Voltage Regulation of Power Distribution System with Interconnected Distributed Generators

Abstract

In the recent years the electrical power utilities are undergoing rapid restructuring process worldwide. Indeed, with deregulation, advancement in technologies and concern about the environmental impacts, competition is particularly fostered in the generation side thus allowing increased interconnection of generating units to the utility networks. These generating sources are called as distributed generators (DG) and defined as the plant which is directly connected to distribution network and is not centrally planned and dispatched. 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. In order to achieve these benefits with large penetration of DG source in existing utility networks several technical problems are to be fronted such as voltage regulation, islanding of DG, degradation of system reliability, power quality problems, protection and stability of the network. These issues need to be resolved, to pave the way for a sustainable energy future based on a large share of DG and hence a lot of research effort is required. Among the above issues the voltage rise problems have been reported as the foremost concern against the connection of large amounts of distributed generators to mediumvoltage and low-voltage distribution networks. The distribution systems have been planned and designed for unidirectional power flow and operated at constant voltage levels. The connection of the large amount of DG systems to the utility may reverse the power flow resulting in voltage rise above the statutory limits. Present network design practice is to limit the generator capacity to the level at which the upper voltage limit is not exceeded with maximum generation and minimum load. This can lead to a reduction in connectable generation capacity, under utilization of appropriate generation sites. The conventional voltageregulation methods of distribution system are designed with unidirectional power flow in mind and are not going to be effective in presence of a significant number of DG systems. Thus there is a need to redesign these methods to take care of bidirectional power flow or new methods have to be developed to accommodate the large number of DG systems. Thus, development of new voltage control devices/schemes have the potential to revolutionize the control of distribution network. In this thesis steady state voltage rise problem in a distribution networks interconnected with DG is examined. Case studies are presented using simulation results to study the impact of location, magnitude and operating condition of the integrated DG. The impact of other factors such as voltage at the primary substation, distance from the primary substation, demand on the system, type of loads and loading conditions on voltage level of distribution system are also analyzed. A comprehensive study on voltage control in a distribution system by taking in to account a number of DG systems and capacitors under various conditions is also presented in this work. In order to prepare the distribution networks for larger penetration of DG systems, effective voltage regulation methods are required so as to keep the voltage levels within the limits. In this work sensitivity analysis and participation factor based approaches for voltage regulation of typical distribution system are presented. The formulation for determination of voltage sensitivity index for voltage control is also given. The effectiveness of the developed approaches for voltage regulation of typical IEEE 69 bus distribution is analyzed using case studies. The OLTC, switched capacitors and DG systems are considered to regulate the voltage level in the distribution system in this study. Both methods can be used to handle all types of radial distribution system structures regardless of the system size proficiently. Individual control of various voltage regulating device such as OLTC, shunt capacitor including DG in a distribution system may cause unnecessary operations, and consequentlywear, energy consumption as well as voltage disturbances. Thus coordination among these devices for effective voltage regulation can lead increase in number of DG interconnection to utility network. In this work a coordinated regulation method using genetic algorithm is developed. Genetic algorithm is utilized to determine the optimal amount of operation for individual voltage regulating device for given distribution network. The performance of the developed method is analyzed using simulation results through two case studies. The sample load conditions using time varying load profile is considered. The multiple voltage regulating devices such as OLTC, LRT, SC, SVC including DG system is considered in this study. The input load data is very critical for distribution system load flow studies. In most cases time varying load data is required for accurate load flow analysis such as hourly load day for a day or a month. The use of this large data for load flow analysis can complicate the solution approaches. Thus in most of the cases, for validation of voltage regulation methods few samples of data are considered instead of considering the entire load profile. This approach may not accurately represent the load variation of entire profile for load follow analysis. In this work a fuzzy clustering technique for load profile generations is presented. Load profile generation using fuzzy clustering can be more realistic case rather than considering sample load conditions from set of time varying data. Fuzzy clustering is used to find 3 prototypes of hourly load data for a day (24 vectors) instead of considering sample load conditions directly from profile. Load profile generation using fuzzy clustering can be more realistic case as it considers the similarity of variation for entire set of load data. The performance of coordinated voltage control method is analyzed using the load profile generated using fuzzy cluster in this work. Distribution systems are usually unbalanced due to unbalanced loading of the different phases. Besides industrial or domestic customers some distributed generators can also impose an unbalanced operation of electrical networks. Load flow analysis of balanced radial distribution systems will be inefficient to solve the unbalanced cases and the distributionsystems need to be analyzed on a three phase basis. In this thesis load flow solution for a unbalanced system using forward and back sweep method has been presented. The detailed modeling of various components of the distribution system for the unbalanced load flow solution is also given. The performance of the coordinated voltage regulation method using two Case studies using IEEE 13 and 25 bus unbalanced distribution is analyzed through simulation. The simulation results are reported to evaluate the effectiveness of the developed method in voltage regulation of unbalanced distribution system.