Abdi Abdirahman, Ahmed Abd, Stephen Garcia
supervised by Dewei Xu, M.Sc., Ph.D. Tsinghua, P.Eng
More and more power converters for renewable energy are connected to the grid. The testing of the power converters is becoming more and more important nowadays. The project is to develop a testing equipment to simulate the grid and isolate the power converter from the main grid operation.
The objective is to develop a bi-directional converter to simulate the grid operation and provide an isolated grid for power converter testing for the following specifications: Power rating: 10kW energy bidirectional. Input voltage from grid: Three phase 208VAC, 60Hz. Output voltage at isolated grid: three-phase, 208VAC, 60Hz. The voltage is variable from 0 to 208V. Voltage THD: < 5% Voltage regulation at full load: 5% Simulated testing environment: LVRT test with E-ON LVRT voltage profile.
In order to achieve this the design of the project corresponds to two converters connected back to back, after extensive research, and after understanding the different types of control schemes that exist for converters we decided to implement the Voltage Oriented Control (VOC) scheme for the converter that will be connected directly to the grid, and also it was determined to use a Direct Voltage Control for the converter that is going to be connected to the isolated voltage coming out of the rectifier. We decided on the VOC because of Fixed switching Freq. (easier design input filter)and also because of advanced PWM strategies can be used which derives in cheaper A/D converters, VOC guarantees high dynamic and static performance via an internal current control loop, but the quality depends mainly on the current control strategy. This strategy guarantees a fast transient response and high static performance via internal current loop, consequently the performance depends on the quality of the current control loop, and we can find strategies that can be applied for current control. A widely used scheme for control is the dq synchronous controller, where the regulated currents are DC quantities. This eliminates steady-state errors. VOC also carry disadvantages the main one being the coordinate transformation and decoupling between active and reactive components is required. But for this we have a solution in our design we will be implementing a decoupled controller.
On the isolated voltage side we will be implementing the Direct Voltage Control scheme, since on the isolated side we don't have a direct way to measure the angles for the dq-transformations, we will generate this angle from the reference frequency of our system, in our design basically, transformation block receives the synchronous angle and transform the three phase local bus voltage into d-q reference frame. Two lead-lag regulators are employed to directly regulate the d and q component voltages. The target of the control system is to force the bus voltage follow the reference voltage through a fixed frequency transformation. The frequency is indirectly controlled through the transformation block by the given frequency. Reference QUOTE and QUOTE are the desired bus voltage and the fixed frequency is the desired system operating frequency. If the system outputs follow the references, then active and reactive power balances are maintained. The system can be decoupled on a d-q reference frame.
The controller for each axis uses a single loop to directly regulate the voltage. In this case, compared with conventional power control with cascaded loops, it has a much faster dynamic performance with even higher system bandwidth. Two lead-lag regulators are employed to regulated the d and q component voltages to obtain a higher phase margin and highest control bandwidth The reference voltages can be arbitrary selected as long as the magnitude of the voltage is desired voltage. Usually the d-component reference voltage QUOTE is selected as the required local bus voltage, while the q-component reference voltage QUOTE is set to be zero. Space vector modulation (SVM) method is adapted to generate desired PWM gating signals.
Project targeted applications: Testing of power converters. The project is to develop a testing equipment to simulate the grid and isolate the power converter from the main grid operation.