Hiren H. Patel

MATLAB-Based Modeling to Study the Effects of Partial Shading on PV Array Characteristics

The performance of a photovoltaic (PV) array is affected by temperature, solar insolation, shading, and array configuration. Often, the PV arrays get shadowed, completely or partially, by the passing clouds, neighboring buildings and towers, trees, and utility and telephone poles. The situation is of particular interest in case of large PV installations such as those used in distributed power generation schemes. Under partially shaded conditions, the PV characteristics get more complex with multiple peaks. Yet, it is very important to understand and predict them in order to extract the maximum possible power. This paper presents a MATLAB-based modeling and simulation scheme suitable for studying the I-V and P-V characteristics of a PV array under a nonuniform insolation due to partial shading. It can also be used for developing and evaluating new maximum power point tracking techniques, especially for partially shaded conditions. The proposed models conveniently interface with the models of power electronic converters, which is a very useful feature. It can also be used as a tool to study the effects of shading patterns on PV panels having different configurations. It is observed that, for a given number of PV modules, the array configuration (how many modules in series and how many in parallel) significantly affects the maximum available power under partially shaded conditions. This is another aspect to which the developed tool can be applied. The model has been experimentally validated and the usefulness of this research is highlighted with the help of several illustrations. The MATLAB code of the developed model is freely available for download.

Maximum Power Point Tracking Scheme for PV Systems Operating Under Partially Shaded Conditions

Current-voltage and power-voltage characteristics of large photovoltaic (PV) arrays under partially shaded conditions are characterized by multiple steps and peaks. This makes the tracking of the actual maximum power point (MPP) [global peak (GP)] a difficult task. In addition, most of the existing schemes are unable to extract maximum power from the PV array under these conditions. This paper proposes a novel algorithm to track the global power peak under partially shaded conditions. The formulation of the algorithm is based on several critical observations made out of an extensive study of the PV characteristics and the behavior of the global and local peaks under partially shaded conditions. The proposed algorithm works in conjunction with a DC-DC converter to track the GP. In order to accelerate the tracking speed, a feedforward control scheme for operating the DC-DC converter is also proposed, which uses the reference voltage information from the tracking algorithm to shift the operation toward the MPP. The tracking time with this controller is about one-tenth as compared to a conventional controller. All the observations and conclusions, including simulation and experimental results, are presented.

MPPT Scheme for a PV-Fed Single-Phase Single-Stage Grid-Connected Inverter Operating in CCM With Only One Current Sensor

The cost and efficiency of a photovoltaic (PV)-based grid-connected system depends upon the number of components and stages involved in the power conversion. This has led to the development of several single-stage configurations that can perform voltage transformation, maximum power point tracking (MPPT), inversion, and current shaping-all in one stage. Such configurations would usually require at least a couple of current and voltage sensors and a relatively complex control strategy. With a view to minimize the overall cost and control complexity, this paper presents a novel MPPT scheme with reduced number of sensors. The proposed scheme is applicable to any single-stage, single-phase grid-connected inverter operating in continuous conduction mode (CCM). The operation in CCM is desirable as it drastically reduces the stress on the components. Unlike other MPPT methods, which sense both PV arrays output current and voltage, only PV arrays output voltage is required to be sensed to implement MPPT. Only one current sensor is used for shaping the buck-boost inductor current as well as for MPPT. The information about power output of the array is obtained indirectly from arrays voltage and the inductor current amplitude. Detailed analysis and the flowchart of the algorithm for the proposed scheme are included. Simulation and experimental results are also presented to highlight the usefulness of the scheme.

A Single-Stage Single-Phase Transformer-Less Doubly Grounded Grid-Connected PV Interface

A transformer provides galvanic isolation and grounding of the photovoltaic (PV) array in a PV-fed grid-connected inverter. Inclusion of the transformer, however, may increase the cost and/or bulk of the system. To overcome this drawback, a single-phase, single-stage [no extra converter for voltage boost or maximum power point tracking (MPPT)], doubly grounded, transformer-less PV interface, based on the buck-boost principle, is presented. The configuration is compact and uses lesser components. Only one (undivided) PV source and one buck-boost inductor are used and shared between the two half cycles, which prevents asymmetrical operation and parameter mismatch problems. Total harmonic distortion and DC component of the current supplied to the grid is low, compared to existing topologies and conform to standards like IEEE 1547. A brief review of the existing, transformer-less, grid-connected inverter topologies is also included. It is demonstrated that, as compared to the split PV source topology, the proposed configuration is more effective in MPPT and array utilization. Design and analysis of the inverter in discontinuous conduction mode is carried out. Simulation and experimental results are presented.

Control of a Stand-Alone Inverter-Based Distributed Generation Source for Voltage Regulation and Harmonic Compensation

Operation of a distributed-generation (DG) source, producing sinusoidal output voltage and supplying nonlinear loads is characterized by a nonsinusoidal voltage at the point of common coupling (PCC). The situation gets particularly aggravated during the islanding mode, when the grid power is not available. This deteriorates the performance of other loads connected in parallel. A scheme based on nonsinusoidal pulsewidth-modulated control of a stand-alone inverter-based DG source is proposed to regulate and produce a sinusoidal voltage at the PCC. It is demonstrated that without any current sensor or compensating device, the individual harmonic components and THD of the supply voltage at the PCC can be reduced to the desired level. Simulations have been performed to study the viability of the proposed scheme and all of the results are presented.

PV Based Distributed Generation with Compensation Feature Under Unbalanced and Non-linear Load Conditions for a 3-ϕ, 4 Wire System

With the increased penetration of Distributed Resources (DR) into the grid, it is desirable and advantageous to tap additional benefits out of these resources besides their main function of active power generation. This paper presents a Photo Voltaic (PV) fed system which can not only feed active power into the grid, but can also compensate for unbalanced, non-linear loads connected to the power grid. The interface with the grid is realized using three, 1-ϕ inverters. The proposed system is capable of compensating the negative and zero sequence harmonics of the load currents and to achieve a zero neutral current. Extensive simulations have been performed to study the viability of the proposed system, including the maximum power point tracking (MPPT) performance and response of the configuration to step disturbances in linear load, unbalanced operation and sudden application of shading effect. All the results are presented.

Modified dual second-order generalised integrator FLL for synchronization of a distributed generator to a weak grid

Performance of grid-connected converter relies on fast and precise estimation of phase angle and amplitude of the fundamental component of grid voltage signal to guarantee the proper synchronization of DG in modern power systems. However, the extent to which the frequency is estimated is dependent on the type of grid voltage abnormalities and structure of the phase-locked loop or frequency locked loop control schemes. Among various control schemes, second-order generalized integrator based frequency-locked loop (SOGI-FLL) is reported to have the most promising performance even under grid voltage abnormalities like sag, swell, harmonics, imbalance, frequency variations etc. except the dc offset. This paper presents a modified dual second-order generalized integrator frequency-locked loop (MDSOGI-FLL) for three-phase systems to cope with the non-ideal three-phase grid voltages having all type of abnormalities including dc offset. The complexity in control scheme is almost the same as the standard dual SOGI-FLL, but the performance is enhanced. Simulation results show that the proposed MDSOGI-FLL is effective under all abnormal grid voltage conditions. The results are also compared with that obtained by DSOGI-FLL to justify the superior performance.

An effective power management strategy for photovoltaic based distributed generation

Grid-connected photovoltaic systems are usually designed to provide active powers. The statutory standards or the grid-code does not allow the injection of reactive power. However, many countries have relaxed or are in process of relaxing the standards. Hence, it is highly desirable that the capacity of the inverters to control reactive power be utilized. The reactive power that a PV can supply is constrained by the maximum (active) power that PV can deliver and changes with the environmental conditions. A reactive power sharing algorithm is proposed that not only ensures proper distribution of reactive power amongst the PV inverters but also is able to supply the maximum power generated by PV to the grid. It ensures that the inverters operate at nearly equal apparent power leading to nearly equal percentage utilization of the inverters. This results into uniform heating of the similar devices of the various inverters, thereby preventing the overloading of any inverter. The effectiveness of the algorithm over other conventional algorithms is displayed through the results obtained in MATLAB/Simulink.

Enhancing output power of PV array operating under non-uniform conditions

Current in series connected photovoltaic modules receiving different irradiance is limited by the module that receives the least irradiance. This reduces output power of PV array operating under partially shaded conditions as the module that receives higher irradiance do not operate at their maximum power point. Current compensation can be applied to all the modules to match the current of modules receiving different irradiance. With an aim to reduce the number of power converters required for current compensation, the paper presents an algorithm of current compensation for the Total-Cross-Tied (TCT) configuration. The algorithm is simple to implement and requires less sensors. Simulation results obtained in MATLAB/Simulink are included to demonstrate the effectiveness of the proposed approach.

Performance of shunt active power filter with DSOGI-FLL Under distorted grid voltage

Proliferation of non-linear loads and power-electronics based distributed generators has aggravated the issue of harmonic and its adverse effects on the electrical utility and various equipment connected to the utility. To mitigate the effects of the harmonics, it is advisable to eliminate them or to maintain them within the statutory limits. This objective can be effectively achieved using Shunt Active Power Filter (SAPF). However, it is a challenging task to control the SAPF to effectively compensate the harmonics, especially when the grid voltage is characterized by different abnormalities like presence of harmonics, imbalance in supply, imbalance in load, dc offset etc. The SAPFs effectiveness depends on the accurate generation of the reference current. Some of the conventional methods fail to generate the desired reference current under some of these grid abnormalities, ultimately leading to deviation from a balanced set of sinusoidal grid currents. The paper presents the technique to compute the reference compensating current using second-order generalized integrator (SOGI), frequency-locked loop (FLL) and instantaneous power theory (p-q theory) to control SAPF. This approach, which obtains the reference current from the voltage and current components derived through dual SOGI-FLL, can effectively and quickly compensate the harmonics even under unbalance conditions. The systems dynamic performance is tested for a three-phase three-wire network under different grid abnormalities and the results are reported and analyzed.