GRID INTERCONNECTION AND ISLANDING OPERATION OF 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 ofDG 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 ofDG 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 and finding new design for distribution system. These issues need to be resolved, to pave the way for a sustainable energy future based on a large share of DG. Hence a lot of research effort is required. This thesis deals with some of these issues. The present trend in DG technology is towards smaller DG system, with capacity less than 500 kW. An excellent example is a new and fast growing microturbine generation system (MTG). These generation systems are more reliable, have higher operating efficiency and ultra low emission levels. They can operate on multi-fuels and are proving to be a supplement to i traditional forms of power generation whether it is stand alone, mobile, remote or interconnected with the utility applications. Microturbines rotate at a very high speed in the range of 60,000 to 1, 20,000 rpm and uses permanent magnet synchronous machine to generate electric power. These generators need power electronic converters for interface to the grid. Modeling of these DG systems is a significant challenge mainly due to the power electronic interface and control. In this thesis, a simulation model of a single shaft MTG system suitable for grid connection is developed in Matlab/simulink. The MTG model consists of models of microturbine, permanent magnet synchronous machine and power electronic converter interface with control. The model considers bidirectional power flow between the grid and MTG system using back to back voltage source converter topology. This avoids the need for separate starting arrangements to launch the microturbine. The PQ converter control strategy for grid connected mode of operation has been developed using slow outer loop DC link voltage and fast inner loop current control structure. Through simulation, performance of the developed model has been tested by connecting it to distribution network. A 480 V, 60 Hz distribution network is used for the study. The performance of the model is studied under various grid disturbances such as voltage unbalance, harmonic pollution etc. The phase locked loop (PLL) structure used in the converter control provides accurate estimation of the phase angle even under voltage unbalance condition. The simulation results show that the developed model performance is not affected by the grid disturbances considered in this study and has the ability to adjust the supply depending on the commanded power, within MTG system rating. One of the major concerns in operating DG in grid connected mode is the possibility of islanding of DGdue to grid disturbances such as network faults. Islanding occurs when a portion of the distribution system becomes electrically isolated from the remaining part of the power system, yet continues to be energized by the distributed generators connected to the isolated subsystem. The present practice is to disconnect DG whenever islanding takes place for safety and security reasons. But there is an increasing trend to operate DG in intentional islanding mode, to provide continuous and reliable powerto customers during grid outages. As a result the IEEE Std. 1547 -2003 states, one of its tasks for future consideration is to implement the intentional islanding of DG systems. Thus there is a need for seamless switching of the DG n operation between grid connected and islanding modes without down time, during the outages of the utility. This is necessary mainly for power electronic based converter DG systems. In this thesis an interface control required for intentional islanding operation of MTG system is developed. A novel scheme for seamless transfer of MTG system operation between grid connected and islanding mode and vice versa is proposed, using the estimated phase angle error obtained by phase locked loop (PLL). The presented scheme consists of islanding detection and re-closure algorithm. The devised islanding detection technique can accurately detect the grid disturbance and transfer the DG interface control to islanding mode. The presented re-closure method is an improvement over the existing method due to the fact that, it does not require the system to de-energize before re-synchronizing and reconnecting to the utility. The presented scheme does not negatively effect the DG or utility operations and can work even under matching DG and load power ratings. The developed scheme has been tested through simulation, to study the performance of the developed model of MTG system in grid connected and intentional islanding modes. Interconnecting large number of small DG systems with diverse characteristics to low voltage network causes many problems. The emerging microgrid concept is an effective solution to integrate these micro DG sources. The microgrid concept assumes a cluster of loads and micro-generation sources together with storage devices (flywheels, energy capacitors and batteries), operating as a single controllable module of a distribution system. Such systems can operate in parallel with the utility network or in an islanded mode. To the customers, the microgrid is designed to meet their special needs and provide additional benefits such as improved power quality and reliability, increased efficiency through co-generation and local voltage support. The operation and control of a wide range of DG systems in microgrid has a lot of research potential. In this thesis, dynamic model of a typical microgrid consisting of MTG system, a synchronous generator based DG (diesel generator) and wind generation system is presented. The simulation results show that, operation of MTG system as a motor at the starting and later as generator does not cause any disturbance in the microgrid. The performance of converter based MTG unit, synchronous DG and induction generator based wind system are studied both in grid iii connected and in islanding modes. One of the important features of microgrid is its islanding mode of operation during disturbance in the utility. Both planned and unplanned grid outage conditions are considered to study the performance of the microgrid. A critical factor in islanded operation of microgrid is to regulate its voltage and frequency and to ensure proper load sharing among the various sources. The simulation results show that converter based DG system acts fast during islanding mode to control the voltage and frequency variations. The studies also indicate the need for storage devices and load shedding for controlling the fast and long frequency deviations. Another important aspect in interconnecting large number of generators to distribution network is the rise in the steady state voltage level due to reverse power flow. Existing networks are designed to provide constant voltage supply to the customers with in the statutory limit. Present practice of limiting the generation capacity to control the voltage rise, leads to under utilization of DG units. The conventional voltage regulation methods of distribution system are designed with unidirectional power flow in mind. Thus there is a need to modify these methods to take care of bidirectional power flow or new methods have to be developed to accommodate the large number of DG systems. In this context, it is necessary to explore the control of DG reactivepower output for the regulationof the distribution systemvoltage. In this thesis, steady state voltage rise problem in a distribution system due to connection of synchronous basedDGsystem is analyzed. A combined method using conventional automatic voltage and power factor control with heuristic rule set, for excitation control of the synchronous based DG system is discussed. The simulation results show that the combined method gives betterperformance compared to automatic power factor control method. Also, a new fuzzy logic based excitation power factor control method for multi DG environment has been devised. In this method, individual DG systems co-ordinate in voltage regulation of the distribution network, based on their participation factor. The participation factor for each DG system is determined from voltage sensitivity analysis and is used for obtaining reference voltage. The fuzzy logic controller determines the excitation power factor based on the error between the reference voltage and terminal voltage of the DG. The simulation results on a test system show that the proposed method gives better performance compared to the method without coordination between individual DG systems.