INVESTIGATIONS ON CURRENT CONTROL DISTRIBUTED GENERATION SYSTEMS

Abstract

Modern power system is envisaged to incorporate microgrids in the power grids to realize efficiently flexible and reliable flow of power by effectively utilizing renewable energy sources and storage systems. The microgrids is often required to operate both in grid connected and islanded mode. Typically in electrical network alternators coupled with front end transformers act as voltage source, and subject to critical control for its entry and exit from the system. Moreover, the control of these alternators is based on the reaction of the grid in terms of frequency depicting the power fluctuations and reactive power support to sustain terminal voltage constant. Such control become obvious due to rotating element in the alternator which retards the speed and in-turn the frequency of the generated supply on load, and increase for light load conditions. But Voltage Source Converters (VSCs) being static device offer no indication in frequency change upon different loading conditions. Moreover, the response of VSC is too fast, therefore, controlling it based on reaction from the grid would cause large transients. Voltage control, identical to conventional Power System based on droop characteristics has been prescribed by many researchers, but, one due to vicinity of sources (with very low impedance between two micro sources) and estimation of power angle, there exist a large chance of circulating current which may have cascading effect. The best alternative for such micro sources in a micro grid is through current control. This thesis investigates current control schemes for operation of micro source in grid connected and islanded mode. Typically, a master distributed generation (DG) unit and current controlled slave DGs to share the load as per their rating, and absorbs the transients arising from load perturbations and DG source switching is considered for investigation. Master DG is operated in current control while in grid connected operation and is operated with PWM switching to present a voltage source for slave DGs which are always operated in direct current control mode either under central command generating references for different DGs based on sensed voltage and by decentralized control by sensing the current at different nodes. Simulated results under MATLAB environment are presented to validate the effectiveness of the control scheme. Both load perturbations and outage of DGs is incorporated while accessing the performance evaluation of the proposed control scheme, both in grid connected and islanded mode.