Island detection method for single-phase photovoltaic grid-connected inverter

Abstract

The invention relates to an island detection method for a single-phase photovoltaic grid-connected inverter. The detection method specifically comprises the following steps: S1, establishing a photovoltaic cell mathematical model for modeling; various output parameters of the photovoltaic cell in the new environment are calculated according to the temperature and the illumination radiation intensity of the photovoltaic cell; s2, establishing a maximum power point tracking control model, so that the controller enables the photovoltaic cell to work at the maximum power point; and S3, in a normal working state of the power grid, periodically and uninterruptedly performing frequency disturbance on the output voltage of the inverter in positive and negative directions so as to eliminate the balance effect of the load property on a single frequency disturbance direction, and judging whether an island occurs or not by judging whether the frequency of the output voltage of the inverter exceeds an allowable frequency specified by a grid-connected standard or not. According to the detection method, the detection failure caused by the load property of the inverter can be effectively avoided, and the detection effect of the islanding effect is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of inverters, and more particularly to an island detection method for a single-phase photovoltaic grid-connected inverter. Background Art

[0002] When the photovoltaic grid-connected power generation system is connected to the public power grid, if the public power grid stops supplying power to the local area due to fault maintenance or misoperation, the grid-connected system cannot detect the power outage of the public power grid and continues to operate in parallel, which will cause the local load to be in a state of receiving power. At this time, the photovoltaic grid-connected power generation system and the load form an isolated system, and this phenomenon is called the island effect (island phenomenon).

[0003] Since the photovoltaic power generation system works in parallel with the power grid, the power grid may stop working due to failures, equipment maintenance or operational errors. In other words, the islanding effect is a common problem in photovoltaic grid-connected power generation systems. When the islanding phenomenon occurs, the load properties of the inverter have a certain impact on the frequency of the inverter output voltage. At present, the failure of islanding detection is often caused by the problem of the inverter load properties.

[0004] Patent publication number CN201911015412.1 discloses an island detection method, including the following steps: collecting the voltage signal of the grid-connected point and obtaining its angle orientation error value; proportionally differentially adjusting the angle orientation error value to obtain the disturbance amount of the reactive power instruction of the grid-connected inverter; superimposing the disturbance amount of the reactive power instruction on the reactive current instruction of the grid-connected inverter to control the reactive power output of the grid-connected inverter; real-time detection of various parameters of the grid-connected point voltage, and comparing them with a preset threshold range. If any of the parameters exceeds the threshold range, it is determined that an island has occurred.

[0005] The technical solution disclosed in the prior art detects various parameters of the grid connection point voltage in real time and compares them with the preset threshold range, thereby eliminating the blind spot of island detection and accurately performing inverter island detection. However, it cannot solve the problem of detection failure caused by the nature of the inverter load. Summary of the invention

[0006] The technical problem solved by the present invention is to overcome the problems of the prior art and provide an island detection method for a single-phase photovoltaic grid-connected inverter that can effectively avoid detection failure caused by the nature of the inverter load.

[0007] The purpose of the present invention is achieved through the following technical solutions:

[0008] Disclosed is an island detection method for a single-phase photovoltaic grid-connected inverter, the detection method specifically comprising the following steps:

[0009] S1: Establish a mathematical model of photovoltaic cells; calculate the output parameters of photovoltaic cells under the new environment based on the temperature and light radiation intensity of the photovoltaic cells;

[0010] S2: Establish a maximum power point tracking control model so that the controller makes the photovoltaic cell work at the maximum power point;

[0011] S3: When the power grid is in normal working condition, the inverter output voltage is periodically and continuously disturbed in both positive and negative directions to eliminate the balancing effect of the load properties on the single frequency disturbance direction. By judging whether the inverter output voltage frequency exceeds the allowable frequency specified by the grid-connected standard, it is determined whether an islanding occurs.

[0012] Preferably, the step S1 comprises the following steps:

[0013] S11: Calculate the battery temperature difference; the battery temperature difference ΔT is calculated as follows:

[0014] ΔT=TT ref

[0015] Where T is the current temperature, T ref The battery operating temperature under standard conditions;

[0016] S12: Calculate the light intensity difference. The calculation formula of the light intensity difference ΔS is as follows:

[0017]

[0018] Where S is the current light intensity, S ref is the light intensity value under standard conditions;

[0019] S13: Calculate the open circuit voltage of the photovoltaic cell; the calculation formula of the open circuit voltage is as follows:

[0020] U′ oc =U oc (1-λΔT)ln(e+βΔS)

[0021] Where U oc is the open circuit voltage of the photovoltaic cell under standard test conditions, β and λ are constants, and their typical values ​​are β = 0.5, λ = 0.00288 / ℃;

[0022] S14: Calculate the short-circuit current of the photovoltaic cell; the calculation formula of the short-circuit current is as follows:

[0023]

[0024] In the formula, I sc is the short-circuit current of the photovoltaic cell under standard test conditions, α is a constant, and its typical value is 0.0025 / ℃;

[0025] S15: Calculate the maximum power point current of the photovoltaic cell; the current calculation formula when working at the maximum power point is as follows:

[0026]

[0027] In the formula, I m It is the maximum power point current of the photovoltaic cell under standard test conditions;

[0028] S16: Calculate the maximum power point voltage of the photovoltaic cell; the voltage calculation formula when working at the maximum power point is as follows:

[0029] U′ m =U m (1-λΔT)ln(e+βΔS)

[0030] Where U m It is the maximum power point voltage of the photovoltaic cell under standard test conditions.

[0031] Preferably, step S2 comprises the following steps:

[0032] S21: Detect the output current and output voltage I of the current photovoltaic power generation system at the current working point (k) and U (k) ;

[0033] S22: The instantaneous output power of the photovoltaic cell P = UI, and the voltage U at both ends is derived as follows:

[0034]

[0035] S23: At the maximum power point When the system operates at the maximum power point

[0036] when When , the current operating point is on the left side of the maximum power point, and the operating voltage needs to be increased to move to the right;

[0037] when When , the current working point is at the maximum working point, and the working voltage needs to be kept unchanged;

[0038] when , the current operating point is on the right side of the maximum power point, and the operating voltage also needs to be reduced to move to the left.

[0039] Preferably, step S3 comprises the following steps:

[0040] S31: Add disturbance signals cf1 and cf2;

[0041] S32: Determine the magnitude relationship of the absolute values ​​of the errors |Δf1| and |Δf2| between the grid-connected current frequency output by the inverter and the voltage frequency at the common coupling point PCC. If |Δf1|>|Δf2|, select cf1 as the next feedback enhancement signal, continue to apply disturbance to the frequency of the inverter output voltage, and enter S33;

[0042] S33: A new disturbance signal can be obtained by calculating according to the disturbance signal formula of the positive feedback active frequency shift method;

[0043] S34: Determine whether the current inverter output voltage frequency f1 exceeds the allowable frequency f specified by the grid connection standard island If it exceeds the limit, it is determined that an island has occurred and the connection between the inverter and the grid is cut off;

[0044] S35: Similar to increasing the disturbance cf1, a new periodic disturbance signal can be obtained by calculating according to the disturbance signal formula of the active frequency shift method of positive feedback;

[0045] S36: Determine whether the current inverter output voltage frequency f2 exceeds the allowable frequency f specified by the grid connection standard island If it exceeds, it is determined that an island has occurred and the connection between the inverter and the grid is immediately cut off.

[0046] Preferably, in step S31, the disturbance signals cf1 and cf2 are set to 5% and -5% respectively.

[0047] Preferably, in step S32, if |Δf1|<|Δf2|, the cf2 disturbance is increased and the process proceeds to S35.

[0048] Preferably, the disturbance calculation formula of the active frequency shift method with positive feedback in step S33 is:

[0049] cf k =cf k-1 +F(ΔW K )

[0050] Where cf k-1 is the disturbance signal of the inverter in the previous cycle, cf k is the disturbance signal of this period, F(ΔW K ) is a feedback signal applied according to the change of the inverter output voltage frequency;

[0051] Furthermore, a new disturbance calculation formula can be obtained:

[0052] cf 1(k+1) =cf 1(k) +Δcf1

[0053] In the formula, cf 1(k) , cf 1(k+1)They represent the voltage frequency of the PCC point in the previous cycle and the current cycle of cf1 respectively, and Δcf1 is the feedback signal.

[0054] Preferably, in step S34, if f1 does not exceed the permissible frequency f specified in the grid connection standard island , then return to S33 to continue enhancing the cf1 disturbance until an island fault is found.

[0055] Preferably, in step S35, a new periodic disturbance signal calculation formula is:

[0056] cf 2(k+1) =cf 2(k) +Δcf2

[0057] In the formula, cf 2(k) , cf 2(k+1) They represent the voltage frequency of the PCC point in the previous cycle and the current cycle of cf2 respectively, and Δcf2 is the feedback signal.

[0058] Preferably, in step S36, if the current inverter output voltage frequency f2 does not exceed the permissible frequency f specified by the grid connection standard, island , then return to S35 to continue to enhance the cf2 disturbance until an island fault is found.

[0059] It has the following beneficial effects:

[0060] 1) After adopting the periodic disturbance AFDPF detection method, when the islanding effect occurs, for any load, after the grid is disconnected, the frequency of the inverter output voltage will always change significantly, thus eliminating the dependence of the fixed direction CF active frequency drift method on the load. It effectively avoids the detection failure caused by the nature of the inverter load and improves the detection effect of the islanding effect.

[0061] 2) The system applies positive feedback to the disturbance signal cf with a large frequency error to further expand the frequency error, thereby reducing the detection blind area and further improving the detection effect of the island effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 A control flow chart of the conductivity increment method of an island detection method for a single-phase photovoltaic grid-connected inverter of the present invention

[0063] Figure 2 A principle block diagram of an islanding detection method for a single-phase photovoltaic grid-connected inverter of the present invention;

[0064] Figure 3 A control program flow chart of an island detection method for a single-phase photovoltaic grid-connected inverter of the present invention;

[0065] Figure 4It is a flow chart of an island detection method for a single-phase photovoltaic grid-connected inverter of the present invention;

[0066] Figure 5 A diagram of grid voltage and output current of an island detection method for a single-phase photovoltaic grid-connected inverter according to the present invention. DETAILED DESCRIPTION

[0067] In order to clearly illustrate the technical features of the present solution, the present invention is described in detail below through specific implementation methods and in conjunction with the accompanying drawings.

[0068] In the following description, many specific details are set forth to facilitate a full understanding of the present application. However, the present application may also be implemented in other ways different from those described herein. Therefore, the scope of protection of the present application is not limited to the specific embodiments disclosed below.

[0069] In addition, in the description of the present application, it should be understood that the terms "center", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "axial", "radial", "circumferential" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first" and "second" are used only for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, the features defined as "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present application, the meaning of "multiple" is two or more, unless otherwise clearly and specifically defined.

[0070] In this application, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, an electrical connection, or a communication; it can be a direct connection, or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0071] In the present application, unless otherwise clearly specified and limited, a first feature "above" or "below" a second feature may be that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in an appropriate manner in any one or more embodiments or examples.

[0072] Example 1

[0073] Disclosed is an island detection method for a single-phase photovoltaic grid-connected inverter, wherein the control strategy specifically comprises the following steps:

[0074] S1: Establish a mathematical model of photovoltaic cells; calculate the output parameters of photovoltaic cells under the new environment based on the temperature and light radiation intensity of the photovoltaic cells;

[0075] S2: Establish a maximum power point tracking control model so that the controller makes the photovoltaic cell work at the maximum power point;

[0076] S3: When the power grid is in normal working condition, the inverter output voltage is periodically and continuously disturbed in both positive and negative directions to eliminate the balancing effect of the load properties on the single frequency disturbance direction. By judging whether the inverter output voltage frequency exceeds the allowable frequency specified by the grid-connected standard, it is determined whether an islanding occurs.

[0077] In this embodiment, after adopting the periodic disturbance AFDPF detection method, when the islanding effect occurs, for any load, after the grid is disconnected, the frequency of the inverter output voltage will always change significantly, thereby eliminating the dependence of the fixed direction cf active frequency drift method on the load. At the same time, the system applies positive feedback to the disturbance signal cf with a large frequency error to further expand the frequency error, thereby reducing the detection blind area and improving the detection effect of the islanding effect.

[0078] Example 2

[0079] Disclosed is an island detection method for a single-phase photovoltaic grid-connected inverter, wherein the control strategy specifically comprises the following steps:

[0080] S1: Establish a mathematical model of photovoltaic cells; calculate the output parameters of photovoltaic cells under the new environment based on the temperature and light radiation intensity of the photovoltaic cells;

[0081] S2: Establish a maximum power point tracking control model so that the controller makes the photovoltaic cell work at the maximum power point;

[0082] S3: When the power grid is in normal working condition, the inverter output voltage is periodically and continuously disturbed in both positive and negative directions to eliminate the balancing effect of the load properties on the single frequency disturbance direction. By judging whether the inverter output voltage frequency exceeds the allowable frequency specified by the grid-connected standard, it is determined whether an islanding occurs.

[0083] The difference between this embodiment and embodiment 1 is that:

[0084] Under the premise of ensuring the accuracy of the engineering mathematical model, the output characteristic equation of the current is appropriately simplified and deformed when designing photovoltaic cells. When the external light intensity and temperature change, the output characteristics of the photovoltaic cell will inevitably be affected. This requires modifying the model of the photovoltaic cell to describe its characteristic curve under different environments. Based on the reference photovoltaic cell temperature and light radiation intensity, the various output parameters of the photovoltaic cell under the new environment are calculated.

[0085] Step S1 specifically includes the following steps:

[0086] S11: Temperature difference calculation

[0087] The calculation formula of temperature difference ΔT is as follows:

[0088] ΔT=TT ref

[0089] Where T is the current temperature, T ref It is the battery operating temperature under standard conditions.

[0090] S12: Light intensity difference calculation

[0091] The calculation formula of light intensity difference ΔS is as follows:

[0092]

[0093] Where S is the current light intensity, S ref It is the light intensity value under standard conditions.

[0094] S13: Open circuit voltage calculation

[0095] The open circuit voltage is calculated as follows:

[0096] U′ oc =U oc (1-λΔT)ln(e+βΔS)

[0097] Where U ocis the open circuit voltage of the photovoltaic cell under standard test conditions. β and λ are constants, and their typical values ​​are β = 0.5 and λ = 0.00288 / ℃.

[0098] S14: Short-circuit current calculation

[0099] The short-circuit current is calculated as follows:

[0100]

[0101] In the formula, I sc is the short-circuit current of the photovoltaic cell under standard test conditions, α is a constant, and its typical value is 0.0025 / ℃.

[0102] S155: Maximum power point current calculation

[0103] When working at the maximum power point, the current calculation formula is as follows:

[0104]

[0105] In the formula, I m It is the maximum power point current of the photovoltaic cell under standard test conditions.

[0106] S16: Maximum power point voltage calculation

[0107] When working at the maximum power point, the voltage is calculated as follows:

[0108] U′ m =U m (1-λΔT)ln(e+βΔS)

[0109] Where U m It is the maximum power point voltage of the photovoltaic cell under standard test conditions.

[0110] Example 3

[0111] Disclosed is an island detection method for a single-phase photovoltaic grid-connected inverter, wherein the control strategy specifically comprises the following steps:

[0112] S1: Establish a mathematical model of photovoltaic cells; calculate the output parameters of photovoltaic cells under the new environment based on the temperature and light radiation intensity of the photovoltaic cells;

[0113] S2: Establish a maximum power point tracking control model so that the controller makes the photovoltaic cell work at the maximum power point;

[0114] S3: When the power grid is in normal working condition, the inverter output voltage is periodically and continuously disturbed in both positive and negative directions to eliminate the balancing effect of the load properties on the single frequency disturbance direction. By judging whether the inverter output voltage frequency exceeds the allowable frequency specified by the grid-connected standard, it is determined whether an islanding occurs.

[0115] The difference between this embodiment and the above embodiment is that the maximum power tracking control enables the photovoltaic power generation system to have the ability to self-optimize, by monitoring the current output voltage and current value of the photovoltaic cell, and through analysis of a certain control algorithm, it is determined whether the current working state is at the maximum power point. Then, the working voltage of the photovoltaic cell is adjusted by changing the equivalent load impedance. Regardless of changes in the external environment or load, the controller can also make the photovoltaic cell work at the maximum power point.

[0116] This embodiment adopts the conductance increment method to perform tracking control.

[0117] S21: Detect the output current and output voltage I of the current photovoltaic power generation system at the current working point (k) and U (k) .

[0118] S22: The instantaneous output power of the photovoltaic cell P = UI, and the voltage U at both ends is derived as follows:

[0119]

[0120] S23: At the maximum power point When the system operates at the maximum power point From this, we can derive the following basis for determining the maximum power point:

[0121] when When , the current operating point is on the left side of the maximum power point, and the operating voltage needs to be increased to move to the right;

[0122] when When , the current operating point is at the maximum operating point, and the operating voltage needs to be kept unchanged.

[0123] when , the current operating point is on the right side of the maximum power point, and the operating voltage also needs to be reduced to move to the left.

[0124] The so-called periodic disturbance AFDPF islanding effect detection method refers to periodically and continuously performing frequency disturbances in both positive and negative directions on the inverter output voltage under normal grid operation to eliminate the balancing effect of load properties on a single frequency disturbance direction.

[0125] S31: Add disturbance signals cf1 and cf2. Research shows that in the active frequency disturbance method, if cf is within ±5%, the harmonics of the inverter output power can meet the relevant requirements of the grid connection standard and basically meet the requirements of island detection. Therefore, in the present invention, the two disturbance signals cf1 and cf2 are set to 5% and -5% respectively.

[0126] S32: Determine the magnitude relationship of the absolute values ​​of the errors |Δf1| and |Δf2| between the grid-connected current frequency output by the inverter and the voltage frequency at the common coupling point PCC. If |Δf1|>|Δf2|, select cf1 as the next feedback enhancement signal, continue to apply disturbance to the frequency of the inverter output voltage, and enter S33; otherwise, increase the cf2 disturbance and enter S35.

[0127] S33: A new disturbance signal can be obtained by calculating according to the disturbance signal formula of the positive feedback active frequency shift method, wherein the disturbance calculation formula of the positive feedback active frequency shift method is:

[0128] cf k =cf k-1 +F(ΔW K )

[0129] Where cf k-1 is the disturbance signal of the inverter in the previous cycle, cf k is the disturbance signal of this period, F(ΔW K ) is the feedback signal applied according to the change of the inverter output voltage frequency.

[0130] Furthermore, a new disturbance calculation formula can be obtained:

[0131] cf 1(k+1) =cf 1(k) +Δcf1

[0132] In the formula, cf 1(k) , cf 1(k+1) They represent the voltage frequency of the PCC point in the previous cycle and the current cycle of cf1 respectively, and Δcf1 is the feedback signal.

[0133] S34: Determine whether the current inverter output voltage frequency f1 exceeds the allowable frequency f specified by the grid connection standard island If it exceeds, it is determined that an island fault occurs and the connection between the inverter and the grid is cut off; otherwise, the process returns to S33 to continue to enhance the cf1 disturbance until an island fault is found.

[0134] S35: Similar to increasing the disturbance cf1, a new periodic disturbance signal calculation formula can be obtained by calculating the disturbance signal formula of the active frequency shift method of positive feedback;

[0135] cf 2(k+1) =cf2(k) +Δcf2

[0136] In the formula, cf 2(k) , cf 2(k+1) They represent the voltage frequency of the PCC point in the previous cycle and the current cycle of cf2 respectively, and Δcf2 is the feedback signal.

[0137] S36: Determine whether the current inverter output voltage frequency f2 exceeds the allowable frequency f specified by the grid connection standard island If it exceeds, it is determined that an island fault occurs, and the connection between the inverter and the grid is immediately cut off; otherwise, the process returns to S35 to continue to enhance the cf2 disturbance until an island fault is found.

[0138] To verify that under the control of this method, the inverter output frequency changes faster, thereby exceeding the grid-connected standard in a shorter period of time, triggering the protection circuit and cutting off the connection between the grid and the inverter.

[0139] The system was simulated and analyzed based on the Matlab / Simulink simulation platform.

[0140] The simulation parameters are as follows: the rated output power of the single-phase photovoltaic grid-connected system is 1.5kw, the load is an RLC parallel load that is balanced with the output power of the single-phase photovoltaic grid-connected system and has a quality factor of 2.5, the grid voltage is 220V, the voltage upper threshold of the voltage protection threshold is 450V, and the voltage lower threshold is 150V. The rated frequency of the grid is 50Hz, the frequency upper threshold of the frequency domain threshold of the frequency protection is 50.5Hz, and the frequency lower threshold is 49.5Hz. The simulated fault condition is: at 0.4s, the grid is powered off, and the single-phase photovoltaic grid-connected system and the load form a self-sufficient island. The island detection method must detect the island within the subsequent 0.3s, that is, the high-speed automatic reclosing time, so as to achieve effective island protection.

[0141] Figure 2 The grid-connected voltage and output current waveforms when an island occurs in the simulation using the method of the present invention are given. Figure 2 It can be seen that within 0.1 seconds, the inverter has stopped current and voltage output, which fully meets the design requirements.

[0142] Beneficial effects of the present invention: The islanding detection method of a single-phase photovoltaic grid-connected inverter of the present invention adopts a periodic disturbance AFDPF detection method. When the islanding effect occurs, for any load, after the grid is disconnected, the frequency of the inverter output voltage will always change significantly, thereby eliminating the dependence of the fixed direction cf active frequency drift method on the load. At the same time, the system applies positive feedback to the disturbance signal cf with a large frequency error to further expand the frequency error, thereby reducing the detection blind area and improving the detection effect of the islanding effect.

[0143] Obviously, the above embodiments are merely examples for clearly illustrating the technical solutions of the present invention, and are not intended to limit the implementation methods of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all implementation methods here. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for detecting an islanding of a single-phase photovoltaic grid-connected inverter, characterized in that: The detection method specifically comprises the following steps: S1: Establish a mathematical model of photovoltaic cells; calculate the output parameters of photovoltaic cells under the new environment based on the temperature and light radiation intensity of the photovoltaic cells; S2: Establish a maximum power point tracking control model so that the controller makes the photovoltaic cell work at the maximum power point; S3: When the power grid is in normal working condition, the inverter output voltage is periodically and continuously disturbed in both positive and negative directions to eliminate the balancing effect of the load properties on the single frequency disturbance direction. By judging whether the inverter output voltage frequency exceeds the allowable frequency specified by the grid-connected standard, it is determined whether an islanding occurs.

2. The island detection method for a single-phase photovoltaic grid-connected inverter according to claim 1, characterized in that: The step S1 comprises the following steps: S11: Calculate the battery temperature difference; the battery temperature difference ΔT is calculated as follows: ΔT=T-T ref Where T is the current temperature, T ref The battery operating temperature under standard conditions; S12: Calculate the light intensity difference. The calculation formula of the light intensity difference ΔS is as follows: Where S is the current light intensity, S ref is the light intensity value under standard conditions; S13: Calculate the open circuit voltage of the photovoltaic cell; the calculation formula of the open circuit voltage is as follows: U′ oc =U oc (1-λΔT)ln(e+βΔS) Where U oc is the open circuit voltage of the photovoltaic cell under standard test conditions, β and λ are constants, and their typical values ​​are β = 0.5, λ = 0.00288 / ℃; S14: Calculate the short-circuit current of the photovoltaic cell; the calculation formula of the short-circuit current is as follows: In the formula, I sc is the short-circuit current of the photovoltaic cell under standard test conditions, α is a constant, and its typical value is 0.0025 / ℃; S15: Calculate the maximum power point current of the photovoltaic cell; the current calculation formula when working at the maximum power point is as follows: In the formula, I m It is the maximum power point current of the photovoltaic cell under standard test conditions; S16: Calculate the maximum power point voltage of the photovoltaic cell; the voltage calculation formula when working at the maximum power point is as follows: U′ m =U m (1-λΔT)ln(e+βΔS) Where U m It is the maximum power point voltage of the photovoltaic cell under standard test conditions.

3. The island detection method for a single-phase photovoltaic grid-connected inverter according to claim 1, characterized in that: The step S2 comprises the following steps: S21: Detect the output current and output voltage I of the current photovoltaic power generation system at the current working point (k) and U (k) ; S22: The instantaneous output power of the photovoltaic cell P = UI. The voltage U at both ends is derived as follows: S23: At the maximum power point When the system operates at the maximum power point when When , the current operating point is on the left side of the maximum power point, and the operating voltage needs to be increased to move to the right; when When , the current working point is at the maximum working point, and the working voltage needs to be kept unchanged; when , the current operating point is on the right side of the maximum power point, and the operating voltage also needs to be reduced to move to the left.

4. The island detection method for a single-phase photovoltaic grid-connected inverter according to claim 1, characterized in that: The step S3 comprises the following steps: S31: Add disturbance signals cf1 and cf2; S32: Determine the magnitude relationship of the absolute values ​​of the errors |Δf1| and |Δf2| between the grid-connected current frequency output by the inverter and the voltage frequency at the common coupling point PCC. If |Δf1|>|Δf2|, select cf1 as the next feedback enhancement signal, continue to apply disturbance to the frequency of the inverter output voltage, and enter S33; S33: A new disturbance signal can be obtained by calculating according to the disturbance signal formula of the positive feedback active frequency shift method; S34: Determine whether the current inverter output voltage frequency f1 exceeds the allowable frequency f specified by the grid connection standard island If it exceeds, it is determined that an island occurs and the connection between the inverter and the grid is cut off; S35: Similar to increasing the disturbance cf1, a new periodic disturbance signal can be obtained by calculating according to the disturbance signal formula of the active frequency shift method of positive feedback; S36: Determine whether the current inverter output voltage frequency f2 exceeds the allowable frequency f specified by the grid connection standard island If it exceeds, it is determined that an island has occurred and the connection between the inverter and the grid is immediately cut off.

5. The island detection method for a single-phase photovoltaic grid-connected inverter according to claim 4, characterized in that: In step S31, the disturbance signals cf1 and cf2 are set to 5% and -5% respectively.

6. The island detection method for a single-phase photovoltaic grid-connected inverter according to claim 4, characterized in that: If |Δf1|<|Δf2| in the step S32, the cf2 disturbance is increased and the process goes to S35.

7. The island detection method for a single-phase photovoltaic grid-connected inverter according to claim 4, characterized in that: The disturbance calculation formula of the active frequency shift method with positive feedback in step S33 is: cf k =cf k-1 +F(ΔW K ) Where cf k-1 is the disturbance signal of the inverter in the previous cycle, cf k is the disturbance signal of this period, F(ΔW K ) is a feedback signal applied according to the change of the inverter output voltage frequency; Furthermore, a new disturbance calculation formula can be obtained: cf 1(k+1) =cf 1(k) +Δcf1 In the formula, cf 1(t) , cf 1(k+1) They represent the voltage frequency of the PCC point in the previous cycle and the current cycle of cf1 respectively, and Δcf1 is the feedback signal.

8. The island detection method for a single-phase photovoltaic grid-connected inverter according to claim 4, characterized in that: In step S34, if f1 does not exceed the permissible frequency f specified by the grid connection standard island , then return to S33 to continue enhancing the cf1 disturbance until an island fault is found.

9. The island detection method for a single-phase photovoltaic grid-connected inverter according to claim 4, characterized in that: In step S35, a new periodic disturbance signal calculation formula is provided; cf 2(k+1) =cf 2(k) +Δcf2 In the formula, cf 2(k) , cf 2(k+1) They represent the voltage frequency of the PCC point in the previous cycle and the current cycle of cf2 respectively, and Δcf2 is the feedback signal.

10. The island detection method for a single-phase photovoltaic grid-connected inverter according to claim 4, characterized in that: In step S36, if the current inverter output voltage frequency f2 does not exceed the permissible frequency f specified by the grid connection standard island , then return to S35 to continue to enhance the cf2 disturbance until an island fault is found.

Images