New Control Strategy for Three-Phase Grid-Connected LCL Inverters without a Phase-Locked Loop


 The three-phase synchronous reference structure phase-locked loop (SRF-PLL) is widely used for synchronization use in power systems. In this paper, a new control strategy for three-phase grid-connected LCL inverters without a PLL is given. According to the new strategy, a current reference can be produce by using the immediate power control scheme and the proposed positive-sequence voltage detector.


Through logical search, it is determined that a high-quality grid current can be created by introducing the new control strategy. In addition, a kind of independent control for reactive power can be produce under unbalanced and distorted grid conditions. Finally, the excellent work of the planned control strategy is confirm by means of simulation and experimental results.


1.Control strategy

2.Grid-connected inverters

3. Instantaneous power control scheme

4.LCL filter

5.Positive-sequence voltage detector




Fig. 1. Block diagram of the control system with LCL filter.



 Fig. 2. Simulation results of the proposed control system. (a) Generated current reference signals. (b) A-phase grid voltage and three-phase current. (c) Actual active and reactive powers.


Fig. 3. Experimental results of the positive-sequence voltage detector under actual grid operating conditions. (a) Utility voltage and the detected positive-sequence signals. (b) Harmonic spectrum of the utility voltage. (c) Harmonic spectrum of the detected positive-sequence signals.


Fig. 4. Experimental results of a step in the reactive power reference. (a) A-phase grid voltage and three-phase current. (b) A-phase grid voltage and A-phase current.


A new control form for three-phase grid-connected voltage source inverters (VSI) with an LCL-filter is planned. By using the immediate power control scheme and the planned positive-sequence voltage detector, the current reference can be indirectly generated, which avoids the complex PLL.


The performance of the proposed system for three-phase grid-connected VSIs is display via simulation results, which show a significant growth in both the steady state and transient action. The same action is experimentally confirmed. The fast dynamic response to a reference step is not troubled by the inclusion of additional control loops. Good work is guaranteed even under unbalanced and distorted grid voltages.


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[4] R. Inzunza, T. Sumiya, Y. Fujii, and E. Ikawa, “Parallel connection of grid-connected LCL inverters for MW-scaled photovoltaic systems,” in Proc. IEEE IPEC, pp. 1988-1993, 2010.

[5] T. Noguchi, H. Tomiki, S. Kondo, and I. Takahashi, “Direct power control of PWM converter without power-source voltage sensors,” IEEE Trans. Ind. Appl., Vol. 34, No. 3, pp. 473-479, Mar./ Jun. 1998.

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