Abstract
This paper presents a robust multi-input multi-output (MIMO) state-feedback control scheme for a photovoltaic (PV) inverter connected to a weak grid. For a weak grid, the point of common coupling (PCC) voltage is very sensitive to the power disturbances and it is dynamically coupled to the PLL dynamics. So far, most of the control methods do not take into accounts these couplings. Therefore, in this paper, the MIMO controller was designed taking into account the dynamics of the phase-locked loop (PLL) and coupling effects between PCC voltage and the active power to enhance the system’s robustness. As result, the robust performance of the PV inverter interfaced to a weak grid was yielded. In addition, the sensitivity of the system to PLL was eliminated, allowing the use of larger PLL bandwidth even in a very weak grid. Based on the eigenvalues analysis method, a comparative study between the proposed control method and the conventional vector control method was performed. The proposed method is verified with simulations in PLECS and real-time simulations in the RT Box. The results show that the proposed MIMO control method preserves the system stability robustness against any change of grid strength, generated power and PLL bandwidth.
Highlights
Renewable energy sources such as photovoltaic systems, wind power, etc. are increasingly integrated into the grid through Voltage Source Converters (VSC) to overcome the climate change issues related to conventional energy sources for instance coal, fuels, etc.The large PV systems are located far away from the load centre and require a long transmission line to interface to the main grid [1]
The DC-link voltage follows the reference Vmpp obtained from the maximum power point tracking (MPPT) and the point of common coupling (PCC) voltage follows its reference values
When a step change of solar insolation occurs at 1 s, with G jumps to 950 W/m2 corresponding to steady-state of 0.85 PU and 0.235 PU active and reactive power, respectively, and 1.01 PU maximum PV voltage, the rapidly dying oscillations occur for conventional control method, while for the proposed multi-input multi-output (MIMO) system there are no induced oscillations
Summary
Renewable energy sources such as photovoltaic systems, wind power, etc. are increasingly integrated into the grid through Voltage Source Converters (VSC) to overcome the climate change issues related to conventional energy sources for instance coal, fuels, etc. From the analytical results in [6], the interaction of the PLL and DVC reduces the damping of low-frequency oscillations and it may result in the lower-frequency instabilities, which are identified to range in frequency of the outer-loop control (AVC and DVC) and PLL [4,5] Such instabilities can be amplified by the AVC required to stabilise the VSC when it interacts with the weak grid by providing the required reactive power to compensate for the voltage drop across the line impedance. [18] proposed an H∞ robust control for the AVC loop to stabilise the system under a very weak grid while considering the PLL dynamics on PCC link voltage.
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