Análise de estabilidade para inversores LCL conectados à rede elétrica baseada em expressões analíticas e estimativas de impedâncias no domínio síncrono DQ

Abstract

The diversification in the energy matrix has been driving the emergence of new forms of electricity generation, often integrated into the grid through the inverter. The control of this device is usually designed without considering the impact of the connection with the grid. One approach to assess the stability of systems employing inverters controlled in the synchronous dq reference frame is to apply the generalized Nyquist criterion (GNC) to the product between the grid impedance matrix and the inverse of the inverter impedance matrix. This work proposes obtaining the impedance matrices of the LCL inverter and the electrical grid. The impedance matrix of the LCL inverter is established through an analytical expression. The matrix is coupled, and its components exhibit resonance peaks caused by the filtering elements. The impact of three different controllers on the inverter impedance is studied. Two of them are improvements over a standard control technique. The results obtained with a hardware-in-the-loop (HIL) system and an experimental setup validate the developed models. They confirm that the Zqq component of the matrix behaves like a negative resistor within the bandwidth of the control phase-locked loop (PLL), similar to what is reported for the L inverter. Interestingly, the Zqd component also exhibits this behavior, which may further impair the external stability of the system. The grid impedance matrix is estimated using disturbance signals, with pseudo-random binary reference current levels imposed by the inverter. Frequency domain analyses reveal the need for an exclusive PLL (EPLL) for the estimation of this matrix, whose bandwidth must be designed within a frequency range lower than those of interest in the estimation. Selecting a wider bandwidth can lead to incorrect assessments of system stability.