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Virtual Synchronous Machine Based Control of Photovoltaic (PV) Inverters

Switching model of three-phase PV inverter
Fig. 1. Switching model of three-phase PV inverter
Coal-fueled power generation is being retired by several utilities with the advent of natural gas as an energy source. Further, variety in power generation is sought by utilities to a point that renewable energy sources are being integrated into the power grid and causing a decrease in system inertia. As such, the response of the grid is fluctuations in frequency, or the voltage is slower based on the amount of kinetic energy stored in the rotating mass of the generation sources.

Conventional photovoltaic (PV) inverters utilize phase-locked loops (PLL) connected at the point of common coupling (PCC) to synchronize to the grid. However, fluctuations in the grid frequency and voltage will propagate through the control to the PV inverter which can trip the PV farm offline. A recently proposed control scheme suggests duplicating PV inverters synchronous machines via virtual inertial and impedance. The performance of the virtual synchronous machine (VSM)-based control is assessed by building the average model of the PV inverter in the abc-frame and dq-frame. In the abc-frame, the response of the PV inverter to grid events, such as faults, can be modeled by perturbing the frequency at the PCC (Fig. 1). These results are compared to the PLL control to assess the functionality of the VSM control during contingencies, and to determine if the propagation is mitigated by the VSM-based control. In the dq-frame, stability analysis of the PV inverter with VSM control can be calculated and modeled using the dq-frame output impedance of the inverter and the linearization tool in MATLAB. These results can then be compared to the conventional PLL control to assess the overall performance of the VSM control.

This paper assesses the performance of the VSM control scheme for a PV inverter. Specifically, the inverter coupled with the response of the control to fluctuations in the grid due to planned and unplanned events, in addition to stability analysis of the inverter based on the dq-frame output impedance using the generalized Nyquist stability criterion. Additionally, these results are compared to the conventional PLL to analyze whether or not this new control scheme better synergizes the PV farm with the grid.
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