Synchronised control of grid-interactive inverters using stationary frame resonant regulators
Creators: name: Nazib, Afif
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400803
subject: Electrical energy generation (incl. renewables, excl. photovoltaics)

description: Over the last two decades grid-interactive inverters have emerged as an essential component to merge distributed generation (DG) into electrical grids, configured as both grid-connected and standalone structures. These systems operate in two modes; grid-following and grid-forming. For grid-following inverters, the closed loop current regulator and synchronisation unit are the two important components. However these components require careful tuning to achieve stable operation. In addition, a complex control structure is often required to accommodate grid imbalance and distortion. For standalone or grid-forming inverters, the control of parallel-connected grid-forming inverters poses more challenges. Until now, multiloop droop-based voltage-controlled strategies have been the main approach proposed to achieve parallel inverter operation in an islanded microgrid. However, the efficacy of such strategies is still under investigation, particularly since the droop response of a DG inverter is an artificial response that mimics the natural response of a rotating machine.<br><br>The synchronisation unit, typically the phase locked loop (PLL) is essential for both grid-connected and standalone inverters to accurately estimate the grid phase angle and frequency. Its implementation and performance have been widely discussed in the literature; with the unanimous conclusion that a trade-off between the PLL dynamic response and disturbance rejection is required for a standard system. While complex synchronisation techniques have been developed to overcome these limitations, they introduce practical implementation issues, consequently leading to a more complicated overall control structure with potential stability issues. <br><br>This thesis now explores an integrated self-synchronising control strategy for grid-interactive inverters that has simplicity and excellent robustness to adverse grid conditions compared to the traditional multi-loop control strategies. The first part of the thesis focuses on grid-connected inverter operation using a stationary frame Proportional Resonant (PR) current regulator with harmonic compensators (HC). To begin, the limitation of a standard PLL for direct synchronisation using voltage sensors is investigated for an inductive (L) filtered system, illustrating the trade-off between the synchronisation dynamic response and disturbance rejection. The principles of self-synchronisation, which synchronises the PLL to the resonant regulator modulation command instead of the measured grid voltage, are established, and the benefits of the approach are demonstrated.<br><br>Next, the thesis extends the self-synchronising approach to an inductive-capacitive-inductive (LCL) filtered system. The limitation of a voltage-sensing high bandwidth PLL for an LCL filtered system using inverter-side current regulation is demonstrated under distorted grid conditions. A novel control strategy based on the self-synchronising approach is then proposed to achieve indirect grid current regulation with high disturbance rejection under distorted grid conditions, using only the inverter-side currents. The thesis continues by enhancing the self-synchronising control strategy for unbalanced grid conditions. It first shows the effect of the self-synchronising control strategy under unbalanced grid conditions and then develops a simple approach to achieve accurate synchronisation, current regulation and robustness even under extreme unbalanced grid conditions.<br><br>The thesis then identifies that the PLL within the self-synchronising approach can be eliminated and proposes instead a self-synchronising current control strategy with integrated frequency tracking. The self-synchronising principle is developed in detail and synchronisation without a PLL is demonstrated with its bandwidth solely dependent on the PR current regulator bandwidth. The outcome is an integrated self-synchronising current control strategy for grid-connected inverters which is fast, simple and robust against distorted grid voltages and varying grid frequency.<br><br>The final part of the thesis looks at grid-forming inverter operation. A new control approach which utilizes the PR controller intrinsic droop response is proposed to achieve power sharing with voltage and current loop control and a PLL. This intrinsic droop mechanism is then explained. Finally, the self-synchronsing approach is incorporated into the PR voltage controller to achieve a self-synchronising controller for an islanded microgrid which is simple but very effective compared to traditional multiloop control strategies.