Sergiu Spataru

On the Perturb-and-Observe and Incremental Conductance MPPT Methods for PV Systems

This paper presents a detailed analysis of the two most well-known hill-climbing maximum power point tracking (MPPT) algorithms: the perturb-and-observe (P&O) and incremental conductance (INC). The purpose of the analysis is to clarify some common misconceptions in the literature regarding these two trackers, therefore helping the selection process for a suitable MPPT for both researchers and industry. The two methods are thoroughly analyzed both from a mathematical and practical implementation point of view. Their mathematical analysis reveals that there is no difference between the two. This has been confirmed by experimental tests according to the EN 50530 standard, resulting in a deviation between their efficiencies of 0.13% in dynamic and as low as 0.02% under static conditions. The results show that despite the common opinion in the literature, the P&O and INC are equivalent.

Stable and optimal fuzzy control of a laboratory Antilock Braking System

This paper discusse four new Takagi-Sugeno fuzzy controllers (T-S FCs) for the longitudinal slip control of an Antilock Braking System laboratory equipment. Two discrete-time dynamic Takagi-Sugeno fuzzy models of the controlled plant are derived based on the parameters in the consequents of the rules using the domains of the input variables, and doing the local linearization of the plant model. The original T-S FCs are designed by parallel distributed compensation to obtain the state feedback gain matrices in the consequents of the rules. Two T-S FCs are tuned by imposing relaxed stability conditions to the fuzzy control systems (FCSs) and the other two T-S FCs are tuned by the linear-quadratic regulator approach applied to each rule. Linear matrix inequalities are solved to guarantee the global stability of the FCSs. Real-time experimental results validate the original T-S FCs and design approaches.

Detection of increased series losses in PV arrays using Fuzzy Inference Systems

There are well-defined methods to measure the (increased) series resistance of PV panels in controlled laboratory conditions. However, the presence of various irradiance levels and partial shadows, in case of an outdoor installation, may affect the series resistance estimation. This paper focuses on the estimation of increased series resistance in outdoor conditions in various irradiance levels and partial shadow conditions. A diagnostic method that is able to detect an increase in series resistance based on fuzzy classifiers is presented and experimentally validated.

Photovoltaic array condition monitoring based on online regression of performance model

Photovoltaic (PV) system performance can be degraded by a series of factors affecting the PV generator, such as partial shadows, soiling, increased series resistance and shunting of the cells. This concern has led to greater interest in improving PV system operation and availability through automatic supervision and condition monitoring of the PV system components, especially for small PV installations, where no specialized personnel is present at the site. This work proposes a PV array condition monitoring system based on a PV array performance model. The system is parameterized online, using regression modeling, from PV array production, plane-of-array irradiance, and module temperature measurements, acquired during an initial learning phase of the system. After the model has been parameterized automatically, the condition monitoring system enters the normal operation phase, where the performance model is used to predict the power output of the PV array. Utilizing the predicted and measured PV array output power values, the condition monitoring system is able to detect power losses above 5%, occurring in the PV array.

Elucidating PID Degradation Mechanisms and In Situ Dark I–V Monitoring for Modeling Degradation Rate in CdTe Thin-Film Modules

A progression of potential-induced degradation (PID) mechanisms is observed in CdTe modules, which are dependent on the stress level and moisture ingress. This includes shunting, junction degradation, and two different manifestations of series resistance. The dark I-V method for in situ characterization of Pmax based on superposition was adapted for the thin-film modules undergoing PID in view of the degradation mechanisms observed. An exponential model based on module temperature and relative humidity (RH) was fit to the PID rate for multiple stress levels in chamber tests and validated by predicting the observed degradation of the module type in the field.

Power ramp limitation and frequency support in large scale PVPPs without storage

Photovoltaic (PV) power generation started to become a mature technology and large scale PV Power Plants (PVPPs) operating in Maximum Power Point Tracking (MPPT) are not a solution anymore. During changes in the meteorological conditions, PVPPs output is directly influenced creating high power ramps inducing short-term voltage and frequency instabilities into the power system. System operators in countries with increased PV penetration started to become more and more conservative regarding their operation creating strict Grid Codes (GCs). As a proven fact European Network of Transmission System Operators for Electricity (ENTSO-E) pushed the limit one step forward involving PVPPs to contribute with ancillary services even to frequency support. The goal of the paper is to present PVPPs with power ramp limitation and frequency support capabilities without involving any storage equipment. This is achieved by setting the PVPP under its MPP and ensuring a certain amount of reserve. The nature of the reserve has a dynamic character, being released during transients in irradiance and frequency. The entire components of the system are modeled in RSCAD and the performance evaluated on a Real Time Digital Simulator (RTDS).

Quantifying solar cell cracks in photovoltaic modules by electroluminescence imaging

This article proposes a method for quantifying the percentage of partially and totally disconnected solar cell cracks by analyzing electroluminescence images of the photovoltaic module taken under high- and low-current forward bias. The method is based on the analysis of the modules electroluminescence intensity distribution, applied at module and cell level. These concepts are demonstrated on a crystalline silicon photovoltaic module that was subjected to several rounds of mechanical loading and humidity-freeze cycling, causing increasing levels of solar cell cracks. The proposed method can be used as a diagnostic tool to rate cell damage or quality of modules after transportation. Moreover, the method can be automated and used in quality control for module manufacturers, installers, or as a diagnostic tool by plant operators and diagnostic service providers.

Effects of photovoltaic module soiling on glass surface resistance and potential-induced degradation

The sheet resistance of three soil types (Arizona road dust, soot, and sea salt) on glass were measured by the transmission line method as a function of relative humidity (RH) between 39% and 95% at 60°C. Sea salt yielded a 3.5 orders of magnitude decrease in resistance on the glass surface when the RH was increased over this RH range. Arizona road dust showed reduced sheet resistance at lower RH, but with less humidity sensitivity over the range tested. The soot sample did not show significant resistivity change compared to the unsoiled control. Photovoltaic modules with sea salt on their faces were step-stressed between 25% and 95% RH at 60°C applying -1000 V bias to the active cell circuit. Leakage current from the cell circuit to ground ranged between two and ten times higher than that of the unsoiled controls. Degradation rate of modules with salt on the surface increased with increasing RH and time.

Remote and centralized monitoring of PV power plants

This paper presents the concept and operating principles of a low-cost and flexible monitoring system for PV plants. Compared to classical solutions which can require dedicated hardware and/or specialized data logging systems, the monitoring system we propose allows parallel monitoring of PV plants with different architectures and locations by taking advantage of the intrinsic monitoring capabilities of the inverters and their internet connectivity. The backbone of the system is a software system capable of collecting production measurements and current-voltage (I-V) characteristic curve measurements from the inverters within each PV plant. The monitoring software stores the PV measurements in a data warehouse optimized for managing and data mining large amounts of data, from where it can be later visualized, analyzed and exported. By combining PV production measurements data with I-V curve measurements the diagnostic and condition monitoring capabilities of the PV system can be greatly enhanced. The practical implementation and operation of the monitoring system is demonstrated with a study case system deployed at Aalborg University.

Model-based fuzzy control solutions for a laboratory Antilock Braking System

This paper gives two original model-based fuzzy control solutions dedicated to the longitudinal slip control of Antilock Braking System laboratory equipment. The parallel distributed compensation leads to linear matrix inequalities which guarantee the global stability of the fuzzy control systems. Real-time experimental results validate the new fuzzy control solutions.