Pallavi Bharadwaj

Sequential Optimization for PV Panel Parameter Estimation

The output of a solar photovoltaic (PV) system can be predicted using a parameterized model. The parameters can be obtained from the datasheet of the panel or from the measured current-voltage (I-V) characteristics. Measurement-based parameter estimation is shown to provide better results, as compared with the datasheet-based method. This effect magnifies for older panels due to the deterioration of panel characteristics with time, leading to a deviation from the datasheet specifications. Least-squares minimization is applied on the measured data using the Levenberg-Marquardt algorithm (LMA), trust region reflective Newton, and steepest descent optimization, which are compared on the basis of accuracy and speed. Low computational complexity and high parameter accuracy is ensured by introducing a novel sequential parameter estimation, in which two of the parameters are analytically evaluated, and a separate sequential evaluation of the shunt resistance parameter is performed. The predicted maximum power from the proposed method matches the experimental measurements with high accuracy.

Direct duty ratio controlled MPPT algorithm for boost converter in continuous and discontinuous modes of operation

Demand of increased lifetime, compact size and reduced cost of PV systems has led to the incorporation of LCL filters in the boost converter of a grid connected PV system. Additional filtering offered at the input by LCL filter reduces the inductance of boost converter. This calls for an algorithm which can track the maximum power in both discontinuous (DCM) and continuous conduction modes (CCM) of boost converter operation accurately. Direct duty ratio based perturb and observe algorithm for MPPT has been implemented in real time using VHDL based FPGA. The sensitivity of the algorithm to DCM and CCM operation of converter is analysed. It is shown that the duty ratio increment used in the MPPT algorithm should be evaluated based on the sensitivity factor. The direct duty ratio based approach is found to work well with both modes of operation, even with mode transitions.

High performance buck-boost converter based PV characterisation set-up

Photovoltaic panel characterisation finds extensive use in output prediction, efficiency estimation and defect detection in photovoltaic (PV) industry. Out of the various characterisation methods available, the use of switched mode power converters is one of the most attractive solutions, owing to high speed, low power loss and additional maximum power point tracking ability. Duty ratio variation for the controlled switch, from zero to one, leads to complete current-voltage curve tracing from open circuit to short circuit condition in case of a buck-boost converter. Therefore an IGBT based buck-boost converter is shown to work as a PWM controlled electronic load for the PV array characterisation. An input filter is further added to prevent the injection of high frequency current into the PV source. This filter is designed with a high resonance frequency which ensures small filter size as well as high speed characterisation of the PV array.

Design, fabrication and evaluation of solar irradiation meter

Measurement of solar irradiation finds extensive use in photovoltaic(PV) panel characterisation and efficiency estimation. A solar cell based light irradiation meter provides better dynamic performance and is cost effective as compared to a thermopile based pyranometer. To predict the output of a PV panel accurately, it is important to match the spectral response of solar cell sensor with PV panel. Monocrystalline and polycrystalline silicon solar cell based irradiation meters have been developed in laboratory. Their relative performance is evaluated under different irradiation and temperature conditions and calibrated against standard commercial solar irradiation meter with good results.

Design and fabrication of switching characterization set-up for GaN FETs

Wide band gap devices offer significant advantages such as high power density, fast switching and high efficiency. It is important to understand the trade-off involved in switching loss, conduction loss and reverse conduction characteristics for power converter design. Switching characterisation is essential for understanding the behaviour of new enhancement mode Gallium Nitride (GaN) power transistors that are available today. A theoretical method of switching loss estimation is presented in this work along with the development of a double pulse tester (DPT)-board for switching measurements. Careful layout design and measurement methodology is necessary for low measurement noise due to circuit parasitics. The designed DPT board has additional features for synchronous operation, short circuit test and variable temperature measurements. Additionally, the design of an air-core toroidal cage coil inductor is discussed, which ensures non saturated inductive switching of the device, even at high pulsed currents.

Hybrid synchronous DC-DC buck power converter using Si and GaN transistors

Usage of power converters in a vast variety of equipments from cellphones, laptops to automobiles, aeroplanes and satellites is increasing. Different synchronous buck converter configurations are considered in this work using Silicon (Si) devices and Gallium Nitride (GaN) devices of equal current ratings. The performance of these converters in terms of power losses and overall efficiency are compared. It is shown that the usage of GaN devices instead of Si devices reduces the power losses and improves the overall efficiency of the power converter. A hybrid synchronous buck converter topology consisting of one Si device and one GaN device is shown to have the lowest rated power loss for switches with similar ratings. The usage of GaN device as active switch and the Si device as synchronous diode is beneficial and has the highest efficiency among the possible converter configurations with an improvement in efficiency by 3–5% for rated conditions.