INDUCTION GENERATOR CONTROLLER TECHNOLOGY
The controller may be broadly divided into three sections, namely, the power section, sensing circuits, and the control section. Power transistors using IGBT’s or MOSFET’s are used in the power section of the generator controller in a conventional multi-phase configuration, the number of phases being the same as the number of phases in the generator winding. Anti-parallel diodes are connected across each of the transistor.
The DC rail is connected to a power capacitor. An additional power inverter is used when an AC output at a constant frequency such as 60 or 50 Hz. is required. Sensing of currents and voltages is provided at the load as well as in the power section of the controller. In addition, the speed of the shaft is measured. All the parameters sensed by the sensing circuits are conditioned by filtering and digitizing as required. The control section receives the information provided by the sensors.
The parametric model of the generator is incorporated in the control section. In conjunction with a PID control algorithm, appropriate switching commands for the power transistors are generated in the control section. This creates the necessary frequency and amplitude of the excitation currents that flow in the induction generator windings and are induced into the squirrel cage rotor. The control section also includes protective functions such as over-current, over-voltage, and over-temperature protection circuitry. Figure 10 shows the controller in a block diagram.
INDUCTION GENERATOR CONTROLLER BENEFITS
When compared to PM and SR generator controllers, induction generator controller offers the following benefits:
i) Sensing: The control of induction generator slip requires precise measurement of speed.
On the other hand, the control of SR generator requires precise measurement of the rotor position. This is a much more difficult task to accomplish than the measurement of speed.
ii) Switching and control speed: For the SR generator, the operating frequency is extremely
high, in the range of 6 kHz. at 60000 RPM. This requires high speed switching of power transistors. The switching commands also must be provided at a high rate. For the induction generator, the operating frequency is in the 1 kHz. to 2 kHz. range at 60000 RPM depending upon whether 2 pole or 4 pole generator design is selected. The switching rate for the power transistors can be lowered in a reasonable range.
iii) Power Section Sizing: In the case of PM generators, due to the wide variation in the
voltage output, complexities are introduced in the controller requiring voltage boost mechanisms. The power electronic components must function at high stress levels. In the SR generator controller, high rates of change of currents and voltages result in high stress levels for the power electronic devices. The induction generator has a well-regulated sinusoidal output that can be conditioned without using highly stressed electronic components.
Overall it is believed that the controller for the induction generator is more robust, smaller in size, and cost less than the controller for PM or SR generators in the power range under consideration.
STATUS OF CURRENT TECHNOLOGY
Electrodynamics Associates, Inc. is currently developing a 125-200 kW induction generator to operate at 62000 RPM on an SBIR Phase II contract from AFRL/WPAFB, Dayton, OH. The generator is an air-cooled design. An identical machine has built to operate as a motor. The generator and the motor are mounted on a base plate and coupled together. An optical speed counter is attached at one shaft extension. Figure 1 1 shows the photograph of this assembly.
Controllers for both the generator and the motor have been developed for the test purposes. The control functions are embedded in a software model and the PC in the loop system using MathWorksTM software packages is used. The controller set up is shown in Figure 12.
At the time of writing this paper, the motor generator test set up is operational and tests have been completed to 67 kW power output from the generator at 24000 RPM. The generated power on the dc bus is fed back into the motor, so that only the losses in the motor generator set are provided from the utility bus. Tests are continuing at higher speeds to demonstrate the rated power by the end of the current calendar year. During the next phase of this project, improvements in the controller are planned. Use will be made of current technology DSP’S or ASIC’S along with more compact power electronic components to reduce the controller size.
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