Group SD0804
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Rotor Controller for Doubly-Fed Induction Generator
The main focus of the project will be designing a controller that will sense the speed of a doubly-fed induction generator and inject a variable-frequency current to maintain a constant frequency for the voltage induced in the stator. The goal will be to provide a consistent 60 Hz output frequency so the stator output can be connected to the existing power grid.
The Team:
- Rob Scheeler - Mat Arnold - Chris Woodard
The Big Picture
The diagram on the left illustrates how the DFIG controller is implemented in the wind generation system. Our project takes place in the "DC" region of the picture. We are responsible for calculating the three-phase signal that is sent back to the rotor windings on the DFIG.
The diagram on the right is the overview of our project and shows how the signal traverses the system:
- The optical tachometer monitors the mechanical speed of the rotor
- The dsPIC reads the signal and determines the proper output signal
- The hex buffer steps up the 5v signal from the dsPIC to 12v for the driver
- Each driver uses the high and low input signals to drive a pair of power MOSFETs
- The MOSFETs, powered by the rectified three-phase current, send the signal to the rotor windings
Project Requirements
- Two levels of DC voltage (5 & 12V) will be required to power the DSPIC, Buffer, Drivers and shut down circuitry. This voltage will be obtained from the AC grid through a transformer and rectification.
- An optical tachometer will be necessary to sense the speed of the rotor and will be monitored by the PIC. The PIC will calculate the frequency of the signal needed to be injected to the rotor to maintain constant amplitude 60 Hz output frequency.
- A protection circuit will be implemented to shut off the MOSFET drivers in the case of over-current on power MOSFET legs.
- Sinusoidal Pulse Width Modulation will be implemented to remove unwanted harmonics produced by “dumb” half-cycle pulsing of the rotor injection signal.
- PIC must be able to maintain real-time operation to properly monitor optical tachometer and calculate/adjust rotor injection frequency.
- The unit will be built to operate on the 175W induction motor in the power lab (Part of Lab-volt Station), but will ultimately be useful to a variety of motor sizes assuming power MOSFET’s and 3-Phase Rectifier are not overloaded.
- The final product will be housed in an enclosure for safety and mobility. At this time there are no pressing size constraints.
Implementation Options Considered
The biggest decision that needed to be made in the initial stages of the project was how to provide the three phase pulses to the MOSFET drivers that would supply the injected rotor current. The following explains the pros and cons of each of the considered solutions.
PIC Processor: One option was to use a PIC processor and C programming to read the rotor frequency and generate appropriate pulse sequence. This option provided the most scalability, and user-friendliness. We decided to use this option in our implementation because the only downside was the speed, which still adequately met the requirements.
Flip-Flops: The alternative solution was to use a timer in conjunction with a series of flip flops that would generate the same pulses according to the rotor frequency. This hardware implementation would be much faster than using a PIC, but if we needed to make changes it would require hardware replacement. We did not have as much experience with this solution, so we concluded that the PIC was a much better option.
Solution:
- DSPIC: The DSPIC offers more capabilities than the PIC we were originally using. It can handle each of the three PWM output signals as well as their complements. The DSPIC also offers dead time control which will allow us to create a gap in between the high and low switching signals to prevent a short circuit. The following spreadsheet outlines the main advantages to choosing the dsPIC:
All the Pieces
This section will explain the different parts of the project, and how they relate to the project as a whole.
Low-Voltage Supply: The power supply unit for this project will provide two separate voltages capable of carrying currents of up to three amps each. The 12v output will supply the voltage for the drivers and the power MOSFETS. The 5v supply will provide the voltage for the PIC processor.
High-Voltage Supply The high voltage supply across the MOSFETs is supplied by the transformed input of a 120 V AC. The voltage passes through a three-phase rectifier and the DC voltage is then cleaned up by a large capacitor.
PIC Processor: The DSPIC will read the frequency input from the rotor of the generator and calculate the necessary current that needs to be injected back into the rotor. The PIC will feed a buffer and three separate drivers that will step up the voltage and current since the PIC cannot provide enough current on its own.
Driver: The hex buffer will step up the 5v signal from the PIC to a 12v signal for the drivers. The drivers will supply the signal with enough current to the power MOSFET gates.
H-Bridge: The power MOSFETs will amplify the signal from the drivers to provide the necessary three phase current that will feed the main power grid. They MOSFETs are mounted on a heat sink to provide for proper heat dissipation.
Shut-Down The shut down circuitry will monitor the current though each leg of the H-Bridge. If the current exceeds 10 amps, the circuit will shut off the drivers until the problem is resolved and the circuit is manually reset.
Lab-Volt Station Connection
The controller is designed to integrate with the Lab-Volt stations in the power lab. The main advantage to using the Lab-Volt station is the data acquisition via the Lab-Volt software. We were able to monitor our three input currents and one phase of the output voltage on the Lab-Volt oscilloscope. The following diagram shows how the controller is interfaced with the station, further information is provided in the controllers user manual.
Gantt
Budget
The following is a link to the Excel version of the final budget for our project
