Photovoltaic power station DC boost converter bus capacitor aging fault early warning method

Abstract

The invention discloses a photovoltaic power station DC boost converter bus capacitor aging fault early warning method. The method comprises the following steps: acquiring a voltage ub on a common DC bus and a current iO on a DC cable; judging zero crossing point time tn of the current iO; calculating N (N-1) / 2 groups of results; judging an area zero crossing point t'q formed by the current iO and the time axis; calculating three variables when t is equal to tm; calculating (Q-1) groups of results; calculating to obtain points in the three-dimensional space; according to a least square method fitting plane y = f (x, z, # imgabs0 #), determining omega 1, omega 2 and omega 3; obtaining Rx and Cx; calculating the ratio of D to D0; when the ratio of D to D0 is greater than a set threshold value alpha, determining that the bus capacitor has failed; otherwise, determining that the bus capacitor does not fail. The method can effectively avoid the influence of the parameter change of the DC cable, and achieves the timely judgment and early warning of the aging fault of the bus capacitor of the DC boost converter.

Description

Technical Field

[0001] The present invention relates to the field of photovoltaic power station equipment monitoring and operation and maintenance, and in particular to a photovoltaic power station DC boost converter bus capacitor aging fault early warning method. Background Art

[0002] In photovoltaic power plants, DC boost converters play a vital role. By converting the low-voltage DC power generated by photovoltaic modules into higher-voltage DC power, the maximum power point tracking technology is integrated to ensure maximum energy capture and conversion efficiency. The bus capacitor inside can smooth the output voltage, reduce voltage fluctuations and provide instantaneous power compensation. However, bus capacitors often fail, including overheating, voltage breakdown and excessive ripple current. The causes of these failures are usually: during long-term operation, the electrolyte of the bus capacitor dries up and the dielectric degrades, resulting in a decrease in capacitance, which eventually leads to failure. For these failures, regular inspections and maintenance are usually required to ensure the safe operation of the photovoltaic power plant.

[0003] The aging state of the bus capacitor of the DC boost converter can be determined by its equivalent series resistance ( R x ) and the equivalent series capacitance ( C x ) quantified. Usually, R x Double or C x When it decreases by 20%, it can be determined that the bus capacitor has aged. Considering the limitations of the sensors installed on site in actual photovoltaic power plants, namely, the current on the bus capacitor of the DC boost converter is unknown, and only the current output by the DC boost converter can be obtained; the voltage on the DC bus capacitor is not a standard DC value, but a quasi-DC value due to the power fluctuation of the photovoltaic array, which limits the application and implementation of many existing methods.

[0004] In response to the actual situation of the above project, it is necessary to use the electrical quantities measured by the sensors installed on site to realize the equivalent electrical parameters of the bus capacitor of the DC boost converter. R x and C x The invention patent "Online Diagnosis Method of DC Bus Capacitor of Photovoltaic System DCDC Converter" (Application No.: CN202310124370.5) proposes a method to diagnose the equivalent electrical parameters of bus capacitors. C x The estimation method is C xWhen the estimated value drops to 80% of the initial value, a fault protection signal is issued. However, the method (1) proposed in this patent can only estimate C x Cannot be estimated R x ; (2) C x The estimation accuracy is not high and is affected by the changes in DC cable parameters; (3) R x The sensitivity of aging status diagnosis is insufficient, and there is a lack of a sensitive aging indicator to provide timely fault warning. Summary of the invention

[0005] In view of the defects in the prior art, the present invention proposes a method for early warning of aging faults of bus capacitors in a photovoltaic power station DC boost converter. The electrical quantity measured by the sensors installed on site, i.e., the voltage on the public DC bus, is used to measure the voltage on the common DC bus. u b and the current on the DC cable i O , to achieve the equivalent electrical parameters of the DC boost converter bus capacitor R x and C x accurate estimate of R x and C x Based on the changing trends of the two, a highly sensitive aging fault judgment index is proposed D , effectively avoiding the influence of changes in DC cable parameters and realizing timely judgment and early warning of aging faults of bus capacitors in DC boost converters.

[0006] The technical solution adopted by the present invention is: a method for early warning of aging fault of bus capacitor of DC boost converter of photovoltaic power station, the method comprising the following steps: Step 1: Get the voltage on the common DC bus u b and the current on the DC cable i O ; Step 2: Determine the current i O Zero crossing time t n ; Step 3: Calculation N ( N -1) / 2 groups of results, that is ; Step 4: Determine the current i O The area formed with the time axis crosses zerot′ q ; Calculate the three variables when t=tm, namely ; Step 5: Calculation Q -1 set of results, i.e. ; Step 6: Calculation , , Get a point in three-dimensional space and ; Step 7: Fit the plane using the least squares method ,Sure ω 1. ω 2 and ω 3; Step 8: Get R x and C x ; Step 9: Calculate the aging evaluation index D ; Step 10: Calculation D and D 0 ratio; when D and D When the ratio is greater than the set threshold α, it is determined that the bus capacitor has failed; otherwise, it is determined that the bus capacitor has not failed.

[0007] Furthermore, in step 1, the common DC bus voltage u b and DC cable current i O Has the following relationship: ; in, l 0 is the equivalent inductance parameter of the DC cable as a whole.

[0008] Furthermore, the step 2 specifically includes, according to formula (1), recording i O The zero crossing time is t n ,Right now i O ( t n ) = 0, where 1≤ n < n + x ≤ N。

[0009] Furthermore, the step 3 specifically includes:N indivual t n The voltage and current measurement results corresponding to the time are obtained N ( N -1) / 2 groups related u b and i O The calculation result is ; in, .

[0010] Furthermore, the step 4 specifically includes, according to formula (1), recording i O The area enclosed by the time axis is equal to 0 at the time t′ q ,Right now ,in .

[0011] Further, the step 5 specifically includes, according to Q indivual t′ q The voltage and current measurement results corresponding to the time are obtained Q -1 group related u b and i O The calculation result is .

[0012] Further, the step 6 specifically includes: , , , get the point in three-dimensional space and .

[0013] Furthermore, the step 7 specifically includes fitting a three-dimensional plane using the least squares method ; The three-dimensional plane has the following expression ; in, is a three-dimensional plane column vector, ω 1, ω 2 and ω 3 is the three-dimensional plane coefficient, and l 0, C x and R x Has the following relationship ; Therefore, according to the three-dimensional plane expression obtained by fitting, we get ω 1, ω 2 and ω 3; The step 8 is to calculate according to formula (5) R x and C x .

[0014] Further, the step 9 specifically includes defining an aging fault judgment index D , with the following expression ; In conventional photovoltaic power plants, l The magnitude of 0 is 10 -5 ~10 -3 , l The square of 0 is 10 -10 ~10 -6 Therefore l The square of 0 is much smaller than the value 1, then ; According to formula (7), D It is considered to be irrelevant to the DC cable parameters, so the aging status diagnosis results will not be affected by changes in DC cable parameters.

[0015] Further, the step 9 specifically includes defining the aging evaluation index of the output side bus capacitor when it is healthy as D 0; when D and D 0 ratio is greater than the set threshold α, it is determined that the bus capacitor is about to fail, that is, .

[0016] Beneficial technical effects: The present invention proposes a method for early warning of aging fault of bus capacitors of DC boost converters in photovoltaic power stations, which uses the electrical quantity measured by sensors installed on site, namely, the voltage on the public DC bus. u b and the current on the DC cable i O , to achieve the equivalent electrical parameters of the DC boost converter bus capacitor R x and C x accurate estimate of R x and C x Based on the changing trends of the two, a highly sensitive aging fault judgment index is proposedD , effectively avoiding the influence of changes in DC cable parameters and realizing timely judgment and early warning of aging faults of bus capacitors in DC boost converters. BRIEF DESCRIPTION OF THE DRAWINGS The present invention has the following drawings: Figure 1 Typical topology diagram of photovoltaic power station; Figure 2 Equivalent circuit diagram of boost DC converter and DC cable; Figure 3 Equivalent circuit diagram of the bus capacitor on the output side; Figure 4 Schematic diagram; Figure 5 Schematic diagram; Figure 6 and Distribution diagram and fitted three-dimensional plane schematic diagram; Figure 7 u b and i O Schematic diagram; Figure 8 Five groups of bus capacitors in aging state and Distribution diagram and fitted three-dimensional plane schematic diagram. DETAILED DESCRIPTION

[0018] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the following is a brief description of the present invention in conjunction with the attached Figure 1-8 The present application is further described in detail with specific implementation methods.

[0019] The present invention is a method for early warning of aging fault of bus capacitor of DC boost converter of photovoltaic power station, which comprises the following principles and steps: Figure 1 The typical topology diagram of a photovoltaic power station includes photovoltaic panels, DC boost converters, DC cables, public DC bus, inverters and three-phase power grids. Figure 2 , the research object of this patent is the output side bus capacitor in the DC boost converter. Figure 3 , the bus capacitor on the output side can be equivalent to an equivalent resistor R x and equivalent capacitance C x , during the aging process of bus capacitors, R x There is an increasing trend, C x Showing a decreasing trend.

[0020] During the operation of the photovoltaic power station, the DC boost converter will automatically exit when the light intensity decreases every day. Figure 4 , t = t 0), the common DC bus voltage measured by the sensor installed on site u b and DC cable current i O Has the following relationship: ; in, l 0 is the equivalent inductance parameter of the DC cable as a whole, according to Figure 4 ,remember i O The zero crossing time is t n ,Right now i O ( t n ) = 0, where 1≤ n < n + x ≤ N .according to N indivual t n The voltage and current measurement results corresponding to the time can be obtained N ( N -1) / 2 groups related u b and i O The calculation result is ; in, .

[0021] according to Figure 5 ,remember i O The area enclosed by the time axis is equal to 0 at the time t′ q ,Right now ,in .according to Q indivual t′ q The voltage and current measurement results corresponding to the time can be obtained Q -1 group related u b and i O The calculation result is ; make , , , , we can get several points in three-dimensional space and ,like Figure 6 As shown. Among them, represent , represent Based on these points, a three-dimensional plane is fitted using the least squares method. The three-dimensional plane has the following expression ; in, is a three-dimensional plane column vector, ω 1, ω 2 and ω 3 is the three-dimensional plane coefficient, and l 0, C x and R x Has the following relationship ; Therefore, according to the three-dimensional plane expression obtained by fitting, we can get ω 1, ω 2 and ω 3, and then calculate according to formula (5) R x and C x .

[0022] Define an aging fault judgment indicator D , with the following expression ; In conventional photovoltaic power plants, l The magnitude of 0 is 10 -5 ~10 -3 , so ,but ; According to formula (7), D It is considered to be irrelevant to the DC cable parameters, so the aging status diagnosis results will not be affected by changes in DC cable parameters.

[0023] The aging evaluation index of the output bus capacitor when it is healthy is defined as D 0. When D and D 0 ratio is greater than the set threshold α, it is determined that the bus capacitor is about to fail, that is, ; Application examples: like Figure 7 and Figure 8 As shown in the figure, a photovoltaic power station model is constructed in the simulation software MATLAB / Simulink. Parameters are set in the simulation model, DC cable parameters l 0=2*10 -4 H, output bus capacitor in the gradual aging process R x Increase, C x Set the aging status of five groups of bus capacitors, and the simulation parameters in the model are: GI: R x =32 mΩ and C x =2.5 mF; G-II: R x =40 mΩ and C x =2.375 mF; G-III: R x =48 mΩ and C x =2.25 mF; G-IV: R x =56 mΩ and C x =2.125 mF; GV: R x = 64 mΩ and C x =2.0 mF( R Compared to health value, it is doubled; C x 20% reduction) In the simulation model, the DC boost converter is exited at 0.3s. When the bus capacitor is healthy, the voltage on the common DC bus is u b and the current on the DC cable i O like Figure 1 shown.

[0024] Pick h =1000, α=1.2.

[0025] According to the implementation steps, we can get ω 1, ω 2 and ω3 and R x , C x The calculation results are as follows: According to the above results, R x and C x The calculation results match the parameters set in the model. D / D 0 is less than 1.2, GV is medium D / D 0 is greater than 1.2, so it is determined that the bus capacitor of this group has developed an aging fault.

[0026] The above embodiments describe the technical solutions of the present invention in detail. Obviously, the present invention is not limited to the described embodiments. Based on the embodiments of the present invention, people familiar with the technical field can also make various changes accordingly, but any changes that are equivalent to or similar to the present invention belong to the scope of protection of the present invention. The contents not described in detail in this specification belong to the prior art known to professional and technical personnel in the field.

Claims

1. A photovoltaic power station DC boost converter bus capacitor aging fault early warning method, characterized in that: The method comprises the following steps: Step 1: Get the voltage on the common DC bus u b and the current on the DC cable i O ; Step 2: Determine the current i O Zero crossing time t n ; Step 3: Calculation N ( N -1) / 2 groups of results, that is ; Step 4: Determine the current i O The area formed with the time axis crosses zero t′ q ; Calculate the three variables when t=tm, namely ; Step 5: Calculation Q -1 set of results, i.e. ; Step 6: Calculation , , get the point in three-dimensional space and ; Step 7: Fit the plane using the least squares method y = f ( x , z , ),Sure ω 1. ω 2 and ω 3; Step 8: Get R x and C x ; Step 9: Calculate the aging assessment index D ; Step 10: Calculation D and D 0 ratio; when D and D When the ratio is greater than the set threshold α, it is determined that the bus capacitor has failed; otherwise, it is determined that the bus capacitor has not failed.

2. A photovoltaic power station DC boost converter bus capacitor aging fault early warning method as claimed in claim 1, characterized in that: In step 1, the common DC bus voltage u b and DC cable current i O Has the following relationship: ； in, l 0 is the equivalent inductance parameter of the DC cable as a whole.

3. A photovoltaic power station DC boost converter bus capacitor aging fault early warning method as claimed in claim 2, characterized in that: The step 2 specifically includes, according to formula (1), recording i O The zero crossing time is t n ,Right now i O ( t n ) =0, where 1≤ n < n + x ≤ N。 4. A photovoltaic power station DC boost converter bus capacitor aging fault early warning method as claimed in claim 3, characterized in that: The step 3 specifically includes: N indivual t n The voltage and current measurement results corresponding to the time are obtained N ( N -1) / 2 groups related u b and i O The calculation result is ； in, .

5. A photovoltaic power station DC boost converter bus capacitor aging fault early warning method as claimed in claim 4, characterized in that: The step 4 specifically includes, according to formula (1), recording i O The area enclosed by the time axis is equal to 0 at the time t′ q ,Right now ,in .

6. A photovoltaic power station DC boost converter bus capacitor aging fault early warning method as claimed in claim 5, characterized in that: The step 5 specifically includes: Q indivual t′ q The voltage and current measurement results corresponding to the time are obtained Q -1 group related u b and i O The calculation result is 。 7. A photovoltaic power station DC boost converter bus capacitor aging fault early warning method as claimed in claim 6, characterized in that: The step 6 specifically includes: , , get the point in three-dimensional space and .

8. A photovoltaic power station DC boost converter bus capacitor aging fault early warning method as claimed in claim 7, characterized in that: The step 7 specifically includes fitting a three-dimensional plane using the least squares method. y = f ( x , z , ); the three-dimensional plane has the following expression ; in, is a three-dimensional plane column vector, ω 1, ω 2 and ω 3 is the three-dimensional plane coefficient, and l 0, C x and R x Has the following relationship ; Therefore, according to the three-dimensional plane expression obtained by fitting, we get ω 1, ω 2 and ω 3; The step 8 is to calculate according to formula (5) R x and C x .

9. A photovoltaic power station DC boost converter bus capacitor aging fault early warning method as claimed in claim 8, characterized in that: The step 9 specifically includes defining an aging fault judgment index. D , with the following expression ; In conventional photovoltaic power plants, l The magnitude of 0 is 10 -5 ~10 -3 , l The square of 0 is 10 -10 ~10 -6 ; Therefore l The square of 0 is much smaller than the value 1, then ; According to formula (7), D It is considered to be irrelevant to the DC cable parameters, so the aging status diagnosis results will not be affected by changes in DC cable parameters.

10. A photovoltaic power station DC boost converter bus capacitor aging fault early warning method as claimed in claim 9, characterized in that: The step 9 specifically includes defining the aging evaluation index of the output side bus capacitor when it is healthy as: D 0; when D and D 0 ratio is greater than the set threshold α, it is determined that the bus capacitor is about to fail, that is, 。

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