Abstract
For inverters, a phase-locked loop (PLL) is usually needed for the grid synchronization. Typically, for the single-phase inverters, the orthogonal-signal-generators based PLLs (e.g., delay-based PLL) can be used. However, if the grid at the point of common coupling (PCC) exhibits a large grid impedance, the inverter may not work well or even be unstable. In order to work satisfactorily in the very weak grid, this study aims to formulate a robust PLL. At first, by modeling the inverter output impedance and considering the frequency coupling effect, the stability of the typical delay-based PLL has been analyzed and the reason for the performance degradation has been explained. Then, based on analyzing the differences of PLL blocks under different PCC conditions, the robust PLL with the grid current feedforward is discussed. Compared with the typical PLL, the improvement of the system behaviors in the weak grid cases is mainly attributed to the extra term on the numerator of output impedance, which is introduced by the current feedforward of the proposed PLL. The selection of control parameters has been emphasized for maintaining the high robustness. At last, selected comparative waveforms have verified that the single-phase inverter can perform well even with the large grid impedance, without the grid impedance estimation.
Highlights
In the distributed power generation systems (DPGSs), the grid-connected inverter with an LCL or LCL-Trap filter is the key interface [1], [2]
The main work and contributions are: 1) investigating the system stability based on the inverter output impedance model, through which the reason why the typical phase-locked loop (PLL) cannot assure a stable operation in the case of large grid impedance is explained; 2) proposing an improved PLL, where the grid current feedforward and its design enhance the robustness by increasing the magnitudes and phases of the output impedance within a wide frequency range; 3) providing comparative test results, which can verify that the proposed PLL control works well without the grid impedance estimation and the proposed PLL is suitable for the weak grid application
For the grid-connected inverter, the PLL has a non-negligible interaction with the grid impedance
Summary
In the distributed power generation systems (DPGSs), the grid-connected inverter with an LCL or LCL-Trap filter is the key interface [1], [2]. In [31] where a very low PLL bandwidth is used (i.e., 18 Hz), an impedance compensator has been proposed to extend the operation range of a three-phase inverter in the weak grid. The main work and contributions are: 1) investigating the system stability based on the inverter output impedance model, through which the reason why the typical PLL cannot assure a stable operation in the case of large grid impedance is explained; 2) proposing an improved PLL, where the grid current feedforward and its design enhance the robustness by increasing the magnitudes and phases of the output impedance within a wide frequency range; 3) providing comparative test results, which can verify that the proposed PLL control works well without the grid impedance estimation and the proposed PLL is suitable for the weak grid application.
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