Modeling and Performance Analysis of Microgrid with Wind and Photovoltaic Based Distributed Generation Systems

Abstract

The advancement in technology, environmental concerns, emerging power markets and deregulation of electric power utilities are leading to increased interconnection of distributed generators (DGs) to the utility system. The different types of DGs, such as microturbines and fuel cells in addition to the traditional solar and wind power are creating significant new opportunities. The benefits of interconnection of these generators are improved reliability, power quality, efficiency, alleviation of system constraints along with the environmental benefits. Due to the growing momentum towards sustainable energy developments and considering these benefits it is expected that a large number of DG systems will be interconnected to the power system in the coming years. Interconnecting large number of small DG systems with diverse characteristics to low voltage network causes many problems. The microgrid is a section of network operating in a systematic way, comprising sufficient generating resources in the autonomous or grid connected mode in an efficient and controlled way. The microgrid has more control flexibilities and larger power capacity to fulfil power quality requirements and system reliability. Along with generation sources microgrid also consists of storage devices such as flywheels, batteries and super capacitors. The wind and photovoltaic (PV) power generation are two of the most promising renewable energy technologies. Hybridizing wind and solar power sources together with storage batteries to cover the periods of time without sun or wind provides a realistic form of power generation. Currently variable speed drives (VSD), lighting, batteries and electronics constitute major part of the load. The DC power can be supplied to these loads from microgrid system with usage of minimum converters with decreased losses. In this research work, a microgrid with wind and PV as the energy resources with single 3 phase inverter considering both DC and AC loads, which can reduce the multiple conversion stages has been implemented. Both wind generator and PV panels are controlled to operate at their maximum power point. The wind power system presented in this work uses the permanent magnet synchronous generator (PMSG), because of its property of self excitation, which allows operation at a high power factor and high efficiency. It also results in smaller size, minimum weight and higherii torque to size ratio. This work mainly focuses on mathematical modeling, control schemes for operation of microgrid with energy storage devices for both isolated and grid connected mode under various generation and load conditions. Based on the dynamic component models, a simulation model for the microgrid system has been implemented in the Matlab/Simulink environment. In this work, the load following performance of microgrid system is studied in an isolated mode of operation. The microgrid model with integration of wind and PV energy system with battery energy storage devices has been implemented. The battery is thus controlled to provide the deficit power when the combined wind and PV energy sources cannot meet the net power demand. All three energy systems are connected in parallel to a common DC bus line through three different DC/DC converters. The performance study is analyzed with consideration of DC loads, nonlinear and induction motor loads for variable nature of the individual DG source. In this thesis the simulation results for evaluation of the performance of the microgrid system in grid interconnected mode of operation using case studies are also presented. For grid integrated microgrid system, grid behaves as backup energy source. The overall power management strategy for coordinating the power flows among the different energy sources is presented in the thesis. The results show that the overall power management strategy is effective and the power flows among the different energy sources and the load demand is balanced successfully. Also the performance of the microgrid system is studied under grid perturbations conditions. The common grid perturbations considered in this study are balanced voltage dip, voltage unbalance and harmonic distortions. The simulation result reported in this work also shows that, the performance of the model presented is not affected by the grid disturbances considered. The last part of the dissertation focuses on power smoothing of the grid integrated microgrid system using ultracapacitors, also with combination of battery and ultracapacitors. The battery performance can be improved in terms of the power density by combining ultracapacitors with batteries which are typically low power devices. The power obtained from wind and PV system varies with the changes in weather conditions. Especially in weak power systems with large penetration of intermittent renewable energy (RE) generation sources into the utility grid, mayiii introduce adverse effects on the utility grid. To compensate or absorb the difference between the generated power and the required grid power, energy storage systems are used. Most of the technical literatures discuss the control performance of battery storage devices used for power smoothing of renewable power sources such as wind or PV power system. However in this research, the control schemes have been developed for power smoothing of the grid integrated microgrid system using ultracapacitors and combination of battery and ultracapacitors. In order to observe the real-time performance of energy storage system in smoothing the output power fluctuations, the practical site data for wind speed and solar irradiation are considered. The final result of proposed control strategy is a smooth and ramp controlled power output that can be injected to the grid.