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Sunday, 2 July 2017

Multilevel Inverter for Grid-Connected PV System Employing Digital PI Controller

This paper presents a single-phase five-level photovoltaic (PV) inverter topology for grid-connected PV systems with a novel pulse width-modulated (PWM) control scheme. Two reference signals identical to each other with an offset equivalent to the amplitude of the triangular carrier signal were used to generate PWM signals for the switches. A digital proportional–integral current control algorithm is implemented in DSP TMS320F2812 to keep the current injected into the grid sinusoidal and to have high dynamic performance with rapidly changing atmospheric conditions. The inverter offers much less total harmonic distortion and can operate at near-unity power factor. The proposed system is verified through simulation and is implemented in a prototype, and the experimental results are compared with that with the conventional single-phase three-level grid-connected PWM inverter.

1.      DSP TMS320F2812
2.      Grid connected
3.      Photovoltaic (PV)
4.      Proportional–integral (PI) current control
5.      Pulse width modulated (PWM) inverter.



Fig. 1. Single-phase five-level inverter topology.


Fig. 2. Inverter output voltage (Vinv) and grid current (Ig) for different values of M. (a) Vinv forM <0.5. (b) Ig forM <0.5. (c) Vinv forM >1.0.(d) Ig forM >1.0. (e) Vinv for 0.5 M 1.0. (f) Ig for 0.5 M 1.0.

Fig. 3. Step response of the PI current control scheme.

This paper presented a single-phase multilevel inverter for PV application. It utilizes two reference signals and a carrier signal to generate PWM switching signals. The circuit topology, modulation law, and operational principle of the proposed inverter were analyzed in detail. A digital PI current control algorithm is implemented in DSP TMS320F2812 to optimize the performance of the inverter. Experimental results indicate that the THD of the five-level inverter is much lesser than that of the conventional three-level inverter. Furthermore, both the grid voltage and the grid current are in phase at near-unity power factor.

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