MODELLING, DESIGN AND DEVELOPMENT OF PV BASED MICROGRID

Abstract

In recent years, with the exhaustion of fossil fuels and increasing public awareness about the use of green energy, the renewable energy has gained popularity and is emerging as an important source of energy. Also, the electrical power grid is on the threshold of a paradigm shift from centralized power generation, transmission and massive electric grids to distributed generation (DG). DG basically uses small-scale generators, like photovoltaic (PV) panels, wind turbine, fuel cells, small and micro hydropower, diesel generator set, etc., and is confined to small distribution networks to produce power close to the end users. Renewable energy sources (RES) are the important constituents of DG and provide electricity with higher reliability and security and have fewer harmful environmental consequences than traditional power generators. With increased penetration of DGs into the traditional grid system, it is required to resolve the technical and operational problems viz. power quality, voltage instability, fault identification and clearing, etc. brought by the DG deployment. PV based microgrid may be one of the solutions to meet these challenges. A microgrid is a group of interconnected loads and distributed generators within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid. It can be connected and disconnected from the grid to enable it to operate either in grid-tied or standalone mode. Grid-tied PV based microgrid can be either single-stage or two-stage depending on the technical requirements. In single stage configuration, PV array is directly connected to a DC/AC converter whereas in two-stage configuration an additional DC/DC converter with maximum power point tracking (MPPT) capability is connected in between the PV array and DC/AC converter and provides the desired DC voltage to the inverter. This research work aims at modelling, design and development of a two-stage threephase grid-tied PV based microgrid. In order to predict the behaviour of the designed system under steady state as well as in the dynamic state, modelling of the overall system has been carried out. Steady state response of the PV based microgrid is studied from its mathematical model comprising of the governing equations of the designed vii

system. Characterization studies viz. sensitivity and reliability analysis are the performance indicators of any system. Accordingly, the sensitivity analysis of the designed system has been performed and the sensitivity functions of the major components, i.e. solar cell and converter have been developed. Also, component and system level reliability analysis have been performed for the system under consideration. In the present work, the two stages of power conversion consist of boost converter and grid interfacing inverter. The DC-DC converter is used to boost the output voltage of the PV array to the required DC link voltage level along with the functionality of tracking the maximum power obtained from PV array under varying irradiation and temperature. The PV inverter is used to convert the generated DC voltage to AC of required voltage and frequency and to maintain the power balance between DG, load and grid. The interfacing control algorithms are used to control the inverter for its efficient utilization and grid synchronization. Conventional control algorithms use feedback controller like proportional integral (PI) controller for DC-link voltage control. These controllers are not best suited for nonlinear systems like PV based microgrid as the overshoots and long settling time in their response are inevitable. In order to overcome the drawbacks of the conventional algorithms, an intelligent asymmetrical fuzzy logic (AFL) based interfacing control algorithm and feedforwardfeedback adaptive interfacing control algorithm are proposed and developed for the PV inverter. The proposed algorithms also improves the utilization of the proposed system by incorporating additional features of active power filter (APF), VAR generation, and load balancing in the inverter. Grid interconnection of PV based microgrid has the advantage of efficient utilization of generated power. But the technical requirements from the utility grid side need to be satisfied to ensure the safety of the operators and the reliability of the utility grid. According to IEEE Std 1547-2003, one such technical requirement of the grid interconnection is the response of the microgrid to islanding. This research work proposes a novel islanding detection algorithm based on the estimation and analysis of negative sequence components of the voltage (Vneg) at the point of common coupling (PCC). Wavelet packet transform (WPT) is used for the features extraction from Vneg viii

components. The binary tree classifier is used to discriminate between other disturbances and islanding condition. The proposed algorithm is capable of detecting islanding events even under the worst-case scenario, where the inverter output power is nearly equal to the local load consumption. Also, the proposed method is faster than the existing passive detection methods. A standalone PV system can be used efficiently and economically to feed household loads, the majority of which works on DC power, such as LED (Light Emitting Diodes) lights, BLDC (Brushless DC) drives, mobile phones, computers, televisions, etc. Standalone PV system feeding DC power directly to loads can be an attractive solution to locally utilize DC electricity with minimum distribution and conversion losses. This concept has recently resulted in a novel grid system known as DC nanogrid. A DC nanogrid supplies the residential and commercial loads which may operate on AC or DC voltage of different utilization levels. Interfacing such variety of loads and controlling power flow between these loads presents an interesting challenge. Multiple dedicated converters can serve the purpose, but they exhibit the problems of power flow coordination, low efficiency, higher component count, and the increased size of the system. The last objective of this research is to develop innovative multi-terminal voltage converters for renewable-energy applications. A PV based multi-terminal DC nanogrid is developed using dual-input single-output (DISO) and single-input dualoutput (SIDO) converter configurations with improved reliability and efficiency. The characterization studies of these converters such as sensitivity analysis and reliability analysis have been carried out. Also, the performance of the developed converter configurations are investigated using MATLAB along with Simulink toolbox. The SIDO converter configurations are experimentally validated using a 100W prototype, built and tested in the laboratory of DTU for practical applications. The research work presented in the thesis is expected to provide good exposure to design, development and control of the grid-tied PV based microgrid and DC nanogrid