NEGATIVE FIELD FORCING ON ALTERNATOR
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Joined: Sep 2010
29-12-2010, 04:58 PM
Prabhu sankaranarayanan.G., Venkatesan.S
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Deregulation of the electric utility has created unprecedented opportunities for the growth in the market for alternator. In order to extract the maximum benefit from this situation improvement must be made in the control and protection features incorporated in alternator. The automatic voltage regulator has a key role to improve alternator performance. This project and implimentation investigates the current and voltage characteristics of the alternator at the times of load acceptance and rejection and introduces proposed AVR excitation system. This project and implimentation uses instantaneous output voltage information in order to get a fast dynamic response when a sudden load changes. In this technique to replace phase controlled rectifier with a PWM controlled rectifier and the dynamic performance can be achieved by PID controller.
The crux of this project and implimentation lies in the area of field forcing on the rotor of the alternator. At present control systems are able to force the field upwards when accepting a load. The theory behind this technique is quite simple. When a load is applied, the alternator output voltage will drop unless action is taken. To regulate the voltage an automatic voltage regulator is used to increase the voltage supplied to the DC exciter stator winding. This will subsequently increase the current in the rotor of the alternator and the output voltage will then rise in accordance with the load. The present method works well for load acceptance however the problem becomes apparent upon observation of load rejection using the same AVR. When the load is suddenly removed the field current cannot be forced downward therefore the current will decay in accordance with the time constant of the machine.
This is quite slow in comparison to load acceptance and can be seen quite clearly in figure 1. Notice that the voltage waveform during load acceptance returns to the nominal value much quicker than for load rejection. This project and implimentation will investigate this problem.