Voltage Equalization of DC Link capacitors in a CHB-Based PET under the Unbalance Voltage Conditions

The unbalance currents injection by power electronic transformers in order to improve the unbalance voltage conditions of the network causes an imbalanced voltage in the DC link capacitors and creates an active powers inequality in the different phases. In this paper, the proposed control structure independently controls the active power of each phase by modifying the power angle between the voltages of the grid and transformer. Based on the presented analytical relations, the amplitude and phase of the desired zero component current can be calculated. The power angle variations in the different phases of the first stage create a zero-sequence current by the proposed control system which is equal to the result of analytical relations. The simulation results in two modes of the voltage imbalance and network fault show that by injecting the improvement negative currents, the active powers and the DC voltage of capacitors are significantly affected so these Divergences limit the transformer improvement capacity and create vulnerabilities for DC capacitors. After applying the proposed control method, proper convergence is created in the powers flow and DC capacitors voltage. Experimental results also show the convergence of these parameters after applying the proposed control method. Also, the conformity of the zero component by the proposed control structure with the calculated values from the presented analytical relations is clearly visible from the laboratory results.