MODELLING AND ANALYSIS OF HYBRID MICROGRID SYSTEM

Abstract

Electrical practices for the power industry are enormously changing and those advancements will mark an evolution of new principles and techniques in the future, predominantly related to the planning and operation of the electricity generation and distribution. The proliferation of electricity requirements escalates the use of renewable energy sources (RESs) in the present and future energy technology. Therefore, the emphasis on the networks, which comprises a combination of RESs and their integration into the main grid has tremulously increased. RESs such as wind, photovoltaic, fuel cell have made a significant contribution to the world. Due to their eco-friendly nature, flexibility and worldwide availability, RESs have proved to be an attractive solution for designing modern grids. The proposed research work entitled “Modelling and Analysis of Hybrid Microgrid System” focuses on microgrid (MG) control to enhance its power quality performance. Microgrids are a crucial component of a modern power system and are capable of supplying growing energy needs as well as providing flexibility. Microgrids offer a platform to interface the RES and energy storage elements. The conceptualization of microgrid is based on the integration of several energy sources using power electronics equipment. Thus, the power electronic interface is a key feature in realizing the revolutionary concept of the microgrid. At the same time, the interfacing of RES with the grid through power electronic equipment also raise the concern about the reliable and stable operation. Various configurations of the power electronics converters such as rectifiers, inverters, bidirectional converters are integral elements of microgrid (MG). Power conditioning devices add reliability and functionality to the MG system. Reliability allows large penetration of RESs while functionality ensures the optimal usage of RES in an MG. However, power electronic converters also increase the system cost due to complex technical control and increased losses in the system. Increased interconnection of RES may lead to severe voltage distortions and voltage fluctuations. Furthermore, the contribution of nonlinear load in the power system is high and leads to power quality (PQ) problems such as harmonic injection in voltages and currents in the system. The continued presence of PQ problems deteriorates the quality of power at the user end which is highly undesirable, hence solutions to PQ problems are mandatory for a healthy power system. A promising solution for the mitigation of PQ problems is Shunt Active Power Filter (SAPF). SAPF is a shunt connected voltage source converter with a DC-link capacitor. It is capable of mitigating PQ issues such as voltage unbalancing, current harmonics and poor power factor. It also gives superior functional characteristics such as faster response, low cost, reduced size and it is also able to deliver reactive power efficiently and improves transient condition response. However, SAPF does not contribute in terms of the active power requirement of the load, so it must be integrated with the RESs as a cost-effective solution to mitigate PQ problems. With these considerations in the present study, three RESs, PV, wind and fuel cell are considered and modelled to frame an MG that can deliver good quality of supply at the consumer end. Further, MG is studied under the varying RESs generation, as RESs are not a constant source of power and their power generation directly dependent on the environmental conditions. Simulation results have been investigated under nonlinear load, unbalanced nonlinear load, linear load under varying environmental conditions using models developed in MATLAB/SIMULINK platform. Both grid-connected microgrid and standalone microgrid PQ issues are addressed. To develop grid-connected microgrid, different RES viz. photovoltaic, wind and fuel cell have been interfaced at a common DC-link and then with the grid through VSC. In a stand-alone microgrid, PV and wind are primary energy sources, FC and battery are secondary energy sources and electrolyzer as a dump load is used. Initially, detailed modelling and characteristics of interfaced RESs are discussed. Further, wind-photovoltaic-fuel cell powered grid-connected microgrid has been developed. The developed MG has been tested and analyzed using conventional control techniques of VSC such as synchronous reference frame theory (SRFT), v power balance theory (PBT) with the simulated models of RESs. Further, new control techniques for DC-link voltage control based on adaptive neuro-fuzzy inference system (ANFIS), hybrid-fuzzy-PI controller (H-FLC), Takagi-Sugeno Fuzzy Logic Controller (TSFLC) DC-link voltage controller and fuzzy controller have been developed, analyzed and compared with conventional PI-based DC-link voltage control scheme. New control techniques for VSC to improve the convergence rate and to achieve DC offset rejection have been designed and developed in this research work. These techniques have been implemented in grid-connected microgrid interfaced with wind power generation source, PV power generation system and fuel cell power generation. These techniques include hyperbolic tangent least mean square (H-LMS), adaptive neuro-fuzzy inference system least mean square (ANFIS-LMS), zero attracting quaternion least mean square (ZAQ-LMS) and modified shrinkage widely linear complex-valued LMS (MSWL-CLMS). PQ issues such as load unbalancing, current harmonics, reactive power compensation and power factor correction have been considered. Detailed simulation results have been recorded and analyzed for load changes and environmental variations. Subsequently, the hybrid microgrid integrated with electric vehicle (EV) load at DC-link is developed and analyzed using the self-tuning filter (STF) control technique of VSC. EV is in plug and play role, it can be charged through RESs and on the other hand, it allows the owner to sell EV energy to the grid for revenue generation. In an electrical power system, the presence of unbalanced grid voltage is also a major PQ problem. The grid voltage can be highly distorted due to large grid impedance. Double second-order generalized integrator (DSOGI) filter to extract positive sequence voltage from the grid voltage is proposed and for the reference currents extraction, self-tuning filter (STF) is implemented. The prime characteristic of the MG is its capability of the seamless transition from grid-connected mode to standalone mode and vice-versa. The designed MG is capable of going into standalone operation during the grid outage and exhibits seamless synchronization with the grid after removal of the grid fault. Synchronization operation has been carried out using Low-Pass Notch Filter PLL (LNP-PLL) based grid-synchronization control. Grid synchronization allows the practical implementation of such a system in the real-world on a large scale. Besides the modelling and analysis of grid-connected microgrid and its control, a standalone microgrid consisting of photovoltaic-wind power-fuel cell, battery and electrolyzer (dump load) has been developed. New power controllers and control strategy has been presented for the standalone system. Developed controllers are simple in design and based on the measurement of DC-link voltage and current. The proposed hybrid system is suitable to achieve power equilibrium through the developed controllers. Power-sharing is based on the regulation of DC-link voltage and state of charge in the battery. FC is capable to deliver power to load when power generation of PV and wind is less than the load requirement. Under all the aforementioned cases PQ issues for the presented microgrid have been mitigated successfully.