A high voltage ride-through method for single-stage photovoltaic inverter

Abstract

The present invention relates to a single-stage photovoltaic inverter high voltage ride-through method, belonging to the technical field of AC / DC converter optimization control. The additional controller of the present invention is based on system parameters and the measured grid voltage amplitude U peak , the instantaneous value of the photovoltaic system output power P PV Calculate the minimum DC voltage U required to avoid overmodulation min , U min That is, the minimum value of the DC voltage required for the high voltage ride-through of the single-stage photovoltaic inverter system, and it is compared with the photovoltaic maximum power point voltage, and the result obtained is used as the reference DC voltage of the power control system. The present invention can avoid the risk of overmodulation, and realize the tolerance operation of the single-stage photovoltaic system under the condition of temporary rise of the grid voltage by reasonably adjusting the DC side voltage in the single-stage photovoltaic system, thereby ensuring the safe and stable operation of the single-stage photovoltaic inverter and maximizing the utilization of the energy generated by the photovoltaic system.

Description

Technical Field

[0001] The invention relates to a high voltage ride-through method for a single-stage photovoltaic inverter, and belongs to the technical field of AC / DC converter optimization control. Background Art

[0002] Compared with the traditional two-stage photovoltaic inverter, the single-stage photovoltaic inverter has the outstanding advantages of simple structure, high working efficiency, compact system size and low manufacturing cost. With these technical advantages, the proportion of single-stage photovoltaic inverters and systems in the market has gradually increased, and they are considered to be a topological structure with great development prospects in applications such as distributed power generation, photovoltaic local support, and DC power distribution systems. However, when the single-stage photovoltaic inverter system is working, changes in light or temperature will cause the output power of the photovoltaic system to fluctuate continuously. In order to maximize the use of the power generated by the photovoltaic system, the maximum power point tracking control (Maximum Power Point Tracking, MPPT) will adjust the DC side voltage. Unlike the traditional two-stage photovoltaic system, the DC side voltage of the single-stage photovoltaic system is not a fixed value, which may cause over-modulation problems when the grid voltage rises temporarily, making it difficult to complete high voltage ride-through, and ultimately causing waveform distortion.

[0003] At present, domestic and foreign research has paid little attention to the above issues. The overmodulation risk assessment criteria for single-stage photovoltaic inverter systems mostly directly adopt the modulation ratio calculation method of two-stage photovoltaic inverter systems. However, the existing methods have defects. For example, the setting of the DC side voltage does not take into account the influence of the light intensity and the corresponding maximum power point tracking method on the DC voltage, and does not consider the high voltage ride-through problem when the grid voltage rises temporarily. The method of modifying the main circuit topology and adding auxiliary circuits will not only increase power loss, but also increase cost and design complexity. In addition, most of the existing research on the high voltage ride-through problem of single-stage photovoltaic inverter systems is based on proportional-integral control, which has insufficient dynamic response speed and cannot respond to the grid voltage rise in time. These problems have become bottlenecks that restrict the application of single-stage inverters in photovoltaic system grid connection. Summary of the invention

[0004] In view of the above problems, the present invention provides a high voltage ride-through method for a single-stage photovoltaic inverter. An additional controller is used to calculate the minimum DC voltage required to complete high voltage ride-through without overmodulation problems, and the minimum DC voltage is compared with the photovoltaic maximum power point voltage. The result is used as the reference DC voltage of the power control system, which ensures the safe and stable operation of the single-stage photovoltaic inverter and maximizes the utilization of the energy generated by the photovoltaic system.

[0005] The technical solution of the present invention is as follows:

[0006] A single-stage photovoltaic inverter high voltage ride-through method is implemented by a control system of the single-stage photovoltaic inverter. The control system includes a maximum power point tracking controller, an additional controller and a power control system. The maximum power point tracking controller calculates a reference voltage U MPPT The additional controller is based on the system parameters (angular frequency ω, resistance value R of the grid-connected filter f And the inductance value L of the grid-connected filter f etc.) and the measured grid voltage amplitude U peak , the instantaneous value of the photovoltaic system output power P PV Calculate the minimum DC voltage U required to avoid overmodulation min , U min This is the minimum DC voltage required for high voltage ride-through of a single-stage photovoltaic inverter system;

[0007] Will U min with U MPPT For comparison, when U MPPT Greater than or equal to U min When the single-stage photovoltaic inverter operates in the maximum power tracking mode, U MPPT As DC voltage reference value U Ref ; When the grid voltage rises temporarily, U min Increase, when U MPPT Less than U min When the single-stage photovoltaic inverter switches from the maximum power tracking mode to the high voltage ride-through mode, U min Alternative U MPPT As DC voltage reference value U Ref , in order to achieve the tolerance operation of single-stage photovoltaic inverter under the condition of grid voltage surge;

[0008] The power control system is based on the output current i and DC voltage U cap The reference value and predicted value corresponding to the output power P control the main circuit.

[0009] Preferably, in the maximum power point tracking controller, the output voltage U of the photovoltaic array is input PV and output current I PV , and then calculate the instantaneous power P of photovoltaic output PV , the calculation method is:

[0010] P PV =U PV I PV (1)

[0011] The instantaneous power P output by photovoltaic at the current moment PV The instantaneous value P of the previous sampling period (k-1)The comparison is performed, and the comparison result generates the reference voltage U for the next sampling period according to the perturbation observation method. MPPT .

[0012] Preferably, the additional controller calculates U min for:

[0013]

[0014] Where ω is the angular frequency, R f is the resistance value of the grid-connected filter, L f is the inductance value of the grid-connected filter.

[0015] Preferably, U min The derivation process is:

[0016] S1: Taking phase A of the ABC three-phase voltage as an example, according to formula (2), the instantaneous measured value of the grid-side voltage u ga The expression of the instantaneous power P output of photovoltaic is PV Substituting this into the equation, we can derive the AC current i ga Expressions for instantaneous values;

[0017] u ga (t) = U peak cosωt (2)

[0018]

[0019] In the formula, t represents the current time, U peak represents the measured grid voltage amplitude, ω represents the angular frequency, P A Represents the instantaneous active power of phase A;

[0020] S2: Based on Kirchhoff's law, derive the electromotive force e at the output port of a single-stage photovoltaic inverter a The relationship between the AC current on the grid side;

[0021]

[0022] In the formula, R f is the resistance value of the grid-connected filter, L f is the inductance value of the grid-connected filter;

[0023] Substituting formula (3) into formula (4), we get

[0024] e a (t)=Acosωt+Bsinωt (5)

[0025] In the formula,

[0026]

[0027] Rewrite equation (5) into the form of cosine function, and we get

[0028]

[0029] In the formula,

[0030]

[0031] S3: Calculate the value of the modulation ratio M, using the space voltage vector pulse width modulation technology based on the three-phase three-level diode neutral point clamped topology. The modulation ratio calculation formula can be expressed as:

[0032]

[0033] Where U dc Represents the DC side voltage of a single-stage photovoltaic inverter;

[0034] According to the modulation ratio calculation formula (9), substituting into formula (5)-formula (8), the minimum DC voltage required to avoid overmodulation can be obtained: min :

[0035]

[0036] Preferably, the power control system controls the output current i and the DC voltage U cap The reference value of the output current i is determined by the reference value calculator according to the instantaneous value P of the output power of the photovoltaic system. PV With grid voltage U g Calculated; DC voltage U cap Reference value U Ref The maximum power point tracking controller output result U MPPT The additional controller output result U min The reference value of output power P is the instantaneous value of the photovoltaic system output power P PV At the same time, the prediction value calculator will calculate the instantaneous measured value i of the inverter output current at the current moment o The instantaneous measured value of the inverter output voltage e at the current moment o Calculate the predicted values ​​of output current, DC voltage and output power in the next sampling period;

[0037] Afterwards, the output current prediction value is subtracted from the current reference value, the DC voltage prediction value is subtracted from the DC voltage reference value, and the output power prediction value is subtracted from the output power reference value, so as to obtain the error values ​​of the three control targets;

[0038] The error values ​​of the three control objectives will be multiplied by the weight factor and then brought into the value function of the model predictive control module. The weight factor is adjusted according to the system parameters and control response requirements. The higher the priority of the control objective, the larger the weight factor. The value function in the model predictive control module can be expressed as J = f(i,U cap ,P), where f(i,U cap ,P) represents the mathematical expression of the error values ​​of the three control targets of output current, DC voltage and output power multiplied by the weight factor and added together, J is the calculated value of the value function, and the minimum value of the value function J min The corresponding switch state will be used as an insulated gate bipolar translator (IGBT) trigger signal to trigger the insulated gate bipolar translator (GBT) in the main circuit.

[0039] In the present invention, when the proposed single-stage photovoltaic converter is used under the condition of grid voltage transient rise, the additional controller will adjust the grid voltage amplitude U according to the system parameters such as angular frequency ω, inductance L, resistance R and the measured grid voltage amplitude U peak , the instantaneous value of the photovoltaic system output power P PV Calculate the minimum DC voltage U required to avoid overmodulation min , which is the minimum DC voltage required for high voltage ride-through of a single-stage photovoltaic inverter system. If the actual operating voltage of a single-stage photovoltaic system is less than this value, it can be considered that overmodulation has occurred, so U min It can be used as a basis for overmodulation risk assessment. Figure 1 in, f(U peak ,P PV ) represents the calculation function of the minimum value of the DC voltage required for high voltage ride-through, and the input parameter is the measured grid voltage amplitude U peak , the instantaneous value of the photovoltaic system output power P PV . stM≤1 means that the modulation ratio is less than 1, that is, the condition of no overmodulation occurs is satisfied. Then, U min The maximum power point tracking controller output result U MPPT The maximum power point tracking controller output result U MPPT Greater than or equal to the minimum DC voltage required for high voltage ride-through U min The PV inverter will operate in the maximum power point tracking mode. When the maximum power point tracking controller outputs the result U MPPT Less than the minimum DC voltage required for high voltage ride-through U minWhen the PV inverter switches from the maximum power tracking mode to the high voltage ride-through mode, the PV inverter will prioritize the high voltage ride-through under the condition of grid voltage swell without causing over-modulation. Therefore, the reference value of the DC voltage will change according to the grid voltage swell.

[0040] For any details not provided in the present invention, please refer to the prior art.

[0041] The beneficial effects of the present invention are:

[0042] The present invention can achieve the tolerance operation of the single-stage photovoltaic system under the condition of grid voltage surge by reasonably adjusting the DC side voltage in the single-stage photovoltaic system without the risk of over-modulation. Ref To reduce the modulation ratio, thereby improving the high voltage ride-through capability of the single-stage photovoltaic inverter. In the implementation process, the present invention introduces an overmodulation risk assessment method based on the minimum required voltage for high voltage ride-through (U min with U MPPT Comparison of) and operation mode switching mechanism to ensure the safe and stable operation of single-stage photovoltaic inverters and maximize the use of energy generated by the photovoltaic system. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The drawings in the specification, which constitute a part of the present application, are used to provide further understanding of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute improper limitations on the present application.

[0044] Figure 1 A control block diagram of a high voltage ride-through method for a single-stage photovoltaic inverter according to the present invention;

[0045] Figure 2 For U min Calculation results under example parameters;

[0046] Figure 3 This is the output characteristic curve of the photovoltaic series JAM5-72-165;

[0047] Figure 4 This is the relationship between the maximum power point voltage and the required minimum DC voltage under normal grid conditions;

[0048] Figure 5 It is the relationship diagram between the maximum power point voltage and the required minimum DC voltage under the condition of temporary rise of grid voltage;

[0049] Figure 6 It is the operation mode diagram of single-stage inverter under different working conditions;

[0050] Figure 7The waveform diagram of the photovoltaic system under normal grid conditions, where (a) is AC voltage, (b) is AC current, (c) is AC power, (d) is DC voltage, (e) is DC current, and (f) is DC power;

[0051] Figure 8 is the grid voltage "U g =1.15pu"U g =1.3pu” condition, waveform diagram of the single-stage photovoltaic inverter high voltage ride-through method without using the proposed method, where (a) is AC voltage, (b) is AC current, (c) is AC power, (d) is DC voltage, (e) is DC current, and (f) is DC power;

[0052] Fig. 9 is the grid voltage "U g =1.3pu”, waveforms of the high voltage ride-through method of the proposed single-stage photovoltaic inverter, where (a) is the AC voltage, (b) is the AC current, (c) is the AC power, (d) is the DC voltage, (e) is the DC current, and (f) is the DC power. DETAILED DESCRIPTION

[0053] In order to enable those skilled in the art to better understand the technical solutions in this specification, the technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with the drawings in the implementation of this specification, but are not limited to this. Anything not fully described in the present invention shall be based on the conventional technology in the art.

[0054] Example 1

[0055] A single-stage photovoltaic inverter high voltage ride-through method, the overall block diagram is as follows Figure 1 As shown, it is realized by the control system of a single-stage photovoltaic inverter. The control system includes a maximum power point tracking controller, an additional controller and a power control system. The maximum power point tracking controller calculates the reference voltage U MPPT The additional controller is based on the system parameters (angular frequency ω, resistance value R of the grid-connected filter f And the inductance value L of the grid-connected filter f etc.) and the measured grid voltage amplitude U peak , the instantaneous value of the photovoltaic system output power P PV Calculate the minimum DC voltage U required to avoid overmodulation min , U min This is the minimum DC voltage required for high voltage ride-through of a single-stage photovoltaic inverter system;

[0056] Will U min with U MPPT For comparison, when U MPPT Greater than or equal to Umin When the single-stage photovoltaic inverter operates in the maximum power tracking mode, U MPPT As DC voltage reference value U Ref ; When the grid voltage rises temporarily, U min Increase, when U MPPT Less than U min When the single-stage photovoltaic inverter switches from the maximum power tracking mode to the high voltage ride-through mode, U min Alternative U MPPT As DC voltage reference value U Ref , in order to achieve the tolerance operation of single-stage photovoltaic inverter under the condition of temporary grid voltage rise.

[0057] The power control system is based on the output current i and DC voltage U cap The reference value and predicted value corresponding to the output power P control the main circuit.

[0058] like Figure 1 As shown in FIG. 1 , when the proposed single-stage photovoltaic converter is used under the condition of grid voltage transient rise, the additional controller will adjust the grid voltage amplitude U according to the system parameters such as angular frequency ω, inductance L, resistance R and the measured grid voltage amplitude U peak , the instantaneous value of the photovoltaic system output power P PV Calculate the minimum DC voltage U required to avoid overmodulation min , which is the minimum DC voltage required for high voltage ride-through of a single-stage photovoltaic inverter system. If the actual operating voltage of a single-stage photovoltaic system is less than this value, it can be considered that overmodulation has occurred, so U min It can be used as a basis for overmodulation risk assessment. Figure 1 in, f(U peak ,P PV ) represents the calculation function of the minimum value of the DC voltage required for high voltage ride-through, and the input parameter is the measured grid voltage amplitude U peak , the instantaneous value of the photovoltaic system output power P PV . stM≤1 means that the modulation ratio is less than 1, that is, the condition of no overmodulation occurs is satisfied. Then, U min The maximum power point tracking controller output result U MPPT The maximum power point tracking controller output result U MPPT Greater than or equal to the minimum DC voltage required for high voltage ride-through U min The PV inverter will operate in the maximum power point tracking mode. When the maximum power point tracking controller outputs the result U MPPT Less than the minimum DC voltage required for high voltage ride-through U minWhen the PV inverter switches from the maximum power tracking mode to the high voltage ride-through mode, the PV inverter will prioritize the high voltage ride-through under the condition of grid voltage swell without causing over-modulation. Therefore, the reference value of the DC voltage will change according to the grid voltage swell.

[0059] Example 2

[0060] A single-stage photovoltaic inverter high voltage ride-through method, as described in Example 1, except that in the maximum power point tracking controller, the output voltage U of the photovoltaic array is input PV and output current I PV , and then calculate the instantaneous power P of photovoltaic output PV , the calculation method is:

[0061] P PV =U PV I PV (1)

[0062] The instantaneous power P output by photovoltaic at the current moment PV The instantaneous value P of the previous sampling period (k-1) The comparison is performed, and the comparison result generates the reference voltage U for the next sampling period according to the perturbation observation method. MPPT .

[0063] Example 3

[0064] A single-stage photovoltaic inverter high voltage ride-through method, as described in Example 2, except that the additional controller calculates the U min for:

[0065]

[0066] Where ω is the angular frequency, R f is the resistance value of the grid-connected filter, L f is the inductance value of the grid-connected filter.

[0067] The derivation process is:

[0068] S1: Taking phase A of the ABC three-phase voltage as an example, according to formula (2), the instantaneous measured value of the grid-side voltage u ga The expression of the instantaneous power P output of photovoltaic is PV Substituting this into the equation, we can derive the AC current i ga Expressions for instantaneous values;

[0069] u ga (t) = U peak cosωt (2)

[0070]

[0071] In the formula, t represents the current time, U peak represents the measured grid voltage amplitude, ω represents the angular frequency, P A Represents the instantaneous active power of phase A;

[0072] S2: Based on Kirchhoff's law, derive the electromotive force e at the output port of a single-stage photovoltaic inverter a The relationship between the AC current on the grid side;

[0073]

[0074] In the formula, R f is the resistance value of the grid-connected filter, L f is the inductance value of the grid-connected filter;

[0075] Substituting formula (3) into formula (4), we get

[0076] e a (t)=Acosωt+Bsinωt (5)

[0077] In the formula,

[0078]

[0079] Rewrite equation (5) into the form of cosine function, and we get

[0080]

[0081] In the formula,

[0082]

[0083] S3: Calculate the value of the modulation ratio M, using the space voltage vector pulse width modulation technology based on the three-phase three-level diode neutral point clamped topology. The modulation ratio calculation formula can be expressed as:

[0084]

[0085] Where U dc Represents the DC side voltage of a single-stage photovoltaic inverter;

[0086] According to the modulation ratio calculation formula (9), substituting into formula (5)-formula (8), the minimum DC voltage required to avoid overmodulation can be obtained: min :

[0087]

[0088] Example 4

[0089] A single-stage photovoltaic inverter high voltage ride-through method, as described in Example 3, except that the power control system controls the output current i and the DC voltage U cap The reference value of the output current i is determined by the reference value calculator according to the instantaneous value P of the output power of the photovoltaic system. PV With grid voltage U g Calculated; DC voltage U cap Reference value U Ref The maximum power point tracking controller output result U MPPT The additional controller output result U min The reference value of output power P is the instantaneous value of the photovoltaic system output power P PV At the same time, the prediction value calculator will calculate the instantaneous measured value i of the inverter output current at the current moment o The instantaneous measured value of the inverter output voltage e at the current moment o Calculate the predicted values ​​of output current, DC voltage and output power in the next sampling period;

[0090] Afterwards, the output current prediction value is subtracted from the current reference value, the DC voltage prediction value is subtracted from the DC voltage reference value, and the output power prediction value is subtracted from the output power reference value, so as to obtain the error values ​​of the three control targets;

[0091] The error values ​​of the three control objectives will be multiplied by the weight factor and then brought into the value function of the model predictive control module. The weight factor is adjusted according to the system parameters and control response requirements. The higher the priority of the control objective, the larger the weight factor. The value function in the model predictive control module can be expressed as J = f(i,U cap ,P), where f(i,U cap ,P) represents the mathematical expression of the error values ​​of the three control targets of output current, DC voltage and output power multiplied by the weight factor and added together, J is the calculated value of the value function, and the minimum value of the value function J min The corresponding switch state will be used as an insulated gate bipolar translator (IGBT) trigger signal to trigger the insulated gate bipolar translator (GBT) in the main circuit.

[0092] In order to verify the accuracy and feasibility of the high voltage ride-through method for a single-stage photovoltaic inverter proposed in this patent, the following is verified in combination with an embodiment. The main circuit parameters of the photovoltaic system in the embodiment are shown in Table 1.

[0093] Table 1 Parameters of main circuit of photovoltaic system

[0094]

[0095] The relationship between the minimum DC voltage required for high voltage ride-through and the grid voltage and power described in the method of the present invention is as follows: Figure 2 As shown, P PV Represents the instantaneous value of the photovoltaic system output power, U peak Represents the grid voltage amplitude, U min Represents the minimum value of the DC voltage required to prevent overmodulation. Figure 2 As shown in Figure 1, the minimum DC voltage required under different working conditions varies between 269.44V and 815.10V, which means that when the grid voltage fluctuates, the photovoltaic array cannot always maintain maximum power control. The results show that under almost all working conditions, the grid voltage is the main factor affecting the required DC voltage. It should be noted that when the grid voltage is around 0.5pu and the photovoltaic output power is around 1.0pu, as the grid voltage increases, the required DC voltage first decreases and then increases, which reflects the effect of the filter voltage drop on the minimum DC voltage required for high voltage ride-through, which is consistent with the conclusion of formula (10).

[0096] In the embodiment, the photovoltaic array is composed of a JAM5-72-165 photovoltaic panel having 18 series modules and 4 parallel panels. The ideal power-voltage curve of the photovoltaic array output characteristic is as follows: Figure 3 shown. Figure 3 In, P PV Represents the instantaneous value of the photovoltaic system output power, U PV Represents the output voltage of the photovoltaic array. The maximum power point voltage of the photovoltaic array in the embodiment is 651.6V, and the maximum output power is 11890W.

[0097] The present invention proposes two working modes to adjust the control method of the photovoltaic system:

[0098] Operation mode I: The required DC voltage is lower than the maximum power point voltage of the photovoltaic array (U MPPT ). In this mode, MPPT control is enabled to convert as much power as possible from the PV array to the grid. The relationship between the maximum power point voltage and the minimum voltage required for high voltage ride-through is as follows: Figure 4 shown.

[0099] Operation mode II: When the grid voltage rises temporarily, the required DC voltage is higher than the maximum power point voltage of the PV array, and the PV system switches to high voltage ride-through mode. In this mode, to avoid overmodulation, the MPPT is temporarily disabled. The relationship between the maximum power point voltage and the minimum required voltage for high voltage ride-through is as follows: Figure 5 At this time, the minimum DC voltage required for high voltage ride-through will replace the photovoltaic maximum power point voltage as the reference voltage of the power control system.

[0100] exist Figure 4The maximum power point tracking controller outputs U MPPT (651.6V) is greater than the minimum DC voltage required for high voltage ride-through U min (544.59V), therefore, the maximum power point tracking controller output result is U MPPT (651.6V) will be used as the reference voltage for the power control system. Figure 5 The maximum power point tracking controller outputs U MPPT (651.6V) is less than the minimum DC voltage required for high voltage ride-through U min (696.43V), therefore, the minimum DC voltage required for high voltage ride-through is U min (696.43V) will be used as the reference voltage for the power control system.

[0101] Figure 6 Schematic diagram of the working mode of a single-stage photovoltaic inverter under different working conditions. Figure 6 In, P PV Represents the instantaneous value of the photovoltaic system output power, U peak Represents the grid voltage amplitude, U min Represents the minimum DC voltage required for high voltage ride-through. Without changing the single-stage system topology or the control strategy of the power control system, when the grid-side voltage rises temporarily, the voltage at the maximum power point will be lower than the minimum DC voltage required for high voltage ride-through, resulting in overmodulation. This method sets a voltage alarm when the voltage approaches the limit value. If there is a risk of overmodulation, it switches from mode I to mode II, and reasonably abandons the photovoltaic array to avoid the risk of overmodulation, complete voltage ride-through and ensure voltage quality.

[0102] To further verify the effectiveness of the proposed method, the waveforms before and after the operation method under grid voltage swell conditions using a single-stage PV inverter are provided. Figure 7 The waveform diagram of the photovoltaic system under normal grid conditions is given. Figure 8 Given the grid voltage "U g =1.15pu"U g =1.3pu” without using the proposed single-stage PV inverter high voltage ride-through method. Fig. 9 Given the grid voltage "U g =1.3pu” condition, using the proposed single-stage photovoltaic inverter high voltage ride-through method waveform diagram. Figures 7 to 9 The six sub-figures (a) to (f) in the figure represent AC voltage, AC current, AC power (where P represents AC active power and Q represents AC reactive power), DC voltage, DC current, and DC power, respectively.

[0103] According to the method for calculating the minimum DC voltage required for voltage ride-through proposed in the present invention, when the light intensity and ambient temperature remain unchanged, if the DC side voltage is controlled at the maximum power point, the maximum allowable grid voltage is 1.18pu, that is, it can still operate when the grid side voltage temporarily rises to 1.15pu, but overmodulation will occur when the grid side voltage temporarily rises to 1.3pu.

[0104] The analysis of the data is as follows:

[0105] First, in Figure 8 In the corresponding operating condition, the grid-side voltage is increased to 1.15pu and 1.3pu from t=0.5s to t=0.7s, and then restored to 1.0pu at t=0.9s. Figure 7 and Figure 8 It can be seen from the figure that during the period from t = 0.5s to t = 0.7s, the DC power output by the photovoltaic system and the AC power output by the inverter are Figure 7 The AC side current waveform still maintains a sinusoidal shape. In addition, the DC side voltage still maintains the maximum power point voltage, proving that the inverter can operate under this condition.

[0106] Secondly, from Figure 8 It can be seen that during the period from t=0.7s to t=0.9s, the low DC voltage caused an overmodulation problem, which caused the AC power output of the inverter to fluctuate and the AC side current waveform to be distorted, which did not meet the requirements of high voltage crossing. It should be noted that since the DC side voltage reference value is still the result of maximum power point tracking control at this time, and the power control module takes the minimum AC current error as the main control target, the DC side voltage and current are forced to change, and the system becomes further unstable. This result proves that under the conditions of the system parameters and the grid voltage increase, the grid-connected inverter cannot complete the high voltage crossing, which is consistent with the results of the overmodulation criterion mentioned in this patent.

[0107] Third, after adding the additional controller, the grid-connected inverter is no longer controlled at the maximum power point voltage. Fig. 9 In the example, the grid voltage is increased to 1.3 pu from t = 0.5 s and restored to 1.0 pu at t = 0.9 s. When the grid voltage temporarily rises to 1.3 pu, the DC reference voltage provided by the additional controller is 696.4 V. This value is obtained by substituting the grid voltage amplitude into equation (10) and calculating it together with the power-voltage equation of the photovoltaic output characteristics. Fig. 9It can be seen from the operation results that by reasonably discarding part of the power generated by the photovoltaic array, the AC side current maintains a sinusoidal waveform, no overmodulation occurs, and no reactive power is generated. After t=0.9s, the grid-side voltage is restored, and the photovoltaic system completes the high voltage ride-through during the temporary rise of the grid voltage, which proves the effectiveness of the method proposed in this patent.

[0108] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A high voltage ride-through method for a single-stage photovoltaic inverter, characterized in that: It is realized by the control system of a single-stage photovoltaic inverter. The control system includes a maximum power point tracking controller, an additional controller and a power control system. The maximum power point tracking controller calculates the reference voltage U MPPT The additional controller is based on the system parameters and the measured grid voltage amplitude U peak , the instantaneous value of the photovoltaic system output power P PV Calculate the minimum DC voltage U required to avoid overmodulation min , U min This is the minimum DC voltage required for high voltage ride-through of a single-stage photovoltaic inverter system; Will U min with U MPPT For comparison, when U MPPT Greater than or equal to U min When the single-stage photovoltaic inverter operates in the maximum power tracking mode, U MPPT As DC voltage reference value U Ref ; When the grid voltage rises temporarily, U min Increase, when U MPPT Less than U min When the single-stage photovoltaic inverter switches from the maximum power tracking mode to the high voltage ride-through mode, U min Alternative U MPPT As DC voltage reference value U Ref , in order to achieve the tolerance operation of single-stage photovoltaic inverter under the condition of grid voltage surge; The power control system is based on the output current i and DC voltage U cap The reference value and predicted value corresponding to the output power P control the main circuit; U calculated by the additional controller min for: Where ω is the angular frequency, R f is the resistance value of the grid-connected filter, L f is the inductance value of the grid-connected filter; U min The derivation process is: S1: Taking phase A of the ABC three-phase voltage as an example, according to formula (2), the instantaneous measured value of the grid-side voltage u ga The expression of the instantaneous power P output of photovoltaic is PV Substituting this into the equation, we can derive the AC current i ga Expressions for instantaneous values; u ga (t)=U peak cosωt (2) In the formula, t represents the current time, U peak represents the measured grid voltage amplitude, ω represents the angular frequency, P A Represents the instantaneous active power of phase A; S2: Based on Kirchhoff's law, derive the electromotive force e at the output port of a single-stage photovoltaic inverter a The relationship between the AC current on the grid side; In the formula, R f is the resistance value of the grid-connected filter, L f is the inductance value of the grid-connected filter; Substituting formula (3) into formula (4), we get e a (t)=Acosωt+Bsinωt (5) In the formula, Rewrite equation (5) into the form of cosine function, and we get In the formula, S3: Calculate the value of the modulation ratio M, using the space voltage vector pulse width modulation technology based on the three-phase three-level diode neutral point clamp topology. The modulation ratio calculation formula is expressed as: Where U dc Represents the DC side voltage of a single-stage photovoltaic inverter; According to the modulation ratio calculation formula (9), substituting into formula (5)-formula (8), the minimum DC voltage required for no overmodulation is obtained: min :

2. The high voltage ride through method for a single-stage photovoltaic inverter according to claim 1, characterized in that: In the maximum power point tracking controller, the output voltage U of the photovoltaic array is input PV and output current I PV , and then calculate the instantaneous power P of photovoltaic output PV , the calculation method is: P PV =U PV AND PV (1) The instantaneous power P output by photovoltaic at the current moment PV The instantaneous value P of the previous sampling period (k-1) The comparison is performed, and the comparison result generates the reference voltage U for the next sampling period according to the perturbation observation method. MPPT .

3. The single-stage photovoltaic inverter high voltage ride-through method according to claim 2, characterized in that: The power control system controls the output current i and DC voltage U cap The reference value of the output current i is determined by the reference value calculator according to the instantaneous value P of the output power of the photovoltaic system. PV With grid voltage U g Calculated; DC voltage U cap Reference value U Ref The maximum power point tracking controller output result U MPPT The additional controller output result U min The reference value of output power P is the instantaneous value of the photovoltaic system output power P PV At the same time, the prediction value calculator will calculate the instantaneous measured value i of the inverter output current at the current moment o The instantaneous measured value of the inverter output voltage e at the current moment o Calculate the predicted values ​​of output current, DC voltage and output power in the next sampling period; Afterwards, the output current prediction value is subtracted from the current reference value, the DC voltage prediction value is subtracted from the DC voltage reference value, and the output power prediction value is subtracted from the output power reference value, so as to obtain the error values ​​of the three control targets; The error values ​​of the three control objectives will be multiplied by the weight factor and then brought into the value function of the model predictive control module. The higher the priority of the control objective, the larger the weight factor. The value function in the model predictive control module is expressed as J = f(i,U cap ,P), where f(i,U cap ,P) represents the mathematical expression of the error values ​​of the three control targets of output current, DC voltage and output power multiplied by the weight factor and added together, J is the calculated value of the value function, and the minimum value of the value function J min The corresponding switch state will be used as an insulated gate transistor trigger signal to trigger the insulated gate transistor in the main circuit.

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