The basic method of controller design for LCL filtered grid-connected inverter in continuous domain has been reviewed above.
Open image in new window Fig. 1 Topology of a three-phase LCL filtered grid-connected inverter.
Open image in new window Fig. 3 Single-phase equivalent circuit of LCL filtered grid-connected inverter with virtual resistor connected in parallel with filter capacitor.
In this paper, Sect. 2 introduces a typical model of an LCL filtered grid-connected inverter, and the traditional method to evaluate CCF active damping is reviewed briefly.
High-performance LCL filtered grid-connected converters should have fast dynamic response, sufficient switching frequency attenuation, and good damping performance while preserving stability.
For the convenience of analysis, an equivalent circuit is used for a single-phase LCL filtered grid-connected inverter, as shown in Fig. 3.
In this paper, a critical value of the CCF active damping coefficient k damp_c is proposed to construct a stability evaluation method for LCL filtered grid-connected converters.
In summary, the literature does not provide a stability evaluation method for digitally controlled LCL filtered grid-connected inverters that accounts for computation and PWM delays, nor does it provide a general guideline for selecting controller parameters to ensure stability.
The open-loop transfer function T A(s) in the s-domain can be expressed as: T_{text{A}} (s) = frac{{G_{text{i}} left( s right)}}{{s^{3} L_{1} L_{2} C + s^{2} L_{2} Ck_{text{damp}} + sleft( {L_{1} + L_{2} } right)}} (1) Open image in new window Fig. 4 Block diagram of single-phase LCL filtered grid-connected inverter with CCF active damping.
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