© Imperial College LondonPage 1 A Review and Proposal on Controller Design for a DC/AC Power Converter Xinxin Wang Control and Power Group

© Imperial College LondonPage 2 Outline 1.Background 2.The two-loop control system review 3.A new discrete voltage controller design and switching procedure design 4.DSP implementation 5.Conclusion

© Imperial College LondonPage 3 1. Background The distributed generation (DG) is developing rapidly. Power converters, such as IGBTs, are used as the interfaces between DGs and local loads. H ∞ repetitive control theory is used to design a controller for the DC/AC power converter.

© Imperial College LondonPage 4 System Modelling Two-loop control system Repetitive control system Formulation of the H-infinity problem Calculation of active and reactive power Active and reactive power controller 2. Two-loop control system review The work in this section has been done before. Two published papers, ‘H ∞ Repetitive Control of DC-AC Converters in Microgrids’, G. Weiss, et. al. (2004) and ’Decoupling control of the active and reactive power for a grid-connected three- phase dc-ac inverter’, J. Liang, et. al. (2003), are related to this section.

© Imperial College LondonPage 5 System Modelling back

© Imperial College LondonPage 6 Two-loop control system

© Imperial College LondonPage 7 Repetitive control system

© Imperial College LondonPage 8 Formulation of the H-infinity problem

© Imperial College LondonPage 9 Calculation of active and reactive power We assume that the grid voltage is the reference phasor, with angle zero: From the figure of the system modeling,system modeling

© Imperial College LondonPage 10 Assume that the we know the angle of the equivalent impedance, Here, Z g is the equivalent impedance of the grid interface inductor and short distance of the transmission line.

© Imperial College LondonPage 11 Active and reactive power controller

© Imperial College LondonPage 12 Shortcoming of the previous voltage controller Formulation of H ∞ problem for the new controller Simulation results of the new discrete controller Switching procedure design and simulation results 3. The new discrete voltage controller design and switching procedure design

© Imperial College LondonPage 13 Shortcoming of the previous voltage controller back

© Imperial College LondonPage 14 Formulation of H ∞ problem for the new controller The new block is W g which is of PI type.

© Imperial College LondonPage 15 Simulation results There is no DC component in the tracking error. Compared to the previous tracking error, this new controller has a better performance. tracking error

© Imperial College LondonPage 16 Switching procedure design In some cases, S g and S c should be switched on and off. This is the simplified circuit.

© Imperial College LondonPage 17 Grid disconnected and connected while the converter is working ‘Grid up’ : The breakdown is over. Then set the reference voltage V ref = V g 1, which is the fundamental component of the grid voltage V g. Now, S c is closed and S g is open. ‘Grid connected’ : Set active and reactive power reference P ref = 0, Q ref = 0. Connect the grid to the micro-grid. Now both S c and S g are closed. ‘Delayed grid connected’ : Set P ref and Q ref to desired values.

© Imperial College LondonPage 18 Simulation results control signals tracking error

© Imperial College LondonPage 19 Converter disconnected and connected while the grid is working ‘Converter up’ : Assume the converter is not connected. Now, S g is closed and S c is open. Measure the active and the reactive power P m, Q m. ‘Converter connected’ : Set active and reactive power reference P ref = P m, Q ref = Q m. Connect the converter to the micro-grid. Now both S c and S g were closed. ‘Delayed converter connected’ : Change P ref and Q ref to desired values.

© Imperial College LondonPage 20 Simulation results control signals tracking error

© Imperial College LondonPage 21 Two DSPs introduction. The controller structure with two DSPs. Parallel communication between the two DSPs. Layout of the printed circuit board in Protel. 4. DSP implementation of the controller

© Imperial College LondonPage 22 Two digital signal processors (DSPs) from TI TM, a fixed- point DSP LF2407A and a high speed floating-point DSP C6713, are used. The power controller, voltage controller and neutral-point controller are implemented by C6713. The LF2407A is used to implement PLL, monitor the protection and transmit the values of voltages and currents. External memory interface (EMIF) of LF2407A and host port interface (HPI) of C6713 are used to do the parallel communication between the two DSPs. Introduction of the two DSPs

© Imperial College LondonPage 23 The controller structure with two DSPs.

© Imperial College LondonPage 24 Parallel communication between the two DSPs

© Imperial College LondonPage 25 Layout of the printed circuit board in Protel Three connectors are used. One is for the connection to HPI, and the other two are for the connection to EMIF.

© Imperial College LondonPage 26 Summary of the work to date Future work 5. Conclusion

© Imperial College LondonPage 27 Review the voltage and power controller design. Design a new discrete voltage controller for the converter system. Design the switching procedures of the grid or converter disconnected and connected to the local loads. Do the parallel communication between the two DSPs. Summary of the work to date

© Imperial College LondonPage 28 Finish DSP implementation of the control system in the experiment. Design a power controller using dq transformation. Design control system for converters in parallel. Future work Short term:

© Imperial College LondonPage 29 Stator power Finish the embedding of the whole system which includes the wind turbine, the DFIG, the back to back converters and the grid. A wind turbine blade experiences a variety of loads which occur at specific frequencies, leading to the output power variations. Such as: Tower shadow; Frictions due to the gearboxes and drive train. Long term:

© Imperial College LondonPage 30 Generalization of the internal model principle. Such model can reject (track) different disturbances (references) with different fundamental frequencies.

© Imperial College LondonPage 31