grid-connected power generation system when the grid voltage drops, improve the system dynamic response speed, and maintain the DC side voltage stability.

[0005] In order to solve the above technical problems, a low voltage ride-through control method for photovoltaic inverter based on improved MPPT is proposed, including:

[0006] A photovoltaic energy flow mapping and prediction model is established to provide prediction data. A PCC voltage full feedforward scheme that suppresses grid-side overcurrent is adopted to eliminate the impact of PCC voltage and build a stable operating environment for low-voltage ride-through. An adjustment coefficient is introduced to improve MPPT control, adjust the photovoltaic voltage reference value and DC bus voltage, and limit the DC bus voltage. When the DC bus voltage is effectively limited, reactive power is injected to respond to grid faults and input into the Boost circuit.

[0007] As a preferred solution of the photovoltaic inverter low voltage ride-through control method based on improved MPPT described in the present invention, the photovoltaic energy flow mapping and prediction model includes collecting the output power of the inverter and preprocessing:

[0008]

[0009] Among them, P norm (t) is the normalized PV output power at time t, P pv (t) is the actual photovoltaic output power at time t, c is the slope parameter of the normalized curve, μ P is the mean value of photovoltaic output power. Construct dynamic eigenvector X(t):

[0010]

[0011] Where ill(t) is the light intensity at time t, ω is the frequency related to the light intensity, capturing the frequency component of light changes. Tem(t) is the ambient temperature at time t, Tem ref is the reference temperature, H(t) is the humidity value at time t, H ref is the reference humidity, P load (t-1) is the load power value at time t-1.

[0012] Nonlinear Dynamic Modeling of Photovoltaic Output:

[0013]

[0014] Among them, C1, C2, and C3 are constant parameters with different sensitivities and characteristic effects, and η represents the nonlinear relationship of light intensity. represents the exponential decay effect of temperature on PV output power, ∈(t) is the error term at time t, reflecting the difference between the model prediction and the actual output.

[0015] Make predictions based on the feature vector and dynamic model output:

[0016]

[0017] Among them, f represents the function that takes the feature vector and historical output power as input to construct the final energy flow prediction model:

[0018]

[0019] in, is the predicted PV output power at time t+Δt, F is the number of historical moments considered, f is the step size variable, λ is the attenuation constant that controls the intensity of the influence of historical data on the current prediction, Z is the normalization constant, A represents the importance coefficient of the normalized output in the final prediction, O is the number of features contained in the feature vector, and w o is the weight coefficient of the lth feature, reflecting the contribution of the lth feature to the prediction. o (t) is the lth feature in the feature vector.

[0020] The duty cycle of the pulse width modulation signal is calculated based on the energy flow prediction model.

[0021] As a preferred solution of the photovoltaic inverter low voltage ride-through control method based on improved MPPT described in the present invention, wherein: the PCC voltage full feedforward solution includes, by adding a full feedforward path of the PCC voltage, eliminating the influence of the PCC voltage on the grid-side current, the full feedforward of the PCC point voltage is completed in a hybrid coordinate system, and the feedforward function G is derived. ff , where K pwm is the duty cycle of the PWM signal:

[0022]

[0023] Using historical voltage data V pcc (t) Predict the voltage at the future time

[0024]

[0025] Among them, V PCC (t-iΔt) is the voltage value at the historical moment t-iΔt, j is a different time point in the past, Δt is the predicted time interval, n is the number of historical time points considered, s represents the continuous time points in the time interval, t c is the time constant, which controls the speed of voltage decay. i is a dynamic parameter of the importance of historical voltage values ​​to prediction, γ is a frequency parameter of periodic changes in time, and e is the base of the natural logarithm.

[0026] Real-time monitoring of the current voltage V pcc (t), and calculate the feedback signal error E(t):

[0027]

[0028] The feedforward function is adjusted by feedback compensation, where α is the weighting factor:

[0029]

[0030] Derive the final feedforward function:

[0031]

[0032] Among them, K VCC is the gain factor used to adjust the feedback signal.

[0033] When the PCC point voltage is fully fed forward, the PCC point voltage no longer affects the grid-side current, and no grid-side overcurrent occurs. At this time, the low voltage ride-through operating environment becomes stable.

[0034] As a preferred solution of the photovoltaic inverter low voltage ride through control method based on improved MPPT described in the present invention, wherein: the MPPT control improvement includes starting to adjust when the low voltage ride through operating environment reaches stability, adding an adjustment coefficient δ on the basis of traditional MPPT control, and constructing a new reference value V pv And meet:

[0035] V pv =δ*V dc (t)

[0036] V MPP <V pv <V oc

[0037] Among them, V dc (t) is the DC bus voltage, V MPP is the voltage of the photovoltaic panel at the maximum power point, V pv is the photovoltaic panel voltage, V oc is the open circuit voltage of the photovoltaic panel.

[0038] The improved MPPT control includes three regulation situations: When the photovoltaic power is greater than the active power, V dc (t stores excess energy, V pv Increased, defined as zone 1, the power generated by the PV modules decreases.

[0039] When the photovoltaic power is less than the active power, V dc (t) emits energy to stabilize the system, V pv The power output of the photovoltaic modules increases.

[0040] When the photovoltaic power is equal to the active power, it is defined as zone 3, and the energy on the photovoltaic side and the grid side is balanced.

[0041] Under the condition of real-time monitoring of photovoltaic power and active power, when it is detected that the photovoltaic power P pv crosses the threshold T, a region switching mechanism is triggered: when P pv >T, it switches to Region 1; when P pv <T, it switches to Region 2; when P pv =T, it remains in Region 3. Among them, the dynamic threshold T is:

[0042]

[0043] Among them, a is the photovoltaic system performance optimization coefficient determined through historical data, is the average load power at a time point before the current time, and b is the baseline offset set according to the actual voltage demand.

[0044] After switching to different regions, the adjustment coefficient δ of each region is updated respectively:

[0045]

[0046] Among them, δ1, δ2, and δ3 are the updated adjustment coefficients of the three regions respectively, c is the adjustment sensitivity factor, P target is the target power, P actual is the actual output power, P load is the load demand.

[0047] The updated adjustment coefficient is re-input into the calculation formula for constructing the new reference value V pv to obtain the updated reference value and satisfy V MPP <V pv <V oc .

[0048] As a preferred solution of the low-voltage ride-through control method for a photovoltaic inverter based on improved MPPT according to the present invention, among them: the limitation of the DC bus voltage includes constructing an improved DC bus voltage limitation model and adjusting the current voltage V pcc (t) through a dynamic feedback mechanism:

[0049]

[0050] Among them, ΔV pcc (t) is the current voltage adjustment amount, ε is the gain coefficient of the deviation from the reference value, θ is the gain coefficient of the voltage change rate, V dc (t) is the DC bus voltage at the current time point t, U pv is the newly constructed reference value, and k is the parameter of the control sensitivity.

[0051] Filter the information of the voltage adjustment amount:

[0052]

[0053] Where S(t) represents a complex filtering function based on historical voltage and load, M is the number of past time series considered, and V load (tm) represents the load voltage at time tm.

[0054] The adjusted DC bus voltage formula is:

[0055] V dc (t) new =V dc (t-1)+ΔV pcc (t)+S(t)

[0056] Get the final voltage limit calculation:

[0057]

[0058] According to the above model, V limit (t) is the new DC bus voltage limit value, is the limiting factor to ensure that the voltage is within a reasonable range, V existing (t) is the current voltage at time t, V existing (t) = V pcc (t)+ΔV pcc (t).

[0059] If V dc (t) new If the limit value is exceeded, the control system will adjust the output power and the connected load to keep the DC bus voltage within a safe range.

[0060] As a preferred solution of the photovoltaic inverter low voltage ride through control method based on improved MPPT described in the present invention, the response to the grid fault includes that when the grid fails, the voltage outer loop needs to be disconnected and the active current reference value is recalculated, and it does not exceed 1.1 times the rated value:

[0061]

[0062] Among them, i N is the grid-connected current rating, i dref and i qref They are respectively the active current reference value and reactive current reference value of the grid-side current.

[0063] Inject a certain amount of reactive power into the grid connection point and restore the grid voltage through reactive power compensation:

[0064] Q=V PCC (t)·IQ (t)

[0065] Among them, I Q (t) is the reactive current.

[0066] Control logic for reactive power generation:

[0067] Qtarget=Kq·(V PCC (t)-Vref)

[0068] At this time, the reactive component of the grid-connected current is not 0. It is adjusted in real time according to the degree of grid voltage drop. The reactive current instruction satisfies:

[0069]

[0070] Among them, U T is the per unit value of the grid-connected voltage.

[0071] As a preferred solution of the photovoltaic inverter low voltage ride through control method based on improved MPPT described in the present invention, wherein: the input Boost circuit includes a dynamic model of the Boost circuit represented by a state space model:

[0072]

[0073] Among them, V boost is the Boost output voltage, V in is the input voltage, L is the inductance, R is the circuit impedance, I boost (t) is the current in the Boost circuit at time t.

[0074] When the Boost circuit reaches steady state, the relationship between input and output is:

[0075]

[0076] Among them, U0 and U pv are the output voltage and input voltage of the Boost circuit respectively.

[0077] Conduct an overall system assessment:

[0078]

[0079] Y is the evaluation cost function, which measures the deviation between the system output and the target value. The smaller the better. out (t is the actual output power of the inverter at time t, P target is the desired target output power, Q(t) is the actual reactive power at time t, Q target is the target reactive power, is the target DC bus voltage ttotal is the total time frame for the evaluation.

[0080] After evaluation, if the evaluation cost function Y exceeds the preset threshold Y th , system parameters need to be updated:

[0081]

[0082] Among them, Δδ is the update amount of the MPPT adjustment coefficient, ΔV is a parameter that controls the size of the update. pv is the update amount of the photovoltaic voltage reference value, P is the parameter controlling the size of the update amount, To evaluate the partial derivative of the cost function Y with respect to the MPPT adjustment coefficient, it reflects the sensitivity to the change of δ. To evaluate the partial derivative of the cost function Y with respect to the photovoltaic voltage reference value, it reflects the effect of V pv Sensitivity to change.

[0083] Another object of the present invention is to provide a photovoltaic inverter low voltage ride-through control system based on improved MPPT. The present invention improves the stability and reliability of the photovoltaic power generation system under low voltage conditions to maximize energy output and ensure system safety, effectively respond to grid fluctuations and failures, and integrates the collaboration of various modules. The system realizes efficient and reliable photovoltaic power generation, promotes the widespread application of renewable energy, and contributes to the goal of sustainable development.

[0084] As a preferred solution of the photovoltaic inverter low voltage ride-through control system based on improved MPPT described in the present invention, it is characterized by including a photovoltaic energy flow mapping and prediction module, a PCC voltage full feedforward control module, an MPPT control module, a DC bus voltage limiting module and a power grid fault response module.

[0085] The photovoltaic energy flow mapping and prediction module collects the output power and environmental parameters of the inverter, obtains standardized photovoltaic power data, establishes dynamic feature vectors, constructs a nonlinear relationship model with photovoltaic output, and combines the feature vectors and historical data to predict the photovoltaic output power at future times.

[0086] The PCC voltage full feedforward control module derives a feedforward function in a hybrid coordinate system, eliminates the influence of the PCC voltage, monitors the current PCC voltage in real time, calculates the error of the feedback signal, adjusts the feedforward function through a weighting factor, controls the environment to be stable, and obtains the final feedforward function by combining monitoring and feedback information.

[0087] The MPPT control module is divided into three areas according to the relationship between photovoltaic power and active power, calculates dynamic thresholds, and establishes a regional switching mechanism to monitor photovoltaic power in real time, trigger regional switching, and update the status when it is detected that the power crosses the threshold, and update the adjustment coefficient according to the operation requirements of different areas.

[0088] The DC bus voltage limiting module calculates the adjustment amount of the deviation from the reference value, uses the gain coefficient for adjustment, and adjusts the output power and load in real time when the voltage exceeds the limit value.

[0089] The grid fault response module identifies grid faults, disconnects from the grid when a fault occurs, recalculates the active current reference value and ensures that it does not exceed 1.1 times the rated value, injects an appropriate amount of reactive power into the grid connection point, restores the grid voltage, adjusts the reactive current command according to the grid demand, controls the dynamic response of the Boost circuit, and ensures a stable output voltage.

[0090] A computer device comprises a memory and a processor, wherein the memory stores a computer program, and is characterized in that when the processor executes the computer program, the steps of the photovoltaic inverter low voltage ride-through control method based on improved MPPT are implemented.

[0091] A computer-readable storage medium having a computer program stored thereon, characterized in that when the computer program is executed by a processor, the steps of the photovoltaic inverter low voltage ride-through control method based on improved MPPT are implemented.

[0092] Beneficial effects of the present invention: The present invention improves the MPPT control in the photovoltaic BOOST boost circuit, so that the photovoltaic power will automatically converge to the active power according to the proposed control scheme, and the DC bus overvoltage will be limited; to avoid LVRT failure, the improved current loop control strategy is used to effectively suppress the overshoot of the inductor current when the voltage drops, thereby reducing the risk of grid-side overcurrent and eliminating DC bus voltage overvoltage. In addition, the quality of the grid-side current is also improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0093] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work, among which:

[0094] Figure 1 An overall flow chart of a photovoltaic inverter low voltage ride through control method based on improved MPPT is provided for one embodiment of the present invention.

[0095] Figure 2 An improved current loop control block diagram of a photovoltaic inverter low voltage ride through control method based on improved MPPT is provided in one embodiment of the present invention.

[0096] Figure 3 A DSOGI phase-locked loop schematic diagram of a photovoltaic inverter low voltage ride-through control method based on improved MPPT provided in one embodiment of the present invention.

[0097] Figure 4 An improved MPPT control diagram of a photovoltaic inverter low voltage ride through control method based on improved MPPT is provided in one embodiment of the present invention.

[0098] Figure 5 The voltage and current waveforms at the PCC of a photovoltaic grid-connected inverter system of a photovoltaic inverter low voltage ride-through control method based on an improved MPPT provided by an embodiment of the present invention.

[0099] Figure 6 An embodiment of the present invention provides active power and reactive power waveforms of a photovoltaic grid-connected inverter system based on an improved MPPT photovoltaic inverter low voltage ride-through control method.

[0100] Figure 7 An overall control block diagram of a photovoltaic inverter low voltage ride through control system based on improved MPPT provided by one embodiment of the present invention.

[0101] Figure 8 A photovoltaic inverter low voltage ride through control block diagram of a photovoltaic inverter low voltage ride through control system based on an improved MPPT is provided in one embodiment of the present invention. DETAILED DESCRIPTION

[0102] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary persons in the art without creative work should fall within the scope of protection of the present invention.

[0103] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0104] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor is it an embodiment that is mutually exclusive with other embodiments, either individually or selectively.

[0105] The present invention is described in detail with reference to schematic diagrams. When describing the embodiments of the present invention, for the sake of convenience, the cross-sectional diagrams showing the device structure will not be partially enlarged according to the general scale, and the schematic diagrams are only examples, which should not limit the scope of protection of the present invention. In addition, in actual production, the three-dimensional dimensions of length, width and depth should be included.

[0106] At the same time, in the description of the present invention, it should be noted that the directions or positional relationships indicated by the terms "upper, lower, inner and outer" are based on the directions or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as limiting the present invention. In addition, the terms "first, second or third" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0107] In the present invention, unless otherwise clearly specified and limited, the terms "install, connect, connect" should be understood in a broad sense, for example: it can be a fixed connection, a detachable connection or an integral connection; it can also be a mechanical connection, an electrical connection or a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0108] Example 1, reference Figure 1-Figure 4 , which is the first embodiment of the present invention, and provides a photovoltaic inverter low voltage ride through control method based on improved MPPT, comprising:

[0109] like Figure 2 As shown, the present invention adds a feedforward function Gff(s) to the traditional grid-side current control module. When the grid voltage drops, the grid-side overcurrent is mainly caused by the instantaneous drop in the PCC point voltage. In order to eliminate the influence of the PCC voltage on the grid-side current, a full feedforward path of the PCC voltage is introduced.

[0110] like Figure 3 As shown, the present invention adopts DSOGI. When the grid voltage drops asymmetrically, due to the presence of negative sequence components, SRF-PLL cannot accurately keep the same phase with the grid voltage. For the grid synchronization problem under the asymmetrical drop of grid voltage, DSOGI-PLL is an alternative and better solution.

[0111] like Figure 4 As shown, the present invention adds another pin S1 under the traditional MPPT control, and corrects the photovoltaic voltage reference according to the DC bus voltage in order to achieve power balance during LVRT.

[0112] S1: Establish photovoltaic energy flow mapping and prediction model and provide prediction data.

[0113] Furthermore, the output power of the inverter is collected and pre-processed:

[0114]

[0115] Among them, P norm (t) is the normalized PV output power at time t, P pv (t) is the actual photovoltaic output power at time t, c is the slope parameter of the normalized curve, μ P is the mean value of photovoltaic output power. Construct dynamic eigenvector X(t):

[0116]

[0117] Where ill(t) is the light intensity at time t, ω is the frequency related to the light intensity, capturing the frequency component of light changes. Tem(t) is the ambient temperature at time t, Tem ref is the reference temperature, H(t) is the humidity value at time t, H ref is the reference humidity, P load (t-1) is the load power value at time t-1.

[0118] Nonlinear Dynamic Modeling of Photovoltaic Output:

[0119]

[0120] Among them, C1, C2, and C3 are constant parameters with different sensitivities and characteristic effects, and η represents the nonlinear relationship of light intensity. represents the exponential decay effect of temperature on PV output power, ∈(t) is the error term at time t, reflecting the difference between the model prediction and the actual output.

[0121] Make predictions based on the feature vector and dynamic model output:

[0122]

[0123] Among them, f represents the function that takes the feature vector and historical output power as input to construct the final energy flow prediction model:

[0124]

[0125] in, is the predicted PV output power at time t+Δt, F is the number of historical moments considered, f is the step size variable, λ is the attenuation constant that controls the intensity of the influence of historical data on the current prediction, Z is the normalization constant, A represents the importance coefficient of the normalized output in the final prediction, O is the number of features contained in the feature vector, and w o is the weight coefficient of the lth feature, reflecting the contribution of the lth feature to the prediction. o (t) is the lth feature in the feature vector.

[0126] The duty cycle of the pulse width modulation signal is calculated based on the energy flow prediction model.

[0127] Preferably, the input power of the system is P in , the output power is P out , set P in =V d I d , where V d is the input voltage, I d is the input current; ideally, the input power and output power are equal, that is:

[0128]

[0129] Using a converter where the output voltage V out With duty cycle K pwm Related:

[0130] V out =K pwm ·V d

[0131] Further deduction:

[0132]

[0133] Among them, I out is the output current, determined by the load characteristics.

[0134] For example, input voltage V d =400V, output current I out =10A The predicted energy flow is

[0135]

[0136] The storage ratio is 75%.

[0137] S2: Adopt the PCC voltage full feedforward solution to suppress grid-side overcurrent to eliminate the impact of PCC voltage and build a stable operation environment for low voltage ride through.

[0138] Furthermore, by adding a full feedforward path for the PCC voltage, the influence of the PCC voltage on the grid-side current is eliminated. The full feedforward of the PCC point voltage is completed in the hybrid coordinate system, and the feedforward function G is derived. ff , where K pwm is the duty cycle of the PWM signal:

[0139]

[0140] Using historical voltage data V pcc (t) Predict the voltage at the future time

[0141]

[0142] Among them, V PCC (t-iΔt) is the voltage value at the historical moment t-iΔt, j is a different time point in the past, Δt is the predicted time interval, n is the number of historical time points considered, s represents the continuous time points in the time interval, t c is the time constant, which controls the speed of voltage decay. i is a dynamic parameter of the importance of historical voltage values ​​to prediction, γ is a frequency parameter of periodic changes in time, and e is the base of the natural logarithm.

[0143] Real-time monitoring of the current voltage V pcc (t), and calculate the feedback signal error E(t):

[0144]

[0145] The feedforward function is adjusted by feedback compensation, where α is the weighting factor:

[0146]

[0147] Derive the final feedforward function:

[0148]

[0149] Among them, K VCC is the gain factor used to adjust the feedback signal.

[0150] When the PCC point voltage is fully fed forward, the PCC point voltage no longer affects the grid-side current, and no grid-side overcurrent occurs. At this time, the low voltage ride-through operating environment becomes stable.

[0151] S3: Introduce the adjustment coefficient to improve the MPPT control, adjust the photovoltaic voltage reference value and the DC bus voltage, and limit the DC bus voltage.

[0152] Furthermore, when the low-voltage ride-through operating environment reaches stability, start the adjustment. On the basis of the traditional MPPT control, add an adjustment coefficient δ to construct a new reference value V pv and satisfy:

[0153] V pv = δ * V dc (t)

[0154] V MPP < V pv < V oc

[0155] wherein, V dc (t) is the DC bus voltage, V MPP is the voltage of the photovoltaic panel at the maximum power point, V pv is the voltage of the photovoltaic panel, V oc is the open-circuit voltage of the photovoltaic panel.

[0156] The improved MPPT control includes three adjustment cases: when the photovoltaic power is greater than the active power, V dc (t) stores the excess energy, V pv rises, which is defined as Region 1, and the power generated by the photovoltaic module decreases.

[0157] When the photovoltaic power is less than the active power, V dc (t) emits energy to stabilize the system, V pv drops, which is defined as Region 2, and the power generated by the photovoltaic module increases.

[0158] When the photovoltaic power is equal to the active power, it is defined as Region 3, and the energy balance between the photovoltaic side and the grid side is achieved.

[0159] Under the condition of real-time monitoring of the photovoltaic power and the active power, when it is detected that the photovoltaic power P pv crosses the threshold T, trigger the region switching mechanism: when P pv > T, switch to Region 1, when P pv < T, switch to Region 2, when P pv = T, maintain in Region 3, wherein, the dynamic threshold T is:

[0160]

[0161] wherein, a is the photovoltaic system performance optimization coefficient determined through historical data, is the average load power at the time point before the current time, and b is the baseline offset set according to the actual voltage demand.

[0162] After switching to different regions, update the adjustment coefficient δ of each region respectively:

[0163]

[0164] Among them, δ1, δ2 and δ3 are the update adjustment coefficients of the three regions respectively, c is the adjustment sensitivity factor, P target is the target power, P actual is the actual output power, P load is the load demand.

[0165] Re-enter the updated adjustment coefficient to construct the new reference value V pv The updated reference value is obtained in the calculation formula and satisfies V MPP <V pv <V oc .

[0166] S4: When the DC bus voltage is effectively limited, reactive power is injected to cope with grid faults and input into the Boost circuit.

[0167] Furthermore, an improved DC bus voltage limiting model is constructed to adjust the current voltage V through a dynamic feedback mechanism. pcc (t):

[0168]

[0169] Where, ΔV pcc (t) is the current voltage regulation, ε is the gain coefficient of the deviation from the reference value, θ is the gain coefficient of the voltage change rate, V dc (t) is the DC bus voltage at the current time point t, U pv is the new reference value constructed, and k is the parameter that controls the sensitivity.

[0170] Filter the voltage regulation information:

[0171]

[0172] Where S(t) represents a complex filtering function based on historical voltage and load, M is the number of past time series considered, and V load (tm) represents the load voltage at time tm.

[0173] The adjusted DC bus voltage formula is:

[0174] V dc (t) new =V dc (t-1)+ΔV pcc (t)+S(t)

[0175] Get the final voltage limit calculation:

[0176]

[0177] According to the above model, V limit (t) is the new DC bus voltage limit value, is the limiting factor to ensure that the voltage is within a reasonable range, V existing (t) is the current voltage at time t, V existing (t) = V pcc (t)+ΔV pcc (t).

[0178] If V dc (t) new If the limit value is exceeded, the control system will adjust the output power and the connected load to keep the DC bus voltage within a safe range.

[0179] It should be noted that when a grid fault occurs, the voltage outer loop needs to be disconnected and the active current reference value needs to be recalculated and not exceed 1.1 times the rated value:

[0180]

[0181] Among them, i N is the grid-connected current rating, i dref and i qref They are respectively the active current reference value and reactive current reference value of the grid-side current.

[0182] Inject a certain amount of reactive power into the grid connection point and restore the grid voltage through reactive power compensation:

[0183] Q=V PCC (t)·I Q (t)

[0184] Among them, I Q (t) is the reactive current.

[0185] Control logic for reactive power generation:

[0186] Q target =K q ·(V PCC (t)-V ref )

[0187] At this time, the reactive component of the grid-connected current is not 0. It is adjusted in real time according to the degree of grid voltage drop. The reactive current instruction satisfies:

[0188]

[0189] Among them, U T is the per unit value of the grid-connected voltage.

[0190] It should also be noted that the dynamic model of the Boost circuit is represented by a state space model:

[0191]

[0192] Among them, V boost is the Boost output voltage, V in is the input voltage, L is the inductance, R is the circuit impedance, I boost (t) is the current in the Boost circuit at time t.

[0193] When the Boost circuit reaches steady state, the relationship between input and output is:

[0194]

[0195] Among them, U0 and U pv They are the output voltage and input voltage of the Boost circuit respectively.

[0196] Conduct an overall system assessment:

[0197]

[0198] Y is the evaluation cost function, which measures the deviation between the system output and the target value. The smaller the better. out (t is the actual output power of the inverter at time t, P target is the desired target output power, Q(t) is the actual reactive power at time t, Q target is the target reactive power, is the target DC bus voltage t total is the total time frame for the evaluation.

[0199] After evaluation, if the evaluation cost function Y exceeds the preset threshold Y th , system parameters need to be updated:

[0200]

[0201] Among them, Δδ is the update amount of the MPPT adjustment coefficient, ΔV is a parameter that controls the size of the update. pv is the update amount of the photovoltaic voltage reference value, P is the parameter controlling the size of the update amount, To evaluate the partial derivative of the cost function Y with respect to the MPPT adjustment coefficient, it reflects the sensitivity to the change of δ; To evaluate the partial derivative of the cost function Y with respect to the photovoltaic voltage reference value, it reflects the effect of V pv Sensitivity to change.

[0202] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

[0203] Example 2, reference Figure 5-Figure 6 An embodiment of the present invention provides a photovoltaic inverter low voltage ride through control method based on improved MPPT. In order to verify the beneficial effects of the present invention, scientific demonstration is carried out through experiments.

[0204] Figure 5 This is a simulated DC voltage waveform using the strategy proposed in this invention. The simulated working condition is set as follows: the voltage drops at 0.4s and recovers at 0.6s. From the simulated waveform, it can be seen that the improved MPPT control scheme can effectively suppress the overshoot of the inductor current when the voltage drops, thereby reducing the risk of overcurrent on the grid side.

[0205] Figure 6 The simulated photovoltaic grid-connected system outputs active power and reactive power using the strategy proposed in the present invention. It can be seen from the simulated waveform that when the grid voltage drops and VPCC recovers, the active power and reactive power under improved control recover faster.

[0206] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

[0207] Embodiment 3, the third embodiment of the present invention, is different from the first two embodiments in that:

[0208] If the functions are implemented in the form of software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art or the part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium, including several instructions for a computer device (which can be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the methods described in each embodiment of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), disk or optical disk, and other media that can store program codes.

[0209] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as an ordered list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by an instruction execution system, device or apparatus (such as a computer-based system, a system including a processor, or other system that can fetch instructions from an instruction execution system, device or apparatus and execute instructions), or in conjunction with such instruction execution systems, devices or apparatuses. For the purposes of this specification, "computer-readable medium" can be any device that can contain, store, communicate, propagate or transmit a program for use by an instruction execution system, device or apparatus, or in conjunction with such instruction execution systems, devices or apparatuses.

[0210] More specific examples of computer-readable media (a non-exhaustive list) include the following: an electrical connection with one or more wires (electronic device), a portable computer disk case (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable and programmable read-only memory (EPROM or flash memory), an optical fiber device, and a portable compact disk read-only memory (CDROM). In addition, the computer-readable medium may even be a paper or other suitable medium on which the program is printed, since the program may be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, deciphering or, if necessary, processing in another suitable manner, and then stored in a computer memory.

[0211] It should be understood that the various parts of the present invention can be implemented by hardware, software, firmware or a combination thereof. In the above-mentioned embodiments, a plurality of steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented by hardware, as in another embodiment, it can be implemented by any one of the following technologies known in the art or their combination: a discrete logic circuit having a logic gate circuit for implementing a logic function for a data signal, a dedicated integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0212] Example 4, reference Figure 7-Figure 8 , which is the fourth embodiment of the present invention, provides a photovoltaic inverter low voltage ride-through control system based on improved MPPT, including a photovoltaic energy flow mapping and prediction module, a PCC voltage full feedforward control module, an MPPT control module, a DC bus voltage limiting module and a power grid fault response module.

[0213] The photovoltaic energy flow mapping and prediction module collects the output power and environmental parameters of the inverter, obtains standardized photovoltaic power data, establishes dynamic feature vectors, constructs a nonlinear relationship model with photovoltaic output, and combines feature vectors and historical data to predict photovoltaic output power at future times.

[0214] The PCC voltage full feedforward control module derives the feedforward function in a hybrid coordinate system, eliminates the influence of the PCC voltage, monitors the current PCC voltage in real time, and calculates the error of the feedback signal. The feedforward function is adjusted by the weighting factor to stabilize the control environment, and the final feedforward function is obtained by combining the monitoring and feedback information.

[0215] The MPPT control module divides the photovoltaic power into three areas according to the relationship between the photovoltaic power and the active power, calculates the dynamic threshold, and establishes a regional switching mechanism to monitor the photovoltaic power in real time, trigger regional switching, and update the status when it detects that the power crosses the threshold, and update the adjustment coefficient according to the operation requirements of different areas.

[0216] The DC bus voltage limit module calculates the adjustment amount of the deviation from the reference value and uses the gain coefficient for adjustment. When the voltage exceeds the limit value, the output power and load are adjusted in real time.

[0217] The grid fault response module identifies grid faults. When a fault occurs, it disconnects from the grid, recalculates the active current reference value and ensures that it does not exceed 1.1 times the rated value, injects an appropriate amount of reactive power into the grid connection point, restores the grid voltage, adjusts the reactive current command according to the grid demand, controls the dynamic response of the Boost circuit, and ensures a stable output voltage.

[0218] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A photovoltaic inverter low voltage ride-through control method based on improved MPPT, characterized in that: include, Establish photovoltaic energy flow mapping and prediction models and provide prediction data; Adopt the PCC voltage full feed-forward solution to suppress grid-side overcurrent to eliminate the impact of PCC voltage and build a stable operation environment for low voltage ride-through; Introduce adjustment coefficient to improve MPPT control, adjust the photovoltaic voltage reference value and DC bus voltage, and limit the DC bus voltage; When the DC bus voltage is effectively limited, reactive power is injected to cope with grid faults and input into the Boost circuit.

2. The photovoltaic inverter low voltage ride through control method based on improved MPPT as claimed in claim 1, characterized in that: The photovoltaic energy flow mapping and prediction model includes collecting the output power of the inverter and preprocessing it: Among them, P norm (t) is the normalized PV output power at time t, P pv (t) is the actual photovoltaic output power at time t, c is the slope parameter of the normalized curve, μ P is the mean value of photovoltaic output power; construct dynamic eigenvector X(t): Where, ill(t) is the light intensity at time t, ω is the frequency related to the light intensity, capturing the frequency component of the light change; Tem(t) is the ambient temperature at time t, Tem ref is the reference temperature, H(t) is the humidity value at time t, H ref is the reference humidity, P load )t-1) is the load power value at time t-1; Nonlinear Dynamic Modeling of Photovoltaic Output: Among them, C1, C2, and C3 are constant parameters with different sensitivities and characteristic effects, and η represents the nonlinear relationship of light intensity. represents the exponential decay effect of temperature on PV output power, ∈(t) is the error term at time t, reflecting the difference between the model prediction and the actual output; Make predictions based on the feature vector and dynamic model output: Among them, f represents the function that takes the feature vector and historical output power as input to construct the final energy flow prediction model: in, is the predicted PV output power at time t+Δt, F is the number of historical moments considered, f is the step size variable, λ is the attenuation constant that controls the intensity of the influence of historical data on the current prediction, Z is the normalization constant, A represents the importance coefficient of the normalized output in the final prediction, O is the number of features contained in the feature vector, and w o is the weight coefficient of the lth feature, reflecting the contribution of the lth feature to the prediction; X o )t) is the lth feature in the feature vector; The duty cycle of the pulse width modulation signal is calculated based on the energy flow prediction model.

3. The photovoltaic inverter low voltage ride through control method based on improved MPPT as claimed in claim 2, characterized in that: The PCC voltage full feedforward scheme includes: by adding a full feedforward path for the PCC voltage, eliminating the influence of the PCC voltage on the grid-side current; the full feedforward of the PCC point voltage is completed in a hybrid coordinate system; and the feedforward function G is derived. ff , where K pwm is the duty cycle of the PWM signal: Using historical voltage data V pcc (t) Predict the voltage at the future time Among them, V PCC (t-iΔt) is the voltage value at the historical moment t-iΔt, j is a different time point in the past, Δt is the predicted time interval, n is the number of historical time points considered, s represents the continuous time points in the time interval, t c is a time constant that controls the speed of voltage decay; βi is a dynamic parameter of historical voltage value importance for prediction, γ is a frequency parameter of time periodic change, and e is the base of natural logarithm; Real-time monitoring of the current voltage V pcc (t), and calculate the feedback signal error E(t): The feedforward function is adjusted by feedback compensation, where α is the weighting factor: Derive the final feedforward function: Among them, K VCC is the gain factor used to adjust the feedback signal; When the PCC point voltage is fully fed forward, the PCC point voltage no longer affects the grid-side current, and no grid-side overcurrent occurs. At this time, the low voltage ride-through operating environment becomes stable.

4. The photovoltaic inverter low voltage ride through control method based on improved MPPT as claimed in claim 3, characterized in that: The MPPT control improvement includes starting to adjust when the low-voltage ride-through operating environment reaches stability, adding an adjustment coefficient δ on the basis of traditional MPPT control, and constructing a new reference value V pv And meet: V pv =δ*V dc (t) In MPP <V pv <V oc Among them, V dc (t) is the DC bus voltage, V MPP is the voltage of the photovoltaic panel at the maximum power point, V pv is the photovoltaic panel voltage, V oc is the open circuit voltage of the photovoltaic panel; The improved MPPT control includes three regulation situations: When the photovoltaic power is greater than the active power, V dc (t) stores excess energy, V pv Increase, defined as zone 1, the power generated by the photovoltaic modules decreases; When the photovoltaic power is less than the active power, V dc (t) emits energy to stabilize the system, V pv Reduced, specified as zone 2, the power emitted by the PV modules increases; When the PV power is equal to the active power, it is defined as zone 3, and the PV side and grid side energy are balanced; Under the condition of real-time monitoring of photovoltaic power and active power, when it is detected that the photovoltaic power P pv crosses the threshold value T, a region switching mechanism is triggered: when P pv >T, it switches to Region 1; when P pv <T, it switches to Region 2; when P pv =T, it remains in Region 3. Among them, the dynamic threshold value T is: Among them, a is the photovoltaic system performance optimization coefficient determined by historical data, is the average load power at the time point before the current time, and b is the baseline offset set according to the actual voltage demand; When switching to different areas, update the adjustment coefficient δ of each area separately: Among them, δ1, δ2 and δ3 are the update adjustment coefficients of the three regions respectively, c is the adjustment sensitivity factor, P target is the target power, P actual is the actual output power, P load is the load demand; Re-enter the updated adjustment coefficient to construct the new reference value V pv The updated reference value is obtained in the calculation formula and satisfies V MPP <V pv <V oc .

5. The photovoltaic inverter low voltage ride through control method based on improved MPPT as claimed in claim 4, characterized in that: The limiting of the DC bus voltage includes constructing an improved DC bus voltage limiting model and adjusting the current voltage V through a dynamic feedback mechanism. pcc (t): Where, ΔV pcc (t) is the current voltage regulation, ε is the gain coefficient of the deviation from the reference value, θ is the gain coefficient of the voltage change rate, V dc (t) is the DC bus voltage at the current time point t, U pv is the new reference value constructed, k is the parameter controlling the sensitivity; Filter the voltage regulation information: Where S(t) represents a complex filtering function based on historical voltage and load, M is the number of past time series considered, and V load (tm) represents the load voltage at time tm; The adjusted DC bus voltage formula is: V dc (t) new =V dc (t-1)+ΔV pcc (t)+S(t) Get the final voltage limit calculation: According to the above model, V limit (t) is the new DC bus voltage limit value, is the limiting factor to ensure that the voltage is within a reasonable range, V existing (t) is the current voltage at time t, V existing (t) = V pcc (t)+ΔV pcc (t); If V dc (t) new If the limit value is exceeded, the control system will adjust the output power and the connected load to keep the DC bus voltage within a safe range.

6. The photovoltaic inverter low voltage ride through control method based on improved MPPT as claimed in claim 5, characterized in that: The response to the grid failure includes that when the grid fails, the voltage outer loop needs to be disconnected and the active current reference value is recalculated and does not exceed 1.1 times the rated value: Among them, i N is the grid-connected current rating, i dref and i qref They are respectively the active current reference value and the reactive current reference value of the grid-side current; Inject a certain amount of reactive power into the grid connection point and restore the grid voltage through reactive power compensation: Q=V PCC (t)·I Q (t) Among them, I Q (t) is the reactive current; Control logic for reactive power generation: Q target =K q ·(V PCC (t)-V ref ) At this time, the reactive component of the grid-connected current is not 0. It is adjusted in real time according to the degree of grid voltage drop. The reactive current instruction satisfies: Among them, U T is the per unit value of the grid-connected voltage.

7. The photovoltaic inverter low voltage ride through control method based on improved MPPT as claimed in claim 6, characterized in that: The input Boost circuit includes a dynamic model of the Boost circuit represented by a state space model: Among them, V boost is the Boost output voltage, V in is the input voltage, L is the inductance, R is the circuit impedance, I boost (t) is the current in the Boost circuit at time t; When the Boost circuit reaches steady state, the relationship between input and output is: Among them, U0 and U pv are the output voltage and input voltage of the Boost circuit respectively; Conduct an overall system assessment: Y is the evaluation cost function, which measures the deviation between the system output and the target value. The smaller the better. P out (t) is the actual output power of the inverter at time t, P target is the desired target output power, Q(t) is the actual reactive power at time t, Q target is the target reactive power, is the target DC bus voltage t total The total time frame for the assessment; After evaluation, if the evaluation cost function Y exceeds the preset threshold Y th , system parameters need to be updated: Among them, Δδ is the update amount of the MPPT adjustment coefficient, ΔV is a parameter that controls the size of the update. pv is the update amount of the photovoltaic voltage reference value, P is the parameter controlling the size of the update amount, To evaluate the partial derivative of the cost function Y with respect to the MPPT adjustment coefficient, it reflects the sensitivity to the change of δ; To evaluate the partial derivative of the cost function Y with respect to the photovoltaic voltage reference value, it reflects the effect of V pv Sensitivity to change.

8. A system using the photovoltaic inverter low voltage ride through control method based on improved MPPT as claimed in any one of claims 1 to 7, characterized in that: It includes photovoltaic energy flow mapping and prediction module, PCC voltage full feedforward control module, MPPT control module, DC bus voltage limiting module and grid fault response module; The photovoltaic energy flow mapping and prediction module collects the output power and environmental parameters of the inverter, obtains standardized photovoltaic power data, establishes dynamic feature vectors, constructs a nonlinear relationship model with photovoltaic output, and combines the feature vectors and historical data to predict the photovoltaic output power at future times; The PCC voltage full feedforward control module derives the feedforward function in a hybrid coordinate system, eliminates the influence of the PCC voltage, monitors the current PCC voltage in real time, calculates the error of the feedback signal, adjusts the feedforward function through a weighting factor, controls the environment to be stable, and obtains the final feedforward function by combining monitoring and feedback information; The MPPT control module is divided into three areas according to the relationship between photovoltaic power and active power, calculates dynamic thresholds, and establishes a regional switching mechanism to monitor photovoltaic power in real time, trigger regional switching, and perform status updates when power crossing the threshold is detected, and update the adjustment coefficient according to the operation requirements of different areas; The DC bus voltage limiting module calculates the adjustment amount of the deviation from the reference value, uses the gain coefficient for adjustment, and adjusts the output power and load in real time when the voltage exceeds the limit value; The grid fault response module identifies grid faults, disconnects from the grid when a fault occurs, recalculates the active current reference value and ensures that it does not exceed 1.1 times the rated value, injects an appropriate amount of reactive power into the grid connection point, restores the grid voltage, adjusts the reactive current command according to the grid demand, controls the dynamic response of the Boost circuit, and ensures a stable output voltage.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: When the processor executes the computer program, the steps of the photovoltaic inverter low voltage ride through control method based on improved MPPT are implemented as described in any one of claims 1 to 7.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the photovoltaic inverter low voltage ride through control method based on improved MPPT are implemented as described in any one of claims 1 to 7.

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