JP7102245B2 - Diagnostic method and diagnostic equipment for solar cell modules - Google Patents

Description

The present invention relates to a diagnostic method and a diagnostic device for a solar cell module used for photovoltaic power generation.

Photovoltaic power generation consists of many parts such as solar cell modules, mounts, wiring, power conditioners, and transformers. Since a photovoltaic power generation site is usually expected to be used for a long period of 20 years or more, it is necessary to ensure a stable equipment operation status throughout the period of use. For this reason, proper maintenance of equipment is important, and among them, the solar cell module is placed in a harsh environment exposed to direct sunlight and wind and rain, so ensuring long-term reliability is the most important component. Therefore, equipment maintenance at the power generation site requires a technique for correctly grasping the deterioration state of the solar cell module.

The silicon-based solar cell module has a structure in which silicon cells having a PN junction are arranged in a plane and they are connected in series by wiring called a bus bar. The cells connected by wiring are embedded in resin, sandwiched between tempered glass and a back sheet, and the ends are protected by an aluminum frame. FIG. 1 shows an equivalent circuit of the solar cell module. 11 is a current source in which the cell absorbs light to generate a current, 12 is a diode corresponding to the PN junction of the cell, 13 is a shunt resistance R _sh corresponding to the leakage current path between terminals, and 14 is the wiring inside the module. The series resistance R _s .

Deterioration and failure of the solar cell module range from glass breakage to failure of the wiring take-out part for extracting electricity from the cell series circuit called the junction box to the outside. Abnormalities with large changes in appearance and electrical output can be easily detected, but abnormalities that progress slowly are difficult to detect. Many of these abnormalities are caused inside the solar cell module and are reflected in the equivalent circuit of the solar cell module. For example, the deterioration of the resin that seals the cell appears as a decrease in the current from the current source 11 because the light transmittance is lowered due to the coloring of the resin due to the deterioration. Further, wiring deterioration such as solder peeling due to expansion and contraction of the member appears as an increase in the series resistance R _s . The shunt resistance R _sh is an element corresponding to the leakage current path between the terminals of the solar cell module, and its value is affected by a plurality of factors such as the inside of the PN junction and the resin.

The series resistance R _s increases with the deterioration of the solar cell module, but the amount of increase in the series resistance R _s and the amount of decrease in the output of the solar cell module are generally in a linear relationship, and from the initial stage of deterioration to the output of the solar cell module. A large impact. On the other hand, the shunt resistance R _sh decreases due to the deterioration of the solar cell module, but if the deterioration of the shunt resistance R _sh does not progress significantly, the effect on the solar cell module output cannot be seen, and the solar cell module output once increases. It has the characteristic that it decreases sharply when it begins to decrease. The most remarkable deterioration of the shunt resistance R _sh is known as the PID (Potential Induced Degradation) phenomenon. PID is a phenomenon in which ions caused by glass move under the influence of an electric field (potential) and accumulate at the cell interface, thereby deteriorating the diode characteristics of the PN junction. Since the PID rapidly deteriorated in a short period of time and the output also decreased sharply, it was relatively easy to detect. However, since it is not only the PID phenomenon that causes deterioration of the shunt resistance R _sh , the shunt resistance R _sh is a parameter that needs to be continuously monitored in the maintenance of photovoltaic power generation.

Typical inspection methods used for maintenance of solar cell modules include visual inspection, IV (current-voltage) characteristic inspection, and infrared image inspection. Among these, the IV characteristic test has the advantage that it is relatively easy and the deterioration can be quantified. It is also possible to isolate the deterioration of each of the series resistance R _s and the shunt resistance R _sh , which is difficult for the IV characteristic inspection by visual inspection or infrared image inspection. Since the deterioration of the series resistance R _s and the decrease of the module output are generally in a proportional relationship, if the cause of the deterioration can be identified as the series resistance R _s , the progress of the subsequent output decrease can be roughly estimated, and there is a need for countermeasures. And the time can be foreseen. If the cause of the deterioration is the shunt resistance R _sh , if it is due to the PID phenomenon, the output may decrease rapidly, so it is possible to take measures such as focusing on progress monitoring.

However, in the case of deterioration of the shunt resistance R _sh , a plurality of factors such as a resin for sealing the cell can be considered as the cause of the deterioration in addition to the PN junction of the cell that causes the PID phenomenon. Therefore, from the viewpoint of solar cell module maintenance, it is desirable to further isolate the cause of deterioration of the shunt resistor R _sh . If the PN junction of the cell is deteriorated, the progress is relatively fast and it is necessary to take immediate action, whereas if the resin is deteriorated, the progress is relatively slow because it is caused by the change over time due to light irradiation. This is because it can be expected to exist and the measures to be taken are different. However, in the IV characteristic inspection, it is difficult to isolate the cause because the same element (shunt resistance 13) is represented on the equivalent circuit regardless of whether the cause is a PN junction or a resin.

EL (Electroluminescence) inspection, insulation resistance measurement, impedance measurement method, and the like are known as methods for isolating the cause of deterioration of the shunt resistance R _sh . The EL inspection is a method of capturing light emitted by a band-to-band transition in a silicon cell caused by passing a current between the terminals of a solar cell module with a camera. When deterioration occurs in the PN junction of the cell, the emission brightness decreases due to the increase in non-emission transition, so that the deterioration of the cell can be visualized. However, it is difficult to quantitatively grasp the deterioration because the leakage current that does not pass through the cell also causes a decrease in the emission brightness. In addition, since a power source having a relatively large capacity is required to pass a current between the terminals, it is usually used for indoor inspection, and it is difficult to inspect at an actual power generation site.

Insulation resistance measurement is a method of inspecting the insulation property of a resin by measuring the insulation resistance between one terminal of the solar cell module or both short-circuited terminals and the frame. When the resin deteriorates and the insulating property deteriorates, the leakage current between the terminals increases, so that the change in the shunt resistance R _sh due to the current path other than the cell can be grasped. However, it is difficult to quantitatively grasp the deterioration because the measurement of insulation resistance is greatly affected by the temperature, humidity, and the state of dirt on the module surface.

Impedance measurement is a method of examining the frequency characteristics of impedance by applying a minute AC signal between the terminals of the solar cell module. Patent Document 1 discloses a method for diagnosing a solar cell module using impedance frequency characteristics between both terminals of the solar cell module and between the terminals and the frame. According to it, first, the frequency spectrum of the impedance between terminals or between terminals and frames is acquired. From the obtained frequency characteristics, the values (equivalent circuit constants) of the elements constituting the equivalent circuit to be measured are obtained by fitting. In the fitting, the equivalent circuit constants are selected so as to best reproduce the entire frequency spectrum. Therefore, the constant of each element of the equivalent circuit has one value in the entire measured frequency domain. Next, these values are standardized by the equivalent circuit constants acquired in the undegraded state. Deterioration judgment is performed by comparing the standardized value with a preset threshold value.

International release 2015/1633329

S. M. Sze & KWOK K. NG, “Physics of semiconductor devices (Third Edition)” John Wiley & Sons, Inc., Publication, USA, April 10, 2006, p.119

Deterioration of solar cell modules has the potential risk of causing a sharp drop in output, depending on the cause. Therefore, an inspection technique capable of detecting deterioration and isolating the cause is required. Although the IV characteristic test can detect deterioration, it is not possible to isolate the cause when the shunt resistance R _sh has deteriorated. Further, according to the impedance measurement disclosed in Patent Document 1, the cause of deterioration of the shunt resistance R _sh can be isolated, but it is standardized by the circuit constant in the state where it is not deteriorated in this process. Therefore, information on the circuit constant in the non-deteriorated state is required, and if this value is unknown, the deterioration cannot be diagnosed. When undertaking the maintenance business of a photovoltaic power generation site from the middle, it can easily occur that the information in the initial state of the solar cell module does not exist.

Therefore, it is necessary to develop a method that can detect the deterioration of the shunt resistance R _sh of the solar cell module and isolate the cause without using the past data such as the data in the state where the module is not deteriorated.

In the method for diagnosing a solar cell module according to an embodiment of the present invention, the solar cell module is placed in a light-shielded state, and the frequency f of the input AC signal is continuously changed to obtain the impedance Z and the phase angle θ of the solar cell module. The frequency spectrum is acquired, and the frequency spectrum of the capacitance Cp constituting the equivalent circuit of the solar cell module in the light - _shielded state is calculated from the acquired frequency spectrum of the impedance Z and the phase angle θ of the solar cell module, and the solar cell module. In the frequency _spectrum of the capacitance Cp , the solar cell module is diagnosed by comparing the capacitance value on the lower frequency side than the cutoff frequency with the capacitance value on the higher frequency side than the cutoff frequency. The frequency f of the AC signal is a frequency range in which the variables caused by the wiring of the solar cell module in the equivalent circuit can be ignored, and the capacitance C _p of the solar cell module is calculated as −sinθ / 2πf | Z |. ..

Deterioration of cells constituting the solar cell module can be detected without using past data such as data in a non-deteriorated state.

Other challenges and novel features will become apparent from the description and accompanying drawings herein.

It is an equivalent circuit diagram of a solar cell module. This is a configuration example of a solar power generation system. It is an equivalent circuit diagram of a solar cell module in a non-power generation state. This is an example of a measurement circuit that measures the impedance of a solar cell module. This is a measurement example of the frequency spectrum of impedance Z. This is an example of measuring the frequency spectrum of the phase angle θ. This is the relational expression between the impedance, phase angle and circuit constant of the solar cell equivalent circuit in the non-power generation state. It is the equivalent circuit diagram of the solar cell module which limited the frequency domain and simplified. It is a frequency spectrum of parallel resistance calculated from the measured value. It is a frequency spectrum of capacitance calculated from the measured value. This is a measurement example of the frequency spectrum of impedance Z. This is an example of measuring the frequency spectrum of the phase angle θ. It is a frequency spectrum of parallel resistance calculated from the measured value. It is a frequency spectrum of capacitance calculated from the measured value. It is a figure which shows the temperature dependence of the parallel resistance in a normal module and a deteriorated module. It is a figure which shows the bias voltage Vdc dependence of C_high and C_low in a normal module and a deteriorated module. It is a figure which shows the temperature dependence of the frequency spectrum of a capacitance C _p . This is an example of the hardware configuration of the solar cell module diagnostic system. It is a flowchart which determines the presence or absence of deterioration of a solar cell module. It is a flowchart which determines the presence or absence of deterioration of a solar cell module. It is a figure which shows the temperature dependence of the forward resistance _RF of a PN junction obtained theoretically. It is a flowchart which determines the presence or absence of deterioration of a solar cell module.

In this embodiment, the deterioration of the cell of the solar cell module is detected by impedance measurement.

FIG. 2 shows a configuration example of a relatively large-scale solar power generation system. The unit in which the solar cell modules shown in 21 are connected in series is called a string 22, and a plurality of strings 22 are connected in parallel in the junction box 24. A circuit in which strings 22 are connected in parallel is called an array 23. The plurality of arrays 23 are further connected in the current collector box, and are input to the power conditioner (PCS) 25 from there. The PCS 25 converts the DC power output from the solar cell into AC and sends it to the system 27. The PCS 25 may be connected to the system 27 via an isolation transformer 26.

The equivalent circuit 10 of the solar cell module is as shown in FIG. 1, and a current is generated by the incident of sunlight on the cells in the solar cell module. It is represented by the fact that the current is supplied from the ideal constant current source 11 to be supplied. A part of the supplied current is supplied to the load from the P terminal 15 via the diode 12 and the rest via the series resistance 14. The current supplied from the load returns to the current source 11 through the N terminal 16. The leakage current generated in the solar cell module flows through the path of the shunt resistor 13. In principle, the impedance characteristics of the solar cell module can be measured even in the power generation state, but the measurement in the non-power generation state is easier from the viewpoint of improving the measurement circuit and measurement accuracy.

Therefore, when measuring the impedance of the solar cell module at the power generation site, the measurement is performed with the surface of the solar cell module (the glass surface side where sunlight is incident) shielded from light. FIG. 3 shows an equivalent circuit 30 of the solar cell module when an AC signal is applied between the PN terminals in a light-shielded state, that is, in a state where the solar cell module does not generate power. Since it is in a non-power generation state, the current source 11 disappears, and the diode 12 corresponding to the PN junction is represented by the capacitance C _p 31 and the parallel resistance R _p 32. The shunt resistance _Rsh in the equivalent circuit in the power generation state represented all the resistance components corresponding to the resistance to the current passing through the PN junction and the path of the leakage current passing through the resin in the cell and around the cell, but in the non-power generation state. The parallel resistance R _p 32 in the equivalent circuit of is a resistance representing the resistance component to the current of the PN junction path and the resistance component of the leakage current path caused by the cell. However, the resistance of the PN junction path and the resistance of the other leakage current paths are connected in parallel, but the resistance of the leakage current path other than the PN junction path is extremely small compared to the resistance of the PN junction path, so the parallel resistance. R _p roughly corresponds to the resistance of the PN junction path. Therefore, the portion 33 in which the capacitance C _p 31 and the parallel resistance R _p 32 are combined corresponds to the PN junction. In the element caused by wiring, the inductance L _s 34, which is visible due to the AC component, is newly added to the series resistance R _s 14.

When measuring the impedance characteristics of the solar cell module, take necessary safety measures such as stopping the PCS and disconnecting the string circuit including the module to be measured. Then, a corrugated cardboard is used to cover the entire light receiving surface of the module to be measured. As a result, the solar cell is placed in a light-shielded state to stop power generation. The method used to create a light-shielded state is not limited to this. Instead of corrugated cardboard, a light-shielding curtain or a blue sheet may be covered with a light-shielding sheet or rubber made by folding multiple sheets, or measurement may be performed at night.

After leaving it in that state for a while and waiting for the cell temperature to stabilize, the frequency spectrum of the impedance of the solar cell module is acquired. For the measurement, a measuring device such as an LCR meter or an impedance analyzer can be used, and in particular, a device capable of continuously changing the frequency of the input signal (AC signal) input to the solar cell module is used. FIG. 4 shows an example of a measurement circuit for measuring the impedance of the solar cell module. It has a sine wave oscillator 41, a current-voltage converter 42, and a vector voltage ratio measuring unit 43 as main components of the measurement circuit, and an AC signal generated from the sine wave oscillator 41 is one of the solar cell modules 40. The output signal input from the terminal (P terminal 15) and output from the other terminal (N terminal 16) is input to the current-voltage converter 42. The sine wave oscillator 41 includes a bias power supply 51, an oscillator 52, and a synthesis circuit 53 that synthesizes a DC bias Vdc from the bias power supply 51 and an oscillation signal from the oscillator 52 to output an AC signal. The current-voltage converter 42 uses the potential of the reference resistor 54 that converts the output signal (current signal) output from the solar cell module 40 into a voltage and the potential of the connection point L between the solar cell module 40 and the reference resistor 54 as the reference potential. It has a control amplifier 55 that controls the voltage. The vector voltage ratio measuring unit 43 includes a first voltmeter 56 that measures the AC voltage between the P and N terminals of the solar cell module 40, and a second voltmeter 57 that measures the AC voltage across the reference resistor 54. , The ratio meter 58 for obtaining the ratio between the first voltmeter 56 and the second voltmeter 57.

The impedance Z and the phase angle θ of the solar cell module 40 are measured by the measurement circuit shown in FIG. Impedance Z is calculated as the ratio of the magnitude of the AC voltage of the solar cell module 40 measured by the first voltmeter 56 to the magnitude of the AC current of the solar cell module 40 obtained from the control amount of the control amplifier 55. Further, the phase angle θ is determined from the phase difference between the AC voltage of the solar cell module 40 obtained by the first voltmeter 56 and the AC current of the solar cell module 40 obtained by the second voltmeter 57 by the ratio meter 58. It is calculated.

Using the measurement circuit shown in FIG. 4, the measurement voltage level of a 60-cell polycrystalline silicon solar cell module commercially available for industrial use is set to 0.1 V (effective value of input signal (AC signal)). , DC bias Vdc is 0V, the frequency of the input signal is changed from 1Hz to 100kHz, and the frequency spectrum of impedance Z and phase angle θ is acquired. Further, the DC bias Vdc setting is changed to 0.1, 0.5, 1.0, 1.5, 2.0, 2.5V, and the frequency spectrum is acquired with each setting. This is because the solar cell module is provided with a bypass diode, so when a reverse bias is applied to the PN terminal, the bypass diode is turned on in a relatively small voltage range, and the desired characteristics cannot be obtained. Therefore, by superimposing the DC bias Vdc on the input AC signal and making the amplitude of the AC signal sufficiently small, all the applied voltages are kept at positive values to prevent the bias diode from turning on, and DC. The inclination of the -IV characteristic is also measured in the region where the diode characteristic of the solar cell module can be regarded as substantially constant. The frequency spectrum of the measured impedance Z of the solar cell module is shown in FIG. 5A, and the frequency spectrum of the phase angle θ is shown in FIG. 5B.

The equivalent circuit constant is obtained from these results, but in the prior art, the obtained spectrum was obtained by fitting using the impedance expression obtained from the equivalent circuit. FIG. 6 shows the relational expression between the impedance, the phase angle and the circuit constant of the equivalent circuit of the solar cell module in the non-power generation state. Specifically, the impedance Z is expressed as a complex number, and the absolute value | Z | of Z and the phase angle θ are expressed using the real part (Re (Z)) and the imaginary part (Im (Z)) of Z, respectively (Im (Z)). It is represented by equation 1). Further, Re (Z) and Im (Z) are represented by (Equation 2) by using the circuit constants of the equivalent circuit 30 shown in FIG. While the equivalent circuit 30 includes four variables of capacitance C _p , parallel resistance R _p , series resistance R _s , and inductance L _s , the equation for solving them is the impedance Z shown in (Equation 1). Since the absolute value | Z | and the equation of the phase angle θ are two, it cannot be solved analytically, and it is necessary to obtain the circuit constant by fitting. The notation "absolute value | Z | of impedance Z" is used here in relation to the complex representation of impedance, and is the value of impedance Z measured by the measurement circuit.

In this embodiment, no fitting is used to obtain the circuit constants. Originally, in a solar cell module, the series resistance R _s is at most several mΩ to several Ω, which is negligible as compared with the shunt resistance R _sh or the parallel resistance R _p of several kΩ to several tens of kΩ. Further, the inductance L _s is also about several μH. With this magnitude, the influence on the frequency spectrum of impedance Z appears in the high frequency region where the impedance of the capacitance C _p is sufficiently smaller than that of the shunt resistance R _sh or the parallel resistance R _p . Is. In the case of a solar cell module, the region is a frequency region of about 100 kHz or higher. This means that when the frequency spectrum is fitted by the conventional method, the values of the series resistance R _s and the inductance L _s affect the higher frequency side than about 100 kHz, and the series resistance R _s and the inductance L, which will be described later, are affected. It can be confirmed by calculating the real part and the imaginary part of the impedance Z for the equivalent circuit ignoring _s and comparing it with the real part and the imaginary part obtained by the fitting which is the conventional method.

Therefore, in the frequency spectrum measurement shown in FIGS. 5A and 5B, the frequency of the input signal is limited to 100 kHz. Within this frequency range, there is no problem even if the series resistance R _s and the inductance L _s are ignored. FIG. 7 shows a simplified equivalent circuit diagram ignoring these. The real part and the imaginary part of the impedance Z with respect to the equivalent circuit 70 are represented by (Equation 3) shown in FIG. By substituting this equation into (Equation 1), the absolute value | Z | of impedance Z and the phase angle θ can be obtained. The feature of (Equation 3) is that (Equation 2) is represented by four variables, whereas it is represented by two variables (parallel resistance R _p , capacitance C _p ). Since the absolute value | Z | of the impedance Z shown as (Equation 1) and the two equations for the phase angle θ are given to the two variables, the variables can be obtained analytically. Specifically, (Equation 4) is obtained by substituting (Equation 3) into (Equation 1) to obtain the variables R _p and C _p . Here, ω = 2πf (f is the frequency of the input signal). The equivalent circuit constants (R _p , C _p ) can be obtained by substituting the measured impedance Z and the phase angle θ into (Equation 4). As can be seen from the equation, R _p and C _p have frequency dependence.

The series resistance R _s and inductance L _s ignored from the equivalent circuit cannot be obtained, but it is a great advantage that the equivalent circuit constant can be obtained without using fitting. When finding by fitting, there are cases where the reproducibility of the experimental value by the fitting function is insufficient, and when the degree of agreement between the function value in one frequency domain and the experimental value is increased, the degree of agreement in another frequency region decreases. Occurs. Although there are certain guidelines such as minimizing the sum of the residual squares of the values obtained from the experimental data and the fitting function in order to reduce the arbitrariness of the fitting, aside from the uniformity of the procedure, the result is a satisfactory fitting. I do not guarantee that it will be given.

Another advantage of the measurement method of this embodiment is that the measurement system can be simplified by limiting the frequency range for measuring the frequency spectrum to a relatively low frequency region. For example, in order to measure the frequency range up to the MHz order, an expensive device is required, which is generally a large device, and it is necessary to pay attention to the electrical connection. In particular, when measuring at an outdoor power generation site, the influence of noise becomes large, which is disadvantageous in terms of accuracy. On the other hand, if the frequency range is about 100 kHz or less, measurement can be performed with a relatively simple device, and the influence of connection and noise on the measurement result is small.

The equivalent circuit constants (R _p , C _p ) calculated based on (Equation 4) from the frequency spectrum measurements shown in FIGS. 5A and 5B are shown in FIGS. 8A and 8B. FIG. 8A is the frequency spectrum of the parallel resistance R _p , and FIG. 8B is the frequency spectrum of the capacitance C _p .

Here, from the expression of (Equation 4) in FIG. 6, when the phase angle θ is near 0 °, the parallel resistance R _p to | Z |, and most of the absolute value | Z | of the impedance Z is the parallel resistance R _p . It can be seen that the influence of the parallel resistance R _p is carried into the capacitance C _p through the denominator | Z |. On the other hand, when the phase angle θ is in the vicinity of −90 °, the relationship of the absolute value | Z | to 1 / (ωC _p ) of the impedance Z is established. Therefore, the capacitance C _p is responsible for the absolute value | Z | of the impedance Z when θ is in the vicinity of −90 °, and the parallel resistance R _p is affected by the capacitance C _p through the molecule | Z |. You can see that it is included.

From the measurement result of the phase angle θ shown in FIG. 5B, the R _p value in the frequency domain where the phase angle θ is in the vicinity of 0 ° is set to “R _p _low” in FIG. 8A, and the phase angle θ is in the vicinity of −90 °. The C _p value in the frequency domain is represented as "C _p _high" in FIG. 8B. Here, in R _p _low and C _p _high, the contribution of each element occupies most of the absolute value | Z | of the impedance Z of the equivalent circuit. When the phase angle θ is near 0 °, the absolute value | Z | of the impedance Z is determined exclusively by R _p _low, and there is almost no contribution of C _p . When the phase angle θ is near −90 °, the absolute value | Z | of the impedance Z is determined exclusively by C _p _high, and there is almost no contribution of R _p . That is, it is considered that R _p _low and C _p _ high are frequency-independent circuit constants R _p and C _p that were conventionally attempted to be calculated by fitting. This can be confirmed from the fact that the Mott-Schottky plot can be used to determine the junction capacity of the cells connected in series that make up the module for C _p _high.

The same method is then applied to modules with degraded cells. As a module in which the cell was deteriorated, a module in which the output was deteriorated by about 15% due to potential-induced deterioration (PID) was used as a test module. The measurement results are shown in FIGS. 9A and 9B. FIG. 9A is a frequency spectrum of the impedance Z of the deteriorated solar cell module, and FIG. 9B is a frequency spectrum of the phase angle θ of the deteriorated solar cell module. The equivalent circuit constants (R _p , C _p ) calculated based on (Equation 4) from the frequency spectrum measurements shown in FIGS. 9A and 9B are shown in FIGS. 10A and 10B. FIG. 10A is a frequency spectrum of the parallel resistance _Rp , and FIG. 10B is a frequency spectrum of the capacitance _Cp . Comparing FIGS. 8A and 10A, it can be seen that the value of R _p _low is significantly reduced, whereas the value of C _p _ high is hardly changed when comparing FIGS. 8B and 10B. From this, it is expected that the deterioration of the solar cell module can be detected by R _p _low. However, as shown in FIG. 11, the parallel resistance R _p has an extremely large temperature dependence, and when the module temperature is high, the difference in the parallel resistance R _p between the normal module and the deteriorated module is almost eliminated. It is not uncommon for the cell temperature to rise to about 70 ° C. in an environment exposed to direct sunlight outdoors, and it is difficult to detect deterioration by simply comparing _{Rp_low} . Moreover, the difference in R _p _low becomes smaller from that of the normal module as the degree of deterioration of the solar cell module becomes smaller, so that the deterioration detection becomes more difficult. On the other hand, since C _p _high has a small difference between the normal module and the deteriorated module, it is difficult to use it for deterioration detection because high measurement accuracy is required.

Therefore, in this embodiment, attention is paid to the capacitance on the low frequency side (expressed as "Cp _{_low} "). C _p _low is a capacitance value seen on the low frequency side of the frequency spectrum of the capacitance C _p shown in FIGS. 8B and 10B. C _p _low has a larger value than C _p _high and shows a substantially constant value in a certain frequency range. Further, C _p _high can be classified by the cutoff frequency fc described below.

The cutoff frequency fc is defined as follows. In the equivalent circuit shown in FIG. 7, when an AC signal is input to the terminal, when the signal frequency is low, the impedance (1 / (ωC _p )) of the capacitance C _p is large, so that the signal is preferentially parallel resistance R _p . Pass through. On the other hand, when the signal frequency is high, on the contrary, the impedance of the capacitance C _p becomes small, so that the signal preferentially passes through the capacitance C _p . In the intermediate frequency region, there is a frequency in which the impedance 1 / (ωC _p ) of the capacitance C _p and the impedance R _p of the parallel capacitance R _p are equal, and this frequency is defined as the cutoff frequency fc. Θ = tan ^-1 ( _{−ωC p} R _p ) can be easily derived from (Equation 1) and (Equation 3) in _FIG . It can be seen that the frequency at which θ is −45 ° corresponds to the cutoff frequency fc.

The value of the capacitance Cp at which the capacitance is substantially constant on the frequency side lower than the cutoff frequency fc is defined as _{C_low} , and the value of the capacitance at which the capacitance is substantially constant on the frequency side higher than the cutoff frequency fc is defined as C_high. do. For example, in the frequency spectrum of the phase angle θ shown in FIG. 5B, the frequency at which the phase angle is −45 ° is about several tens of Hz. On the other hand, in the frequency spectrum of the capacitance C _p shown in FIG. 8B, the vicinity of several tens of Hz corresponds to the region where the capacitance C _p is changing, and is approximately 150 nF on the high frequency side and 200 nF on the low frequency side. It shows a constant value. Therefore, C_low of this solar cell module is about 200 nF, and C_high is about 150 nF. FIG. 12 shows the results of measuring C_low and C_high obtained by dividing them by the cutoff frequency fc by changing the bias voltage Vdc for the normal module and the deteriorated module. It can be seen that the change in C_low is larger than the change in C_high before and after deterioration. C _ high is approximately equal to C _p _ high, and as explained above, C _p _ high can be regarded as the cell junction capacitance C _p . The junction capacitance _Cp of the cell is basically determined by the doping profile of the PN junction of the solar cell module. Therefore, in order for this value to change significantly, it is necessary to change the doping profile of the PN junction, but such a change requires extremely high energy, and with the energy generated by use as a solar cell. No such change will occur. On the other hand, since the phase angle _θ≈0 on the low frequency side, Cp to 1 / | Z | and | Z | to _Rp are obtained from (Equation 4) in FIG. It is _p , and C _p increases as R _p decreases due to deterioration. That is, C _p _ low becomes large, reflecting that R _p _ low becomes significantly small due to the deterioration of the solar module.

As shown in FIG. 11, the temperature dependence of the parallel resistance R _p is very large, and the magnitude of R _p changes by an order of magnitude with a temperature change of several tens of degrees, so that the temperature reached by the cell during use at the power generation site. Even in the range, the magnitudes of the parallel resistance R _p of the normal module and the deteriorated module are almost the same. On the other hand, the capacitance Cp hardly changes in the same temperature range (T = 0 to 80 ° _C. ) as shown in FIG. From (Equation 4) in FIG. 6, R _p to 1 / ω C _p , and the capacitance C _p does not change much even though the parallel resistance R _p fluctuates greatly with the temperature change. This is because ω (= 2πf) changes with the temperature change, so that the cutoff frequency fc changes so as to cancel the change in the parallel resistance _Rp .

As shown in FIG. 12, C_high shows almost the same value in the normal module and the deteriorated module. Furthermore, for normal modules, C_high and C_low are approximately the same size. This reflects that in the normal module, even if _Cp is calculated by the equation of (Equation 4) in FIG. 6, the original junction capacitance value can be obtained in the target frequency range.

In this way, C_high shows almost the same value in the normal module and the deteriorated module, whereas in the normal module, C_low is almost the same as C_high, but it changes greatly when it deteriorates. Moreover, the temperature dependence of both C_high and C_low is small. Therefore, in the first embodiment, the degree of deterioration of the solar cell module is determined by comparing the value of C_high and the value of C_low. This makes it possible to determine the degree of deterioration of the solar cell module to be evaluated even if it does not have the measured values at the normal time.

FIG. 14 shows an example of the hardware configuration of the solar cell module diagnostic system for diagnosing the deterioration of the solar cell module according to this embodiment. The solar cell evaluation device 100 includes a processor 101, a main memory 102, an auxiliary memory 103, an input / output interface 104, a display interface 105, an I / O port 106, and a network interface 107, which are connected by a bus 108. The input / output interface 104 is connected to an input device 109 such as a keyboard or a mouse, and the display interface 105 is connected to the display 110 to realize a GUI. The network interface 107 is an interface for connecting to a network. The auxiliary storage 103 is usually composed of a non-volatile memory such as an HDD, a ROM, or a flash memory, and stores a program executed by the solar cell evaluation device 100, data to be processed by the program, and the like. The main memory 102 is composed of RAM, and temporarily stores a program, data necessary for executing the program, and the like according to instructions from the processor 101. The processor 101 executes a program loaded from the auxiliary storage 103 into the main memory 102. The solar cell evaluation device 100 can be realized by, for example, a PC (Personal Computer) or a server.

The auxiliary storage 103 stores measurement data 120, other data, a solar cell module diagnostic program 122, and other programs. The solar cell module diagnostic program 122 includes an impedance measuring unit 122a and a deterioration evaluation unit 122b as its main parts.

The measuring device 130 is connected to the solar cell evaluation device 100 via the I / O port 106. The measuring device 130 includes the measuring circuit shown in FIG. 4, and the solar cell module 131 to be diagnosed is connected to the measuring device 130. The measuring device 130 acquires the frequency spectrum of the impedance Z and the phase angle θ of the solar cell module 131.

FIG. 15 shows a flowchart of determining whether or not the solar cell module has deteriorated, which is executed by the solar cell module diagnostic program 122. The impedance measurement unit 122a sets necessary measurement parameters such as a frequency measurement range and a bias voltage Vdc, acquires a frequency spectrum of impedance Z and phase angle θ of the solar cell module 131 by the measurement device 130, and stores it as measurement data 120. (S01). Further, the frequency spectrum of the impedance Z and the phase angle θ is converted into the frequency spectrum of the parallel resistance R _p and the capacitance C _p , and these are also stored as the measurement data 120 (S02). The deterioration evaluation unit 122b diagnoses the solar cell module using these measurement data 120. When the deterioration evaluation unit 122b makes a diagnosis based on the capacitance C _p , the conversion of the parallel resistance R _p to the frequency spectrum may be omitted.

Although FIGS. 9A and 9B are shown as measurement examples of the deterioration module, when the deterioration further progresses, the phase angle θ does not reach −90 ° in the frequency spectrum of the phase angle θ in the frequency range of 100 kHz or less and is significantly separated. There are cases where the value increases toward a positive value or the cutoff frequency fc does not exist in the frequency range. In such a case, it means that the capacitance C _p of the PN junction is significantly deteriorated. Therefore, it is considered that the solar cell module is deteriorated without the diagnosis of this embodiment. Can be judged.

Therefore, θ _min , which is the minimum value of the phase angle θ, is detected (S03), and it is evaluated whether θ _min is within a predetermined value in the vicinity of −90 ° (S04). Further, the cutoff frequency fc is detected (S05). If θ _min does not reach the predetermined threshold value or more at −90 ° (N at S04) or the cutoff frequency fc is not within the measurement range (N at S06), it is diagnosed that the solar cell module 131 has deteriorated. (S10). On the other hand, when these are not applicable, the determination described in this embodiment is performed. First, C_high and C_low are detected using the cutoff frequency fc (S07), and the detected C_high and C_low are compared (S08). For the comparison between C_high and C_low, the difference (| C_high-C_low |) may be taken, or the ratio (C_low / C_high) may be calculated and used. A threshold value is set in advance according to the measurement error and the degree of deterioration to be detected. If the difference (ratio) between C_high and C_low is smaller than the threshold value, it is diagnosed that there is no deterioration (S09), and C_high and C_low are combined. When the difference (ratio) is equal to or greater than the threshold value, it is diagnosed that there is deterioration (S10). The obtained diagnostic result is displayed on the display 110, for example (S11). In this way, the deterioration of the solar cell module can be diagnosed even if there is no measured value in the normal state.

Another flowchart for determining the presence or absence of deterioration of the solar cell module is shown in FIG. In the flowchart of FIG. 16, the deterioration of the solar cell module is determined only by C_low. The PN junction profile cannot be changed significantly for solar cells that require the maximum power generation efficiency with an inexpensive manufacturing process, and it is considered that the difference in junction capacity _Cp is not so large for silicon-based solar cells. .. That is, regardless of the cell maker, it can be expected that the values of C_high will be almost similar. In the flow of FIG. 16, C_high assumes a predetermined value for the silicon-based solar cell, and deterioration determination is performed based on whether C_low is within a predetermined range based on the assumed C_high. In the flowchart of FIG. 16, steps for performing processing common to the flowchart of FIG. 15 are indicated by the same reference numerals, and duplicate description will be omitted. In this modification, C_low is detected from the measurement data (S17), and the presence or absence of deterioration is determined based on whether C_low is smaller than a predetermined threshold value (S18). According to this method, there is an advantage that the frequency range acquired in step S01 can be further narrowed.

As the second embodiment, a method of determining the deterioration of the solar cell module using the parallel resistance _Rp will be described. In this method, deterioration is determined by comparing the theoretical value of the parallel resistance _Rp with the actually measured value.

First, the theoretical value of the parallel resistance R _p is obtained. The parallel resistance R _p corresponds to the resistance value obtained from the slope of the DC-IV characteristic of the PN junction of the solar cell module. However, since the solar cell module is equipped with a bypass diode, when measuring the impedance of the solar cell module, if a reverse bias is applied to the PN terminal, the bypass diode turns on in a relatively small voltage range, which is desired. The characteristics cannot be acquired. Therefore, as described in the first embodiment, when measuring the impedance of the solar cell module, the DC bias Vdc is superimposed on the input AC signal. Furthermore, by making the amplitude of the AC signal sufficiently small, all the applied voltages are kept at positive values to prevent the bias diode from turning on, and the gradient of the DC-IV characteristic is also adjusted by the diode characteristic of the solar cell module. It can be measured in a region that can be regarded as substantially constant. With such a device, the parallel resistance _{R p} obtained from the impedance measurement can be regarded as corresponding to the forward resistance RF of the PN junction.

For example, p. 119 describes a theoretical method for determining the forward resistance _RF . According to this method, the forward resistance RF at an arbitrary temperature with respect to the DC bias _Vdc can be calculated. The values obtained by theory are not accurate enough to reproduce the measured values with sufficient accuracy, but the temperature dependence is generally reproduced. Therefore, in this embodiment, the temperature dependence obtained from the theoretical value is used for determining the deterioration of the solar cell module. FIG. 17 is a diagram showing the temperature dependence of the theoretically obtained forward resistance R _F value ( _RF (T)) with reference to the forward resistance R _F value ( _RF (0)) at T = 0 ° C. Is. Specifically, assuming a Si solar cell using a standard phosphorus-doped p-type substrate, the physical property values are donor concentration 1 × 10 ²¹ (1 / cm ³ ) and acceptor concentration 1 × 10 ¹⁶ (1 / cm ³ ). , Effective state density of conduction band 2.8 × 10 ¹⁹ (1 / cm ³ ), Effective state density of valence band 2.65 × 10 ¹⁹ (1 / cm ³ ), Electron mobility 120 (cm ² / Vs) , The lifetimes of electrons and holes are both set to 1 ms, and the calculation is made in consideration of the temperature dependence of the intrinsic semiconductor carrier density, electron diffusion coefficient, hole diffusion coefficient, band gap, and hole mobility. Other physical constants use well-known representative values.

Next, the parallel resistance R _p is actually measured. After light-shielding the module to be measured and allowing an appropriate time, the temperature of the solar cell module is measured. In this embodiment, a non-contact infrared laser thermometer is used, but a contact measurement using a thermocouple or the like may be used. After that, the frequency spectra of the impedance Z and the phase angle θ are measured with respect to the solar cell module to be inspected in the same manner as in the first embodiment. The measurement data is converted into the frequency spectrum of the parallel resistance R _p , and the R _p value (R _p _low) of the lowest frequency is obtained. As described in Example 1, R _p _low is the value of the frequency-independent circuit constant R _p . Therefore, the R _p value obtained from the measured temperature T (° C.) and the temperature dependence of the forward resistance RF theoretical value shown in FIG. 17 is used to determine the _{R p} value at a specific temperature at which deterioration determination is performed. Convert to R _p value. As shown in FIG. 17, if the temperature range is relatively narrow, about 0 to 80 ° C., the temperature dependence can be regarded as a straight line by logarithmically plotting the vertical axis. For example, if the measured temperature is T ₁ (° C.) and the obtained R _p is R _p (T ₁ ), the R _p value (R _p (T)) at the temperature T (° C.) can be calculated by the following equation. ..
log [R _p (T)] = A × (T ₁ -T) + log [R _p (T ₁ )]
Here, A is the average slope of the temperature dependence shown in FIG. In this way, the R _{p value obtained from the data measured at an arbitrary temperature can be converted into the R p value} at a specific temperature.

As shown in Example 1, when the temperature dependence of the normal module and the deteriorated module is compared, as shown in FIG. 11, the difference between the two becomes small in the temperature range of 50 ° C. or higher, and the presence or absence of deterioration can be determined. It gets harder. Further, since the value of the parallel resistance _Rp changes according to the degree of deterioration of the module, the positional relationship between the normal module and the deteriorated module shown in FIG. 11 also changes. As the degree of deterioration becomes smaller, the line of the deteriorated module approaches the line of the normal module, so that the temperature range in which the deterioration can be determined becomes narrower. Ultimately, the temperature range is determined in consideration of the amount of deterioration to be detected, but it is safe to measure at a temperature of approximately 40 ° C. or lower.

Deterioration is determined by converting an experimentally obtained R _p value into an R _p value at a specific temperature, for example, 25 ° C., and comparing the value with a predetermined threshold value. The threshold value may be appropriately determined from the degree of deterioration to be detected, measurement error, and the like.

The system configuration for executing the second embodiment is the same as that of the first embodiment. The deterioration evaluation unit 122b in the solar cell module diagnosis program is configured to convert the R _p value actually measured as described above into the R _p value at a predetermined temperature to determine the deterioration.

However, in the second embodiment, the deterioration determination can be greatly simplified. FIG. 18 shows a flowchart for determining the presence or absence of deterioration of the solar cell module. In the second embodiment, the parallel resistance _Rp value obtained from the impedance Z of the lowest frequency is used for the deterioration determination. Therefore, unlike the first embodiment, it is not necessary to acquire the frequency spectrum of the impedance Z, and the measurement can be remarkably simplified as compared with the first embodiment. For example, a handheld simple LCR meter cannot be used in the method of Example 1 because impedance measurement cannot be performed while continuously changing the frequency of the input signal. On the other hand, in the second embodiment, it is sufficient to measure the impedance at a specific frequency at which the phase angle θ can be regarded as 0. Therefore, the R _p value at a low frequency is obtained by using such a simple LCR meter, and the specific temperature is obtained. By converting to the value of, deterioration diagnosis of the solar cell module becomes possible. That is, it is possible to make a simple diagnosis of the solar cell module by using a simple battery-powered LCR meter.

First, the solar cell module is placed in a light-shielded state, and an AC signal having a predetermined frequency at which the phase angle θ can be regarded as 0 is input to measure the impedance of the solar cell module (S21). Further, the temperature when the impedance of the solar cell module is acquired is measured (S22).

Using the acquired impedance of the solar cell module, the parallel resistance R _p'that constitutes the equivalent circuit of the solar cell module in the light-shielded state is calculated (S23). At this time, since the phase angle θ = 0, it can be calculated as R _p '= | Z | according to (Equation 4) in FIG. Since the parallel resistance R _p'has a large temperature dependence as described above, the parallel resistance value obtained in step S23 is converted into a parallel resistance value having a predetermined temperature. Therefore, in order to convert the parallel resistance R _{p'to the parallel resistance R p at} a predetermined temperature, the solar cell evaluation device 100 of the second embodiment holds the temperature dependence information of the forward resistance of the PN junction of the solar cell module in advance. ing. As described above, this temperature dependence information can be theoretically calculated based on the physical parameters of the solar cell. Using this temperature dependence information, the parallel resistance R _{p'is converted into the parallel resistance R p at} a predetermined temperature (S24), and the deterioration of the solar cell module is determined based on the parallel resistance R _p (S25 to S28). .. For example, when the parallel resistance R _p exceeds the threshold value R _p _ref, it is determined that there is no deterioration, and when the parallel resistance R _p is equal to or less than the threshold value R _p _ref, it is determined that there is deterioration. As shown in FIG. 11, the lower the module temperature, the wider the difference between the normal module and the deteriorated module. Therefore, it is desirable that the temperature at which the resistance values are compared is lower than the temperature at which the impedance measurement is performed.

As described in Example 1, the temperature dependence of the parallel resistance _Rp and the temperature dependence of the cutoff frequency fc have a high correlation. Therefore, it is possible to perform the deterioration determination based on the cutoff frequency fc as well as the deterioration determination based on the parallel resistance _Rp .

10, 30, 70 ... Equivalent circuit, 11 ... Current source, 12 ... Diode, 13 ... Shunt resistance, 14 ... Series resistance, 15 ... P terminal, 16 ... N terminal, 21, 40, 131 ... Solar cell module, 22 ... String, 23 ... array, 24 ... junction box, 25 ... power conditioner, 26 ... isolated transformer, 27 ... system, 31,71 ... capacitance, 32,72 ... parallel resistance, 34 ... inductance, 41 ... sinusoidal oscillator, 42 ... current-voltage converter, 43 ... vector voltage ratio measuring unit, 51 ... bias power supply, 52 ... oscillator, 53 ... synthesis circuit, 54 ... reference resistance, 55 ... control amplifier, 56, 57 ... voltmeter, 58 ... ratio Total, 100 ... Solar cell evaluation device, 101 ... Processor, 102 ... Main storage, 103 ... Auxiliary storage, 104 ... Input / output interface, 105 ... Display interface, 106 ... I / O port, 107 ... Network interface, 108 ... Bus, 109 ... Input device, 110 ... Display, 120 ... Measurement data, 122 ... Solar cell module diagnostic program, 130 ... Measuring equipment.

Claims (10)

With the solar cell module in a light-shielded state, the frequency f of the input AC signal is continuously changed to acquire the frequency spectrum of the impedance Z and the phase angle θ of the solar cell module.
From the acquired frequency spectra of the impedance Z and the phase angle θ of the solar cell module, the frequency spectrum of the capacitance Cp constituting the equivalent circuit of the solar cell module in the light - _shielded state was calculated.
In the frequency _spectrum of the capacitance Cp of the solar cell module, the sun is compared with the capacitance value on the low frequency side of the cutoff frequency and the capacitance value on the high frequency side of the cutoff frequency. Diagnose the battery module and
The frequency f of the AC signal is a frequency range in which the variable caused by the wiring of the solar cell module in the equivalent circuit can be ignored, and the capacitance Cp of the solar cell module is _−sinθ / 2πf | Z |. Diagnostic method of solar cell module to calculate . In claim 1,
The cutoff frequency is a frequency at which the phase angle θ of the solar cell module is −45 °, which is a method for diagnosing the solar cell module. In claim 2,
A method for diagnosing a solar cell module in which the frequency that gives the capacitance value on the high frequency side is a frequency of 100 kHz or less. In claim 3,
When the frequency spectrum of the phase angle θ of the solar cell module does not reach −90 ° at −90 ° or more in the frequency range of 100 kHz or less, or when the cutoff frequency does not exist in the frequency range of 100 kHz or less. Is a method for diagnosing a solar cell module that determines that the solar cell module is deteriorated. In claim 2 ,
A predetermined value is assumed in advance as the capacitance value on the high frequency side of the cutoff frequency.
A method for diagnosing a solar cell module for diagnosing the solar cell module by comparing a capacitance value on the lower frequency side than the cutoff frequency with the predetermined value. It is a diagnostic device for solar cell modules.
With the processor
Memory and
The frequency spectrum of the impedance Z and the phase angle θ of the solar cell module acquired by continuously changing the frequency f of the input AC signal with the solar cell module in a light-shielded state, and the frequency spectrum read into the memory and read by the processor. Has auxiliary memory to store the solar cell module diagnostic program to be executed,
The solar cell module diagnostic program has a deterioration evaluation unit.
The deterioration evaluation unit has a cutoff frequency in the frequency spectrum of the capacitance Cp that constitutes the equivalent circuit of the solar cell module in the light - _shielded state, which is calculated from the frequency spectra of the impedance Z and the phase angle θ of the solar cell module. The solar cell module is diagnosed by comparing the capacitance value on the lower frequency side with the capacitance value on the higher frequency side than the cutoff frequency .
The frequency f of the AC signal is a frequency range in which the variable caused by the wiring of the solar cell module in the equivalent circuit can be ignored, and the capacitance Cp of the solar cell module is _−sinθ / 2πf | Z |. Diagnostic device for solar cell modules to calculate . In claim 6 ,
The cutoff frequency is a frequency at which the phase angle θ of the solar cell module is −45 °, and is a diagnostic device for the solar cell module. In claim 6 ,
A measuring device to which the solar cell module in a light-shielded state is connected is connected to the diagnostic device.
The solar cell module diagnostic program has an impedance measuring unit and has an impedance measuring unit.
The impedance measuring unit is a diagnostic device for a solar cell module that sets measurement parameters for the measuring device and acquires a frequency spectrum of the impedance Z and the phase angle θ of the solar cell module measured by the measuring device. In claim 7 .
The deterioration evaluation unit assumes a predetermined value in advance as the capacitance value on the high frequency side of the cutoff frequency, and sets the capacitance value on the low frequency side of the cutoff frequency and the predetermined value. A solar cell module diagnostic device that diagnoses the solar cell module by comparison. In claim 7.
A diagnostic device for a solar cell module in which the frequency that gives the capacitance value on the high frequency side is a frequency of 100 kHz or less.

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