This article presents an in-situ comparative analysis and power quality tests of a newly developed photovoltaic charging system for e-bikes. The various control methods of the inverter are modeled and a single-phase grid-connected inverter is tested under different conditions. Models are constituted for two current control methods; the proportional resonance and the synchronous rotating frames. In order to determine the influence of the control parameters, the system is analyzed analytically in the time domain as well as in the frequency domain by simulation. The tests indicated the resonance instability of the photovoltaic inverter. The passivity impedance-based stability criterion is applied in order to analyze the phenomenon of resonance instability. In conclusion, the phase-locked loop (PLL) bandwidth and control parameters of the current loop have a major effect on the output admittance of the inverter, which should be adjusted to make the system stable.
Grid-forming converters are attracting attention for their significant advantages in terms of stability in a weak grid and simulated inertia. However, while they offer great flexibility due to the use of power semiconductors, they are also affected by their low current-carrying capacity. This means that during a fault, instead of the usual voltage control, a current limiting control is active, which changes the dynamic performance of the converter and influences transient stability. This manuscript focuses on the dynamic performance of grid-forming converters during the restart phase at the post-fault period, and proposes an initial phase threshold to prevent the converter from going into current saturation. Based on this, the manuscript proposes several restart strategies during the post-fault period, by using some fast resynchronization methods in order to meet the requirements of the converter’s stable operation and fast active power restoration. Finally, the above findings and the proposed strategies are validated by a joint control hardware-in-the-loop system.
The precise control of output power by grid-connected converters relies on the correct identification and tracking of a grid voltage’s phase at the converter terminal. During severe grid faults, large disturbances cause the converter’s operating point to move away from the stable equilibrium point during normal operation. This leads to oscillations of both the active and reactive power fed into the grid. Using large-signal modelling, this study investigated the converter’s dynamic processes during and after such fault situations. The investigation considered the influence of the converter’s phase-locked loop (PLL), responsible for phase tracking, as well as that of the DC link on the converter-grid system, which has a major influence on the active power exchange with the grid. On this basis, this study also focused on the reactive current reference’s influence during and after fault clearing. Furthermore, an easily implementable strategy for reactive current injection, leading to minimum power oscillations, was presented. The results and the optimized strategies were validated via controller hardware-in-the-loop tests.
Maintaining frequency stability is one of the central objectives of power grid operation. While this task is currently primarily done by employing stored rotational energy, in a converter-dominated grid fed by renewables, sources, such as wind and photovoltaic, must be involved in the frequency control of the power grid in order to maintain a stable, efficient grid operation in the process of achieving carbon neutrality. However, due to the lack of rotational inertia reserves, the converter requires additional energy storage devices to respond to the grid’s frequency regulation requirements. For modeling a converter-dominated grid, the behavior of such additional short-time storage must be modeled properly in order to obtain realistic simulation results, which allow drawing conclusions about the frequency stability behavior of the grid. This paper investigates the boundaries of the dynamic performance of the output current of the energy storage device so that the converter can achieve the function of frequency regulation in a more economical manner. In this paper, a dynamic supporting converter based on a phase-locked loop and a grid-forming converter, as well as the DC link of the converter containing an energy storage system, are modeled. On this basis, the optimal boundaries of the dynamic performance of the output current of the energy storage device are investigated. It is concluded that not only the proper sizing of grid-supporting energy storage devices is important for proper grid operation, but the dynamic behavior also has to be modeled and designed properly in order to guarantee a stable operation under all circumstances.
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