Online Detection Of The Typical Hot-Spot Phenomenon In A PV System With Adaptive Control Strategies

In recent years,it has been reported that a new type of hot-spot phenomenon with extremely high temperature usually appears in photovoltaic(PV)generation systems.This type of hot-spot phenomenon may lead to irreversible cell destruction and accelerate the rate of thermal degradation within a short period of time,even a fire in severe cases.It has been found that such hot-spots frequently appear in partially shaded PV modules containing crystal defects.Among all known crystal defects,the low-resistance defects usually occur at cracks,intersecting junctions,and the edge of cells.These defects are associated with high conductivity that occurs in conjunction with a reverse bias when under external shadows.From electrical characteristics,it can be explained that some equivalent paths resulting from such defective regions are provided for current to flow more easily.That is,a high and prolonged reverse current can flow through these defective regions,meaning that there exists an equivalent low resistance at defective regions.In order to ensure the safe operation of the PV generation system,this study mainly focuses on the real-time detection and suppression of hot-spots resulting from low-resistance defects,so as to eliminate any further risk of hot-spot phenomenon in time.Firstly,in order to understand the formation mechanism of such hot-spots,this study demonstrated the role of the equivalent resistance in current diversion when under partial shadow by establishing an equivalent model of the defective cell with the various reverse characteristics of PN junctions.In addition,three working modes(?,?,?)of the PV generation system were divided due to the magnetite of the current and the formation conditions of such hot-spots during power generation were analyzed in detail using an equivalent diagram of a PV string which was under control of the conventional maximum power tracking algorithm,P&O.Moreover,the simulation was employed to show the thermal power distribution when under different external shadow conditions.Accordingly,on the basis of simulation results,it is clear that there is a working area on the thermal power distribution in which the heating power is almost 0 when under partial shading conditions,that is,it is a safe area.By shifting the operating point of the PV generation system from the maximum power point to the safe area,ensuring that the PV generation system could operate in a safe state with zero thermal power,except for full shadow.In particular,the operating voltage at the junction of modes ? and ? is defined as optimal safe operating voltage.Since the system operates at this voltage,the thermal power can be sufficiently suppressed while allowing the output power generation maximized.On the other hand,in order to detect low-resistance defects induced hot spots in real time,this study proposed a transient scanning-based hybrid control method with the conventional P&O method.Since the transient scanning is involved in the P&O algorithm,the hot-spots can be accurately detected according to the ohmic reverse characteristic of the low resistance defects presented in operating mode ?.In addition,according to the shadow conditions may occur,the control modes corresponding to three main cases of no shadow,partially shaded normal module,and partially shaded defective module are respectively taken into consideration and designed.Then,the feasibility of this hybrid control method for online hot-spot detection and suppression was verified by a series of experimental trials,along with the effectiveness was verified by thermography.From the experimental results,it indicated that under the control of the hybrid algorithm,even if the defective module is partially shaded by the external shadow,the PV generation system can still operate in a safe state.However,this hybrid method can only be used to detect such hot-spots in a full PV string.That is,although this method can detect hot-spots and suppress the concerning nonzero thermal power in a full string,it is difficult to identify the detailed defective modules.Moreover,by shifting the operating point closer to an open circuit to suppress the generation of thermal power,which results in power loss.Therefore,as an application,this study also proposed a distributed control system based on the structure of cascaded DC-DC converters and its control method.Similarly,it has been experimentally verified that in the proposed distributed system,the defective modules can be quickly identified by scanning each module instantaneously.Besides,when compared with the hybrid method,the module-level hot-spot suppression can also reduce the unexpected power loss significantly,to a certain extent.Finally,a conclusion of this study was given.The low-resistance defects induced hot-spots can be accurately and effectively detected and suppressed in real time with both of these two control methods proposed in this study.It is worth noting that using the hybrid method can detect the hot-spots in a full string avoid adding any other external devices.Yet,the distributed control method can be regarded as an expansion of the former.In the original control system,the defective modules can be further quickly identified by installing power electronic devices individually.More significantly,in practical applications,it is essential to achieve a rescannable balance among user's requirements,cost,efficiency and other factors when making a choice between the two methods.