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Advanced Brushless Excitation System with Dynamic De-Excitation Capability

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Brushless excitation system of synchronous generators suffers from a poor dynamic de-excitation which is highly required during a load rejection. This is mainly due to the rotating diode bridge that cannot quickly de-excite the generator field winding by negative excitation voltages. This paper presents an advanced excitation system composed by the conventional brushless excitation structure controlled by a positive voltage regulator, and a quick de-excitation structure associated with a negative controller. The dynamic control of the de-excitation structure is studied using two negative controllers: the first one is a PID controller, and the second one is a hysteresis comparator based on the monitoring of the generator voltage level. Nevertheless, the positive controller is designed using the H∞ control law. The complete system stability, in the presence of two controllers, is ensured by the design of a positive H∞ controller that considers the dynamic operation of the de-excitation structure as a perturbation to be rejected. In this paper, the influence of the de-excitation structure response time on the voltage regulation performance is also analyzed. By using a validated simulator of an 11.5 kVA generator, a comparison between the proposed excitation system and the conventional one is performed. The results clearly show, for different load conditions, the advantages of the new excitation system by reducing the generator voltage overshoot and response time, during a sudden load rejection or after a fault detection. In addition, the study shows that the negative PID controller has better de-excitation behavior than the hysteresis comparator.
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Excitation System; Synchronous Generator; Brushless Excitation; Voltage Regulation; H∞ Controls; Feedback Control Systems

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