A hybrid synchronous/fixed reference frame pll for phase synchronization with unbalanced three-phase grid conditions

Control of power flow to or from the 3-phase grid requires accurate and fast estimation of both the positive sequence and the phase of the grid voltage, for proper timing of the power control devices. Unbalanced conditions will degrade the quality of this estimation. Following a Clarke transformation of the 3-phase to 2-phase αβ voltage signal, the positive sequence can be extracted by phase shifting the a and β signals by -90° and +90°, respectively. A hybrid scheme is proposed, embedding both a conventional two-phase PLL augmented with adaptive synchronous estimation of the positive sequence amplitude, and a generalized integrator to estimate the negative sequence. The two components are configured in parallel, according to the internal model principle, within a frequency adaptive, quadrature signal feedback structure. This results in faster response, and reduced signal processing complexity, making it suitable for DSP implementation in a wide range of applications. Parameter tuning is also simplified to suit application requirements. Experimental results are presented to validate the proposed method.