Dinâmica durante faltas de microturbinas a gás conectadas ao sistema de distribuição de energia elétrica

Abstract

The growing number of connections to electric distribution systems have increased the need for modernization of conventional grids. The integration of distributed generation (DG) into each systems requires a complete understanding of it’s impact on distribution feeders. This work deals with the dynamic characterization of a distributed generation system based on gas microturbines (MTG) during voltage sags transients caused by eletrical faults in the grid. The caracterization considers the influence of feeder’s characteristics and the system operating point and observes electrical and mechanical variables of the generation system evaluated in cases of electrical fault in the grid. First approximated equations are derived to represent the feedback loops involved in the microturbine control system which includes rotor speed, DC-link voltage and generator side converter current controls. Then, simulations of the complete nonlinear model are performed for diferent fault and operation conditions of the system. This thesis also deals with LVRT (low voltage ride through) capability from this generation system. The low LVRT capability is verified by evaluating the critical time taken for the direct current (DC) link voltage of the back-to-back converter to reach an overvoltage limit during the grid’s fault. Aiming the expansion of the LVRT capability of the system, a control strategy is proposed, which acts on the speed control loop of the synchronous generator during momentary voltage sags in the grid preventing DC-link overvoltage. This solution allows the continuous operation under voltage sags in the grid without involving energy dissipation in the DC link. Simulation results are presented for a 30kW generation system, showing suitable dynamics and effectiveness in providing LVRT capability to the system in all cases tested, avoiding overvoltage in the DC link.