1. Introduction
The annual global market for photovoltaic (PV) solar energy has increased considerably in recent years. Specifically, in 2019, the figure of 100 GW was exceeded. The total accumulated capacity has been increased by 25%, reaching the value of 5785 GW. This supposes a substantial growth if it is compared with the 15 GW of the previous decade. The increased demand in emerging markets and Europe, due to a large extent to the continuous price reductions, has offset a substantial decline in the market in China that had consequences worldwide [
1].
Self-consumption remained an important driver of the market for new systems distributed in some regions, and corporate purchases of PV solar energy expanded considerably, particularly in the United States and Europe. Around the world, mining, manufacturing, and other industries were installing PV (and other renewable) solar plants to supply power for their operations [
1]. In the year 2018, investment was made in a new, more efficient production capacity and additional advances in solar PV technology. At the end of 2018, at least 32 countries had an accumulated capacity of 1 GW or more. PV solar technology played a significant and growing role in the generation of electricity in several countries, including Honduras (12.1%), Italy, and Greece (both around 8.2%), and by the end of 2018, one in five Australian households generated at least part of their electricity with solar PV [
1].
In this scenario of the PV solar sector, it seems that the large production plants, as well as their energy production, will play a very important role. One of the major focuses of interest is the monitoring, inspection, and maintenance of PV solar plants, regardless of their power [
2,
3,
4]. Operation and maintenance (O&M) are the main saving points for investors in solar PV, and for this reason, in recent years, there has been a greater emphasis on advanced techniques for PV systems design, operation, and maintenance [
5].
PV power plants are exposed to numerous types of problems that can cause performance degradation due to elevated operating temperatures [
6], but the main and most worrying are those related to PV modules and PV inverters, as concluded after the presentation of some research in which sixty-three real PV solar plants were analyzed [
7]. Classically, there are numerous fault detection techniques for PV modules. Perhaps the most widely used method is infrared thermography (IRT), for hot spot detection [
8,
9]. Some authors use the thermographic images obtained by drones to subsequently use artificial intelligence systems for automatic fault detection [
10]. Another important technique is based on electroluminescence (EL) images [
11], where in some cases, low-cost systems are used [
12], and in many cases, the use of drones is also possible [
5]. All these techniques make an estimation of the type of faults from the images obtained.
Concerning the measurements in a module or PV plant, the I-V curves are the most interesting, since these curves provide accurate information about possible faults or degradation of the module or set of modules without the need of any estimation process [
13,
14]. Therefore, being able to have I-V curves of the individual modules allows decisions on the replacement of the specific faulty or degraded device. This decision is better taken based on a direct measurement than based on an estimation. Recalling the phrase by William Thomson Kelvin (Lord Kelvin), “
if you cannot measure it, you cannot improve it”, it is critical to have actual measurements of PV systems, and if possible, at the PV module level.
The necessity of shutting down the PV plant area during the traditional I-V electrical inspection with commercial testers has led to an important reduction of the use of this technique applied to the module level. It is more frequently used at the string level, as it is much easier to disconnect the strings at the combiner boxes to perform the I-V curve tests than disconnecting each single module. In any case, the great workforce and time required for this traditional I-V inspection has been the reason for only checking a single percentage of the strings of the plants in the preventive maintenance plan performed nowadays by the operators, which is usually 5% of the strings per year [
7,
15,
16]. Moreover, the identified underperforming strings with this methodology will have to be inspected afterwards to find the specific location of the defective modules. This subsequent examination has been traditionally done using manual thermography. As will be seen, the two proposed current–voltage (I-V) devices extremely facilitate this inspection in comparison with the commercial testers, performing online I-V curves and minimizing the workforce and the energy losses during the inspection.
There are many studies on I-V curve tracing systems in PV modules in the literature and several commercial testers. The different strategies are reviewed in this section. Some authors propose a monitoring system of a PV module based on a microprocessor where, in addition to the electrical parameters (V and I), the system measures irradiance and temperature [
17], but it requires the module to be disconnected from the corresponding string.
In [
18], an I-V tracer with a measuring strategy close to one of those proposed in this article is presented, but it is much more complex and expensive. It uses a relay to isolate the PV module under measurements from the string and connect it to the tracing device, and one minicomputer controls the entire system. Authors use the device to study the degradations of the PV modules (discoloration, cracks, and Potential Induced Degradation—PID) using the I-V curve.
Other authors propose an offline I-V curve tracer with extended capabilities that is able to trace I-V curves for a single PV cell [
19]. This equipment provides great versatility, since it allows cell (or associations of them) traces to be made.
In [
20], an inexpensive I-V curve tracer is presented. The device is based on an open-source platform with an integrated microcontroller, which allows electrical measurements and machine to-machine (M2M) communications.
Other authors present a portable I-V tracer for the characterization of PV modules outdoors [
21]. The work is validated in a real environment, and this portable device is capable of making an I-V trace of a 6kV string.
There is a patent that proposes carrying out the I-V trace of a PV module, or a string, involving the PV inverter in the measurement and control [
22]. The inverter has a first switch to disconnect its output from any device connected to it, a second switch to disconnect the input direct current (DC), and a discharge device to release any residual load in the input capacitor. The inverter also includes a voltage sensor and a current sensor on the DC side, as well as a data logger to record the I-V trace.
Another patent raises an I-V curve tracer through a fixed-value inductive or capacitive charging device [
23]. The patent proposes the correlation between voltage and current samples using a voltage digital value as the address of a memory module and the current as the content of that address.
The patent in [
24] also proposes a distributed online I-V tracer that includes an energy-storage device in order to keep the voltage supplied by the module under test while the measurements are made.
In recent years, some authors have done comparative work between different devices (commercial and ad hoc for research). Specifically, many devices, both commercial and research oriented, are presented in document [
25]. Regarding those dedicated to research, the authors present five different methods: variable resistive load method, capacitive load method, electronic load method, four-quadrant power supply method, and DC–DC converter method. The authors create the following performance indicators: performance factor, deviation on critical points and full curve, fill factor, voltage and current ratio, and slope deviation. In this way, different devices can be compared objectively. The authors conclude that the DC–DC converter method shows the advantage of flexibility and effectiveness to synthesize the merits of fast speed, high accuracy, low-cost solution, and ease of control, but the high-frequency switching operation causes ripples and increases the difficulty of data acquisition. The dynamics of power converters also affect the operational speed for I-V curve tracing, but these issues can be overcome by improving power conditioners and control strategies. For all the methods analysed, the measurement points were unevenly distributed on the I-V curve, and the issue should be resolved to improve the acquisition accuracy and resolution.
An online I-V for a complete string is proposed in [
26]. In this case, the proposed online I-V tracer topology does not require the plant to be brought offline to obtain the I-V trace. The circuit topology allows energy stored in the capacitor during the trace to be delivered to the load in the following state. The load does not experience any power interruption while the panels are isolated from the load during the trace. The circuit is retrofittable at the junction box level.
In summary, and after the analysis of the documents mentioned, it is possible to affirm that most of the proposed devices are quite invasive and time consuming, in the sense that in order to perform the measurements, it is necessary to disconnect the PV modules from the power line, and therefore, to interrupt the electrical circuit to the PV inverter, or they are complex and quite difficult to integrate in a real scenario because of its size, complexity, and cost. Therefore, research gaps have been identified. Then, the development and validation of low-cost, simple, and small devices that allow the layout of a PV module integrated into a power string, without the need to disconnect it (avoiding re-starting protocols and losses of production), seems a leap forward.
Often, solutions proposed in the research field for distributed monitoring and diagnostics, despite being fully operative, are difficult to be implemented in a real industrial environment because of their price.
A future commercial implementation of such a system depends strongly on the in-module electronics cost, since no monitoring and diagnostics system cost should represent a significant percentage of the module cost itself. For this reason, one of the main goals of this work is to obtain an electronic system that is not only able to accomplish online individual I-V tracings with quality but also to produce cost-effective hardware.
With the aim of improving the research gaps found, authors have developed and presented two different low-cost methods for performing not only individual I-V traces of the PV module but also concomitant measures of the partial voltages corresponding to the (usually three) substrings of the panel, each of them protected by means of a bypass diode, and all these measures being carried out online (without the need to disconnect the PV module from the string to which it is connected). The first strategy proposes installing a sweep block for each PV module (the authors propose calling this the distributed strategy), and a second one, installing a common sweeping block for all the modules composing the power string, which will be referred to as the mixed strategy.
The main goal of both designs will be the low-cost and simplicity of the electronics, especially the ones installed within each module, due to its strong impact in the total cost of the system. Following this line, the authors have proposed the implementation of all the blocks previously mentioned (at least partially) with a low-cost 8 bit microcontroller. Of course, some extra external components will be necessary, but these will be only two inexpensive power MOSFET transistors, a linear voltage regulator, and a few passive components. For experiments, the communications block relies on a small Bluetooth module controlled via RS232 protocol from the microcontroller, but for the final version, our group is now developing a Power Line Communication protocol specific for PV wirings fully controlled in firmware from the same microcontroller, minimizing cost and complexity. Both developed strategies are compared with other actual I-V tracer devices regarding complexity, size, online or distributed measurement capability, power supply requirements, accuracy, resolution, and cost, with the objective of highlighting the relevance of the developed strategies.
As it will be detailed later, both methods have some advantages and drawbacks, and they will be suitable for different situations. In addition, this work has a second aim, which is to make a comparison between both methods presented, focusing on the advantages and disadvantages of each one of them.
The paper consists of the following sections:
Section 2 presents the materials and methods used in the article;
Section 3 presents the experimental results of the two models presented;
Section 4 raises a discussion and comparison of both methods; and
Section 5 shows the main conclusions and future work related to the research presented.
3. Results
For validating the curves obtained from the two prototypes described above, the commercial tester “Solar IV-e” from manufacturer HT (ref. HT 0255) has been used. The curves from this commercial tester have been compared with the prototype’s ones.
Measurements have been made in our local plant for the two strategies described before, and in order to validate the resulting I-V curves, measurements have been carried out successively with our prototypes and with the commercial tester.
Every measurement has been made in the same conditions for which each device has been intended, that is to say:
With the module disconnected from the string for the commercial tester measurements,
Under production with the prototype card connected to one module within the string, and the string connected to the inverter for distributed strategy,
Over the whole string (11 modules in series) disconnected from the inverter for the mixed strategy.
These prototypes aim at being able to get the same I-V characteristics under their own operating conditions as the commercial instrument does over an isolated module.
The time between the prototype and commercial tester measurements has been reduced as much as possible, and measurements have been performed in the central hours of a clear day, so they should be taken under the same irradiance and temperature conditions. Irradiance and temperature have been recorded for every measurement and checked out to be equal (within a 1% difference between the commercial tester and the prototype) for the same module. This way, a good comparison can be made between the commercial tester and prototype measurements.
In
Table 3, the irradiation level, ambient temperature, and module temperature conditions in which the measurements have been taken are shown.
3.1. Distributed Strategy Results
The electronics corresponding to the distributed strategy have worked as expected when an I-V tracing was demanded over one of the eleven modules forming the string of our mini-plant. The short voltage drop (during about 40 ms) was overcome by the power inverter without any power interruption. In our experiments, the string voltage drop was about 9% of the nominal value (one module shorted among eleven), which is close to a typical ratio in power plants. The low-pass filtering associated with the wiring inductance and the usual capacitive input stage of the inverters allow this strategy to work properly without the need of any storage device supplying energy during the measurement lapse, such as the one proposed in reference [
24]. This is a key point to keep the electronic cost at its minimum, as the storage device (battery or condenser big enough) will have a price over the total cost of the whole card without it.
Figure 10 shows the most representative I-V curves obtained with the distributed strategy compared with the commercial tester ones. Each graph displays dashed yellow curves corresponding to RAW data from the prototype and continuous red curves with data points superimposed for commercial tester data.
The measurements have been carried out in our experimental plant, always with the string under production, over each of the modules composing the string. The modules in our plant have quite different performances, and several of them show different kinds of faults, resulting in a variety of curve shapes. Representative curves of modules with different types of faults and faultless ones are shown.
The fit between the commercial tester curves and the prototype ones has been evaluated by calculating the orthogonal distance between them. This led us to deviations lower than 0.5% except for the first sample from the commercial tester next to the short circuit. In this zone, next to the short circuit point, all the curves from the commercial tester show a straight segment between the first and second samples that is almost 3 V apart. The first sample from the commercial tester seems to be always misaligned with the prototype curve (sometimes over, sometimes under it), and direct measurement of the short circuit current corresponding to each curve shows that the current value from the prototype is the correct one. For this reason, this first sample has been discarded when comparing the curves, and the orthogonal deviations between prototype and commercial tester are kept within 0.5%. This percentage is the same as the uncertainty reported for the commercial tester, and it is small enough for diagnosis and monitoring purposes.
As shown in the graphs, the resolution along the prototype curves is higher than that from the commercial tester except for a short segment between 5 V and 14 V, where both resolutions are approximately equal. Regarding the resolution, it must be noted that the commercial tester takes 150 points/curve, but our prototype takes 330 points/curve. This reduced resolution of the prototype in the mid-early section of the curve is due to the fact that the firmware-controlled control loop (needed for performing the load sweep) takes some time to lock the desired voltage/time slope, as a consequence of the limited processing speed of the ultra-low-cost microcontroller used. Anyway, the resolution next to the short circuit point is as high as that out of this mid-early zone, allowing for a good determination of module parameters that very much depend on those next to the short-circuit zone.
3.2. Mixed Strategy Results
The system using the mixed strategy has also worked successfully. In this case, the measurements have been carried out in our experimental plant by disconnecting the string from the inverter and connecting it to the sweeping electronics common for all the strings. The synchronization between the sweep card and the measurement cards installed in every module has been implemented for the experiment via RS232 protocol over a Bluetooth device
The sweeping card (that in this case manages about 500 V, corresponding to the whole string) was stressed by forcing up to 5000 sweeping cycles one second apart, and no fault was found.
Figure 11 shows the most representative I-V curves obtained with the mixed strategy from the same modules as with the previous strategy, and the comparison with the commercial tester ones. Each graph displays dashed yellow curves corresponding to RAW data from the prototype and continuous blue lines corresponding to commercial tester data.
The curve from module 7 has been added in this second set of comparison curves because it reveals the effect of a mixed strategy prototype curve not reaching the zero voltage point, which is due to the issue described before regarding the asymmetry between the modules within the string. In the case of module 7, when all the strings are pushed to short-circuit, its best performance allows this module to remain as an active generator and its minimum voltage to reach about 6 volts, while most of the other modules behave as dissipative loads.
In this second set of curves, the first sample from the commercial tester shows the same strange behavior pointed out before; thus, it must be also disregarded for comparison. For the curves obtained with the mixed strategy, the orthogonal distance between the commercial tester curves and the prototype ones shows deviations lower than 0.8%, which are slightly bigger than the previous ones but small enough for monitoring and diagnosis purposes.
Regarding this mixed strategy, no control firmware of the sweeping process has been implemented, since a capacitive load has been used. This result in a lower resolution in the upper zone of the curves obtained than the one in the right zone, but anyway, the resolution is higher than the one of the commercial tester, and it is high enough for diagnosis purposes. The lack of a sweeping control for this strategy could be supplied by an adaptive capture rate programmed in firmware in a fashion that resolution in different segments of the curve could be equalized.
4. Discussion and Comparison
The word “online” along this document means that all the measurements are made without the need of a physical disconnection of the individual modules from the string. It could be considered that there is a short (in the order of milliseconds) disconnection for measurements, especially regarding the mixed strategy, but the fact that in both strategies, this disconnection is automated through semiconductor devices and short enough allows for the plant not to be stopped and power production uninterrupted, so the authors think that the term “online” is perfectly appropriate.
The reason for defining the two topologies presented is to try to meet the specific needs of two kinds of solar plant installations: small or domestic solar plants, whose power production depends on just one modules string, and big industrial solar plants, with many strings connected, where a short unavailability of one string can be assumed with monitoring and diagnostics purposes.
Both I-V tracing strategies described lead to curves with a reasonably good fitting in relation to the commercial tester ones, as it can be seen in
Figure 10 and
Figure 11, which are accurate enough for monitoring and diagnosis purposes.
The main goal of our work regarding the low cost of the system and the simplicity of the electronics in order to make its commercial introduction feasible has been successfully accomplished. Supposing an average PV module price of about 100 Euros, the cost of the distributed strategy cards is around 5–7% of the module price, and for the mixed strategy, the cost is around 4–5% (charging each module card with the corresponding fraction of the common sweeping card cost). On the other hand, for previously proposed online distributed I-V tracers based in microcomputers such as the Raspberry Pi, the cost of only these systems is around the 30% of the module price, or in the case of the systems that need an energy-storage device, it is approximately priced over the total cost of the cards proposed by the authors. Anyway, the complexity, physical size, and price of all the I-V tracers in the literature are over the corresponding characteristics for the two systems proposed here.
In the following
Table 4, a comparison of I-V tracer devices regarding complexity, size, online or distributed measurement capability, power supply requirements, accuracy, resolution, and cost is performed. This table includes the comparison of a commercial I-V tracer [
27], the two online tracers proposed in [
18,
26], and the two strategies proposed by the authors in this paper. As it can be seen in the last row of
Table 4, the main goal of both designs, which was the low cost and simplicity of the electronics, has been achieved, since the total cost of the system is much lower than the cost of the current devices. The cost of our distributed strategy is 10€, and the one of our mixed strategy is 10€ per module and 30€ for the central card, versus 4500€ for the commercial HT Solar I-Ve tracer [
27], which are 0.22% and 0.88% of the commercial price, respectively.
This success regarding the low cost, simplicity, and reduced size of the electronics makes it possible to consider the integration of the electronic cards within the PV modules. Therefore, the proposed strategies are really relevant for an actual PV system, leading to the possibility of a full monitored solar plant at the PV module level, which will be difficult to achieve based on bigger and more expensive devices such as those previously proposed in the literature.
Both methods have some advantages and drawbacks, and they will be suitable for different situations. In the following paragraphs, a comparison between both methods focusing on the advantages and disadvantages of each of them and their optimal scenarios is presented.
When the total cost of the system per module is the main goal, the mixed strategy is the cheaper option, but as mentioned above, it makes the complete string unavailable during the measurement, so it is better for those big plants with several panel strings connected in parallel, which guarantees the continuity of power production regardless of the disconnection of the string under measurement.
Furthermore, this mixed strategy uses sweeping electronics that operate at voltage levels typically between 400 V and 800 V (that of a complete string), and this usually means a reduced life for some components (as the power MOSFET and capacitor).
In addition, the choice of capacitive load as a sweeping device for mixed strategy makes the control of the sweeping slopes impossible, which in general will vary in an unpredictable way due to the complex shape of the string I-V curve, leading to successive steps in the capacitor charging current and the corresponding successive changes in the voltage slope. In practice, this lack of sweeping control does not represent a serious drawback, since if the sampling frequency is high enough, an adjustment is possible of the time between samples, in order to keep increments of voltage and current within reasonable values, and in any case, curves that will allow accurate diagnosis of the modules.
Regarding the distributed strategy, it makes the module under measurement unavailable during the short time necessary for the I-V tracing (typically 40 ms–100 ms), but the measurement sequence proposed guarantees that the power production of the panels string will be uninterrupted.
In terms of energy loss due to the I-V tracing process, both strategies have the same ratio loss/I-V trace, since both take the same time to perform each tracing.
Namely, supposing that one I-V trace (taking 40 ms) is done each day and a daily production time of 7 h in average, the order of daily energy loss could be approximated as 40/(7 × 3600000) = 0.00016%, which is almost negligible.
In small or domestic plants with just one string, the distributed strategy is the only choice if it is preferable for the energy production to be uninterrupted, since the mixed one makes the string unavailable for production during the measurement.
5. Conclusions and Future Work
This work has demonstrated the possibility of low-cost online I-V tracing viability at the module level with a minimum invasion, presenting two successful different strategies that lead to good quality measurement results. The main goal of our work regarding the low cost of the system and the simplicity of the electronics in order to make its commercial introduction feasible has been successfully accomplished. The cost of our distributed strategy is 10€, and the one of our mixed strategy 10€ per module and 30€ for the central card, versus 4500€ for other commercial tracers, which supposes 0.22% and 0.88% of the commercial price, respectively.
When the total cost of the system per module is the main goal, the mixed strategy is the cheaper option, but it makes the complete string unavailable during the measurement, so it is better intended for those big plants with several panel strings connected in parallel, which guarantees the continuity of power production regardless of the disconnection of the string under measurement. In small or domestic plants with just one string, the distributed strategy is the only choice if it is preferable for the energy production to be uninterrupted, since the mixed one makes the string unavailable for production during the measurement.
For both strategies, reliable low-cost communications play an important role, and because of this, our research group is currently developing a PV specific power line communication (PLC) system, which aims at implementing a half-duplex communication hardware over the existing power wires and a corresponding firmware loaded in the same low-cost microcontroller used for the I-V tracing, leading to a minimal circuitry (and cost) of the board.
This communication system must support the signaling and control of electronic boards and data transfer, and ideally, it could be fully controlled by firmware (even for the modulation/demodulation processes).
Finally, the I-V measurements made from our prototypes (and eventually any other data taken from sensors within the plant) must lead to the development of prediction and/or decision tools.
Following this line, our group will focus the research on three main streams:
Improved strategies and increased applications,
Sensors integration and the development of a reliable low-cost power line communication system,
Development of multi-criteria tools intended to help in the decision strategies leading to keeping production efficiency within PV plants.