Composite Current Space Vector Based Powerline Communication (PLC) Method For Grid Connected Inverters In AC Microgrids
Power distribution facilities all over the world have been committed towards making the grids smarter in order to reduce the risks of grid failures and provide an affordable, reliable, and sustainable supply of electricity to the end consumers. The smart grid concept involves incorporation of monitoring, analysis and control functions into the existing power distribution infrastructure. One of the foremost steps in realizing the smart grid concept is the integration of information and communication technologies with power system engineering. Various communication technologies are available, out of which Powerline Communication (PLC) has been found to be most suitable owing to its least intensiveness on additional infrastructure. Existing methods use PLC as a separate communication physical layer to establish com- munication between components in a micro/sub-micro grid. However, these methods poses the problems of a separate physical layer requirement to establish communication between inverters, attenuation of the information signal by the EMI filters present in various loads and equipments connected to the micro grid, requirement of signal repeaters at regular distance intervals and requirement of a separate server for monitoring and control. In order to simultaneously utilize the incorporation of front end inverters into the grid and achieve inter-inverter communication, a PLC method for the grid connected inverters based on a harmonic injection into the grid current is proposed in this thesis. The harmonic injection is accomplished by considering the grid current as a composite vector with components rotating at different speeds. The lower harmonic spectrum space can be chosen to avoid the attenuation problems associated with the EMI filters. In the proposed method, as the choice of the harmonic space is flexible, it is possible to even adopt a dynamically changing harmonic space to optimize THD. The advantage of the method is that it simultaneously achieves communication along with grid interfacing of DGs without any requirement of extra hardware. Also, since the principle of information exchange amongst inverters is the same as that of the power transfer, there is no added complexity involved in the inverter control system due to the proposed PLC method. The principle of the Composite Space Vector on which the proposed PLC method is based upon has been explained in detail along with the frame transformation equations. The control scheme to achieve the power transfer and the information exchange for the grid connected inverters is explained. The design procedure for various circuit elements and the control loop parameters has been explained. The thesis also discusses the various factors affecting the choice of the modulating signal and the speed of communication achievable in the proposed PLC method. For both the three phase and single phase systems, simulation results have been presented for the proposed PLC method under different grid conditions and different harmonics as the modulating signals. The simulations have been performed using the MATLAB SIMULINK SimPowerSystems toolbox. The simulation results have been experimentally verified through a laboratory prototype. The laboratory prototype consists of individual IGBT based inverters controlled through the Texas Instruments TMS320F2812 DSP based digital controller.