Abstract
The output LC filter of a photovoltaic (PV) string three-level grid-tied inverter that connects the filter capacitor neutral point to dc-link capacitor neutral point can reduce the common-mode (CM) current injected to the grid by letting the CM current circulate within the inverter. However, the internal CM current may resonate because of the existence of the resonant frequency of the internal CM LC circuit. Compared with the traditional continuous pulse-width modulation (CPWM), the resonance can be worse if discontinuous pulse-width modulation (DPWM) is applied, for the zero sequence quantity of DPWM contains more harmonics than that of CPWM. In this paper, a virtual negative resistor based common mode current resonance suppression method for a three-level grid-tied inverter is proposed to overcome the CM current resonance problem in DPWM application. Different positions of the virtual negative resistor in the equivalent CM circuit with different feedback variables are analyzed theoretically. The virtual negative resistor connected in series with the inductor in the equivalent CM circuit is selected to damp the CM current resonance for simplification and damping performance. Different from the implementation in CPWM where a pair of small voltage vectors exist and are used to adjust the CM voltage directly, the proposed method for DPWM application is implemented indirectly by adding the CM adjustment quantity to differential-mode (DM) control quantity with appropriate coefficients. Depending on the sector of DM control quantity in the α β reference frame, the coefficients are calculated using one of three specific voltage vectors. Experimental results are given to demonstrate the effectiveness of theoretical analyses and the proposed method.
Highlights
Among all renewable energy sources, photovoltaic (PV) systems have experienced rapid growth both in residential and commercial applications
The originality lies in two points: (1) a virtual negative resistor based CM current resonance suppression method and (2) its implementation in discontinuous pulse-width modulation (DPWM) application
The zero sequence quantity of continuous pulse-width modulation (CPWM) and DPWM according to different modulation index and time are shown in Figure 4a,c, respectively
Summary
Among all renewable energy sources, photovoltaic (PV) systems have experienced rapid growth both in residential and commercial applications. The CM current resonance suppression methods for inverter using the ILCL filter are proposed in [13,14] to reduce the leakage current injected to gird. In [13], the CM voltage which is injected into DM voltage to extend the inverter modulation index is controlled to suppression CM current resonance Both the methods are useful and effective, and are based on a pair of small voltage vectors to adjust the CM voltage. A virtual negative resistor based AD (VNRBAD) method is proposed to damp the CM current resonance for a three-level inverter with an improved LC (ILC) filter in DPWM application. The originality lies in two points: (1) a virtual negative resistor based CM current resonance suppression method and (2) its implementation in DPWM application.
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