Here, a single-stage cascaded H-bridge (CHB) inverter is presented for grid-connected photovoltaic (PV) systems. The CHB inverter has separate DC links and allows individual control of PV arrays. The conversion efficiency is high and the harmonic generation is lower than conventional PV inverters. Although the CHB inverter is a good candidate for injection of solar power into grid, its control issues have not been completely solved. One of the main challenges in the CHB inverter is the harmonic generation when the connected PV arrays to the H-bridge cells have different amounts of insolation. This study deals with the asymmetrical operating conditions of PV arrays (or H-bridge cells) in the CHB inverter and presents an analytical equation for determination of cells’ modulation indices based on PV arrays data. Then, a control loop is added to the tracking algorithm of conventional control systems to determine whether an H-bridge cell is in the linear modulation or not. In the case of overmodulation, the corresponding DC link voltage is increased by the controller to bring it back to the linear region. The validity of new method is confirmed by simulations and experiments on a seven-level 1.7 kW CHB inverter.
Fig. 1. Grid-connected PV inverter based on the CHB inverter
EXPECTED SIMULATION RESULTS:
Fig. 2. Injected current to grid
(a) By the presented algorithm ,(b) By the proposed strategy in this paper
Fig. 3. Evaluation of proposed control system behaviour before and after applying the new strategy (a) Modulation indices, (b) Modulating waveforms, (c) Arrays DC link voltages and reference values, (d) Total injected power to the grid
Fig. 4. Dynamic behaviour of the proposed control system under change of irradiance level of the first PV array
Fig. 5. Dynamic behaviour of the proposed control system under grid voltage swell and non-uniform distribution of irradiances
In this paper, a modified control strategy was proposed for the CHB inverter in the grid-connected PV applications. Based on the circuit analysis, a mathematical relation was derived for determination of cells’ operating conditions in the CHB inverter. This relation shows the value of cells’ modulation indices based on the PV system data. Accordingly, a modified control strategy was proposed to extend the operating range of the CHB inverter under heavy mismatching conditions. In this method, the condition of each H-bridge is checked continuously and when a cell enters to the overmodulation region, its voltage is gradually increased to bring it back to the linear region. This modification helps to prevent the interruption of CHB inverter due to extra harmonic generation in the overmodulation region. The proposed method can be easily applied to the already existing control systems to increase their operating range under asymmetric conditions.
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