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

Abstract

The deregulation of electric power utilities, advancement in technology, environmental concerns and emerging power markets are leading to increased interconnection of distributed generators to the utility system. Various new types of distributed generator systems, such as microturbines and fuel cells in addition to the more traditional solar and wind power are creating significant new opportunities for the integration of diverse DG systems to the utility. Inter connection of these generators will offer a number of benefits such as improved reliability, power quality, efficiency, alleviation of system constraints along with the environmental benefits. With these benefits and due to the growing momentum towards sustainable energy developments, it is expected that a large number of DG systems will be interconnected to the power system in the coming years. Unlike centralized power plants, the DG units are directly connected to the distribution system; most often at the customer end. The existing distribution networks are designed and operated in radial configuration with unidirectional power flow from centralized generating station to customers. The increase in interconnection of DG to utility networks can lead to reverse power flow violating fundamental assumption in their design. This creates complexity in operation and control of existing distribution networks and offers many technical challenges for successful introduction of DG systems. Some of the technical issues are islanding of DG, voltage regulation, protection and stability of the network. Some of the solutions to these problems include designing of standard interface control for individual DG systems by taking care of their diverse characteristics, finding new ways to/or install and control these DG systems. In this regard a particularly promising and emerging solution is the microgrid concept of integrate large number of distributed generation resources to the grid. The microgrid is a systematic way of operating a section of network, comprising sufficient generating resources in grid connected or autonomous mode in an efficient, deliberate and controlled way. The microgrid has larger power capacity and more control flexibilities to fulfill the system reliability and power quality requirements, in addition to all the inherited advantages of a single DG system. Along with generation sources microgrid also consists of storage devices such as flywheels, batteries and super capacitors. The operation and conitrol of microgrid can be very challenging due to diverse characteristics of DG systems and storage devices integrated in the microgrid and also presence of ac and dc loads. Important aspect of the microgrid is the possible combination of renewable energy sources with inevitable uncertainty in the output such as wind and solar with reliable energy sources like fuel cell and microturbine systems along with storage devices. In this regard promising configuration of the microgrid is the combination of wind and fuel cell based DG system along with storage device. In this work solid oxide fuel cell (SOFC) and wind based microgrid system along with battery, UC and electrolyzer as energy storage devices has been implemented. The dynamic models of SOFC, wind, ultra capacitor(UC), battery and electrolyzer are presented. The wind system considered in this work employs the permanent magnet synchronous generator (PMSG). The detailed modeling of the power electronic converters for interfacing the generation and storage devices considered in the microgrid are also given. In this work SOFC, wind system, UC, battery and dc loads are integrated at common dc-link to reduces multiple energy conversion losses. The battery and UC are integrated at common dc-link through bidirectional dc-dc converters. The electrolyzer and dc loads are connected to the dc link through the buck converter interface. The modelling of the control schemes to coordinate and manage the operation of the DG systems and storage devices in the microgrid are also presented in this work. The presented microgrid model is integrated to the utility network through the 3 phase voltage source inverter along with necessary control scheme. Matlab/Simulink/SimpowerSystem environment is used in this work implementing the microgrid system and study the performance of the same in grid connected and isolated mode of operation. The performance of the microgrid in grid connected mode considering only wind system with and without storage device is presented in this work. The maximum power point tracking (MPPT) method is employed in the wind system to extract maximum power under different wind speed conditions. The performance results obtained through the simulation show the output power fluctuation due to variation in the wind speed. The wind system performance has been studied with battery and battery-UC coordinated operation. The freiiquent charge and discharge cycle of battery due to change in wind speed is also shown through the simulation results, which reduces life of the battery. In this work the battery instant discharge is controlled by using ultracapacitor for varying wind speed by coordinated operation of battery and UC. Dump load is also used to maintain the power balance between generation and demand in this work. The effectiveness of the coordinated control of UC and battery for wind system in microgrid is studied through simulation results. The wind power output variation can be effectively compensated using the SOFC along with the ultracapacitor and electrolyser. Thus microgrid with SOFC, wind system, ultracapacitor and electrolyzer is implemented in this work. The electrolyser considered in this work can effectively utilize the wind power for producing hydrogen during increased wind speed. The hydrogen is required for fuel cell operation. The simulation results are presented and analysed to study the performance of the microgrid with above configuration. The simulation results show the effectiveness ultra capacitor in compensating slow dynamic response of the fuel cell. The ultracapacitor instant supplies the any increase in load demand while SOFC gradually increase the power delivery. The inherent slow dynamic response of the fuel cell can adversely effect the load. The effective control and coordination of SOFC and UC can supplement each other and avoid adverse effect on the load. The simulation are also presented to show the effective utilization of increased wind power output during high wind speed condition to produce hydrogen through electrolyser. The performance of ac/dc microgrid based on SOFC, wind, ultracapacitor, battery and electrolyzer is studied in this work. In this study two separate DC loads one at 48V and another 380V is considered apart from the ac load at the grid side. The control schemes are implemented to coordinate the operation of energy sources and storage devices to supply the load efficiently. The performance of the microgrid is studied through simulation results both in grid connected and islanded mode of operation. Typical case studies are presented in this research work considering various scenarios of feeding ac and dc loads in the microgrid. The simulation results presented in the case studies highlight the changes in SOFC output power due to change in wind speed, limited options to charge the storage devices,load control during less generation and charging iiiof UC by battery. The simulation results show the effective operation of UC in compensating slow response of SOFC while controlling battery discharge rate. The results presented also show the performance of battery in supplying minimum load when SOFC is disconnected and also generation of hydrogen using electrolyser during more generation by wind and less load condition.