Photovoltaic string output power optimization method in hot spot fault state

Abstract

The invention provides a photovoltaic string output power optimization method in a hot spot fault state, and the method comprises the steps: scanning an I-V curve of a photovoltaic string, obtaining voltage and current data, carrying out the single-peak and multi-peak judgment through the obtained data, employing different control methods according to different curve forms, employing an MPPT algorithm when a detection curve is a single peak, and carrying out the optimization of the output power of the photovoltaic string in a hot spot fault state. Controlling the string to output at the maximum power point; when multiple peaks of the curve are detected, performing parameter calculation on the curve by using a multi-peak parameter calculation method to obtain a final peak fault photo-generated current value # imgabs0 #, comparing the # imgabs1 # with the maximum power point current In obtained by scanning and the local maximum power point current In, and adopting different control strategies according to different comparison results to obtain a final peak fault photo-generated current value # imgabs1 #; the maximum power output can be carried out on the premise that the string does not generate a hot spot fault.

Description

Technical Field:

[0001] The present invention belongs to the field of optimizing the output power of photovoltaic strings, and particularly relates to a method for optimizing the output power of a photovoltaic string in a hot spot fault state. Background Art:

[0002] With the continuous development of photovoltaic power generation technology, the installed capacity of photovoltaic power stations has been continuously increasing globally. This trend not only reflects the profound understanding of environmental protection by humans but also marks a major transformation of the energy structure. However, with the rapid increase in the installed capacity of photovoltaic power generation, the operation and maintenance methods of traditional power stations have gradually exposed their limitations.

[0003] The existing string output control methods mainly rely on MPPT. Traditional MPPT control methods include the perturbation and observation method, the fixed voltage method, and the conductance increment method. The existing MPPT technology is relatively mature and can better achieve maximum power point tracking in the shadow state. However, in the shadow occlusion state, using this technology may cause hot spot faults, reduce the component life, accelerate its aging, and even cause safety problems. The existing hot spot prevention methods mainly focus on photovoltaic reconstruction and changing the design circuit, and there is no solution to this problem from the perspective of power point optimization.

[0004] Therefore, a hot spot suppression control algorithm is proposed. The algorithm first scans the I-V curve of the photovoltaic string to obtain voltage and current data, and uses these data to judge single-peak and multi-peak. According to different curve forms, different control methods are adopted. When the detected curve is single-peak, the MPPT algorithm is used to control the string to output at the maximum power point; when the detected curve is multi-peak, the multi-peak parameter calculation method is used to calculate the parameters of the curve to obtain the photocurrent value of the last peak fault. Compare with the maximum power point current I obtained from the previous scan m According to different comparison results, different control strategies are adopted, so that the string can output maximum power without generating hot spot faults. Summary of the Invention:

[0005] The present invention proposes a method for optimizing the output power of a photovoltaic string in a hot spot fault state, which is characterized in that it optimizes the power point output of the photovoltaic string in the occlusion state or hot spot fault state, so that the string can output maximum power without exacerbating the hot spot fault.

[0006] To achieve the above object, the present invention adopts the following technical solutions, and the steps are as follows:

[0007] Step 1: Taking 1 minute as a cycle, obtain the voltage-current curve data of the photovoltaic string at the current moment by changing the duty cycle D from 0 to 1, and record the voltage U at the maximum power point. m, current I m and the voltage U at the local maximum power point of the last peak n , current I n , take the I-V curve data points (V i , I i ), i = 1, 2,......, n, and calculate the difference Δ(dI / dU) between the data of adjacent two points on the curve.

[0008] Step 2: Use the slope principle to judge the number of peaks of the I-V curve. If Δ(dI / dU) > 0, the curve is a single peak. If there is Δ(dI / dU) < 0, the curve is a multi-peak. At the same time, determine the turning point current I of the curve according to the position and quantity of its negative sampling points k and the number of string peaks.

[0009] Step 3: When the curve is a single peak, directly perform maximum power output using the conductance increment method. When (dI / dU + I / U) > 0, U < U m , at this time, increase the duty cycle to increase the voltage. When (dI / dU + I / U) < 0, U > U m , at this time, decrease the duty cycle to decrease the voltage. When (dI / dU + I / U) = 0, the output power at this time is the maximum power P m .

[0010] Step 4: When the curve is a multi-peak, it is necessary to separate the I-V curve from the first peak and split the multi-peak curve into multiple single-peak curves. And use the 6-point method to calculate the parameters n 1 , R s1 , I ph1 , R sh1 , I o1 , and then perform sampling calculations in turn until the photocurrent value of the faulty cell corresponding to the last peak is calculated

[0011] Step 5: Compare the magnitudes of I m and to determine the optimized target current I c . When , P c_string = 0, there is no hot spot hazard, and it should directly generate electricity according to the maximum working point current. At this time, I c = I m ; when , the power consumption of the string P c_string > 0. To eliminate the hot spot hazard, the working point current should be controlled at and below. To ensure the maximum power output of the string without hot spot hazard, the working point current should be controlled at the local maximum power point of the last peak. At this time, I c = I n . Control the working point current to I by changing the duty cyclec , until the end of the cycle.

[0012] Step Six: Enter the next cycle and repeat the above operations to ensure that the string can operate at the optimized target current I under varying environmental conditions c to achieve the purpose of maximum power output without exacerbating the hot spot fault. Description of the Drawings:

[0013] Figure 1 Flowchart of a method for optimizing the output power of a photovoltaic string in a hot spot fault state according to an exemplary embodiment of the present specification;

[0014] Figure 2 Change in the adjacent slope difference Δ(dI / dU) between the single peak and the triple peak of the photovoltaic string according to an exemplary embodiment of the present specification;

[0015] Figure 3 Sampling point diagram for calculating the parameters of the first peak of the photovoltaic string according to an exemplary embodiment of the present specification;

[0016] Figure 4 Sampling point diagram for calculating the parameters of the second peak of the photovoltaic string according to an exemplary embodiment of the present specification. Detailed Implementation Manner:

[0017] The present invention will be further described below in conjunction with the accompanying drawings and examples. It should be understood that the embodiments described herein are only used to illustrate and explain the present invention and are not used to limit the present invention.

[0018] The present invention provides a method for optimizing the output power of a photovoltaic string in a hot spot fault state, characterized in that the power point output of the photovoltaic string in an occluded state or a hot spot fault state is optimized, so that the string can perform maximum power output without exacerbating the hot spot fault. The specific steps include:

[0019] Step One: Based on the real-time characteristics of the photovoltaic string, with one minute as a complete test cycle, by adjusting the duty cycle D of the PWM signal from 0 to 1, the working state of the photovoltaic string is dynamically changed, and the output voltage and current values of the photovoltaic string at different duty cycles are synchronously recorded to draw a complete I-V characteristic curve. Record the voltage U m , current I m at the maximum power point, as well as the voltage U n , current I n at the local maximum power point of the last peak. To further analyze the characteristics of the I-V curve, all valid data points (V i , I i ) on the I-V curve are selected, i = 1, 2,..., n, and the difference Δ(dI / dU) between adjacent two-point data of the curve is calculated. As shown in Equation (1):

[0020]

[0021] Step 2: Assume that the I-V curve of the photovoltaic string has m data points. Then the number of adjacent slope differences Δ(dI / dU) is n, where n = m - 2. The changes in the adjacent slope differences Δ(dI / dU) for single-peak and triple-peak curves are as Figure 2 shown.

[0022] From Figure 2 it can be seen that for a single-peak I-V curve, from the short-circuit current point to the open-circuit voltage point, the slope of each point gradually decreases. Therefore, Δ(dI / dU) is always positive and its value gradually decreases. For a triple-peak I-V curve, there are two turning points I cut1 and I cut2 . For the two points near the turning point, the trend of the slope change will change, and Δ(dI / dU) will become negative, corresponding to Figure 2 points A and B in, and these two points can be marked as turning points at the same time, which can be used as the basis for judging multi-peak curves.

[0023] Therefore, to determine the number of peaks k of the string and the turning point current I cuti of the curve, it is only necessary to calculate the adjacent slope difference Δ(dI / dU) and record the number i of negative sampling points and their positions, where the number of peaks k of the string = i + 1.

[0024] Step 3: When the curve is single-peak, use the conductance increment method to calculate dI and dU and the current I and voltage U at present. Track the maximum power point by calculating the conductance difference between two adjacent points of the string curve. The power expression of the photovoltaic string is shown in Equation (2):

[0025] P = UI (2)

[0026] Taking the derivative of Equation (2) gives Equation (3):

[0027]

[0028] When dP / dU is zero, the output power of the photovoltaic string is the largest, and Equation (4) can be obtained:

[0029]

[0030] When (dI / dU + I / U) < 0, that is, -I / U > dI / dU, the output of the photovoltaic string is on the right side of the maximum power point, and U > U m , and at this time, the duty ratio is reduced to reduce the voltage;

[0031] When (dI / dU + I / U) > 0, that is, -I / U < dI / dU, the output of the photovoltaic string is on the left side of the maximum power point, and U < U m , and at this time, the duty ratio is increased to increase the voltage;

[0032] When (dI / dU + I / U) = 0, that is, -I / U = dI / dU, the output of the photovoltaic string is at the maximum power point, and the output power at this time is the maximum power P m .

[0033] By comparing the magnitudes of -I / U and dI / dU and controlling the duty cycle, the photovoltaic string can be controlled to output at the maximum power point.

[0034] Step 4: When the curve has multiple peaks, it is necessary to separate the I-V curve from the first peak and split the multi-peak curve into multiple single-peak curves. Six points are sampled from the first peak of the string for parameter calculation, which are the short-circuit current point (0, I sc ), the point near the short-circuit current (δU, I sc + δI), any sampling point A (U a1 , I a1 ), any sampling point B (U b1 , I b1 ), any sampling point C (U c1 , I c1 ), and the point near sampling point C (U c1 + δU, I c1 + δI). The sampling point diagram is as shown in Figure 3 Figure.

[0035] The output characteristics of the first peak of the photovoltaic string are composed of the output characteristics of normal cells in normal units. Therefore, the forward expression of the photovoltaic cell is as shown in Equation (5):

[0036]

[0037] Taking the partial derivative of the current in Equation (5), Equation (6) is obtained:

[0038]

[0039] Substituting the exponential term in Equation (5) into Equation (6) and eliminating the exponential term in Equation (6), Equation (7) is obtained

[0040]

[0041] Taking the short-circuit current point (0, I sc ) and substituting it into Equation (5) to obtain the expression (8) of the first peak I ph1 :

[0042]

[0043] Taking the short-circuit current point (0, I sc ), the point near the short-circuit current point (δU, I sc + δI), and calculating the derivative R sho1 at the short-circuit current point=(δU - 0) / (I sc +δI - I sc ). Combining the short - circuit current point (0, I sc ), substituting it into Equation (6), the expression of the first peak R sh1 is obtained as Expression (9):

[0044]

[0045] Take any sampling point C(U c1 , I c1 ) and the point near sampling point C (U c1 +δU, I c1 +δI). Calculate the derivative R sco1 at point C as R c1 =(U c1 +δU - U sc ) / (I sc +δI - I c1 ). Combining point C (U c1 , I sh ), substituting it into Equation (6), since R sho1 ≈R s >>R sc , and I ph ≈I s1 , the expression of the first peak R

[0046]

[0047] Substitute the open - circuit voltage point (U oc , 0) into Equation (5), combine it with Equation (8), eliminate I ph1 . At the same time, since R sh ≈R sho1 >>R s , U oc >>I sc R s Equation (11) is obtained as follows:

[0048]

[0049] Take any sampling point A (U a1 , I a1 ) and substitute it into Equation (5), combine it with Equation (8) to eliminate I ph1 , combine it with Equation (9) to eliminate the exponential term. At the same time, since R sh ≈R sho1 >>R s The expression of I o1 is obtained as Expression (12):

[0050]

[0051] Combining Equation (11) and Equation (12) to eliminate I o1 , Equation (13) can be obtained:

[0052]

[0053] To avoid the open-circuit voltage point (U oc , 0), take any point B (U b1 , I b1 ) to replace the open-circuit voltage point, and Equation (14) can be obtained:

[0054]

[0055] Substitute Expression (10) into Equation (14), and a quadratic equation in one variable P(n 1 κT / q) 1 +R(n 2 κT / q)+G = 0 containing only n 1 can be obtained, and the ideality factor n 1 of the diode can be obtained as shown in Equation (15):

[0056]

[0057] After calculating the ideality factor n 1 of the diode from Equation (15), substitute n 1 into Equation (7) to obtain the series resistance R s1 , and rely on Equations (8), (9), and (12) to obtain the photocurrent I ph1 , the shunt resistance R sh1 , and the reverse saturation current I o1 of the diode respectively. So far, the calculation of the five parameters of the first peak is completed.

[0058] To calculate the parameters of the second peak, sample 7 points from the second peak of the string for parameter calculation. Arbitrarily sample three points A (U 2 , I a2 ), B a2 (U 2 , I b2 ), C b2 (U 2 , I c2 ) within c2 . Arbitrarily sample 3 points D (U 1 , I d1 ), D d1 (U 1 , I d1 ), F d1 (U 1 , I f1 ), and sample the point near F1 (U f1 )f1 +δU, I f1 +δI). The sampling points are shown in Figure 4 the figure below.

[0059] The output characteristics of the second peak of the photovoltaic string are composed of the output characteristics of the normal cells in the normal units and the output characteristics of the normal and faulty cells in the first type of faulty units. When the normal and faulty cells in the first type of faulty units both output in the forward direction, their output voltage U' unit2 is as shown in Equation (16):

[0060]

[0061] When the output current of the string is the output characteristics of the first type of faulty units are composed of the forward output characteristics of the normal cells and the reverse output characteristics of the faulty cells in the faulty units, and their output voltage is as shown in Equation (17):

[0062]

[0063] Subtracting Equation (17) from Equation (16) gives Equation (18):

[0064]

[0065] The parameter values of the normal cells in the second type of faulty units are the same as those of the normal cells in the first peak, i.e., {I ph2 ; R sh2 ; R s2 ; I o2 ; n 2} = {I ph1 ; R sh1 ; R s1 ; I o1 ; n 1}, and U' unit2 can be calculated from the parameters of the first peak and can be obtained by sampling. Therefore, Equation (18) can be regarded as a ternary linear equation for solving and . Arbitrarily select the data of three points A 2 (U a2 , I a2 ), B 2 (U b2 , I b2 ), and C 2 (U c2 , I c2 ) on a2 ; ΔU b2 ; ΔU c2 ​} data, and then use Cramer's rule to solve the equations. The value of is shown in formula (19):

[0066]

[0067] When the string output current is When , the output characteristic of the first type of faulty unit is formed by the superposition of the forward output characteristic of the normal battery in the faulty unit and the forward output characteristic of the faulty battery, as shown in formula (20):

[0068]

[0069] The faulty cell parameters calculated by equation (19) are stripped from the normal cell output characteristics in equation (20), and equation (21) can be obtained:

[0070]

[0071] According to formula (21), Randomly sample 3 points in the interval D 2 (U d2 , I d2 ), E 2 (U e2 , I e2 ), F 2 (U f2 , I f2 ) and the nearby points of F2 (U f2 +δU,I f2 +δI). Then, the remaining parameters of the faulty battery cell in the first type of faulty unit are obtained by the first peak parameter calculation method of equations (10) to (15). Derivative R at point F2 sco2 =(U f2 +δU-U f2 ) / (I f2 +δI-I f2 ). The parameter calculation formula is shown in formula (22):

[0072]

[0073] The string is composed of photovoltaic units connected in series. Except for the first peak, the output characteristics of the subsequent k-th peak are superimposed by the output characteristics of the normal cells and the faulty cells in the faulty unit. Therefore, it is only necessary to Sampling point A k (U ak , I ak ), B k (U bk , I bk)、C k (U ck ,I ck ), calculate the

[0074] of the k-th type of unit by combining formula (19). m and to determine the optimized target current I c . When , the power consumption of the string P c_string = 0, there is no hot spot risk, and it should generate electricity directly according to the maximum power point current. At this time, I c = I m ; when , P c_string > 0. To eliminate the hot spot risk, the operating point current should be controlled at and below. To ensure the maximum power output of the string without hot spot risk, the operating point current should be controlled at the local maximum power point of the last peak. At this time, I c = I n . Compare the output current I of the string with the optimized target current I c , and determine the disturbance direction of the voltage by judging the magnitudes of I and I c . When I < I c , increase the duty cycle to increase the voltage. When I > I c , decrease the duty cycle to decrease the voltage. If I = I c , it proves that the output power optimization has been achieved.

[0075] Step 6: Enter the next cycle and repeat the above operations to ensure that the string can operate at the optimized target current I c under varying environmental conditions, so as to achieve the purpose of maximum power output without exacerbating the hot spot fault.

[0076] The above is the specific implementation method in engineering applications, but the present invention is not limited to the described method. The basic principle method of the present invention is based on the above basic solution, and changes, modifications, substitutions, and deformations that do not depart from its principle and spirit still fall within the protection scope of the present invention.

Claims

1. A method for optimizing the output power of photovoltaic strings under hot spot fault conditions, characterized in that Optimizing the power point output of photovoltaic strings in the shaded state or hot spot fault state can enable the strings to output the maximum power without exacerbating the hot spot fault. The specific steps are as follows: Step 1: With a one-minute cycle, obtain the voltage and current curve data of the photovoltaic string at the current moment by changing the duty cycle D from 0 to 1, and record the voltage U at the maximum power point m 、Current I m And the voltage U at the local maximum power point at the end peak n 、Current I n , take the IV curve data point (Vi, Ii), i = 1, 2, ..., n, and calculate the difference Δ (dI / dU) between two adjacent points on the curve. Step 2: Use the slope principle to determine the number of peaks of the IV curve. If Δ(dI / dU) is greater than 0, the curve is single-peaked. If Δ(dI / dU) is less than 0, the curve is multi-peaked. At the same time, the turning point current I of the curve is determined according to the position and number of negative sampling points. k And the number of group peaks. Step 3: When the curve is a single peak, use the conductivity increment method to directly obtain the maximum power output. When (dI / dU+I / U)>0, U<U m At this time, the duty cycle is increased to increase the voltage. When (dI / dU+I / U)<0, U>U m At this time, the duty cycle is reduced to reduce the voltage. When (dI / dU+I / U)=0, the output power is the maximum power P m . Step 4: When the curve has multiple peaks, it is necessary to separate the IV curve from the first peak and split the multi-peak curve into multiple single-peak curves. And use the 6-point method to calculate the first peak parameters n1 and R s1 ,I ph1 , R sh1 ,I o1 Then, the sampling points are calculated in sequence until the photocurrent value of the faulty unit corresponding to the final peak is calculated. Step 5: Comparison I m and The size of the optimized target current I c .when When P c_string = 0, no hot spot risk, power generation should be directly based on the maximum operating point current. At this time, I c =I m ;when When the string power consumption is P c_string >0, in order to eliminate the hidden danger of hot spots, the working point current should be controlled at To ensure the maximum output power of the string without hot spot hazards, the operating point current should be controlled at the local maximum power point at the end peak. c =I n By changing the duty cycle, the operating point current is controlled to I c , until the cycle ends. Step 6: Enter the next cycle and repeat the above operation to ensure that the string can work at the optimized target current I under changing environment. c In order to achieve the purpose of maximum power output without aggravating hot spot failure.

2. The method according to claim 1, characterized in that: The string power consumption P described in step 5 c_string , the operating current and voltage of the PV strings and the model parameters of normal components and hot spot components are combined to represent the sum of the power consumed by each faulty unit. Its calculation is shown in formula (1):

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