Doctoral Thesis Defense Session No. 50 of PPGEE - Stability analysis for grid-connected LCL inverters based on analytical expressions and impedance estimates in the synchronous domain DQ

STUDENT: DIEGO OLIVEIRA CARDOSO

DATE: 08/16/2024

TIME: 2:00 PM

LOCATION: Universidade Federal da Bahia - Escola Politécnica

 

TITLE: Stability analysis for grid-connected LCL inverters based on analytical expressions and impedance estimates in the DQ synchronous domain

 

KEYWORDS: Grid-connected LCL inverter; Generalized Nyquist criterion; Synchronous reference 𝑑𝑞; Impedance models; Pseudo-random binary disturbances; Phase-locked loops.

 

ABSTRACT:

The diversification of the energy matrix has been driving the emergence of new forms of electricity generation, often integrated into the power grid by means of an inverter. The control of this device is usually designed without considering the impact of the connection to the grid. One approach to assess the stability of connected systems that employ inverters controlled in the synchronous reference frame 𝑑𝑞 is to apply the generalized Nyquist criterion (GNC) to the product between the grid impedance matrix and the inverse of the inverter impedance matrix. In this work, we propose to obtain the impedance matrices of the LCL inverter and the power grid. The impedance matrix of the LCL inverter is established by means of an analytical expression. The matrix is ​​coupled and its components present 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 system and an experimental setup validate the developed models. They confirm that the 𝑍𝑞𝑞 component of the matrix behaves as a negative resistor within the control phase-locked loop (PLL) bandwidth (CPLL), similar to what is reported for the L inverter. Interestingly, the 𝑍𝑞𝑑 component also presents this behavior, which can further impair the external stability of the system. The grid impedance matrix is ​​estimated using disturbance signals, with binary pseudo-random reference current levels, imposed by the inverter. Frequency domain analyses reveal the need for a dedicated PLL (EPLL) for the estimation of this matrix, whose passband should be designed within a smaller frequency range than those of interest in the estimation. Selecting a wider band can lead to incorrect assessments of the system stability.

 

MEMBERS OF THE BOARD:

FABIANO FRAGOSO COSTA (ADVISOR)      UFBA (President)

ANDRE PIRES NOBREGA TAHIM                UFBA

DARLAN ALEXANDRIA FERNANDES           UFPB

EDUARDO LENZ CEZAR                            ITA

ROGERIO GASPAR DE ALMEIDA                UFPB

Em 07/08/2024