Control of a flexible microgrid during both modes of operations with presence of nonlinear loads

Abstract

During microgrid islanded operations, the current waveform at the point of common coupling (PCC) is deteriorated when distributed generation resources supply nonlinear loads. This alteration generates voltage distortion that can affect both the distribution generation system and the connected loads. Another encountered problem in microgrids is the transition phase during islanded operations. In fact, during this step, the voltage controller used in zero level control switches off while the current controller used to control the current during grid-tied mode turns on. This transition exhibits a voltage spike during the switching process if the output of both controllers is not equal, such effect can impact the stability of the global microgrid system. In this paper, to enhance the power quality of the electrical power system in presence of nonlinear loads during islanded and grid connected operations, a fuzzy logic controller is used in the zero level control to get an accurate voltage regulation and to reject disturbances that may affect the control system. Moreover, the same fuzzy controller is kept during grid connected mode to control the current, by setting the scale factors of the fuzzy coefficients corresponding to the appropriate control during the transition phase, a smooth transition is ensured. This advantage reduces the number of controllers and makes the architecture less complex since one controller is required for both modes to reach the desired performances. Furthermore, a parallel active filter (PAF) based on pq theory is used to reduce the harmonic contents of the currents line and the PCC voltage during islanded mode. This combination between the fuzzy control and the PAF allows to strengthen the control and to get a good total harmonic distortion (THD) during islanded operations. In this context, the hierarchical control is analyzed and the control parameters are designed. Finally, the microgrid test including three voltage source inverters (VSI) supplying two loads and two nonlinear loads are simulated under Matlab/Simulink environment to investigate the performances of the control structure.