Publication
The use of gas turbines in the power generation industry has been increasing. Their demand represents more than 50% of the world market of thermal power plants. Electric power generators using gas turbines as power sources are connected to the turbines through a mechanical gearbox, in order to adapt their synchronous speed to the optimal rotation speed of the turbine, which is very often much higher than the synchronous speed. However, due to direct network connection, the generator speed cannot be variable: it is imposed by the network and constant. To overcome this problem, we propose to replace the mechanical gearbox by a flexible electronic solution which offers a high efficiency. Using this approach, the turbine is directly connected to the synchronous generator, which is connected to the grid through an indirect static frequency converter. However, this type of converter is not common in this application because of very high switching losses due to the high frequency of the PWM technique used for its control. In this paper, a new control strategy is proposed for the multi-level converter, characterized by its high efficiency due to the use of square-wave modulation. The main advantage of this mode is the quasi absence of switching losses. In this mode, only the frequency can be varied between the input and the output voltage, but their magnitudes are not freely controllable. A voltage magnitude adaptation can be done by the generator's excitation. The produced active and reactive power can be controlled by the generator excitation as well as both the angle shift between the generator and rectifier voltages and between the inverter and network voltages. Simulation and experimental results for different operating points highlight the capabilities of the proposed control strategy. These include the ability to operate with unity power factor and better current quality. (C) 2016 The Authors. Published by Elsevier Ltd.
Fernando Porté Agel, Guiyue Duan, Daniele Gattari