voltage, and the short circuit current of string-connected photovoltaic (PV) panels. The system can substitute the junction box, is fully self-powered, and does not ...
Effective Real-Time Performance Monitoring and Diagnostics of Individual Panels in PV Plants P. Guerriero*, V. d’Alessandro*, L. Petrazzuoli**, G. Vallone**, and S. Daliento* *Department of Electrical Engineering and Information Technology, University of Naples Federico II, via Claudio 21, 80125 Naples, Italy **IER s.r.l., Maddaloni, Italy not only malfunctioning events, but also presence of snow and dust, as well as shading occurrences due to neighboring buildings, TV antennas, or passing clouds, without disassembling the string. These systems are designed to be mounted on selected panels (by enriching or even substituting the junction box) in a threefold way, namely, (i) as diagnostic tools: after the detection of a problem in the string performed by the inverter, they allow discovering the module that originates the fault; (ii) as permanent functionality and performance evaluators: in this case, they keep under control the operation of the modules in the PV plant during their entire life-cycle; (iii) as anti-theft devices. The innovative feature of the system proposed in [9] is that the selected panel can be measured by electronically disconnecting it from the string, which allows the detection of the open circuit voltage Voc and the short circuit current Isc, useful e.g., for cell modeling purposes [10]. In this paper, we present an improved version of this circuit, which is also suited to measure the operating voltage Vpanel and current Ipanel of the monitored module. As a result, further information on the behavior of the PV plant under shading conditions, as well as on the inverter operation, can be gained. A wide experimental campaign is conducted on two different PV plants to prove the reliability and usefulness of the proposed system.
Abstract—In this paper, we present an improved version of a recently-developed monitoring circuit devised to measure the operating voltage and current, the open circuit voltage, and the short circuit current of string-connected photovoltaic (PV) panels. The system can substitute the junction box, is fully self-powered, and does not require additional cables thanks to a wireless communication. An extensive experimental campaign is performed to prove the reliability and usefulness of the system for continuous monitoring of PV plants; in particular, the capability to detect the effects of partial shading conditions on a PV string is highlighted. Index Terms—Monitoring photovoltaic plant, reliability.
system,
partial
shading,
I. INTRODUCTION Remote monitoring of PV systems is an important issue for the PV community. The long life-cycle of installed plants, concurrently with the low demand for servicing, justifies the need for only automatic real-time fault detection. Moreover, financing companies require guarantees about the long-term energy production, which can only be obtained if all parts of the system retain their own performances. Nowadays a monitoring action is usually performed by inverters only at the “low granularity” string level [1] due to a twofold reason: (1) individual panels have relatively low fault probability in their life-cycle (about 15% of the PV system failures [2]); (2) each problem at panel level propagates to the whole string [3]. However, in medium/high power plants the number of panels can rise to many thousands, thus increasing the possibility of maintenance query; besides, if problems arise from a malfunctioning module, its location is very hard to identify and often requires string disassembling. An approach to tackle this issue relies on satellitebased systems, which carry out the monitoring of PV plants by determining irradiation maps and comparing the collected data with simulations [4], [5]. However, results are significantly dependent on the theoretical models employed; in addition, the application of this method is unavoidably limited to satellite-observed plants. As an alternative strategy, wireless self-powered monitoring circuits operating at the “high granularity” single-panel resolution have been conceived and developed by SEM s.r.l. [6]-[8] and by a joint collaborative effort between IER s.r.l. and University of Naples [9] in order to detect
978-1-4673-4430-2/13/$31.00 ©2013 IEEE
II. THE MONITORING SYSTEM The schematic diagram and a picture of the SMD version of the monitoring circuit are reported in Figs. 1 and 2, respectively. Each block is allotted to accomplish a specific task. The supply section comprises a linear charge circuit, a dc-dc converter that provides the requested voltages to the system, and an array of supercapacitors, which are widely used in energy harvesting applications [11], [12]. The charge circuit adapts the operating voltages of the panel to levels suited for the super-capacitor charging and the dc-dc conversion, and derives a small amount of current (§20 mA) from the panel to sustain normal system operation, during which the super-capacitors are charged with a current limited by a feedback network. The charge circuit is fully deactivated throughout the measurement period not to affect the operating point of the panel; in this condition, the system is sustained by the energy provided by the super-capacitors. A low-power 8-bit microcontroller unit
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(MCU) supervises the tasks (namely, measurement phases/durations, data acquisition, and wireless connectivity).
PV panel
charge circuit
SuperCap + DC/DC
measurement circuit
MCU wireless
The current sensing section is deactivated by keeping the transistors MI1 and MI2 dry. It is to be remarked that the voltage sensing block is designed so as to offer a high input impedance not to perturb the panel operating point forced by the inverter. (2) After deactivating the voltage sensing section, transistor Md is turned off and device MI2 is enabled to allow the flow of the panel current Ipanel through the current sensing section, where it is converted into a voltage that is acquired by the MCU ADC. (3) The current sensing block is isolated by switching transistor MI2 off, which fully disconnects the module from the string so that Ipanel=0 A and Vpanel=Voc, while the bypass diode mounted on the monitoring circuit offers an alternative path to the current Istring. MOSFET MV is then enabled to favor the Voc measurement. (4) The panel is still disconnected. MV is turned off and MI1 is triggered to put the module in parallel with the current sensing block, which can be assimilated to a short circuit due to the very low resistance. As a result, the current Isc can be detected. The data acquisition is carried out after a few hundreds of s in order to avoid the expected current oscillations due to the transition of Ipanel from 0 A to Isc.
wireless network
disconnection circuit
monitoring circuit
Fig. 1. Schematic diagram of the monitoring circuit.
The following considerations are in order: The whole measurement period lasts only 3 ms. Compared to the variant presented in [9], also the measurements of the operating voltage Vpanel and current Ipanel are made possible thanks to the mere addition of the MI2 path. x As concerns the bypass case (ii), only Voc and Isc are determined.
x x
Fig. 2. Monitoring circuit in an SMD assembly.
The system is equipped with a disconnection circuit based on a switch, i.e., a power MOSFET connected in series with the panel (hereinafter referred to as Md). This circuit is devised to isolate the panel from the string, and is either (i) intentionally activated by the MCU or (ii) automatically enabled when the panel is under bypass conditions (i.e., due to shading). It is worth noting that, in spite of the panel exclusion, the string current keeps flowing through a bypass diode mounted in the circuit. In case (i), the disconnection takes place immediately before the measurement stage to allow the detection of Isc and Voc. In comparison with the version presented in [9], in case (ii) the disconnection circuit imposes a positive voltage drop across the bypassed panel, thus making it suited to supply energy to the system; as a result, the module can be monitored even under bypass conditions. The measurement circuit is sketched in Fig. 3, along with a pictorial representation of the measurement phases corresponding to case (i). As can be seen, this circuit is composed by voltage and current sensing sections controlled by the MCU through a switch network based on power MOSFETs. The phases can be described as follows: (1) The panel is still connected in the string (the power MOSFET Md is active and the bypass diode is switched off). In order to measure the operating voltage Vpanel of the module, the MCU enables the device MV to connect the panel to the voltage sensing block, which adapts Vpanel to the MCU ADC range.
Finally, the measured results are transmitted via wireless network to a coordinator circuit that collects data from all panels of the field and allows recognizing possible malfunctions. Further details on the wireless communication can be found in [9]. Fig. 4 details the evolutions of the voltages and currents playing a role during the measurement procedure. As can be seen, the loss of energy production is almost negligible, in spite of the panel disconnection. III. MEASUREMENT RESULTS Several experiments were carried out to prove the reliability of the proposed system and the usefulness of a continuous monitoring of PV plants. In this section, we show selected results illustrating the evaluation of (a) performance (Voc, Isc) and operating conditions (Vpanel, Ipanel) of individual panels belonging to a string, and (b) the influence of the interaction between string and energy conversion system. A. First experiment: architectural shading detection The first experiment was performed by applying the monitoring circuit to a string comprising 10 50Wp panels, installed on the roof of the Department of
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Electrical Engineering and Information Technology in Naples, and affected by architectural shading. Fig. 5 details the results collected during a whole measurement day for the outer modules of the string, namely, 1 and 10. As expected, Voc (Fig. 5a) was found to be much less shading-sensitive than Isc (Fig. 5b), which can be therefore reviewed as a reliable shading indicator. In particular, the Isc evolution on the monitored panels is essential to accurately follow the movement of the shadow along the string. Conversely, monitoring the Voc behavior is helpful to assess various key parameters, as e.g., maximum producible power and module temperature [13]. An inspection of Fig. 5b reveals that panel 10 produces an Isc much lower than that of panel 1 in the morning, thus suggesting a shadow occurrence; this is confirmed by the picture taken at 10 AM shown in Fig. 5c. Conversely, panels 1-3 are impacted by the shadow in the afternoon, and the PV current of panel 10 exceeds that of panel 1.
MV voltage sensing
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B. Second experiment: bypass effects and inverter temporary failure
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Small-sized architectural shading (due to e.g., chimneys, TV antennas, poles) is commonly believed to entail a negligible degradation in the yield of domestic roof PV plants, since the energy loss is erroneously associated to the shaded area. The second experiment was conceived to explore this effect by means of the proposed approach. The analysis was conducted on a PV string composed of 10 210Wp modules and installed on the roof of the IER factory; each panel is divided into three subpanels connected in series, each of which is provided with a bypass power diode [14]. The influence of a smallarea, yet complex-shaped, architectural shading was emulated by placing a hang-coat in the close proximity of a string portion (Fig. 6). Fig. 7 depicts the Voc and Isc data monitored for the shadow-affected modules (namely, 710) during a whole day; the measurements – repeated for various days – were carried out with a fine acquisition rate, i.e., every 90 s. The inspection of the Isc results (Fig. 7b) proves that the proposed strategy allows straightforwardly distinguishing an architectural shading from an atmospheric one. A shading effect is observed from 10:00 AM to noon, where panel 9 is affected by an Isc drop (up to 30% of the sunny value despite the small shaded area) proving that all its subpanels are shaded; another one occurs between 12 AM and 3 PM and involves all the monitored panels. The first shading was
2 MCU
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Fig. 4. Evolution of the key (a) voltages and (b) currents against time during the measurement process; the phases 1, 2, 3, 4 illustrated in Fig. 3 are highlighted.
voltage sensing
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Istring=Ipanel
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Istring Fig. 3. Behavior of the monitoring circuit for the detection of (1) Vpanel, (2) Ipanel, (3) Voc, and (4) Isc.
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classified as architectural due to a twofold reason: (1) it focuses on only one module, and (2) it was shown to be localized in the same time range regardless of the day. Conversely, the second shading was ascribed to clouds, since it is shared by all modules and it does not regularly arise in specific hours of the day.
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Fig. 7. Experimental (a) open circuit voltage Voc and (b) short circuit current Isc vs. time, as measured during a whole day by the monitoring circuit for the modules of the front row of the string mounted on the IER roof shown in Fig. 6.
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The previous analyses demonstrate that the Isc monitoring allows identifying and classifying partial shading effects. Further details on the behavior of the PV plant under shading conditions can be gained thanks to the measurements of the operating voltage Vpanel and current Ipanel, possible in the latest circuit variant. In addition, the evolutions of these signals provide information about the interaction between inverter and PV plant, and therefore can be used to examine the effects of bypass activation and MPPT failures [15]. Fig. 8 illustrates the behavior of Vpanel (along with Voc) and Ipanel corresponding to the same day of Fig. 7. The sudden Vpanel collapse involving modules 8 (9:00 AM – 10:30 AM) and 9 (9:30 AM – 3:00 PM) reveals that the bypass action is activated over these time windows. In order to more clearly explain the observed behavior, one can consider the evolution of Vpanel corresponding to panel 7 – not suffering from architectural shading – as a reference. A drop of nearly ѿ of the open circuit voltage of the panel is expected for each shaded subpanel compared to the reference module; a progressive shading of more subpanels gives a typical ladder shape to the Vpanel vs. time evolution. From the inspection of Fig. 8a, it can be evinced that the shadow projected by the hangcoat first affects module 8 by impacting up to two subpanels, and then module 9 by progressively involving all subpanels (consistently with the Isc drop shown in
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Time [Hours]
Fig. 5. Experimental (a) open circuit voltage Voc and (b) short circuit current Isc as a function of time, as measured during a whole day by the monitoring circuit for the outer panels of (c) a 10-panel string mounted on the roof of the Department of Electrical Engineering and Information Technology in Naples.
Fig. 6. PV string installed on the IER roof with a hang-coat placed to emulate a small-area architectural shading.
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Fig. 7b). It should be noted that panel 8 is sunny after 11 AM, whilst panel 9 is still suffering from architectural shading involving first two subpanels and then only one; this cannot be diagnosed by a mere observation of the short circuit current Isc, which does not reduce thanks to the activation of the bypass diodes.
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Fig. 9. Effect of the occurrence of partial shading: (a) typical step sequence performed by the MPPT algorithm; (b) evolution of the operating voltage of selected panels. Fig. 9b is the magnification of the evidenced area in Fig. 8a.
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IV. CONCLUSION
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In this paper, an enriched version of a monitoring system designed for individual panels embedded in installed PV plants has been presented. The circuit is suited to measure various key parameters of the panel, namely, short circuit current, open circuit voltage, and actual operating point, without affecting the energy production. A wide experimental investigation has been performed on two PV plants in order to explore the features of the system. It has been demonstrated that a proper inspection of the collected data allows gaining a complete insight into the behavior of the PV plant and its interaction with the inverter under shading conditions. In particular, the shadow propagation along the string, the number of shaded subpanels belonging to a chosen module, as well as the effects of the MPPT action can be easily observed and understood. It can be concluded that the proposed circuit represents a powerful tool for high granularity diagnostics or real-time performance evaluation in PV plants.
Time [Hours]
Fig. 8. Experimental (a) voltages Vpanel, Voc, and (b) current Ipanel against time for panels 7, 8, 9 belonging to the front row of the string installed on the IER roof shown in Fig. 6.
The presented monitoring system is also suited to highlight temporary MPPT failures performed by the inverter on a PV plant under partial shading conditions. Fig. 9a shows the typical step sequence followed by the MPPT algorithm. At beginning, the string is completely sunny (1); then the produced power suddenly decreases due to a local irradiance fall induced by shading [16] (2); subsequently, the MPPT algorithm forces and increase in Vpanel by assuming a uniform irradiance condition and undesirably tracks a local maximum instead of the absolute one located at a lower voltage (3); finally, the MPPT algorithm drives the operating point close to the absolute maximum by reducing the voltage (4). This behavior allows explaining the transient Vpanel data collected by the monitoring circuit and depicted in Fig. 9b: after a shading occurrence on one subpanel belonging to module 8 at around 9:10 AM, the inverter pushes the other panels to a local maximum by increasing the operating voltage.
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