DFIG Controller

The goal of this project is to design and implement a rotor side converter (RSC) that monitors a doubly-fed induction generators (DFIGs) torque, current and voltage output in order to command an input waveform that will stabilize the DFIG with the grid at 60Hz. The completion of this project will aid in the advancement of wind energy systems by allowing further research to be conducted on doubly-fed inductions generators.

Background
As the principal generator used in Type III wind energy conversion systems, the doubly fed induction generator (DFIG) is widely accepted in today's wind energy industry. As society shifts toward renewable energy, these systems are becoming more prevalent. This brings an increasing need for research into how these systems interact with the grid. To achieve this end, the University of Idaho has created a wind turbine simulator which uses an AC induction motor to drive a DFIG at variable speeds and monitor power delivery and stability. In order to use this test bench, a full featured controller is needed operator the DFIG properly. That is the motivation for this project. The DFIG is essentially a wound rotor induction generator in which the rotor circuit can be controlled by external devices to achieve variable speed operation. The stator of the generator is connected to the grid through a transformer, whereas the rotor connection to the grid is done through power converters, harmonic filters, and the transformer. The stator of the generator delivers power from the wind turbine to the grid. In the rotor, the power can be delivered from the rotor to the grid and vice versa through rotor-side converter depending on rotor speed, and power factors. Two of the main components in the control system of the DFIG are the grid-side converter (GSC) and the rotor-side converter (RSC). The GSC is responsible for managing power delivery to the grid and generating a stable DC rail value from the grid to power the inverters. The RSC is responsible for controlling the power applied to the rotor in order to facilitate smooth, clean, and efficient power generation. The scope of this project is assembling and testing the components to create the RSC to be used with the wind turbine simulator test bench that the University of Idaho has built.

Deliverables
The focus for the first semester is understanding the systems involved and getting components ordered and tested. The process for this is the following: Once hardware procurement and testing is completed, the plan for the second semester is the following: This outlines the process, and the final deliverables will be:
 * Learning to understand systems involved and components required
 * Developing and verifying a design for the system using simulations
 * Developing specifications and selecting hardware components based on design and verification
 * Ordering components and testing individually to ensure expected function and specification compliance
 * Perform component integration and subsystem tests
 * Develop control code and test sub routines involved
 * Complete any fabrication of custom components
 * Mount components to power electronics stand
 * Final integration and full scale tests
 * 1) A capable Microcontroller programmed with the required algorithms to run on the wind turbine simulator
 * 2) Power electronics components including Gate Driver, IGBTs, snubbers, rectifiers, etc.
 * 3) System connection components including any breakout boards and cables required to connect components
 * 4) Isolation components, in this case Optic Fiber links for noise immunity and isolation of high and low voltage components
 * 5) Sensors connected and tested in the final layout

Specifications and Constraints
Specifications for the project are listed below:
 * Prototype must be modular
 * Prototype must be robust
 * Utilize startup and fail safe procedures in code
 * Incorporate adequate isolation

Constraints of the RSC design were as follows:
 * Integration must be designed to handle a DC bus voltage value up to 340V
 * Must be able to stabilize a generator operating in a range of 0-60Hz with the grid (ie. 60Hz)
 * Read sensor inputs and command the 3-phase inverter at a minimum frequency of 20kHz

=Project Outline= Below you will find the schedule that was followed to complete this project. The gantt chart shown is a broad outline of the design steps taken. A detailed version can be found in the additional documents section of this page.

=Project Learning= Since our team members weren't really familiar with DFIG, we had to do a lot of readings and researches to get the ideas of how DFIG works and control over it. Also, it is a hard project that was out there for several semesters and even graduate students worked on it, our progress can be very slow. We took three weeks in the power lab to test the machine in sub-synchronous and super-synchronous mode and did the locked-rotor test, the no-load test and the DC test. Below is the result we got for our first parameter tests.



= Design Evaluation and Verification =

Space Vector Modulation and Controller
Evaluation of the Space Vector Modulation (SVM) algorithm began with studying and understanding the basic concept during the learning phase. Equations for our implementation of SVM were collected and used to create a more detailed simulation that accurately tested the implementation of SVM that will be used for the physical test bench. Essentially, this test consists of utilizing SVM in current control mode, determining the PWM scheme to use, devising an algorithm for causing the fewest switching events possible, and finally testing how this interacts with an inductive load. To test active current control, a PI controller was implemented in the simulation. The three phase current measured in the load was converted to Direct-Quadrature (dq) axis rotating reference frame in order to have stable value (i.e. current values are essentially constants), then it is not difficult to implement a PI controller that can maintain the currents very close to the command, even as the command changes. Next, we decided to use center aligned PWM with three channels (one for each phase leg) with six outputs (three positive and three negative or complementary). This makes applying the derived vector straightforward and provides protection for power electronics by maintaining the switch configuration in one of the eight possible safe states. Using center aligned PWM also naturally allows us to reduce the number of switching events (by ensuring that all states are not toggled to on/off at the beginning of each cycle) while also staying in each state for the correct amount of time. In each cycle, the SVM algorithm generates a time for each of three states. These consist of two adjacent non zero states and a zero state (the remaining time for a cycle). There are two zero states where there is no net current flow through the balanced three phase load (i.e. states 000 and 111). By properly utilizing these zero states, we are able to ensure that only one phase leg changes state at a time. With the PWM waveform generated, the simulation tests how this waveform would interact with an inductive load. To test this, we generate a transfer function for each phase of the load. We feed the voltage waveform generated by PWM through the transfer function in order to obtain a current estimate. This is done in each phase and is used to calculate the overall current in the dq rotating reference. This overall current is compared to the command to generate an error function which is used by the PI controller to actively adjust the command to control the current phase and magnitude. After completing this simulation and gaining a reasonable understanding of the control system in general, it was possible to develop a list of requirements for the microcontroller to use in the project. The notable requirements are:
 * Clock speed >100 MHz with PWM capable timers at >20 MHz in order to provide at least 9 bits of resolution at a fast switching frequency of 40 KHz (for standard PWM). *Note: 9 bits = 512 steps = max 0.66 V/step @ 340 V DC rail
 * Advanced PWM capability including 6 channels (automatic 3-phase complementary a bonus) and center aligned mode.
 * At least 6 simultaneous ADC channels with at least 12 bits of resolution
 * Various communications protocolos including I2C, SPI, CAN for interfacing with other modules
 * Hardware math accelerators such as FPUs and MACs (usable without assembly wrappers is a bonus)
 * Free IDE with plentiful documentation, code samples, libraries, and developer community

Power Electronics
Power electronics evaluation began by familiarizing ourselves with the interactions between the devices we would be using. To do so the first step was to investigate the integration of the gate driver with the IGBTs. In our research we came up with two possible solutions for our design: the first solution was to implement a three-phase gate driver with a six pack IGBT module and the second solution was to design the RSC using a three-phase gate driver with three two pack IGBT modules. Each solution also incorporated the use of a high-side and low-side gate driver as a means of initial research to avoid damaging our three-phase gate driver during the learning phase. While both methods are feasible due to the lack of equipment on hand to mount the six pack IGBT module the final design was chosen to utilize three two pack IGBT modules. After deciding on a model for gate drivers and IGBTs, simulation models for each device were able to be plugged into LTSpice (Spice) simulations to verify the design. Spice simulations were initially constructed for a simple single leg design to observe component integration and were later adapted to an H-bridge design. The H-bridge simulation was completed to investigate component integration and the turn on/off delays of each IGBT to ensure the dead time needed between the turn on/off of each leg of IGBTs to avoid faults caused by short circuiting the system. Following verification of the integration of the gate driver and IGBTs the next power electronics design revolved around incorporating the protection circuits and components. Protection for our design focused on two main topics: snubbers and MOVs. Snubber circuits were chosen in order to minimize the switching losses of our design while also protecting the IGBTs by minimizing the losses of each IGBT and therefore reducing heat transfer in the devices making the design more robust. From the calculations and the use of the principle of duality to determine component values needed for the turn off and turn on snubbers, the components were integrated into the original single leg Spice simulation to verify functionality. During this process many reiterations were conducted after slight changes in inductor, capacitor and resistor values of the snubber circuits in order to manipulate the voltage and current waveforms of the IGBT in a manner that produced an acceptable power loss in the device. Alterations of these components were also made in order to produce voltage and current values through the IGBT below the device ratings. This resulted in the following finalized values for the snubber circuit: 50 Ohm resistors, 1.2uH inductors, .12uF capacitors. Performing calculations led to an average power of just 38W due to the short interval of time in which the 4kW were being produced. Alongside snubbers the other means of protection mentioned were the use of MOVs. MOVs were utilized as an extra layer of protection for the IGBTs in order to act as a path for current to flow if an unexpected spike were to occur. Due to this the MOV was selected based on its Varistor voltage that falls in the range of 420-520V, its continuous rms voltage of 300V, and its peak pulse current of 100A for an 8/20us current waveform. The last two design components to be simulated for verification and understanding purposes were the three-phase inverter and the high voltage (HV) side power supply. To complete the three-phase inverter simulation an ideal circuit had to be implemented in order for Spice to run in a timely manner due to the complexity of timing involved with the fully integrated design of the three-phase inverter. A schematic for the ideal three-phase inverter can be found in Appendix D along with its results. From the schematic you can observe that the ideal three-phase inverter did not include the chosen components, instead it utilized ideal sources and switches. Due to this fact this simulation served as a means of understanding and having a visual representation of the end goal of this project and reference for tests in the lab. Concluding the power electronics verification stage was a simulation to validate component selection for the HV power supply which consisted of a 120:12V transformer, a full wave rectifier, smoothing capacitors, and two DC-DC converters for supplying power to the receivers and transmitter on the HV side and the fault pin of the gate driver. To view the schematic and results for the Spice simulation regarding the HV power supply see Appendix E. In Appendix E you will see the schematic which utilizes a voltage source to represent the voltage seen from the output of the 120:12V transformer and a current source depicting the total load current being drawn by the receivers/transmitter, DC-DC converters and the gate drive. While the DC-DC converters are needed to supply power to the receivers/transmitter and fault pin of the gate driver, the gate driver is able to be powered directly from the output of the full wave rectifier. This can be accomplished since the rectifier is producing a voltage just under 16V which meets the gate driver supply voltage range of 13-17.5V.

= Hardware Procurement =

Optic Links
= Implementation and Testing =

=Previous project revising= Starting from the second semester, our team focused on both hardware and software by first revising previous project done by graduate students and former senior design teams. For the hardware, we looked at a grid side converter(GSC) design built by Rebecca Dong, it consists of power converter module, DC/DC converter, voltage/current sensor and a single 16V power supply. We came up with a brief schematic of this GSC and learned the function of each component of the board, even though our goal is to design a converter for the rotor side, we can still use it as a reference. However, this board is too complicated, we want to build a controller system with multiple boards perform individual tasks, so we turn to a project done in 1996 led by Dr.Joseph Law, this project is a controller system for a induction machine, we particularly looked at the gate driver delay circuit and the gate driver board. In the delay circuit, 2 sets of input go into 1 of the 3 lines that are used to drive the gate of the power converter, the purpose of the delay circuit is to prevent the situation when the upper and lower transistor of the inverter closed at the same time to cause a short around the DC bus. The delay circuit then sends its output information to the gate driver board through fiber optics to eliminate the interference of other equipment in this system. The gate driver board converts fiber optic light signals to pulse signals to trigger the transistor and provide reference for the emitter on each of the transistor.

=Field orientation Control= Using an already provided assembly language for a squirrel cage induction machine partial c++ code was developed that can be used in conjunction with hardware decided to use in the future.

=PID control= To start we first developed a simple RL circuit with current across the inductor as output and simple step function as voltage input and developed simulink Matlab files for it. In future using the simulink files provided by DR. Law, which can be found in project portfolio Matlab code needs to be developed in simple C/C++ language that can be easily imported to any hardware decided to use.

=Final Design=

=Future work= Future work needed to be completed for this project can be broken down into five topics, all of which have been partially completed but have some advancements needing to be implemented. These topics include PCB fabrication, high voltage side stand configuration, low voltage side stand configuration, coding, and system testing. Tasks involved with the future work associated with PCB fabrication are as follows: The PCB board needing to be printed is for the gate driver and its external components utilized in the protection abilities that the gate driver device provides. Steps needing to be completed to finish the high voltage side of the prototype include: Low voltage side stand configurations tasks needing to be completed are: In terms of coding the only task needing to be completed is to write and verify the field oriented control algorithm. Following these tasks the last item involved with future work is to run tests utilizing the wind turbine simulator and the RSC design. It is advised to run a few small scale tests verifying system integration before performing tests with the wind turbine simulator.
 * PCB printing (gerber files provided)
 * RCin port capacitor chosen
 * ITrip port capacitor chosen
 * ITrip port resistor chosen
 * Mounting the PCB and making gate driver connections (mounting hardware already in place)
 * Twist soldering snubber components (snubber components already in place)
 * Drilling extra holes on the heatsink for proper IGBT mounting
 * Mounting the positive rail
 * Connecting power supply outputs to the optics and gate driver(devices already mounted in a layout that provides easy interconnections to be made)
 * Mounting uC and making connections to the optics and sensors
 * Creating the breakout boards that will aid in the connection process of the uC (schematics for these boards are provided)

=Team Members= Mason Ulrich Major: Electrical Engineering Email: ulri3329@vandals.uidaho.edu Bio: Mason Ulrich was born and raised in Lewiston, Idaho. During his academic career he studied Electrical Engineering with an emphasis on Power. Following his graduation he will be moving to Olympia, Washington to work for Electrical Power Systems Inc. (EPS) as a substation design engineer. In his free time you will find Mason rock climbing, skiing, on top of a mountain, or getting lost in the presence of the beautiful outdoors.

=Additional Documentation=

Gantt Chart
Gantt Chart

Final Report
=Meeting Minutes=